Effect of Ammonium Group on Inhibition of the Acid Corrosion of Steel and Iron by D-Alkylquaternary Ammonium Compounds

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.