The corrosion inhibition of aluminium alloy 7075-T651 with cerium and Mischmetal di phenyl phosphate

Previous studies have shown that cerium diphenyl phosphate (Cedpp) ₃ is a very effective inhibitor of corrosion of aluminium alloys in chloride solutions. This paper describes the results of further studies using electrochemical and constant immersion corrosion tests to compare the effectiveness of Ce(dpp) ₃ and Mischmetal diphenyl phosphate Mm(dpp) ₃ as inhibitors of corrosion pitting on AA7075-T651 aluminium alloy. The results shows that both Ce(dpp) ₃ and Mm(dpp) ₃ are excellent inhibitors of pitting corrosion of this alloy in very aggressive environments of continuously aerated 0.1M and 1.0M sodium chloride (NaCl) solutions. Polarisation tests indicate that these compounds act as a cathodic inhibitors by reducing the rate of the oxygen reduction reaction, which results in a decreased corrosion current density and a separation of the corrosion potential from the pitting potential. This inhibition is thought to be due to the formation of a surface film consisting of rare earth metal oxide, aluminium oxide and a cerium-aluminium organo-phosphate complex. Surface analysis data from scanning electron microscopy and X-ray Energy Dispersive Spectroscopy show the complex nature of this protective film. This work further develops our understanding about the mechanisms through which these complex films form, and how inhibition occurs in the presence of these compounds.