Alumina pitting corrosion

Experience shows that in certain cases, pitting corrosion in contact with water may develop during the first weeks of service. For example, in freshwater tanks, significant pitting corrosion can be observed after a few months of service as large pits, several millimetres in diameter and 1-2 mm in depth, covered by white pustules of alumina gel. 

Pitting corrosion in stagnant water has a different aspect than in moving water. Pits are often disseminated, of large diameter (1-5 mm), covered with voluminous white pustules of alumina gel, and sometimes with a deposit of hard, light yellow scale made up of carbonates. When scraping this deposit, a pit can be found, the depth of which may exceed 1 or 2 mm (Figure D.1.15). Analysis has shown that an acidic medium is retained under these scale nodules [35]. 

Aluminium corrosion results in the formation of alumina Al(OH)3, which is insoluble in water and precipitates as a white gel, which is found in corrosion pits as white gelatinous flakes. Upon corrosion, 27 g of aluminium will form 33.6 1 of hydrogen. 

Unlike other metals, corrosion of aluminium is always eye-catching because the corrosion pits are covered with white, voluminous and gelatinous pustules of alumina gel A1(0H)3. These pustules are much bigger than the underlying cavity. 

AlumiDa is a gelatinous white gel that covers corrosion pits. After several weeks of exposure to air, part of the water will evaporate and the alumina will look like a white powder. Whether dry or not, the alumina adheres well to the metal surface. 

This is the most commonly encountered form of corrosion for aluminum alloys. In certain near-neutral aqueous solutions, a pit once initiated will continue to propagate into the metal due to acidification of the solution within the pit. The acidic conditions limit the formation of protective alumina films that usually prevent pit growth [38]. 

The pitting- and crevice-corrosion processes eventually penetrate the foils and result in exposure of the graphite/aluminum composite matrix below, at which point the corrosion rate becomes extremely accelerated. Corrosion has been shown to proceed preferentially along foil/foil, wire/wire, and wire/foil interfaces in the composite (Ref 10). Severe exfoliation occurs because of wedging of the hydrated alumina (Al2(OH)3) corrosion products within the composite. Figure 4 shows an example of sev graphite/aluminum corrosion (known as catastrophic failure). This catastrophic condition can occur within 30 days in seawater after exposure of the graphite-aluminum matrix. Catastrophic failure in the marine atmosphere and in splash/spray environments is less rapd than in seawater. 

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