Microstructural Analysis of Corrosion Damage of LY12M Lap Joint in Aging Aircraft

摘要

The LY12M lap joint of aging aircraft was torn down and then inspected, which had serviced for more than twenty years and had retired recently. Both two- and three-dimensional metallographic montages were prepare to characterize the corrosion damages and to determine the extent of corrosion penetrations. In addition to the optical microscopic analyses, energy dispersive X-ray spectroscopy (EDS) was used to analyze the composition of specific corrosion by-products. Based on the results presented in this paper, it can be concluded that corrosion inside lap joints is very complex. Some forms of corrosion observed were exfoliation corrosion, pitting, preferential dissolution, and intergranular corrosion. Different forms and degrees of corrosion based upon corrosion damage location. Small pockets of intergranular corrosion were founded under exfoliated grains. This observation suggests that the practice of grinding out areas of corrosion is no guarantee of completely removing the corrosion. A fatigue crack can initiate immediately when a localized region of corrosion of an appropriate size is introduced, and the measurement of vertical or pillowing displacement along is not sufficient to assess the structural integrity of lap joints.