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Increase of Salt Fog Corrosion Resistance of Plasma Nitrided Steel by Pulsed Plasma Post-Oxidation

Plasma nitriding has demonstrated to be a good alternative to increase surface hardness, wear and abrasion resistance of AISI 4340 steel. Unfortunately, corrosion resistance of the obtained nitrided layer is poor as a consequence of its high grade of porosity. In the present work, pulsed plasma post-oxidation process is analyzed as an alternative technique to increase corrosion resistance of plasma nitrided AISI 4340. The effect of post-oxidation process duration (15, 30 and 60 minutes) on the corrosion performance of samples when they are exposed to a rich chloride fog under controlled temperature and pressure conditions was evaluated. Results showed that post-oxidized during 15 minutes samples have the best corrosion performance. After 21 h of exposition to 3.5 % NaCl fog, the presence of red rust on post-oxidized during 15 minutes samples was reduced to around 1/16 part of the only nitrided sample. According with scanning electron microscopy and X-ray diffraction analysis, the increase in the corrosion resistance of post-oxidized samples is related to the occurrence of an oxidized thin layer constituted mainly by magnetite kind iron oxide.