The inhibition of the corrosion of Al, Al-6061 and Al-Cu in chloride free aqueous media .1. Passivation in acid solutions

The electrochemical behaviour of Al, Al-6061 and AI-Cu was investigated in solutions of different pH. The inhibition of the corrosion processes occurring at the electrode/electrolyte interface in solutions of berate buffers (pH2) was studied. Molybdates and dichromates show pronounced inhibition action which was investigated and discussed. X-ray photoelectron spectroscopy (XPS) experiments have shown that AI-Cu alloys have remarkable Cu peaks on the material surface. Exposure of the different electrodes to solutions of pH2 containing MoO42-, Cr2O72- and SO42- With the same concentration leads to the incorporation of Cr and Mo in the passive film matrix whereas sulpher was not incorporated. EIS experiments and polarization measurements have shown that the corrosion behaviour of the electrode material is dependent on the concentration of the passivating ion in the solution. The effectiveness of Cr2O72- in the inhibition of the corrosion processes occurring at the electrode surface is attributed to the accommodation of this anion on the flawed regions of the naturally occurring passive film and the powerful oxidizing properties of the anion leading to the formation of a stable passive film. SEM investigations have shown that the presence of Cu on the Al-Cu alloy surface initiates flawed regions which are responsible for the increased corrosion rate of this material. (C) 1997 Elsevier Science Ltd.