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王 “ ‘ · 五丫 了 。 。 二 ‘ 了 孟 。 二 日 。 ， 。 一 一 一 ， “ 。 五 一 。 ， 五 一 五 ， 丫 “ 一 ， 一 了 夕。 · 二 户。 二 ‘ 居 子 了 广 口 书 广招 、 。 ， 斌 ‘ 之 几。 。 。 ， · 甜 。 嘴 飞 ， ， 尹 伙 ‘ ‘ 。 月 吃 同尸脚尸翻沪 尸衬， ， 创‘ 」‘ 压口口 叮古，勺 翔尸卜 一 曰 ， 沙 」 门 盯 长维 · ，，二 卜 。 。 · 乞厂 ” 盆 】 ” 厂， 差 ， ‘ 一 城卜门口 ， 冲 山 亏， 竺亡二玉舀州卜 卜‘ 日，州 目翌知 片只日卜分山 一，一一 － 已。为 卜击门泣 ︸ ，。 、、 。 嗡信 言 ， 卜 二 。 二 ， ， 丫 一 “ 丫。 ， 。 一 。 了 ， 乓 尸