The corrosion of untreated aluminium products is due to local decomposition of the thin, passivating oxide film. Dry air refers to air with a relative humidity less than 60 per cent, which in Sweden only occurs on a few warm days in early summer. In the winter when the temperature drops below freezing point, no metals are subject to corrosion, as the process requires water in a fluid state.
Untreated aluminium products that is exposed to moisture during transport and handling must immediately be unwrapped and dried off. Goods that have become very cold, e.g. during winter transport, and which are transferred to heated premises, may develop condensation that can cause corrosion.
Condensed water vapour forms on metal surfaces that are cooler than the ambient air. Crevice corrosion can occur in narrow fluid-filled crevices (0.02–0.5 mm) between untreated aluminium surfaces because of variable oxygen potential (irregular air supply) and can cause patchy discolouration.
Damage from corrosion during the handling of untreated aluminium products can also be caused by clammy hands, where moisture is transferred to the aluminium surface by way of fingerprints. Due to the presence of sodium chloride and other electrolytes, sweat can cause potentially corrosive patches to appear. The effect of sweat varies depending on the individual. It is always better to wear protective gloves when handling untreated extrusions. In other instances, fingerprint marks occur in connection with anodisation.
It is a good idea to always unwrap aluminium products on delivery and then to store it in a dry, clean and well-ventilated place.
Fretting corrosion is a form of corrosion that occurs when aluminium extrusions rub against one another during transport, for example. When the oxide layer is rubbed away, a new one is formed, which will eventually cause black spots of aluminium oxide to appear at the point of contact.
The supplier is not responsible for corrosion that occurs after delivery to the customer caused by incorrect storage or handling.