Effect of Zr Additions on the Oxidation Kinetics of FeCrAlY Alloys in Low and High pO₂ Gases

Alloys of nominal composition Fe-20Cr-5Al-0.05Y (mass pct) with and without a 0.03 pct Zr addition were isothermally oxidized at temperatures of 1473 to 1573 K (1200 to 1300 °C) in both flowing Ar-O₂ and Ar-H₂-H₂O. Continuous thermogravimetric (TG) measurements of oxygen uptake showed the rates to be faster in both gases for Zr-containing than for the Zr-free alloy. Scale growth on the Zr-bearing alloy by inward oxygen diffusion is accelerated by the fine alumina grain size and the formation of elongated zirconia particles in the two-phase scales. Zirconia formation ceases when the supply of metal in the thin alloy substrate is exhausted, and subsequent single-phase alumina growth proceeds at a slower rate. A simple diffusion model is shown to account semiquantitatively for zirconium enrichment in alumina scales. The presence of hydrogen modifies both oxygen transport in the scale and its permeability in the alloy.