Corrosion behaviour, microhardness and surface characterisation of low energy, high current ion implanted austenitic stainless steel

This work presents the results of low energy nitrogen ion implantation of A ISI 304 type stainless steel (SS) at moderate temperatures of about 400 deg reesC. The nitrogen ions are extracted from a Kauffman type ion source at a voltage of 1.2 keV, the ion current is 1 mA cm (-2). The ion dose of 2.3 x 10(19) ions cm (-2) corresponds to 1 h of treatment. A metastable fcc inte rstitial solid solution with a high nitrogen content of about 30 at.% is fo rmed. This phase combines high hardness with good corrosion resistance. The nitrogen profiles were obtained by glow discharge optical spectrometry (GD OS) and the microstructural state was examined by X-ray diffraction and sca nning electron microscopy. Microhardness measurements show a significant in crease of hardness (700 Hv 0.015 against about 200 Hv 0.015 for the untreat ed SS), which allows an estimation of the nitriding depth of about 3 mum wh ich is in good agreement with the GDOS profiles. The corrosion behaviour wa s studied in both acid and chloride solutions by potentiodynamic measuremen ts. It appears that the presence of nitrogen modifies the corrosion behavio ur in acid media and the stainless character is maintained. But such a high dose of implantation (2.3 x 10(19) ions cm (-2)) induces an increase of th e metallic contact area (i.e. the roughness) with the electrolyte and thus an increasing total current. In chloride media the very high nitrogen conte nt would be able to modify the repassivation process in such a way that the potential range in which repassivating pitting occurs is broadened. (C) 20 01 Elsevier Science B.V. All rights reserved.