The Competition Mechanism Of Passivation-Activation Of Surface And Tunnel Walls In The D.C. Etching Of Aluminum Foils For Electrolytic Capacitors

A competition mechanism throughout the tunnel etching of aluminum foils is established in this study, namely, the competition between the attack to passive surface by aggressive anions and the passivation caused by passivators; dissolution competition between side wall of tunnels and overall surface of foils. To demonstrate the competition mechanisms, the etching conditions influencing the morphology of tunnels, surface modification of foils, redemption and regeneration of passive state were investigated. The relationship between capacitance and corrosion conditions in the enlargement of tunnels justifies the latter competition mechanismThe varied etching conditions were implemented by enhancing the passivating characteristics and viscosity, changing the A13+ concentration, temperature of etchants and current pulsation. Passivating acid in the etchant is conducive to the instant passivation of exposed areas on the foil surface, and hence sustains the balance of competition between aggressive anions and passivators, providing pit nucleatiori sites continuously. Increasing viscosity of etchant promotes the passivation of active areas. Uniform tunnel distribution is related to certain A13+ concentration avoiding large pits at the beginning stage of etching. The higher concentration of vitriol, the lower critical temperature of tunnel growth, and the lowest critical temperature so far can be reduced to 35 . Tunnel distribution can be improved with 6% pulsation of etching current.The surface modification of foils contributes to the activation of surface before etching, and consequently provides more pit sites. The redemption and regeneration of passive film on foil surface lead to continuous growth of tunnels. Anodized in oxide-forming acid, the etched foil shows higher capacitance. Implanting copper at M/O interface by means of anodic oxiding simulates the original foil surface.The enlargement of tunnels is determined to some extent by the dissolution competition of foil surface and wall of tunnels. Surface destroyed by large current density at the first stage of etching tends to dissolve than tunnel walls. The addition of surfactant to etchant can protect surface from intense dissolution, and keep arelative similarity of dissolving activity between surface and tunnel walls. In order to gain a satisfactory enlargement of tunnels, the etchant temperature should be controlled at 70-90 .