Passivation and chloride-induced corrosion behavior of Zn-Al coated steel in belitic calcium sulfoaluminate cement system

Abstract

As one of the low-carbon alternatives to ordinary Portland cement (OPC), belitic calcium sulfoaluminate (BCSA) cement has several benefits, such as rapid strength gain, low shrinkage and high durability. However, insufficient passivity was achieved for steel in BCSA cement due to its low alkalinity and high sulfates content. Despite the fact that these features are detrimental to the passivity of carbon steel, it should be underlined that the low-alkaline pore solutions of BCSA cement may prevent the dissolution of galvanized coating for galvanized steel, implying that galvanized steel may exhibit superior corrosion resistance when embedded in BCSA cement compared to OPC. To verify this hypothesis, the passivation behavior and chloride resistance were investigated for a Zn-Al coated steel in the OPC-BCSA blended system (abbreviated as PSA), including extract solutions and mortars, using electrochemical tests and microstructural characterization methods. The results indicate that the Zn-Al coated steel shows comparable passivation behavior in OPC and PSA systems, though it performs slightly differently in extract solutions and mortars. Nevertheless, compared to the OPC solution, improved chloride resistance is observed for the Zn-Al coated steel in the PSA solution, which is largely attributed to the presence of a stable retained Zn-Al coating. Furthermore, owing to the formation of a more compact steel-mortar interface, the enhanced corrosion resistance is more pronounced for the Zn-Al coated steel in the PSA mortar with the R_c + R_ct value of 351.0 kΩ cm², which is remarkably higher than that in the OPC mortar (9.2 kΩ cm²) after long-term chloride attack.