Localised corrosion behaviour of high and low nitrogen Cr‐Mn steels

It is well established that addition of nitrogen to steels improves the pitting corrosion resistance of steels. To elucidate the role of nitrogen, electrochemical and surface analytical studies were carried out on Cr‐Mn steels with low (0.0075 %) and high (0.9 %) nitrogen contents. The incubation time for pitting, measured in sodium sulphate solutions containing different concentrations of chloride was higher for the high nitrogen steel. The pit initiation process was found to be first order with respect to chloride concentration. The passive current density was one order lower than that of nitrogen free steel. The grain broundary attack observed in low nitrogen steel was not present in high nitrogen steel. XPS analysis indicated nitrogen enrichment at the surface in the passive layer. This may be one reason for the higher pitting resistance of nitrogen containing steel besides formation of ammonium ions and rising pH.