The formation of phosphate coatings on nitrided stainless steel

Abstract X10Cr18Ni9Ti stainless steel was plasma nitrided at 600°C or 575°C for 9 h and then subjected to the phosphating in zinc and manganese/iron phosphate baths. Depth profile analysis by glow discharge optical emission spectrometry (GDOES) showed that the coatings obtained in the Mn/Fe phosphate bath were about 10 μm thick and were enriched in chromium. Surface analyses by GDOES and Auger electron spectroscopy indicated that the outer layers of the coatings were composed mainly of the components from the bath, whereas the constituents from the steel prevailed in deeper layers. Anodic behaviour of the phosphated and post-treated steel was examined in 0.1 M Na2SO4 of pH 3.0 or 6.4 by measuring polarisation curves and linear polarisation resistance. It was found that the phosphating with a subsequent chromate passivation and oil impregnation significantly improved the corrosion resistance of the nitrided steel, imparting the resistance up to an order of magnitude higher than that of the unnitrided stainless steel. The ability of the nitrided stainless steel to undergo the phosphating can be related to its enhanced anodic reactivity.