Efficiency of organic acids and their anions in retarding the dissolution of aluminium

Abstract

The effect of a number of aliphatic, aromatic and amino acids, as well as of their anions, on the dissolution of Al in 2N HC1 and in 1·5N NaOH was studied by the thermometric method. In both media the additives caused a decrease in the maximum reaction temperature and a corresponding reduction in the reaction number (R.N.). Curves representing the variation of the percentage decrease in R.N. with the logarithm of the molar concentration of the additives were constructed. These reveal, in the case of aliphatic acids, that the additives are adsorbed through their carboxylic groups, and that dissolution-inhibition is independent of the chain length. Aromatic acids, on the other hand, appear to lie flat on the surface of the corroding metal. Amino acids in HCI are stronger corrosion inhibitors than the corresponding aliphatic acids, suggesting that adsorption occurs through both the protonated −COOH and −NH 2 groups. In acid solutions all additives act as cathodic inhibitors. The acid anions are weaker corrosion restrainers in NaOH than the corresponding acids in HCl. Anomalies in the R.N.-log C curves of amino acids in alkaline solutions are recorded and are explained on the basis of a change in the mode of adsorption of the additive anions.