Corrosion resistance of Si-Al-bearing ultrafine-grained weathering steel.

In the Ultra-steel project at the National Institute for Materials Science (NIMS), which run from 1996 to 2005, high-Si-Al-content ultrafine-grained (UFG) weathering steel was developed by grain refinement and weathering guidance. It was found that this steel has excellent strength, toughness and corrosion resistance. Samples were prepared by multi pass warm rolling at temperatures between 773 and 873 K. The grain size of steel rolled at 873 K was about 1 μ m, and the tensile strength (TS) and elongation (EL) had excellent values of 800 MPa and 20%, respectively. In general, steels with high Si and Al contents exhibit inferior toughness to carbon steel (SM); however, the toughness of the developed sample was markedly improved by grain refinement. Cyclic corrosion tests in the presence of chloride ions confirmed that the developed steel exhibited excellent corrosion resistance, superior to that of SM. Electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) analyses showed that Si and Al mainly exist in the inner rust layer. Si and Al were identified as existing in the Si2 + and Al3 + states in the nanoscale complex oxides constituting the inner rust layer. Electrochemical impedance spectroscopy(EIS) measurement showed that the corrosion reaction resistance (Rt) of the developed steel was much greater than that of SM. In the developed steel, the nanoscale complex oxides were formed in the inner rust layer, which increased Rt, and resulted in the excellent corrosion resistance.