Impedance imaging for prediction and detection of airframe corrosion

Electrochemical impedance spectroscopy (EIS) is widely recognized as a versatile technique for evaluating the kinetics of corrosion process and, in particular, the degree of protection afforded by oxide, organic and other insulating coatings. In this work we have used the EIS method to study the behavior of coated and uncoated airframe aluminum alloy 2024-T3 corroding in 0.5M aqueous NaCl; crevice corrosion in this alloy has also been studied via use of simulated joints.^ The EIS spectra observed on coated specimens yield a characteristic, two time constant spectrum that can be modeled by a circuit incorporating a geometric coating capacitance, a pore resistance, a polarization resistance and a double layer resistance and the decrease of coating capacitance and polarization resistance can be related to the degradation of the coating.^ The EIS spectra for coated 2024-T3 aluminum with simulated joint showing crevice corrosion display three semicircles on the Nyquist plot, which can be modeled by a circuit with three time constant. For the third time constant, a crevice-related capacitance and a charge transfer resistance corresponding to the crevice were used to model the impedance spectra with good fitting results; and values of the crevice resistance can be justified with results from measuring the surface morphology changes for simple crevice geometry created by placing a micro slide on top of 2024-T3 aluminum in aqueous 0.5M NaCl solution.^ The corrosion behavior in aqueous 0.5M NaCl of 2024-T3 aluminum treated with a wool wax (lanolin) based corrosion preventive coating was studied via EIS and DC electrode polarization. The wool wax coating was found to be very corrosion preventative for this alloy in the aqueous saline environment whether applied to freshly prepared surfaces or to corroded specimens removed from simulated service.^ An EIS procedure can be designed that contributes a new NDE technique for the detection of corrosion on aircraft structures. This is especially sensitive to crevice corrosion which cannot be detected by other NDE methods.