Revealing the inner rules of PREN from electronic aspect by first-principles calculations

Highlights: • The PREN is related to surface acidity and electronic property of passive film. • Incorporation of Cr, Mo, N in Fe₂O₃ weakens surface acidity and sensitivity to Cl⁻. • The negatively-charged acceptor (N) center in passive film could repel Cl⁻. This study provides insight into the empirical PREN formula by studying the effects of alloying elements (Cr, Mo, N, Ti) on the surface acidity and electronic structures of α-Fe₂O₃. Incorporation of Cr, Mo and N into α-Fe₂O₃ (0001) surface can weaken the surface acidity and reduce the susceptibility to Cl⁻, while the influence of Ti is opposite. The doped N into α-Fe₂O₃ behaves as an acceptor and lowers the concentration of electron carriers due to compensation. A negatively-charged center has formed to repel Cl⁻ after the compensation and ionization of acceptor. The mechanism of pitting resistance improving by Cr, Mo or N is proposed.