The effect of electrochemical reactions on the surface film composition and corrosion resistance of aluminum in acid solutions
Cathodic polarization was found to reduce the hydrogen transport resistance of the surface oxide film on aluminum, at potentials around -1.65V vs. Ag/AgCl/4M KCl in 0.1M HCl solution. Reduction of film resistance was shown to be due to change of film composition as a consequence of cathodic charging. Formation of surface film was also found at potentials where the film resistance was reduced. The film, which was shown to contain appreciable amounts of water, was found to be an ohmic proton conductor with the conductivity at the same order of magnitude as precipitated aluminum hydroxide;The cathodic product detected by ex-situ infrared spectroscopy is an aluminum oxide or hydroxide which contains water. The peak areas for three major bands increased linearly with cathodic charge suggesting that the film maintains an approximately uniform composition as it become thicker. The spectra for the cathodic products compare most favorably to that of the porous, amorphous oxide films formed by anodizing in acid solution. Aluminum ions in the film are highly coordinated to hydroxyl groups. The OH groups or H[subscript]2O molecules of the film are readily accessible to exchange by D[subscript]2O indicating that the film has an "open" structure;Cathodic charging of aluminum in acid solution induced an anodic current transient in the passive potential region, accompanied by electrode mass increase, which was primarily due to water absorption into the film. The anodic current decay, by three orders of magnitude, was due to an increase of the film resistance. The reaction with the metal surface of water which had been stored in the film during cathodic charging, to form aluminum oxide, was considered to be responsible for the increase of resistance;The rates of pit initiation and pit growth during the first 30 ms anodic polarization at -0.4V, above the pitting potential, were also enhanced by the prior cathodic charging during the first 30 ms. Pits initiate over a longer period of time with greater oxide film mass. The pitting current diminished if oxidation reaction was allowed to take place more than 20 ms at -0.9V before the potential was stepped to -0.4V. Increase of film resistance by oxidation reaction below the pitting potential was responsible for the decrease of rates of pit initiation and pit growth.