Met. Mater. Int., doi: 10.1007/s12540-016-5601-0

Corrosion of AISI 430 Ferritic Stainless Steel in (N2/3.1%H2O/2.42%H2S)Mixed Gas at 600-800 °C Min Jung Kim and Dong Bok Lee* School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Korea (received date: 3 November 2015 / accepted date: 21 January 2016) The AISI 430 ferritic stainless steel with a composition of Fe-16.5Cr-0.5Mn-0.6Si-0.06C in wt% was corroded at 600, 700 and 800 °C for up to 30 h in 1 atm of (N2/3.1%H2O/2.42%H2S)-mixed gas. It displayed poor corrosion resistance because of H2O/H2S. The hydrogen dissolution in the scales owing to H2O/H2S, and the predominant formation of sulfides owing to H2S made the scale highly susceptible to cracking and spallation. The sulfur potential in the mixed gas was so high that sulfides formed predominantly. Sulfides with fast growth rates always overgrew oxides to constitute the main scale. Iron corroded to FeS via the outward diffusion of Fe2+ ions to form the outer scale, which led to the formation of Kirkendall voids in the scale. Chromium corroded to (Fe, Cr)-mixed sulfides or Cr2S3 in the inner scale. The scales grew fast, and were highly fragile, porous, and susceptible to spallation. Keywords: alloys, oxidation, sulfidation, corrosion, AISI 430 stainless steel

1. INTRODUCTION AISI 430 is a ferritic, high Cr stainless steel with good corrosion resistance, mechanical properties, and formability. In order to utilize AISI 430 ferritic stainless steel (FSS) as high-temperature structural components, it is important to study its corrosion resistance in oxidizing and/or sulfidizing environments. Generally, sulfidation is much more problematic than oxidation, because metal sulfides grow fast, are less adherent, and melt at lower temperatures than the corresponding oxides [1-3]. In this study, AISI 430 FSS corroded between 600 and 800 °C for up to 30 h in N2/H2O/H2S-mixed gas in which oxidation and sulfidation can occur simultaneously. Such a mixed gas is frequently encountered in coal gasification systems and petrochemical units, which operate at high temperatures under highly corrosive environments [4,5]. The high-temperature corrosion behavior of AISI 430 FSS in air [6,7], wet air [8], N2/16.5%O2/20%H2O [9-11], and marine environment [12] was previously studied. When AISI 430 FSS oxidized in air at 800 °C for 200 h, a bilayered scale that consisted of an outer (Cr,Fe,Mn)3O4 spinel and an inner Cr2O3 scale formed [6,7]. When the hot air was mixed with 500 ppm NaCl(g), a bilayered scale that consisted of an outer Fe2O3 scale and an inner Cr2O3-rich scale formed [12]. However, the corrosion behavior of AISI 430 FSS in N2/H2O/H2S-mixed gas was not yet adequately studied. Water vapor that is invariably present in the synthetic *Corresponding author: [email protected] KIM and Springer

gas, coal gas and combustion gas accelerates corrosion rates, decreases the plasticity of scale, and develops voids. H2S that dissociates into sulfur and hydrogen is quite aggressive, because sulfur forms sulfides, and hydrogen reduces the sulfide scale, developing porous scales where metal and sulfur ions diffuse fast. H2(g) and H2O(g) transports through voids, cavities, and cracks in the scale. In addition, atomic hydrogen released from H2O/H2S-mixed gas penetrates the scale and the metal interstitially, forms hydrogen clusters, and causes hydrogen embrittlement [13,14]. Therefore, the corrosion of metals in H2O/H2S-mixed is quite serious, very complex, and still not satisfactorily understood. In this study, the corrosion rates, scales, and corrosion mechanism of AISI 430 FSS in N2/H2O/ H2S-mixed gas at high temperatures were characterized.

2. EXPERIMENTAL PROCEDURE Small coupons of AISI 430 FSS plate with a nominal composition of Fe-16.5Cr-0.5Mn-0.6Si-0.06C in wt% were ground to 1000 grit using SiC abrasion paper, ultrasonically cleaned in acetone and methanol, and corroded at 600, 700, and 800 °C for up to 30 h in a quartz tube under 1 atm of flowing (N2/ 3.1%H2O/2.42%H2S)-mixed gas, and cooled to room temperature. Since the current corrosion test condition was so severe even to corrode platinum much, the thermogravimetric analyzer could not be used. The corrosion test gas composition was attained by blowing (N2/2.5%H2S)-mixed gas at 1 atm through a water bath kept at 25 °C into the quartz tube centrally located in an electric furnace. The purity of the N2 and

Corrosion of AISI 430 Ferritic Stainless Steel in (N2/3.1%H2O/2.42%H2S)-Mixed Gas at 600-800 oC

H2S gas used was 99.999%, and 99.5%, respectively. The corroded test coupons were characterized by a scanning electron microscope (SEM), an X-ray diffractometer (XRD) with CuKα radiation, and an electron probe microanalyzer (EPMA).

3. RESULTS AND DISCUSSION Figure 1 shows the average thickness of scales formed on AISI 430 FSS after corrosion at 600-800 °C for 10-30 h in (N2/3.1%H2O/2.42%H2S)-mixed gas. Corrosion rates increased with an increase in the corrosion temperature. AISI 430 FSS corroded almost linearly, indicating that the scales had little protectiveness. When AISI 430 FSS oxidized in air, the thickness of scales after oxidation at 600, 700, and 800 °C for 30 h was only 0.5, 1.1, and 2.2 μm, respectively, due to the formation of protective oxide scales. However, accelerated corrosion in N2/H2O/H2S-mixed gas was attributed to formation of sulfides owing to H2S, and hydrogen dissolution in the scales owing to H2O/H2S. Figure 2 shows SEM/EDS results of AISI 430 FSS after corrosion at 600 °C for 10 h. Loosely adherent scale lumps were scattered on the surface of the thin scale, indicating that corrosion occurred locally to a small extent during the early corrosion stage (Fig. 2(a)). They can be divided into the outer scale (O.S.) and inner scale (I.S.) (Fig. 2(b)). The composition of the outer and inner scale was 50Fe-50S and 39Fe-29Cr-32S in at%, respectively, according to the EDS analysis (Figs. 2(c) and (d)). This indicated that the outer and inner scale consisted of FeS and the (Fe,Cr)-mixed sulfide, respectively. It is noted that AISI 430 FSS formed Cr2O3-rich scales in the oxidizing atmosphere [6,7]. FeS has a very high concentration of cation vacancies so that it grows rapidly via the outward diffusion 2+ of Fe ions [2]. This depleted Fe, and thereby enriched Cr underneath, facilitating the formation of the inner (Fe, Cr)-mixed 2+ sulfide. The outward diffusion of Fe ions also formed voids at the scale-metal interface. Voids can act as stress concentration sites to generate cracks in the scale. Cracks also generate owing to the thermal stress arisen by the thermal expansion

Fig. 1. Thickness of scales formed on AISI 430 FSS after corrosion at 600, 700, and 800 °C for 10-30 h in (N2/3.1%H2O/2.42%H2S)-mixed gas.

Fig. 2. AISI 430 FSS after corrosion at 600 °C for 10 h. (a) SEM top view, (b) SEM cross-sectional image, (c) EDS spectrum of the outer scale, and (d) EDS spectrum of the inner scale.

coefficient mismatch between the outer and inner scale, the growth stress developed in the scale, and the incorporation of hydrogen in the scale with a limited plasticity [14]. The scales formed in this study were inevitably susceptible to cracking and spallation. Figure 3 shows SEM/EDS results of AISI 430 FSS after corrosion at 600 °C for 30 h. As corrosion proceeded, scale lumps kept nucleating at the surface, grew, and interconnected to cover the whole surface, as shown in Fig. 3(a). Cracks propagated inter- and trans-granularly across the outer scale, owing to the large thermal and growth stress generated, voids formed, and the incorporation of hydrogen in the scale. The scale shown in Fig. 3(b) shows vertical and horizontal cracks and numerous voids particularly at the lower part of the outer FeS scale. The scale consisted primarily of the outer FeS scale (Fig. 3(c)) and the inner Cr2S3 scale (Fig. 3(d)). When the rapidly growing FeS grains met with neighbouring FeS grains, cracks generated easily. Cracks also generated at the interface of the outer/ inner scale, and inside the inner scale. The outer scale was thicker than the inner scale, owing to the high nonstoichiometry of FeS (Fig. 3(b)). The formation of the outer, thick 2+ FeS scale through the rapid outward diffusion of Fe ions

Min Jung Kim and Dong Bok Lee

Fig. 4. AISI 430 FSS after corrosion at 700 °C for 30 h. (a) SEM crosssectional image, (b1-5) EDS maps of (a), (c) EDS line profiles along A-B shown in (a), (d) XRD pattern of the outer scale, and (e) XRD pattern of the inner scale.

Fig. 3. AISI 430 FSS after corrosion at 600 °C for 30 h. (a) SEM top view, (b) SEM cross-sectional image, (c) EDS spectrum of the outer scale, and (d) EDS spectrum of the inner scale.

changed the inner scale from the (Fe,Cr)-mixed sulfide (Fig. 2(d)) to Cr2S3 (Fig. 3(d)). Figure 4 shows SEM/EDS/XRD results of AISI 430 FSS after corrosion at 700 °C for 30 h. The thick scale detached from the alloy owing to the increased thermal and growth stress, voids formed, and the hydrogen dissolution in the scale (Fig. 4(a)). Many flaws such as voids and cracks developed especially in the outer FeS scale in order to relieve the large stress developed. During corrosion and the subsequent cooling stage, more scales spalled with an increase in the thickness. In Fig. 4, the marker test was performed by manually spraying fine Pd powder onto the sample surface prior to corrosion in order to understand the corrosion mechanism of AISI 430 FSS in N2/H2O/H2S-mixed gas. The bottom of Pd (see the Pd map shown in Fig. 4(b)) corresponded to the initial sample surface. From Figs. 4(b) and (c), it is seen that the scale can be divided into the outer FeS scale (O.S.), the middle Cr-rich sulfide scale with some iron sulfides (M.S.), and the inner Cr-rich sulfide scale that also has some iron sulfides (I.S.).

The outer FeS scale was identified in Fig. 4(d). In Fig. 4(e), which was taken after grinding off most of the scale, the α-Fe matrix and the Cr2S3 scale were detected without any iron sulfides, implying that the middle and the inner scale consisted primarily of Cr2S3. The Pd map shown in Fig. 4(b) indicates that some Pd powder was carried outwardly up to the surface, and also accumulated along the outer FeS grain boundary, portraying individual, coarse FeS grains that formed 2+ by the outward migration of Fe ions. It also indicates that the thin middle scale that was surrounded by Pd was formed 3+ by the outward diffusion of Cr ions and a lesser amount of 2+ Fe ions. The oxygen line profile shown in Fig. 4(c) indicates that a small amount of oxygen was present mostly in the middle scale, inner scale, and subscale area. It is noted that oxygen was originally present in N2/H2O/H2S-mixed gas as an impurity. The inner scale was formed primarily by the inward diffusion of sulfur and some oxygen. In principle, Cr2S3 with a defect structure of Cr2+xS3 grows by the outward diffusion of interstitial 3+ cations [2]. However, Cr ions could not have a chance to diffuse outward due probably to the fast sulfidation rate. Instead, 22S and O ions diffused inwardly through easy-diffusion paths such as cracks, voids, and grain boundaries, and formed the inner Cr2S3 scale having some iron sulfides. Clearly, not oxidation but sulfidation prevailed in this study because of H2S in the gas. The inner grains were much finer than the outer FeS grains, because Cr2S3 grew slower than FeS.

Corrosion of AISI 430 Ferritic Stainless Steel in (N2/3.1%H2O/2.42%H2S)-Mixed Gas at 600-800 oC

Fig. 5. AISI 430 FSS after corrosion at 800 °C for 25 h. (a) SEM top view, (b) EPMA cross-sectional image, and (c) EPMA line profiles of along A-B shown in (b).

Figure 5 shows SEM/EPMA results of AISI 430 FSS after corrosion at 800 °C for 25 h. The surface was covered with fully grown, facetted, coarse FeS grains (Fig. 5(a)). Voids in the outer FeS scale were coarse because the fast growing FeS grains could not deform plastically to accommodate the

anisotropic volume expansion. Voids were also conspicuous in the inner scale (Fig. 5(b)). Here, the whole scale detached from the matrix. Cracks propagated inter- and trans-granularly (Fig. 5(b)). The formation of cracks and voids around the outer FeS scale was also outlined in Figs. 2-4. Figure 5(c) indicates the outer FeS scale with some Cr-sulfides (O.S.), the middle Cr2S3 scale with some iron sulfides (M.S.), and the inner Cr2S3 scale with some iron sulfides (I.S.). The formation of the outer FeS scale led to the formation of the middle and inner scale rich in Cr2S3. Unlike the outer FeS that grew at the free surface, sulfides and a lesser amount of oxides formed competitively in the middle and inner scale. The scale thickness decreased in the order of the outer, inner, and middle scale. Since the consumption of sulphur in the outer FeS scale increased the oxygen potential underneath, oxygen was weakly present in the lower part of the scale. Unlike Si that segregated in the inner scale due to the immobility of its sulfide or oxide, another minor alloying element, Mn, was rather uniformly corroded in the whole scale. Since the defect structures of the Mn-sulfide and oxide are Mn1-yS and Mn1-xO, respectively, Mn can exist in the outer scale through its outward diffusion [2]. When Fe-Si alloys oxidized, silicon generally segregated around the scale/matrix interface as SiO2 or Fe2SiO4 [15]. The kind and amount of scales depend on the amount of elements in the alloy, the thermodynamic stability and growth rates of the sulfides or oxides, and the gas composition employed. As listed in Table 1, oxides are thermodynamically more stable than the corresponding sulfides [16]. However, the sulfur potential in (N2/3.1%H2O/2.42%H2S)-mixed gas was high enough to form sulfides with fast growth rates. Sulfides always overgrew oxides to constitute the main scale. When sulfides form instead of oxides, growth stress develops more owing to the large volume expansion as can be seen from the PillingBedworth ratio, which means the ratio of oxide to metal volume. Not only the dominant corrosion of Fe and Cr but also the subsidiary corrosion of minor alloying elements such as 0.5% Mn and 0.6% Si led to the accumulation of the large growth stress in the scale. Also, a large thermal stress developed owing to the mismatch in thermal expansion coefficients between the thick scale and the alloy during cooling

Table 1. Standard free energy change of the given reaction, ΔG° (kJ/mol of S2 or O2) [16] and Pilling-Bedworth ratio [17] ΔG° at 800 °C P-B ratio (%) Reaction ΔG° at 600 °C 2Fe(s)+S22FeS(s) 2Fe(s)+O22FeO(s) 2Cr(s)+S22CrS(s) 4/3Cr(s)+O22/3Cr2O3(s) 2Mn(s)+S22MnS(s) 2Mn(s)+O22MnO(s) Si(s)+S2SiS2(s) Si(s)+O2SiO2(s)

-207.3 -429.6 -315.3 -605.6 -439.2 -642.1 -201.9 -752.5

-186.8 -404.8 -292.6 -571.9 -415.8 -612.8 -176.4 -716.4

261 169 [17] 240 200 [17] 284 175 [17] 379 213 [17]

Min Jung Kim and Dong Bok Lee

after corrosion. In addition, the dissolution of hydrogen in the scale accelerated the breakage and spallation of scales. Hence, AISI 430 FSS was nonprotective in this study.

4. CONCLUSION AISI 430 FSS corroded fast, almost linearly at 600-800 °C in (N2/3.1%H2O/2.42%H2S)-mixed gas. During the early stage, scale lumps that consisted primarily of the outer FeS and inner (Fe, Cr)-mixed sulfides formed. They were fragile and loosely adherent. As corrosion progressed, scale lumps grew to the bilayered scale that consisted primarily of the outer FeS and the inner Cr2S3. Further corrosion led to the formation of the triple layered scale, which consisted primarily of the outer FeS with some Cr-sulfides, the middle Cr2S3 with some iron sulfides, and the inner Cr2S3 with some iron sulfides. The outer and middle scales grew by the outward diffusion of cations, while the inner scale grew by the inward diffusion of anions. The scales were vulnerable to the cracking, spallation, and void formation.

ACKNOWLEDEMENT This research was supported by Basic Science Research Program through NRF funded by the Ministry of Education (No. 2013R1A1A2064793: M.J. Kim) and KETEP grant (No. 20143030050070: D.B. Lee) funded by the Korea government Ministry of Trade, Industry and Energy.

REFERENCES 1. S. Mrowec and M. Wedrychowska, Oxid. Met. 13, 481 (1979). 2. S. Mrowec and K. Przybylski, Oxid. Met. 23, 107 (1985).

3. X. Cheng, Z. Jiang, D. Wei, J. Zhao, B. J. Monaghan, R. J. Longbottom, and L. Jiang, Met. Mater. Int. 21, 2 (2015). 4. R. John, Shreir’s Corrosion, 4th ed., (eds. R. A. Cottis, M. J. Graham, R. Lindsay, S. B. Lyon, J. A. Richardson, J. D. Scantlebury, and F. H. Stott), p. 240, Elsevier, UK (2010). 5. A. S. Khanna, Introduction to High Temperature Oxidation and Corrosion, p.135, ASM Int., USA (2002). 6. J. Rufner, P. Gannon, P. White, M. Deibert, S. Teintze, R. Smith, and H. Chen, Int. J. Hydrogen Energy 33, 1392 (2008). 7. H. Ebrahimifar and M. Zandrahimi, Solid State Ionics 183, 71 (2011). 8. M. P. Brady, M. Fayek, J. R. Keiser, H. M. Meyer, K. L. More, L. M. Anovitz, D. J. Wesolowski, and D. R. Cole, Corros. Sci. 53, 1633 (2011). 9. I. Saeki, H. Konno, R. Furuichi, T. Nakamura, K. Mabuchi, and M. Itoh, Corros. Sci. 40, 91 (1998). 10. D. B. Lee, Met. Mater. Int. 20, 2 (2014). 11. D. B. Lee and M. J. Kim, Korean J. Met. Mater. 53, 6 (2015). 12. H. H. Liu, W. J. Cheng, and C. J. Wang, Appl. Surf. Sci. 257, 10645 (2011). 13. D. J. Young, High Temperature Oxidation and Corrosion of Metals, p. 455, Elsevier, UK (2008). 14. N. Birks and G. H. Meier, Introduction to High Temperature Oxidation of Metals, p. 176, Cambridge University Press, UK (1983). 15. D. B. Lee, Korean J. Met. Mater. 52, 899 (2013). 16. I. Barin, Thermochemical Data of Pure Substances, pp. 444-909, VCH, Germany (1989). 17. E. McCafferty, Introduction to Corrosion Science, p. 236, Springer, NY (2010).