STRUCTURE AND PROPERTIES OF PLASMA-NITRIDED STAINLESS-STEEL

A series of plasma-nitriding experiments has been conducted on AISI 304L austenitic stainless steel at temperatures ranging from 400 to 600 degrees C using a pulsed d.c. plasma with various pulse duration/repetition ratios in an N-2-H-2 gas mixture. The structure and composition of the plasma-nitrided surface layer were analysed by means of X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), optical microscopy and microhardness testing. The corrosion behaviour of the S phase was also investigated. The maximum Knoop hardness after plasma nitriding is about 1400 HK 0.01 and the maximum thickness of the compound layer is 34 mu m. XRD patterns show that the surface layer consists of the S phase only. The corrosion performance was tested in 0.05 M Na2SO4 solution at pH 3.3 and in neutral 3.5% NaCl solution at ambient temperature. Potentiostatic and potentiodynamic experiments yielded slightly higher passive corrosion currents for plasma-nitrided 304L. Fitting corrosion in neutral electrolytes containing chloride ions was observed only for untreated 304L. The passive layers formed on both types of samples were similar in constitution and thickness as determined from XPS sputter profiles.