In situ Investigations of Near Phase Boundary Water Diffusion of Ageing Adhesive Bonds by Scanning Kelvin Probe
Authors: Andreon Barbara Priscila , Plagemann Peter 
Published: February 2020

Many industries nowadays rely on the adhesive bonding technology for their success, most notably aerospace, automotive, construction, and the electronics industry. One of the main advantages of the adhesives is the ability to bond dissimilar materials and the more efficient load transfer in comparison to other joining methods. Several industrial applications request the adhesive joint to withstand loads under different environmental conditions such as high moisture, and extreme temperatures. Thus, predicting the lifetime of adhesively bonded joints under moist environments is crucial to support tailoring the properties for the respective application demand. Relevant effects of water in adhesively bonded components may be strength degradation and introduction of stresses. Knowing the behaviour of the water in adhesive joints may contribute to directing the design of bonded components, lead to greater confidence and increase the use of adhesives in the industry.
A significant cause of joint weakening is the effect of water in the interface between adhesive and substrates. There is evidence that the transport kinetics of water in the adhesive bulk do not correspond to the kinetics in the adhesive joint, studies show that water can be absorbed much faster by the interface than by the bulk. [1, 2] The study of the kinetics of water absorption in the interface is a particular challenge, and is also gravimetrically hardly detectable. A powerful and established method which can detect directly the corrosion induced delamination of polymer coatings is the scanning Kelvin probe (SKP). [3]
In the present work the SKP is used to study the water uptake of adhesive bonds. To determine the water uptake processes of an adhesive joint by means of SKP, an adhesive is applied to a metal surface, and as second substrate a very thin non-conductive barrier layer is used. Several scanning Kelvin probe experiments are carried out, under different environmental conditions, to define which are the most suitable
adhesives, substrates and barrier layers to investigate the water diffusion in situ and near the interface of an adhesive joint. The potential difference distribution is measured time-dependent in order to obtain information about the water absorption kinetics. The tests are accompanied by analytical and electrochemical methods, e.g., dynamic vapour sorption and electrochemical impedance spectroscopy.
[1] Zanni-Deffarges, M. P.; Shanahan, M.E.R. (1995): Diffusion of water into an epoxy adhesive. Comparison between bulk behaviour and adhesive joints. In International Journal of Adhesion and Adhesives 15 (3), pp. 137–142. DOI: 10.1016/0143-7496(95)91624-F.
[2] Wapner, K.; Grundmeier, G. (2004): Spatially resolved measurements of the diffusion of water in a model adhesive/silicon lap joint using FTIR-transmission-microscopy. In International Journal of Adhesion and Adhesives 24 (3), pp. 193–200. DOI: 10.1016/j.ijadhadh.2003.09.008.
[3] Hausbrand, R.; Stratmann, M.; Rohwerder, M. (2009): Corrosion of zinc–magnesium coatings. Mechanism of paint delamination. In Corrosion Science 51 (9), pp. 2107–2114. DOI: 10.1016/j.corsci.2009.05.042.