W. Kai

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700-900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H2/H2S/H2O mixture with varying sulfur partial pressures of 10−7−10−5 atm. and oxygen partial pressures of 10−24−10−2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and −10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe1−xS) and an inner complex scale of FeAl2S4 and FeS. Alloys having intermediate Al contents (Fe-18 and −28 at.% Al) formed scales that consisted of mostly iron sulfide and Al2O3 as well as minor a amount of FeAl2S4. The amount of Al2O3 increased with increasing Al content. The Fe 40 at.% Al formed only Al2O3 at 700°C, while most Al2O3 and some FeS were detected at T≥800°C. The formation of Al2O3 was responsible for the reduction of the corrosion rates.

High temperature corrosion behavior of iron aluminides containing ternary additions in H2/H2S/H2O mixed gases

Abstract The high-temperature corrosion behavior of four iron aluminides containing Fe–18Al (in at.%) and three Fe–18Al–5M alloys (where M was Cr, Mn, or Mo) was studied over the temperature range of 700–900°C in a H 2 /H 2 S/H 2 O atmosphere. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with decreasing temperature, and the addition of ternary elements resulted in various decreases in the corrosion rate compared with Fe–18Al. It was found that Mo was effective to improve the corrosion resistance at T ≤800°C while Cr had a better improvement at 900°C. The scales formed on all iron aluminides were strongly dependent on temperature and ternary addition.

Effects of sulfur pressure on the sulfidation behavior of 310 stainless steel

High-temperature sulfidation behavior of 310 stainless steel was studied over the temperature range of 700–900°C above a pure sulfur pool with the sulfurvapor range of 10−4–10−1 atm. The corrosion kinetics followed the parabolic rate law in all cases. The corrosion rates increased with increasing temperature and sulfur pressure. The scales formed on 310 stainless steel were complex and multilayered. The outer scale consisted of iron sulfide (with dissolved Cr), (Fe, Ni)9S8 and chromium sulfides (Cr2S3 and Cr3S4 with dissolved Fe), while the inner layer was a heterophasic mixture of Cr2S3, Cr3S4, NiCr2S4, and Fe1−xS. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations (Fe, Ni, and Cr), and the inner scale grew by the inward transport of sulfur. The formation of Cr2S3, Cr3S4, and NiCr2S4 partly blocked the transport of iron through the inner scale, resulting in a reduction of the corrosion rates as compared with the results in the literature.

The Corrosion of Titanium Aluminides in H2/H2S/H2O Atmospheres at 800–1000°C

The corrosion behavior of three Ti–Al intermetallics containing 20, 30, and 40 wt.% Al was studied over the temperature range 800–1000°C in a H2/H2S/H2O gas mixture. Ti–20Al and Ti–40Al alloys had the single-phase structure of Ti3Al and TiAl, respectively, while Ti–30Al was a two-phase mixture of Ti3Al+TiAl. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but decreased with increasing Al content. The Ti–40Al alloy exhibited the best corrosion resistance among all alloys studied. The scales formed on Ti–Al intermetallics were heterophasic and duplex, consisting of an outer-scale layer of pure α-TiO2 and an inner layer of α-TiO2 with minor amounts of α-Al2O3 and Til-xS. The amount of α-Al2O3, which increased with increasing Al content, is responsible for the reduction of the corrosion rates as compared with those of pure Ti oxides.

The high-temperature corrosion of Fe-28Al base alloys containing Cr and Y additions in H2/H2S/H2O mixed gases

The high-temperature corrosion behavior of three Fe–28Al–xCr (where x=2, 5, and 10 at.%) ternary and Fe–28Al–5Cr–0.5Y quaternary alloys was studied over the temperature range of 700–900 °C in H2/H2S/H2O mixed gases. The results showed that the corrosion kinetics followed the parabolic rate law in all cases. The parabolic rate constants decreased with decreasing temperature, but fluctuated with increasing Cr-content. It was found that Fe–28Al–5Cr revealed the best performance to improve the corrosion resistance compared with Fe–28Al in the ternary alloys studied. The Fe–28Al–10Cr alloy showed poor corrosion resistance, and whose corrosion rates were only slightly lower than those of Fe–28Al and much higher than those of low-Cr alloys. The addition of Y did not provide any significant reduction in the corrosion rates as compared with those of Fe–28Al–5Cr at T≤800 °C. The scales formed on Fe–28Al–xCr were strongly dependent on temperature and Cr-content, consisting of most Al2O3, some FeS, and minor Cr2S3/Cr3S4 (for 2Cr- and 5Cr-content alloys) and of most FeS, minor amounts of Cr2S3/Cr3S4 and Al2O3 (for Fe–28Al–10Cr). The scales formed on the Y-containing alloy were nearly identical to those of Fe–28Al–5Cr.

Sulfidation Behavior of Inconel 738 Superalloy at 500–900°C

The high-temperature sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500–900°C in pure sulfur vapor over the range 102–104 Pa. The sulfidation kinetics followed the parabolic rate law in all cases. The sulfidation rates increased with increasing temperature and sulfur pressure. The scales formed were bilayered and temperature-dependent. At T≤700°C, the outer scale consisted of mostly NiS (with dissolved Co) and minor (CoS2 and NiCo2S4, while the inner layer was a heterophasic mixture of NiS, NiCo2S4, and minor amounts of Al2S3 and chromium sulfide (Cr2S3/Cr3S4). At T≥750°C, the outer scale consisted of mostly Ni3S2 (with dissolved Co) and minor amounts of Co3S4 and Cr2S3/Cr3S4, while the inner layer was a complex, heterophasic mixture of Ni3S2, Cr2S3/Cr3S4, CoCr2S4, and minor Al2S3. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations and the inner scale grew by the inward transport of sulfur. The formation of Al2S3 and Cr2S3/Cr3S4 partly blocked the transport of cations through the inner scale and consequently reduced the sulfidation rates as compared to pure nickel.

The high-temperature corrosion behavior of Nb-Al alloys in a H2/H2S/H2O gas mixture

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H2/H2S/H2O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of α-Nb and Nb3Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO2 layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO2 and minor amounts of Nb2O5, NbS2, and γ-Al2O3, while the scales formed on Nb-25Al consisted of mostly Nb2O5 and some γ-Al2O3. The scales formed on Nb-75Al consisted of mostly α-Al2O3 and Nb3S4 atT ≤900°C, and mostly α-Al2O3 , Nb3S4 and some AlNbO4 at 1000°C. The formation of α-Al2O3 and Nb3S4 resulted in a significant reduction of the corrosion rates.

The sulfidation of Inconel 738 in H2/H2S/Ar gas mixture at 500-900°C

The sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500–900°C in three various ratios of H2/H2S/Ar atmospheres having sulfur partial pressures in the range 10−4–102 Pa. The sulfidation kinetics followed the parabolic rate law in all cases, and the sulfidation rates increased with increasing temperature and sulfur partial pressure. In general, the scales formed were heterophasic and duplex, consisting of an outer layer composed mostly of nickel sulfides (mostly NiS and Ni3S2, and minor Ni7S6 detected at T > 750°C) plus some CoS2/Co3S4, CoMo2S4, NiCo2S4, and minor chromium sulfide (Cr2S3/Cr3S4), while the inner layer contained a mixture of nickel sulfides and minor amounts ofA12S3 and Cr2S3/Cr3S4. The amount of NiS decreased and that of Ni3S2 increased with increasing temperature. The dependence of the sulfidation rate constants (Kp) and Ps(2)(the n value) was significantly higher at T ≥ 850°C as compared to that measured at lower temperatures (T ≤ 850°C).

Corrosion behavior of Y-Al alloys in a H2/H2S/H2O gas mixture at 800-950°C

The corrosion behavior of pure Y and two Y–Al alloys containing 5 and 10 wt.% Al was studied over the temperature range 800–950°C in a H2/H2S/H2O gas mixture. Both alloys had the two-phase structure of Y+Y2Al. With the exception of Y–10Al, for which a kinetics inversion was observed between 800°C and higher temperatures (T≥ 850°C), the parabolic rate constants generally increased with increasing temperature, but decreased with increasing Al content. The scales formed on pure Y and the Y–Al alloys were single but heterophasic, consisting of mostly Y2O3 and minor Y2O2S. XRD results showed no evidence of Al2O3 and pure sulfides. The formation of Y2O3 and Y2O2S on Y–10Al at 800°C resulted in a subsurface phase transformation from Y+Y2Al to YAl2 and broke the structural integrity of the scale, being responsible for the fast corrosion rate.

Effects of Mo and Al additions on the sulfidation behavior of 310 stainless steel

The high-temperature sulfidation behavior of 310stainless steel (310SS) with Mo and Al additions (up to10 at.%) was studied over the temperature range700-900°C in pure-sulfur vapor over the range of 10-3 to 10-1 atm. Thecorrosion kinetics followed the parabolic rate law inall cases and the sulfidation rates increased withincreasing temperature and sulfur pressure. Thesulfidation rates decreased with increasing Mo and Al contents and it wasfound that the addition of 10 at.% Mo resulted in themost pronounced reduction among the alloys studied. Thescales formed on 310SS with Mo additions were complex, consisting of an outer layer of ironsulfide (with dissolved Cr), (Fe,Ni)9S8, andCr2S3/Cr3S4(with dissolved Fe), and an inner heterophasic layer ofFe1-xS,Cr2S3/Cr3S4,NiCr2S4,Fe1.25Mo6S7.7, FeMo2S4, andMoS2. The scales formed on 310SS with Mo andAl additions had a similar mixture as above, except thatAl0.55Mo2S4 was alsoobserved in the inner layer. The formation ofMoS2 andAl0.55Mo2S4 partly blocked the transport of cations throughthe inner scale, resulting in the reduction of thesulfidation rates compared to 310SS.