Study of corrosion resistance properties of nitrided carbon steel using radiofrequency N₂/H₂ cold plasma process

C38 carbon steel have been plasma-nitrided using a radiofrequency cold plasma discharge treatment in order to investigate the influence of gas composition on corrosion behaviour of nitrided substrates. The investigated C38 steel was nitrided by a RF plasma discharge treatment using two different gas mixtures (75% N₂/25% H₂ and 25% N₂/75% H₂) at different times of plasma-treatment on non-heated substrates. Electron Probe Microanalysis (EPMA) showed that the nitrided layer formed using 75% N₂/25% H₂ gas mixture was thicker compared to those formed in the case of 25% N₂/75% H₂ or pure N₂. The modifications of the corrosion resistance characteristics of plasma-nitrided C38 steel in 1M HCl solution were investigated by weight loss measurements and ac impedance technique. The results obtained from these two evaluation methods were in good agreement. It was shown that the nitriding treatment in both cases (75% N₂/25% H₂ and 25% N₂/75% H₂) improves the corrosion resistance of investigated carbon steel, while the better performance is obtained for the 75% N₂/25% H₂ gas mixture. X-ray photoelectron spectroscopy (XPS) was carried out before and after immersion in corrosive medium in order to establish the mechanism of corrosion inhibition using N₂/H₂ cold plasma nitriding process.