Aluminium corrosion in hydrochloric acid solutions and the effect of some organic inhibitors

Electrochemical behaviour of pure aluminium in HCl solutions of different concentration, in presence and absence of organic inhibitors has been studied by potentiostatic and potentiodynamic methods. The evolution of the electrode surface was examined by optical microscope. The resistance of aluminium against corrosion in solutions with low concentrations of HCl can be attributed to a rapidly formed surface oxide. The presence of the chloride ions creates extensive localized attack. In strong acidic solutions no passive-active transition up to high overvoltages was observed. The increase of HCl concentration led, in all the cases, to the increase of the dissolution rate of aluminium in according to the dissolution proposed mechanism and Pourbaix diagram. The existence of an oxide layer on the metal at an appreciable coverage and thickness leads to anomalously high anodic and cathodic Tafel slopes (at high overpotential), which is not the case for potentials in the range of about 70 mV around the stationary corrosion potential. The addition of surfactants leads in all cases to the inhibition of the corrosion process. The results obtained indicate that, Tween 20 and 81 are more effective to inhibit the corrosion of pure aluminium than hexadecylpyridinium bromide (HDPB). Under the critical micelle concentration (CMC), the inhibition of used surfactants is negligible. At a concentration higher than the CMC, the inhibiting action increases rapidly. The inhibition process was attributed to the formation of adsorbed film on the metal surface that protects the metal against corrosive agents. The sigmoidal shape of the adsorption isotherm confirm the applicability of Frumkin's equation to describe the adsorption process of the three surfactants tested in 0.5 M HCl solution on an aluminium electrode. (C) 2002 Elsevier Science B.V All rights reserved.