Aluminium alloys:

• Comparatively low density; high resistance; oxidising substances, e.g. atmospheric oxygen, cause formation of an aluminium oxide layer on the surface which protects the underlying aluminium from further oxidation and corrosion (passivation).
• The structure of the oxide layer depends on the respective storage conditions, namely is relatively undefined.
• Suitable treatment can remove the oxide layer and form a new, defined, porous layer firmly attached to the base material. This is the reason for the good bonding properties, in particular with phenolic and epoxy resin adhesives. These classes of adhesives form chemical bonds with the aluminium oxide, meaning the bonded joints have good long-term stability.
• Corrosion processes can, however, also result in detachment of the aluminium oxide from the base material (“bondline corrosion“).