Steam effects on cyclic oxidation of chromia forming alloys

High temperature oxidation of Fe-Cr alloys with Cr levels of 9-30 wt% has been investigated by using isothermal and cyclic experiments at 600, 700 and 950°C. The mixed gases Ar-20%O2, Ar-20%O2H20, Ar-5%O2-20%H20, Ar-10%H2-5%H20 and Ar-10% H2-20%H20 at total pressure of I atm were used. The effect of lanthanum alloying on Fe-Cr alloy oxidation was also studied. In general, increasing alloy chromium levels reduced the oxidation rate. In isothermal oxidation at 700°C, a protective chromia layer was formed in dry oxygen for Fe9Cr. Alloys with Cr≥17% formed protective chromia layers in all gas mixtures. In thermal cyclic oxidation, at high Po2, the 9%Cr alloy exhibited breakaway oxidation at an early stage of the reaction at all reaction temperatures. However, for Cr≥17%, no breakaway oxidation occurred, and the weight gains were very low at temperatures of 950 and 700° C. At 600°C, breakaway oxidation was observed on all Fe-Cr alloys in Ar-20%O2 and Ar-20%O2-5%H20 gases due to scale spallation and the low alloy Cr diffusivity which slows the chromia scale re-healing. Introducing water vapour lo Ar-02 and Ar-H2 accelerated the oxidation of alloys containing ≤17% Cr. However, this accelerating effect was not evident for Fe25Cr alloy at 700 and 950°C. At low Po2, breakaway oxidation of alloys with Cr≤17% occurred, very early in Ar-10%H2-20%H20 and after 300 cycles in Ar-IO%H2-5%H20. The addition of lanthanum to Fe9Cr and Fe 17Cr led to increased oxidation rates due to enhanced inward oxygen transport, leading to faster scaling, and rapid consumption of chromium at the alloy surface in high H20containing gases in Ar-5%O2 and Ar-10%H2 at 700°C. However, the presence of lanthanum in Fe25Cr-0.09La retarded the growth of both Fe-rich oxide and chromia, which in turn decreased the oxidation rate