Pitting and pitting inhibition of iron in sodium sulphate solutions

The anodic behaviour of high purity iron in 0.5 M sodium sulphate solutions was studied. Experiments were made in both acid and alkaline solutions (pH 2.7, pH 9.0, pH 10.0 without buffers; and pH 9.2 with borate buffer). Anodic polarization curves, and surface scratching experiments, showed pitting potentials in 0.5 M Na₂SO₄ pH 9.0 and pH 10.0 solutions. Their values were very close to the corrosion potential obtained in a 0.5 M Na₂SO₄, pH 2.7, pit-like solution. The pitting potential in a borate buffered 0.5 M Na₂SO₄ solution was 50 mV higher than that in the unbuffered solutions. The pitting inhibition potential measured in a 0.5 M Na₂SO₄ solution, pH 10.0, was very close to the passivation potential found in the pit-like solution. All these facts can be explained by the localized acidification mechanism for pitting. The pitting potential is the minimum potential at which an acidified solution can be produced and maintained in contact with the dissolving metal. Similarly the pitting inhibition potential is the electrode potential at which the metal becomes passive in the pit-like solution.