Corrosion in all its variety
The word ‘corrosion’ is one that everyone knows. What forms on iron is brown rust, on copper roofs a greenish patina and on aluminium parts a white tarnish. What appears so familiar, turns out when we look more closely to be complex.
Corrosion has many forms. It can be two-dimensional. It can start locally in holes or gaps and carry itself on in cracks. Such ageing can also start inside materials (hydrogen embrittlement). Whenever two metals are in contact with each other, the less noble of the two starts – if there is water present – to disintegrate. Mistakes in the design or manufacture of a component aid such processes, as does abrasion, tensile stress and wear during the component’s use.
Electrochemical corrosion is frequently encountered: a film of moisture from water is sufficient to start off oxidation processes. Salts and acids accelerate this. The chemical processes change in sea water or soil, as they do too in the presence of micro-organisms. Chemical corrosion without any water is also possible.
The multitude of corrosion types has been summarised by the International Standards Organisation (ISO) in standard 8044. The English version reads: “BS EN ISO 8044 – Corrosion of metals and alloys. Basic terms and definitions” and exists in the latest version from 2015.
And: no protective coat can fully stop corrosion. It is only possible to appreciably slow down the corrosion process to a greater or lesser degree. An appropriate coating system helps to do that.