Tafel Slopes and Chemical Structure of Inhibitors of the Acid Corrosion of Steel

Previous work in this Laboratory showed that many inhibitors of the acid corrosion of steel cause large changes in the anodic Tafel slope, b_a, particularly in 0·5 M H₂SO₄. It was found that cationic inhibitors, such as long-chain n-alkylquaternary ammonium compounds, mostly reduced b_a whereas non-ionic compounds, such as thiourea derivatives, increased it. With the latter there was some differentiation according to the substituent, the aliphatic derivatives giving the largest increases and the aromatic derivatives the smallest.

The decreases in b_a caused by the n-alkylquaternary ammonium compounds are considered to be due to their initial adsorption at anodic sites and also to the structure of their adsorbed layers which involves a layer of positive charges at the metal surface. The non-ionic compounds, on the other hand, appear to be adsorbed generally over the metal surface, causing inhibition mainly by steric hindrance to the anodic and cathodic reactions. These differences are considered in relation to their possible effects on the potential energy curve for the transition of Fe²⁺ ions from the steel surface to the aqueous layer.

The adsorption of the positively charged n-alkylquaternary ammonium ions on the metal surface stimulates the co-adsorption of Br⁻ ions, and this effect increases with the length of the alkyl chain. With the C_l6 and C_l8 compounds the Br⁻ ion is adsorbed in such high concentration that the positive field of the adsorbed quaternary ammonium ions is largely neutralised, so that these compounds behave more like non-ionic inhibitors and give increases in b^a.