An Electrical Model for the Cathodically Charged Aluminum Electrode

Abstract

A mathematical model was formulated for the surface film on aluminum which considered the film to consist of an inner barrier high-field conducting layer and an outer, porous, ohmically conducting layer. It included all relevant capacitive and faradaic processes, and incorporated the nonlinearity of conduction and reaction rate expressions. The model was used to investigate structural changes produced by cathodic polarization in acid solutions, which has been found to promote anodic pitting strongly in ac etching processes. The film's structural parameters were fit to experimental current transients following anodic potential steps after cathodic charging. The model was found to represent experimental current decays realistically over several orders of magnitude variation of current density (factor of 103) and time (factor of 105). To a first approximation, cathodic activation could be described in terms of a decrease of the inner layer thickness (δ) from about 30 to 15 or 20 Å and an increase of the porosity (p) to 0.02. These changes were interpreted as being due to the rapid penetration of pores into the outer portion of the initial film. The pores may have been produced by electrochemical dissolution of the oxide during cathodic current flow through the film. The promotion of pitting by cathodic charging in chloride solutions suggests that some fraction of the pores may penetrate to the metal. A mathematical model was formulated for the surface film on aluminum which considered the film to consist of an inner barrier high-field conducting layer and an outer, porous, ohmically conducting layer. It included all relevant capacitive and faradaic processes, and incorporated the nonlinearity of conduction and reaction rate expressions. The model was used to investigate structural changes produced by cathodic polarization in acid solutions, which has been found to promote anodic pitting strongly in ac etching processes. The film's structural parameters were fit to experimental current transients following anodic potential steps after cathodic charging. The model was found to represent experimental current decays realistically over several orders of magnitude variation of current density (factor of 10) and time (factor of 10). To a first approximation,, cathodic activation could be described in terms of a decrease of the inner layer thickness (8) from about 30 to 15 or 20 A and an increase of the porosity (p) to 0.02. These changes were interpreted as being dueto the rapid penetration of pores into the outer portion of the initial film. The pores may have been produced by electrochemical dissolution of the oxide during cathodic current flow through the film. The promotion of pitting by cathodic charging in chloride solutions suggests that some fraction of the pores may penetrate to the metal. IntroductionSurface films on structural metals generally have a high electrical resistance which protects them from corrosion. However, the film resistance presents an obstacle to their use in applications where reactiv...

Fabrication of alumina films with laminated structures by ac anodization

“…The role of the cathodic half-cycle in the ac etch process has been explained in terms of (i) the formation of a precipitated aluminum oxyhydrate ‘etch film’ due to the increase in the local pH during the cathodic half-cycle [76, 91]; (ii) the hydration of the surface oxide film [92–94]; or (iii) the mechanical deterioration of the surface oxide film due to hydrogen gas generation [95–97]. In ac electrograining, Dyer and Alwitt [91] proposed a pit-on-pit propagation mode.…”

Behavior of interfacial micro-defects in aluminum and its oxide film

“…The film after cathodic polarization is likely porous based on the deuterium exchange experiment by Lin 48 , and film conductance measurements using the QCM 49 . Wang and Hebert 50 investigated the structure change of oxide film on aluminum due to cathodic charging using transient electrochemical measurements. They found a decrease of the inner pore-free layer thickness and an increase of porosity of the outer porous layer.…”

The roles of the surface oxide film and metal-oxide interfacial defects in corrosion initiation on aluminum

“…From Quartz Crystal Microbalance (QCM) experiments [9], they found further indications that no significant oxide dissolution occurs during the cathodic polarization period. Wang and Hebert [10] found that cathodic polarization leads to a decrease in barrier layer thickness and an increase in porosity. Their work is described in greater detail below.…”

“…After studying the initial stages of cathodic breakdown of thin anodic aluminum oxide films, Hassel and Lohrengel [10] proposed a mechanism for cathodic breakdown of the film.…”

“…10 The effect of dissolution time in 5 % H3PO4 solution at 85 °C on the barrier layer thickness of the oxide film as-received foils. Both types of the foils were pretreated in the H3PO4 bath for different times.…”