Effect of sulfates on passivation in alkaline environments

Abstract

Studies investigating passivation behaviour and corrosion resistance in simulated alkaline environments have typically been conducted in solutions containing sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)2), with limited study of the effect of sulfates (SO42−). These sulfates are known to be present in the pore solution of materials like Portland cement-based concrete and may also ingress by the exposure to sulfate-rich environments. This study investigates the passivation behaviour and potentiodynamic polarisation behaviour of a high-strength carbon steel and high-strength stainless steels (austenitic and duplex) in saturated calcium hydroxide solutions containing up to 2000 ppm SO42−. Results indicate a negligible impact on the passivation behaviours of single-phase carbon steel and austenitic stainless steels. However, in duplex stainless steels, 1000 ppm of SO42− promotes passivation, whereas higher concentrations of 2000 ppm inhibit passivation. Additional research is necessary to correlate the altered passivation behaviour resulting from the presence of SO42− with chloride-induced corrosion resistance.