Detection of Moisture in Adhesive Bonds Using Electrochemical Impedance and Dielectric Spectroscopies

Abstract

The ingress of moisture into an adhesive bonded structure can lead to a variety of effects. Moisture has the ability to plasticize the adhesive but may also segregate into microvoids. The water in the voids will have less of an effect as it does not plasticise the matrix and is unable to influence the interface between the resin and the substrate. Only if the T g of the matrix drops below ambient will it allow expansion of the voids and then there will be consequential lowering of the fatigue characteristics of the resin. Water also has the ability to interact with metallic surfaces leading to changes in the nature of the interaction between adhesive and adherend. Water has a large dipole signature and is easily studied in terms of its effect on the electrical properties of bonded joint structures. In this paper, applications of electrochemical impedance and dielectric spectroscopies are described. Both techniques measure the electrical response of the joint, however the nature of the measurement performed allows slightly different, but complementary, approaches to be adopted. Application of the techniques to similar joint structures illustrates the similarities and differences of the two approaches. The article demonstrates the value of using electrical methods to probe the changes that occur in joints on ageing and emphasizes the value of these methods in developing and understanding of the phenomena that are responsible for loss of bond integrity.

Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: State of the art

“…Wahab et al [77] also found that the initial diffusion rate was twice as rapid into laminated adhesive resin discs compared with bulk adhesive. Investigations implementing more advanced methods, such as Fourier transform infrared spectroscopy (FTIR) [78,79], nuclear reaction analysis [81] and electrochemical impedance spectroscopy (EIS) [82,83] also confirm the previous findings. This implies that employing the diffusion coefficient of bulk adhesive specimens would result in an underestimation of the actual moisture content at the interface of steel and adhesive.…”

“…This variation, plus uncertainties resulting from many different combinations of adherents, surface preparations and adhesive materials, has made interfacial diffusion a complicated phenomenon to characterise. However, it is generally accepted that the diffusion process is much faster at the interface compared with bulk adhesive [76][77][78][79][80][81][82][83]. Zanni-Deffarges and Shanahan [76] compared the diffusion rates in bulk adhesive specimens and bonded joints and found that it was greater for adhesive joints.…”