The pitting of mild steel in phosphate-borate solutions in the presence of sodium sulphate

The localized corrosion of mild steel in neutral buffered solutions containing Na₂SO₁ was studied using potentiostatic and potentiodynamic techniques complemented by scanning electron microscopy. The breakdown potential changes linearly with the logarithm of Na₂SO₄ concentration. The logarithm of the induction time for pit initiation decreases linearly with the reciprocal of the applied potential, until the inhibition potential region is reached. Open circuit potential decay measured shows that the apparent thickness of the passive oxide film decreases at potential values more negative than the breakdown potential. The pitting current density is close to the diffusion limiting current of iron dissolution through a FeSO₄ film. Results suggest that two-dimensional FeSO₄ salt islands are formed before pitting, as it has been postulated for iron in halide-containing solutions.