Initiation of localized steel corrosion in alkaline sulfate solution

The role of sulfate ions on corrosion of steel in alkaline solutions (including cementitious materials) has been contested in the literature due to the effects of the many exposures, material and test conditions. Recent observations of steel corrosion in cementitious grouts in post-tensioned bridges coinciding with elevated sulfate-ion concentrations have prompted the development of material specifications concerning segregated grout. Electrochemical testing of steel in alkaline sulfate solutions were conducted to elucidate the impairment of the passive layer. The increase in the anodic current exchange density at higher sulfate ion concentrations resulted in the development of electronegative potentials and high corrosion rates. The numerous distinct electrochemical noise events in solutions with > 1,300 ppm SO42− were characteristic of metastable pitting and pitting. The results of the testing identified sulfate levels that impair the passive layer and provided support for recent specifications that limited sulfate ion concentrations in deficient grout.