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.