Improvement of the corrosion resistance of low-pressure nitrided and post-oxidized steels by a polymer impregnation final treatment

NITRALOX® is a modern, low-pressure gas nitriding process developed to improve both wear and corrosion resistance. In such a process, nitrided parts undergo an oxidizing step consecutively realized in the same furnace. The whole sequence is called an oxynitriding treatment. After being oxynitrided, samples may be impregnated with a polymer layer to bring about further enhancement of the corrosion resistance. The corrosion characteristics of nitrided 34CrMo4 steel samples were studied in 3% (by weight) NaCl solution, using steady-state electrochemical techniques and electrochemical impedance spectroscopy. Corrosion resistance was evaluated after each step of the process, i.e. nitriding, oxidation and then polymer impregnation, and the results were correlated to the steel surface structure and morphology. From this study, it appears that the sole application of a nitriding treatment does not significantly improve the corrosion resistance of steel, and may even promote localized corrosion. The post oxidation step leads to a slight decrease in the corrosion rate because it partially seals the porous nitrided layer. However, the best corrosion performances are obtained thanks to the final polymer impregnation step, which significantly decreases (by three decades) the corrosion rate of the substrate. Impedance diagrams performed after each treatment also reveal new information on the corrosion mechanisms of nitrided steel.