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Pitting is used to describe the formation of small pits on the surface of a metal or alloy. Pitting corrosion is a very severe form of localised corrosion. A characteristic of pitting is that only small spots on the metal surface corrode, but due to the small ratio of the anodic on cathodic surface area, the corrosion pits can grow quickly resulting ultimately in perforation of the metallic structure.

Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped. The number of pits that initiate depends on the alloy. For example for a 1199 alloy in the order of 100 pits cm−2 are initiated, while for a 4% copper containing alloy the number of pits is in the order of 1000 pits cm−2. Hence, alloys from the series of Al-Cu and Al-Cu-Mg alloys (those in which Cu is the major alloying element), are especially prone to pitting.

Image of Pit Propagation Mechanisms Image of Pit Propagation Mechanisms of Al 2

First pic: Pit on a Cu-containing alloy; Second pic: Pitting corrosion with relatively deep attack on an unclad alloy.

Pit Propagation Mechanisms of Al

The formulation of the mechanism of pitting corrosion of aluminium is very complex. Although, it is generally accepted that there are two steps to the mechanism

Pit initiation


The initiation of pitting corrosion is often linked to the presence of local defects at the metal surface such as flaws in the oxide or segregates of alloy elements, and the presence of aggressive anions such as chlorides in the environment. The chlorides are believed to locally disrupt the oxide, preferably at pre-existing weak spots, resulting in microfissures of several nm in diameter.

Pit propagation

Amongst the many initiated pits only a few will propagate. The proposed mechanism shown here supports recent studies indicating that pitting corrosion of aluminium is a discontinuous process.

The overall reaction equation can be described as follows
2Al + 3H2O + 3/2·O2 → 2Al(OH)3

The pit propagation process is built up of different intermediate steps
In the initiated pit, aluminium is dissolved : Al -->Al3+ + 3e−, and the Al3+ cations react with Cl− anions to form a complex intermediate species AlCl4−. Hydrolysis of this species results in acidification of the bottom of the pit to a pH < 3 (due to formation of H+). This highly aggressive environment results in autopropagation of the pit.
Al3+ cations concentrated at the bottom of the pit diffuse out of the pit, where they encounter a more alkaline environment due to the cathodic process of hydrogen gas evolution : 6H+ + 6e− → 3H2, or water reduction:2/3·O2 + 3H2O + 6e → 6OH−

As a result aluminium hydroxide is formed and precipitates at the borders of the pit. This deposit of white corrosion product grows to eventually block off the entrance to the pit for further ionic exchange processes.
 
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