Simultaneous Aluminizing and Chromizing of Steels to Form (Fe, Cr)3Al Coatings

Abstract

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°C and 1150°C yields dense and uniform ferrite coatings of about 400-μm 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 was made to add a trace of Ce to the Al+Cr content of the coating by introducing Ce oxide into the pack, but there is no evidence that this doping was achieved. Upon cyclic oxidation in air at 700°C, the coated steel exhibits a negligible 0.085 mg/cm2 weight gain for 1900 one-hour cycles. Virtually no attack was observed on coated steels tested in a simulated boiler atmosphere at 500°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°C for 1000 hr.