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 tests (aluminum-aluminum bonds only) were performed. Upon moisture absorption, the impedance spectra change shape with the low-frequency region becoming resistive. The low-frequency impedance decreases by several orders of magnitude, depending on the adhesive and the experimental conditions. For bonds with 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 potential joint deterioration and enables condition-based maintenance. For bonds with smooth interfaces with little or no physical bonding (mechanical interlocking), crack propagation can proceed interfacially with minimal moisture absorption. A comparison of the incubation times for Forest Products Laboratory (FPL, or sulfuric acid-sodium dichromate) etched surfaces, both bonded to epoxy adhesives and freely exposed to water or humidity at different temperatures, shows that hydration occurs with the same activation energy and, hence, the same mechanism, independent of whether or not the surface is covered with adhesive. However, the pre-exponential factor in the rate constant is dependent on the concentration of free moisture at the interface so that the hydration rate varies by several orders of magnitude.