Comparison of the high-temperature oxidation of uncoated and CVD-FBR aluminized AISI-304 stainless steel

Aluminide coatings were obtained by means of the CVD-FBR technique at 525degreesC for 1.5 hr under a reactive-gas mixture composed of 10 vol.% H-2 + 1 vol.% HCl, the rest being Ar as the fluidizing inert gas. Subsequent heat treatment at temperatures to 900degreesC was conducted to enhance interdiffusion of the components. As a result, substoichiometric Ni-Al phases were found to form. Uncoated and coated plus heat-treated specimens were then subjected to 950degreesC oxidation for up to about 200 hr under isothermal conditions. The coated plus heat-treated specimens had much lower oxidation rates than the uncoated ones because of the formation of protective alumina scales. Loss of protective behavior occurred only by spalling of the alumina scales upon cooling from the oxidation temperature. The higher oxidation rates of uncoated specimens ha e been attributed to nodule formation and minor subsequent spalling of the oxide scale.