Surface Films Produced by Cathodic Polarization of Aluminum

Ching-Feng Lin
1994 Journal of the Electrochemical Society  
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
more » ... t 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. ABSTRACT 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/cm z 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. ABSTRACT 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 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.
doi:10.1149/1.2054716 fatcat:uhjonr4x5vfkpi7ejirn5npwgy