Influence of high power diode laser surface melting on the pitting corrosion resistance of type 316L stainless steel

Abstract

High Power Diode Lasers (HPDL) are becoming more and more attractive for industrial materials processing because of their high efficiency, low running costs, small sizes and low weight. Surface melting experiments have been carried out on 316L steel with a 1 kW HPDL, with application to modify its pitting corrosion resistance in NaCl 0.05 M. Surface modifications have been investigated with optical microscopy for the microstructure, microprobe analysis for the chemical content and X Ray Diffraction for phase transformations and residual stresses. Heat conduction characteristics, analysed with a 2D Finite element code, have driven to a 28% calculated absorption of the laser light generating nearly 400 μm melted depth. A refinement and homogenization of structure together with δ-ferrite transformation and the dissolution of inclusions was found in the melted thickness, driving to enhanced pitting resistance (nearly + 0.2 V on the pitting potential values, and factor 2 decrease of the passive current density). This pitting resistance, investigated at different depths below the surface, was found to be little affected by the δ ferrite content (6% estimated value), and the fineness of the microstructure, but depreciated by the surface state without post-polishing. Therefore, it is believed that localized corrosion improvements can be mainly attributed to the dissolution of Al-base and Mn-base detrimental inclusions, despite the generation of up to 6% δ ferrite susceptible to drive to enhanced galvanic couplings with γ phase.