Stainless Steel 304 and Carbon Mild Steel A36 Activity in Chloride-Containing Hybrid Pumice-Portland Cement Extract Pore Environment

The effect of chlorides on the corrosion activities of SS304 and carbon steel A36 was investigated during immersion in a hybrid pumice-Portland cement extract solution, containing high concentration of chlorides (5 g L-1 NaCl), in order to simulate the concrete-pore marine environment. The hybrid pumice-Portland cement (HB1) has been considered an alternative "green" cement system. The initial pH of the extract (12.99) decreased to 9.5 after 14 days, inducing a severe corrosion risk for A36, as suggested by the very negative corrosion potential (OCP ≈ -363 mV). Meanwhile, the SS304 tended to passivate and its OCP shifted to positive values (≈+72 mV). Consequently, the surface of the A36 presented a corrosion layer mainly of FeOOH, while that of the SS304 was composed of Cr2O3, Fe3O4 and NiO, according to the SEM-EDS and XPS analysis. An extended area of an almost uniform corrosion attack was observed on the A36 surface, due to the less protective Fe-corrosion products, while the SS304 surface presented some small pits of ≈1 µm. Based on electrochemical impedance measurements, the polarization resistance (Rp) and thickness of the passive layer were calculated. The Rp of the SS304 surface increased by two orders of magnitude up to ≈11,080 kΩ cm2, and the thickness of the layer reached ≈1.5 nm after 30 days of immersion. The Rp of carbon steel was ≈2.5 kΩ cm2 due to the less protective properties of its corrosion products.