A series of plasma-nitriding experiments has been conducted on AISI 30
4L austenitic stainless steel at temperatures ranging from 400 to 600
degrees C using a pulsed d.c. plasma with various pulse duration/repet
ition 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 d
iffraction (XRD), scanning electron microscopy, X-ray photoelectron sp
ectroscopy (XPS), optical microscopy and microhardness testing. The co
rrosion behaviour of the S phase was also investigated. The maximum Kn
oop hardness after plasma nitriding is about 1400 HK 0.01 and the maxi
mum thickness of the compound layer is 34 mu m. XRD patterns show that
the surface layer consists of the S phase only. The corrosion perform
ance was tested in 0.05 M Na2SO4 solution at pH 3.3 and in neutral 3.5
% NaCl solution at ambient temperature. Potentiostatic and potentiodyn
amic experiments yielded slightly higher passive corrosion currents fo
r plasma-nitrided 304L. Fitting corrosion in neutral electrolytes cont
aining chloride ions was observed only for untreated 304L. The passive
layers formed on both types of samples were similar in constitution a
nd thickness as determined from XPS sputter profiles.