Development and Oxidation of a RexOy- Modified Chromizing Coating

Novel reactive element oxide RexOy (Re=Ce,Y)- modifed chromizing coatings have been developed by the first step of co-electrodeposition of Ni with CeO2 or Y2O3 particles on a mild carbon steel and the subsequent step of diffusional chromizing on electrodeposi ted Ni-CeO2/or Y2O3 composite films using a halide-activated pack-cementation method at 1120 o C for 4 h. By comparison, the coating processing was also performed on the carbon steel and on an electrodeposited Ni film without RexOy addition. The thickness of the chromizing coating obtained was approximately 75μm on the carbon steel and in a range of 55~60 μm on the other electrodeposited films with and without RexOy. The former chromizing coating was mainly composed of α(Cr,Fe) phase and the latter coatings mainly composed of α(Cr, Ni) phase. All chromizing coatings were double- layered. However, the outer layer of coatings on the composite films was thicker and finer grained, in which the distribution of the RexOy particles was observed. The oxidation at 940 o C showed that the RexOy-modifed chromizing coatings were profoundly oxidation resistant as compared to the other two RexOy-free coatings. It is proposed that the better performance of the RexOy-modifed chromizing coating was associated with dispersion of the RexOy particles, which exerted reactive element effect on the growth of chromia scale.