Electrochemical behaviour and corrosion inhibition of Al, Al-6061 and Al–Cu in neutral aqueous solutions

The electrochemical behaviour of Al and two of its widely used alloys, Al-6061 and Al–Cu, was investigated in neutral solutions at pH 7. The corrosion inhibition of these materials was studied in neutral solutions using sulphates, molybdates and dichromates as passivators. The effectiveness of dichromates as passivators for Al and its alloys in these solutions was discussed. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to explain the role of the passivating anion in the corrosion inhibition process. It was found that Cr and Mo are incorporated in the barrier film on Al or its alloys after immersion in neutral solutions containing these anions. Sulphate ions are adsorbed on the barrier film surface or present in the film in a very low concentration compared to dichromates and molybdates. The inhibition efficiency increases as the concentration of the passivating anion in the solution increases. Scanning electron microscopy (SEM) has shown that the Al–Cu alloy has remarkable flawed regions which are due to the presence of Cu on the alloy surface, as was confirmed by XPS. The flawed regions completely disappear after immersion in solutions containing dichromates and partially disappear after immersion in solutions containing molybdates.