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

1974; Taylor & Francis; Volume: 9; Issue: 4 Linguagem: Inglês

10.1179/000705974798321224

2048-2892

R. Driver, R. J. Meakins,

Hydrogen embrittlement and corrosion behaviors in metals

Previous work in this Laboratory showed that many inhibitors of the acid corrosion of steel cause large changes in the anodic Tafel slope, ba, particularly in 0·5 M H2SO4. It was found that cationic inhibitors, such as long-chain n-alkylquaternary ammonium compounds, mostly reduced ba 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 ba 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 Cl6 and Cl8 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 ba.