Recent Applications of Scanning Microscopy in Surface Engineering 2020View this Special Issue
Research Article | Open Access
Yanbing Tang, Xinwang Shen, Zhihong Liu, Ying Li, "Effects of the Different Solid Deposits on the Corrosion Behavior of Pure Fe in Water Vapor at 500°C", Scanning, vol. 2020, Article ID 6280725, 8 pages, 2020. https://doi.org/10.1155/2020/6280725
Effects of the Different Solid Deposits on the Corrosion Behavior of Pure Fe in Water Vapor at 500°C
A comprehensive corrosion investigation of pure Fe in an environment of solid sodium salt deposit (i.e., NaCl or Na2SO4) with mixtures of H2O and O2 at 500°C was conducted by mass gain measurement, X-ray diffraction (XRD), scanning electron microscope (SEM), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that corrosion rates were accelerated with solid NaCl or Na2SO4 deposit due to their reaction with the formed protective scale of Fe2O3 and subsequently resulted in its breakdown. The corrosion rate of pure Fe with solid NaCl is higher than that with solid Na2SO4 because of the lower activation energy () for chemical reaction of Fe in solid NaCl+H2O+O2 (i.e., 140.5 kJ/mol) than that in solid Na2SO4+H2O+O2 (i.e., 200.9 kJ/mol). Notably, the electrochemical corrosion rate of pure Fe with solid NaCl deposit, , was a little lower than that with solid Na2SO4 deposit.
Corrosion of metal materials is severe in the environment with solid salt deposit and dry or wet O2 at medium and high temperatures [1–18], especially for turbine blades in planes or ships and power boilers. Due to their excellent mechanical properties, low cost, and ease of machining, pure Fe and its alloys are the popular materials that were investigated in solid alkali chloride deposit in dry or wet O2 [1–7]. Most researchers [2, 5, 6] thought that the solid NaCl could react with Fe2O3 to generate Cl2 via the reaction, . Then, Cl2 could react with Fe to form FeCl(s), i.e., . As such, FeCl2(s) would continuously evaporate under low vapor pressure at high temperature. The FeCl2 vapor would diffuse outward through cracks and pores of the scale. Finally, the FeCl2 vapor would react with O2 to form Fe3O4 and/or Fe2O3 when they met during the process. Apparently, the solid NaCl could react with the oxidation scale that formed in dry or wet air at medium and high temperatures and lead to the breakdown of the protective scale to accelerate the corrosion rate of materials. Folkenson et al.  proposed a new mechanism as follows. Solid KCl reacts with O2 and H2O to generate chloride ions (i.e., ) and subsequently react with iron ions to form FeCl2(s). Cao et al.  postulated a hypothesis of “dynamic water film” in which H2O molecules were continuously being absorbed on and evaporated from the surface of the material. The electrochemical corrosion might occur in the dynamic water film, which accelerates the corrosion of metal . Shu et al.  used impedance spectroscopy to investigate the corrosion mechanisms of pure Fe and pure Cr with solid NaCl deposit in water vapor at 600°C. According to the analysis of the resistance and capacitance of corrosion scale, the electrochemical corrosion was proved to occur in the corrosion environments. After that, Tang et al.  investigated the interaction between chemical reactions and electrochemical reactions of pure Fe in this corrosion environment. The chemical reactions and electrochemical reactions follow “ce mechanism,” in which Fe and Fe2O3 first react chemically with NaCl, water vapor, and oxygen to generate HCl (g). Then, the HCl (g) reacts with pure Fe electrochemically via a one-electron electrochemical reduction to form H2.
Compared to the alkali chlorides, the corrosion mechanisms of metal/alloys with solid sulfate are lacking. However, the corrosion mechanisms are focused on molten sulfate. Recently, many studies have been carried out on the corrosion behavior of metals/alloys in a molten Na2SO4 environment [19–25] and many corrosion mechanisms have been proposed. One of the well-known mechanisms is the sulfidation model [19, 20], in which the formation of sulfides accelerates the corrosion. The other one is the acidic-basic fluxing [19, 21–24] mechanism, in which dissolution of the protective oxide scales, due to formation of basic Na2O, was considered the reason for the accelerated corrosion. Moreover, based on the electrochemical mechanism , corrosion was considered an electrochemical reaction in which the transfer of electrons accelerated the corrosion. Tang et al.  investigated the corrosion behavior of pure Fe under solid Na2SO4 deposit in wet oxygen flow at 500°C. The results showed that the corrosion of Fe includes chemical corrosion and electrochemical corrosion. The chemical reaction and electrochemical reaction follows the “ce mechanism.” Fe and Fe2O3 first react chemically with Na2SO4, water vapor, and oxygen to generate H2SO4 (g). And then, the H2SO4 (g) reacts with pure Fe electrochemically via a one-electron electrochemical reduction to form H2. The coeffect between deposited solid Na2SO4 and H2O+O2 certainly exists and significantly accelerates the corrosion of pure Fe.
NaCl and Na2SO4 are normal corrosive mediums. The corrosion of materials in solid salt environment depends on the anion of the salt [18, 27]. However, the effects of the different solid deposits (NaCl and Na2SO4) on the corrosion behavior of pure Fe in water vapor are still unclear. In this paper, the corrosion differences of pure Fe with solid NaCl and solid Na2SO4 deposit in water vapor were comparatively studied to cognize the corrosion behaviors of materials in the corrosion profoundly.
The pure Fe (99.9%) was used as experimental specimen. The metallography of the specimen is shown in Figure 1. The microstructure of pure Fe is ferrite. The maximum grain size is about 100 μm. Before the experiment, the sample was ground using silicon-carbide abrasive papers down to 1000 grit, degreased in acetone then ethanol, and dried in air before use. The NaCl and Na2SO4 are of analytical purity (≥99.5%). The solid salt was deposited on the preheated Fe sample surface by repeatedly brushing and drying a salt-saturated solution. The mass of salt was about 4 mg/cm2. The temperature of the furnace was controlled at 500°C. H2O came from an 80°C water bath. Pure O2 was passed through the glass bubbler with a flux of 200 ml/min.
The corrosion test was carried out in a thermal balance . To prevent the H2O from condensing in the upper part of the thermal balance, a counterflow of N2 was passed through the apparatus at 150 ml/min. After the furnace was heated to the desired temperature and the gas flow was stabilized, the specimen was quickly suspended into the furnace tube, and the test was started. All the measurements were carried out at ambient pressure. After the tests, the specimens were further examined by XRD and SEM.
A special three-electrode system was built for the electrochemical measurements in this particular environment . To decrease the resistance of the solution and obtain a uniform electric field, the reference electrodes consisted of four platinum wires, each with a diameter 0.4 mm, and the counter electrode was a circular strip of platinum foil about 2 mm wide. All potential values in this paper were reported versus this platinum reference electrode. The Fe working electrode was a rod 10 mm long and 5 mm diameter. The three electrodes were placed in quartz tubes, which acted as insulators. All the gaps were sealed by high-temperature inorganic glue. The three-electrode system after solid NaCl and solid Na2SO4 deposition was directly put into the furnace at the desired temperature and with water vapor for electrochemical measurements.
A PAR2273 Electrochemical Measurement System manufactured by EG&G was used for all electrochemical measurements, which also has the function to compensate the resistance between reference electrode and working electrode. In the galvanic corrosion measurement, the ratio of anodic area to cathodic area is 1 : 2. In the potentiodynamic polarization measurements, the measurements were carried out after 1000 s in the corrosion environment for obtaining an electrochemical stability and the scan rate was 1 mV/s. The resistance between reference and working electrodes was compensated during measurements according to the design of electrochemical system and testing work station. All measurements were repeated more than three times.
3. Results and Discussion
Figure 2 shows the mass gain of pure Fe as a function of time at 500°C with and without solid NaCl or Na2SO4  in O2 containing water vapor. As is seen from Figure 2, the corrosion of pure Fe is accelerated with solid NaCl or Na2SO4 deposit. Compared to the case with solid Na2SO4 deposit, the corrosion rate of pure Fe with solid NaCl deposit is slightly higher at all-time duration.
In our previous studies [1, 13], it is found that the corrosion of pure Fe in both corrosion environments includes a chemical corrosion process and an electrochemical corrosion process, while the overall corrosion is dominated by the chemical corrosion process with a percentage of over 90%. Herein, we investigated the differences of chemical corrosion that is influenced by NaCl and Na2SO4.
To affect the chemical corrosion rate, there are two aspects: (a) the protection of scale on the surface of pure Fe. The compact and integrated scale can restrain the corrosion of substrate. (b) The activity of corrosion reactants. As it is known, the corrosion rate would increase with a decreasing active energy. The details of the effects are discussed as follows.
The scale includes solid salt deposition scale (NaCl or Na2SO4) and corrosion scale on the surface of pure Fe. Figures 3(a) and 3(b) show the surface morphologies of solid NaCl and Na2SO4, respectively, before corrosion test. The results showed that both salt scales are loose and porous. However, the solid NaCl film was much looser and more porous than solid Na2SO4 film, which led to an easy transport of H2O and O2 to the interface of pure Fe and solid NaCl film, promoting the chemical corrosion process of pure Fe.
The corrosion of materials with solid salt deposit in water vapor is different with that in aqueous solution. The corrosion scale would stay on the surface of substrate, which should restrain the corrosion of substrate. Figure 4 shows the cross-sectional morphologies of pure Fe after 10 h corrosion at 500°C in (Figure 4(a)) and (Figure 4(b)) . It indicated that the corrosion scale formed on the surface of pure Fe was loose and porous in both corrosion environments. A number of volatile species are formed in the corrosion process, which could contribute to the formation of the loose and porous corrosion scale . As a matter of fact, some green deposits were observed on the tube inner surface of the furnace after many hours of experiments, confirming the formation of volatile species. However, the scale formed in solid is looser and higher porosity than that formed in solid . This indicated that the reactants (H2O and O2) could be easier to transport through the corrosion scale formed in the environment with NaCl. Eventually, it promotes the chemical corrosion process of pure Fe.
The components of the corrosion scale formed on the surface of pure Fe in solid or  after 10 h corrosion are shown in Figure 5. The components of the scales in the two corrosion environments are remarkably different. The component of the scale on the surface of pure Fe formed in solid is hematite that mainly consists of Fe2O3, while the component of the scale on the surface of pure Fe formed in solid mainly consists of Fe3O4 with a little of Fe2O3. According to the published research , the generation of Fe2O3 or Fe3O4 is closely relative to oxygen pressure. Fe2O3 would be generated at a relatively high oxygen pressure, while Fe3O4 would be generated at a relatively low oxygen pressure. From Figures 3 and 4, the NaCl scale is looser with higher porosity than Na2SO4 scale; meanwhile, the corrosion scale formed in the case of was also looser with higher porosity than that formed in the case of . The oxygen could transport inward through the corrosion scale and solid NaCl scale easily. The oxygen pressure in the corrosion scale that formed in is higher than that formed in . This is the reason why the components of the corrosion scales in the two corrosion environments were different.
The corrosion mechanism of pure Fe in the two corrosion environments could be understood on the basis of the components, morphologies of the corrosion scales, and published research.
For the case of solid NaCl, firstly, NaCl reacts with Fe2O3 and H2O to generate Na2Fe2O4 and HCl .
The Cl2 could react with Fe to form FeCl2 .
The vapor pressure for FeCl2(s) is at 500°C. A continuous evaporation will take place .
For the case of solid Na2SO4, firstly, Na2SO4 reacts with Fe2O3 and H2O to generate Na2Fe2O4 and H2SO4 .
The generated H2SO4 could react with Fe to form FeSO4.
According to the XRD results, the FeSO4 could react with O2 and H2O to form Fe3O4 and Fe2O3.
The generation of H2SO4 and H2 led to the formation of many holes and cracks in the scale (see Figure 4(b)).
According to the morphologies shown in Figures 3 and 4, the more porous NaCl scale and corrosion scale formed in the promoted the chemical corrosion process. However, this did not fully explain why the corrosion rate of pure Fe in was higher than that in . The activation energy is a key parameter to estimate the chemical reaction rate. The lower the activation energy, the more atoms, ions, or molecules of substances are activated to transition state. Therefore, the rate of chemical reaction increases with decreases in chemical reaction activation energy.
Both corrosion mechanisms of pure Fe in solid and solid corrosion environments follow the ce mechanism [1, 26]. For the ce mechanism, the relationship between phase angle and frequency could be given as Equation (9) . where is used for representing for phase angle, and for chemical reaction rate constants, for angular frequency, for diffusion coefficient, for apparent heterogeneous rate constant, for activity coefficient, for charge transfer coefficient, for number of electrons transferred, for applied d. c. potential, for reversible half-wave potential, and , , and for their conventional electrochemical meanings. The value of can be calculated using Equation (9). Figure 6 shows the relationship between frequency and phase angles of pure Fe in solid at 500°C. The plots have a maximum. It suggests that the corrosion mechanism of pure Fe in the two corrosion environments involves the interaction of the chemical and the electrochemical reactions, which is similar with pure Fe in solid . The calculated values of for pure Fe in solid and solid corrosion environments are 0.230 sec-1 and 0.031 sec-1, respectively. Therefore, the chemical corrosion rate of pure Fe in is higher than that in solid because of its higher chemical reaction rate constant in the case with solid NaCl.
Chemical reaction rate is closely related with the activation energy. The lower the activation energy is, the higher the chemical reaction rate is. According to the logarithmic Arrhenius equation, the rate constant () dependence of temperature () is given by the relationship () : where is used for representing for the rate constant, for a temperature-independent constant (often called the frequency factor), for the absolute temperature, for the universal gas constant, and for the activation energy. According to Equation (15), a plot of vs. gives a straight line with slope of -. The values of for Fe in solid corrosion environments can be obtained from the slope of Figures 7. The value is 200.9 kJ/mol. The value of for Fe in solid corrosion environments is 140.5 kJ/mol . The lower activation energy of Fe in solid accounts for its higher chemical reaction and the higher overall corrosion rate.
The electrochemical corrosion of pure Fe in solid corrosion environments has been shown in our earlier studies [1, 26]. The potentiodynamic polarization plot of pure Fe in solid at 500°C is shown in Figure 8. The anodic current densities of pure Fe in both two corrosion environments increase linearly with anodic potential increasing in the active polarization zone, which can be attributed to active dissolution in the aqueous environment, because the loose and porous corrosion scale could not inhibit the electrochemical corrosion. The cathodic reaction rate of pure Fe in solid is higher than that in . The electrochemical corrosion rates () were calculated by fitting the potentiodynamic polarization curves in the active polarization zones. The electrochemical corrosion current density () obtained was and for pure Fe in solid and solid , respectively. The amount of Fe corroded by electrochemical reaction was obtained using Faraday’s rule. After the calculation, the chemical reaction rates of pure Fe in the two corrosion environments within 1 h are 0.036 g/h/cm2 and 0.041 g/h/cm2, respectively. It must illustrate that the calculation time herein is in one hour, because the potentiodynamic polarization measurements were carried out within one hour, and there is no significantly variety of the electrochemical corrosion rate in one hour. This was proved by presented authors used an electrochemical instrument named CMB 1510B (based on weak polarization theory) manufactured by State Key Laboratory for Corrosion and Protection, to measure the electrochemical corrosion rate every 4 minutes during the whole corrosion reaction. The electrochemical corrosion rate of pure Fe in solid is slightly lower than that in solid .
As is well-known, charge transfer is the fundamental characteristic of the electrochemical reaction [30, 31]. The corrosion scale and solid salt scale are the key influence factors for electrochemical reaction rate. The corrosion scale of pure Fe formed in solid is looser and more porous than those formed in solid (see Figure 4), and the solid NaCl scale is also looser and more porous than solid Na2SO4 scale (see Figure 3). The HCl could volatilize and diffuse through the looser and more porous corrosion scale and NaCl scale easily. Thus, the cathodic reaction rate of pure Fe in solid is higher than that in solid . As a consequence, the electrochemical corrosion rate of pure Fe in solid is higher than that in solid .
As a consequence, the electrochemical corrosion rate of pure Fe in solid is higher than that of it in solid . In addition, the components of the corrosion scales formed on the surface of pure Fe in solid and solid after 10 h corrosion are shown in Figure 5. The component of the scales on the surface of pure Fe formed in solid is hematite that mainly consists of Fe2O3, while the component of the scales on the surface of pure Fe formed in solid is Magnetite that mainly consists of Fe3O4. The protection of Fe2O3 is well than that of Fe3O4 , which also inhibit the electrochemical corrosion rate of pure Fe in solid corrosion environment.
The corrosion rate of the pure Fe is significantly accelerated under a NaCl or Na2SO4 deposit in an atmosphere of at 500°C. Both the salts of NaCl and Na2SO4 could react with Fe2O3 to result in a breakdown of the protective scale and subsequently accelerate the corrosion rate of pure Fe.
Compared to the case in solid , the corrosion rate of pure Fe is much higher in solid . The activation energy () for chemical reaction of pure Fe in solid is 200.9 kJ/mol, which is higher than that of pure Fe in solid .
The percentage contribution of the electrochemical reactions in total corrosion is insignificant. It was also found that the electrochemical corrosion rate of pure Fe with solid NaCl deposit was , which was a little lower than that with solid Na2SO4 deposit.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was supported by the National Natural Science Fund of China under the contract No. 51371181 and No. 51671198.
- Y. B. Tang, L. Liu, Y. Li, and F. H. Wang, “Evidence for the occurrence of electrochemical reactions and their interaction with chemical reactions during the corrosion of pure Fe with solid NaCl deposit in water vapor at 600 °C,” Electrochemistry Communications, vol. 12, no. 2, pp. 191–193, 2010.
- Y. H. Shu, F. H. Wang, and W. T. Wu, “Synergistic Effect of NaCl and Water Vapor on the Corrosion of 1Cr-11Ni-2W-2Mo-V Steel at 500-700°C,” Oxidation of Metals, vol. 51, no. 1/2, pp. 97–110, 1999.
- M. Cao, L. Liu, Z. F. Yu, L. Fan, L. Ying, and F. H. Wang, “Studies on the corrosion behavior of Fe-20Cr alloy in NaCl solution spray at 600 °C,” Corrosion Science, vol. 133, pp. 165–177, 2018.
- F. H. Wang and Y. H. Shu, “Influence of Cr content on the corrosion of Fe-Cr alloys: the synergistic effect of NaCl and water vapor,” Oxidation of Metals, vol. 59, no. 3/4, pp. 201–214, 2003.
- L. Liu, Y. Li, C. L. Zeng, and F. H. Wang, “Electrochemical impedance spectroscopy (EIS) studies of the corrosion of pure Fe and Cr at 600 °C under solid NaCl deposit in H2O,” Electrochimca Acta, vol. 51, no. 22, pp. 4736–4743, 2006.
- H. J. Grabke, E. Reese, and M. Spiegel, “The effects of chlorides, hydrogen chloride, and sulfur dioxide in the oxidation of steels below deposits,” Corrosion Science., vol. 37, no. 7, pp. 1023–1043, 1995.
- N. Folkeson, T. Jonsson, M. Halvarsson, L. G. Johansson, and J. E. Svensson, “The influence of small amounts of KCl(s) on the high temperature corrosion of a Fe-2.25Cr-1Mo steel at 400 and 500°C,” Materials and Corrosion, vol. 62, no. 7, pp. 606–615, 2011.
- Y. H. Shu, F. H. Wang, and W. T. Wu, “Corrosion behavior of Ti60 alloy coated with a solid NaCl deposit in O2 plus water vapor at 500–700 °C,” Oxidation of Metals, vol. 52, no. 5/6, pp. 463–473, 1999.
- L. Fan, L. Liu, Z. F. Yu, M. Cao, Y. Li, and F. H. Wang, “Corrosion behavior of Ti60 alloy under a solid NaCl deposit in wet oxygen flow at 600 °C,” Scientific Reports, vol. 6, no. 1, pp. 1–12, 2016.
- Y. H. Shu, F. H. Wang, and W. T. Wu, “Corrosion behavior of pure Cr with a solid NaCl deposit in O2 plus water vapor,” Oxidation of Metals, vol. 54, no. 5/6, pp. 457–471, 2000.
- F. H. Wang, S. J. Geng, and S. L. Zhu, “Corrosion behavior of a sputtered K38G nanocrystalline coating with a solid NaCl deposit in wet oxygen at 600 to 700 oC,” Oxidation of Metals, vol. 58, no. 1/2, pp. 185–195, 2002.
- C. Wang, F. Jiang, and F. H. Wang, “Corrosion inhibition of 304 stainless steel by nano-sized Ti/silicone coatings in an environment containing NaCl and water vapor at 400–600°C,” Oxidation of Metals, vol. 62, no. 1/2, pp. 1–13, 2004.
- Y. B. Tang, L. Liu, Y. Li, and F. H. Wang, “The electrochemical corrosion mechanisms of pure Cr with NaCl deposit in water vapor at 600 oC,” Journal of The Electrochemical Society, vol. 158, no. 8, pp. C237–C241, 2011.
- L. Fan, L. Liu, M. Cao et al., “Corrosion behavior of pure Ti under a solid NaCl deposit in a wet oxygen flow at 600 °C,” Metals, vol. 6, no. 4, pp. 72–82, 2016.
- L. Fan, L. Liu, Y. Cui et al., “Effect of streaming water vapor on the corrosion behavior of Ti60 alloy under a solid NaCl deposit in water vapor at 600 degrees C,” Corrosion Scencei, vol. 160, article 108177, pp. 165–178, 2019.
- E. Reese and H. J. Grabke, “Einfluß von Natriumchlorid auf die oxidation von hochlegierten Chrom- und Chrom-Nickel-Stählen,” Materials and Corrosion, vol. 44, no. 2, pp. 41–47, 1993.
- P. Kofstad, High Temperature Corrosion, Elsevier Applied Science, London, British, 1998.
- Y. Shinata and Y. Nishi, “NaCl-induced accelerated oxidation of chromium,” Oxidation of Metals, vol. 26, no. 3-4, pp. 201–212, 1986.
- R. A. Rapp, “Whitney Award Lecture—1986:Chemistry and electrochemistry of the hot corrosion of metals,” Corrosion, vol. 42, no. 10, pp. 568–577, 1986.
- E. L. Simons, G. V. Browning, and H. A. Liebhafsky, “Sodium sulfate in gas turbines,” Corrosion, vol. 11, no. 12, pp. 17–26, 1955.
- N. S. Bornstein and M. A. Decrescente, “The role of sodium and sulfur in the accelerated oxidation phenomena-sulfidation,” Corrosion, vol. 26, no. 7, pp. 309–314, 1970.
- N. S. Bornstein and M. A. Decrescente, “The role of sodium in the accelerated oxidation phenomenon termed sulfidation,” Metallurgical and Materials Transactions, vol. 2, no. 10, pp. 2875–2883, 1971.
- J. A. Goebel and F. S. Pettit, “The influence of sulfides on the oxidation behavior of nickel-base alloys,” Metallurgical and Materials Transactions, vol. 1, no. 12, pp. 3421–3429, 1970.
- J. A. Goebel and F. S. Pettit, “Na2SO4-induced accelerated oxidation (hot corrosion) of nickel,” Metallurgical and Materials Transactions, vol. 1, no. 7, pp. 1943–1954, 1970.
- M. Li, High Temperature Corrosion of Metals, Metallurgical Industry Press, Beijing, China, 2001.
- Y. B. Tang, L. Liu, L. Fan, Y. Li, and F. H. Wang, “The corrosion behavior of pure iron under solid Na2SO4 deposit in wet oxygen flow at 500 °C,” Materials, vol. 7, no. 9, pp. 6144–6157, 2014.
- A. Zahs, M. Spiegel, and H. J. Grabke, “Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing at mospheres at 400-700oC,” Corrosion Science, vol. 42, no. 6, pp. 1093–1122, 2000.
- D. E. Smith, “Alternating current polarography of electrode processes wih coupled homogeneous chemical Reactions. I. Theory for systems with first-order preceding, following, and catalytic chemical reactions,” Analytical Chemistry., vol. 35, no. 6, pp. 602–609, 1963.
- L. Z. Chen and Y. H. Zhang, Physical Chemistry, Shanghai Science and Technology, Shanghai, 2005.
- Y. X. Qiao, D. K. Xu, S. Wang et al., “Effect of hydrogen charging on microstructural evolution and corrosion behavior of Ti-4Al-2V-1Mo-1Fe alloy,” Journal of Materials Science & Technology, vol. 60, pp. 168–176, 2021.
- Y. X. Qiao, Z. H. Tian, X. Cai et al., “Cavitation erosion behaviors of a nickel-free high-nitrogen stainless steel,” Tribology Letters, vol. 67, no. 1, pp. 1–9, 2019.
- C. Gleitzer, “Electrical properties of anhydrous iron Oxides,” Key Engineering Materials, vol. 125-126, pp. 355–418, 1996.
Copyright © 2020 Yanbing Tang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.