Inhibitive Effect of Tannic Acid on the Corrosion of Copper in Acid Solutions

Abstract

The behaviour of copper has been studied in lM solutions of HCl, HNO₃ and an HCl–HNO₃ mixture, containing various amounts of tannic acid (0 · 25, 0·5 and 1·0%). As tannic acid is a mixture of galloylglucoses, the same acids containing gallic acid (0·1, 0·2 and 0·5%) or glucose (0·1, 0·5 and 1·0%) have also been studied.

Glucose inhibits the corrosion of copper in HNO₃ solution to a very slight extent and has no effect in other solutions. Gallic and tannic acids have no effect in HCl but, at the highest concentrations tested, show an inhibitive efficiency of 80% and 70%, respectively, in HNO₃ and about 65–70% in HCl–HNO₃ mixtures.

Gallic acid may be considered to be a cathodic inhibitor that does not interfere directly with the cathodic process but modifies the corrosive environment. Tannic acid behaves both as a cathodic inhibitor and also as an anodic inhibitor due to the formation of an oxidation compound that is adsorbed on the copper surface.