Inhibition effects of bismuth(III) chloride (BiCl3), bismuth(III) iodide (BiI3), and a mixture of BiI3 and benzyl thiocyanate (C6H5CH2SCN or BTC) on corrosion of iron (Fe) in 0.5 M sulfuric acid (H2SO4) at elevated temperatures were investigated using polarization measurements. The film formed on the Fe surface was analyzed by x-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA). Because the anodic process of Fe corrosion was not suppressed, BiCl3 was an ineffective inhibitor at > 70°C. Since the anodic process was inhibited by specific adsorption of I, BiI3 at 1 × 10−4 M was highly efficient for inhibition of Fe corrosion in 0.5 M H2SO4 at 90°C. The synergistic inhibitory effect of 1 × 10−4 M BiI3 and 4 × 10−3 M BTC resulted in a significantly high inhibitor efficiency (Ieff) of 99.1% for Fe corrosion in 0.5 M H2SO4 at 90°C. The cathodic process was suppressed by covering most of the surface with metallic bismuth (Bi). The anodic process was inhibited by coverage with the oxidative addition product of BTC at small spots uncoated with the Bi layer.

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