Nitriding and oxidative finishing
Nitriding and nitrocarburizing - Industrial Processes and Practices

Combination layers, whose electrochemical behavior is cathodic, are highly resistant to corrosion, especially the most carbon-rich ε-layers (9% carbon by weight), but they have the disadvantage of being porous. Any through-porosity can lead to corrosion pitting to the point of promoting corrosion of the more anodic substrate, in a manner comparable to the corrosion observed with chromium deposits, which are generally fissured. For this reason, we're looking to improve corrosion behavior through oxidative passivation. The growth of Fe₃O₄-type compounds on the combination layer, and inside the porosities, enhances corrosion resistance through the passivation effect and by plugging porosities due to oxide growth. Post-oxidation is also seen as anticipating the decomposition of unstable nitrides. In fact, oxidation is not sufficient, and optimum corrosion...