Scanning Kelvin probe blister studies of the delamination of epoxy films on organosilane modified ZnMgAl alloy coated steel

Abstract

The scanning Kelvin probe blister test (SKP‐BT) was applied to study the corrosive delamination of model epoxy films applied to ZnMgAl alloy coated steel substrates. This test allowed the superimposition of mechanical and corrosive load. Moreover, ultra‐thin 3‐aminoproptriethoxysilane (γ‐APS) films were studied as adhesion promoting and corrosion protecting films, which inhibit the delamination process. The surface of the ZnMgAl alloy coating with and without applied ultra‐thin organosilane thin films were analyzed by polarization modulation infrared reflection absorption spectroscopy (PM‐IRRAS) and X‐ray photoelectron spectroscopy (XPS). The spectroscopic results show films that are only few nanometer thick, and therefore cannot act as efficient barrier films. However, peel‐force studies and SKP‐BT results show an effective inhibition of the delamination process of the epoxy film for the interface which was chemically modified by γ‐APS. The effect of the interfacial layer is mainly assigned to the higher wet‐adhesion based on the interaction of silanol groups with thin surface oxides and carbonates of the alloy. Raman spectroscopic analysis showed that the delamination on samples without organosilane thin films is based on a preceding cathode and a subsequent mixed corrosion. Only a minor cathodic delamination was observed on γ‐APS modified samples.