Valuation of the pitting corrosion resistance of nitrocarburized and oxidized steel by salt spray tests and electrochemical corrosion investigations

摘要

The pitting corrosion resistance of nitrocarburized and oxidized steel 2OMn Cr5 was characterized by electrochemical investigations in 500 NaCl solution and the salt spray test and discussed in relation to (N+C)-concentration-depth-profile of the compound layer. The described mechanism of the pitting corrosion in neutral, chloride media is valid for both corrosion tests. A direct correlation between pitting potential and time for first visible pits after salt spray was obtained at different oxidized specimens with little pitting corrosion resistance. Specimens with high resistance achieve test periods of 140 up to 480 h in the salt spray, however the pitting potentials have been detected only in the range of 1200 up to 1270 mV because of the beginning oxygen generation. The best results will be obtained by an oxidation at 450℃. The hereby achieved high (N+C)-concentration of the ε-phase beneath the surface enables a better adaptation of the oxide layer.