Metal dissolution and passivation in etch pits and tunnels on aluminum
In this work the mechanisms for initial growth and repassivation within cubic etch pits and etch tunnels were studied. Constant current and step current reduction experiments were performed to understand the current sources which supply the external current and the potential driving force for repassivation;During the first 35 ms of a constant current etch of 12.89 mA/cm[superscript]2, the potential transient goes through a maximum approximately 1.6 V above the open circuit potential. It was found that the external current was supplied by a capacitive charging current and a metal dissolution current. For samples which had been pretreated with HCl prior to the etch, it was found that the initial growth of the pits is due a currentless mechanism in parallel with the metal dissolution current density. In contrast, a sample which had not been pretreated gave evidence of electrochemical dissolution only. Vacancy condensation was discussed as a possible currentless mechanism;The results from the step current reduction experiments were used to determine the kinetics of metal dissolution and repassivation. A decrease in the surface over-potential relative to the repassivation potential is associated with an increase in the passivation velocity and a decrease in the metal dissolution current density within a pit. A mathematical model which predicted the potential transient after a step current reduction was developed. A mechanism involving competitive adsorption between specifically adsorbed chloride ions and oxygen, similar to that proposed by Kolotyrkin (1) and Uhlig and Bohni (2), was utilized in this investigation;A study of the pit and tunnel morphology was also used to study the potential driving force for repassivation. These observations gave evidence that nucleation induces the rapid passivation of small etch pits and the sidewalls of tunnels. A computer simulation which assumed that each pit and tunnel experienced the same potential driving force for repassivation was unable to predict the experimentally observed pit and tunnel morphology. Thus it was concluded that local potential changes accompanying pit nucleation are responsible for the rapid passivation that occurs. ftn (1) Ja. Kolotyrkin, J. Electrochem. Soc., 29, 363 (1989). (2) H. H. Uhlig and H. Bohni, J. Electrochem. Soc., 116, 906 (1969).