Use of impedance spectroscopy to study the integrity of the aluminium oxide films in mercury embrittled aluminium

Mercury embrittlement is a significant issue in the gas processing industry, where the precipitation of mercury from the gas stream in cryogenic heat exchangers can lead to embrittlement of the aluminium structure and it has been the cause of several significant failures. This paper studies the use of impedance spectroscopy to examine the interface between mercury droplets and 5083 aluminium. As-received, mechanically polished and artificially aged samples were examined using a two electrode mode with a zero bias. The Nyquist results indicated that as-received and polished and aged samples behaved as an Rs-C-Rp type circuit, indicating that the oxide film could be modelled as a capacitor and resistor in parallel. Using the dielectric constant of alumina, the capacitance results yielded oxide films of thickness between 2.2 and 50 nm, depending on the degree of aging of the sample. By gradually increasing the voltage amplitude, it was found that the interface broke down at a field strength of approximately 10MV/m, which is similar to the dielectric field strength of alumina. Immediately after polishing, however, no film was found and the interface appeared to be a simple short circuit. A series of bend tests coupled with the frequency response analyser were used to demonstrate that the film remained continuous beyond the point at which plastic deformation occurred and in many cases up until the point at which embrittlement occurred. These results have confirmed that the oxide film on aluminium can effectively separate mercury from the underlying aluminium alloy and that impedance spectroscopy is a useful tool for studying the stability of the interface.