Abstract
Although carbide coated steels prepared by immersion in a fused borax bath have a high corrosion resistance, local corrosion often appears on them. As one of preventive means against local corrosion, steels were plated before carbide coating in the present study. Investigations were made of the surface layers formed on chromium-, nickel-, and copper- plated steels immersed in a fused borax bath containing powders of Fe-V, V2O5+B4C, Fe-Nb or Cr, and of the corrosion resistance of the treated steels. A layer of VC, NbC or (Cr, Fe)7C3+(Cr, Fe)23C6 was found to form on chromium- and nickel- plated steels, by a reaction between V, Nb or Cr, dissolved in the borax and carbon atoms supplied through the plated layer from the steel matrix. In this immersion process, chromium- and nickel- plated layers transformed into a carbide layer of (Cr, Fe)7C3 and a solid solution layer of Fe-Ni-C, respectively. Double carbide layers of VC or NbC+(Cr, Fe)7C3 were observed on a chromium- plated steel. In Cr- plated steel immersed in a borax bath containing Cr powders, a (Cr, Fe)7C3+(Cr, Fe)23C6 layer was formed on a (Cr, Fe)7C3 layer, but these carbide layers were observed as a single layer under a microscope. In copper-plated steel, any carbide layer was not formed. Chromium- plating gave a protective effect on the generation of local corrosion of carbide coated steels in 36% HCl aqueous solution, but nickel- plating gave little effect.
