The influence of phosphate on repassivation of 304 stainless steel in neutral chloride solution

Electrochemical and ESCA techniques have been used to study the influence of phosphate on passivation and repassivation of 304 stainless steel exposed for two hours at room temperature to neutral chloride containing solutions: 0.2 M NaCl, 0.003 M citric acid with and without 0.014 M Na2HPO4. Exposed to the phosphate-free solution the passive film (17 Å thick) formed on the surface of the steel consists of an outer 6 Å thick (Fe0.6IIICr0.4III)(OH)3 layer and an inner 11 Å thick. (Fe0.3IIFe0.2IIICr0.5III)2O2.7 layer. In the phosphate-containing solution the overall thickness of the film becomes 22 Å. The inner oxide layer is found to be 8 Å and its composition is about the same as the inner oxide layer formed in the phosphate-free solution. The outer layer formed on the surface in the phosphate-containing solution is 14 Å thick and it consists of CrPO4 · 2H2O mixed with Fe-Cr-hydroxide. The dynamic polarisations show that phosphate has no effect on the current either in the active range or in the passive range. Phosphate does not influence the pitting potential of the steel; in the two solutions the pitting potential is 300 mV (sce). Chronoamperometric recordings detect higher frequency of formation of metastable pits during exposure to the phosphate-free solution than to the phosphate-containing solution. The repassivation potentials of the steel in the phosphate-free and in the phosphate-containing solutions are −100 mV and 0 mV (sce), respectively. It is concluded that the positive effect of phosphate is that it enhances the repassivation of initiated pits. At pit initiation the acidity in the pit volume increases due to hydrolysis of hydrated cations. With phosphate present the increase in acidity will be limited due to formation of phosphoric acid. The condition for repassivation is thereby improved.