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、 ，、 ， 从 ， ， 五 ， 五 ， ， 五 ， 。 一 ， 五 。 。 五 · ， · ， ， ， · ， · ， 。 · · ， 一 二 · ， · ， 。 。 ， ， 从 。 ‘ 几 。 、 、 、 、 。 。 一 叭 · 五 ， 盯 。 ， 厂 。 · · 五 ， 牡 。 住