Pitting Corrosion Currents on Steel in Relation to the Concentration of the Inhibitive and Corrosive Anions under Natural Corrosion Conditions

The changes with time in the pitting corrosion current density on a steel electrode with the concentration of both the inhibitive anions (CrO₄ ²⁻, HPO₄ ²⁻ and WO₄ ²⁻) and aggressive anions (Cl⁻, Br⁻ and I⁻) was followed using a simple electrolytic cell. In chromate solutions the pitting corrosion currents started to flow after an induction period, τ, which varied, in one and the same inhibitor solution, with the concentration of the aggressive anion, according to the expression: log τ =a−b log C_agg.

The pitting corrosion currents finally reached steady-state values which depended on the type and concentration of both the inhibitive and corrosive anions. At a constant inhibitor concentration, the corrosion current varied with the concentration of the aggressive anion according to: log i_corr = a_l + b₁ log C_agg, and with constant aggressive ion concentration according to: log i_corr =a₂ - b₂ log C_inh Comparison was made between the experimentally obtained values of a_l (a₂ and b₁ (b₂), and the corresponding computed values. The a₂ values show that the corrosivity of the three aggressive anions decreases in the order: Cl⁻ > Br⁻ > I⁻; on the other hand the inhibitive efficiency of the inhibiting anions decreases in the order: CrO₄ ²⁻ > HPO₄ ²⁻ > WO₄ ²⁻.