Polarization and impedance studies on the effects of anion and cation inhibitors on corrosion of iron in an electrolytic anhydrous methanol solution of ferric chloride

Inhibition mechanisms of anionic and cationic inhibitors for corrosion of iron in an anhydrous methanol solution of 0.1M LiClO4 and 0.01M FeCl3 (H2O < 80 ppm) were investigated by polarization and impedance measurements of an iron electrode, the surface of which was prepared under an argon atmosphere in a dry box. Because of specific adsorption on the surface, Br- mostly suppressed the anodic Cl--catalyzed iron dissolution. The cathodic process, reduction of Fe3+ to Fe2+, was mainly inhibited by adsorption of alkyltrimethylammonium ion C(n)TMA(+) via an electrostatic interaction between the cation and the negatively charged surface. Joint adsorption of Br- and C(n)TMA(+) on the surface results in synergistic inhibition of both processes. Iodide ion was less effective for iron corrosion in the FeCl3 methanol solution than Br- because of I- oxidation with Fe3+ to form I-3(-). Thiocyanate ion stimulated the anodic iron dissolution process through a SCN--catalyzed path.