Effect of Gas Nitriding on the Corrosion Resistance of Carbon Steel and Empirical Electron Theory Analysis of Compound Layer

Gas oxynitrocarburizing was carried out on Q235 carbon steel at low temperature. scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemical methods were used to study the effect of oxynitrocarburizing on corrosion resistance of Q235 steel. And the empirical electron theory (EET) was applied to evaluate the structure of valence electrons and the energy of covalent bonds. The results showed that the corrosion resistance of Q235 carbon steel could be obviously improved by gas oxynitrocarburizing. The calculated results reveal that the largest bond strength n_A and the bond energy E_a in the oxynitrocarburized layer were markedly higher than those in α-Fe after gas oxynitrocarburizing, which indicates the close relationship between corrosion and structure of valence electrons. Meanwhile, the n_A and E_a of main bonds increased due to the N atom dissolved in α-Fe, which enhanced the phase stability of the oxynitrocarburized layer. This might be the primary mechanism for the excellent corrosion resistance of the oxynitrocarburized Q235 carbon steel.