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卜 ， 了 一 。 ， 一 ， 一 一 一 ， ‘ 洲 洲产 ， 。 · 了 ‘ 。 功 · 产 了 ， · 劣 · ， ， 扭 ， ， ， 一 · · ， ， 砰 ， ， ， ， ， · 口 爪 · · ， ， ， · · ， 尹