Prior cathodic polarization was found to increase pit densities on aluminum by a factor of 100 during the first 20 ms of anodic polarization above the pit initiation potential, in 1N HC1 at 65~ Enhancements of the average pit growth current densities were also observed. Immersion of aluminum in solutions of HC1, or NaOH followed by HC1, before cathodic polarization, was also necessary to obtain cathodic enhancement of pitting. The total pit volume at 20 ms was approximately the faradaic equivalent volume of the cathodic charge, calculated assuming three cathodic electrons resulted in the removal of one A1 atom during anodic polarization. This result suggests that the cathodic reaction may produce a species which participates in the initial stages of anodic dissolution. * Electrochemical Society Student Member. ** Electrochemical Society Active Member.

Cathodic polarization of aluminum in acid solution produces a surface film which was studied using infrared reflectance spectroscopy (IRS), and quartz crystal microbalance (QCM), and capacitance measurements. According to the QCM results, deposition of the film began after passage of 7.2 mC/cm2 of cathodic charge at a potential of −2.0 V. This charge was consistent with IRS and capacitance measurements. The film grew at an approximately constant rate with time, indicating that its ionic conduction resistance is small. Also, the linear increase of the reciprocal capacitance as a function of film mass is consistent with film growth occurring uniformly across the electrode surface. IRS showed that the cathodic film is an amorphous aluminum hydroxide or oxyhydroxide which contains absorbed water; QCM stripping measurements found that there was at least one water molecule per aluminum ion. This extensive hydration is perhaps related to the relatively low ion transport resistance. Mass transport calculations indicated that the film was formed by direct electrochemical growth and not by precipitation.

The electrochemical behavior of aluminum during cathodic polarization was investigated with the quartz crystal microbalance, to identify changes in the electrical conduction properties of the surface film, which result in strongly enhanced electrochemical reaction rates. As a consequence of cathodic charging at potentials more negative than about −1.45 V vs. NHE in 0.1M solution, the surface film transforms from a high field electrical conductor to an ohmic conductor, and then begins to grow. The critical potential for forming this ohmically conducting film agrees with the potential below which aluminum hydroxide is expected to be more stable than aluminum oxide, near the metal/film interface. The conductivity of the cathodic film is within an order of magnitude of the proton conductivity of bulk hydrated aluminum hydroxide, . When the potential is stepped above the open-circuit potential subsequent to cathodic charging, there is a characteristic current decay during several seconds, after which the conductivity is three orders of magnitude smaller than at the cathodic potential. A mechanism is given, based on calculated overpotentials for interfacial reaction and transport processes, through which the oxide film transforms to hydroxide at cathodic potentials. J. Electrochem. Soc., Vol. 141, No. 1, January 1994 9 The Electrochemical Society, Inc. 51. P. Tarte, Spectrochim. Acta, 23A, 2127 (1967). 52. J. Newman, Electrochemical Systems, p. 267, Changes Produced by Cathodic Polarization in the Electrical Conduction Behavior of Surface Films on AluminumChing-Feng Lin a and Kurt R. Hebert* Department of Chemical Engineering, Iowa State University, Ames, Iowa 50011 ABSTRACTThe electrochemical behavior of aluminum during cathodic polarization was investigated with the quartz crystal microbalance, to identify changes in the electrical conduction properties of the surface film, which result in strongly enhanced electrochemical reaction rates. As a consequence of cathodic charging at potentials more negative than about -1.45 V vs. NHE in 0.1M HC1 solution, the surface film transforms from a high field electrical conductor to an ohmic conductor, and then begins to grow. The critical potential for forming this ohmically conducting film agrees with the potential below which aluminum hydroxide is expected to be more stable than aluminum oxide, near the metal/film interface. The conductivity of the cathodic film is within an order of magnitude of the proton conductivity of bulk hydrated aluminum hydroxide, AI(OH)3. H20. When the potential is stepped above the open-circuit potential subsequent to cathodic charging, there is a characteristic current decay during several seconds, after which the conductivity is three orders of magnitude smaller than at the cathodic potential. A mechanism is given, based on calculated overpotentials for interracial reaction and transport processes, through which the oxide film transforms to hydroxide at cathodic potentials.

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 KCI in 0.1 M HCl solution. Five minutes immersion of samples in the electrolyte solution prior to polarization was also necessary for the increase of cathodic current. 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 mechanism through which the oxide film resistance reduced during cathodic charging in acid solution is discussed.