Pitting corrosion behaviour of large area laser surface treated 304L stainless–steel

Previous work on laser surface melting of austenitic stainless–steels for the improvement of localised corrosion resistance has been performed using CO2 lasers with a circular beam profile, generating melt tracks of width 1–5 mm. In large area applications, overlapping of individual tracks is often required. This results in in-homogeneity of microstructure at the overlapped regions that may adversely influence corrosion properties. This work investigates the microstructure and corrosion characteristics of laser-melted single tracks of AISI 304L stainless–steel using a 3 kW Nd:YAG laser with a 13 mm wide line beam profile. The microstructures, as a function of laser operating conditions, were characterised by means of scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction. The corresponding pitting corrosion resistances were evaluated using potentiodynamic polarisation in deaerated 3.5% NaCl solution at 25 °C. The results showed that the microstructure and solidification mode were dependent on the cooling rate. Formation of δ-ferrite, dissolution of inclusions and promotion of preferred orientation of γ-austenite along the [200] direction were found in the melted zone, leading to enhanced pitting potentials, with a shift up to +315 mV. Compared with the literature, the 304L SS treated by the line beam profile produced a more uniform microstructure and consistent corrosion behaviour, with greater improvement of the pitting potential.