High power diode laser (HPDL) surface melting and laser peening (LP) experiments have been carried out with application to modify the pitting corrosion resistance of 316L austenitic stainless steel in Na+Cl− environment, by thermal (HPDL), mechanical (LP with coating) or thermo-mechanical effects (LP without coating).
Surface modifications have been analyzed with Optical microscopy, SIMS, ΕΡMΑ, and XRD measurements, in order to ensure a global understanding of surface states after both laser treatments. Electrochemical testings perfonned in NaCl 0.05 M have consisted of free potential recordings and anodic polarizations at 2 mV/s.
The results show that HPDL surface meltings carried out with a 1 kW diode at 25 kW/cm2, drive to a 400 µm affected depth with a surface refinement (5 µm dendrite arm spacings), a dissolution of inclusions (Si, Fe, Ca, Al aggregates) and the generation of 5-6 % δ ferrite, together with a small compressive stress field (-100 MPa). The resulting effect is a +220 mV anodic shift of pitting potentials, attributed to the dissolution of inclusions, despite the occurrence of a small Cr segregation.
LP experiments with Nd:YAG pulsed lasers between 5 and 25 GW/cm2 (3 ns and 10 ns durations) drive to enhanced compressive stresses (-500 MPa), deformation bands systems, without modification of the chemical content. The resulting effect is an increase of pitting potentials (up to +100 mV) due to a modification of inclusion-matrix interfaces. Additionnaly, XPS investigations of the passive film reveal thicker passive film layers on LP surfaces, less prone to damage by Cl- ions. This reveals a mechanochemical effect of LP, not demonstrated without coating, due to ablation effects on the metal surface.