Pitting and crevice corrosion of iron aluminides in a mild acid-chloride solution

Aqueous corrosion (pitting and crevice) characteristics of iron aluminides in a mild acid-chloride (Cl^-) solution (200 ppm Cl^- [5.5 × 10^-3 M sodium chloride {NaCl}] at pH 4 [6.25 × 10^-5 M sulfuric acid {H₂SO₄}]) were studied as functions of alloying additions and thermomechanical processing methods. Electrochemical results indicated beneficial effects of chromium (Cr) and molybdenum (Mo). Immersion tests confirmed the electrochemical results and revealed the time dependence of localized corrosion behavior. The crevice corrosion behavior of iron aluminides was investigated using electrochemical and immersion crevice tests. The difference between crevice and pitting susceptibilities was minimal for the Mo-containing iron aluminide but significant for the iron aluminides without Mo. X-ray photoelectron spectroscopy (XPS) of surfaces passivated in the mild acid-chloride solution revealed the passive films of iron aluminides were composed mainly of oxidized aluminum (Al₂O₃) that coexisted with chromium oxide (Cr₂O₃) and molybdenum oxide (MoO₃). XPS results also indicated the role of Mo in improving the localized corrosion resistance of the Cr-containing iron aluminides. Mo facilitated the formation of a high Al content in the passive film and impeded the adsorption of Cl^- on the surface.