High Temperature Corrosion of Fe-25Cr and Fe-17Cr-1.5Si-0.5Al Alloys in Oxidizing and Sulfidizing Enviornments

D0I:10.13374/i.issm1001-053x.1989.06.035 第11卷第6期 北京科技大学学推 Vol.11 No.6 1989年11月 Journal of University of Science and Technology Beijing No7.1989 High Temperature Corrosion of Fe-25Cr and Fe-17Cr-1.5Si-0,5Al Alloys in Oxidizing and Sulfidizing Enviornments' Wang C@irong(王才荣)，。Gong Maoxiu(公茂秀)， Zhang Wengi(张文奇)，Zhu Rizhang（来日彰)。 ABSTRACT:The simultaneous oxidation and sulfidation of Fe 25Cr and Fe- 17Cr-1.5Si-0.5Al alloys was studied at 1023K and 1223K in Ha-H,O-H,S gas mixtures.The kinetic boundary which indicates the transition from oxide to sulfide has heen found in these two alloys.The critical oxygen partial pressures of Fe17Cr1.5Si0.5Al alloys were systematically lower than those of Fe-25Cr alloy.The reaction kinetics were measured by the stainless steel spring balance, and the reaction products were characterized by X-ray diffraction and scanning electron microscopy.The reaction rate usually decreased with the increase of the oxygen partial pressure at the constant sulfur partial pressure.The exista- nce of silicon plays an important role to suppress the sulfidation of Fel7Cr alloy. KEY WORDS:high temperature corrosion,Fe-Cr alloy,oxidizing,sulfidizing In technical processes,metallic materials are often exposed to the gases con- taining more then one oxidant.The combustion gases of fossil fuels or gases formed by coal gasification processes contain compounds of element O,S,C. Generally,the sulfidation is the main reason for the degradation of metallic materials at high temperatures in industrial environments.The understanding of the simultaneous sulfidation and oxidation,or the suppression of sulfide forma- tion,in particular,is of technical importance. Recent investigationst 1-51 have shown that all the iron-nickel and cobalt-based technical and model alloys exhibit in such atmospheres the existance of simulta- necus sulfidation and oxidation.In the simultaneous sulfidation and oxidati.n processes there exists a transition from oxide to sulfide.This transition is also Manuscript Received Jan.24,1989 ..Dept.of Surface Science and Corrosion Engineering ···Shandong Institute of Metallurgical Design-Rescarch 604

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c.lled the kinetic boundary!6).The pure sulfidztion and pure oxidation has been found when the oxygen partial pressure is greatly lower or higher than the critical one. The purpose of this paper is to examine the influence of addition agents in Fe-Cr alloy,mainly silicon,on the transition from oxide formation to the for- mation of sulfide-rich corrosion products. 1 Experimental The alloys were melted in a vacuum furnace,and the compositions of alloys were as follow (wt%):Fe25Cr:Fe-base,Cr-24.49,Si-0,Al-0,C-0.018,RE-0; Fc25Cr1.5Si0.5Al:Fe-base,Cr-16.75,Si-1.47,A1-0.46,C-0.10,RE-0.05. The latter was cast into rod of 20mm in diameter.Fe25Cr alloy was further hct rolled at 1050C after casting into rod cf 100mm in diameter.All the rods were annealed for 2.5h at 950C,and then machined into 12mm in diameter, finally cut into slice specimen of 612mm x 1.5mm.All the specimens were drilled near the end to make hcles of 1.2mm in diameter fer suspension.Each of the specimen was mechanically polished to obtain the smooth surface. The kinetic me:surements were carried cut in an apparatus,in which weigt of specimen were weighed by a stainless steel spring.High purity gases (HaS- 99.5%,H2-99.998%,Ar-99.997%)were used.The rates of the gases were monitored by flow meters to obtain the desired partial pressure of oxygen and sulfur. The water composition was made by bubbling argon with the constant flow rate through two water containers.The temperatures of the first container immersed in a thermostat controlled within +0.2C is higher than that of the second container immersed in another thermostat controlled also within t0.2.C. Thus the supersaturated water composition was made in the first container and condensed in the second container.and finally the saturated water compcsition at the same temperature with the second thermestat can be obtained. The temperature of the furnace was controlled with a Pt-Pt-10%Ru thermo- ccup!e within +2C.The experiments were carried out at 1023K and 1223K, respectively. The scales were examined by X-ray diffraction and scanning electron micro- scpy· 2 Experimental Results 2.1 Kinetics Corrosion kinetics of Fe25Cr and Fe17Cr1.5Si0,5A lalloys in H2-H20-H2S 605

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gas mixtures at a constant temperature of 1223K are shown in Fig.1a and Fig. 1b respectively.The sulfur partial pressure was kept constant at a value of 6.22x 10=5kPa and the oxygen partial pressure was varied from 0 to 2,07x 101s kPa. From above results,it can be known that there exists a kinetic boundary. This kinetic boundary corresponds to the transition from oxide formation to the formation of sulfide-rich corrosion products.Due to the kinetic reasons (such as diffusion and deffects etc.),it can not be concluded that there is no sulfide formation at all when the kinetic rate is relatively low.The kinetic boundary just shows that the sulfide-rich scales or protective oxide layer will be formed.In fact,the metastable sulfides are usually formed when the oxygen partial pressure is relatively high.The existance of metastable sulfide is the main reason that the kinetic boundary or transition from oxide to sulfide is not completely understood. Because the weight gain data can give a first indication of whether the oxi- dation or sulfidation had mainly occured,it is convenient to chose such critical partial pressures of oxygen and sulfur as the data of transition from oxide to sulfide.According to this discussion,the transition data of Fe25Cr and Fe17 Cr1.5Si0.5Al alloys are chosen as o=1.66x10*1kPa and Po,=2.07x10-4 2000 2000 %3.512210-7MPa 22171017MPa 1500 9351017MPa 1500 8,.16207Mpa ,1.98时1 1000 60 612n0 1000 MPa o20 500 500 40 80 120 160 200 ,min 40 80120 160 200 min (a) (b) Fig,1 Change of the weight gain of alloys with oxygen partial pressure (ps2=6.22x 10-5kPa) a.Fe25Cr alloy, b.Fe17Cr1,5Si0.5Al alloy kPa respectively,at 1223K and ps2=6.22x 10-5kPa.Such kind of data can be used to predict which alloy is more convenient for the industrial uses. The transition (or kinetic boundary)curves of Fe25Cr and Fe17Cr1,5Si 606

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0.5Al alloys at 1023K and 1223K are shown in Fig.2a and Fig.2b.From above results,it can be known that the critical oxygen partial pressure of Fel7- Cr1.5Si0.5Al alloy are systematical lower than those of Fe25Cr alloy.Thus it can be concluded that the Fe17Cr1.5Si0.5Al alloy is better than Fe25Cr alloy in suppresion of sulfidation at these conditions -5.0 -6.0 -5.0 -7,0 ●Fe-25Cr-1.5Si-0.5A里 -6.0 Fe-25Cr-1.5si-0.5A alloy OFe-25Cr alloy o Fe-25Cr alloy -8.0h -22.0-21.0 -20.0-19.0-18.0 -17.0-16,0 -15.0-14.0-13.0 .1og(Foz/101kPa) log(P /101kPa) (a) (b) Fig.2 The kinetic boundary of Fe25Cr and Fe17Cr1,5Si0.5Al alloys,a.1023K;b.1223K 2.2 X-ray Diffracticn The phases of various reaction products formed on Fe-25Cr and Fe17Cr1,5 Si0.5Al alloys were different and varied with the gas composition as can be seen in Table 1 and Table 2 which list some representative analysis.The scales formed on Fe25Cr alloy consisted mainly of CrxSy,FeCr2S,Cr2Os when the gas composition was near to transition value.When the oxygen partial pressure is greatly higher than the critical one,the main phase is Cr2O3 with a small amount of FeCr2O,and CrxSy.The scales formed on Fel7Cr1.5Si0.5Al alloy consisted mainly of CrxSy, FeCraS,CraO,and FeaSiO when the gas composition was near to the trasition value.When oxygen partial pressure is greatly higher than the critical one,the main phases are Cr2O,and Fe2SiO.with a small amount of FeCr2S,and CrxSy. Table 1 X-ray diffraction of corrosion products of Fe25Cr alloy Treatment conditions temp. po21ps2 period K kPa h Phase 1023 1.47×10-18/3.28×10-6 1.5 FeCr2S,CrxSy,Cr2O3 1223 1.51×10-12/1.92×10-3 2.0 CrxSy,Cr2O3,FeCr2S 1223 1.48×10-14/6.22×10-8 2.0 Cr203,CrxSy 607

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Table 2 X-ray diffraction of corrosion products of Fe17Cr1,5Si0,5Al alloy Treatment conditions temp. po2lpsa period K kPa h Phases 1223 0/1.92×10-3 2.0 FeS,FeCr2S 1223 9.74×10-16/6.23×10-52.5 CrxSy,Fe2SiO,Cr2O3,FeCraS 1023 9.78×10-19/2.2×10-8 1.5 Cr2Os,Fe2SiO,CrxSy 1023 7.32×10-18/9.12×10-4 2.0 Cr203,Fe2SiO,CrxSy,FeCr2S Fig.3 The sulfide with a small Fig.4 The sulfide and the oxide amount of oxide surface surface formed on Fe17Cr formed on Fe17Cr1.5Si 1.5Si0,5Al alloy.1223K, 0.5Al alloy.1023K,po2= p02=2.45×10-16kPa,ps2 2.02×10-18kPa,ps2=5.70 =6.22×10-7kPa,2h ×10-5kPa,2.5h Fig.5 The oxide with a small amount of sulfide surface formed on Fe17Cr1.5Si- C.5Al alloy.1023K,po2= 4.15×10-17kPa,ps2=9.11 ×10-6kPa,3h 608

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2.3 Surface Morphology As it is known,the sulfide usually grows several orders of magnitude faster than the oxide and the sulfide has more defects than the oxide.As the consequ- ence,in the surface of the scale,the formed sulfide often has the coarse and defective morphology,meanwhile the oxide has the smooth,thin and protective one.Therefore,it is possible to distinguish the oxides from the sulfides in their morphologies. As shown in Fig.3,the coarse sulfides which are mainly composed of FeCr2 S,and CrxSy are formed on Fe25Cr and Fel7Cr1.5Si0.5Al alloys in the simu- Itanecus sulfidation and oxidation as the oxygen partial pressure is near to the critical one.The smooth,thin and protective oxides are formed in the small islands of the surface and inner layer.When the oxygen partial pressure increa- ses,the sulifide particles become the small one,and the amount of sulfide decre- ases,as indicated in Fig.3,Fig.4,and Fig.5.From these figures,it can be known that the small sphere-like metastab'e sulfide can overgrow on the oxide substrate.In the relatively high oxygen partial pressure,the contineous oxide layer can form in the surface of the scale. EDX analysis shows that the composition of the oxide scale formed on Fe 25Cr alloy is mainly composed of Cr and O with a small amount of Fe,mean- while the composition of the oxide scale formed on Fel7Cr1.5Si0.5Al alloy is mainly composed of Cr,Fe,Si and O with a small amount of Al.The oxide Fe2SiO,can form in the relatively lower oxygen partial pressure than the oxide Cr203 3 Discussion The existence of kinetic boundary in Fe25Cr and Fel7Cr1.5Si0.5Al alloys indicates that the formation of the oxide scale can suppress the sulfide formation. The kinetic boundary depends mainly on the O and S partial pressures of the ) gas and on the compcsitions of the alloys.Such situation indicates the existence of the competeticn between sulfidation and oxidation. The experimental results show that when the corrosion rate is relatively low, there also exists sulfides in the surface of the scale.The formation of the meta- stable sulfide is mainly due to the locally variation of sulfur,oxygen and metal activities in the surface of the scale.The very fast nucleation and growth rate of sulfides is another important reason that the metastable sulfide can locally exist.Since the rapid growth of sulfide must require the existence of fast diffusion path,such as the existence of sulfide which connects the substrate and 609

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outer layer or the existence of crack or fissure in oxide,it can be concluded that the metal ionic diffusion through oxide layer is the rate controlling process when the corrosion rate is very low. The X-ray and SEM analysis of the outer surfaces of the scales confirms the coexistence of both oxides and sulfides.The occurrence of simultaneous sulfida- tion and oxidation is mainly due to the kinetic reasons and the variation of sulfur, oxygen and metal activities. The FeS phase had not been found in the simultaneous sulfidation and oxida- tion atmospheres in these two alloys.The occurrence of this phenomenon is influenced by the formation of oxide layer which will diminish the diffusion rate of iron ion in the scale.Besides,the reason that the formation of the metastable sulfides FeCr2S and CrxSy in the simultaneous sulfidation and oxida- tion of the alloys is also related to the diffusion processes of metal ions in the oxide layer. 4 Conclusion (1)The critical oxygen partial pressures of Fe17Cr1.5Si0.5Al alloy is syste- matically lower than those of Fe25Cr alloy.The Fe17Cr1.5Si0.5Al alloy exhi- bits more resistant then Fe25Cr alloy in the lower oxygen and high sulfur partial pressure atmospheres. (2)The scale analysis confirms the good influence of FeaSiO on the corro- sion resistance.The existence of silicon plays an important role to suppress the sulfidation. (3)The metastable sulfide can coexist with the oxide in the very larger oxygen partial pressure range.The formation of oxide and sulfide mainly depe- nds on the O and S pressure. (4)The experimental results show that the formation of oxide layer can suppress the sulfide formation. REFERENCES 1 Wang D,Douglass D L.Oxid.Met.,1983;20:111 2 Hindam H,Wittle D P.Corrosion,1982;38:32 3 Rao D B,Jacob K T.Met.Trans.,1983;14A:295 4 Papaiacovou P,Grabke H J.Werstpffe und Korrosion,1985;36:320 5 Huang TT,Lin Y C,et al.J Electrochem.Soc.,1984;131:219 6 Rahmel A,Schorr M,et al.Oxid.Met.,1987;27:199 610

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