Analysis of corrosion pits in weld seams of internally threaded water-cooled wall tubes in subcritical boilers

Analysis of corrosion pits in weld seams of internally threaded water-cooled wall tubes in subcritical boilers | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Analysis of corrosion pits in weld seams of internally threaded water-cooled wall tubes in subcritical boilers Linfei XIAO This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7250163/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract For the problem of corrosion pits in the welds of internally threaded water-cooled wall tubes in a subcritical boiler, the failure mechanism under the synergistic effect of structural mutation and high-temperature hydrogen corrosion was revealed by means of laser confocal inspection, metallurgical examination, XRD analysis, and EDS analysis. The study shows that the fire-side weld of the front wall in the burnout wind region causes flow field distortion due to root protrusion and groove, which induces Cl − and Si 4+ /Al 3+ enrichment, forming pitting corrosion and subscale corrosion. at the same time, high-temperature hydrogen corrosion is triggered by the wall temperature of more than 400 ℃, microcracks at the near inner-wall grain boundaries are expanded and form mesh defects, and the scale layer exacerbates the local temperature rise to form the vicious circle of “high-temperature corrosion-crack expansion” and “high-temperature corrosion-crack expansion”. "Vicious cycle. A four-stage corrosion pit evolution model is proposed, and it is suggested to optimize the weld runner design and strictly control the water quality and heat load to block the failure chain. internally threaded water-cooled wall hydrogen corrosion structural mutation flow field distortion subcritical boiler Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1 Introduction With the widespread implementation of flexibility retrofit in coal-fired units, deep wide-load peaking operation has become the norm [ 1 – 2 ]. This operation mode leads to frequent over-temperature of water-cooled wall tubes and significantly increases the alternating thermal stresses on the heated surfaces [ 3 – 4 ], which in turn triggers a variety of failure modes, including fatigue cracking failure [ 5 ], transverse cracking failure [ 6 ], and hydrogen corrosion downgauging failure [ 7 ]. High-temperature hydrogen corrosion is an important mode of water-cooled wall failure, and studies have shown that high-temperature hydrogen corrosion inducing factors mainly include high-temperature corrosion of water-cooled wall caused by oxygen-rich combustion conditions [ 8 – 9 ], localized overheating due to flame center deviation [ 10 – 11 ], and fire-side reducing atmosphere exacerbated by low-nitrogen combustion [ 12 – 13 ], which affects the safe in-service performance of water-cooled wall tubes. Regarding the mechanism of high-temperature hydrogen corrosion, Chevreux N [ 14 ] and Nazarov V [ 15 ] showed that the formation of methane cavities in carbide-enriched zones and their induced localized stress concentrations are the key factors for microcracks sprouting, and Alshahrani M [ 16 ] further revealed the role of the thermodynamic stability of the carbide in the modulation of the hydrogen erosion resistance. Notably, Goritskii M [ 17 ] found that the proportion of intergranular features on brittle fracture surfaces due to hydrogen erosion can be abruptly increased to 50.5%, and this microstructural degradation echoes the dislocation creep-controlled cavity growth mechanism revealed by D Han [ 18 ] through finite element simulations. When the material is in a complex stress state, as Chen [ 19 ] pointed out the multiple effects of Nb atoms on the hydrogen behavior, the hydrogen corrosion resistance of the material exhibits significant nonlinear characteristics. The corrosion case of water-cooled wall tubes in a thermal power plant B&WB-1025/18.44-M boiler provides engineering evidence for the above mechanism study. The boiler belongs to subcritical parameter, intermediate reheat, natural circulation single-drum Π-shaped boiler. The water-cooled wall adopts membrane-type all-welded structure, and the pipe section from the inflection point of the cold ash hopper to the angle of the folded flame adopts internally threaded pipe, with the specification of Ф60×6.5mm, the material of which is SA-210C, and the chemical composition of which is shown in Table 1 . After 131840 hours of operation of the internally threaded water-cooled wall pipe in the exhaust air area, endoscopic inspection revealed that. corrosion pits appeared in the middle of the weld joints of the fire-side of the front wall, presenting the typical characteristics of hydrogen corrosion, which was similar to that of the water-cooled wall of the Mahmoud [ 20 ]. The localized superheat-induced creep damage model reported by Khedr [ 20 ] and the temperature field-stress field coupling model established by Mavzovin V S [ 21 ] are highly consistent. In particular, the special stress distribution of the internally threaded pipe structure in the region of heat load concentration may have accelerated the process of methane pressure-driven microcrack extension as described by Nazarov V V [ 15 ]. The preferential appearance of corrosion pits in the middle of the weld in this case further verifies the applicability of the grain boundary diffusion-assisted cavity growth theory in the region of welded joints, and provides a new reference for failure analysis for water-cooled wall water-cooled corrosion protection against high-temperature hydrogen corrosion in subcritical boilers. Table 1 Chemical composition of SA-210C water-cooled wall tube Elements C Si Mn Fe Chemical composition(wt.%) 0.25 0.26 0.83 98.66 2 Experimental materials and methods Corrosion pit specimens and comparison tube specimens were intercepted at a height of 30 m in the combustion air area of the boiler front wall for systematic analysis. An Olympus LEXT OLS5100 laser confocal microscope was used to scan the three-dimensional morphology of the corrosion pits, with a sampling frequency of 3000 Hz, to determine the diameter and depth parameters. A Thermo Niton XL2 XRF spectrometer was used to characterize the chemical composition of the water-cooled wall tubes. In the sample pretreatment stage, 180# to 800# series sandpaper was used sequentially for gradient grinding, and finally 2µm diamond polishing solution was used to complete the fine polishing, and 4% nitric acid alcohol solution was selected for corrosion treatment. For the fire-side scale samples, after the special cleaning solution treatment and drying, the electronic balance was used to weigh accurately, and the Malvern Panaco Epsilon 4 energy dispersive fluorescence spectrometer was used for compositional analysis. The observation of microstructure was accomplished using a Leica DMI3000M inverted metallurgical microscope, while the micro-morphology and compositional analysis was realized by a Zeiss ZEM20 field emission scanning electron microscope. The physical characterization of the corrosion products was carried out by a D8 DISCOVER X-ray diffractometer, with the test conditions set as follows. scanning angle range of 10°-90°, θ-2θ working mode, tube voltage of 35kV, tube current of 40mA, scanning step length of 150 s/step, and a total of 4 steps of data collection. 3 Results and Analysis 3.1 Macroscopy Macroscopic examination of the water-cooled wall tube after autopsy, the macroscopic morphology of the inner and outer walls as shown in Fig. 1 a, the specimen to the fire-side of the outer wall of the presence of obvious flue gas corrosion characteristics, corrosion products in the form of white deposits attached to the surface of the tube wall. It is worth noting that the corrosion pits are mainly concentrated in the middle of the weld seam on the fireward side of the inner wall of the water-cooled wall tube, and their macroscopic morphology shows the circular characteristics after the vapor vortex scouring. The three-dimensional morphology of the corrosion pits was scanned by laser confocal microscope, and the measurement by body microscope showed that the maximum diameter of the corrosion pits was 15.7 mm, see Fig. 1 b, and the depth of the corrosion pits was further scanned by laser confocal system in two transverse and one longitudinal scans, see Fig. 1 c, and the results showed that the maximum depth of the corrosion pits was 3.9 mm, and the thickness of the wall thickness was reduced by 60%. Water-cooled wall pipe strength calculations show that the nominal wall thickness of the pipe is 3.64mm, while the current remaining wall thickness has been significantly lower than the strength of the design requirements of the critical value. This severe wall thinning not only significantly reduces the load carrying capacity of the material, but may also lead to potential stress concentrations and risk of pipe burst. 3.2 Chemical sampling analysis Along the weld cutting water-cooled wall pipe samples at both ends of the base material, and chemical cleaning, such as Fig. 2 a, the comparison of the pipe and corrosion pits pipe at both ends of the base material in the cleaning of the inner surface shows a metallic luster, no obvious corrosion pits, a phenomenon that shows that the corrosion defects have a distinctive localized characteristics, mainly concentrated in the middle area of the weld seam. The scale samples of the tube samples were analyzed, and the results showed that the amount of scale on the fire-side was about 35% higher than that on the back-side. The scale amount of the fire-side and back-side of the comparison tube was 117.55 g/m 2 and 87.02 g/m 2 respectively, and there was not much difference between the results of the corrosion pit tube and the scale amount of the comparison tube, with the scale amount being in the same order of magnitude, and all the measured values of the scale amount were much lower than the chemical cleaning thresholds stipulated in the “Chemical Cleaning Guidelines for Boilers in Thermal Power Plants” (DL/T 794–2012) (the main steam pressure > 15.6 MPa operation of the water-cooled wall of the ladle furnace scale amount > 250 g/m 2 ), indicating that the wall scaling is not the main cause of corrosion. The chemical composition of the inner wall scale samples is shown in Table 2 , and the results show that. the main components of the scale samples are Fe 2 O 3 , SiO 2 and Al 2 O 3 , which is consistent with the typical characteristics of the deposition of metal oxidation products and silica-aluminum compounds carried by the feed water in the course of boiler operation. The compositional differences between different tube samples were all within the same order of magnitude, further verifying that the formation of corrosion pits was not directly related to the overall fouling condition. Therefore, the formation of corrosion pits in the middle of the weld is the result of local factors rather than an overall scaling or corrosion problem. the weld area may have special conditions such as structural mutation or stress concentration, which makes it more susceptible to localized corrosion under the same operating environment. the higher scaling on the fire-side is related to the higher heat load and more favorable deposition conditions on that side, but the overall scaling level is still in the normal range. According to the laboratory's historical furnace water analysis report records, 36 months prior to the discovery of the corrosion defect, there was an intermittent exceedance of the chlorine ion content of the furnace water that lasted for one month. Monitoring data show that the fluctuation range of chloride ion concentration during this period was 0.68–0.90 mg/L, with the highest value exceeding the limit of chloride ion content (≤ 0.4 mg/L) specified in the standard “Quality of Water Vapor of Thermal Generating Units and Steam Power Equipment” (GB/T 12145 − 2016) by 125%. The power plant has gradually reduced the chlorine ion concentration of the furnace water to 0.204 mg/L by increasing the operation and adjustment measures of the continuous discharge volume, and finally reached the standard requirement stably. The optimized operation of the continuous discharge system can effectively control the concentration of chloride ions in the furnace water, but the short-term exposure to high concentrations of chloride ions may accelerate the process of pitting corrosion of metal materials, laying a hidden danger for the formation of subsequent corrosion defects. Table 2 Analysis of the composition of scale samples of water-cooled wall comparison tube and corrosion pit tube Tubular Scale Sample Composition Fe 2 O 3 SiO 2 Al 2 O 3 Cr 2 O 3 SO 3 ZnO CaO MnO Content (wt.%) comparison tube 74.71 9.77 7.29 2.41 1.39 1.21 0.84 0.78 corrosion Pit Tube 73.94 9.88 7.09 3.14 1.52 1.52 0.66 0.83 3.3 Microstructural analysis Axial observation of the metallographic organization of both sides of the corrosion pit, see Fig. 3 a. The corrosion pit along the weld axial middle presents an oval pit shape, the inner wall of the pit is attached with serious oxide skin, the maximum thickness of the oxide skin is up to 2 mm, the formation of the oxide skin is the metal at high temperature in the environment of chemical reaction with oxygen, and the thickening of its not only reflecting the long-term role of the high temperature environment, but also on the tube wall of the further corrosion process. At the bottom of the corrosion pits, there are four circumferential macroscopic cracks, with a maximum crack depth of about 916 µm, and Fig. b shows that there are vertically distributed along-crystalline microcracks on both sides of the macroscopic cracks. The parent material organization is shown in Fig. 3 c and d. The organization is ferrite plus pearlite, and the content of pearlite decreases gradually from the core to the inner wall, and the pearlite in the inner wall disappears completely, which is replaced by the filling of the cracks by the oxidation products. Moreover, the microcracks in the range of 250 µm near the inner wall have been completely filled with oxides, indicating that this region has experienced a long and severe oxidation process. At the same time, different degrees of decarburization occurred near the inner wall, and the degree of decarburization gradually increased from the core to the inner wall, which is a reaction driven by high temperature [ 22 ]. Under the high temperature environment, the hydrogen atoms may react with the carbon in the steel, so that the carbon element gradually escapes from the metal, which results in decarburization. Black stripes of intergranular microcracks existed on the inner surface of the weld area and the parent material of 200 mm on both sides of the weld, and the intergranular microcracks in the parent material were distributed in the form of a network, while the intergranular microcracks at the weld showed a worm-like distribution, and the emergence of the intergranular microcracks further weakened the strength and integrity of the metal material, which may become a channel for further intrusion of corrosive media, accelerating the damage of the water-cooled wall. The main cracks of the weld and the micro-morphology of the base material were observed under scanning electron microscope and analyzed by EDS surface scanning, and the results are shown in Fig. 4 and Table 3 .As shown in Fig. 4 a, the weld A area shows typical oxide filling and hole morphology, indicating that the base material has undergone significant oxidative corrosion under high temperature and high pressure environment. The microstructure of the base metal in Fig. 4 c shows reticulated along-crystalline microcracks and macrocracks, with cracks expanding from the inner wall to the outer wall, and the aggregation of microcracks causes the generation of macrocracks.The EDS surface-scanning analysis results show a significant decrease in the carbon content in the weld A area and in the base metal near the cracks, which confirms the decarburization phenomenon caused by high-temperature hydrogen corrosion. This phenomenon is consistent with the hydrogen corrosion mechanism. at high temperatures (> 400°C), water vapor reacts with iron to form atomic hydrogen ([H]), which diffuses into the steel and reacts with the carburizer (Fe 3 C) to form methane (CH 4 ). As CH₄ accumulates at grain boundaries and forms high-pressure micropores [ 17 ], it ultimately induces the initiation and extension of cracks along the grain. Table 3 EDS surface scanning element results for weld and base material Elements Fe O C Mn Si 其他 Content (wt.%) Figure 4 b 77.03 16.11 4.01 0.81 0.60 1.44 Figure 4 c 95.10 - 3.72 0.88 0.30 - 3.4 Analysis of corrosion products Through anatomical observation of the corrosion pits of the water-cooled wall tubes, it was found that the corrosion pits showed an obvious layered structure (Fig. 5 a). X-ray diffraction (XRD) was used to analyze the physical phases of the corrosion products at the bottom, middle and top of the corrosion pits, which are shown in Fig. 5 b. As can be seen from the figure, the physical phases of the main peak are mainly dominated by Fe 2 O 3 and Fe 3 O 4 , while the corrosion products also contain part of FeS, FeCl 2 and FeSO 3 , of which Fe 2 O 3 is a common product of the oxidation of Fe under an aerobic environment, and Fe 3 O 4 is preferentially generated by water-cooled wall under high temperature, low oxygen or reducing environment. the appearance of FeS implies that the corrosion product is a common product of the oxidation of Fe in an aerobic environment. Low oxygen or reducing environment preferentially generated. the appearance of FeS implies that there may be sulfur-containing substances in the corrosive environment, the sulfur-containing components of the thermal power plant fuel in the combustion process and the subsequent steam cycle and other processes, it is possible to produce corrosive sulfur-containing media involved in the corrosion reaction. FeCl 2 may be associated with the historical furnace water Cl − exceeds the standard. FeSO 3 may be in the complex chain of corrosion reaction by sulfur-containing material further reaction generation. The mass percentage of each material phase was calculated by using the reference intensity ratio (RIR) method, which is shown in Eq. ( 1 ). The XRD patterns were split-peak fitted by the BG function to obtain the characteristic peak intensities of each material phase, and the RIR values of the corresponding material phases were found against the PDF cards, and the results are shown in Table 4 , which quantitatively analyzed the distribution of the material phases of the corrosion products in each layer, as shown in Fig. 5 c. The results show that the bottom corrosion products are dominated by Fe 3 O 4 (54.80%), indicating that the region is in a high-temperature reducing environment, and the iron matrix mainly reacts with water vapor or sulfide to generate Fe 3 O 4 (reaction formula. 3Fe + 4H 2 O→Fe 3 O 4 + 4H 2 ). The Fe 3 O 4 content in the middle and upper corrosion products decreased to 45.93% and 46.49%, respectively, while the Fe 2 O 3 content increased significantly in the upper part (33.34%), indicating that the upper part was closer to an oxidizing atmosphere, which prompted the further oxidation of Fe 3 O 4 (4Fe 3 O 4 + O 2 →6Fe 2 O 3 ).The presence of FeS, FeCl 2 and FeSO 3 indicates that the corrosion process The presence of sulfur and chlorine involved in the corrosion process may originate from sulfides (H 2 S/SO 2 ) and chlorides (HCl) in the fuel, which form an acidic corrosive environment locally (Fe + H 2 S→FeS + H₂). The corrosion stratification mechanism can be summarized as follows. the bottom is dominated by Fe 3 O 4 due to high-temperature oxygen deficiency. the middle is the transition zone, where Fe 3 O 4 coexists with Fe 2 O 3 and the upper part has an elevated proportion of Fe 2 O 3 due to enhanced oxygen diffusion. This phenomenon reflects the direct influence of oxygen concentration gradient on the composition of corrosion products in the burnout wind region, and confirms the role of sulfur/chlorine synergistic corrosion. 1 where V x is the mass percentage of substance phase i . I i is the diffraction peak intensity of substance phase i , and RIR i is the reference intensity ratio of substance phase i . Table 4 R-values and peak intensities of XRD physical phases Serial number Phase 2θ (diffraction angle) RIR hkl Diffraction peak intensity. Bottom oxide Middle oxide Top oxide 1 FeSO 3 24.13 1.33 12−1 65 81 126 2 Fe 3 O 4 30.095 2.12 400 76 256 104 3 Fe 2 O 3 33.157 3.29 104 109 347 355 4 Fe 3 O 4 35.482 5.03 311 376 751 296 5 FeCl 2 35.429 4.71 104 373 750 295 6 Fe 3 O 4 40.205 2.03 424 109 102 120 7 Fe 3 O 4 43.123 5.03 400 131 152 134 8 FeS 43.126 3.96 114 130 150 132 9 Fe 3 O 4 49.462 3.26 024 114 145 185 10 Fe 3 O 4 53.499 5.03 422 88 140 80 11 Fe 2 O 3 54.066 3.29 116 146 140 236 12 Fe 3 O 4 57.031 5.03 511 192 238 170 13 Fe 3 O 4 62.627 5.03 440 311 409 290 14 Fe 2 O 3 64.002 3.29 300 146 120 192 3.5 Structural analysis of welded seams To reveal the general pattern of corrosion pits in the weld seams of the water-cooled wall in the overfire air zone, tube sections adjacent to the corrosion pits were cut and sampled, and the macroscopic morphology of the tube samples was analyzed, as shown in Fig. 6 . Macroscopic examination revealed that the layered yellow substance attached to the inner surface of the tube samples, confirmed by spectral analysis (Mn = 1.18 wt.%, Fe = 98.59 wt.%), was an adherent formed by the cooling of high-temperature molten iron during the tube cutting process, rather than a corrosion product. As shown in the figure, the weld seams of the water-cooled wall tubes on the fire-side in Figs. 6 a and b exhibit no signs of corrosion pits, with a smaller backing layer spacing of 3.22 mm. The weld root is flush with the base material, and the transition zone from the internal threads to the weld is smoothly treated with a relief groove, showing no significant structural mutations. In contrast, the weld seams of the water-cooled wall tubes on the fire-side in Figs. 6 c and d display obvious corrosion pits, with an axial length of approximately 26.58 mm, and the axial diameter terminates at the root of the relief groove. The spacing of the weld backing layer significantly increases to 9.98 mm, which is 3.1 times the width of the comparison tube weld. Additionally, the weld root protrudes 2.53 mm from the base material, forming a groove structure with a depth of about 1.1 mm. In Fig. 6 c and d, the structural mutation at the root of the weld on the fire-side significantly deteriorates the local flow field characteristics. Hou[ 23 ] also confirmed this through simulation using a two-phase flow model, with the governing equations employing the incompressible Navier-Stokes equations, as shown in Eq. ( 2 ). The results indicate that (1) Due to the presence of internal threads and the structural mutation of the weld, a larger low-velocity zone exists near the inner wall weld of the water-cooled wall tube [ 24 ]. (2) Vortices occur at the root area of the weld connected to the threaded channel, leading to backflow. This flow field distortion promotes the enrichment of aggressive ions such as Cl⁻ in the vortex region [ 25 ], forming a local high-concentration corrosive environment. Simultaneously, the protruding weld root experiences enhanced turbulent shear forces, making the oxide film more prone to detachment, ultimately leading to preferential corrosion in this area. The research findings confirm that geometric factors such as the smoothness of the weld root and the curvature radius of the transition zone are key factors controlling the corrosion susceptibility of water-cooled wall tubes. In engineering practice, it is essential to strictly avoid designs with abrupt structural changes. $$\:\text{ρ}\text{}\text{dfrac}\text{∂}\text{u}\text{∂}\text{t}\text{−∇⋅}\eta\left(\text{∇}\text{u}\text{+(∇}\text{u}{\text{)}}^{\text{T}}\right)\text{+}\text{ρ}\left(\text{u}\text{⋅∇}\right)\text{u}\text{+∇}\text{p}\text{=}\text{F}$$ 2 where ρ is the density, u is the velocity, η is the dynamical viscosity. p is the pressure. and F is the source term. 3.6 Discussion Combined with the above test, it is analyzed that the main reason for the corrosion pits in the weld seam of the inner threaded water-cooled wall is. the sudden change of the weld seam structure caused by the change of the water-cooled wall media flow field. the secondary reason is the high temperature of the exhaust air area, the high load on the fire-side, the occurrence of high-temperature hydrogen corrosion, and the Cl − exceeding the standard of the water quality caused by the buildup of impurities in the weld to form the corrosion of the under-scaling. (1) Structural mutation of weld seam destroys flow field changes. Internal threaded water-cooled wall pipe to the fire-side of the weld there are significant structural differences, compared to the pipe weld (Fig. 6 a, b) bottoming layer spacing is small, the root and the base material level, the internal thread retractor groove is smooth, the flow path is continuous. and corrosion pit pipe weld (Fig. 6 c, d) bottoming layer spacing of up to 3.1 times the comparison tube, the root protrudes from the base material of 2.53mm and the formation of a depth of 1.1mm notch. Structural changes in the water-cooled wall pipe medium flow field drastically changed, on the one hand, caused by low-flow velocity area and flow separation, the sudden change in the fluid flow is blocked, the kinetic energy is converted into pressure energy, the formation of low-flow velocity area, the sudden change in flow velocity will lead to flow separation phenomenon, resulting in the medium in the middle of the weld stagnation, for the aggressiveness of the ions to gather to create the conditions. On the other hand, the vortex and reflux phenomenon is triggered, the threaded channel and weld groove interface, the sudden change of the flow channel morphology induced vortex. Vortex area fluid movement disorder, not only impede the normal flow of media, but also due to the vortex effect of Cl − , carried in the feed water Si 4+ , Al 3+ continued to enrich the formation of local corrosive environments and scale deposition of the environment, the water quality of the historical data show that the furnace water Cl − had exceeded the standard of 0.9 mg/L, exceeding the national standard of 125%, pitting corrosion, the generation of FeCl 2 , the occurrence of Sub-catalyzed acidification, the generation of HCl, the reaction is shown in the formula (3) and (4). $$\:\text{Fe}\text{+2}\text{C}{\text{l}}^{\text{−}}\text{→}\text{FeC}{\text{l}}_{\text{2}}\text{+2}{\text{e}}^{\text{−}}$$ 3 $$\:\text{FeC}{\text{l}}_{\text{2}}\text{+}{\text{H}}_{\text{2}}\text{O}\text{→}\text{Fe}\text{(}\text{OH}{\text{)}}_{\text{2}}\text{+2}\text{HCl}$$ 4 (2) High load in the exhaust air area triggers high temperature corrosion. Burnout wind area to the fire-side of the water-cooled wall scale is larger, higher heat load, when the fire-side of the water-cooled wall pipe wall temperature is greater than 400 ℃, the tube produces vapor stratification or steam stagnation, the occurrence of the reaction of the formula (5) to generate [H]. According to the Arrhenius formula, the reaction rate increases exponentially with increasing temperature, the reaction is also more violent, so that the metal oxide film generation and destruction of the equilibrium tilted to the “rapid destruction”. Generated free [H] can not be quickly taken away by the medium, under the action of high temperature along the lattice and grain boundaries to the fire-side of the matrix to the internal diffusion, and with the steel Fe3C, free C reaction, see formula (6) and (7), to generate CH 4 , corrosion pits at the bottom of the Fe 3 O 4 began to be deposited. Continuous generation of H from the inner surface to the outer surface of the thickness of the direction of the existence of gradient distribution characteristics [ 26 ], the surface of the C is first consumed to form the surface decarburization phenomenon, CH 4 aggregated at grain boundaries, induced intergranular cracks sprouting [ 15 ], cracks will trigger a significant stress concentration effect. and at the same time, due to the accumulation of scale thermal conductivity is poor, it will lead to the wall of the pipe wall local temperature rises sharply, which further accelerated the corrosion process. Thus, a vicious cycle of "high-temperature hydrogen corrosion - crack expansion - corrosion pit aggravation" is formed. the microcracks gradually expand and connect with each other to form a reticulation along the crystalline crack defects shown in Fig. 3 (c). $$\:\text{Fe}\text{+}{\text{H}}_{\text{2}}\text{O}\text{→}{\text{Fe}}_{\text{3}}{\text{O}}_{\text{4}}\text{+8[}\text{H}\text{]}$$ 5 $$\:\text{2}{\text{H}}_{\text{2}}\text{+}{\text{Fe}}_{\text{3}}\text{C}\text{→3}\text{Fe}\text{+}\text{C}{\text{H}}_{\text{4}}\text{↑}$$ 6 $$\:\text{2}{\text{H}}_{\text{2}}\text{+}\text{C}\text{→}\text{C}{\text{H}}_{\text{4}}\text{↑}$$ 7 Not only that, S-containing substances generated by fuel combustion, as well as Cl − containing substances in water vapor, are more likely to decompose into reactive S²-, Cl − at high temperatures, and these ions migrate to the water-cooled wall along with the vapor cycle, and synergize with the high-temperature environment, respectively, to occur in the reaction of formula (8) and formula (3), exacerbating the local corrosion. $$\:\text{Fe}\text{+}{\text{H}}_{\text{2}}\text{S}\text{→}\text{FeS}\text{+}{\text{H}}_{\text{2}}\text{}$$ 8 In summary, the formation of corrosion pits can be attributed to four stages. (1) The initial stage. the weld root structure mutation, triggered by the flow field distortion, Cl − enrichment, pitting formation. (2) The development stage. pitting pits at the accumulation of sediment, oxide film stripping, high temperature hydrogen corrosion and the formation of C and CH 4 , along the crystalline microcracks sprouting. (3) Accelerated stage. the accumulation of scale leads to wall temperature rise, corrosion pits into the “scale accumulation → high temperature hydrogen corrosion → corrosion pits intensified” vicious cycle, until the formation of macro corrosion pits. If no intervention, will enter (4) Failure stage. along the crystal microcracks through the formation of macro-cracks, the strength and toughness of the pipe attenuation [ 27 ], when unable to withstand the working stress, the pipe burst. 4 Conclusion The main reason for the formation of corrosion pits in female threaded water-cooled wall weld is due to the root protrusion, the root groove leads to structural mutation, triggering the flow field distortion, prompting the continuous enrichment of Cl − and the Si 4+ and Al 3+ in the feed water to form a localized corrosive environment and the subscale deposition environment. the high load of the combustion wind region triggers the water-cooled wall pipe high-temperature hydrogen corrosion, and the sediment generated by the reaction of the Cl − exceeding the standard in water quality is a secondary reason for the formation of the corrosion pits. The accumulation of under-scale corrosion is the secondary reason for the formation of corrosion pits. It is recommended to optimize the design of weld seams and accurately control the water quality and heat load, so that the corrosion chain can be blocked from the source and the long-term safe operation of the unit can be guaranteed. Declarations Conflict of interest The authors declare no conflict of interest. Author Contribution Xiao Linfei is the sole author and corresponding author of this paper, responsible for the experiments, data processing, manuscript writing, manuscript review, and submission of the article. Acknowledgments The results of this research were obtained from the water-cooled wall safety assessment project of Datang Pucheng Power Generation Co Ltd (Project No. DTXBY-CLS-I-[2024]006), which was funded by China Datang Group Corporation. The authors would like to thank the Datang Northwest Electric Power Testing and Research Institute for supporting this experiment. Data Availability Statement The data generated and analyzed during this study have been presented in this thesis and its supplementary information document . References ZHANG Guangcai, ZHOU Ke, LIU Honggang, et al. Practice of deep peak load regulation for a 600 MW supercritical concurrent boiler[J]. Thermal Power Generation, 2018, 47(5). 83-88. HU Jiangen, TONG Jialin, MAO Jianbo, et al. The research of the comparison of deep peak regulation capacity for typical coal-fired boilers[J]. Boiler Technology, 2019, 50(6). 59-64. JIANG Xin, JIANG Caisheng. Influence of deep peak load regulation on metal life of generating set[J]. Thermoelectric Technology, 2020(1). 12-16. ZHANG Guangcai, ZHOU Ke, LU Fen, et al. Discussions on deep peaking technology of coal-fired power plants[J]. Thermal Power Generation, 2017, 46(9). 17-23. 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CHEN Youfu, XU Songmei, GUAN Shipian, et al. Relationship between furnace flame center position and waterwall outlet temperature[J]. Thermal Power Generation, 2018, 47(06). 71-77. GAO Quan, ZHANG Junying, QIU Jihua, et al. Study on characteristics of high-temperature corrosion in coal-fired utility boilers[J]. Journal of Engineering for Thermal Energy and Power, 2007, 22(3). 292-296. ZHAO Hong, WEI Yong. Mechanism and influencing factors of high-temperature fire-side corrosion on water-cooled walls in coal-fired boilers[J]. Power Engineering, 2002, (02). 1700-1704. Chevreux N , Flament C , Gillia O ,et al.Understanding the Phenomenon of High Temperature Hydrogen Attack (HTHA) Responsible for Ferrito-Pearlitic Steels Damage[J].High Temperature Corrosion of Materials, 2024, 101(5).1225-1236. Nazarov V V .Localized Destruction Criterion before the Onset of High-Temperature Hydrogen Corrosion of a Hollow Steel Cylinder[J].Steel in Translation,2025,54(11).1122-1129. M.A.M. A ,S.W. O ,G. D , et al.Mitigating high temperature hydrogen attack with interphase precipitation[J].International Journal of Hydrogen Energy,2024,50(PA).189-198. Goritskii V M , Shneiderov G R , Goritskii O V .Effect of Hydrogen Corrosion on the Brittle Fracture Resistance of an Oil Line Pipe 530 mm in Diameter Made of 12GSB Ferritic–Pearlitic Steel[J].Russian Metallurgy (Metally), 2020, 2020(4).461-469. Han D ,Gao Y ,Loya E P , et al.A mechanistic interpretation of Nelson curves for PVP failures under high temperature hydrogen attack[J].Mechanics of Materials,2024,196.105079-105079. Chen L ,Ji H ,Su B , et al.The role of Nb in enhancing the corrosion resistance of U-Nb alloy to hydrogen[J].Corrosion Science,2025,243112594-112594. Mahmoud K ,Walaa A ,M. N , et al.Metallurgical analysis of ASME SA213 T12 boiler vertical water-wall tubes failure[J].Engineering Failure Analysis,2023,145 Mavzovin V S , Ovchinnikov I G .Modeling of deformation and fracture processes of unevenly heated round plate subjected to high-temperature hydrogen corrosion[J].E3S Web of Conferences, 2024, 535(000).8 Poorhaydari ,Kioumars.A Comprehensive Examination of High-Temperature Hydrogen Attack—A Review of over a Century of Investigations[J].Journal of Materials Engineering and Performance,2021,30(11).1-34. HOU Xiangsong, SHI Yixiang, YANG Jingbiao. Analysis of scaling and corrosion on water-cooled wall in pulverized coal boiler and its causes[J]. Power System Engineering, 2007, (02). 26-28. Shao Y , Hacker J M .Local similarity relationships in a horizontally inhomogeneous boundary layer[J].Boundary-Layer Meteorology, 1990, 52(1-2).17-40. YANG K ,Hwang J ,Bremhorst K , et al.Numerical Investigation of Turbulent Flow around a Rotating Stepped Cylinder for Corrosion Study[J].The Canadian Journal of Chemical Engineering,2003,81(1).26-36. YANG Z ,XIA G ,Walker S M , et al.High temperature oxidation/corrosion behavior of metals and alloys under a hydrogen gradient[J].International Journal of Hydrogen Energy,2006,32(16).3770-3777. Mostert R J , Zyl A V , Pretorius C C E ,et al.Criteria for the onset of structural integrity degradation due to high-temperature hydrogen attack of a carbon- manganese steel[J].Procedia Structural Integrity, 2024, 54.381-389. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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17:46:51","extension":"xml","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":93589,"visible":true,"origin":"","legend":"","description":"","filename":"84128c48f38e44f18c595bd2ad2792ee1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/7ec638e04a81ad48459cd893.xml"},{"id":91895527,"identity":"8e6dac8a-5ccc-4074-b6e2-f479e19a1fc6","added_by":"auto","created_at":"2025-09-22 17:46:51","extension":"html","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":98782,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/a266a26a11a5b721622312b6.html"},{"id":91895925,"identity":"6f09013f-989c-4272-b4c4-6576eb7096a9","added_by":"auto","created_at":"2025-09-22 17:54:51","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":195319,"visible":true,"origin":"","legend":"\u003cp\u003eMacroscopic inspection content of water-cooled wall corrosion pits, \u003cstrong\u003ea\u003c/strong\u003e Macroscopic morphology of inner and outer walls after corrosion pit water-cooled wall specimens were dissected. \u003cstrong\u003eb\u003c/strong\u003e Corrosion pit morphology. \u003cstrong\u003ec\u003c/strong\u003e Corrosion pit laser confocal depth detection.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/8a2ab03b3b81b5df70158da2.jpeg"},{"id":91895505,"identity":"37da3daf-0280-4191-928d-c5020b838a23","added_by":"auto","created_at":"2025-09-22 17:46:50","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":109433,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the inner wall of water-cooled wall before and after chemical cleaning on the fire-side. \u003cstrong\u003ea\u003c/strong\u003e Comparison tube before and after cleaning on the fire-side. \u003cstrong\u003eb\u003c/strong\u003e Corrosion pit tube before and after cleaning on the fire-side. \u003cstrong\u003ec\u003c/strong\u003eAmount of scale on the inner wall\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/66ff13a83b5fcc38d75cc986.jpeg"},{"id":91895928,"identity":"fcf546df-6744-4cd6-b9d7-dbf5944cfd06","added_by":"auto","created_at":"2025-09-22 17:54:51","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":269798,"visible":true,"origin":"","legend":"\u003cp\u003eMetallographic organization of corrosion pits in water-cooled wall welds. \u003cstrong\u003ea\u003c/strong\u003e Macroscopic metallographic organization of corrosion pits. \u003cstrong\u003eb\u003c/strong\u003e Metallographic organization of large cracks. \u003cstrong\u003ec\u003c/strong\u003e Metallographic organization at the inner wall of the base material. \u003cstrong\u003ed\u003c/strong\u003e Metallographic organization of the core of the base material\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/606189a68e6704ceed0f9d65.jpeg"},{"id":91896095,"identity":"87e24d38-a33c-4d9f-8ec7-054aeb5efe82","added_by":"auto","created_at":"2025-09-22 18:02:51","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":283770,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructural morphology of the corrosion crater region. \u003cstrong\u003ea\u003c/strong\u003e Microstructure morphology at the weld crack. \u003cstrong\u003eb\u003c/strong\u003e EDS surface scanning results of the weld crack A region. \u003cstrong\u003ec\u003c/strong\u003e Crack and hole morphology of the base material on the inner surface and EDS surface scanning result\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/39e78e1e706d76fad35784e8.jpeg"},{"id":91896096,"identity":"f4862f1a-a9fa-44cd-bb37-dd35dd975c9e","added_by":"auto","created_at":"2025-09-22 18:02:51","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":169269,"visible":true,"origin":"","legend":"\u003cp\u003eXRD characterization of physical phases of corrosion products of corrosion pits. \u003cstrong\u003ea\u003c/strong\u003e Graphical representation of XRD sites of corrosion products. \u003cstrong\u003eb\u003c/strong\u003e Characterization of XRD physical phases of different sites. \u003cstrong\u003ec\u003c/strong\u003e Statistical graph of mass percentage of different physical phases\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/1b49fdd536bf1d9fdbd15e18.jpeg"},{"id":91895932,"identity":"2c7acbdc-7308-490c-bca6-db87929a181f","added_by":"auto","created_at":"2025-09-22 17:54:51","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":320139,"visible":true,"origin":"","legend":"\u003cp\u003eStructural inspection of water-cooled wall welds. \u003cstrong\u003ea, b\u003c/strong\u003e Physical drawings of comparative water-cooled wall pipe welds in the exhaust air area. \u003cstrong\u003ec, d\u003c/strong\u003ePhysical drawings of water-cooled wall pipe welds in the corrosion pits in the exhaust air area\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/d0b461c8f197e64c301af943.jpeg"},{"id":93356427,"identity":"2274ba79-8528-43eb-ba05-ba8d43adca5e","added_by":"auto","created_at":"2025-10-13 01:32:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2124680,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7250163/v1/a4173cd8-2df3-4b0b-925f-4fa6fef97e5d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Analysis of corrosion pits in weld seams of internally threaded water-cooled wall tubes in subcritical boilers","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eWith the widespread implementation of flexibility retrofit in coal-fired units, deep wide-load peaking operation has become the norm [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. This operation mode leads to frequent over-temperature of water-cooled wall tubes and significantly increases the alternating thermal stresses on the heated surfaces [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], which in turn triggers a variety of failure modes, including fatigue cracking failure [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], transverse cracking failure [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], and hydrogen corrosion downgauging failure [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eHigh-temperature hydrogen corrosion is an important mode of water-cooled wall failure, and studies have shown that high-temperature hydrogen corrosion inducing factors mainly include high-temperature corrosion of water-cooled wall caused by oxygen-rich combustion conditions [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], localized overheating due to flame center deviation [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], and fire-side reducing atmosphere exacerbated by low-nitrogen combustion [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], which affects the safe in-service performance of water-cooled wall tubes. Regarding the mechanism of high-temperature hydrogen corrosion, Chevreux N [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] and Nazarov V [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] showed that the formation of methane cavities in carbide-enriched zones and their induced localized stress concentrations are the key factors for microcracks sprouting, and Alshahrani M [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] further revealed the role of the thermodynamic stability of the carbide in the modulation of the hydrogen erosion resistance. Notably, Goritskii M [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] found that the proportion of intergranular features on brittle fracture surfaces due to hydrogen erosion can be abruptly increased to 50.5%, and this microstructural degradation echoes the dislocation creep-controlled cavity growth mechanism revealed by D Han [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] through finite element simulations. When the material is in a complex stress state, as Chen [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] pointed out the multiple effects of Nb atoms on the hydrogen behavior, the hydrogen corrosion resistance of the material exhibits significant nonlinear characteristics.\u003c/p\u003e\u003cp\u003eThe corrosion case of water-cooled wall tubes in a thermal power plant B\u0026amp;WB-1025/18.44-M boiler provides engineering evidence for the above mechanism study. The boiler belongs to subcritical parameter, intermediate reheat, natural circulation single-drum Π-shaped boiler. The water-cooled wall adopts membrane-type all-welded structure, and the pipe section from the inflection point of the cold ash hopper to the angle of the folded flame adopts internally threaded pipe, with the specification of Ф60\u0026times;6.5mm, the material of which is SA-210C, and the chemical composition of which is shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. After 131840 hours of operation of the internally threaded water-cooled wall pipe in the exhaust air area, endoscopic inspection revealed that. corrosion pits appeared in the middle of the weld joints of the fire-side of the front wall, presenting the typical characteristics of hydrogen corrosion, which was similar to that of the water-cooled wall of the Mahmoud [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The localized superheat-induced creep damage model reported by Khedr [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] and the temperature field-stress field coupling model established by Mavzovin V S [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] are highly consistent. In particular, the special stress distribution of the internally threaded pipe structure in the region of heat load concentration may have accelerated the process of methane pressure-driven microcrack extension as described by Nazarov V V [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The preferential appearance of corrosion pits in the middle of the weld in this case further verifies the applicability of the grain boundary diffusion-assisted cavity growth theory in the region of welded joints, and provides a new reference for failure analysis for water-cooled wall water-cooled corrosion protection against high-temperature hydrogen corrosion in subcritical boilers.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eChemical composition of SA-210C water-cooled wall tube\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eElements\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMn\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFe\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eChemical composition(wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e98.66\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"2 Experimental materials and methods","content":"\u003cp\u003eCorrosion pit specimens and comparison tube specimens were intercepted at a height of 30 m in the combustion air area of the boiler front wall for systematic analysis. An Olympus LEXT OLS5100 laser confocal microscope was used to scan the three-dimensional morphology of the corrosion pits, with a sampling frequency of 3000 Hz, to determine the diameter and depth parameters. A Thermo Niton XL2 XRF spectrometer was used to characterize the chemical composition of the water-cooled wall tubes. In the sample pretreatment stage, 180# to 800# series sandpaper was used sequentially for gradient grinding, and finally 2\u0026micro;m diamond polishing solution was used to complete the fine polishing, and 4% nitric acid alcohol solution was selected for corrosion treatment. For the fire-side scale samples, after the special cleaning solution treatment and drying, the electronic balance was used to weigh accurately, and the Malvern Panaco Epsilon 4 energy dispersive fluorescence spectrometer was used for compositional analysis. The observation of microstructure was accomplished using a Leica DMI3000M inverted metallurgical microscope, while the micro-morphology and compositional analysis was realized by a Zeiss ZEM20 field emission scanning electron microscope. The physical characterization of the corrosion products was carried out by a D8 DISCOVER X-ray diffractometer, with the test conditions set as follows. scanning angle range of 10\u0026deg;-90\u0026deg;, θ-2θ working mode, tube voltage of 35kV, tube current of 40mA, scanning step length of 150 s/step, and a total of 4 steps of data collection.\u003c/p\u003e"},{"header":"3 Results and Analysis","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Macroscopy\u003c/h2\u003e\u003cp\u003eMacroscopic examination of the water-cooled wall tube after autopsy, the macroscopic morphology of the inner and outer walls as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, the specimen to the fire-side of the outer wall of the presence of obvious flue gas corrosion characteristics, corrosion products in the form of white deposits attached to the surface of the tube wall. It is worth noting that the corrosion pits are mainly concentrated in the middle of the weld seam on the fireward side of the inner wall of the water-cooled wall tube, and their macroscopic morphology shows the circular characteristics after the vapor vortex scouring. The three-dimensional morphology of the corrosion pits was scanned by laser confocal microscope, and the measurement by body microscope showed that the maximum diameter of the corrosion pits was 15.7 mm, see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, and the depth of the corrosion pits was further scanned by laser confocal system in two transverse and one longitudinal scans, see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, and the results showed that the maximum depth of the corrosion pits was 3.9 mm, and the thickness of the wall thickness was reduced by 60%. Water-cooled wall pipe strength calculations show that the nominal wall thickness of the pipe is 3.64mm, while the current remaining wall thickness has been significantly lower than the strength of the design requirements of the critical value. This severe wall thinning not only significantly reduces the load carrying capacity of the material, but may also lead to potential stress concentrations and risk of pipe burst.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Chemical sampling analysis\u003c/h2\u003e\u003cp\u003eAlong the weld cutting water-cooled wall pipe samples at both ends of the base material, and chemical cleaning, such as Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, the comparison of the pipe and corrosion pits pipe at both ends of the base material in the cleaning of the inner surface shows a metallic luster, no obvious corrosion pits, a phenomenon that shows that the corrosion defects have a distinctive localized characteristics, mainly concentrated in the middle area of the weld seam.\u003c/p\u003e\u003cp\u003eThe scale samples of the tube samples were analyzed, and the results showed that the amount of scale on the fire-side was about 35% higher than that on the back-side. The scale amount of the fire-side and back-side of the comparison tube was 117.55 g/m\u003csup\u003e2\u003c/sup\u003e and 87.02 g/m\u003csup\u003e2\u003c/sup\u003e respectively, and there was not much difference between the results of the corrosion pit tube and the scale amount of the comparison tube, with the scale amount being in the same order of magnitude, and all the measured values of the scale amount were much lower than the chemical cleaning thresholds stipulated in the \u0026ldquo;Chemical Cleaning Guidelines for Boilers in Thermal Power Plants\u0026rdquo; (DL/T 794\u0026ndash;2012) (the main steam pressure\u0026thinsp;\u0026gt;\u0026thinsp;15.6 MPa operation of the water-cooled wall of the ladle furnace scale amount\u0026thinsp;\u0026gt;\u0026thinsp;250 g/m\u003csup\u003e2\u003c/sup\u003e), indicating that the wall scaling is not the main cause of corrosion. The chemical composition of the inner wall scale samples is shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, and the results show that. the main components of the scale samples are Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, SiO\u003csub\u003e2\u003c/sub\u003e and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, which is consistent with the typical characteristics of the deposition of metal oxidation products and silica-aluminum compounds carried by the feed water in the course of boiler operation. The compositional differences between different tube samples were all within the same order of magnitude, further verifying that the formation of corrosion pits was not directly related to the overall fouling condition. Therefore, the formation of corrosion pits in the middle of the weld is the result of local factors rather than an overall scaling or corrosion problem. the weld area may have special conditions such as structural mutation or stress concentration, which makes it more susceptible to localized corrosion under the same operating environment. the higher scaling on the fire-side is related to the higher heat load and more favorable deposition conditions on that side, but the overall scaling level is still in the normal range.\u003c/p\u003e\u003cp\u003eAccording to the laboratory's historical furnace water analysis report records, 36 months prior to the discovery of the corrosion defect, there was an intermittent exceedance of the chlorine ion content of the furnace water that lasted for one month. Monitoring data show that the fluctuation range of chloride ion concentration during this period was 0.68\u0026ndash;0.90 mg/L, with the highest value exceeding the limit of chloride ion content (\u0026le;\u0026thinsp;0.4 mg/L) specified in the standard \u0026ldquo;Quality of Water Vapor of Thermal Generating Units and Steam Power Equipment\u0026rdquo; (GB/T 12145\u0026thinsp;\u0026minus;\u0026thinsp;2016) by 125%. The power plant has gradually reduced the chlorine ion concentration of the furnace water to 0.204 mg/L by increasing the operation and adjustment measures of the continuous discharge volume, and finally reached the standard requirement stably. The optimized operation of the continuous discharge system can effectively control the concentration of chloride ions in the furnace water, but the short-term exposure to high concentrations of chloride ions may accelerate the process of pitting corrosion of metal materials, laying a hidden danger for the formation of subsequent corrosion defects.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eAnalysis of the composition of scale samples of water-cooled wall comparison tube and corrosion pit tube\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"10\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003eTubular Scale Sample Composition\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eZnO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003eCaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c10\"\u003e\u003cp\u003eMnO\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eContent (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ecomparison tube\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e74.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e9.77\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.41\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e1.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e1.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.84\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e\u003cp\u003e0.78\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ecorrosion Pit Tube\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e73.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e9.88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.09\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e1.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e1.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e\u003cp\u003e0.83\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Microstructural analysis\u003c/h2\u003e\u003cp\u003eAxial observation of the metallographic organization of both sides of the corrosion pit, see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. The corrosion pit along the weld axial middle presents an oval pit shape, the inner wall of the pit is attached with serious oxide skin, the maximum thickness of the oxide skin is up to 2 mm, the formation of the oxide skin is the metal at high temperature in the environment of chemical reaction with oxygen, and the thickening of its not only reflecting the long-term role of the high temperature environment, but also on the tube wall of the further corrosion process. At the bottom of the corrosion pits, there are four circumferential macroscopic cracks, with a maximum crack depth of about 916 \u0026micro;m, and Fig. b shows that there are vertically distributed along-crystalline microcracks on both sides of the macroscopic cracks. The parent material organization is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec and d. The organization is ferrite plus pearlite, and the content of pearlite decreases gradually from the core to the inner wall, and the pearlite in the inner wall disappears completely, which is replaced by the filling of the cracks by the oxidation products. Moreover, the microcracks in the range of 250 \u0026micro;m near the inner wall have been completely filled with oxides, indicating that this region has experienced a long and severe oxidation process. At the same time, different degrees of decarburization occurred near the inner wall, and the degree of decarburization gradually increased from the core to the inner wall, which is a reaction driven by high temperature [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Under the high temperature environment, the hydrogen atoms may react with the carbon in the steel, so that the carbon element gradually escapes from the metal, which results in decarburization. Black stripes of intergranular microcracks existed on the inner surface of the weld area and the parent material of 200 mm on both sides of the weld, and the intergranular microcracks in the parent material were distributed in the form of a network, while the intergranular microcracks at the weld showed a worm-like distribution, and the emergence of the intergranular microcracks further weakened the strength and integrity of the metal material, which may become a channel for further intrusion of corrosive media, accelerating the damage of the water-cooled wall.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe main cracks of the weld and the micro-morphology of the base material were observed under scanning electron microscope and analyzed by EDS surface scanning, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, the weld A area shows typical oxide filling and hole morphology, indicating that the base material has undergone significant oxidative corrosion under high temperature and high pressure environment. The microstructure of the base metal in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec shows reticulated along-crystalline microcracks and macrocracks, with cracks expanding from the inner wall to the outer wall, and the aggregation of microcracks causes the generation of macrocracks.The EDS surface-scanning analysis results show a significant decrease in the carbon content in the weld A area and in the base metal near the cracks, which confirms the decarburization phenomenon caused by high-temperature hydrogen corrosion. This phenomenon is consistent with the hydrogen corrosion mechanism. at high temperatures (\u0026gt;\u0026thinsp;400\u0026deg;C), water vapor reacts with iron to form atomic hydrogen ([H]), which diffuses into the steel and reacts with the carburizer (Fe\u003csub\u003e3\u003c/sub\u003eC) to form methane (CH\u003csub\u003e4\u003c/sub\u003e). As CH₄ accumulates at grain boundaries and forms high-pressure micropores [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], it ultimately induces the initiation and extension of cracks along the grain.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEDS surface scanning element results for weld and base material\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003eElements\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFe\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eMn\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003e其他\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eContent (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e77.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e16.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e4.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e95.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e3.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Analysis of corrosion products\u003c/h2\u003e\u003cp\u003eThrough anatomical observation of the corrosion pits of the water-cooled wall tubes, it was found that the corrosion pits showed an obvious layered structure (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). X-ray diffraction (XRD) was used to analyze the physical phases of the corrosion products at the bottom, middle and top of the corrosion pits, which are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. As can be seen from the figure, the physical phases of the main peak are mainly dominated by Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, while the corrosion products also contain part of FeS, FeCl\u003csub\u003e2\u003c/sub\u003e and FeSO\u003csub\u003e3\u003c/sub\u003e, of which Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is a common product of the oxidation of Fe under an aerobic environment, and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is preferentially generated by water-cooled wall under high temperature, low oxygen or reducing environment. the appearance of FeS implies that the corrosion product is a common product of the oxidation of Fe in an aerobic environment. Low oxygen or reducing environment preferentially generated. the appearance of FeS implies that there may be sulfur-containing substances in the corrosive environment, the sulfur-containing components of the thermal power plant fuel in the combustion process and the subsequent steam cycle and other processes, it is possible to produce corrosive sulfur-containing media involved in the corrosion reaction. FeCl\u003csub\u003e2\u003c/sub\u003e may be associated with the historical furnace water Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e exceeds the standard. FeSO\u003csub\u003e3\u003c/sub\u003e may be in the complex chain of corrosion reaction by sulfur-containing material further reaction generation.\u003c/p\u003e\u003cp\u003eThe mass percentage of each material phase was calculated by using the reference intensity ratio (RIR) method, which is shown in Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The XRD patterns were split-peak fitted by the BG function to obtain the characteristic peak intensities of each material phase, and the RIR values of the corresponding material phases were found against the PDF cards, and the results are shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, which quantitatively analyzed the distribution of the material phases of the corrosion products in each layer, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec. The results show that the bottom corrosion products are dominated by Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (54.80%), indicating that the region is in a high-temperature reducing environment, and the iron matrix mainly reacts with water vapor or sulfide to generate Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (reaction formula. 3Fe\u0026thinsp;+\u0026thinsp;4H\u003csub\u003e2\u003c/sub\u003eO\u0026rarr;Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;4H\u003csub\u003e2\u003c/sub\u003e). The Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e content in the middle and upper corrosion products decreased to 45.93% and 46.49%, respectively, while the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content increased significantly in the upper part (33.34%), indicating that the upper part was closer to an oxidizing atmosphere, which prompted the further oxidation of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (4Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e\u0026rarr;6Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e).The presence of FeS, FeCl\u003csub\u003e2\u003c/sub\u003e and FeSO\u003csub\u003e3\u003c/sub\u003e indicates that the corrosion process The presence of sulfur and chlorine involved in the corrosion process may originate from sulfides (H\u003csub\u003e2\u003c/sub\u003eS/SO\u003csub\u003e2\u003c/sub\u003e) and chlorides (HCl) in the fuel, which form an acidic corrosive environment locally (Fe\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eS\u0026rarr;FeS\u0026thinsp;+\u0026thinsp;H₂).\u003c/p\u003e\u003cp\u003eThe corrosion stratification mechanism can be summarized as follows. the bottom is dominated by Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e due to high-temperature oxygen deficiency. the middle is the transition zone, where Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e coexists with Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and the upper part has an elevated proportion of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e due to enhanced oxygen diffusion. This phenomenon reflects the direct influence of oxygen concentration gradient on the composition of corrosion products in the burnout wind region, and confirms the role of sulfur/chlorine synergistic corrosion.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAowAAABDCAYAAAAWElSeAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAAFiUAABYlAUlSJPAAABNaSURBVHhe7d1NbBPH+wfwbypVaoVDGm0OJOZCrJSmKpBAwwkhGgdBwJWqqkDUF4NUUd4uNMkB2kNLDwhUG7gEBSWVeDkkVTgUxSGVfsSiFafQvIAqVYmVIAG2IzUpAu8BTvO/sP7bE9tr74tf4u9HslT22dnOZh95xzO7MxVCCAEiIiIiojTekDcQERERESVig5GIiIiIMmKDkYhKRjQahcvlQkVFRdLH5XIhGo3KuxMRkUXYYCSiklFbW4u5uTn09fXFtw0MDGBubg61tbVJ+xIRkXXYYCSikrNmzRoAgNfrRUdHhxwmIiKLscFYhjisR6VuaGgIALBv3z45RERENmCDsQxxWI9KmaqqCIfDqK+vx5YtW+QwERHZgA3GMsZhPSpFoVAI09PT2LZtG3/gEBHlCRuMZYzDelSKgsEglpaWsHHjRjlEREQ2YYOxTHFYj0qRqqoYHR2FoihobW2Vw0REZBM2GMsUh/WoFGl529TUhIaGBjlMREQ2YYOxTHFYj0qRlrft7e1wOBxymIiIbMIGYxnisB6VIuYtEVHhsMFYhjisR6WIeUtEVDhsMBqgTXytTXQ9ODiImpoa+P1+edeixGG98pGYq6FQCF1dXWhra4OqqvG8DQQCcrGixLwlIiocNhgNqK2txb179+B2u/HHH38AAK5evYqHDx/KuxYdDuuVl8Rc/fPPP3H06FG8ePECv/76K1avXo29e/diZmZGLlZ0jORtIBDg6kVERBZhg9GgiYkJrF69Gm+88QY6OjowNDRUEi+QcFiv/ExMTODJkyfYvn07VFXFm2++iVWrVmH79u0Ih8NYv369XKSoqKoKv9+PsbExVFVVyeG0PB4PVy8iIrLIimowplsj2YqPPNw8NDSEf//9F3v27IGqqnj58mXWPR+FVI7DevnMi2zls05DQ0Noa2tDQ0MDgsEg3n77bXg8HoRCIbx48aKo5+H0+/2orKzEjz/+CACYn5/H5s2b48Pq6fj9ftTU1GBqakoOERGRASuqwVhbW4ubN29CUZT4NkVRMDk5CSFEzp/Z2Vm0tLQk/T/w+mb/zz//4OTJk3A4HAiFQgBQ9D12uQzrBQIB3ZtyqchXXuQiX3WKRqOIRCL4/PPP49dfy9tgMIjGxsai7oHr6upadq5CCNy5cyfjD54jR47g008/jS9/SURE5qyoBiMANDc34+rVq/Eb8dLSEnbu3Gmop0HrkXG73UnPJy4sLGDdunXxBmIwGMT777+PhYWFhNLmqaqKixcvor+/Xw7lLNdhPY/Ho3tTNqu/vx8XL1403CiNRqPYuXMnotEoVFVFW1tbUk9bYg9TPvJCj3w981GniYkJ1NXVoba2FrFYDBUVFfEexYcPH6K9vb2o89aoUCiEyspKWxrDenmrF5+amkJNTU1Srq6UH2dEtHItazCmuvGmGury+/3L4sXytqXH48HVq1fj/15aWsJnn31m6OF3h8OBn3/+GTU1NfFtwWAQW7dujTemHj58iHA4jFWrViWUNCcajaK1tRVPnz5FR0eHHM5JrsN6gUAAFRUVOHjwoByyVEdHB54+fYpPPvkkZT30JDaGNG63G7FYDEIILC4uorm5OR6zOy8ySXc97a7TzMxM/NnaxL+X1ks+MzNj6Y+CdOeZb8FgEHV1dfJmS+jlrV68ubkZi4uLEEIgFovB7XbLuxARFR+RxuTkpFAURQAQnZ2dIhaLybuIgYEBAUC43W4xOzsrhwvO5/MJAPFPS0uLiEQi8m66YrGY+Omnn1L+DewQiURES0uL8Pl8cihvvF6vGB4eljfbwufzGbo2iXWMxWLC7XYLt9ute53ynRfZXM9818kO2ZxnPsRiMbF//34xOTkphyyll7d6cZFj3hJReVlcXBTnz58Xc3NzckhXb2+vGBkZkTebsqyHUdPQ0ICmpiYoioIvv/wyZS9EOByG1+vFb7/9VpTP73V1dcHn88X/ff/+fXz11Vcpf/Vn4nA4bOutSOXUqVNobGxEV1eXHMoL7bm3fL0M0dXVhcbGRpw6dUoOpWWmjvnOi2yuZ77rZIdszjMfYrEYqqqqbP9O0stbvTgRUToPHjzAF198ga+//hr19fVyGPPz8/j+++/TjuQcOXIET548wfHjx+WQcXILUhOJRER9fb2or69P+Qt5eHi4ZH4Ve73epN4br9dbtPUeHh4WiqLY3juSyfDwsPB6vfJmW2k92tn2avp8vqSeLCM9NfnIi1yvZz7qZIdcz9NO+cxfvbzVixvJWyJa2aanp4XL5RKLi4tySMzNzYnz58+L6upqAUDs2rVL3iXJsWPHxLFjx+TNhhhqME5OTop33323KG4O2YjFYilvxMVGu3kUum75HI7W5HLjTDXkmEt5jd15YeR62l0nOxg5TzvlM3/18s5snIjKy+LionC5XGJgYEAOJRkZGcmqwagdr7e3Vw7lLO2Q9MLCAp4/f45169ahsrIyvl1VVZw7dw5+vz/phYJi5nA40NPTk/Rw+fXr15fNV1dod+/exdjYWEEnANeGep1OZ87DoWY4HA60t7djbGwMd+/elcNJrJrGyO68MHI97a6THYycp9X8fj/6+/sxNTWFly9fYseOHfIuttDLW704EVGiCxcu4L///ks71Kx577335E0pKYqCb775BqdPn8b8/LwczknaBmM4HMbS0hKcTmfS84tXrlzBW2+9BY/Hk7R/sXM4HLhx40bSXHXd3d1FdSMeGhoCgIKuvLGwsIDHjx8jGo2mfG7VTtp5a3+HdOS31M2wMy+MXk8762QHo+dppfXr1+Pw4cM4cuQILl26ZEluZEsvb/XiRER4/Vzi2bNndRuLAFI+15jOrl278OzZs6Tn5I1I22DU1pdN7DUIBAIYHx9HT09Pwp7mpJqeJ9PHzE2ztrYWt27dSvpDd3d3F8V0QKqqIhwOQ1EUOJ1OOZw3zc3NmJmZwZ49e+SQ7ZxOJxRFQTgcTtu7qaoqxsfHdScez4UdeWH2etpRJzuYPU+reDweCCEwPj5uy9yLmejlrV6ciAgAbt68CQDYvn27HDJl06ZNqK6uxuDgoBzKSdoGoywajaKnpwenTp2y9Nd7upUc0n3MvoGZaoWNQ4cOGZos2UqxWAyPHj1CVVVV2a5OsWbNGlRVVeHRo0eIxWJyGLBwOFpmdV5YcT2trpMdrDjPUqeXt3pxIiIkjEKsXbtWDpm2detWPHv2DLdv35ZDWUvbYNRWi1i/fj1UVcXJkydx4sSJknluMZPm5mb873//S1phIxwOy7tREbJyOFpWjHlRjHUiIiLr/fXXXwCAxsZGOWTaO++8AwD4+++/5VDWUjYY5WGmK1euYO3atSX33GImzc3NOH36NADA5/OVxLnJw/Ol/DEiGo3i999/x+effy6HLFOMeVGMdcqFfO1L+UNEZId79+7F/ztxVMkqmzdvBl53uhiVssGYOMwUjUYxPj6OM2fOyLuVtGg0isuXL8Pr9Zoe5s4XeXi+lD9GpFoK0GrFmBfFWKdcyNe+lD9EROUqZYNRMz8/j2+//dby5xYT5fOll0Ta6gvnzp2TQwVRWVmJdevW4fnz51hYWJDDZSHdVE6aoaEh7Nu3T95sKavywsrraVWd7GDleZYqvbzVixMRlYKUDUbtCw6vG3R2PreY75de8PqcRkZGcPPmTVt7q3LhcDjgdDrL+hm1dFM5weRSgNmyMi+sup5W1skOVp1nKcuUt9nEiYhKQcoGo/YF5/V6S+55KT2BQADd3d24evWqrQ1hI7TeM21Ko3KjnXeqXkS7h6PtyAuz19OOOtnB7HmWukx5iyziRETbtm2L/7fZCbZT0ToBzUxJl7LBODMzA7fbndV8i6qqoq2tDRUVFaipqcHt27fhcrlQU1NTVNN/AMDU1BQOHTpUtC8O7NixA263G6Ojo2U5X9vDhw/hdrtTrtJh53C0XXlh5nraVSc7mDnPlSBT3iKLOBERAHz44YcAgEgkIoeWybVROTExAQD44IMP5FD25LUCjfJ6vfH1UDs7O4tunWltbexCrnebzRrcw8PDQlGUjPusRJOTk0JRFMNrABtdk9fuvDByPe2uU66Yt+np5a1e3GjeEtHK09vbKwBkte6zti8AMTc3J4eXcblcorq6WiwuLsqhrKXsYTRC642srKzERx99VHRDaHa8OBAIBNDW1pZVr0ogEMDmzZuxtLQkh5J4PB7s3bsXly5dkkMr2qVLl7B3796896bZkReJjFxPu+sUCATgcrkQjUbl0DLM28z08lYvTkSk2blzJwDgwYMHciju3r17qKiowNGjR+PbXC4Xdu/enbRfovn5eczNzaGjo8PclD1yC9IMvV/TheLz+Szp/QiFQvKmnEQiEdHS0qJbD63XwefzyaEVyefziZaWFhGJRORQ1oz01OQrL3K5nvmqUy6Yt6np5a1eXBjMWyJaub777jtRXV0tbzalt7dXVFdXZ9UTmYllDcZIJCIOHz4sBgYGRH19fcYvyXwaHh4WAEw3YrXz077UtePmMmyY7Y1XJNxIOjs7M95ItOFLrWta/uzevVuMjIzIxYTIUFa7funi2Rw7G9rjC3o31WzkeuO1Ky/SyeZ65qNORhqkduStyJB/2eRXurL5yF29vNWLJ8o1b4loZVtcXBQulyurYelsWXU8SxqMkUhEtLW1xb8cE59nLCStx9OKHo/h4eFl5+T1enO6uedy49X09fWJvr4+eXMS7QaVeMMUQojZ2dn4TTPT36Cvry9+Ix0YGEiKmT12Jn19feLChQuW5Il2401sFKRrGNmdF5mku575qlMsFhOHDx/WbcgksitvhQX5VYjc1ctbvbh2rRNzNd31IqLyMz09LVwul+keQfG6d/HYsWPyZkNMNxgTb9TDw8NJX4aF/BLU6mVFHbTeisTexFSN5MQbQOJHK2fkxputdD2e2vbEG6YsXVlNung2xy42dueFEfms0+TkpOjs7Iz/u9B5K0zmV7qymnTxbI5NRFQo09PT4sCBA6ZeUhkYGLCssSiseOnF4XDgzp07EELA4/GgubkZi4uLEELgzp07BZuo9sSJE3j06BFu3LhhuA4LCwu4ePEiNmzYgPn5eWzcuDEek+cFvHbt2rJJxrXPtWvXEo5qj3RzvTmdTiiKknEljqGhISBFWY2ZYxcbu/PCiHzWKRgMoq6uLv7vQuctTOZXOeUuEZWPTZs2oaenB7/88kvOU+gAwODgIFavXo3Lly/LIePkFuRK4PP5lvWWWPFJHH6Wh6ML2VOj9VCl6i3R60nJVFYvrnfsYpOPvMhVPusUi8XE/v37k/KvkHkrTOZXprJ6cb1jExFRMtM9jMVGWx3DaoqiwOl0AgnL1DmdzviUOoXsqQmFQpiensa2bduSVkJRVTU+zcnx48dTrpKSrqxePJtjF5N85EWu8l2nWCyGqqoqNDQ0xLcVMm9hMr/SldWLZ3NsIiJKtqIajNrqGHZbWFjA48ePEY1Gsx5CnJqawoYNG3D//n1s3rwZgUBA3sWwYDCIpaWlpGG327dvo7W1FWNjY+js7Ey7BrdWNtMQptFjF4t85UUuClGniYkJvHr1Kuuchc15C5P5VQ65S0RUNOQuRyotiS8dJX4URRHd3d1idnZWLhKnlU33NrGZY1PxkR+jKDQz+cXcJSLKrxXVw1iOtGE3r9cLIQQmJyehKAqamprwww8/JA0/yrSyTU1NKfczc2wqDn6/H/39/ZiamsLLly+Laj1jM/nF3CUiyi82GEucPCzX0NCApqYmTE9PIxQKybsn0cq2t7enHKY0c2wqDuvXr8fhw4dx5MgRXLp0KeV1LhQz+cXcJSLKLzYYS5iqqhgdHYWiKGhtbQVeT3PU3t6OpaUlBINBuUhcqrJ68WyPTcXD4/FACIHx8fGiernDTH6lKqsXz/bYRESUGhuMJSzdsFxraysURcHo6Gj8LW5ZurJ68WyOTaTHTH6lK6sXz+bYRESUGhuMJSzdsFw2w2/pyurFszk2kR4z+ZWurF48m2MTEVFqbDCWqFTDbhqHwwGn04mlpaX4fHOJMpXVi2c6diAQQFtbG3tvKCOj+QWdsnpxvWNrAoEAXC4XotGoHCIiKltsMJaoK1euYGxsTN4cp809d/36dQwODsa3q6oKv9+PsbExVFVVJZT4f0aP7fF4CrocJJUGo/llZ+4m8ng8mJubK6pnPomICk6eZ4eKm7akmfxxu90iFovF94tEIqK+vj5p/rkDBw4sK5dY1syxz549K5CwnByRzEx+2Zm7ifM4+ny+ZduIiEiICiGEkBuRREYcPHgQ+/btg8fjkUNEJUFVVXR2duLMmTPsYSQiSsAhabKEtr72li1b5BBRyQiFQqisrGRjkYhIwgYjWWJiYgJ1dXW80VJJCwaDqKurkzcTEZU9NhjJEkNDQ/EXCohKkaqqGB8fX/Z2NRERscFIFtCGo51OJ6fUoZIVi8VQVVWVcjJwIqJyxwYjmbawsIDHjx8jGo1ySh0qWRMTE3j16hVzmIgoBb4lTUTEt/yJiDJiDyMRlS2/34/+/n5MTU3h5cuX2LFjh7wLERGxh5GIylkgEMDHH3+MlpYW3Lp1i2/5ExGlwQYjEREREWXEIWkiIiIiyuj/ANX1+Y6CMlfVAAAAAElFTkSuQmCC\" width=\"652\" height=\"67\"\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e is the mass percentage of substance phase \u003cem\u003ei\u003c/em\u003e. \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e is the diffraction peak intensity of substance phase \u003cem\u003ei\u003c/em\u003e, and \u003cem\u003eRIR\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e is the reference intensity ratio of substance phase \u003cem\u003ei\u003c/em\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eR-values and peak intensities of XRD physical phases\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eSerial number\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003ePhase\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e2θ (diffraction angle)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eRIR\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003ehkl\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e\u003cp\u003eDiffraction peak intensity.\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eBottom oxide\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eMiddle oxide\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eTop oxide\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFeSO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e24.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1.33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e12\u0026minus;1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e126\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e30.095\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e400\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e256\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e104\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e33.157\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e104\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e109\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e347\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e355\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e35.482\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e311\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e376\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e751\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e296\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFeCl\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e35.429\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e4.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e104\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e373\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e750\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e295\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e40.205\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e424\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e109\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e102\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e120\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e43.123\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e400\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e131\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e152\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e134\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFeS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e43.126\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.96\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e114\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e130\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e132\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e49.462\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e024\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e114\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e145\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e185\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e53.499\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e422\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e140\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e80\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e54.066\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e116\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e146\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e140\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e236\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e57.031\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e511\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e192\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e238\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e170\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e62.627\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e440\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e311\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e409\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e290\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e64.002\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e146\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e120\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e192\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Structural analysis of welded seams\u003c/h2\u003e\u003cp\u003eTo reveal the general pattern of corrosion pits in the weld seams of the water-cooled wall in the overfire air zone, tube sections adjacent to the corrosion pits were cut and sampled, and the macroscopic morphology of the tube samples was analyzed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Macroscopic examination revealed that the layered yellow substance attached to the inner surface of the tube samples, confirmed by spectral analysis (Mn\u0026thinsp;=\u0026thinsp;1.18 wt.%, Fe\u0026thinsp;=\u0026thinsp;98.59 wt.%), was an adherent formed by the cooling of high-temperature molten iron during the tube cutting process, rather than a corrosion product. As shown in the figure, the weld seams of the water-cooled wall tubes on the fire-side in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and b exhibit no signs of corrosion pits, with a smaller backing layer spacing of 3.22 mm. The weld root is flush with the base material, and the transition zone from the internal threads to the weld is smoothly treated with a relief groove, showing no significant structural mutations. In contrast, the weld seams of the water-cooled wall tubes on the fire-side in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and d display obvious corrosion pits, with an axial length of approximately 26.58 mm, and the axial diameter terminates at the root of the relief groove. The spacing of the weld backing layer significantly increases to 9.98 mm, which is 3.1 times the width of the comparison tube weld. Additionally, the weld root protrudes 2.53 mm from the base material, forming a groove structure with a depth of about 1.1 mm.\u003c/p\u003e\u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and d, the structural mutation at the root of the weld on the fire-side significantly deteriorates the local flow field characteristics. Hou[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] also confirmed this through simulation using a two-phase flow model, with the governing equations employing the incompressible Navier-Stokes equations, as shown in Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The results indicate that (1) Due to the presence of internal threads and the structural mutation of the weld, a larger low-velocity zone exists near the inner wall weld of the water-cooled wall tube [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. (2) Vortices occur at the root area of the weld connected to the threaded channel, leading to backflow. This flow field distortion promotes the enrichment of aggressive ions such as Cl⁻ in the vortex region [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], forming a local high-concentration corrosive environment. Simultaneously, the protruding weld root experiences enhanced turbulent shear forces, making the oxide film more prone to detachment, ultimately leading to preferential corrosion in this area. The research findings confirm that geometric factors such as the smoothness of the weld root and the curvature radius of the transition zone are key factors controlling the corrosion susceptibility of water-cooled wall tubes. In engineering practice, it is essential to strictly avoid designs with abrupt structural changes.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\text{\u0026rho;}\\text{}\\text{dfrac}\\text{\u0026part;}\\text{u}\\text{\u0026part;}\\text{t}\\text{\u0026minus;\u0026nabla;\u0026sdot;}\\eta\\left(\\text{\u0026nabla;}\\text{u}\\text{+(\u0026nabla;}\\text{u}{\\text{)}}^{\\text{T}}\\right)\\text{+}\\text{\u0026rho;}\\left(\\text{u}\\text{\u0026sdot;\u0026nabla;}\\right)\\text{u}\\text{+\u0026nabla;}\\text{p}\\text{=}\\text{F}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003eρ\u003c/em\u003e is the density, \u003cem\u003eu\u003c/em\u003e is the velocity, \u003cem\u003eη\u003c/em\u003e is the dynamical viscosity. \u003cem\u003ep\u003c/em\u003e is the pressure. and \u003cem\u003eF\u003c/em\u003e is the source term.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.6 Discussion\u003c/h2\u003e\u003cp\u003eCombined with the above test, it is analyzed that the main reason for the corrosion pits in the weld seam of the inner threaded water-cooled wall is. the sudden change of the weld seam structure caused by the change of the water-cooled wall media flow field. the secondary reason is the high temperature of the exhaust air area, the high load on the fire-side, the occurrence of high-temperature hydrogen corrosion, and the Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e exceeding the standard of the water quality caused by the buildup of impurities in the weld to form the corrosion of the under-scaling.\u003c/p\u003e\u003cp\u003e(1) Structural mutation of weld seam destroys flow field changes.\u003c/p\u003e\u003cp\u003eInternal threaded water-cooled wall pipe to the fire-side of the weld there are significant structural differences, compared to the pipe weld (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, b) bottoming layer spacing is small, the root and the base material level, the internal thread retractor groove is smooth, the flow path is continuous. and corrosion pit pipe weld (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, d) bottoming layer spacing of up to 3.1 times the comparison tube, the root protrudes from the base material of 2.53mm and the formation of a depth of 1.1mm notch. Structural changes in the water-cooled wall pipe medium flow field drastically changed, on the one hand, caused by low-flow velocity area and flow separation, the sudden change in the fluid flow is blocked, the kinetic energy is converted into pressure energy, the formation of low-flow velocity area, the sudden change in flow velocity will lead to flow separation phenomenon, resulting in the medium in the middle of the weld stagnation, for the aggressiveness of the ions to gather to create the conditions. On the other hand, the vortex and reflux phenomenon is triggered, the threaded channel and weld groove interface, the sudden change of the flow channel morphology induced vortex. Vortex area fluid movement disorder, not only impede the normal flow of media, but also due to the vortex effect of Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, carried in the feed water Si\u003csup\u003e4+\u003c/sup\u003e, Al\u003csup\u003e3+\u003c/sup\u003e continued to enrich the formation of local corrosive environments and scale deposition of the environment, the water quality of the historical data show that the furnace water Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e had exceeded the standard of 0.9 mg/L, exceeding the national standard of 125%, pitting corrosion, the generation of FeCl\u003csub\u003e2\u003c/sub\u003e, the occurrence of Sub-catalyzed acidification, the generation of HCl, the reaction is shown in the formula (3) and (4).\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\text{Fe}\\text{+2}\\text{C}{\\text{l}}^{\\text{\u0026minus;}}\\text{\u0026rarr;}\\text{FeC}{\\text{l}}_{\\text{2}}\\text{+2}{\\text{e}}^{\\text{\u0026minus;}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\text{FeC}{\\text{l}}_{\\text{2}}\\text{+}{\\text{H}}_{\\text{2}}\\text{O}\\text{\u0026rarr;}\\text{Fe}\\text{(}\\text{OH}{\\text{)}}_{\\text{2}}\\text{+2}\\text{HCl}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e(2) High load in the exhaust air area triggers high temperature corrosion.\u003c/p\u003e\u003cp\u003eBurnout wind area to the fire-side of the water-cooled wall scale is larger, higher heat load, when the fire-side of the water-cooled wall pipe wall temperature is greater than 400 ℃, the tube produces vapor stratification or steam stagnation, the occurrence of the reaction of the formula (5) to generate [H]. According to the Arrhenius formula, the reaction rate increases exponentially with increasing temperature, the reaction is also more violent, so that the metal oxide film generation and destruction of the equilibrium tilted to the \u0026ldquo;rapid destruction\u0026rdquo;. Generated free [H] can not be quickly taken away by the medium, under the action of high temperature along the lattice and grain boundaries to the fire-side of the matrix to the internal diffusion, and with the steel Fe3C, free C reaction, see formula (6) and (7), to generate CH\u003csub\u003e4\u003c/sub\u003e, corrosion pits at the bottom of the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e began to be deposited. Continuous generation of H from the inner surface to the outer surface of the thickness of the direction of the existence of gradient distribution characteristics [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], the surface of the C is first consumed to form the surface decarburization phenomenon, CH\u003csub\u003e4\u003c/sub\u003e aggregated at grain boundaries, induced intergranular cracks sprouting [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], cracks will trigger a significant stress concentration effect. and at the same time, due to the accumulation of scale thermal conductivity is poor, it will lead to the wall of the pipe wall local temperature rises sharply, which further accelerated the corrosion process. Thus, a vicious cycle of \"high-temperature hydrogen corrosion - crack expansion - corrosion pit aggravation\" is formed. the microcracks gradually expand and connect with each other to form a reticulation along the crystalline crack defects shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c).\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:\\text{Fe}\\text{+}{\\text{H}}_{\\text{2}}\\text{O}\\text{\u0026rarr;}{\\text{Fe}}_{\\text{3}}{\\text{O}}_{\\text{4}}\\text{+8[}\\text{H}\\text{]}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$\\:\\text{2}{\\text{H}}_{\\text{2}}\\text{+}{\\text{Fe}}_{\\text{3}}\\text{C}\\text{\u0026rarr;3}\\text{Fe}\\text{+}\\text{C}{\\text{H}}_{\\text{4}}\\text{\u0026uarr;}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$$\\:\\text{2}{\\text{H}}_{\\text{2}}\\text{+}\\text{C}\\text{\u0026rarr;}\\text{C}{\\text{H}}_{\\text{4}}\\text{\u0026uarr;}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eNot only that, S-containing substances generated by fuel combustion, as well as Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e containing substances in water vapor, are more likely to decompose into reactive S\u0026sup2;-, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e at high temperatures, and these ions migrate to the water-cooled wall along with the vapor cycle, and synergize with the high-temperature environment, respectively, to occur in the reaction of formula (8) and formula (3), exacerbating the local corrosion.\u003cdiv id=\"Equ8\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ8\" name=\"EquationSource\"\u003e\n$$\\:\\text{Fe}\\text{+}{\\text{H}}_{\\text{2}}\\text{S}\\text{\u0026rarr;}\\text{FeS}\\text{+}{\\text{H}}_{\\text{2}}\\text{}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eIn summary, the formation of corrosion pits can be attributed to four stages. (1) The initial stage. the weld root structure mutation, triggered by the flow field distortion, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e enrichment, pitting formation. (2) The development stage. pitting pits at the accumulation of sediment, oxide film stripping, high temperature hydrogen corrosion and the formation of C and CH\u003csub\u003e4\u003c/sub\u003e, along the crystalline microcracks sprouting. (3) Accelerated stage. the accumulation of scale leads to wall temperature rise, corrosion pits into the \u0026ldquo;scale accumulation \u0026rarr; high temperature hydrogen corrosion \u0026rarr; corrosion pits intensified\u0026rdquo; vicious cycle, until the formation of macro corrosion pits. If no intervention, will enter (4) Failure stage. along the crystal microcracks through the formation of macro-cracks, the strength and toughness of the pipe attenuation [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], when unable to withstand the working stress, the pipe burst.\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eThe main reason for the formation of corrosion pits in female threaded water-cooled wall weld is due to the root protrusion, the root groove leads to structural mutation, triggering the flow field distortion, prompting the continuous enrichment of Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e and the Si\u003csup\u003e4+\u003c/sup\u003e and Al\u003csup\u003e3+\u003c/sup\u003e in the feed water to form a localized corrosive environment and the subscale deposition environment. the high load of the combustion wind region triggers the water-cooled wall pipe high-temperature hydrogen corrosion, and the sediment generated by the reaction of the Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e exceeding the standard in water quality is a secondary reason for the formation of the corrosion pits. The accumulation of under-scale corrosion is the secondary reason for the formation of corrosion pits. It is recommended to optimize the design of weld seams and accurately control the water quality and heat load, so that the corrosion chain can be blocked from the source and the long-term safe operation of the unit can be guaranteed.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eConflict of interest\u003c/h2\u003e\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eXiao Linfei is the sole author and corresponding author of this paper, responsible for the experiments, data processing, manuscript writing, manuscript review, and submission of the article.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e\u003cp\u003eThe results of this research were obtained from the water-cooled wall safety assessment project of Datang Pucheng Power Generation Co Ltd (Project No. DTXBY-CLS-I-[2024]006), which was funded by China Datang Group Corporation. The authors would like to thank the Datang Northwest Electric Power Testing and Research Institute for supporting this experiment.\u003c/p\u003e\u003ch2\u003eData Availability Statement\u003c/h2\u003e\u003cp\u003eThe data generated and analyzed during this study have been presented in this thesis and its supplementary information document .\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZHANG Guangcai, ZHOU Ke, LIU Honggang, et al. Practice of deep peak load regulation for a 600 MW supercritical concurrent boiler[J]. Thermal Power Generation, 2018, 47(5). 83-88.\u003c/li\u003e\n\u003cli\u003eHU Jiangen, TONG Jialin, MAO Jianbo, et al. The research of the comparison of deep peak regulation capacity for typical coal-fired boilers[J]. Boiler Technology, 2019, 50(6). 59-64.\u003c/li\u003e\n\u003cli\u003eJIANG Xin, JIANG Caisheng. Influence of deep peak load regulation on metal life of generating set[J]. 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Power System Engineering, 2007, (02). 26-28.\u003c/li\u003e\n\u003cli\u003eShao Y , Hacker J M .Local similarity relationships in a horizontally inhomogeneous boundary layer[J].Boundary-Layer Meteorology, 1990, 52(1-2).17-40.\u003c/li\u003e\n\u003cli\u003eYANG K ,Hwang J ,Bremhorst K , et al.Numerical Investigation of Turbulent Flow around a Rotating Stepped Cylinder for Corrosion Study[J].The Canadian Journal of Chemical Engineering,2003,81(1).26-36.\u003c/li\u003e\n\u003cli\u003eYANG Z ,XIA G ,Walker S M , et al.High temperature oxidation/corrosion behavior of metals and alloys under a hydrogen gradient[J].International Journal of Hydrogen Energy,2006,32(16).3770-3777.\u003c/li\u003e\n\u003cli\u003eMostert R J , Zyl A V , Pretorius C C E ,et al.Criteria for the onset of structural integrity degradation due to high-temperature hydrogen attack of a carbon- manganese steel[J].Procedia Structural Integrity, 2024, 54.381-389.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"internally threaded water-cooled wall, hydrogen corrosion, structural mutation, flow field distortion, subcritical boiler","lastPublishedDoi":"10.21203/rs.3.rs-7250163/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7250163/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFor the problem of corrosion pits in the welds of internally threaded water-cooled wall tubes in a subcritical boiler, the failure mechanism under the synergistic effect of structural mutation and high-temperature hydrogen corrosion was revealed by means of laser confocal inspection, metallurgical examination, XRD analysis, and EDS analysis. The study shows that the fire-side weld of the front wall in the burnout wind region causes flow field distortion due to root protrusion and groove, which induces Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e and Si\u003csup\u003e4+\u003c/sup\u003e/Al\u003csup\u003e3+\u003c/sup\u003e enrichment, forming pitting corrosion and subscale corrosion. at the same time, high-temperature hydrogen corrosion is triggered by the wall temperature of more than 400 ℃, microcracks at the near inner-wall grain boundaries are expanded and form mesh defects, and the scale layer exacerbates the local temperature rise to form the vicious circle of \u0026ldquo;high-temperature corrosion-crack expansion\u0026rdquo; and \u0026ldquo;high-temperature corrosion-crack expansion\u0026rdquo;. \"Vicious cycle. A four-stage corrosion pit evolution model is proposed, and it is suggested to optimize the weld runner design and strictly control the water quality and heat load to block the failure chain.\u003c/p\u003e","manuscriptTitle":"Analysis of corrosion pits in weld seams of internally threaded water-cooled wall tubes in subcritical boilers","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-22 17:46:46","doi":"10.21203/rs.3.rs-7250163/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6d555f82-e376-433b-8ac7-8071724d72c4","owner":[],"postedDate":"September 22nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-13T01:23:52+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-22 17:46:46","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7250163","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7250163","identity":"rs-7250163","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}