Studies on Microstructure and Influence of Nitrogen on Pitting Corrosion Susceptibility of Nitrogen-Alloyed 316 SS Weld

The present study investigates the influence of nitrogen on the severity of precipitation and its effect on the localised corrosion susceptibility of type 316 N weld (C = 0.05 wt.%, N = 0.12 wt.%). During thermal aging at the initial stages, although high carbon and aging exists, the pitting corrosion resistance of 316 N weld was improved with nitrogen (N) that is enriched in ferrite that reduced the severity in formation and growth of precipitates with reduced Cr loss there by enhancing pitting corrosion resistance. At longer times of thermal aging, although high carbon and aging for longer hours exist, enhancement in pitting resistance occurred due to nitrogen in ferrite, that reduced the severity in formation and growth of precipitates, allowing small Cr depletion around carbides, dichromium nitride (Cr₂N) precipitation with insufficient Cr depletion. From this study, it was evident that the extent of Cr depletion at carbide/γ at austenite boundaries, and severity in formation and growth of Cr-enriched precipitates depends on Cr content variation arising due to heat input, temperature, time and amount of carbon, or availability of nitrogen in ferrite. The role of weld microstructure, alloying elements, and diffusion of Cr and Mo to the pitting corrosion resistance of 316 SS weld are discussed.