Improving corrosion resistance of selective laser melted 316L stainless steel through ultrasonic severe surface rolling

Selective laser melting (SLM) is one of the common methods of additive manufacturing and herein, ultrasonic severe surface rolling (USSR) is applied to enhance the corrosion resistance of SLM 316L stainless steel (SS) in the 3.5% NaCl solution. USSR produces a thick (∼1 mm) gradient surface layer with a nanostructured, chemically homogeneous, and high-quality surface but a small amount of martensite. The USSR processed 316L SS exhibits significant enhancement in the corrosion resistance compared to the SLM one, as manifested by the greatly improved pitting potential, decreased passive current density, and reduced corrosion current density. The superior corrosion properties stem from the unique gradient structure with a high density of grain boundaries and chemically homogeneous and high-quality surface facilitating the formation of a thicker and more protective passive film that contains larger contents of oxides and hydroxides of Fe and Cr in spite of almost the same total Cr concentration. In addition to the high strength and ductility, the unique gradient structure endows the SLM 316L SS with superior corrosion resistance and long service lifetime in the marine environment, boding well for application to marine equipment and engineering.