When the surface of an aluminium component is attacked down to the bare metal, a stable layer of aluminium oxide will form spontaneously in contact with the air. In the air, this oxide layer will remain perfectly stable in its passive pH zone (approximately 4 to 8.5). Beyond this level, amphoteric metal aluminium becomes sensitive to corrosion because the oxide layer is soluble in a number of acids and bases.
When the aluminium is in its passive pH zone, corrosion tends to occur as pitting or localised corrosion. Corrosion takes place where the oxide layer is breached or under a deposit and is mainly caused by chlorides.
Aluminium is extremely corrosion-resistant in very pure water. However, aluminium is very sensitive to galvanic corrosion when coupled with other, nobler, metals such as copper, lead, nickel and tin. Galvanic corrosion also occurs when the ions of these noble metals are dissolved in water (e.g. 20 ppb Cu2+ will suffice) and are deposited on the surface of aluminium parts. Aluminium is also sensitive to microbiological corrosion.
Aluminium can be used successfully in applications using clean seawater providing the right grade is used. The rate of corrosion tends to increase in inverse proportion to temperature, pH and to the circulation velocity, and in direct proportion to dissolved oxygen and dissolved metal ion concentration.