Voltage-Induced Void Formation in High-Temperature Oxide Scales of Boiler Tubes

Voltage-Induced Void Formation in High-Temperature Oxide Scales of Boiler Tubes | 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 Voltage-Induced Void Formation in High-Temperature Oxide Scales of Boiler Tubes Muhammad Rafiq Haikal Rosdin, Syed Noh Syed Abu Bakar, Abd Malek Abdul Hamid, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4240283/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 The formation of voids in the oxide scale during high temperature oxidation is one of critical issues that leads to poor oxide scale adherence, fouling, spalling and materials loss, which affect substrate’s performance. Visual inspection is currently the sole way to detect corrosion process at high temperature environment. At room temperature, under the standard condition, cathodic protection has been successfully employed to monitor and control the corrosion. Understanding the development of voids in oxide scale is crucial to develop mitigation strategies and predictive maintenance. Thus, this study was intended to serve as a preliminary step to emulate the cathodic protection technique at high temperature. By polarizing the sample, it is postulated that it may affects the diffusivity of cation/anion in oxide scale, which is the rate determining step of the oxidation process. Ueda et al and Maruyama et al has shown that the difference in flux, or chemical potential of the oxygen species is the sole factor for the formation of void in oxide scale in controlled environment. In this study, the amount of voids present was measure directly on T91 alloys exposed at 823 K under various induced voltageT91 alloy which consists of Fe-9%Cr was externally induced with voltages of 0V, 50V and 300V for 43.2 ks, 259.2 ks and 432 ks at 923 K in air ( \({P}_{{O}_{2}}\) ) = 0.21 atm = 2.1 × 10 4 Pa). The presence of oxide layers was analysed using X-Ray Diffraction (XRD) and the void formed was inspected using Scanning Electron Microscopy (SEM). XRD results reveal that peaks of Fe 2 O 3 , Fe 3 O 4 , FeCr 2 O 3 and Cr 2 O 3 were formed on all sample. The parabolic rate constant, K p was calculated as 3.83 × 10 –14 m 2 /s, 2.17 × 10 –14 m 2 /s and 9.25 × 10 –14 m 2 /s respectively, verifying that the reaction occurred was a solid state diffusion. Changes in K p at different induced voltages is clear evidence that the diffusivity was altered by external electrical potential. It was observed that the overall void formation decreased by 17%. Apparently, inducing voltage onto T91 alloy effects the ionic diffusivity and changes the void formation. Conversely, it may be used to promote diffusivity of more inert species such as Cr to form protective layer at early stage of oxidation. high temperature oxidation T91 Alloy Void Formation Induced Voltage Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 1 Introduction The production of hydrogen through steam methane reforming (SMR) has emerged as the most economical method where 95% of global hydrogen production is produced [1] [2] [3] with the efficiency rate of 85% [2], [4], [5]. SMR synthesises hydrocarbons in natural gas to generate syngas by mixing pure methane with superheated steam in the presence of nickel catalyst. The steam was generated using heat recovery steam generator (HRSG) at a temperature of 923K. The most common boiler tube used in HRSG is a seamless ferritic alloy steel tube, ASME SA-213 grade T91, usually referred as T91 alloy boiler tube. As these tubes are subjected to extremely high temperature environment, oxidation of the alloy can occur in the presence of oxygen, resulting in fouling and spalling of oxide scales at the boiler tube. Furthermore, the majority of boiler tubes made must last at least 100,000 hours in service [6]. However, according to Li et. al [7], most of the tubes began to degrade within 4000 hours of service life. This discovery of early degradation than proposed time will majorly affect the boiler safety and operation. To protect the tube against corrosion, a ‘passive layer’ needed to form on the surface to prevent oxygen ions to diffuse even further into the alloy [8], [9]. This passive layer was known as oxide layer where it is inevitable in terms of alloy protection [10]. However, a thick oxide scale formation is not favourable due to the increase of resistance in the scales which can alter the heat transfer coefficients [11]. One potential issue that can arise during oxide scale formation is the development of voids within the scale. The formation of voids can affect the oxidation mechanism and mechanical properties of the scale as well as the metal itself [12]. Various literature reviews discussing on the void formation which has been compiled by Yan et. al. [13]. Most of these literature reviews discussed the formation of voids in a qualitative manner. There are still a few studies that was done quantitative way which has been compiled by Kaderi et. al. [12]. Among the quantitative research was done by Maruyama et. al. [14] wherein he claimed that void formation during high temperature oxidation was closely related to the divergence of ionic fluxes. Therefore, it is possible to investigate the oxidation of metal surfaces in the presence of an oxidising agent by combining Fick's first law with Wagner's oxidation kinetics where the calculations have been discussed by Noguchi and Yakuwa [15]. However, it known that neither of these laws include the influence of an electric field. Schlögl and Helffreich [16] has been theoretically discussed on the change of diffusivity under electrical potential by modifying Nernst-Planck equation. However, to writer’s knowledge, the effect of electric field on the high temperature corrosion is not well explored. Therefore, this study intends to explore the effect of voltage induced on the void formation’s point of view of T91 alloy at high temperature by inducing 0V, 50V and 300V at 923K in normal air. 2 Experimental Methods 2.1 Sample Preparation T91 alloy was cut into smaller pieces before it was being further cut into an average dimension of 10.92 mm × 6.44 mm × 4.24 mm using a precision saw machine. Then, it was grinded using grinding machine with various abrasive paper grades. There were 4 distinct types of abrasive sheets used: P600, P800, P1000, and P2000. The reduction in abrasiveness were shown by the increase in grade number. The sample is initially flattened and the extra oxide layer that had accumulated at the surface is removed using grade P600 abrasive paper, which was then followed by grades P800, P1000, and P2000. After that, a polishing machine was used to polish the ground sample. This machine had a revolving disc with diamond particles implanted on a fabric. The sample is once again polished to create a smooth surface using 0.3 micron-sized particles. Additionally, to minimize surface deformation and provide superior polishing results, alumina fine polishing solution was employed. 2.2 Voltage Induced Experiment Figure 1 shows the schematic diagram for the experimental setup. As for working and counter electrode, platinum wire of 1 metre in length is used due to its high resistivity towards corrosion, high electrical conductivity and chemical reactions. T91 alloy sample will be spot welded onto the working electrode. It is done to ensure that the electrodes remain firmly in place, even when subjected to the corrosive forces and high temperature of the environment. Spot welding also can ensure the voltage can be continuously supplied onto the sample. Then these wires are joined within the ceramic tube and have protruding tips. The glass tube will be inserted within the ceramic tube, with the tips sticking out toward the isothermal zone. To measure precise temperature in the zone, an R-Type thermocouple is placed in the isothermal zone, 5mm from the sample. Then, heat is supplied by the furnace. The platinum electrodes are then connected to Cockroft-Walton circuit or also known as voltage multiplier. The voltage multiplier was designed to generate high voltages from a low DC voltage. A voltmeter will be placed on the platinum wire to identify the real amount of voltage supplied. The amount of voltage supplied onto the sample will be 0V, 50V and 300V. The experiment was conducted at 1 atm (2.1×10 4 Pa of oxygen) with standard dry air composition. For 20 minutes, the temperature was progressively raised to 923K in order to assure stability inside the heated space. The required voltage is then selected, and it is left alone for another 15 minutes to check that the provided voltage is stable. Table ‎2.1 summarized the parameter used for the experiment. Table 1 Parameters used for the experiment. Time (ks) Voltage Supplied (V) 43.2 0 50 300 259.2 0 50 300 432 0 50 300 2.3 Characterization X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were used to characterize the sample. For this experiment, the oxide phases formed were analyzed using XRD with divergence slit of 2.5 degree. The diffraction angle will be set from 20° to 80°. The XRD patterns were compared with α-Fe (JCPDS 00-006-0696), FeO (JCPDS 00-006-0615), Fe 2 O 3 (JCPDS 01-085-0599), Fe 3 O 4 (JCPDS 00-019-0629), FeCr 2 O 4 (JCPDS 00-034-0140) and Cr 2 O 3 (00-034-1479). The samples were mounted individually in a resin after prior to SEM analysis. Then, the sample was cross sectioned for 2 mm using diamond saw before being grinded and polished again using same method mentioned in Section 2.1. On each image, uniformly sized grids were traced, and the percentage area of scale voids was calculated. It was presumed that the scale's volumetric vacancy percentage matched its cross-sectional void percentage [12]. The void percentage was calculated using method proposed by Kaderi et. al. [12], Maruyama et. al [14], [17] and Ueda et. al. [18], [19]. 3 Results and Discussions 3.1 XRD Analysis Figure ‎2 shows the XRD patterns of all sample surfaces at respective conditions. The XRD analysis of raw sample shows the intensity peaks at diffraction angle of 44.8° and 65° which confirms the presence of α-Fe that is in BCC structure [20]. Furthermore, there are also noticeable peaks of Fe 2 O 3 (hematite), Fe 3 O 4 (magnetite), FeCr 2 O 3 (spinel) and Cr 2 O 3 (chromia), confirming that all sample has been oxidized. Meanwhile, samples subjected to 300V for 43.2 ks and all samples exposed for 259.2 ks and 432 ks began to show a less intense peak of magnetite. This is due to their variations in crystal structure as well as the arrangement of atoms. Nasrazadani and Raman [21] stated that magnetite is a mixed valence iron oxide containing both Fe 2+ and Fe 3+ ions in an inverse spinel structure while hematite has a corundum structure with only Fe 3+ ions. The presence of Fe 2+ ions in magnetite causes more disorder in the crystal structure. This causes magnetite to have weaker diffraction peaks compared to hematite. Plus, the dense formation of hematite also can affect the XRD to detect magnetite phase [22]. The peaks of chromia started to increase from the sample exposed at 50V for 43.2 ks until 300V for 432 ks. It verifies that as time goes by, chromia layer starts to develop. However, the peak was not as intense compared to magnetite and hematite. This is because some of the chromium atoms in the sample has successfully bonded with other chromium containing oxides like spinel. Hence, presence of multiple phases can lead to reduction in diffraction peaks. Other than that, the chromium content of T91 alloy was quite low at 9% which can also lead to reduced intensity. A study conducted by Karimi et. al. [23] can reassure the earlier claim. 3.2 SEM Analysis The morphology of the sample at the proposed timeframe was summarized in Figure ‎3. A red dashed line was drawn to indicate the interphase boundary between alloy, inner scale, and outer scale. For ease of discussion, the inner scale will refer to as magnetite while the outer scale is hematite, and the term will be interchangeably used in the discussion. Based on the figure, all samples show the presence of oxide layers, indicating that oxidation process has been successfully occurred even when external voltage was supplied. It was also observed that the voids’ shape was irregular, and some of them were aligned in a lath-like pattern at specific oxide thickness. A clear example of voids and lath-like patterns can be referred to in Fig. 4 . The formation of voids in the inner and outer oxide scale is due to the ionic flux. A study conducted by Ueda et. al. [18]stated that the partial pressure of oxygen can affect the void formation. The higher the oxygen partial pressure, the higher the oxygen chemical potential and hence, a greater number of voids formed. Even though the oxygen partial pressure of this experiment is high (2.1 × 10 4 Pa), there are a presence of voids in the outer scale and only a few voids appear in the inner scale. In addition, Taniguchi [24] proposed that the oxide expanded under a compressive force if the volume of oxide formed to the metal exceeded unity. If the ratio, however, was less than one, the resulting oxide would expand under tensile stress. As a result, a porous oxide coating develops. The oxide layer was either created island-like or layer-by-layer when it was subjected to high temperatures [25]. Figure 5 shows the parabolic plots of the overall oxide thickness in weight gain perspective of the sample. It shows a sharp increase at the beginning phase and then slowly gaining weight up to 432 ks. For the sample exposed to 0V, it appears that the weight gain does not follow the parabolic pattern especially on 259.2 ks of exposure. This was due to oxide spallation which causes it to become fragile during the final weight measurement. Overall, it indicates that almost all oxide scale formed obeys parabolic rate law. From the cross section of the sample, the thickness of oxide scales can be determined to calculate the parabolic rate constant, K p . The K p values for 0V, 50V and 300V were 3.83 × 10 − 14 m 2 /s, 2.17 × 10 − 14 m 2 /s and 9.25 × 10 − 14 m 2 /s respectively. 3.3 Volume Fraction of Voids in Oxide Scale Based on the calculation proposed by the authors in Section 2.3, the graphical representation of volume fraction of void for the sample’s overall phase are as shown in Fig. 6 . The image illustrates that the volume percentage of voids and the governing equations are in good agreement. It also shows that as the exposure time increases, the void formation decreases, and the overall formation of voids has decreased by 17%. One of the reasons for this is that due to the oxide layer is getting thicker over time. As time goes by, the thickness of oxide layer on the surface becomes thicker which will simultaneously hinder the formation of voids. Similarly, as the voltage supplied is increased, void formation on the oxide layers also decreased. Generally, when the sample was exposed to the high temperature environment, oxygen ions from the air will diffuse into the sample. The iron and chromium ions from the sample will also try to diffuse out to the surface. Due to the difference in flux between iron, chromium, and oxygen ions, it will cause some of them to rearrange themselves faster, causing the slower moving ions to have difficulty in moving and redistributing themselves. Thus, it will lead to accumulation of voids. However, as there is presence of external driving force, in this case, external supplied voltage, it has caused the flux to be increased as well. Hence, it will speed up the diffusion process, causing the formation of void to be reduced. In the case of T91 alloy, chromium reacts with oxygen, and it will form a layer of chromia which will act as a passive layer [26]. This layer will prevent further oxidation and corrosion. It is well known that chromia has a much slower diffusion rate than other oxide layers [27]. Therefore, some of the iron ions managed to react with oxygen to form an oxide layer. By inducing voltage onto the sample, the flux of chromia can be accelerated which allows it to reach the surface more quickly and participate in the formation of oxide layer. This will result in the enhanced passive layer where it will also enhance the resistance against void formation and corrosion. This can be seen clearly on Figure ‎7 where the overall void percentage of T91 alloy decreases linearly. A plot of overall void percentage vs voltage is shown in Figure ‎8. When this plot has been done, it shows that the void percentage decreases exponentially as voltage increases. It is again confirmed that the oxidation process has become more active due to the assistance in external voltage that causes more metal ions to be released from the alloy and contributes to breakdown of oxide layers. Analyzing the same figure, it shows that an increase in voltage will result in the void percentage becoming stagnant and achieve a certain limit. This pattern was also similar on Figure ‎9 and 10. This indicates that the void percentage can stay the same if a larger voltage is applied to the sample. However, further research needs to be done to confirm this hypothesis. Based on the graph obtained in Figure ‎6, 7 and 8, further analysis can be done on void percent of hematite and magnetite layer which can be referred on Figure ‎9 to Fig. 14 . A clear distinction can be observed based on this figure where there is higher volume of voids formed in hematite as compared to magnetite. This phenomenon can be explained by several factors. Among them is related to the crystal structure and properties of these two oxide layers. Magnetite has a spinel structure which consists of both Fe 2+ and Fe 3+ ions. This structure allows for a greater number of oxygen vacancies and defects within oxide lattice [28]. Hence, magnetite tends to exhibit higher probability of void formation because it can facilitate the movement of iron ions. On the other hand, hematite has an alpha-corundum crystal structure and contains only Fe 3+ ions. Hematite is more compact and less prone to formation of defects and voids [29]. However, this is not the case when external voltage has been supplied. It appears that the volume fraction of voids for both layers started to decrease as higher amount of voltage was being supplied. Plus, the exponential pattern of volume fraction of void in magnetite phase appears to be steeper compared to hematite. At the exposed temperature, magnetite exhibits an interesting relationship between its magnetic and electrical properties. Referring to Fig. 15 , magnetite is a ferrimagnetic material at which it possessed both magnetic and electrical properties [30]. Due to its mixed-valenced compound, it creates a charged imbalance in the crystal lattice, resulting in the presence of electrical conductivity. As mentioned by Radoń et al. [31], an increase of temperature causes the electrical conductivity of magnetite to significantly enhance. This is because high thermal energy allows for a greater charge carrier, facilitating the movement of all metallic ions in it. Along with external voltage supplied, the induced current will increase the speed of diffusion at magnetite, causing the void formation to decrease. On the contrary, hematite behaves as a paramagnetic at temperature above Neel temperature, (T N ≈ 955K) at which the magnetic moments become disordered, and no long-range magnetic ordering is observed [32]. Figure ‎16 depicts the crystal structure of hematite. Therefore, hematite is considered as insulator or weakly conducting material as well as having a wide bandgap which restricts the flow of electrical current [30], [33]. Due to the mentioned hematite properties, an increase in voltage supplied shows a less significant steep as shown in Fig. 9 and Fig. 10 . Moreover, when the voltage was set up to 300V, their volume fraction of void shows almost linear pattern (Figure ‎10). Overall, the effect of inducing different amount voltage onto T91 alloy shows that as the voltage supplied increase, it has speed up the diffusion rate of magnetite, causing less formation of void and vice versa for hematite. Figure 17 shows a summary of work done on volume fraction of voids on the outermost scale of the sample with respect to oxygen partial pressure, \({P}_{{O}_{2}}\) . A study on void at formation at magnetite scale conducted by Ueda et al. [19] and Maruyama et al. [14] showed a very small range of void formed in the range of 0.05 to 0.06. Note that that these two studies were conducted in a controlled environment. In contrast, a study on void formation of NiO scale by Akiba et al. [36] and void on Fe-Cr-Ni alloy by Kaderi et al. [12] was done in normal air and comparable with this work. Note that the volume fraction of void on the outer scale has a much less magnitude when voltage is supplied. 4 Conclusion T91 alloy has been exposed under high temperature corrosion environment at 923K. The samples were subjected to impressed voltage of 0V, 50V and 300V at 43.2 ks, 259.2 ks and 432 ks. XRD analysis confirms that all oxide layers managed to form in all respective conditions. The cross section for all samples were clearly presented via SEM images. The plot of oxide thickness versus time shows that oxidation following the parabolic law, verifying that the reaction occurred was a solid state diffusion. Overall Kp value for samples exposed at 0V, 50V and 100V were 3.83 × 10 –14 m 2 /s, 2.17 × 10 –14 m 2 /s and 9.25 × 10 –14 m 2 /s respectively. SEM images also revealed that voids managed to form at all samples. Void percentage has been calculated on overall, hematite and magnetite layer. The result shows that overall void formation decreased by 17% and less amount of void observed on magnetite phases as compared to hematite. This is because magnetite’s spinel structure exhibits ferrimagnetism which enhances its magnetic and electrical properties compared to hematite’s alpha-corundum crystal structure. Other than that, void percentage results from this paper have been compared to previous work done by Akiba et al. [36], Maruyama et al. [14], Ueda et al. [19] and Kaderi et al. [12]. Declarations Author Contribution This thesis report represents the collaborative efforts of five authors, each contributing unique skills and expertise to different facets of the research process.M.R.H. Rosdin led the conceptualization and design of the research, outlining the objectives and methodologies. They coordinated data collection and analysis, contributed significantly to the interpretation of findings, and played a key role in drafting and revising the manuscript.M.H. Ani provided expertise in High Temperature Corrosion at boiler tubes, which influenced the theoretical framework development and guided the analysis of results. They contributed to the literature review, critically reviewed the manuscript, and contributed to shaping the discussion section with insightful interpretations.S.N. Syed Abu Bakar contributed to the research design and methodology, ensuring methodological rigor and validity. Their expertise in [mention specific area] brought depth to the interpretation of results and contributed to the synthesis of findings within broader scholarly contexts.A.M Abdul Hamid facilitated access to essential resources, provided guidance on data collection procedures, and contributed to refining the research design. They actively participated in discussions, offered valuable feedback, and contributed to the revision and editing of the manuscript.A.A.M. Ismail provided administrative support throughout the research process, managed logistics, and coordinated communication among team members. They also contributed to proofreading and formatting the manuscript, ensuring its adherence to academic standards.All authors collectively approved the final version of the thesis report and are accountable for the integrity and accuracy of the research presented herein. The diverse contributions of each author were essential in producing this comprehensive work.---Feel free to adjust the details to accurately reflect the contributions of each author in your thesis report. References Z. Khan, S. Yusup, M. Ahmad, C. Vui Soon, Y. Uemura, and K. Sabil, “Review on hydrogen production technologies in Malaysia,” International Journal of Engineering and Technology, vol. 10, no. 2, p. 111, 2010. I. Dincer and C. 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Pascarelli, “Local vibrational dynamics of hematite (α-Fe2O 3) studied by extended x-ray absorption fine structure and molecular dynamics,” Journal of Chemical Physics, vol. 140, no. 22, Jun. 2014, doi: 10.1063/1.4882282 . K. Akiba, M. Ueda, K. Kawamura, and T. Maruyama, “Quantitative prediction of voids formation in a growing nickel oxide scale at 1373 K,” in Materials Transactions, Oct. 2007, pp. 2753–2761. doi: 10.2320/matertrans.MER2007122 . 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-4240283","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":289834292,"identity":"5bf8d828-c2af-4bc7-8213-c38f6ef0cf73","order_by":0,"name":"Muhammad Rafiq Haikal Rosdin","email":"","orcid":"","institution":"International Islamic University Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Muhammad","middleName":"Rafiq Haikal","lastName":"Rosdin","suffix":""},{"id":289834293,"identity":"9c0c4803-483f-4dda-8b03-dff2c3c2ec86","order_by":1,"name":"Syed Noh Syed Abu Bakar","email":"","orcid":"","institution":"International Islamic University Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Syed","middleName":"Noh Syed Abu","lastName":"Bakar","suffix":""},{"id":289834294,"identity":"f4a52414-68e9-4968-a6b4-f5647d158005","order_by":2,"name":"Abd Malek Abdul Hamid","email":"","orcid":"","institution":"International Islamic University Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Abd","middleName":"Malek Abdul","lastName":"Hamid","suffix":""},{"id":289834295,"identity":"197f2871-c09a-44e1-9bbe-b9f7c03c18a8","order_by":3,"name":"Ahmad Abdul Mun’im Ismail","email":"","orcid":"","institution":"International Islamic University Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Ahmad","middleName":"Abdul Mun’im","lastName":"Ismail","suffix":""},{"id":289834296,"identity":"06a1a1ac-1a16-424a-860b-82e135eb447a","order_by":4,"name":"Mohd Hanafi Ani","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxUlEQVRIiWNgGAWjYDACCRBhIyHHxgzmMhOrJU3CmGQtDIkNDMRq0Z3d/OzBjwSL9D52HtMNDBXWiQ1iZwzwajG7c8zcsCdBIreNmcfsBsOZ9MQG6RwCWm4kmEnw/oBqYWw7TIyW9G+SfxIk0tnAWv4RpSXHTJonQSIBoqWBOC1l0jIJEoZtzGxlNxKOpRu3SacVEHLYNsk3CXXy8v2Ht934UGMt2y+dvAGvFlSQAMRsDBz4HYYNsD8gWcsoGAWjYBQMawAAMes/jC1dLowAAAAASUVORK5CYII=","orcid":"","institution":"International Islamic University Malaysia","correspondingAuthor":true,"prefix":"","firstName":"Mohd","middleName":"Hanafi","lastName":"Ani","suffix":""}],"badges":[],"createdAt":"2024-04-09 07:16:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4240283/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4240283/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":54992888,"identity":"4d06e212-f6bf-4c85-863f-6d5b16bc5d74","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":51238,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram for Voltage Induced Experiment.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/21e2f660b2f051f20281be4d.png"},{"id":54992893,"identity":"518b55fc-033d-4dd4-ab09-8ea215e00c4b","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":68646,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of all samples exposed at 650℃ after 43.2, 259.2 and 432 ks of exposure with proposed voltage supplied. The figure also includes XRD pattern of raw sample.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/e9ee519eff5b8aded2bb1bff.png"},{"id":54992890,"identity":"ed4dea4c-7aae-40ab-8ca6-5643763c1ff7","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":343347,"visible":true,"origin":"","legend":"\u003cp\u003eCross sectional micrograph of T91 alloy through various timeframe and voltage. Red dashed line indicates the interphase boundary.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/e4ce47b7867a55f0b6570aec.png"},{"id":54993638,"identity":"39c4a2b1-b015-4f63-9c5c-74fc937ed673","added_by":"auto","created_at":"2024-04-19 17:37:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":379650,"visible":true,"origin":"","legend":"\u003cp\u003eFormation of voids and laths from one of the cross sections exposed to the proposed condition.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/5ee1aebcc8beb0a923592aed.png"},{"id":54992891,"identity":"193f9e2b-1010-4597-a7de-4f8b6e3ab021","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":41866,"visible":true,"origin":"","legend":"\u003cp\u003eOverall oxide thickness vs Time.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/df2bce5b2e2afae6e3c2a3a1.png"},{"id":54993637,"identity":"b199ed01-55ea-4b38-9fa2-bb5298e0e872","added_by":"auto","created_at":"2024-04-19 17:37:08","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":57796,"visible":true,"origin":"","legend":"\u003cp\u003eVolume fraction of void for overall phase of the sample.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/5b05dc29caa0c00e6f4e9586.png"},{"id":54994096,"identity":"45c290cc-4fef-4d20-8019-11d5cb1a07ef","added_by":"auto","created_at":"2024-04-19 17:45:08","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":40015,"visible":true,"origin":"","legend":"\u003cp\u003eOverall void percentage vs Time.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/b4919c911460a3b6b98ea77e.png"},{"id":54992895,"identity":"037f2abc-7326-4d33-b775-669de8cc01a7","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":35797,"visible":true,"origin":"","legend":"\u003cp\u003eOverall void percentage vs Voltage.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/58cd265439d85f922e11e938.png"},{"id":54994095,"identity":"7568b0b7-7f6d-4d5b-bae9-e415f8469ae8","added_by":"auto","created_at":"2024-04-19 17:45:08","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":57745,"visible":true,"origin":"","legend":"\u003cp\u003eVoid Percentage for all samples according to hematite layer.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/93b9184e73d6fff2ce804aed.png"},{"id":54992903,"identity":"91094c4e-2cfd-423f-9dbd-69165283b3e1","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":58967,"visible":true,"origin":"","legend":"\u003cp\u003eVoid Percentage for all samples according to magnetite layer.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/a816674bcd3684357dbfce9c.png"},{"id":54992898,"identity":"7bccd649-db20-4304-94f0-f05c358ca7d2","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":55015,"visible":true,"origin":"","legend":"\u003cp\u003eOverall Void Percentage Decrease vs Time for hematite.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/094dc3f2a1d2e5a52c2a5cc0.png"},{"id":54993641,"identity":"6f0ac6b7-d78f-4b07-b345-f8cd051be878","added_by":"auto","created_at":"2024-04-19 17:37:08","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":41493,"visible":true,"origin":"","legend":"\u003cp\u003eOverall Void Percentage Decrease vs Time for magnetite.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/808c8d0a798e486db1b89f56.png"},{"id":54992901,"identity":"4ae03e16-809e-4ff9-bb31-de4dfae1a585","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":45105,"visible":true,"origin":"","legend":"\u003cp\u003eVoid Percentage Decrease vs Voltage for hematite.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/7e192d074b6192194bf7fe5d.png"},{"id":54992900,"identity":"d2e0f0b8-d61c-4eaa-908a-d8e804ed30e0","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":43523,"visible":true,"origin":"","legend":"\u003cp\u003eVoid Percentage Decrease vs Voltage for magnetite.\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/864e3b801f2162726d4db5b6.png"},{"id":54993643,"identity":"9c31fe58-b343-4cd6-bf0a-29acfa851b8b","added_by":"auto","created_at":"2024-04-19 17:37:08","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":324671,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Illustration of magnetite crystal structure where golden and turquoise balls represent Fe and O respectively (b) Ferrimagnetic spin structure of magnetite [34].\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/44282e8e40eff752065be2e2.png"},{"id":54993642,"identity":"1603a6ed-8732-4a3c-bdce-f46bc7cc3d9b","added_by":"auto","created_at":"2024-04-19 17:37:08","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":277775,"visible":true,"origin":"","legend":"\u003cp\u003eCrystal structure of hematite where silver and red balls represent Fe and O respectively [35].\u003c/p\u003e","description":"","filename":"16.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/d141ffd6a1cc490ab4d0c22c.png"},{"id":54992904,"identity":"3d38c2c1-586e-4c27-a33c-90968316ac86","added_by":"auto","created_at":"2024-04-19 17:29:08","extension":"png","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":70218,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of volume fraction of voids on the outermost scale with other works.\u003c/p\u003e","description":"","filename":"17.png","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/eee16cac5ccce963e838a49b.png"},{"id":57307886,"identity":"9f4e9c85-8b3d-4b8e-a4c9-4398fb678569","added_by":"auto","created_at":"2024-05-29 02:21:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2317736,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4240283/v1/bf3a7780-c08c-49fb-9566-08719a2aa7c4.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Voltage-Induced Void Formation in High-Temperature Oxide Scales of Boiler Tubes","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eThe production of hydrogen through steam methane reforming (SMR) has emerged as the most economical method where 95% of global hydrogen production is produced [1] [2] [3] with the efficiency rate of 85% [2], [4], [5]. SMR synthesises hydrocarbons in natural gas to generate syngas by mixing pure methane with superheated steam in the presence of nickel catalyst. The steam was generated using heat recovery steam generator (HRSG) at a temperature of 923K. The most common boiler tube used in HRSG is a seamless ferritic alloy steel tube, ASME SA-213 grade T91, usually referred as T91 alloy boiler tube. As these tubes are subjected to extremely high temperature environment, oxidation of the alloy can occur in the presence of oxygen, resulting in fouling and spalling of oxide scales at the boiler tube. Furthermore, the majority of boiler tubes made must last at least 100,000 hours in service [6]. However, according to Li et. al [7], most of the tubes began to degrade within 4000 hours of service life. This discovery of early degradation than proposed time will majorly affect the boiler safety and operation.\u003c/p\u003e \u003cp\u003eTo protect the tube against corrosion, a \u0026lsquo;passive layer\u0026rsquo; needed to form on the surface to prevent oxygen ions to diffuse even further into the alloy [8], [9]. This passive layer was known as oxide layer where it is inevitable in terms of alloy protection [10]. However, a thick oxide scale formation is not favourable due to the increase of resistance in the scales which can alter the heat transfer coefficients [11]. One potential issue that can arise during oxide scale formation is the development of voids within the scale. The formation of voids can affect the oxidation mechanism and mechanical properties of the scale as well as the metal itself [12]. Various literature reviews discussing on the void formation which has been compiled by Yan et. al. [13]. Most of these literature reviews discussed the formation of voids in a qualitative manner. There are still a few studies that was done quantitative way which has been compiled by Kaderi et. al. [12].\u003c/p\u003e \u003cp\u003eAmong the quantitative research was done by Maruyama et. al. [14] wherein he claimed that void formation during high temperature oxidation was closely related to the divergence of ionic fluxes. Therefore, it is possible to investigate the oxidation of metal surfaces in the presence of an oxidising agent by combining Fick's first law with Wagner's oxidation kinetics where the calculations have been discussed by Noguchi and Yakuwa [15]. However, it known that neither of these laws include the influence of an electric field. Schl\u0026ouml;gl and Helffreich [16] has been theoretically discussed on the change of diffusivity under electrical potential by modifying Nernst-Planck equation. However, to writer\u0026rsquo;s knowledge, the effect of electric field on the high temperature corrosion is not well explored. Therefore, this study intends to explore the effect of voltage induced on the void formation\u0026rsquo;s point of view of T91 alloy at high temperature by inducing 0V, 50V and 300V at 923K in normal air.\u003c/p\u003e"},{"header":"2 Experimental Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Sample Preparation\u003c/h2\u003e \u003cp\u003eT91 alloy was cut into smaller pieces before it was being further cut into an average dimension of 10.92 mm \u0026times; 6.44 mm \u0026times; 4.24 mm using a precision saw machine. Then, it was grinded using grinding machine with various abrasive paper grades. There were 4 distinct types of abrasive sheets used: P600, P800, P1000, and P2000. The reduction in abrasiveness were shown by the increase in grade number. The sample is initially flattened and the extra oxide layer that had accumulated at the surface is removed using grade P600 abrasive paper, which was then followed by grades P800, P1000, and P2000. After that, a polishing machine was used to polish the ground sample. This machine had a revolving disc with diamond particles implanted on a fabric. The sample is once again polished to create a smooth surface using 0.3 micron-sized particles. Additionally, to minimize surface deformation and provide superior polishing results, alumina fine polishing solution was employed.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Voltage Induced Experiment\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the schematic diagram for the experimental setup. As for working and counter electrode, platinum wire of 1 metre in length is used due to its high resistivity towards corrosion, high electrical conductivity and chemical reactions. T91 alloy sample will be spot welded onto the working electrode. It is done to ensure that the electrodes remain firmly in place, even when subjected to the corrosive forces and high temperature of the environment. Spot welding also can ensure the voltage can be continuously supplied onto the sample. Then these wires are joined within the ceramic tube and have protruding tips. The glass tube will be inserted within the ceramic tube, with the tips sticking out toward the isothermal zone. To measure precise temperature in the zone, an R-Type thermocouple is placed in the isothermal zone, 5mm from the sample. Then, heat is supplied by the furnace.\u003c/p\u003e \u003cp\u003eThe platinum electrodes are then connected to Cockroft-Walton circuit or also known as voltage multiplier. The voltage multiplier was designed to generate high voltages from a low DC voltage. A voltmeter will be placed on the platinum wire to identify the real amount of voltage supplied. The amount of voltage supplied onto the sample will be 0V, 50V and 300V. The experiment was conducted at 1 atm (2.1\u0026times;10\u003csup\u003e4\u003c/sup\u003e Pa of oxygen) with standard dry air composition. For 20 minutes, the temperature was progressively raised to 923K in order to assure stability inside the heated space. The required voltage is then selected, and it is left alone for another 15 minutes to check that the provided voltage is stable. Table \u0026lrm;2.1 summarized the parameter used for the experiment.\u003c/p\u003e \u003cp\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\u003eParameters used for the experiment.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTime (ks)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eVoltage Supplied (V)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e43.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003cp\u003e50\u003c/p\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e259.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003cp\u003e50\u003c/p\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e432\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003cp\u003e50\u003c/p\u003e \u003cp\u003e300\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=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Characterization\u003c/h2\u003e \u003cp\u003eX-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were used to characterize the sample. For this experiment, the oxide phases formed were analyzed using XRD with divergence slit of 2.5 degree. The diffraction angle will be set from 20\u0026deg; to 80\u0026deg;. The XRD patterns were compared with \u003cem\u003eα-Fe\u003c/em\u003e (JCPDS 00-006-0696), \u003cem\u003eFeO\u003c/em\u003e (JCPDS 00-006-0615), \u003cem\u003eFe\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e (JCPDS 01-085-0599), \u003cem\u003eFe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e (JCPDS 00-019-0629), \u003cem\u003eFeCr\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e (JCPDS 00-034-0140) and \u003cem\u003eCr\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e (00-034-1479).\u003c/p\u003e \u003cp\u003eThe samples were mounted individually in a resin after prior to SEM analysis. Then, the sample was cross sectioned for 2 mm using diamond saw before being grinded and polished again using same method mentioned in Section 2.1. On each image, uniformly sized grids were traced, and the percentage area of scale voids was calculated. It was presumed that the scale's volumetric vacancy percentage matched its cross-sectional void percentage [12]. The void percentage was calculated using method proposed by Kaderi et. al. [12], Maruyama et. al [14], [17] and Ueda et. al. [18], [19].\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and Discussions","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 XRD Analysis\u003c/h2\u003e \u003cp\u003eFigure \u0026lrm;2 shows the XRD patterns of all sample surfaces at respective conditions. The XRD analysis of raw sample shows the intensity peaks at diffraction angle of 44.8\u0026deg; and 65\u0026deg; which confirms the presence of \u003cem\u003eα-Fe\u003c/em\u003e that is in BCC structure [20]. Furthermore, there are also noticeable peaks of \u003cem\u003eFe\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e (hematite), \u003cem\u003eFe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e (magnetite), \u003cem\u003eFeCr\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e (spinel) and \u003cem\u003eCr\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e (chromia), confirming that all sample has been oxidized.\u003c/p\u003e \u003cp\u003eMeanwhile, samples subjected to 300V for 43.2 ks and all samples exposed for 259.2 ks and 432 ks began to show a less intense peak of magnetite. This is due to their variations in crystal structure as well as the arrangement of atoms. Nasrazadani and Raman [21] stated that magnetite is a mixed valence iron oxide containing both Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e ions in an inverse spinel structure while hematite has a corundum structure with only Fe\u003csup\u003e3+\u003c/sup\u003e ions. The presence of Fe\u003csup\u003e2+\u003c/sup\u003e ions in magnetite causes more disorder in the crystal structure. This causes magnetite to have weaker diffraction peaks compared to hematite. Plus, the dense formation of hematite also can affect the XRD to detect magnetite phase [22].\u003c/p\u003e \u003cp\u003eThe peaks of chromia started to increase from the sample exposed at 50V for 43.2 ks until 300V for 432 ks. It verifies that as time goes by, chromia layer starts to develop. However, the peak was not as intense compared to magnetite and hematite. This is because some of the chromium atoms in the sample has successfully bonded with other chromium containing oxides like spinel. Hence, presence of multiple phases can lead to reduction in diffraction peaks. Other than that, the chromium content of T91 alloy was quite low at 9% which can also lead to reduced intensity. A study conducted by Karimi et. al. [23] can reassure the earlier claim.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 SEM Analysis\u003c/h2\u003e \u003cp\u003eThe morphology of the sample at the proposed timeframe was summarized in Figure \u0026lrm;3. A red dashed line was drawn to indicate the interphase boundary between alloy, inner scale, and outer scale. For ease of discussion, the inner scale will refer to as magnetite while the outer scale is hematite, and the term will be interchangeably used in the discussion. Based on the figure, all samples show the presence of oxide layers, indicating that oxidation process has been successfully occurred even when external voltage was supplied. It was also observed that the voids\u0026rsquo; shape was irregular, and some of them were aligned in a lath-like pattern at specific oxide thickness. A clear example of voids and lath-like patterns can be referred to in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe formation of voids in the inner and outer oxide scale is due to the ionic flux. A study conducted by Ueda et. al. [18]stated that the partial pressure of oxygen can affect the void formation. The higher the oxygen partial pressure, the higher the oxygen chemical potential and hence, a greater number of voids formed. Even though the oxygen partial pressure of this experiment is high (2.1 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e Pa), there are a presence of voids in the outer scale and only a few voids appear in the inner scale. In addition, Taniguchi [24] proposed that the oxide expanded under a compressive force if the volume of oxide formed to the metal exceeded unity. If the ratio, however, was less than one, the resulting oxide would expand under tensile stress. As a result, a porous oxide coating develops. The oxide layer was either created island-like or layer-by-layer when it was subjected to high temperatures [25].\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the parabolic plots of the overall oxide thickness in weight gain perspective of the sample. It shows a sharp increase at the beginning phase and then slowly gaining weight up to 432 ks. For the sample exposed to 0V, it appears that the weight gain does not follow the parabolic pattern especially on 259.2 ks of exposure. This was due to oxide spallation which causes it to become fragile during the final weight measurement. Overall, it indicates that almost all oxide scale formed obeys parabolic rate law. From the cross section of the sample, the thickness of oxide scales can be determined to calculate the parabolic rate constant, \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e. The \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e values for 0V, 50V and 300V were 3.83 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s, 2.17 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s and 9.25 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Volume Fraction of Voids in Oxide Scale\u003c/h2\u003e \u003cp\u003eBased on the calculation proposed by the authors in Section 2.3, the graphical representation of volume fraction of void for the sample\u0026rsquo;s overall phase are as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The image illustrates that the volume percentage of voids and the governing equations are in good agreement. It also shows that as the exposure time increases, the void formation decreases, and the overall formation of voids has decreased by 17%. One of the reasons for this is that due to the oxide layer is getting thicker over time. As time goes by, the thickness of oxide layer on the surface becomes thicker which will simultaneously hinder the formation of voids.\u003c/p\u003e \u003cp\u003eSimilarly, as the voltage supplied is increased, void formation on the oxide layers also decreased. Generally, when the sample was exposed to the high temperature environment, oxygen ions from the air will diffuse into the sample. The iron and chromium ions from the sample will also try to diffuse out to the surface. Due to the difference in flux between iron, chromium, and oxygen ions, it will cause some of them to rearrange themselves faster, causing the slower moving ions to have difficulty in moving and redistributing themselves. Thus, it will lead to accumulation of voids. However, as there is presence of external driving force, in this case, external supplied voltage, it has caused the flux to be increased as well. Hence, it will speed up the diffusion process, causing the formation of void to be reduced.\u003c/p\u003e \u003cp\u003eIn the case of T91 alloy, chromium reacts with oxygen, and it will form a layer of chromia which will act as a passive layer [26]. This layer will prevent further oxidation and corrosion. It is well known that chromia has a much slower diffusion rate than other oxide layers [27]. Therefore, some of the iron ions managed to react with oxygen to form an oxide layer. By inducing voltage onto the sample, the flux of chromia can be accelerated which allows it to reach the surface more quickly and participate in the formation of oxide layer. This will result in the enhanced passive layer where it will also enhance the resistance against void formation and corrosion. This can be seen clearly on Figure \u0026lrm;7 where the overall void percentage of T91 alloy decreases linearly.\u003c/p\u003e \u003cp\u003eA plot of overall void percentage vs voltage is shown in Figure \u0026lrm;8. When this plot has been done, it shows that the void percentage decreases exponentially as voltage increases. It is again confirmed that the oxidation process has become more active due to the assistance in external voltage that causes more metal ions to be released from the alloy and contributes to breakdown of oxide layers. Analyzing the same figure, it shows that an increase in voltage will result in the void percentage becoming stagnant and achieve a certain limit. This pattern was also similar on Figure \u0026lrm;9 and 10. This indicates that the void percentage can stay the same if a larger voltage is applied to the sample. However, further research needs to be done to confirm this hypothesis.\u003c/p\u003e \u003cp\u003eBased on the graph obtained in Figure \u0026lrm;6, 7 and 8, further analysis can be done on void percent of hematite and magnetite layer which can be referred on Figure \u0026lrm;9 to Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e14\u003c/span\u003e. A clear distinction can be observed based on this figure where there is higher volume of voids formed in hematite as compared to magnetite. This phenomenon can be explained by several factors. Among them is related to the crystal structure and properties of these two oxide layers.\u003c/p\u003e \u003cp\u003eMagnetite has a spinel structure which consists of both Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e ions. This structure allows for a greater number of oxygen vacancies and defects within oxide lattice [28]. Hence, magnetite tends to exhibit higher probability of void formation because it can facilitate the movement of iron ions. On the other hand, hematite has an alpha-corundum crystal structure and contains only Fe\u003csup\u003e3+\u003c/sup\u003e ions. Hematite is more compact and less prone to formation of defects and voids [29]. However, this is not the case when external voltage has been supplied. It appears that the volume fraction of voids for both layers started to decrease as higher amount of voltage was being supplied. Plus, the exponential pattern of volume fraction of void in magnetite phase appears to be steeper compared to hematite.\u003c/p\u003e \u003cp\u003eAt the exposed temperature, magnetite exhibits an interesting relationship between its magnetic and electrical properties. Referring to Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e15\u003c/span\u003e, magnetite is a ferrimagnetic material at which it possessed both magnetic and electrical properties [30]. Due to its mixed-valenced compound, it creates a charged imbalance in the crystal lattice, resulting in the presence of electrical conductivity. As mentioned by Radoń et al. [31], an increase of temperature causes the electrical conductivity of magnetite to significantly enhance. This is because high thermal energy allows for a greater charge carrier, facilitating the movement of all metallic ions in it. Along with external voltage supplied, the induced current will increase the speed of diffusion at magnetite, causing the void formation to decrease.\u003c/p\u003e \u003cp\u003eOn the contrary, hematite behaves as a paramagnetic at temperature above Neel temperature, (T\u003csub\u003eN\u003c/sub\u003e \u0026asymp; 955K) at which the magnetic moments become disordered, and no long-range magnetic ordering is observed [32]. Figure \u0026lrm;16 depicts the crystal structure of hematite. Therefore, hematite is considered as insulator or weakly conducting material as well as having a wide bandgap which restricts the flow of electrical current [30], [33]. Due to the mentioned hematite properties, an increase in voltage supplied shows a less significant steep as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e10\u003c/span\u003e. Moreover, when the voltage was set up to 300V, their volume fraction of void shows almost linear pattern (Figure \u0026lrm;10). Overall, the effect of inducing different amount voltage onto T91 alloy shows that as the voltage supplied increase, it has speed up the diffusion rate of magnetite, causing less formation of void and vice versa for hematite.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e17\u003c/span\u003e shows a summary of work done on volume fraction of voids on the outermost scale of the sample with respect to oxygen partial pressure, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({P}_{{O}_{2}}\\)\u003c/span\u003e\u003c/span\u003e. A study on void at formation at magnetite scale conducted by Ueda et al. [19] and Maruyama et al. [14] showed a very small range of void formed in the range of 0.05 to 0.06. Note that that these two studies were conducted in a controlled environment. In contrast, a study on void formation of NiO scale by Akiba et al. [36] and void on Fe-Cr-Ni alloy by Kaderi et al. [12] was done in normal air and comparable with this work. Note that the volume fraction of void on the outer scale has a much less magnitude when voltage is supplied.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eT91 alloy has been exposed under high temperature corrosion environment at 923K. The samples were subjected to impressed voltage of 0V, 50V and 300V at 43.2 ks, 259.2 ks and 432 ks. XRD analysis confirms that all oxide layers managed to form in all respective conditions. The cross section for all samples were clearly presented via SEM images. The plot of oxide thickness versus time shows that oxidation following the parabolic law, verifying that the reaction occurred was a solid state diffusion. Overall Kp value for samples exposed at 0V, 50V and 100V were 3.83 \u0026times; 10\u003csup\u003e\u0026ndash;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s, 2.17 \u0026times; 10\u003csup\u003e\u0026ndash;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s and 9.25 \u0026times; 10\u003csup\u003e\u0026ndash;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s respectively. SEM images also revealed that voids managed to form at all samples.\u003c/p\u003e \u003cp\u003eVoid percentage has been calculated on overall, hematite and magnetite layer. The result shows that overall void formation decreased by 17% and less amount of void observed on magnetite phases as compared to hematite. This is because magnetite\u0026rsquo;s spinel structure exhibits ferrimagnetism which enhances its magnetic and electrical properties compared to hematite\u0026rsquo;s alpha-corundum crystal structure. Other than that, void percentage results from this paper have been compared to previous work done by Akiba et al. [36], Maruyama et al. [14], Ueda et al. [19] and Kaderi et al. [12].\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eThis thesis report represents the collaborative efforts of five authors, each contributing unique skills and expertise to different facets of the research process.M.R.H. Rosdin led the conceptualization and design of the research, outlining the objectives and methodologies. They coordinated data collection and analysis, contributed significantly to the interpretation of findings, and played a key role in drafting and revising the manuscript.M.H. Ani provided expertise in High Temperature Corrosion at boiler tubes, which influenced the theoretical framework development and guided the analysis of results. They contributed to the literature review, critically reviewed the manuscript, and contributed to shaping the discussion section with insightful interpretations.S.N. Syed Abu Bakar contributed to the research design and methodology, ensuring methodological rigor and validity. Their expertise in [mention specific area] brought depth to the interpretation of results and contributed to the synthesis of findings within broader scholarly contexts.A.M Abdul Hamid facilitated access to essential resources, provided guidance on data collection procedures, and contributed to refining the research design. They actively participated in discussions, offered valuable feedback, and contributed to the revision and editing of the manuscript.A.A.M. Ismail provided administrative support throughout the research process, managed logistics, and coordinated communication among team members. They also contributed to proofreading and formatting the manuscript, ensuring its adherence to academic standards.All authors collectively approved the final version of the thesis report and are accountable for the integrity and accuracy of the research presented herein. The diverse contributions of each author were essential in producing this comprehensive work.---Feel free to adjust the details to accurately reflect the contributions of each author in your thesis report.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eZ. Khan, S. Yusup, M. Ahmad, C. Vui Soon, Y. Uemura, and K. Sabil, \u0026ldquo;Review on hydrogen production technologies in Malaysia,\u0026rdquo; International Journal of Engineering and Technology, vol. 10, no. 2, p. 111, 2010.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eI. Dincer and C. 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Maruyama, \u0026ldquo;Quantitative prediction of voids formation in a growing nickel oxide scale at 1373 K,\u0026rdquo; in Materials Transactions, Oct. 2007, pp. 2753\u0026ndash;2761. doi: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2320/matertrans.MER2007122\u003c/span\u003e\u003cspan address=\"10.2320/matertrans.MER2007122\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\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":"high temperature oxidation, T91 Alloy, Void Formation, Induced Voltage","lastPublishedDoi":"10.21203/rs.3.rs-4240283/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4240283/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe formation of voids in the oxide scale during high temperature oxidation is one of critical issues that leads to poor oxide scale adherence, fouling, spalling and materials loss, which affect substrate\u0026rsquo;s performance. Visual inspection is currently the sole way to detect corrosion process at high temperature environment. At room temperature, under the standard condition, cathodic protection has been successfully employed to monitor and control the corrosion. Understanding the development of voids in oxide scale is crucial to develop mitigation strategies and predictive maintenance. Thus, this study was intended to serve as a preliminary step to emulate the cathodic protection technique at high temperature. By polarizing the sample, it is postulated that it may affects the diffusivity of cation/anion in oxide scale, which is the rate determining step of the oxidation process. Ueda et al and Maruyama et al has shown that the difference in flux, or chemical potential of the oxygen species is the sole factor for the formation of void in oxide scale in controlled environment. In this study, the amount of voids present was measure directly on T91 alloys exposed at 823 K under various induced voltageT91 alloy which consists of Fe-9%Cr was externally induced with voltages of 0V, 50V and 300V for 43.2 ks, 259.2 ks and 432 ks at 923 K in air (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({P}_{{O}_{2}}\\)\u003c/span\u003e\u003c/span\u003e)\u0026thinsp;=\u0026thinsp;0.21 atm\u0026thinsp;=\u0026thinsp;2.1 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e Pa). The presence of oxide layers was analysed using X-Ray Diffraction (XRD) and the void formed was inspected using Scanning Electron Microscopy (SEM). XRD results reveal that peaks of \u003cem\u003eFe\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eFe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eFeCr\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eCr\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e were formed on all sample. The parabolic rate constant, \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e was calculated as 3.83 \u0026times; 10\u003csup\u003e\u0026ndash;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s, 2.17 \u0026times; 10\u003csup\u003e\u0026ndash;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s and 9.25 \u0026times; 10\u003csup\u003e\u0026ndash;14\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e/s respectively, verifying that the reaction occurred was a solid state diffusion. Changes in \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e at different induced voltages is clear evidence that the diffusivity was altered by external electrical potential. It was observed that the overall void formation decreased by 17%. Apparently, inducing voltage onto T91 alloy effects the ionic diffusivity and changes the void formation. Conversely, it may be used to promote diffusivity of more inert species such as \u003cem\u003eCr\u003c/em\u003e to form protective layer at early stage of oxidation.\u003c/p\u003e","manuscriptTitle":"Voltage-Induced Void Formation in High-Temperature Oxide Scales of Boiler Tubes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-19 17:29:03","doi":"10.21203/rs.3.rs-4240283/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":"783e0ba9-d8e7-44eb-b35a-fed79199647c","owner":[],"postedDate":"April 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-05-29T02:12:53+00:00","versionOfRecord":[],"versionCreatedAt":"2024-04-19 17:29:03","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4240283","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4240283","identity":"rs-4240283","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}