THE PITTING CORROSION OF PLASMA-NITRIDED CHROMIUM STEELS IN SULFATE-SOLUTION

3Cr-Mo and 17Cr-2Ni steels were plasma nitrided in two nitrogen-hydrog en gas mixtures containing 25% and 80% nitrogen, respectively. Anodic polarization curves were measured at various depths of the nitrided la yers in 0.05 M Na2SO4 solution, acidified to pH 3.0, at 25-degrees-C. The outer nitride zones and the adjacent thin regions. up to about 10 mum in depth, showed good corrosion resistance and ability to passivat e. The diffusion zones in both steels underwent pitting corrosion and/ or uniform corrosion, depending on the depth. Pitting occurred at dept hs of about 10-30mum, whereas considerable dissolution with pin point etching was observed in deeper regions. It was presumed that pitting c orrosion of the intermediate regions in the halide-free solution appea red under conditions of oxide film formation in the presence of sulpha te salt films.