Corrosion of Post-Tensioned Tendons with Deficient Grout, Part 1: Electrochemical Behavior of Steel in Alkaline Sulfate Solutions

Severe corrosion recently documented in Florida post-tensioned bridges were related to grout segregation that created deficient grout with characteristics of having low chloride content, high sulfate concentration, high pore water pH, and high moisture content. The role of elevated sulfate concentrations in the development of steel corrosion in deficient grout has not been elucidated. As first approach to determine the corrosion mechanism of steel strand in deficient grout, the objective of the research, described here, was to evaluate the role of sulfate ions on the corrosion of steel in alkaline solutions. Steel coupons were exposed to two different alkaline solutions (SPS) simulating pore solutions with pH 12.6 (SPS1) or 13.3 (SPS2) with varying sodium sulfate levels (early fixed content and later increments). Electrochemical experiments included the steady-state condition and potentiodynamic polarization tests. Solution pH was found as an important parameter in the corrosion development of steel in sulfate SPS solutions. In the highly SPS2 solution (pH 13), sulfate ions did not impair passive film growth or stability. In the moderate SPS1 solution (pH 12.6), early fixed sulfate presence could be aggressive by impairing passive film development; however, even high levels of later increments of sulfates cannot depassivate steel. The instability of the passive film in alkaline sulfate solutions resulted in pitting corrosion of steel.