Chapter B.5 - Influence of alloy composition

Abstract

The chemical composition is one of the main parameters governing the corrosion resistance of aluminium and its alloys. However, the effects of adding the same element can vary from one alloy family to another and even being very different, for example, on a cast alloy compared to that on a wrought alloy. All elements added in alloys can not only impact the overall corrosion resistance of the metal to a varying degree but also determine the type of corrosion: pitting corrosion, intergranular corrosion, filiform corrosion, stress corrosion, etc. The action of an element on corrosion resistance is very different according to the state in which it is in the metal structure. The element is in solid solution; it mainly has the effect of modifying the open circuit potential (OCP). However, this modification does not necessarily have a good or bad effect on the corrosion resistance of the alloy. Copper is a classic example: it increases the OCP but degrades the corrosion resistance of aluminium–copper alloys in the form of intermetallics, bonded with aluminium or with other elements present in the alloy. The influence on the corrosion resistance then depends on the electrochemical properties of the intermetallic compound in which it ‘resides’. The corrosion resistance of aluminium alloys depends strongly on their microstructure, including the nature and distribution of intermetallic compounds, the shape and particle size of intermetallics, etc.