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Stern-Geary and Tafel polarisation relationships have been obtained for steel in 0·5 M H2SO4and 1 M HCl containing different concentrations of a wide range of n-alkylquaternary ammonium compounds and thiourea derivatives. The shape of the Stern-Geary polarisation relationship is found to depend on the difference between the anodic and cathodic Tafel slopes, baand bc, in agreement with electrode kinetic theory.For each solution the corrosion rate was derived from the electrochemical data in two different ways:1. Substitution of the Stern–Geary gradient, ∆I/∆E, and the measured values of baand bcin the Stern-Geary equation: II. Extrapolation of the Tafel lines to the point of intersection. The results from the two methods were in good agreement for all of the solutions tested.The corrosion inhibition values calculated from the Stern-Geary corrosion rates were also compared with those measured in separate experiments by weight loss. The agreement was good except for the n-alkylquaternary ammonium compounds in 0·5 M H2SO4, which gave considerable discrepancies at the lower levels of inhibition. This was shown to be due to differences in the pre-immersion treatment of the steel samples.

Previous studies of the inhibition of the acid corrosion of mild steel and pure iron by n-alkyltrimethylammonium (TMA) and n-alkyltriethylammonium (TEA) compounds have been extended to include the corresponding n-alkyltripropylammonium (TPA) and n-alkyltributylammonium (TBA) compounds and the performance ofthefour headgroup types have been compared at different n-alkyl chain lengths. For a given chain length, the percentage inhibition generally increases in the order TMA < TEA < TPA, in line with the expected decrease in mutual repulsion between the adsorbed cations with expanding headgroup size. However, the transition to a TBA headgroup sometimes has an adverse effect, especially at shorter chain lengths. This is possibly due to reduced Van der Waals attraction between the alkyl chains as they are forced apart by the spatial requirements of the large head group.Although the corrosion inhibition increases in the above headgroup sequence for both mild steel and pure iron, the incremental changes are appreciably greater for iron and this could indicate that the rest potential lies closer to the potential of zero charge than it does in the case of steel.Polarisation measurements show that the quaternary ammonium compounds studied in this work are predominantly anodic inhibitors and, with steel in 0·5 M H2SO4, they actually cause appreciable stimulation of the cathodic reaction at the lower levels of inhibition. The possible significance of this in relation to the choice of picking inhibitors is discussed.

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 bawhereas 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 bacaused 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 Cl6and Cl8compounds 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.

Long-chain n-alkyl-trimethyl- and -triethyl-ammonium sulphates give good inhibition of the corrosion of mild steel in 0·5 M H2S04, but little or no inhibition with pure iron. This is probably because mild steel is negatively charged in O·5 M H2SO4whereas pure iron ispositive.Although the sulphates of n-alkyl-trimethy- and -triethyl-ammonium compounds give little inhibition of the corrosion of pure iron in 0·5 M H2SO4, the corresponding bromides give good inhibition. The experimental results suggest that there is a mutual stimulation of adsorption between the bromide and the quaternary ammonium ions.Polarisation measurements show that the quaternary ammonium bromides give a higher proportion of cathodic inhibition with iron than with steel, presumably because of the more restricted cathodic reaction of iron.There are considerable differences between mild steel and pure iron in the effects of the quaternary ammonium bromides on the anodic and cathodic Tafel slopes. These results are discussed in terms of the different mechanisms of adsorption of the inhibitors on steel and iron.The positive charge on pure iron enhances the specific adsorption of Br−and Cl−ions, and they both give appreciable corrosion inhibition in 0·5 M H2SO4. The effect on mild steel may be complicated by competition with HS−ions, since it is found that, whereas Br−ions give some inhibition, the weakly adsorbed Cl−ions actually cause acceleration of corrosion.Thiourea derivatives are effective pickiing inhibitors for steel, but at low concentrations they greatly stimulate the acid attack on pure iron. At higher concentrations, above about 10−5−10−4M, the thiourea derivatives give good inhibition of the acid corrosion of iron.

Differential capacitance and interfacial tension measurements have been carried out at a dropping mercury electrode in 0·5 M H2SO4and M KCl solutions of n-alkylquaternary ammonium bromides containing C3, C12and C16chains paired to trimethyl-, triethyl-, tripropyl-, and tributylammonium headgroups.For n-alkyl chain lengths of 8 and 12 carbon atoms, the adsorption of the quaternary ammonium ions on mercury increases with the size of the headgroup, and the calculated surface concentrations indicate that, at optimum potentials, the larger headgroups are always adsorbed in excess of a monolayer. The compounds with alkyl chains of 16 carbon atoms all show multilayer adsorption behaviour which appears to be practically independent of the headgroup.The results are discussed in relation to the performance of the n-alkylquaternary ammonium compounds as inhibitors of the acid pickling of steel.

Long-chain n-alkylisoquinolinium compounds are very good inhibitors of the corrosion of steel by sulphuric, hydrochloric and hydrobromic acids, giving up to 99% protection in the best cases. It is suggested that their adsorption is aided by interaction between the π-electrons of the heterocyclic ring system and the metal surface.

The oxide films formed on pure iron and mild steel in air at ambient temperatures show marked differences in their behaviour in 0·5 M H2SO4.On pure iron the air-formed oxide film persists for a considerable time in 0·5 M H2SO4, retarding the anodic reaction and giving a more positive corrosion potential than for anaerobically treated samples. It also reduces the effectiveness of corrosion inhibition by quaternary ammonium ions.With mild steel, the air-formed oxide film has the opposite effects. The anodic reaction in 0·5 M H2SO4is stimulated, giving a more negative corrosion potential, and corrosion inhibition by quaternary ammonium bromides is enhanced. From this, and other evidence, it appears that the oxide layer on mild steel is rapidly removed in 0·5 M H2SO4, leaving the anodic sites in a more active state than those on anaerobically treated samples.

Previous experiments with a dropping-mercury electrode indicated that n-alkyltriethylammonium ions are more strongly adsorbed than the corresponding n-alkyltrimethylammonium ions. In parallel with this, it has now been found that the former are better inhibitors of the acid corrosion of steel.At concentrations giving up to 50% corrosion inhibition, the compounds of either series have no effect on the steel electrode potential, indicating that the anodic and cathodic reactions are equally inhibited. This is taken as evidence that the inhibitor ions are adsorbed generally over the metal surface rather than at specific anodic or cathodic sites.With increasing concentration of inhibitor above the level for 50% inhibition, the potential begins to be displaced in a positive direction, suggesting that the anodic reaction is being inhibited more than the cathodic reaction. The results show that the proportion of cathodic inhibition is larger with the triethylammonium than with the trimethylammonium compounds.The cathodic part of the inhibition appears to be due mainly to hindrance of the surface diffusion and recombination of adsorbed hydrogen atoms.