Effects of vanadium (V and IV), molybdenum (VI), and lead (II) on the oxidation kinetics of steel scrap.
Surface impregnation of steel strips with small amounts of the oxides of vanadium (V), molybdenum (VI), and lead (II) increased their rate of oxidation in carbon dioxide atmospheres. The introductionof the accelerator species was effected by pickling of the oxidizable charge for short periods in moderately concentrated aqueous solutions of the sodium salt of the oxyacids. By contrast to the pentavalent vanadium in the metavanadate, the tetravalent state in vanadyl sulfate did not accelerate oxidation. X-ray fluorescence examination of the pickled steel strips indicated the presence of small amounts of the accelerator on the surface layer. The degree of oxidation acceleration could be inversely correlated with the melting point of the promoter oxide. The results are explained on the basis of the hydrolytic decomposition of the pickling agent with subsequent slow interaction with the metal surface, coupled with the tendency of vanadate to form polyanions. The data not only serve to elucidate the mechanism by which catastrophic oxidation takes place, but also show that the promoter species need not be alloyed with the oxidizable metal; surface impregnation by minute amounts of the promoter can still produce catastrophic effects. The oxidation of steel can be more than doubled by this procedure.