Influence of water vapor on the oxidation behavior of aluminized coatings under low oxygen partial pressure

FeCrNi alloy after aluminizing was oxidized at 1000 °C in dry and humid (2.23 vol.% water) H2. Experimental results showed that H2 promotes the formation of θ alumina and its transformation to α alumina. The morphology of surface alumina coating does not change significantly, but the oxidation rate of the aluminized layer accelerates by the addition of water vapor. As a result, more cracks are found beneath the alumina layer when water vapor is present. The addition of water vapor seems having a favorable effect on the selective oxidation of Al and concentration of oxygen vacancy in the aluminized alloys.

► Oxidation of a pack cementation coating of Al on FeCrNi alloy in dry and humid H2.
► H2 promotes the formation of θ alumina and its transformation to α alumina.
► Water vapor induces cracks in the aluminized coating beneath the alumina layer.
► Water vapor accelerates the oxidation of the aluminized coating in H2.