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 investigate d in solutions of different pH. The inhibition of the corrosion proces ses occurring at the electrode/electrolyte interface in solutions of b erate buffers (pH2) was studied. Molybdates and dichromates show prono unced inhibition action which was investigated and discussed. X-ray ph otoelectron spectroscopy (XPS) experiments have shown that AI-Cu alloy s have remarkable Cu peaks on the material surface. Exposure of the di fferent 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. E IS experiments and polarization measurements have shown that the corro sion behaviour of the electrode material is dependent on the concentra tion of the passivating ion in the solution. The effectiveness of Cr2O 72- in the inhibition of the corrosion processes occurring at the elec trode surface is attributed to the accommodation of this anion on the flawed regions of the naturally occurring passive film and the powerfu l oxidizing properties of the anion leading to the formation of a stab le passive film. SEM investigations have shown that the presence of Cu on the Al-Cu alloy surface initiates flawed regions which are respons ible for the increased corrosion rate of this material. (C) 1997 Elsev ier Science Ltd.