Measurements of pitting corrosion currents of zinc in near neutral media

Using a simple model cell the susceptibility of the zinc electrode to pitting corrosion by SO₄²⁻, SO₃²⁻, S₂O₃²⁻ and S²⁻ anions were examined in naturally aerated carbonate solutions. It was found that, pitting started after an induction period, τ, which depended on the type and concentration of the aggressive and passivating anions. The pitting corrosion current increased with time until steady state values were attained. These values depended on both the type and the concentration of the passivating and pitting anions. For the same concentration of the passivating anions, the corrosion current varied with the concentration of the aggressive anion according to the relation: logi_pit.=a₁+b₁logC_agg. At a constant concentration of the aggressive anion, the corrosion current varied with the concentration of the passivating anions according to: logi_pit.=a₂−b₂logC_pass. The constants a₁ (a₂) and b₁ (b₂) were determined for all the systems studied. From the values of a₁ the corrosivity of the sulphur-containing anions is found to decrease in the order SO₄²⁻>SO₃²⁻>S₂O₃²⁻>S²⁻.