Effect of Iron-Containing Intermetallic Particles on the Corrosion Behaviour of Aluminium

The effect of heat treatment on the corrosion behaviour of binary Al-Fe alloys containing iron at levels between 0.04 and 0.42 wt.% was investigated by electrochemical measurements in both acidic and alkaline chloride solutions. Comparing solution heat-treated and quenched materials with samples that had been subsequently annealed to promote precipitation of Al3Fe intermetallic particles, it was found that annealing increases both the cathodic and anodic reactivity. The increased cathodic reactivity is believed to be directly related to the increased available surface area of the iron-containing intermetallic particles acting as preferential sites for oxygen reduction and hydrogen evolution. These particles also act as pit initiation sites. Heat treatment also causes depletion in the solute content of the matrix, increasing its anodic reactivity. When breakdown occurs, crystallographic pits are formed with {100} facets, and are observed to contain numerous intermetallic particles. Fine facetted filaments also radiate out from the periphery of pits. The results demonstrate that the corrosion of "pure" 99.96% Al is thus dominated by the role of iron, which is the main impurity, and its electrochemical behaviour can be controlled by heat treatment.