Activation of Passivated Steel in Sodium Chromate Solution Containing Sodium Chloride

It is shown that, in aqueous solution containing Na₂CrO₄ and NaCl, activation of steel takes place by localized corrosion at a characteristic potential known as the breakthrough potential. The breakthrough potential is related to the chloride ion concentration by the equation 1n C = K₀ – K₁ ψ where C is the concentration of chloride ions and ψ is the metal solution potential and K₀ and K₁ are constants. The E-t curve obtained in Na₂CrO₄-NaCl solution is similar to that obtained in NaOH-NaCl solution in that the potential rises to a peak value and then falls once for all to a negative value. Potential oscillations observed in Na₂HPO₄ - NaCl and Na₂B₄O₇ - NaCl solutions are not observed in Na₂CrO₄ - NaCl solution. One or more corrosion spots are formed at the peak potential in galvanostatic experiments, and these grow with time of application of current. The breakthrough potential is unaffected by an increase in temperature, unlike NaOH-NaCl solution.