Intergranular corrosion of 8090 Al-Li: interpretation by electrochemical impedance spectroscopy

Aluminium-lithium alloys are widely used in the aircraft industry because of their low density and high elastic modulus. The Al-Li alloy 8090 has been well characterised from a mechanical viewpoint, but its corrosion mechanism has not been studied so completely. Recent work has focused on the use of electrochemical techniques to detect the early stages of localised corrosion and to assess the rate of corrosion taking place at specific locations in aircraft. Electrochemical impedance spectroscopy (EIS) has been used to measure and monitor the localised corrosion susceptibility of this Al-Li alloy, which is currently being used in aerospace applications, in solutions of 1 M NaCl + 10 mL H2O2 (30 wt-%) at neutral pH. During the corrosion process, there is an increase in surface inhomogeneity which causes deviations from the ideal behaviour proposed by Randles. A tendency to behave as a porous electrode is evidenced in the Nyquist plot by the presence of a straight segment in the high frequency range. This response is the result of preferential dissolution at grain boundaries, which leads to the development of deep intergranular attack and promotes a non-homogenous signal distribution. At low frequencies a capacitive arc appears that may be related to the chloride adsorption involved in alloy dissolution.