PASSIVATION OF IRON-BASED ALLOYS IN PARA-TOLUENE SULFONIC-ACID

Potentiodynamic and surface analysis techniques were applied to investigate the passivation Of metals in para -toluene sulfonic acids (PTSA). The diffusion-limiting current density was used to calculate the ferrous ion concentration which causes the precipitation of a pseudopassive film. From the surface analysis data, the Cr/Fe ratio in the passive film increases in order of 430, 304, and 316 stainless steels. To analyze the differences, a correction was made to the data so that the observed difference in the oxide film composition is not attributed simply to the Cr and Fe contents of the alloys. This method also was applied to correct the Ni/Fe ratio in the passive film. We conclude that the preferential dissolution of Ni and Fe leads to a Cr enrichment in the passive film.