COMPOSITE FILM FORMATION ON IRON IN SULFURIC-ACID BY BISMUTH(III) CHLORIDE AND BENZYL THIOCYANATE

A synergistic inhibitory effect of bismuth(III) chloride (Bi(III)Cl3) and benzyl thiocyanate (C6H5CHSCN) on corrosion of iron (Fe) in 1 N sulfuric acid (H2SO4) was investigated using polarization and impedance measurements. A composite protective film formed on the Fe surface by a combination of both compounds. The film was analyzed by x-ray photoelectron spectroscopy and electron probe microanalysis. Inhibition efficiency of > 99% was obtained by the formation of the protective film. The film comprised an inner layer of metallic bismuth (Bi) and an outer layer of metallic oxide containing a deposited oxidative-addition product of C6H5CH2SCN. A small spot uncoated with the Bi layer was covered mostly with the product layer, suppressing the anodic process of Fe corrosion. The cathodic process was inhibited by the high hydrogen (H) overpotential of the metallic Bi deposited on most of the substrate surface.