Mechanism of corrosion inhibition of intermetallic Al2Cu in acidic solution

Highlights: • Thin layer of molybdenum oxides inhibits corrosion of intermetallic Al₂Cu in acids. • The protective layer is spontaneously formed by heteropolyoxomolybdates. • Heteropolyoxomolybdate species do not inhibit corrosion of Al₂Cu. • Heteropolyoxomolybdate stabilises aluminium oxide against dissolution. The objective of this work was to study the kinetics of phosphomolybdenum blue formation onto the surface of intermetallic Al₂Cu in orthophosphoric acid solution. This was performed using UV–Vis spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical methods. The latter included measurement of open circuit potential, cyclic voltammograms, polarisation, and chronoamperometric curves as well as impedance spectra. It was shown that a stable form of phosphomolybdenum blue is formed via decomposition of an intermediate in the solution. However, neither phosphomolybdenum blue nor heteropolyoxomolybdate species behave as a corrosion inhibitor. A protective layer of molybdenum oxides formed immediately upon immersion of intermetallic in the corrosive environment, is responsible for corrosion inhibition.