Characterization of the passive properties of 254SMO stainless steel in simulated desulfurized flue gas condensates by electrochemical analysis, XPS and ToF-SIMS
Introduction
Materials used in chimney lining in the thermal power plants experience severe corrosion attacks because of the highly aggressive gases after flue gas desulfurization (FGD), which contains a certain amount of acidic constituents (mainly SO2) with low temperature and high humidity [[1], [2], [3]]. Under the service environment, the flue gas is easy to condense to generate concentrated acidic condensates, in which sulfuric acid is the dominant agent with other contaminants including chlorides, fluorides, and nitrates.
In recent years, the high alloyed steels (super austenitic stainless steels, super duplex stainless steels, and nickel-based alloys) have been recommended to be used in FGD systems to overcome the severe corrosion problems. Zeng et al. [4] reviewed the material selection criterion for the building of flue gas component and manifested that austenitic stainless steels (SS317, 254SMO, and 654SMO) and duplex stainless steels (2205 and 2507) were major acceptable candidates for the flue gas systems in the fossil fuel-powered industries. Shoemaker et al. [5,6] compared the corrosion resistance of these alloys in FGD system by laboratory and field tests and reported that super austenitic steels (Alloy 25-6Mo, Alloy 27-7Mo) and nickel-based alloys (C276) were resistant to the conditions that destroyed duplex steels (2205 and 2507). Similar results were demonstrated by Pettersson et al. [7] who also recommended the super austenitic stainless steels. The chimney lining environment after FGD is different from the FGD system such as the condensate composition and temperature. There is little experience with the use of super austenitic stainless steels as lining materials in chimney at the time. The environmental factors that control the corrosion process of the steels under this particular condition should be illuminated.
Condensate temperature inside the chimney, which always varies between 40 and 80 °C, is one of the most important parameters that affect the passivation behavior of high alloyed steels in terms of the film composition, structure, thickness, electronic properties, and protectiveness [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. Ahn et al. [8] and Jung et al. [9] reported that increase of temperature increased the Fe2+ content and the Fe/Cr ratio in the passive film in alkaline and neutral environment, which was an alternative explanation of the increase in donor density. In acidic electrolytes, the preferential dissolution of Fe is accelerated by temperature, resulting in a lower Fe/Cr ratio at high temperature [21]. The passive film stability formed at different temperatures is also problematic. Huang et al. [10] and Cerdán et al. [11] found that passive film became unstable with increasing temperature because of the formation of a highly disordered amorphous film, while Pallotta et al. [12] suggested that increase in temperature produced a more crystalline passive film which became more resistant to breakdown. The film thickness variation with temperature is more complex since both film growth and dissolution kinetics are altered. Therefore, the thickening [9,13,14], fluctuation [21], and unchanged film thickness [15] are observed with elevated temperature. Moreover, the relationship between the film thickness and corrosion resistance cannot be conformably established [14,17,22]. Due to the above-complicated influences, the critical temperature relating to the passivation properties of the corrosion-resistant alloys is recognized by some authors. Gray et al. [16] reported the critical temperature of 50 °C for C22 alloy in acidic solution, above which the passive currents increased linearly with increasing temperature. Shoesmith et al. [14] also confirmed the same temperature but the passive current increased by only a factor of 2–3 as the temperature increased from 50 to 90 °C. Salah et al. [17] and Cui et al. [21] independently found the critical temperature between 40 and 60 °C in investigating the passivity of stainless steels in acidic solutions and attributed it to the competition between film growth/dissolution and the involvement of oxygen, respectively.
The aforementioned nature of the oxide film formed on stainless steels is responsible for the temperature-dependent behavior. Therefore, the characterization of the passive film, especially the enrichment or depletion of the alloying element, has attracted great interests. Scully et al. [23] investigated the segregation behavior of the alloying element using X-ray photoelectron spectroscopy (XPS) and demonstrated the reason for the high corrosion resistance of a high entropy alloy. Gardin et al. [24,25] combined the XPS and Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) to investigate the surface oxide films on duplex stainless steels. They discussed the quantitative data and built the linear relationship between the two techniques, which provided an effective method for the analysis of the element segregation inside the passive film. The results suggested that molybdenum and nitrogen were enriched in the underlying metal compared to the bulk while chromium and molybdenum were enriched in the native oxide film [24]. Shoesmith et al. [14] used these two techniques and confirmed the enhanced loss of Cr/Mo segregation in the film formed at high temperatures. The condensate temperature was proved to affect the electrochemical passivation properties of stainless steels because of the change in corrosion kinetics and solution oxygen concentration [21]. How the constituting elements in the solution/film/matrix interface respond to the temperature variation of the condensates should be clarified.
In the present work, the passivation properties of a high alloyed 254SMO austenitic stainless steel (hereafter called 254SMO) in the simulated condensates after FGD are investigated with electrochemical and surface analysis. The effect of temperature on the passivation behavior and film characteristics is interpreted. The temperature- and oxygen-related semiconductive property variation is analyzed. The segregation of the element in the surface film, sub-surface layer, and the modified alloy layer is comprehensively discussed.
Section snippets
Materials and solution
The material used in this work was commercial standard 254SMO stainless steel (UNS S31254) with a thickness of 3 mm, supplied by Nippon Yakin Kogyo. The steel was provided in the form of hot-rolled and solution treated at 1200 °C for 1 h, followed by water quenching. Table 1 lists the chemical composition of the 254SMO steel. To reveal the steel microstructure, the specimens were abraded sequentially to 2000 grit SiC paper and then polished with a 1.5 μm diamond abrasive. Then the
Open circuit potential and potentiodynamic polarization curves
Fig. 2a and b display the temperature-dependent OCP and polarization curves of 254SMO stainless steel in the simulated condensates, respectively. The long-term OCP (96 h) is monitored considering the effect of temperature on the OCP is unpredictable and even time-dependent. From literature, Antón et al. [18] and Cardoso et al. [26] reported the positive shift and negative shift of the OCP for nickel-based alloy with increasing temperature, respectively. Salah et al. [17] found that OCP of
Effect of temperature on the electrochemical passivation behavior of 254SMO in the simulated condensates
Fig. 2b confirms that both anodic and cathodic processes are affected by the solution temperature, including the passivation, hydrogen evolution and oxygen reduction reaction. The main objective of this work is characterizing the passive behavior of the 254SMO steel in the simulated condensates and thus the analysis of the cathodic processes is neglected.
Solution temperature affects the active dissolution, passivation, and transpassive dissolution of 254SMO in the simulated condensates.
Conclusion
The passive properties of 254SMO in the simulated desulfurized flue gas condensates in the chimney have been studied and the following conclusions are obtained:
(1) Increase of temperature accelerates the anodic and cathodic reactions simultaneously. The former results in the enhanced transpassive dissolution and surface deterioration, and the latter contributes to the involvement of oxygen reduction.
(2) Donor densities of the film increase generally with increasing temperature, while the
CRediT authorship contribution statement
Yunpeng Dou: Investigation, Visualization. Sike Han: Investigation, Data curation. Liwei Wang: Writing - review & editing. Xin Wang: Methodology, Supervision. Zhongyu Cui: Writing - original draft, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgment
The authors wish to acknowledgement the financial support of National Natural Science Foundation of China (Nos. 51601182, 51701102 and 51901216), the Fundamental Research Funds for the Central Universities (No. 201762008), and the China Postdoctoral Science Foundation (No. 2019M652471).
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