SUBADHESIVE HYDRATION OF ALUMINUM ADHERENDS AND ITS DETECTION BY ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

Hydration of an aluminum oxide surface under an adhesive film was show n to occur when the specimen was immersed for extended periods in hot water. The absence of a crack and lack of access of the water to the s pecimen edges forced the moisture to reach the interface by migrating through the adhesive. The increased volume of the hydration product fo rced it to erupt through the adhesive. This subadhesive hydration is c onclusive proof that hydration can proceed in advance of a crack to pr omote its propagation. Electrochemical impedance spectroscopy (EIS) me asurements on the specimen during exposure readily detected both the i ngress of moisture through the adhesive and the hydration of the subst rate.