Aluminum alloys corrosion intermetallic particles

Instrumentation. The experimental procedure for an AFM equipped with a suitably coated tip has been outlined above. In a study of an aluminum alloy AA2024-T3, intermetallic particles and the matrix phase could be separated clearly [98]. The different surface films on these phases could be associated with their corrosion behavior. Inclusions and their corrosive behavior have been studied with a combination of SKPFM and AFM [101]. The effect of chloride-containing solution on corrosion at the matrix and the intermetallic particles was studied with SKPFM, in addition, light scratching with the AFM in the contact mode was applied to study the effect of the mechanical destabilization [102]. The intermetallic particles dissolved immediately after the film on their surface had been destabilized by mechanical abrasion. 

The corrosion resistance of 3xxx wrought aluminum alloys is very high. The manganese is present in the aluminum matrix as submicroscopic precipitates. The secondary particulate phases are intermetallic compound particles such as MnAl. Good resistance to corrosion of these series is primarily explained electrochemically - the corrosion potential of MnAl is almost the same as that of the aluminum matrix. 

Figure 9-17. Schematic showing the corrosion of aluminum around an aluminum-copper intermetallic particle in an aluminum copper alloy with a copper content of 0.5-2%. The aluminum-copper particle, in the presence of pure aluminum, promotes the reduction of water (shown) or oxygen (not shown). Simultaneously, the reduction reaction causes the pure aluminum to oxidize and then dissolve. This localized corrosion (Al dissolution) results in the formation of pits.

Davoodi, A., Pan, J., Leygraf, C., Norgren, S., The role of intermetallic particles in localized corrosion of an aluminum alloy studied by SKPFM and integrated AFM/SECM. J. Electrochem. Soc. 2008, 155, C211. 

Aluminum alloy microstructures are developed as a result of alloy composition and thermomechanical treatment. From a corrosion perspective, the dominant features of alloy microstructure are grain structure and the distribution of second phase (intermetallic) particles as constituent particles, dispersoids, or precipitates. Such particles have electrochemical characteristics that differ from the behavior of the surrounding alloy matrix, making alloys susceptible to localized forms of corrosion attack that has been termed microgalvanic corrosion. 

Intermetallic particles in aluminum alloys may be either anodic or cathodic relative to the matrix. As a result, two main types of pit morphologies are typically observed. Circumferential pits appear as a ring around a more or less intact particle or particle colony and the corrosion attack is mainly in the matrix phase. This type of morphology arises from localized galvanic attack of the more active matrix promoted by the more noble (cathodic) particle as is shown in Figure 16.6, which also shows the phenomenon with an image collected via optical profilometry. 

The metallurgical characteristics of the aluminum oxide layer also depend on its physical metallurgy, such as defects and metallurgical structure included in the oxide layer. For instance, when intermetallic compound particles as secondary phases are exposed on the surface, a discontinuous oxide film with various defects is often produced at the metal-particle interface. This discontinuous oxide film is weakly or non-protective chemically and physically. Because corrosion is a chemical and electrochemical reaction on the surface, corrosion behavior is readily influenced by surface morphology. The aluminum surface is usually adsorbed or contaminated by water, gases and many kinds of micron-sized substances. Microscopic heterogeneous structures such as vacancies, steps, kinks, and dislocations, and macroscopic heterogeneous structures such as scratches, pits and other superficial blemishes influence the corrosion behavior of aluminum and its alloys to different extents. 

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