Roles of Interstitial Nitrogen, Carbon, and Boron in Steel Corrosion: Generation of Oxyanions and Stabilization of Electronic Structure

The effects of N, C, and B interstitials on the corrosion resistance of Fe were investigated in chloride-free boric-borate solutions at pH 6.0 and 8.0. In potentiodynamic polarization at pH 8.0, the anodic dissolution resistance of Fe-0.3N and Fe-0.3C in the active and passive regions was higher than that of pure Fe. NH₄ ⁺ and NO₂ ^- are considered to be dissolved chemical species that contribute to the higher corrosion resistance of Fe-0.3N. Potentiodynamic polarization measurements in a solution with HCO₃ ^-indicated that HCO₃ ^- also decreases the anodic current densities in the active and passive regions, suggesting that the formation of HCO^- ₃ contributes to the higher corrosion resistance of Fe-0.3C. First-principles calculations showed that the presence of N, C, and B in the Fe-lattice decreases the electronic density of states (DOS) at and near the Fermi level. The consistency between the active dissolution rates and the DOS at and near the Fermi levels of the specimens suggests that the more stable electronic structures occurred by the presence of N and C also result in the suppression of active dissolution of Fe. For Fe-0.3B and Fe-0.006B, the presence of iron boride precipitates promoted localized corrosion.