Laser surface modification for improving localised corrosion resistance of austenitic stainless steels

Abstract

Localised corrosion considerably affects the service life of austenitic stainless steel components. Laser surface modification offers a method of improving the corrosion resistance of the components without affecting the bulk properties by producing a novel microstructure having improved properties on the surface of a material. Surfaces of AISI type 304 stainless steel were laser melted in nitrogen using a continuous wave CO2 laser in order to introduce nitrogen into the surface layer. The pitting resistance, as determined by the potentiodynamic anodic polarisation method in 0·5M NaCl rnedium, indicated an improvement which was attributed to the increased content of nitrogen, confirmed by secondary ion mass spectrometry analysis. Using a pulsed ruby laser, Nd–YAG laser, and multibeam continuous wave CO2 laser, surfaces of sensitised AISI types 316 and 304 stainless steels were modified. The extent of sensitisation (susceptibility to intergranular corrosion) was assessed by the electrochemical potentiokinetic reactivation test, as well as by metallographic and immersion test methods. An improvement in the intergranular corrosion resistance was obtained after laser surface melting. This was attributed to the elimination of the sensitised heterogeneous microstructure at the surface. Pitting resistance also increased significantly, reflecting the formation of a homogeneous microstructure and elimination of susceptible sites for pitting corrosion.