Influence of nitrogen addition on microstructure and pitting corrosion resistance of austenitic weld metals

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Abstract
A study to investigate the role of nitrogen in improving the pitting corrosion resistance of types 316 and 304 stainless steel weld metals has been attempted. Weld metals were prepared by autogenous TIG welding process with argon‐nitrogen mixture as the shielding gas. Delta‐ferrite measurements, made with Magne‐Gage, showed the absence of delta‐ferrite in all the samples of type 316 ss weld metal whereas for type 304 ss it decreased from 3.8 to 0 FN with increase in the levels of nitrogen. Potentiodynamic anodic polarization studies carried out on weld metal samples in deaerated neutral chloride medium showed improved resistance to pitting attack with increase in the amount of nitrogen. This improvement in pitting resistance could be due to the dissolution of nitrogen and formation of inhibiting compounds at the pit sites. The ESCA results of the anodically polarized weld metal samples showed the presence of nitrogen in a compound form. SEM and optical microscopic studies of the pitted samples showed that the initiation sites for pitting attack were triple points, austenite centres and delta‐ferrite/austenite interfaces.