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

Localized corrosion in austenitic stainless steel components considerably affects their service life. Laser surface modification offers a method of improving the corrosion resistance of the components without affecting the bulk properties. This is possible because by laser surface modification, a novel microstructure having improved properties on the surface of a material can be obtained. Surfaces of AISI type 304 stainless steel were laser melted in nitrogen atmosphere using continuous wave CO2 laser in order to introduce nitrogen on the surface layer. The pitting resistance as determined by potentiodynamic anodic polarisation method in 0.5 M NaCl medium indicated an improvement which was attributed to the increased content of nitrogen, confirmed by SIMS analysis. Using pulsed ruby laser, Nd:YAG laser and multi-beam continuous wave CO2 laser, surfaces of sensitized AISI types 316 and 304 stainless steels were modified. The extent of sensitization (susceptibility to intergranular corrosion) was assessed by the electrochemical potentiokinetic reactivation (EPR) test, as well as, by metallographic and immersion test methods. An improvement in the intergranular corrosion resistance after the laser surface melting was obtained. This was attributed to the elimination of the sensitized heterogeneous microstructure at the surface by laser melting. After the laser surface melting, the pitting resistance also increased significantly. This was explained due to the formation of a homogeneous microstructure and elimination of susceptible sites for pitting corrosion after the laser surface melting.