Current distributions and dissolution mechanisms during localized corrosion of steels in alkaline environments

In situ corrosion investigations of iron were carried out in alkaline environments to determine the stages of corrosion that may be encountered with steels in concrete. The electrochemical and chemical processes taking place on Fe have been studied using x-ray absorption, current density mapping of the iron surface and artificial pits. The x-ray absorption near edge spectroscopy was used to observe the conversion of Fe to oxide in hydroxide solutions. The oxide formed was electrochemically active and changes in valence states between 2+ and 3+ were observed during electrochemical cycling between the passive state and hydrogen evolution. The oxide continued to thicken during the cycling with very little dissolution or any conversion back to the metallic state. Current density mapping in chloride/hydroxide solution showed that corrosion took place in highly localized areas confined by the formation of a corrosion product shell. The activity within the shell decayed on removing a supporting cathodic area limiting but reactivated on its replacement. Artificial pits were used to study the anodic processes taking place within the region confined by the corrosion product shell. The local pH and anodic behavior were similar to that observed in neutral or slightly acid bulk solutions. The dissolution within the artificial pits in alkaline bulk solutions was found to be sensitive to the presence of small-quantities of nitrite