Effect of Germanium Additions on the Corrosion Behavior of an AlLi Alloy

The addition of lithium to aluminum and its alloys simultaneously increases strength and reduces density. However, in the past, low elongation to fracture and fracture toughness values have prevented the widespread use of aluminum-lithium alloys. Increased fuel costs in recent years have led to a renewed interest in the weight reduction of aircraft that might be achieved with the use of these alloys. A great deal of research is currently being conducted to improve the mechanical properties; this research includes the addition of small amounts of germanium. The germanium increases the solid solubility of lithium in aluminum and dramatically increases ductility values.

The purpose of this project was to study the effect of germanium on the corrosion properties of an aluminum-lithium alloy. The effect of heat treatment was also considered. Aluminum-lithium precipitates in the following sequence.

AlLi (solid solution) → δ^′(Al₃Li) → δ (AlLi)

The δ^′ phase is a metastable, coherent hardening precipitate that is present in underaged and peak-aged material. Overaging produces the lithium-rich δ phase, which has been identified as detrimental to corrosion resistance.

Three compositions were used in the experiments: (1) 99.98 Al, (2) Al-2.5 Li, and (3) Al-1.9 Li-0.2 Ge. The two alloys were solutionized and quenched, then age hardened to 80% of peak strength for both the underaged and overaged conditions.

Immersion test results indicate few differences in the various samples. The severity of pitting corrosion does not increase significantly beyond 30 days of immersion. Weight loss measurements and optical microscopy indicate that the metals begin to corrode uniformly after an initial period during which pitting corrosion dominates. The alloys also appear to undergo enhanced corrosion at their grain boundaries. Again, this is evident only in the 75- and 120-day samples.

Electrochemical tests demonstrate that the presence of lithium at these concentrations does not significantly affect the electrochemical behavior of aluminum. The underaged ternary alloy is more stable in terms of breakdown, repassivation, and current density (CD) than the pure aluminum or the binary alloy. The results of both the immersion and electrochemical tests will be discussed in terms of corrosion science and microstructural parameters.