Electrochemical Behavior of Steel in Alkaline Sulfate Solutions with Low Level Chloride Concentrations

Abstract

Localized corrosion of steel strand in deficient PT grout have been attributed to the accumulation of grout bleed water and more recently to grout segregation with elevated sulfate concentrations [1-4]. Testing of steel in sulfate alkaline solution and grout with high sulfate concentration showed higher corrosion activity and impairment of passive film development. Anodic polarization results showed that in the alkaline solution (pH 12.6), early fixed sulfate presence could be aggressive by disrupting passive film development. Also, electrochemical noise results showed numerous distinct noise events in solutions with >1,300 ppm SO4 2- that were characteristic of metastable pitting and pitting. Pitting events were sustained in solutions above 10 g Na2SO4/L H2O) [5]. Furthermore, chloride ions have often been associated with strand corrosion and limits are specified in material and building specifications [6]. However, even though strand corrosion in segregated grout in field case studies did not coincide with high chloride concentrations [1]. Testing in grout with low-level chloride concentrations (below conventional threshold values) resulted in elevated corrosion rates in presence of the sulfate ions and indicated that the sulfate presence in the grout may reduce its chloride-ion binding capacity. It was posited that there are synergic effects of sulfate ions and vestigial chloride ions in segregated grout that can promote corrosion activity.The electrochemical behavior of steel in simulated grout pore solutions was investigated to elucidate the effect of low level chloride ions in alkaline sulfate solutions on steel corrosion. The electrochemical noise (EN) technique relating to corrosion of metals in aqueous solution has been developed over the past 50 years and was used here to provide a useful mean to elucidate the localized corrosion behavior of steel subjected to alkaline sulfate solutions with low level of chloride concentration. In the testing here, steel specimen were exposed to alkaline solutions simulating pore solutions with pH 12.6 with varying sodium sulfate levels and low level of sodium chloride. Supplemental electrochemical experiments included the steady-state condition (open circuit potential and linear polarization test) and potentiodynamic polarization tests. References Permeh, K.K. Krishna Vigneshwaran, K. Lau, “Corrosion of Post-Tensioned Tendons with Deficient Grout,” Florida Department of Transportation, Final Report, Contract No. BDV29-977-04, October 20, 2016.K. Krishna Vigneshwaran, S. Permeh, M. Echeverría, K. Lau, and I Lasa. “Corrosion of Post-Tensioned Tendons with Deficient Grout. Part 1. Electrochemical Behavior of Steel in Alkaline Sulfate Solutions.” Corrosion 74 (2018) pp.362-371.Permeh, K.K. Krishna Vigneshwaran, M. Echeverría, K. Lau, and I Lasa. “Corrosion of Post-Tensioned Tendons with Deficient Grout. Part 2. Segregated Grout with Enhanced Sulfate.” Corrosion 74 (2018), pp.457-467.Permeh, K. K . Krishna Vigneshwaran, K.Lau., and I.Lasa. “Corrosion of Post-Tensioned Tendons with Deficient Grout. Part 3: Segregated Grout with Elevated Sulfate and Vestigial Chloride Content.” Corrosion 75 (2019) pp.848-864.Permeh, K. Lau, M. Duncan, and R. Simmons, “Identification of Steel Corrosion in Alkaline Sulfate Solution by Electrochemical Noise” Materials and Corrosion, April 2021.DOI: 10.1002/maco.202112347K. Lee, J. Zielske, An FHWA special study: post-tensioning tendon grout chloride thresholds, Federal highway, Final Report, Contract No. FHWA-HRT-14-039, April 2014.