The corrosion and filming behaviour of aluminium and its alloys in aqueous solutions is sensitive to surface heterogeneities that are frequently microstructural constituents. The aim ofthe present study is to identify and to understand the behaviour at, and in the zone of influence of, hetrogeneities in selected aluminium alloys, in particular second phase particles, after immersion in various environments including deionized water, chloride, acid and inhibitor-containing solutions. The investigation employed transmission electron microscopy (TEM), allied with energy dispersive analysis of X-rays (EDX), and atomic force microscopy (AFM), supported by scanning electron microscopy.

In order to observe directly the behaviour associated with heterogeneities, at appropriate resolution, following exposure to various solutions, two relatively new approaches for specimen preparation for TEM and AFM have been developed based on ultramicrotomy using a diamond knife. One involves immersion of electron transparent ultramicrotomed sections under the selected solutions and consequent examination by TEM and EDX. The second is based on use of the face of the block, from which ultramicrotomed sections had been generated, creating a specimen with a microscopically smooth surface (< 1 nm) for AFM examination.

The study of superpure aluminium (99.99 wt%), after immersion in various solutions, reveals that the initiation and propagation of corrosion proceed at both residual-type flaws, i.e. grain boundaries or sub-grain boundaries, and mechanical-type flaws at damaged regions, i.e. scratches.

Concerning aluminium alloys, observations of ultramicrotomed sections containing various second phase particles, after immersion in deionized water and chloride solution, show that the corrosion behaviour is influenced significantly by second phase particles. The extent of corrosion is related closely to the type (composition) and size of the particles, the compositions of the adjacent alloy matrices and the solutions employed. Detailed examination reveals development of altered regions, showing fine textures in the zones ofinfluence ofthe second phase material into the aluminium matrix immediately adjacent to particles that display nett cathodic behaviour. Such texture, associated with different extents of A1(OH)₃ development, results from local alkalinity generated at the second phase and its consequent effects on the adjacent, filmed aluminium matrix, where the anodic process is located. For particles which show nett anodic behaviour, there is little or no influence on the adjacent alloy matrix;​ in such case, the anodic and cathodic processes are located at the particles where, for example Mg₂Si, undergo selective dissolution of magnesium.

The immersion of specimens containing second phase particles in acid solutions reveals significant thinning of the aluminium matrix adjacent to second phase particles showing nett cathodic behaviour, dissolution of particles with nett anodic behaviour and no change at both the particles with an inert nature and the adjacent matrix. For second phase particles containing silicon (e.g. AlFeSi particles), the nett cathodic behaviour of the particles in chloride solution was not observed during HF pickling;​ instead relatively inert behaviour with respect to the matrix was evident. This arises from the enrichment of silicon at the film/alloy interface during acid pickling;​ in this case, silicon cation species enter the oxide film, but are probably immobile under the field, due to the high value ofthe Si-0 bond energy. This may lead to a film which is more ionically resistive than the usual anodic alumina.

The distinct effects of inhibitor species, e.g. SrCO₄ and CeCl₃, on the behaviour at individual second phase particles and their zones ofinfluence into the matrix have been revealed. During immersion in chloride solution, in the presence of chromate ions, no significant thinning of the matrix immediately adjacent to nett cathodic second phase particles and no loss of nett anodic particles, are evident. In both cases, the matrix and particles appeared unchanged after immersion, but chromium species are detected over the matrix and second phase material. With cerium ions in chloride-containing solution, the zones of influence adjacent to nett cathodic particles are hot revealed;​ cerium is detected at second phase particles. For second phase particles with anodic behaviour, e.g. Mg2Si, dealloying proceeds by selective dissolution ofmagnesium from the particles, which results in the change of nature ofthe particles from anodic to cathodic behaviour;​ the resultant cathodic role allows corrosion inhibition by cerium ions at such particles.