Simultaneous aluminizing and chromizing of steels to form (Fe, Cr)3Al coatings

A cementation pack involving halide activators and elemental Al and Cr powders has been used to achieve the codeposition and diffusion of aluminum and chromium into low-alloy steels. A two-step treatment at 925 degrees C and 1150 degrees C yields dense and uniform ferrite coatings of about 400-mu m thickness, with surface compositions of approximately Fe3Al plus several percent Cr. The two-step heating schedule prevents the formation of a blocking chromium carbide at the substrate surface. An attempt wins mane to add a trace of Ce to the Al+Cr content of the coating by introducing Ce oxide into the pack, bur there is no evidence that this doping was achieved. Upon cyclic oxidation in air at 700 degrees C, the coated steel exhibits a negligible 0.085 mg/cm(2) weight gain for 1900 one-hour cycles. Virtually no attack was observed on coated steels tested in a simulated boiler atmosphere at 500 degrees C for 500 hr. But coatings with a surface composition of only 8 wt.% Al and 6 wt.% Cr suffered limited sulfidation attack in the simulated boiler atmosphere at temperatures higher than 500 degrees C for 1000 hr.