Effect of Oxidation with Lanthanum Sol–Gel Coating on the 330Cb Carburization at 900 °C

Abstract

Corrosion and carburization are the main reasons for the degradation of mechanical properties of conveyor units. In order to improve alloy carburization resistance, an adherent oxide scale can act as a carbon diffusion barrier. Oxidation of Alloy 330Cb in air at 900 °C was performed in order to grow a protective chromia scale. Nevertheless, after oxidation some scale spallation occurred during cooling. Then, argon-annealed lanthanum sol–gel coatings were applied in order to improve the oxide scale adherence. This led to a thinner chromia scale. The convoluted scale structure suggested that a mixed diffusion process was induced by lanthanum. After carburization during 22 h at 900 °C under Ar/10 % CH4, the Cr2O3 scale was converted into carbides and it appears that is necessary to have a thick oxide scale to get a good protection.