Excellent Corrosion Resistance of Dual-Phase Bimodal Stainless Steel

Bimodal microstructures in metals are attractive because they help in achieving high strength and good ductility simultaneously thereby circumventing the strength-ductility paradox. However, there are few reports and limited understanding of the surface degradation behavior of bimodal alloys. Here, the authors report on the exceptional corrosion resistance of bimodal stainless steel developed using a simple single-step processing route. The microstructure of bimodal steel comprises of fine martensite (approximate to 200-400nm) in a matrix of coarse austenitic grains (approximate to 10 mu m). The bimodal steel shows significantly lower corrosion rate of 0.001mm/year in 3.5wt% NaCl solution compared to 0.088mm/year for commercially available SS316L stainless steel. The exceptional corrosion behavior of bimodal steel is attributed to favorable electronic properties of its passive layer. Formation of a depletion zone in the passive layer and higher chromium fraction limits the charge transport across metal-oxide-solution interface. The current microstructural design strategy is potentially transformative for developing high-performance structural materials with superior corrosion resistance.