Rust formation and corrosion performance of Si- and Al-bearing ultrafine grained weathering steel

A high Si, Al type ultrafine-grained (UFG) weathering steel was produced by the multi-pass warm rolling method, and its corrosion resistance was estimated by a cyclic wet/dry corrosion test using chloride ions. The Si- and Al-bearing UFG steel exhibited excellent corrosion resistance in comparison with Si–Mn carbon steel (SM).EPMA and TEM analyses showed that Si and Al mainly exist in nano-scale iron complex oxides in the inner rust layer formed on the developed steel. The Al K X-ray spectrum of Al2O3 and metallic Al had different peak positions due to chemical shifts, and that of the test sample was the same as that of Al2O3 in the EPMA analysis. This result suggested that Al was present in the complex oxides as Al3+. In the same way, Si was identified as an intermediate state such as Si2+ in the complex oxides of the inner rust.EIS (electrochemical impedance spectroscopy) measurement of the corrosion test samples revealed that the rust resistance (Rrust) and corrosion reaction resistance (Rt) of the developed steel were much larger than those of SM. It was found that nano-scale complex iron oxides formed in the lower layer of iron rust in the developed steel, resulting in increased Rrust and Rt, and finally suppressing corrosion.