Localized Corrosion of Carbon Steel in Simulated Concrete Pore Solution: Influence of Chloride Ion and Temperature

Passivity breakdown on carbon steel, which is  the reference candidate material for fabricating high-level nuclear waste containers in Belgium, has been studied and the data are interpreted in terms of the point defect model (PDM).  The near normal distribution in breakdown potential, measured in deaerated simulated concrete pore water solution (pH  12.5) at different chloride concentrations and temperatures are in satisfactory agreement with prediction of the PDM.  An increase in temperature from 25°C to 85°C results in a decrease in localized corrosion resistance according to a linear relationship between the critical breakdown voltage, E_b, and log(chloride activity) for all chloride concentrations.  The linear dependence of the passivity breakdown potential on the square root of potential scan rate (ν^1/2), as predicted by the PDM, yields an estimate of the critical areal concentration of condensed cation vacancies at the metal/film interface (ξ<=5.45×10¹⁴cm^-2 at 85^oC and <=1.07×10¹⁴cm^-2 at 55^oC ) that leads to the passivity breakdown.  The experimental distributions in the breakdown potential for carbon steel at 55^oC and 85^oC are in satisfactory agreement with the relevant calculated distributions.

Acknowledgment: The authors gratefully acknowledge the support of this work by ONDRAF-NIRAS of Belgium.