Citric acid as corrosion inhibitor for aluminium pigment

Aluminium pigments corrode in aqueous alkaline media (e.g., water-borne paints) with the evolution of hydrogen. The hydrogen corrosion of aluminium pigment can be inhibited by the chelating agent citric acid, which is a renewable raw material and absolutely non-toxic. At pH 8 more hydrogen is evolved as at pH 10 which can be explained with the isoelectric point of aluminium, oxide (pH 9) and a chemical reaction of citrate with aluminium. The product of this reaction should be an aluminium(III)-citric acid-chelate which is assumed to be the actual corrosion inhibitor. This assumption can be corroborated by tests with aluminium(III)-citric acid-chelates. With increasing addition of citric acid the hydrogen volumes evolved increase, which could also be explained with a chemical reaction of citrate with aluminium. Agaric acid (alpha-hexadecyl-citric acid) can be considered as a surfactant with the chelating agent citric acid as hydrophilic group and inhibits the corrosion reaction of aluminium pigment better when compared to citric acid. (C) 2003 Elsevier Ltd. All rights reserved.