Cyclic Oxidation of Chromia Forming Alloys in Low and High PO2 Environments
Cyclic oxidation experiments were performed on three Fe-Cr model alloys (Cr: 9, 17, and 25 wt %) in Ar-10%H2-5%H2O (pO2 = 4.6 x 10-22 atm), Ar-20%O2 and Ar-20%O2-5%H2O atmospheres at 700oC. Kinetic investigation showed that in general, increasing Cr content reduced the oxidation rate. At high pO2, in the absence of steam, the 9%Cr alloy exhibited, breakaway oxidation the beginning of the reaction. However, for Cr ≥ 17%, no breakaway oxidation occurred, and the weight gain was very low. Adding steam to Ar-20%O2 accelerated the oxidation rate of alloys with Cr ≤ 17%. However, this accelerating effect was not evident for high chromium content alloys. At low pO2, breakaway oxidation of alloys with chromium levels Cr ≤ 17% occurred after 300 cycles exposure. High Cr content alloys did not undergo significant oxidation. XRD analysis showed that for low Cr content alloys (Cr ≤17%), spinel and wustite were formed in low pO2 gas, while in high pO2 gas, hematite and spinel were produced. In the case of high Cr content alloys, only Cr2O3 was detected. Metallographic examination showed that the spinel was present in the form of internal oxides. A dense chromia layer was observed when at alloy chromium levels above 25%. The reasons for the difference in oxidation behaviour at the two oxygen potentials, and the effect of water vapour are discussed.
Mr. Norinsan Kamil Othman
School of Materials Science & Engineering
University of New South Wales
Sydney NSW 2052 Australia
High Temperature Corrosion of Fe-Cr Alloys
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