Scanning Kelvin Probe study of electrochemical delamination in adhesively bonded joints

Adhesive bonding is a key technology for the construction of lightweight components and its interest in the industry is increasing due to the several advantages when compared to other joining technologies. A major concern is the long-term stability of adhesive joints, especially for metallic substrates exposed to corrosive media. The aim of this work is to investigate the delamination mechanisms occurring in a real closed adhesive joint geometry. To achieve this purpose, the potential distribution at the metal/polymer interface was measured by means of Scanning Kelvin Probe (SKP) through a thin layer of glass. Quantitative measurements of the delamination kinetics were performed, accompanied by XPS inspection of the fracture surfaces. The delamination rate was found to be hundred times slower than in an open joint geometry. Furthermore, the delamination-limiting step for the open joint is the transport of cations, whereas for the closed joint is limited by the ingress of oxygen along the interphase. The cathodic delamination was the mechanism taking place in both cases, however depending on the geometry of the closed joint also the anodic undermining mechanism was found to occur.