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 AISI 304 type stainless steel (SS) at moderate temperatures of about 400°C. 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 2.3×1019 ions cm−2 corresponds to 1 h of treatment. A metastable fcc interstitial solid solution with a high nitrogen content of about 30 at.% is formed. This phase combines high hardness with good corrosion resistance. The nitrogen profiles were obtained by glow discharge optical spectrometry (GDOS) and the microstructural state was examined by X-ray diffraction and scanning electron microscopy. Microhardness measurements show a significant increase of hardness (700 Hv 0.015 against about 200 Hv 0.015 for the untreated SS), which allows an estimation of the nitriding depth of about 3 μm which is in good agreement with the GDOS profiles. The corrosion behaviour was studied in both acid and chloride solutions by potentiodynamic measurements. It appears that the presence of nitrogen modifies the corrosion behaviour in acid media and the stainless character is maintained. But such a high dose of implantation (2.3×1019 ions cm−2) induces an increase of the metallic contact area (i.e. the roughness) with the electrolyte and thus an increasing total current. In chloride media the very high nitrogen content would be able to modify the repassivation process in such a way that the potential range in which repassivating pitting occurs is broadened.