On the Galvanic Coupling of Bulk-AlCu Intermetallic Phases (Al2Cu, Al7Cu2Fe) and Pure Aluminium in NaCl Solution

Aluminium alloys with high mechanical properties, from the 2xxx and 7xxx series, are very sensitive to corrosion because of the presence of numerous intermetallic phases in these materials. It is well known that these phases are responsible of the formation of pits that can spread in bulk alloys via the grain boundaries or lead to generalised corrosion of alloys.

In many studies on corrosion of aluminium alloys, the galvanic coupling between the AlCu intermetallic phases and the aluminium matrix is the main phenomenon, supposed to explain the formation of pits. On the other hand, some authors have clearly highlighted the copper enrichment of some intermetallic phases as Al₇Cu₂Fe (1) and Al₂CuMg (2, 3).

The objective of this work is to study the electrochemical behaviour of Al₂Cu andAl₇Cu₂Fe intermetallic phases in NaCl solution, to characterize the corrosion rate by stationary electrochemical techniques and electrochemical impedance spectroscopy.

FEG-SEM observations, X-ray diffraction and X-ray photoelectron spectroscopy show that the two phases undergo a more or less complex dealloying phenomenon leading to the formation of a nanometric sponge of pure copper, covered with amorphous aluminium hydroxide. This dealloying phenomenon depends on both the copper content in the phase and on its crystallographic structure.

Then, the galvanic current density between the bulk AlCu phases and pure aluminium was monitored in function of immersion time. The comparison between the corrosion current density of the AlCu phases and the galvanic current density show that the galvanic coupling is often negligible compared to the intrinsic corrosion phenomenon of the phase.

So, the formation of pits on AlCu alloys seems to be mainly linked to the particular corrosion mechanism of AlCu intermetallic phases, inducing the formation of holes and pure copper. 

References: 

1. N. Birbilis, M.K Cavanaugh, R.G Buchheit, Corrosion Science 48 (2006) 4202-4215

2. R.G Buchheit, M.A Martinez, L.P Montes, Journal of the Electrochemical Society 147 (2000) 119-124

3. R.G Buchheit, L.P Montes, M.A Martinez, J. Michael, P.F Hlava, Journal of the Electrochemical Society 146 (12) (1999) 4424-4428