Electrochemical sensors for nondestructive evaluation of adhesive bonds

An in-situ corrosion sensor based on electrochemical impedance spectroscopy (EIS) has been used to detect moisture ingress into aluminum-aluminum and aluminum-composite adhesive bonds. Both wedge tests and tensile button test s (aluminum-aluminum bonds only) were performed. Upon moisture absorption, the impedance spectra change shape with the low-frequency region becoming r esistive. The low-frequency impedance decreases by several orders of magnit ude, depending on the adhesive and the experimental conditions. For bonds w ith stable interfaces, such as phosphoric acid anodized (PAA) aluminum, the absorbed moisture causes an initial weakening of the adhesive resulting in reduced strength or small crack propagation. A substantial incubation time prior to substrate hydration and bond degradation allows warning of potent ial joint deterioration and enables condition-based maintenance. For bonds with smooth interfaces with little or no physical bonding (mechanical inter locking), crack propagation can proceed interfacially with minimal moisture absorption. A comparison of the incubation times for Forest Products Labor atory (FPL, or sulfuric acid-sodium dichromate) etched surfaces, both bonde d to epoxy adhesives and freely exposed to water or humidity at different t emperatures, shows that hydration occurs with the same activation energy an d, hence, the same mechanism, independent of whether or not the surface is covered with adhesive. However, the pre-exponential factor in the rate cons tant is dependent on the concentration of free moisture at the interface so that the hydration rate varies by several orders of magnitude.