Mechanism of Pit Growth in Homogeneous Aluminum Alloys

Abstract

Pitting corrosion is a process, which takes place on passive metals and alloys. A characteristic of this type of corrosion is that passivity breaks down at isolated points at the surface and the growth of pits is observed due to locally high rates of metal dissolution. In electrochemical experiments, the growth of pits leads to a rapid rise of the overall current density once a characteristic threshold potential, the pitting potential E(pit) is surpassed. Since the measured current density is composed of the passive current density at the passive surface area and the current density of fast metal dissolution at the pitted area conventional electrochemical tests are not useful for studying metal dissolution inside pits. Some authors tried to overcome this difficulty by working with small wire electrodes. The idea behind was to achieve an electrode state where the whole surface represents a pit and the measured current density becomes, therefore, identical to the real current density inside a pit. However, highly concentrated electrolytes and high potentials must usually be applied to achieve this electrode state [1-3]. Formation of salt films and mass transport control was usually observed under these conditions, which more likely represent the conditions of electropolishing rather than that of a metal suffering pitting corrosion at the corrosion potential.