Effect of Iron Sulfide on Oxidation of Si-Bearing Steel Prior to Reducing Anneal during Galvanizing Process

Effect of pre-treatment by sulfate solution on the oxidation of Si-bearing steel prior to reducing anneal was investigated to improve the galvanizability of the steel in galvanizing. Ammonium sulfate and diluted sulfuric acid accelerate the oxidation of the Si-bearing steel: they produce the same amount of iron oxide as a steel without silicon. Silicate containing iron and manganese, which retards the outer diffusion of iron ions during oxidation, is continuously formed at the interface between an iron oxide film and the steel without ammonium sulfate. FeS, generated by the reaction of SO2 from (NH4)2SO4 and steel, is observed in the iron oxide film near the interface between the iron oxide film and the steel treated with ammonium sulfate. Iron ions diffuse through FeS after this reaction. Ammonium sulfate accelerates the oxidation of the Si-bearing steel not only because FeS acts as the outer diffusion path of iron ions but also because the formation of silicate is delayed due to the generation of SO2.