The impact of HCl on alkali-induced corrosion of stainless steels/FeCrAl alloy at 600 °C: The story after breakaway

The impact of HCl on alkali-induced corrosion of stainless steels/FeCrAl alloy at 600 °C: The story after breakaway | 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 The impact of HCl on alkali-induced corrosion of stainless steels/FeCrAl alloy at 600 °C: The story after breakaway Vicent Ssenteza, Torbjörn Jonsson, Johanna Nockert, Jesper Liske This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4698261/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 22 Jul, 2024 Read the published version in High Temperature Corrosion of Materials → Version 1 posted 7 You are reading this latest preprint version Abstract The impact of Cl on alkali-induced high temperature corrosion of stainless steels/FeCrAl alloys after breakaway oxidation was investigated in a simulated biomass- and waste-fired boiler environment at 600°C. For this investigation, three alloys were exposed to low Cl load environment (H 2 O + KCl) and to high Cl load (H 2 O + KCl + HCl). Post-exposure analysis showed that the stainless steel SVM12 experiences fast oxidation and forms thick double-layered Fe-rich oxide scales. The corrosion attack is further accelerated with addition of HCl for this material with the effect being more pronounced in the inward-growing scale. The FeCrAl and FeCrNi alloys exhibit slower oxidation kinetics after the breakaway corrosion compared to SVM12 in the H 2 O + KCl exposure. Furthermore, in contrast to SVM12, the addition of HCl did not accelerate the corrosion attack on these alloys. It is argued that the properties of the secondary oxide layer formed after breakaway corrosion is important in the continued corrosion resistance against chlorine induced corrosion attack. Especially, the Cr-content in the inner scales is suggested to be important in corrosion mitigation. Oxidation Biomass Breakaway Secondary protection Chlorine Figures Figure 1 Figure 2 Figure 3 Introduction It is well established that combustion of biomass and waste generates corrosive gases/deposits consisting of e.g., alkali salts [ 1 ]. At 600°C and below, this environment is known to destroy the primary protection of stainless steels (Cr 2 O 3 or Cr-rich (Cr, Fe) 2 O 3 )) and FeCrAl alloys (Cr,Al) 2 O 3 ), causing breakaway oxidation [ 2 , 3 ]. This occurs rapidly, within hours or days, in this environment and the alloys will thereby be dependent on the scales formed after breakaway, the so-called secondary protection. The secondary protection is characterized by formation of outward-growing Fe-rich oxide and inward-growing spinel oxides, which composition depends on the alloying elements. In recent research, the concepts of good secondary protection (referring to slow-growing scale formed after breakaway) and poor secondary protection (referring to fast-growing Fe-rich scales) have been introduced to improve the understanding of oxide scale formation after the breakaway phenomenon [ 4 ]. Most of the published literature on corrosion protection at high temperatures has focused on the integrity and lifetime of the primary oxide, coupled to the effect of alloying elements, e.g., Cr/Al [ 4 ], Si [ 5 ], Mo [ 6 ]. However, previous research shows that the mechanisms controlling oxide growth kinetics after breakaway oxidation i.e., the secondary corrosion regime, differ from the primary corrosion regime when it comes to the role of the environment [ 4 ]. The effect of chlorine containing species on the primary corrosion regime (pre-breakaway stage) is well investigated. Alkali chlorides such as KCl, NaCl and PbCl 2 have been reported to breakdown the primary oxide by forming alkali-chromate, the so-called chromate formation mechanism [ 7 ]. HCl has been shown to facilitate formation of metal chlorides through an electrochemical approach which involves the formation of chloride ions at the scale-gas interface and cations at the metal-scale interface via oxidation [ 8 ]. In other work, the role of Cl on the oxidation process has been attributed to its catalytic nature, which promotes the formation of volatile metal chlorides at lower pO 2 , which upon diffusion through the scale to regions of higher pO 2 are converted to respective metal oxides, the so-called active oxidation mechanism [ 3 ]. On the other hand, some investigations have reported that the environment plays little or no role on the oxide scale formation beyond breakaway oxidation [ 9 , 10 ]. Instead, the oxide growth kinetics has been explained by a diffusion-controlled mechanism, which is determined by the diffusivity of the different cations through the spinel [ 11 ]. However, despite the extensive research carried out to understand alkali-induced corrosion, there is limited knowledge on the impact of increased Cl load on the oxide scales formed after breakaway, i.e., on the secondary protection of stainless steels and FeCrAl alloys. The aim of this study is to investigate the impact of an increased Cl load on the scales formed after breakaway on both good secondary protection and poor secondary protection. For this purpose, high-temperature corrosion studies were conducted of three alloys: SVM12 (martensitic stainless steel), Kanthal ® APMT (FeCrAl alloy) and alloy 27Cr33Ni3Mo (austenitic stainless steel), in a laboratory environment containing H 2 O, KCl and HCl at 600°C. Characterization of oxide products were performed using scanning electron microscopy (SEM) in combination with energy-dispersive x-ray (EDX) spectroscopy. Experimental section Materials and sample preparation In this study, three alloys were investigated; SVM12 (martensitic stainless steel) supplied by Vallourec S.A, APMT (FeCrAl alloy) supplied by Kanthal AB and alloy 27Cr33Ni3Mo (austenitic stainless steel) supplied by Alleima AB. The dimensions of the samples were 20x10x2 mm and the chemical compositions are presented in Table 1 . Table 1 Chemical composition in wt-% of alloys used in this study Alloy Fe Cr Ni Al Mo Si Mn SVM12 Bal. 10.5 0.4 0.6 0.6 0.8 Kanthal® APMT Bal. 21 5 3 0.7 0.4 Alloy 27Cr33Ni3Mo Bal. 27 33 3 0.8 2 The samples were prepared by grinding the surfaces with 800 SiC grit paper using a Struers automatic polishing machine. The ground coupons were then cleaned using acetone, followed by ultra-sonic bath and finally cleaned with ethanol. Exposures All exposures were performed isothermally at 600°C for 168 hours. Three environments were investigated: Exposure 1) 5% O 2 + 20% H 2 O + N 2 (Bal.). Exposure 2) as in 1 + 2 mg/cm 2 KCl was pre-deposited on the samples using a spray technique. In addition, an alumina boat filled with KCl(s) was placed upstream the samples at 600°C. A detailed description of the exposure setup can be found here [ 12 ]. Exposure 3) as in 2 + 500 ppm HCl(g). The HCl(g) was introduced into the exposure system from a gas mixture of 5% HCl-95% N 2 and was regulated using a mass flow controller. The amount of HCl(g) passing through the system was double-checked by measuring the Cl content in the collecting bottle (outlet) using ion chromatography. The samples were weighed before and after all the exposures using Sartorius™ scale with the resolution of 1µg. Characterization After exposure, a thin layer of gold was sputtered on the sample surfaces and thin silicon wafer was attached using glue. The coated samples were cut and polished using a Leica EM TXP. Cross-sections of the samples were obtained using a Gatan PECS II broad ion beam (BIB) milling system operated at 8 kV for 6 hours. The corrosion products were analyzed using scanning electron microscopy (SEM) coupled with EDS system and operated at 10 kV for imaging and 20 kV for chemical analysis. Results Gravimetric analysis, onset of breakaway corrosion The onset of breakaway oxidation was studied by exposing the alloys SVM12, APMT and alloy 27Cr33Ni3Mo to H 2 O + KCl at 600°C for 4 hours. Gravimetric analysis showed that all the alloys exhibit significant mass gain corresponding to a calculated oxide scale thickness of at least 5 µm (or 0.8 mg/cm 2 ) for Alloy 27Cr33Ni3Mo, about 6 µm (or 0.9 mg/cm 2 ) for APMT and about 18 µm (or 2.9 mg/cm 2 ) for SVM12. The calculations of oxide thickness from mass gains assume a dense oxide corresponding to the density of Fe 2 O 3 . As the primary oxide scale is in the sub-micron range, all three materials have undergone breakaway corrosion already after 4 hours of exposure. After 4 hours (i.e., post breakaway corrosion) the three materials showcase slightly different oxidation kinetics until the exposure time reaches 168 hours. For SVM12, the mass gain increases between 4 hours and 168 hours with 7.6 mg/cm 2 , APMT with 4.1 mg/cm 2 and Alloy 27Cr33Ni3Mo with 2.6 mg/cm 2 . Thus, the mass gain of SVM12 is about 3 times greater compared to Alloy 27Cr33Ni3Mo after breakaway corrosion, i.e. in the secondary corrosion regime. Gravimetric analysis, after breakaway corrosion The impact of increased Cl load on the oxidation behavior of SVM12, APMT and 27Cr33Ni3Mo after breakaway was investigated by exposing the three materials to an environment with low Cl load, i.e., H 2 O + KCl and to high Cl load, i.e., H 2 O + KCl + HCl at 600°C for 168 hours. In addition, the materials were also exposed to an environment without Cl (i.e., H 2 O only) for reference. Figure 1 shows the mass gain of the materials after 168 hours of exposure. SVM12 exhibits the thickest oxide while APMT and alloy 27Cr33Ni3Mo show thinner oxide thicknesses in all the tested environments. In the absence of Cl (i.e., H 2 O only), SVM12 displays an average mass gain of 2.8 mg/cm 2 which were measured to about 63 µm oxide thickness for the sample in maximum range. With addition of KCl, the mass gain increases to 10.5 mg/cm 2 (or 93 µm oxide thickness). When the Cl load is further increased through the addition of HCl, the mass gain is 16 mg/cm 2 with a measured oxide thickness of 123 µm. The recorded mass gains and oxide thicknesses of APMT and alloy 27Cr33Ni3Mo were considerably lower in all environments. In the absence of Cl (i.e., H 2 O only), APMT displays an average mass gain of 0.015 mg/cm 2 and the alloy 27Cr33Ni3Mo displays mass change of -0.01 mg/cm 2 (represented as green lines due to difference in scale). When exposed to H 2 O + KCl, the mass gains of the alloys increase to 5 mg/cm 2 and 3.4 mg/cm 2 for APMT and alloy 27Cr33Ni3Mo, respectively. When exposed to H 2 O + KCl + HCl, the mass gain and oxide thickness was similar compared to the corresponding exposure in H 2 O + KCl. Microstructural investigation Figure 2 shows SEM-BSE cross-sectional images of the SVM12 steel exposed in H 2 O, H 2 O + KCl and H 2 O + KCl + HCl. This material forms thick and dense oxide scales in all three environments of this study (as determined from SEM analysis on BIB cross-sections). The microstructural investigation shows that the scale is double-layered and consists of an outward-growing and an inward-growing oxide scale. In the H 2 O only exposure, SEM/EDX point analysis (marked as 1 and 2 in the images), line scans and elemental mappings show that the outer scale is composed of a Fe-rich oxide while the inner scales consists of a mixed Fe,Cr oxide. In the presence of H 2 O + KCl, both the outward- and inward-growing scale increases in thickness relative to the H 2 O only environment. Trace amounts of K detected in the lower part of outer layer, whereas trace amounts of Cl is detected in the outer layer as well as within the inner scale, see elemental maps. In H 2 O + KCl + HCl, the inward-growing scale of SVM12 has doubled in thickness compared to the corresponding exposure in H 2 O + KCl. The outward-growing scale is only slightly thicker in H 2 O + KCl + HCl exposure compared to H 2 O + KCl exposure. The inward-growing scale exhibits two distinct microstructures, where the regions closer to the outer/inner scale interface are fully oxidized and dense, while the regions closer to the scale/metal interface consist of mixture of Cr-rich oxide and unoxidized metal. Trace amounts of Cl can be detected in the inner scales in both the H 2 O + KCl and H 2 O + KCl + HCl. Figure 3 shows SEM-BSE cross-sectional images of a) APMT and b) alloy 27Cr33Ni3Mo. SEM/EDX analyses show that both alloys form double-layered scales consisting of Fe-rich outward-growing oxide and mixed (Fe,Cr,Al) oxide and (Fe,Cr) oxide for a) and b), respectively, in the inward-growing scales. APMT forms a 34 µm thick scale in H 2 O + KCl exposure, see Fig. 3 a. At the surface, large K 2 CrO 4 particles embedded with Fe-rich oxide are detected. Beneath this layer, a Fe-rich scale containing large voids is detected. The composition of this oxide is 85 at-%Fe, 10 at-% Cr (cationic-%) and minor elements according to the SEM/EDX point analysis (marked as 1). The inner scale is dense and exhibits non-uniform thickness in the range of 10–18 µm. Elemental analysis (marked as 2) shows that the inner scale is composed of Fe, Cr, Al and O. In the presence of H 2 O + KCl + HCl, this alloy forms a slightly thinner oxide scale compared to the corresponding H 2 O + KCl exposure (∼ 25 µm thick) which is well adherent to the metal. In contrast to the H 2 O + KCl exposure, only KCl particles are detected at the top part of the scale, no K 2 CrO 4 . The inner scale is more enriched in Cr (see line scans and elemental maps) when exposed to H 2 O + KCl + HCl compared to H 2 O + KCl. Trace amounts of Cl in the inner scale can only be detected in the H 2 O + KCl exposure and not in the H 2 O + KCl + HCl exposure. For alloy 27Cr33Ni3Mo (see Fig. 3 b), the oxide scale formed in H 2 O + KCl is ∼ 9 µm and well adherent to the metal. At the surface, the formation of large K 2 CrO 4 particles that contain cracks is prevalent, with an Fe-rich outward-growing oxide tethering the K 2 CrO 4 particles. The inward-growing scale is Cr-enriched close to the metal/scale interface followed by 8 µm deep Cr-depletion zone that is enriched in Ni. In the presence of H 2 O + KCl + HCl, the oxide scale formed is ∼ 9 µm thick and with large KCl particles at the surface. Similar to APMT, no K 2 CrO 4 was detected on alloy 27Cr33Ni3Mo when exposed to H 2 O + KCl + HCl. The outward-growing oxide is Fe-rich with the upper regions embedded in KCl particles. The inward-growing scale is composed of Cr-rich oxide mixed with oxidized metal followed by Cr-depletion zone that is enriched in Ni. Notably, an increased void concentration is observed in these Cr-depleted regions in the presence of HCl. Discussion The aim of this study was to investigate the impact of increased Cl load on the oxide scales formed after breakaway. Based on the results of corrosion test performed for alloys SVM12, APMT and 27Cr33Ni3Mo in low Cl load environment (H 2 O + KCl) and high Cl load environment (H 2 O + KCl + HCl), it is observed that all the three alloys have undergone breakaway oxidation i.e., the slow growing and protective corundum type oxides that normally forms in mild environments have been destroyed and fast-growing iron rich scales are formed instead. Thus, the primary corrosion regime has progressed into the secondary corrosion regime during the first 4 hours of exposure, which is in accordance with previous investigations done on similar alloys [ 4 , 13 ]. Generally, the results show that the effect of increasing chlorine load on the secondary corrosion protection differs among the three alloys investigated. Microstructural investigation reveals that the oxide scales formed on these alloys are dense, which indicates that the scale growth is due to the diffusion of cations and anions in the scale similar to alloys exposed without Cl [ 4 , 14 ]. In fact, the oxide scale morphology did not support the hypotheses of gaseous transport of chlorine through the scale, although trace amounts of Cl were detected in the scales. SVM12 exhibits high mass gains and thick Fe-rich oxide scales under all test conditions indicating the formation of poor secondary corrosion protection. In presence of H 2 O, this material quickly undergoes breakaway oxidation, forming thick double-layered scale. This is in line with results from previous studies investigating the effect of water vapor on similar alloys [ 15 ]. The corrosion attack is increased with addition of KCl leading to fast oxidation and formation of a thicker inward-growing scale. Interestingly, the corrosion attack is further accelerated with addition of Cl in the form of HCl, with the thickness of the inward-growing scale contributing with most of the increased corrosion attack. Thus, it seems that the role of Cl in the scale growth of poor secondary protection of this material is connected to the promotion of oxygen ingress into the metal, as primarily the inward growing part of the oxide is affected. The results indicate that the presence of Cl in the inner scale changes the diffusion properties of elements (e.g., oxygen) through the scale, which was also suggested in [ 16 ]. Attempts to better understand the exact role of chlorine in accelerating corrosion of stainless steels have been performed utilizing 3D-FIB and TEM, but it is still unclear how the presence of chlorine influences the diffusive properties [ 17 ]. Other attempts in explaining the corrosion acceleration of chlorine induced corrosion often refers to active oxidation, where it is suggested that Cl 2 (g) acts as a catalyst and are transports through the oxide scales in cyclic manner via MeCl 2 (g)/Cl 2 (g) reactions [ 3 ]. However, since the oxide scales are dense, it is questionable how a gaseous transport of MeCl 2 (g) and Cl 2 (g) could occur. Furthermore, this mechanism requires that O 2 (g) is disallowed to utilize the same pathways, for the chlorine cycle to work. In the present study, the results clearly show a profound oxygen ingress into the alloy upon addition of HCl, which supports the notion that Cl load further accelerates the corrosion attack on the scales with poor secondary protection. The FeCrAl alloy (APMT) and austenitic stainless steel (alloy 27Cr33Ni3Mo) exhibit improved corrosion resistance in Cl containing environments as revealed by their slow oxidation kinetics and formation of Cr-rich inward-growing scales indicating that these alloys form, in comparison to SVM12, good secondary protection after breakaway. This improved corrosion resistance is attributed to high contents of alloying elements such as Cr, Al and Ni in the alloys (see compositions in Table 1 ) which enables the formation of more protective Cr-rich inward-growing scales, which is in accordance to previous works, e.g., [ 4 ]. It is noted that with addition of HCl, the oxide scale thickness is unaffected or slightly thinner. Instead, the inward-growing oxide is more enriched in Cr with the addition of HCl to the environment. The scale growth for alloys that form good secondary protection is, in addition to being diffusion-controlled, associated with the formation of corrosion products. In the case of H 2 O + KCl, corrosion proceeds by formation of K 2 CrO 4 on the surface. The degree of corrosion resistance is dependent on constant replenishment of Cr to the surface. On the other hand, upon addition of HCl, the formation of K 2 CrO 4 is hindered and instead the KCl is stabilized on the surface. Consequently, alloys exhibiting good secondary protection can experience Cr-enrichment in the inner scales faster with addition of HCl leading to improved corrosion resistance. Conclusion The impact of Cl load on alkali-induced corrosion of SVM12, Kanthal® APMT and alloy 27Cr33Ni3Mo after breakaway oxidation was investigated at 600°C for 168 hours. The FeCr alloy (SVM12) exhibits poor corrosion resistance and forms fast-growing Fe-rich oxide scales that are further accelerated in high chlorine load. The more highly alloyed materials (APMT and alloy 27Cr33Ni3Mo) exhibit improved corrosion resistance in low Cl load forming Cr-rich inner scales and the increased Cl load does not accelerate the corrosion attack on these alloys. Declarations Author Contribution V.S contributed with conceptualization, methodology, investigations, data curation, data validation, writing (original draft, review & editing). T.J contributed with conceptualization, data validation, writing (review & editing). J.N contributed with writing (review & editing). J.L contributed with conceptualization, writing (review & editing), supervision, project administration. Acknowledgements This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 815147 (BELENUS). The study was conducted at the High Temperature Corrosion Center (HTC) at Chalmers University of Technology. Microscopy was performed at the Chalmers Materials Analysis Laboratory (CMAL). 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The influence of a KCl-rich environment on the corrosion attack of 304 L: 3D FIB/SEM and TEM investigations. Corrosion science. 2021;183:109315. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 22 Jul, 2024 Read the published version in High Temperature Corrosion of Materials → Version 1 posted Editorial decision: Accepted 16 Jul, 2024 Reviews received at journal 15 Jul, 2024 Reviewers agreed at journal 15 Jul, 2024 Reviewers invited by journal 09 Jul, 2024 Editor assigned by journal 07 Jul, 2024 Submission checks completed at journal 07 Jul, 2024 First submitted to journal 06 Jul, 2024 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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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4698261","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":327926969,"identity":"d7796f77-9c95-4ecc-a220-d39014bb954c","order_by":0,"name":"Vicent 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°C.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4698261/v1/4a652add6ec424432c68abf0.png"},{"id":61495421,"identity":"daa86093-86be-49ce-8196-787727e766b6","added_by":"auto","created_at":"2024-07-31 11:41:49","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":262338,"visible":true,"origin":"","legend":"\u003cp\u003eSEM-BSE cross-sections images with EDX analysis for SVM12 after 168 hours exposure in H\u003csub\u003e2\u003c/sub\u003eO, H\u003csub\u003e2\u003c/sub\u003eO+KCl and H\u003csub\u003e2\u003c/sub\u003eO+KCl+HCl at 600 °C\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4698261/v1/1fbace521f995118dd0d6694.png"},{"id":61495422,"identity":"d17d2f08-70a7-4116-9230-73ac805b96bf","added_by":"auto","created_at":"2024-07-31 11:41:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":284100,"visible":true,"origin":"","legend":"\u003cp\u003eSEM-BSE cross-sections images with EDX analysis for a) Kanthal\u003csup\u003e®\u003c/sup\u003e APMT and b) alloy 27Cr33Ni3Mo after 168 hours exposure in H\u003csub\u003e2\u003c/sub\u003eO+KCl and H\u003csub\u003e2\u003c/sub\u003eO+KCl+HCl at 600 °C. \u0026nbsp;\u0026nbsp;\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4698261/v1/9f074a642ba29b44c29e48fa.png"},{"id":61499333,"identity":"774dbe11-ae2f-4618-966e-31c46ccaed2d","added_by":"auto","created_at":"2024-07-31 12:23:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":957308,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4698261/v1/90ef4734-463b-4960-bfaa-19d010f7c4d2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The impact of HCl on alkali-induced corrosion of stainless steels/FeCrAl alloy at 600 °C: The story after breakaway","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIt is well established that combustion of biomass and waste generates corrosive gases/deposits consisting of e.g., alkali salts [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. At 600\u0026deg;C and below, this environment is known to destroy the primary protection of stainless steels (Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e or Cr-rich (Cr, Fe)\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)) and FeCrAl alloys (Cr,Al)\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e), causing breakaway oxidation [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. This occurs rapidly, within hours or days, in this environment and the alloys will thereby be dependent on the scales formed after breakaway, the so-called secondary protection. The secondary protection is characterized by formation of outward-growing Fe-rich oxide and inward-growing spinel oxides, which composition depends on the alloying elements. In recent research, the concepts of good secondary protection (referring to slow-growing scale formed after breakaway) and poor secondary protection (referring to fast-growing Fe-rich scales) have been introduced to improve the understanding of oxide scale formation after the breakaway phenomenon [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMost of the published literature on corrosion protection at high temperatures has focused on the integrity and lifetime of the primary oxide, coupled to the effect of alloying elements, e.g., Cr/Al [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], Si [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], Mo [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. However, previous research shows that the mechanisms controlling oxide growth kinetics after breakaway oxidation i.e., the secondary corrosion regime, differ from the primary corrosion regime when it comes to the role of the environment [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe effect of chlorine containing species on the primary corrosion regime (pre-breakaway stage) is well investigated. Alkali chlorides such as KCl, NaCl and PbCl\u003csub\u003e2\u003c/sub\u003e have been reported to breakdown the primary oxide by forming alkali-chromate, the so-called chromate formation mechanism [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. HCl has been shown to facilitate formation of metal chlorides through an electrochemical approach which involves the formation of chloride ions at the scale-gas interface and cations at the metal-scale interface via oxidation [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In other work, the role of Cl on the oxidation process has been attributed to its catalytic nature, which promotes the formation of volatile metal chlorides at lower pO\u003csub\u003e2\u003c/sub\u003e, which upon diffusion through the scale to regions of higher pO\u003csub\u003e2\u003c/sub\u003e are converted to respective metal oxides, the so-called active oxidation mechanism [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. On the other hand, some investigations have reported that the environment plays little or no role on the oxide scale formation beyond breakaway oxidation [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Instead, the oxide growth kinetics has been explained by a diffusion-controlled mechanism, which is determined by the diffusivity of the different cations through the spinel [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHowever, despite the extensive research carried out to understand alkali-induced corrosion, there is limited knowledge on the impact of increased Cl load on the oxide scales formed after breakaway, i.e., on the secondary protection of stainless steels and FeCrAl alloys.\u003c/p\u003e \u003cp\u003eThe aim of this study is to investigate the impact of an increased Cl load on the scales formed after breakaway on both good secondary protection and poor secondary protection. For this purpose, high-temperature corrosion studies were conducted of three alloys: SVM12 (martensitic stainless steel), Kanthal\u003csup\u003e\u0026reg;\u003c/sup\u003e APMT (FeCrAl alloy) and alloy 27Cr33Ni3Mo (austenitic stainless steel), in a laboratory environment containing H\u003csub\u003e2\u003c/sub\u003eO, KCl and HCl at 600\u0026deg;C. Characterization of oxide products were performed using scanning electron microscopy (SEM) in combination with energy-dispersive x-ray (EDX) spectroscopy.\u003c/p\u003e"},{"header":"Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials and sample preparation\u003c/h2\u003e \u003cp\u003eIn this study, three alloys were investigated; SVM12 (martensitic stainless steel) supplied by Vallourec S.A, APMT (FeCrAl alloy) supplied by Kanthal AB and alloy 27Cr33Ni3Mo (austenitic stainless steel) supplied by Alleima AB. The dimensions of the samples were 20x10x2 mm and the chemical compositions are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\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 in wt-% of alloys used in this study\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=\"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=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" 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\" colname=\"c1\"\u003e \u003cp\u003eAlloy\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eMo\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\u003eMn\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSVM12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBal.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKanthal\u0026reg; APMT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBal.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAlloy 27Cr33Ni3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBal.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe samples were prepared by grinding the surfaces with 800 SiC grit paper using a Struers automatic polishing machine. The ground coupons were then cleaned using acetone, followed by ultra-sonic bath and finally cleaned with ethanol.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eExposures\u003c/h2\u003e \u003cp\u003eAll exposures were performed isothermally at 600\u0026deg;C for 168 hours. Three environments were investigated: Exposure 1) 5% O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;20% H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;N\u003csub\u003e2\u003c/sub\u003e (Bal.). Exposure 2) as in 1\u0026thinsp;+\u0026thinsp;2 mg/cm\u003csup\u003e2\u003c/sup\u003e KCl was pre-deposited on the samples using a spray technique. In addition, an alumina boat filled with KCl(s) was placed upstream the samples at 600\u0026deg;C. A detailed description of the exposure setup can be found here [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Exposure 3) as in 2\u0026thinsp;+\u0026thinsp;500 ppm HCl(g). The HCl(g) was introduced into the exposure system from a gas mixture of 5% HCl-95% N\u003csub\u003e2\u003c/sub\u003e and was regulated using a mass flow controller. The amount of HCl(g) passing through the system was double-checked by measuring the Cl content in the collecting bottle (outlet) using ion chromatography. The samples were weighed before and after all the exposures using Sartorius\u0026trade; scale with the resolution of 1\u0026micro;g.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization\u003c/h2\u003e \u003cp\u003eAfter exposure, a thin layer of gold was sputtered on the sample surfaces and thin silicon wafer was attached using glue. The coated samples were cut and polished using a Leica EM TXP. Cross-sections of the samples were obtained using a Gatan PECS II broad ion beam (BIB) milling system operated at 8 kV for 6 hours. The corrosion products were analyzed using scanning electron microscopy (SEM) coupled with EDS system and operated at 10 kV for imaging and 20 kV for chemical analysis.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eGravimetric analysis, onset of breakaway corrosion\u003c/h2\u003e \u003cp\u003eThe onset of breakaway oxidation was studied by exposing the alloys SVM12, APMT and alloy 27Cr33Ni3Mo to H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl at 600\u0026deg;C for 4 hours. Gravimetric analysis showed that all the alloys exhibit significant mass gain corresponding to a calculated oxide scale thickness of at least 5 \u0026micro;m (or 0.8 mg/cm\u003csup\u003e2\u003c/sup\u003e) for Alloy 27Cr33Ni3Mo, about 6 \u0026micro;m (or 0.9 mg/cm\u003csup\u003e2\u003c/sup\u003e) for APMT and about 18 \u0026micro;m (or 2.9 mg/cm\u003csup\u003e2\u003c/sup\u003e) for SVM12. The calculations of oxide thickness from mass gains assume a dense oxide corresponding to the density of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. As the primary oxide scale is in the sub-micron range, all three materials have undergone breakaway corrosion already after 4 hours of exposure. After 4 hours (i.e., post breakaway corrosion) the three materials showcase slightly different oxidation kinetics until the exposure time reaches 168 hours. For SVM12, the mass gain increases between 4 hours and 168 hours with 7.6 mg/cm\u003csup\u003e2\u003c/sup\u003e, APMT with 4.1 mg/cm\u003csup\u003e2\u003c/sup\u003e and Alloy 27Cr33Ni3Mo with 2.6 mg/cm\u003csup\u003e2\u003c/sup\u003e. Thus, the mass gain of SVM12 is about 3 times greater compared to Alloy 27Cr33Ni3Mo after breakaway corrosion, i.e. in the secondary corrosion regime.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eGravimetric analysis, after breakaway corrosion\u003c/h2\u003e \u003cp\u003eThe impact of increased Cl load on the oxidation behavior of SVM12, APMT and 27Cr33Ni3Mo after breakaway was investigated by exposing the three materials to an environment with low Cl load, i.e., H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl and to high Cl load, i.e., H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl at 600\u0026deg;C for 168 hours. In addition, the materials were also exposed to an environment without Cl (i.e., H\u003csub\u003e2\u003c/sub\u003eO only) for reference. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the mass gain of the materials after 168 hours of exposure. SVM12 exhibits the thickest oxide while APMT and alloy 27Cr33Ni3Mo show thinner oxide thicknesses in all the tested environments. In the absence of Cl (i.e., H\u003csub\u003e2\u003c/sub\u003eO only), SVM12 displays an average mass gain of 2.8 mg/cm\u003csup\u003e2\u003c/sup\u003e which were measured to about 63 \u0026micro;m oxide thickness for the sample in maximum range. With addition of KCl, the mass gain increases to 10.5 mg/cm\u003csup\u003e2\u003c/sup\u003e (or 93 \u0026micro;m oxide thickness). When the Cl load is further increased through the addition of HCl, the mass gain is 16 mg/cm\u003csup\u003e2\u003c/sup\u003e with a measured oxide thickness of 123 \u0026micro;m.\u003c/p\u003e \u003cp\u003eThe recorded mass gains and oxide thicknesses of APMT and alloy 27Cr33Ni3Mo were considerably lower in all environments. In the absence of Cl (i.e., H\u003csub\u003e2\u003c/sub\u003eO only), APMT displays an average mass gain of 0.015 mg/cm\u003csup\u003e2\u003c/sup\u003e and the alloy 27Cr33Ni3Mo displays mass change of -0.01 mg/cm\u003csup\u003e2\u003c/sup\u003e (represented as green lines due to difference in scale). When exposed to H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl, the mass gains of the alloys increase to 5 mg/cm\u003csup\u003e2\u003c/sup\u003e and 3.4 mg/cm\u003csup\u003e2\u003c/sup\u003e for APMT and alloy 27Cr33Ni3Mo, respectively. When exposed to H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl, the mass gain and oxide thickness was similar compared to the corresponding exposure in H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eMicrostructural investigation\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows SEM-BSE cross-sectional images of the SVM12 steel exposed in H\u003csub\u003e2\u003c/sub\u003eO, H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl and H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl. This material forms thick and dense oxide scales in all three environments of this study (as determined from SEM analysis on BIB cross-sections). The microstructural investigation shows that the scale is double-layered and consists of an outward-growing and an inward-growing oxide scale. In the H\u003csub\u003e2\u003c/sub\u003eO only exposure, SEM/EDX point analysis (marked as 1 and 2 in the images), line scans and elemental mappings show that the outer scale is composed of a Fe-rich oxide while the inner scales consists of a mixed Fe,Cr oxide. In the presence of H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl, both the outward- and inward-growing scale increases in thickness relative to the H\u003csub\u003e2\u003c/sub\u003eO only environment. Trace amounts of K detected in the lower part of outer layer, whereas trace amounts of Cl is detected in the outer layer as well as within the inner scale, see elemental maps.\u003c/p\u003e \u003cp\u003eIn H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl, the inward-growing scale of SVM12 has doubled in thickness compared to the corresponding exposure in H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl. The outward-growing scale is only slightly thicker in H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl exposure compared to H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl exposure. The inward-growing scale exhibits two distinct microstructures, where the regions closer to the outer/inner scale interface are fully oxidized and dense, while the regions closer to the scale/metal interface consist of mixture of Cr-rich oxide and unoxidized metal. Trace amounts of Cl can be detected in the inner scales in both the H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl and H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows SEM-BSE cross-sectional images of a) APMT and b) alloy 27Cr33Ni3Mo. SEM/EDX analyses show that both alloys form double-layered scales consisting of Fe-rich outward-growing oxide and mixed (Fe,Cr,Al) oxide and (Fe,Cr) oxide for a) and b), respectively, in the inward-growing scales.\u003c/p\u003e \u003cp\u003eAPMT forms a 34 \u0026micro;m thick scale in H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl exposure, see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. At the surface, large K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e particles embedded with Fe-rich oxide are detected. Beneath this layer, a Fe-rich scale containing large voids is detected. The composition of this oxide is 85 at-%Fe, 10 at-% Cr (cationic-%) and minor elements according to the SEM/EDX point analysis (marked as 1). The inner scale is dense and exhibits non-uniform thickness in the range of 10\u0026ndash;18 \u0026micro;m. Elemental analysis (marked as 2) shows that the inner scale is composed of Fe, Cr, Al and O. In the presence of H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl, this alloy forms a slightly thinner oxide scale compared to the corresponding H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl exposure (\u0026sim; 25 \u0026micro;m thick) which is well adherent to the metal. In contrast to the H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl exposure, only KCl particles are detected at the top part of the scale, no K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e. The inner scale is more enriched in Cr (see line scans and elemental maps) when exposed to H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl compared to H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl. Trace amounts of Cl in the inner scale can only be detected in the H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl exposure and not in the H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl exposure.\u003c/p\u003e \u003cp\u003eFor alloy 27Cr33Ni3Mo (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), the oxide scale formed in H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl is \u0026sim; 9 \u0026micro;m and well adherent to the metal. At the surface, the formation of large K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e particles that contain cracks is prevalent, with an Fe-rich outward-growing oxide tethering the K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e particles. The inward-growing scale is Cr-enriched close to the metal/scale interface followed by 8 \u0026micro;m deep Cr-depletion zone that is enriched in Ni. In the presence of H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl, the oxide scale formed is \u0026sim; 9 \u0026micro;m thick and with large KCl particles at the surface. Similar to APMT, no K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e was detected on alloy 27Cr33Ni3Mo when exposed to H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl. The outward-growing oxide is Fe-rich with the upper regions embedded in KCl particles. The inward-growing scale is composed of Cr-rich oxide mixed with oxidized metal followed by Cr-depletion zone that is enriched in Ni. Notably, an increased void concentration is observed in these Cr-depleted regions in the presence of HCl.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe aim of this study was to investigate the impact of increased Cl load on the oxide scales formed after breakaway. Based on the results of corrosion test performed for alloys SVM12, APMT and 27Cr33Ni3Mo in low Cl load environment (H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl) and high Cl load environment (H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl), it is observed that all the three alloys have undergone breakaway oxidation i.e., the slow growing and protective corundum type oxides that normally forms in mild environments have been destroyed and fast-growing iron rich scales are formed instead. Thus, the primary corrosion regime has progressed into the secondary corrosion regime during the first 4 hours of exposure, which is in accordance with previous investigations done on similar alloys [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Generally, the results show that the effect of increasing chlorine load on the secondary corrosion protection differs among the three alloys investigated. Microstructural investigation reveals that the oxide scales formed on these alloys are dense, which indicates that the scale growth is due to the diffusion of cations and anions in the scale similar to alloys exposed without Cl [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. In fact, the oxide scale morphology did not support the hypotheses of gaseous transport of chlorine through the scale, although trace amounts of Cl were detected in the scales.\u003c/p\u003e \u003cp\u003eSVM12 exhibits high mass gains and thick Fe-rich oxide scales under all test conditions indicating the formation of poor secondary corrosion protection. In presence of H\u003csub\u003e2\u003c/sub\u003eO, this material quickly undergoes breakaway oxidation, forming thick double-layered scale. This is in line with results from previous studies investigating the effect of water vapor on similar alloys [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The corrosion attack is increased with addition of KCl leading to fast oxidation and formation of a thicker inward-growing scale. Interestingly, the corrosion attack is further accelerated with addition of Cl in the form of HCl, with the thickness of the inward-growing scale contributing with most of the increased corrosion attack. Thus, it seems that the role of Cl in the scale growth of poor secondary protection of this material is connected to the promotion of oxygen ingress into the metal, as primarily the inward growing part of the oxide is affected. The results indicate that the presence of Cl in the inner scale changes the diffusion properties of elements (e.g., oxygen) through the scale, which was also suggested in [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Attempts to better understand the exact role of chlorine in accelerating corrosion of stainless steels have been performed utilizing 3D-FIB and TEM, but it is still unclear how the presence of chlorine influences the diffusive properties [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Other attempts in explaining the corrosion acceleration of chlorine induced corrosion often refers to active oxidation, where it is suggested that Cl\u003csub\u003e2\u003c/sub\u003e(g) acts as a catalyst and are transports through the oxide scales in cyclic manner via MeCl\u003csub\u003e2\u003c/sub\u003e(g)/Cl\u003csub\u003e2\u003c/sub\u003e(g) reactions [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. However, since the oxide scales are dense, it is questionable how a gaseous transport of MeCl\u003csub\u003e2\u003c/sub\u003e(g) and Cl\u003csub\u003e2\u003c/sub\u003e(g) could occur. Furthermore, this mechanism requires that O\u003csub\u003e2\u003c/sub\u003e(g) is disallowed to utilize the same pathways, for the chlorine cycle to work. In the present study, the results clearly show a profound oxygen ingress into the alloy upon addition of HCl, which supports the notion that Cl load further accelerates the corrosion attack on the scales with poor secondary protection.\u003c/p\u003e \u003cp\u003eThe FeCrAl alloy (APMT) and austenitic stainless steel (alloy 27Cr33Ni3Mo) exhibit improved corrosion resistance in Cl containing environments as revealed by their slow oxidation kinetics and formation of Cr-rich inward-growing scales indicating that these alloys form, in comparison to SVM12, good secondary protection after breakaway. This improved corrosion resistance is attributed to high contents of alloying elements such as Cr, Al and Ni in the alloys (see compositions in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) which enables the formation of more protective Cr-rich inward-growing scales, which is in accordance to previous works, e.g., [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. It is noted that with addition of HCl, the oxide scale thickness is unaffected or slightly thinner. Instead, the inward-growing oxide is more enriched in Cr with the addition of HCl to the environment. The scale growth for alloys that form good secondary protection is, in addition to being diffusion-controlled, associated with the formation of corrosion products. In the case of H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl, corrosion proceeds by formation of K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e on the surface. The degree of corrosion resistance is dependent on constant replenishment of Cr to the surface. On the other hand, upon addition of HCl, the formation of K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e is hindered and instead the KCl is stabilized on the surface. Consequently, alloys exhibiting good secondary protection can experience Cr-enrichment in the inner scales faster with addition of HCl leading to improved corrosion resistance.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe impact of Cl load on alkali-induced corrosion of SVM12, Kanthal\u0026reg; APMT and alloy 27Cr33Ni3Mo after breakaway oxidation was investigated at 600\u0026deg;C for 168 hours. The FeCr alloy (SVM12) exhibits poor corrosion resistance and forms fast-growing Fe-rich oxide scales that are further accelerated in high chlorine load. The more highly alloyed materials (APMT and alloy 27Cr33Ni3Mo) exhibit improved corrosion resistance in low Cl load forming Cr-rich inner scales and the increased Cl load does not accelerate the corrosion attack on these alloys.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eV.S contributed with conceptualization, methodology, investigations, data curation, data validation, writing (original draft, review \u0026amp; editing). T.J contributed with conceptualization, data validation, writing (review \u0026amp; editing). J.N contributed with writing (review \u0026amp; editing). J.L contributed with conceptualization, writing (review \u0026amp; editing), supervision, project administration.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis project has received funding from the European Union\u0026rsquo;s Horizon 2020 research and innovation program under grant agreement No. 815147 (BELENUS). The study was conducted at the High Temperature Corrosion Center (HTC) at Chalmers University of Technology. Microscopy was performed at the Chalmers Materials Analysis Laboratory (CMAL).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMa, W., et al., The fate of chlorine during MSW incineration: Vaporization, transformation, deposition, corrosion and remedies. Progress in Energy and Combustion Science, 2020;76:100789.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFurugaki, T., H. Takahashi, and S. Hayashi, Breakdown of Protective Cr-Rich Oxide Scale Formed on Heat-Resistant Steels for Superheater Tubes in a Waste Power Generation Boiler. High Temperature Corrosion of Materials, 2024;101:61\u0026ndash;78.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGrabke, H., E. Reese, and M. Spiegel, The effects of chlorides, hydrogen chloride, and sulfur dioxide in the oxidation of steels below deposits. Corrosion science, 1995;37:1023\u0026ndash;1043.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePersdotter, A., et al., Beyond breakaway corrosion\u0026ndash;Influence of chromium, nickel and aluminum on corrosion of iron-based alloys at 600 C. Corrosion Science, 2020;177:108961.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eEklund, J., et al., The influence of silicon on the corrosion properties of FeCrAl model alloys in oxidizing environments at 600 C. Corrosion Science, 2018;144:266\u0026ndash;276.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhou, R., et al., Effect of molybdenum addition on oxidation behavior and secondary protection mechanism of FeCrAl coatings. Materials Characterization, 2023;204:113221.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePettersson, J., et al., KCl induced corrosion of a 304-type austenitic stainless steel at 600 C; the role of potassium. Oxidation of Metals, 2005;64:23\u0026ndash;41.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFolkeson, N., L.-G. Johansson, and J.-E. Svensson, Initial stages of the HCl-induced high-temperature corrosion of alloy 310. journal of the electrochemical society, 2007;154:C515.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJonsson, T., et al., Oxidation after breakdown of the chromium-rich scale on stainless steels at high temperature: internal oxidation. Oxidation of metals, 2016;85:509\u0026ndash;536.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOlivas-Ogaz, M., et al., Microstructural study of the influence of KCl and HCl on preformed corrosion product layers on stainless steel. Oxidation of Metals, 2017;87:801\u0026ndash;811.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJonsson, T., et al., High-temperature oxidation of FeCr (Ni) alloys: the behaviour after breakaway. Oxidation of Metals, 2017;87:333\u0026ndash;341.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSsenteza, V., et al., High temperature corrosion resistance of FeCr (Ni, Al) alloys as bulk/overlay weld coatings in the presence of KCl at 600 C. Corrosion Science, 2023;213:110896.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eIsraelsson, N., et al., A microstructural and kinetic investigation of the KCl-induced corrosion of an FeCrAl alloy at 600 C. Oxidation of metals, 2015;84:105\u0026ndash;127.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJonsson, T., et al., Oxidation of Fe\u0026ndash;10Cr in O2 and in O2\u0026thinsp;+\u0026thinsp;H2O environment at 600 C: A microstructural investigation. Corrosion science, 2013;75:326\u0026ndash;336.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQuadakkers WJ, Żurek J. Oxidation in Steam and Steam/Hydrogen Environments. In: Cottis B, Graham M, Lindsay R, Lyon S, Richardson T, Scantlebury D, et al., editors. \u003cem\u003eShreir\u0026rsquo;s Corrosion\u003c/em\u003e. Oxford: Elsevier; 2010:407\u0026ndash;56.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePhother-Simon, J., et al., High-Temperature corrosion of P91/T91, 304L, Sanicro 28 and Inconel 625 exposed at 600\u0026deg; C under continuous KCl deposition. Fuel, 2024;357:130012.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePhother-Simon J, Hanif I, Liske J, Jonsson T. The influence of a KCl-rich environment on the corrosion attack of 304 L: 3D FIB/SEM and TEM investigations. Corrosion science. 2021;183:109315.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Oxidation, Biomass, Breakaway, Secondary protection, Chlorine ","lastPublishedDoi":"10.21203/rs.3.rs-4698261/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4698261/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe impact of Cl on alkali-induced high temperature corrosion of stainless steels/FeCrAl alloys after breakaway oxidation was investigated in a simulated biomass- and waste-fired boiler environment at 600\u0026deg;C. For this investigation, three alloys were exposed to low Cl load environment (H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl) and to high Cl load (H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl\u0026thinsp;+\u0026thinsp;HCl). Post-exposure analysis showed that the stainless steel SVM12 experiences fast oxidation and forms thick double-layered Fe-rich oxide scales. The corrosion attack is further accelerated with addition of HCl for this material with the effect being more pronounced in the inward-growing scale. The FeCrAl and FeCrNi alloys exhibit slower oxidation kinetics after the breakaway corrosion compared to SVM12 in the H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;KCl exposure. Furthermore, in contrast to SVM12, the addition of HCl did not accelerate the corrosion attack on these alloys. It is argued that the properties of the secondary oxide layer formed after breakaway corrosion is important in the continued corrosion resistance against chlorine induced corrosion attack. Especially, the Cr-content in the inner scales is suggested to be important in corrosion mitigation.\u003c/p\u003e","manuscriptTitle":"The impact of HCl on alkali-induced corrosion of stainless steels/FeCrAl alloy at 600 °C: The story after breakaway","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-31 11:41:45","doi":"10.21203/rs.3.rs-4698261/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accepted","date":"2024-07-17T00:30:47+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-07-15T17:24:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"213090698268713543647945482167952885499","date":"2024-07-15T16:44:01+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-07-09T13:59:08+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-07-08T01:33:17+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-07-08T01:33:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"High Temperature Corrosion of Materials","date":"2024-07-06T22:25:44+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1924b8ce-5a66-46d5-abdd-192531319869","owner":[],"postedDate":"July 31st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-07-31T12:23:02+00:00","versionOfRecord":{"articleIdentity":"rs-4698261","link":"https://doi.org/10.1007/s11085-024-10265-8","journal":{"identity":"high-temperature-corrosion-of-materials","isVorOnly":false,"title":"High Temperature Corrosion of Materials"},"publishedOn":"2024-07-22 12:23:02","publishedOnDateReadable":"July 22nd, 2024"},"versionCreatedAt":"2024-07-31 11:41:45","video":"","vorDoi":"10.1007/s11085-024-10265-8","vorDoiUrl":"https://doi.org/10.1007/s11085-024-10265-8","workflowStages":[]},"version":"v1","identity":"rs-4698261","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4698261","identity":"rs-4698261","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}