Assessment of the effect of surface finishing processes on the pitting resistance in saline environments of welded AISI 316L stainless steel

The stability of passive films of welded AISI 316L stainless steel, subjected to different surface finishing treatments, was investigated through anodic polarization measurements in 3.56 wt.% NaCl solution. The AISI 316L specimens, either hot or cold rolled and subsequently welded by TIG (Tungsten Inert Gas), were subjected to various surface finishing treatments (scarfing, mechanical polishing, brightening, pickling, passivation and electropolishing). All specimens were observed through optical and SEM-EDS microscopes, to highlight variations of microstructure and chemical composition of the heat affected zone (HAZ) caused by mechanical and chemical treatments. For each type of treatment, values of the open circuit potential (vs SCE) and pitting potential (vs SCE) were reported. It was observed that the scarfing process produces values of pitting potentials comparable to those obtained from untreated specimens. On the contrary, pickled specimens showed pitting potential values much lower than those treated with chemical passivation. The best pitting resistance performance were shown by the cold rolled series finished by brightening, where the values of pitting potential for both the BM and the HAZ were higher and comparable. The improved pitting resistance of brightened cold rolled samples was attributed to the beneficial role of brightening (due to the ability of this surface treatment to increase the surface chromium content and to form a compact and defect-free protective film), combined with microstructural features of the substrate such as the absence of martensite and the finer grain size.