Etching Behavior of Aluminum in Hydrochloric Acid under Potentiostatic Condition

Abstract

Pit morphology under potentiostatic conditions was investigated for aluminum with (100) crystal orientation in 1.0mol dm^-3 hydrochloric acid solution at 363K.
The potential is considered to be an important factor in determining pit morphology. Increases in anodic potential led to an increase in the number of pits and in tunnel length, and to a decrease in pit diameter within 20C·cm^-2. The transition of pit sites indicated that many pits were formed at an early stage of etching, and then deepened until a limiting length was reached. New pits did not form but the walls of ordinary pits were dissolved as a second step at a higher anodic charge, corresponding to the point of inflection of current density with time.
The phenomenon in which tunnel growth stops abruptly at a limiting length seems to be associated with the increase of Al³⁺in the tunnel. It is considered that tunnel growth stops because the aluminum chloride at the tunnel tips saturates, causing the pitting potential to become more noble.