Passivation behavior of steel in alkali-activated materials and corrosion behavior under carbonation conditions

Abstract

This study investigates the passivation behavior of steel in simulated pore solutions at various hydration stages (early, middle, and late) in alkali-activated materials (AAMs) and the corrosion behavior under simulated carbonation conditions. Portland cement was used as the control group. The research initially focused on studying the passivation behavior of the steel. The results revealed that in the waterglass-activated solution (WGS), a higher Si/Al ratio (>1.5) promoted the formation of an aluminate/silicate adsorption layer on the steel surface, which effectively reduced the negative impact of S^2- on the passive film. In contrast, the NaOH-activated solution (NHS) with a Si/Al ratio lower than 0.5 led to the formation of a loose passive film containing FeS. Both the adsorption layer in WGS and the FeS-containing passive film in NHS showed thicknesses exceeding 30 nm. In the Portland cement solution (PCS), a double-layer passive film with a thickness of less than 10 nm was formed. When subjected to simulated carbonation conditions with reduced pH, the critical pH values for corrosion initiation of the passive film in the WGS and PCS groups were similar, around 8.5. However, the NHS group exhibited a significantly higher critical pH of 12.5. This discrepancy arises because the adsorption layer in WGS and the dense passive film in PCS provided better corrosion resistance, whereas the loose FeS-containing passive film in NHS offered weaker protection.