Comparative study of Fe(Ⅱ)/Fe(Ⅲ) ions reaction in electrochemical polarization process of 304 stainless steel in H₂SO₄ and bipyridine solutions employing experimental and density functional theory methods

Localized corrosion of 304 stainless steel being the significant parts of Starship rocket seriously threatens the long-term service of such aerospace equipment. Scanning electron microscopy, in situ instruments combining electrochemical workstation and Raman spectroscopy, and Density Dunctional Theory (DFT) calculations were employed. The surface morphologies, alloying elements, molecular fingerprint Raman evidence and theoretical mechanism for the localized corrosion of 304 stainless steel during the electrochemical polarization in the mixture solutions containing 0.5 mol/L H₂SO₄ and 2, 2′-bipyridine (bipy) with concentrations of 0.001, 0.010, 0.100 mol/L were discussed. In comparison, the presence of bipy up to 0.100 mol/L in such mixture solutions displayed the neglectable effect on the Fe(Ⅱ)/Fe(Ⅲ) reaction in the polarization process. Raman vibrational frequency around 1492 cm⁻¹ was the evidence of pink color appearance due to the formation of [Fe^Ⅱ(bipy)₃]²⁺. Raman and DFT indicated the yellow color emergence due to the presence of μ-O-[Fe^Ⅲ(bipy)₂(H₂O)]₂⁴⁺ due to the oxidation reaction of [Fe^Ⅱ(bipy)₃]²⁺ with H₂O₂ oxidant, and the dimerization of [Fe^Ⅲ(bipy)₃]³⁺. Furthermore, a quantitative model between [Fe^Ⅱ(bipy)₃]²⁺ concentration and Raman intensity at 1492 cm⁻¹ has been built up. Two linear functions were revealed when [Fe^Ⅱ(bipy)₃]²⁺ concentrations were at 0–0.002 mol/L and 0.002–0.004 mol/L and a concentration error of less than 5% was evidenced in comparison with that investigated by the inductively coupled plasma. The proposed passivation mechanism and quantitative concentration model of 304 stainless steel have certain significance for its corrosion protection and corrosion evaluation.