Aluminum and its alloys are divided into two broad classes, castings and wrought or mechanically worked products. The latter is subdivided into heat-treatable and non-heat-treatable alloys, and into various forms produced by mechanical working. The corrosion resistance of aluminum is dependent upon a protective oxide film. This film is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired.
The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook. Some excerpts of that document are used here.
The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode but by no means always does so. For example, aluminum is quite resistant to concentrated nitric acid. When aluminum is exposed to alkaline conditions corrosion may occur, and when the oxide film is perforated locally, accelerated attack occurs because aluminum is attacked more rapidly than its oxide under alkaline conditions. The result is pitting. In acidic conditions, the oxide is more rapidly attacked than aluminum, and more general attack should result.
Aluminum alloys are also susceptible to hydrogen embrittlement, although the face centered cubic microstructure means that the transport of hydrogen is slower than in high strength steels, and hence the crack growth rate may be lower. The cracking is normally intergranular. As with steels the susceptibility becomes more severe as the strength of the alloy is increased. However, there is also a strong effect of heat treatment and microstructure, and quite high strengths can be obtained with good SCC resistance (as is demonstrated by the use of these alloys in aircraft construction). Any environments that can provide hydrogen can lead to SCC of susceptible alloys, ranging from humid air to salt solution. (reference)
|Quite small changes to the composition of an alloy can have a marked influence on the SCC behavior. For example The figure shown here the effect of copper content on the crack growth rate of a series of Al-Cu-Mg alloys. Some care needs to be exercised in interpreting this graph. The change in copper concentration of the alloy will have a marked effect on the corrosion behavior of the alloy (since the copper-containing precipitates are active cathodic sites), but it will also modify the mechanical properties of the alloy and its response to heat treatment. Consequently, it would probably be possible to change the relative orders of the curves by using different heat treatments. (reference)|