Corrosion of Fe-(9~37) wt. %Cr Alloys at 700–800 °C in (N₂, H₂O, H₂S)-Mixed Gas

Abstract

Fe-(9, 19, 28, 37) wt. %Cr alloys were corroded at 700 and 800 °C for 70 h under 1 atm of N₂, 1 atm of N₂/3.2%H₂O mixed gas, and 1 atm of N₂/3.1%H₂O/2.42%H₂S mixed gas. In this gas composition order, the corrosion rate of Fe-9Cr alloy rapidly increased. Fe-9Cr alloy was always non-protective. In contrast, Fe-(19, 28, 37) wt. %Cr alloys were protective in N₂ and N₂/3.2%H₂O mixed gas because of the formation of the Cr₂O₃ layer. They, however, became nonprotective in N₂/3.1%H₂O/2.42%H₂S mixed gas because sulfidation dominated to form the outer FeS layer and the inner Cr₂S₃ layer containing some FeCr₂S₄.

1. Introduction

Fe-Cr alloys are widely used as high-temperature structural materials. They oxidize when exposed to air or oxygen at high temperatures. When the Cr content in iron was ~5 wt. %, triple oxide layers such as Fe₂O₃/Fe₃O₄/FeO formed, and the oxidation rate was mainly controlled by the growth rate of FeO that formed on the alloy side [1]. The non-stoichiometric wustite grows much faster than the nearly stoichiometric Fe₃O₄ and Fe₂O₃. With an increase in the Cr content to ~10 wt. %, dispersed particles of the FeCr₂O₄ spinel formed more inside the FeO layer, and FeCr₂O₄ particles blocked the diffusion of Fe²⁺ ions to make the FeO layer thinner. With the further increase in the Cr content to ~15 wt. %, a mixed spinel, Fe(Fe,Cr)₂O₄, formed, which decreased the oxidation rate significantly. When the Cr content exceeded ~20 wt. %, the oxidation rate dropped sharply, forming a thin, continuous Cr₂O₃ layer containing a small amount of dissolved Fe ions [1,2,3]. When Fe-Cr alloys were exposed to S₂ gas at 1 atm, an FeS layer formed below 1.86 wt. %Cr, an outer Fe_1–xS layer and an inner (FeS, FeCr₂S₄) mixed layer formed in the range of 1.86–38.3 wt. %Cr, and a solid solution of FeS-Cr₂S₃ formed above 38.3 wt. %Cr [4]. Although Cr decreased the sulfidation rate, even Fe-Cr alloys with high Cr contents displayed insufficient corrosion resistance. This is attributed to the fact that sulfidation rates of common metals are 10–100 times faster than oxidation rates because the sulfides have much larger defect concentrations and lower melting points than the corresponding oxides [5]. The sulfidation of Fe-(20, 25, 30) wt. %Cr steels in 94Ar/5H₂/1H₂S mixed gas at 600 °C for 718 h resulted in the formation of the outer FeS layer and the inner FeCr₂S₄ layer [6]. On the other hand, the corrosion of conventional oxidation-resistant alloys by water vapor and H₂S gas has been a serious problem [3]. Water vapor and H₂S gas release hydrogen atoms, which ingress in the metals interstitially, form hydrogen clusters, and cause hydrogen embrittlement. Water vapor that is present in many industrial gases can form metal hydrides, and change not only the reaction at the scale/metal interface but also the mass transfer in scales, accelerating the corrosion rate [3,7]. In this study, Fe-Cr alloys were corroded at 700 and 800 °C in (N₂, H₂O, H₂S) mixed gas in order to understand their corrosion behavior in hostile (H₂O, H₂S)-containing environments for practical applications. The aim of this study is to examine the influence of the Cr content and the (N₂, H₂O, H₂S)-containing gas on the high-temperature corrosion of Fe-Cr alloys, which has not been adequately investigated before.

2. Experimental Procedures

Four kinds of hot-rolled ferritic Fe-Cr alloy sheets, viz., Fe-(8.5, 18.5, 28.3, 36.9) wt. %Cr, were prepared. They are termed as Fe-(9, 19, 28, 37)Cr, respectively, in this study. They were homogenized at 900 °C for 1 h under vacuum, cut into a size of 2 mm × 10 mm × 15 mm, ground up to a 1000-grit finish with SiC papers, ultrasonically cleaned in acetone, and corroded at 700 and 800 °C for 70 h under 1 atm of total pressure. Each test coupon was suspended by a Pt wire in a quartz reaction tube within the hot zone of an electrical furnace (Ajeon, Seoul, Korea), as shown in Figure 1. Three kinds of corrosion atmospheres were employed, viz. 1 atm of N₂, (0.968 atm of N₂ plus 0.032 atm of H₂O) that was achieved by bubbling the N₂ gas through the water bath kept at 25 °C, and (0.9448 atm of N₂ plus 0.031 atm of H₂O plus 0.0242 atm of H₂S) that was achieved by bubbling N₂ gas through the water bath kept at 25 °C and simultaneously flowing the N₂-5%H₂S gas into the quartz reaction tube. The N₂ gas was 99.999% pure, and H₂S gas was 99.5% pure. Nitrogen gas was blown into the reaction tube during heating and cooling stages. After finishing the corrosion test in N₂, N₂/3.2%H₂O, and N₂/3.1%H₂O/2.42%H₂S gas, the test coupons were furnace-cooled, and characterized by a scanning electron microscope (SEM, Jeol JSM-6390A, Tokyo, Japan), a high-power X-ray diffractometer (XRD, Mac Science M18XHF-SRA, Yokohama, Japan) with Cu-Kα radiation operating at 40 kV and 300 mA, and an electron probe microanalyzer (EPMA, Shimadzu, EPMA 1600, Kyoto, Japan).

3. Results and Discussion

Table 1. Weight gain of Fe-(9, 19, 28, 37)Cr alloys measured after corrosion at 700 and 800 °C for 70 h under 1 atm of N₂, N₂/3.2%H₂O, and N₂/3.1%H₂O/2.42%H₂S gas.

Temp.	Gas	Weight Gain (mg/cm2)
		9Cr	19Cr	28Cr	37Cr
700 °C	N2	195	1–2	1–2	1–2
	N2/H2O	220	1–2	1–2	1–2
	N2/H2O/H2S	2050	470	200	70
800 °C	N2	235	1–2	1–2	1–2
	N2/H2O	400	1–2	1–2	1–2
	N2/H2O/H2S	massive spalling	1530	690	550

lists the weight gains of Fe-(9, 19, 28, 37)Cr alloys due to corrosion at 700 and 800 °C for 70 h, which were measured using a microbalance before and after corrosion. Fe-9Cr always displayed the worst corrosion resistance, gaining excessive weight. For example, Fe-9Cr oxidized fast even in the N₂ gas through the reaction with impurities such as 3 ppm H₂O and 2 ppm O₂ in the N₂ gas (99.999% pure). Fe-9Cr oxidized faster in the N₂/H₂O gas than in the N₂ gas because of water vapor [7]. Water vapor dissociates into oxygen and hydrogen, oxidizes the metal, and forms voids within the oxide scale according to the equation [1,3],

M + H₂O → MO + (2H or H₂)

(1)

Fe-9Cr corroded the most seriously in N₂/H₂O/H₂S gas, because H₂S was much more harmful than H₂O. H₂S dissociates into hydrogen and sulfur. Sulfur forms non-protective metal sulfides according to the following equation:

M + H₂S → MS + (2H or H₂)

(2)

Hydrogen, which is released from H₂S and H₂O, dissolves and ingresses into the alloy and the scale interstitially, generates lattice point defects, forms hydrogen clusters and voids, causes hydrogen embrittlement, produces volatile hydrated species, and accelerates cracking, spallation and fracture of the scale. Hence, no metals are resistant to H₂O/H₂S corrosion. As listed in Table 1, Fe-(19, 29, 37)Cr displayed much better corrosion resistance in N₂ and N₂/3.2%H₂O with weight gains of 1–2 mg/cm² than Fe-9Cr. Fe-(19, 29, 37)Cr formed 0.3- to 1.3-μm-thick, adherent oxide scales. However, even Fe-(19, 29, 37)Cr failed in N₂/3.1%H₂O/2.42%H₂S with large weight gains, forming non-adherent, fragile sulfide scales as thick as 35–750 μm. This scale failure made the weight gains measured in N₂/H₂O/H₂S gas inaccurate. In N₂/H₂O/H₂S gas, the amount of local cracking, spallation and void formation in the scale varied for each test run. Although the accurate measurement of weight gains in N₂/H₂O/H₂S gas was impossible, it was clear that weight gains due to scaling decreased sharply with the addition of Cr.

Figure 2 shows the XRD patterns of scales formed after corrosion at 800 °C for 70 h. The corrosion of Fe-9Cr in N₂ and N₂/H₂O resulted in the formation of Fe₂O₃ and Fe₃O₄, as shown in Figure 2a,b. Oxide scales formed on Fe-9Cr in N₂ and N₂/H₂O were 90 and 100 μm thick, respectively. Since X-rays could not penetrate such thick oxide scales, FeO and Cr-oxides such as FeCr₂O₄, which might form next to the alloy [1], were absent in Figure 2a,b. In contrast, Fe-(19, 28, 37)Cr alloys oxidized at much slower rates in N₂ and N₂/H₂O than Fe-9Cr alloy, as listed in Table 1. Fe-(19, 28, 37)Cr alloys formed the protective Cr₂O₃ scale, as typically shown in Figure 2c,d. Here, the Fe-Cr peaks were strong owing to the thinness of the oxide scales. In Fe-(19, 28, 37)Cr alloys, Cr was dissolved in the α-Fe matrix.

Figure 3 shows the EPMA analytical results on the scales formed on Fe-9Cr after corrosion at 700 °C for 70 h. The oxide scales that formed after corrosion in N₂ and N₂/H₂O were about 90 and 140 μm thick, respectively. The scale morphology and elemental distribution in N₂ gas were similar to those in N₂/H₂O gas, as shown in Figure 3, indicating that the same oxidation mechanism operated in N₂ and N₂/H₂O gas. Voids were sporadically scattered in both oxide scales, below which the oxygen-affected zone (OAZ) existed. Voids formed owing to the volume expansion during scaling, hydrogen released from the water vapor, and the Kirkendall effect arose due to the outward diffusion of cations during scaling. In both oxide scales, the outer layer consisted of iron oxides, while the inner layer consisted of (Fe,Cr) mixed oxides. This indicated that Fe²⁺ and Fe³⁺ ions were more mobile than Cr³⁺ ions. The oxidation in N₂ and N₂/H₂O gas was mainly controlled by the outward diffusion of iron ions through the inner (Fe,Cr) mixed oxide layer. Iron oxidized preferentially in N₂ and N₂/H₂O gas because iron is the base element and its oxide, FeO, is a non-stoichiometric compound with a relatively fast growth rate.

Figure 4 shows the EPMA analytical results on the scales formed on Fe-37Cr after corrosion at 700 °C for 70 h. The oxide scales that formed after corrosion in N₂ and N₂/H₂O were about 0.6 and 1.1 μm thick, respectively. In N₂ and N₂/H₂O gas, the Cr₂O₃ scale formed (Figure 2c,d), in which Fe was dissolved (Figure 4). The complete dissolution of Fe₂O₃ in Cr₂O₃ is possible, because Cr₂O₃ and Fe₂O₃ have the same rhombohedral structure [8]. Like Fe-37Cr, Fe-(19, 28)Cr also formed a thin Cr₂O₃ scale containing some Fe when they corroded in N₂ and N₂/H₂O gas. Once the thin but protective Cr₂O₃ scale formed, the outward diffusion of iron ions was suppressed so that good corrosion resistance was achieved.

In N₂/3.1%H₂O/2.42%H₂S gas, Fe-(9-37)Cr alloys could not form Cr₂O₃, and corroded fast, as typically shown in Figure 5. The scales formed on Fe-(9, 19, 28, 37)Cr alloys consisted primarily of the outer FeS layer (Figure 5a), and the inner Cr₂S₃ layer containing some FeCr₂S₄ (Figure 5b). Since FeS grows fast owing to its high non-stoichiometry, outer FeS grains were coarser than the inner (Cr₂S₃, FeCr₂S₄) mixed grains. In Figure 5c, cracks propagated inter- and trans-granularly due mainly to the excessive growth stress generated in the thick outer scale. The scale shown in Figure 5d was about 100 μm thick, and had cracks and voids. A small amount of Cr was dissolved in the outer FeS layer (Figure 5e). The preferential sulfidation of iron in the outer FeS layer decreased the sulfur potential underneath, and thereby increased the oxygen potential in the inner Cr₂S₃-rich layer, leading to the incorporation of oxygen in the inner Cr₂S₃-rich layer. FeS is a p-type metal-deficit compound, which grows fast by the outward diffusion of Fe²⁺ ions [5,9,10]. Its defect chemical equation is as follows.

$$1/2{\mathrm{O}}_2={\mathrm{O}}_{\mathrm{o}}+2{\mathrm{h}}^{·}+{\mathrm{V}}_{\mathrm{Fe}}^{"}$$

(3)

here, O_O, h^· and ${\mathrm{V}}_{\mathrm{Fe}}^{"}$ mean the O atom on the O site, the electron hole in the valence band with a + 1 charge, and the iron vacancy with a − 2 charge. The defect chemical reaction for the dissolution of Cr₂S₃ in FeS is as follows.

$${\mathrm{Cr}}_2{\mathrm{S}}_3=2{\mathrm{Cr}}_{\mathrm{Fe}}^{·}+{\mathrm{V}}_{\mathrm{Fe}}^{"}+3{\mathrm{S}}_{\mathrm{s}}$$

(4)

Hence, the doping of Cr³⁺ ions would increase the concentration of iron vacancies, leading to the enhancement of the FeS growth. Oxygen was incorporated in the inner Cr₂S₃-rich layer (Figure 5e). However, no oxides were detected in Figure 5b, because their amount was small or oxygen was dissolved in the sulfide scales. Grains in the inner layer were fine owing to the nucleation and growth of Cr₂S₃, together with some FeCr₂S₄ and probably some oxides. In N₂/3.1%H₂O/2.42%H₂S gas, Fe-(9, 19, 28, 37)Cr alloys sulfidized preferentially owing to the high sulfur potential in the test gas.

4. Conclusions

When Fe-9Cr alloy corroded at 700 and 800 °C in N₂ and N₂/3.2%H₂O gas, thick, porous oxide scales formed, which consisted of the outer iron oxide layer and the inner (Fe,Cr) mixed oxide layer. Under the same corrosion condition, Fe-(19, 28, 37)Cr alloys formed thin, dense, protective Cr₂O₃ oxide layers, in which iron was dissolved to a certain extent. In N₂/3.1%H₂O/2.42%H₂S gas, Fe-(9, 19, 28, 37)Cr alloys corroded fast, forming thick, non-adherent, fragile scales, which consisted of the outer FeS layer and the inner Cr₂S₃ layer containing some FeCr₂S₄. The preferential sulfidation of Fe-(9, 19, 28, 37)Cr alloys in the H₂S-containing gas was responsible for the poor corrosion resistance of Fe-(9, 19, 28, 37)Cr alloys.