THE VARIOUS FORMS OF ATMOSPHERIC CORROSION

The most frequent forms of atmospheric corrosion of aluminium are 

Uniform corrosion is not observed because it is so minute that it cannot be measmed. Other forms of corrosion of aluminium and its alloys such as exfoliation corrosion, stress corrosion, etc. are of course possible on age-hardenable alloys of the 2000 and 7000 series when exposed to weathering. However, the selection of alloys and their protection, if present, normally prevents these forms of corrosion from occurring in outdoor applications such as buildings or bridge railings. 

Atmospheric corrosion develops in humid environments with, in general, a pH close to neutral. As a consequence, pitting corrosion is the most commonly observed form of corrosion on aluminium. As discussed below, experience over decades has shown that the rate of pit deepening decreases with time. 

Experience has shown that atmospheric corrosion of aluminium does not always occur. There are many uses in which no pitting corrosion has been detected, even after many years of service. In other words, corrosion of aluminium exposed to weathering is not inevitable. 

Anodising is a means to reduce or even prevent pitting corrosion, if the thickness is sufficient for the exposure environment (see Section C.5.5). 

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