 Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 126 - 129 126 High-temperature Corrosion of T22 Steel in N2/H2S-mixed Gas Min Jung Kim, Dong Bok Lee * School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, South Korea. Received 06 Oct ober 2016; received in revised form 29 December 2016; accept ed 16 January 2017 Abstract ASTM T22 steel (Fe-2.25Cr-1Mo in wt.%) was corroded at 600 and 700 o C for 5-70 h under an atmospheric pressure that consisted of N2-(0.5. 2.5)%H2S-mixed gas. T22 steel cor- roded rapidly, forming outer FeS scales and inner (FeS, FeCr2O4)-mixed scale. The formation of the outer FeS scale facilitated the oxidation of Cr to FeCr2O4 in the inner scales. Since the nonprotective FeS scale was present over the whole scale, T22 steel displayed poor corrosion resistance. Ke ywor ds : Fe-Cr-Mo alloy, T22 steel, corrosion, H2S gas, Sulfidation 1. Introduction The integrated gasification combined cycle (IGCC) power plants are operating in U.S., Japan, Germany, and Netherlands. It is a new technology that turns coal into synthesis gas (syngas) and produces the electricity. It promises low emissions and improved efficiency compared to conventional coal-fired power plants that produce the electricity directly by burning coals [1]. However, one of the main problems in IGCC is the corrosion occurring by the syngas in a gasification unit, be- cause the syngas consisted primarily of the extremely corrosive H2S gas. This limits the operating temperature and the process efficiency of the IGCC power plants. It is noted that the H2S gas has been a major concern in oil refinery plants, high-temperature gas turbines, and petrochemical units. H2S gas dissociates into sulfur and hydrogen ions, and reacts with the steel according to the reaction; H2S+Fe → FeS+H2 [2-4]. Generally, most sulphides are highly nonstoichiometric, and ionic diffusion in the scales is hence quite fast [5,6]. Sulfidation is therefore a quite serious problem. Hydrogen also signifi- cantly decreases the corrosion resistance and mechanical properties of the steel [7-10]. In this study, T22 steel was corroded at 600 and 700 oC for up to 70 h in N2-(0.5, 2.5)%H2S-mixed gas in order to under- stand its corrosion behavior in the H2S-mixed gas. This is important in IGCC power plants, oil refinery plants, high-temperature gas turbines, and petrochemical units. Alt- hough the oxidation behavior of T22 steel was extensively studied [11-12], little is reported about the high-temperature corrosion behavior of T22 steel in H2S-mixed gas. The pur- pose of this study is to investigate the corrosion behavio r of T22 steel in N2/H2S-mixed gas . 2. Method T22 steel plate with a nominal composition of Fe-2.25Cr-1.0Mo-0.45Mn-0.3Si-0.12C in wt% were cut into a size of 2x10x15 mm 3 , ground up to a 1000-grit finish with SiC paper, ultrasonically cleaned in acetone, corroded, and in- spected to examine its corrosion behavior. Each sample was suspended by a Pt wire in a quartz reaction tube positioned vertically inside the hot zone of the vertical electrical furnace, and corroded at 600 and 700 o C for up to 70 h in N2-(0.5, 2.5)%H2S-mixed gas maintained at 1 atm. The employed N2 gas was 99.999% pure, and H2S gas was 99.5% pure. The corroded samples were characterized by a scanning electron microscope (SEM; Jeol JEM-2100F operated at 200 keV), an X-ray diffractometer (XRD) with Cu-Kα radiation operating at 40 kV and 300 mA in θ/2θ configuration, and an electron probe microanalyzer (EPMA). 3. Results and Discussion The corrosion kinetics of T22 steel in N2-(0.5, 2.5)%H2S gas are depicted in Fig. 1. Weight gains were the sum of weight gain due to scaling and weight loss due to scale spallation. They increased with an increase in the temperature and the H2S concentration. The fastest corrosion rate was observed in the sample corroded at 700 o C in N2-2.5%H2S-mixed gas. The almost linear, large weight gains depicted in Fig. 1 indicate vastly fast corrosion kinetics for all the samples. It is noted that local cracking, partial spallation and void formation in the formed scales were unavoidable for all the samples, including at 600 o C in N2-2.5%H2S-mixed gas for 70 h. Such scale fail- ure became more serious as corrosion progressed. Although T22 steel displayed reasonable oxidation resistance in the oxidizing atmospheres, it was non-protective in the harsh H2S-mixed corrosion environment. Fig. 1 Weight gains of T22 steel at 600 and 700 o C in N2-(0.5, 2.5)%H2S-mixed gas Fig. 2(a) indicates that T22 steel consisted mainly of α-Fe. The other minor phase, perlite, was not detected Fig. 2(a) due to its small amount. In this study, the corrosion at 600 and 700 o C for 5-70 h in N2-(0.5, 2.5) %H2S-mixed gas inevitably led to the formation of the outer FeS scale and the inner (FeS, FeCr2O4)-mixed s cale. Fig. 2(b) indicates the outer FeS scale that formed after corrosion at 700 o C for 20 h in N2-2.5%H2S gas. The outer, non-adherent FeS scale was detached off by slightly hitting the sample, and the inner scale was X-rayed as shown in Fig. 2(c). This revealed the inner (FeS, * Corresponding aut hor. Email: dlee@skku.ac.kr http://en.wikipedia.org/wiki/Coal http://en.wikipedia.org/wiki/Syngas http://en.wikipedia.org/wiki/Syngas http://en.wikipedia.org/wiki/Gasification Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 126 - 129 127 Cop y right © TAETI FeCr2O4)-mixed scale, along with the α-Fe matrix phase. The amount of Cr in T22 steel was not large enough to completely cover the matrix surface with the protective Cr2O3 scale. T22 steel reacted with the H2S gas to form FeS, releasing hydrogen according to the equation; Fe(s) +H2S (g) → FeS(s) +H2 (g). Since FeS has a very high concentration of cation vacancies, it grew fast to form the outer scale through the outward diffusion of Fe 2+ ions [9]. The formation of FeS decreased the sulfur potential, and thereby the oxygen potential underneath, facili- tating the formation of the oxides in the inner scale, as shown in Fig. 2(c). The minor alloying elements such as Mo and Mn in T22 steel tended to be expelled from the inner scale, due to their small amount or activity. The distribution of alloying elements depends on the thermodynamic stability of corre- sponding oxides or sulfides and activity of concerning ele- ments. The impurity oxygen in N2-(0.5, 2.5) %H2S-mixed gas reacted with T22 steel according to Eqs. (1) and (2). Fe(s)+1/2 O2 (g) → FeO(s), (1) 2Cr(s)+3/2 O2 (g) → Cr2O3(s). (2) Thermodynamica lly, the o xides are generally more stable than the corresponding sulfides. The spinel is formed by diffusion of Fe 2+ from FeO to Cr2O3 oxides through spinel accompanied by diffusion of hole and evolution of oxygen gas at FeO/spinel interface. The Fe 2+ reacts with Cr2O3 to produce spinel at the spinel/Cr2O3 interface, and Cr 3+ d iffuses with hole to the Cr2O3/gas interface in order to keep elec- tro-neutrality [13]. The formed Fe O and Cr2O3 o xides particles, the solid-state reaction occurred to form the more stable Fe Cr2O4 spinel scale, wh ich gradually dis- persed in external o xide layer. Fe Cr2O4 spinel reacted with FeO and Cr2O3 according to the equation. FeO(s)+ Cr2O3(s) → FeCr2O4(s) Fig. 2 XRD patterns of T22 steel. (a) Before corrosion, (b) The outer scale, and (c) The inner scale that formed after corrosion at 700 o C for 20 h in N2-2.5%H2S gas The morphology of surface scales that formed on T22 steel after corrosion in N2-0.5%H2S gas is shown in Fig. 3. From the early corrosion stage at 600 o C, FeS platelets progressively protruded through the ensuing outward diffusion of Fe 2+ ions over the smooth underlying scale (Figs. 3(a) and (b)). They spalled off easily due to their fast growth rate and incorporation of hydrogen released from the H2S gas. The formed scales were quite fragile so that cracks were seen in Fig. 3(a). At 700 o C, the FeS platelets grew to coarse, protruded FeS grains, as shown in Fig. 3(c). As corrosion progressed, FeS grains grew bigger, leading to the generation of cracks in the surface FeS scale (Fig. 3(d)). The EDS analysis indicated that the outer scale and the inner scale consisted primarily of FeS (Figs. 3(e) and (f)), respectively. Fig. 3 SEM top view of the scales that formed on T22 steel after corrosion in N2-0.5% H2S gas. (a) at 600 o C fo r 5 h, (b ) at 600 o C fo r 40 h, (c) at 700 o C for 5 h, (d) at 700 o C for 40 h, (e) EDS spectrum of spot ①, (f) EDS spectrum of ② The morphology of surface scales that formed on T22 steel after corrosion in N2-2.5%H2S gas is shown in Fig. 4. From the early corrosion stage at 600 o C, coarse, facetted FeS grains covered the whole surface (Fig. 4(a)). They continuously grew bigger as the corrosion progressed (Figs. 4(b)-(d)). In Fig. 4(d), cracks propagated inter- and trans-granularly. With the in- crease of concentration of the H2S gas from 0.5 to 2.5 %, the grains at the surface of the scale became much coarser as shown in Figs. 3 and 4, indicating that the H2S gas accelerated corrosion. Advances in Technology Innovation, vol. 2, no. 4, 2017, pp. 126 - 129 128 Cop y right © TAETI Cop y right © TAETI Cop y right © TAETI Cop y right © TAETI Cop y right © TAETI Fig. 4 SEM top view of the scales that formed on T22 steel after corrosion in N2-2.5% H2S gas. (a) at 600 o C fo r 5 h, (b ) at 600 o C fo r 40 h, (c) at 700 o C for 5 h, (d) at 700 o C for 40 h. Fig. 5 shows SEM/EDS analytica l results of T 22 steel after corrosion in N2-0.5%H2S gas at 600 o C for 70 h. The scale consisted of about 120 μ m-thic k outer scale, and about 70 μ m-thic k inner scale. The outer scale was d e- tached from the inner scale, and vertica l cracks were seen in the inner scale, owing to the la rge stress arisen by (1) the mis match in the therma l e xpansion coeffic ients among the outer scale, inner scale, and the matrix, (2) the difference in the growth rates of various oxides and sulfides, and (3) the hydrogen dissolution in the scale. The EDS analysis indicated that the outer scale and the inner scale consisted prima rily o f FeS (Fig. 5(b)), and (FeS, FeCr2O4) (Fig. 5(c)), respectively. This was consistent of the XRD results de- picted in Fig. 2. FeS platelets protruded over the outer FeS scale (Fig. 5(a)). Fig. 5 SEM/ EDS analytica l results of T22 steel after corrosion in N2-0.5% H2S gas at 600 oC fo r 70 h. (a) cross -sectional image, (b) EDS spectrum of spot ①, (c) EDS spectrum of ②. 4. Conclusions T22 steel was corroded at 600 and 700 o C for up to 70 h in N2/H2S-mixed gas under total pressure of 1 atm. The corrosion occurred almost linearly through the sulfidation, together with oxidation to a less extent. The outer scale consisted primarily of FeS that formed by the outward diffusion of Fe 2+ ions. The inner (FeS, FeCr2O4)-mixed scale formed by the inward diffu- sion of predominantly sulfur and a small amount of oxygen. The outer scale kept growing outwards during corrosion. 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