Kinetic and Microstructural Analyses of Chromia Forming Alloys during Cyclic Oxidation

摘要

Three Fe-Cr model alloys(Cr:9,17,and 25 wt%) were subjected to cyclic oxidation in Ar-10%H_2-5%H_2O(p_(o_2)=3.6×10^(-22) atm),Ar-20%O_2 and Ar-20%O_2-5%H_2O atmospheres at 700℃.In general,increasing alloy chromium levels reduced the oxidation rate.At high p_(o_2),in the absence of water vapour,the 9%Cr alloy exhibited breakaway oxidation at the beginning of the reaction.However, for chromium levels≥17%,no breakaway oxidation occurred,and the weight gain was very low.Adding water vapour to Ar-20%O_2 accelerated the oxidation of alloys containing 17%Cr or less.However,this accelerating effect was not evident for high chromium alloys.At low p_(o_2),breakaway oxidation of alloys with chromium concentration≤17%occurred after 300 cycles exposure.High chromium content alloys did not undergo significant oxidation.XRD analysis showed that for low chromium content alloys(Cr≤17%), spinel and wustite were formed in the low p_(o_2) gas,while in the high p_(o_2) gas,hematite,magnetite and spinel were produced.In the case of high chromium alloys,only Cr_2O_3 was detected.Metallographic examination showed that the spinel was present in the form of internal precipitates.A dense chromia layer was observed at alloy chromium levels above 25%.The reasons for the difference in oxidation behaviour at different oxygen potentials,and the effect of water vapour are discussed in connection with the microstructure of the oxides.