Pitting Corrosion of Zirconium in Chloride Solution

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Abstract
Investigations were conducted on the pitting corrosion of zirconium in chloride solution under potentiostatic conditions. Both stable and unstable pitting were observed. Unstable pitting indicated by anodic current oscillations resulting from repetitive new pit nucleation and repassivation were all ultimately repassivated with time below the stable pit nucleation potential. Only stable pitting was considered in this study. It was observed that although the pit nucleation potential decreases (becomes more active) with an increase in chloride ion concentration, it does not follow a linear relationship with log [Cl−] in NaCl as well as in HCl solutions. The pit nucleation potential was not affected up to a certain pH value, but it shifted thereafter in the noble direction with a further increase in pH and, finally, no pitting corrosion was observed, depending on the bulk chloride ion concentration. An increase in temperature shifts the pit nucleation potential in the noble direction. The activation energy for pit nucleation was found to range from 3.2 to 10.85 kcal/mol. The reaction order leading to the pit nucleation process was 2.0 and 2.4 in low and high chloride concentrations, respectively, and 1.0 at intermediate concentrations. The IR drop of a few mV inside the pit, along with hydrogen gas evolution and black metal powder formation, suggest that the hydrogen evolution results from the reaction of bare surfaces of the ejected metal particles with water while the pits are still in the passive potential range. Pit growth is mass transport limited.

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