Eddy-Current Detection of Pitting Corrosion in Aircraft Lap-Splices

Corrosion is one of the most important limits on the life of transport aircraft. The outer skin of transports consists of thin sheets of aluminum that in many aircraft are fastened at lap-splices. The infiltration of moisture initiates corrosion between the sheets and thus internally erodes the outer covering of the plane. A substantial effort has been undertaken to find ways to detect and quantitatively measure such hidden corrosion. It has been shown that the swept-frequency or pulsed eddy-current impedance can be used to quantitatively measure uniform loss of material (over areas > 1 cm2) in both the first and second layers of the lap-splice [1, 2, 3]. Consequently, eddy-currents form the basis for a quantitative inspection system. However, corrosion does not always result in a uniform loss of material. Sometimes the loss is highly localized and is aptly described as “pitting corrosion”. Consequently, there is a need for understanding the effects of such pits on the eddy-current response.