Grundlegendes Verständnis der Substratkorrosion und der lokalen Schädigungsprozesse in Klebstoff/Oxid/Metall-Grenzflächenphasen
Final Report Abstract
For the achievement of a complete understanding of the ageing and corrosion of adhesively joint steel substrates, both the ageing of a polyurethane reference adhesive and the simultaneous corrosion of a mild steel substrate were studied by combined spectroscopic, electrochemical and microscopic techniques. A corundum blasted mild steel substrate was used as metal substrate in combination with a reference reactive polyurethane adhesive free from additional additives or fillers. The ageing was performed under full immersion in a chloride-free aerated electrolyte at elevated temperature. A clear influence of the water saturated polymer phase and the limited access of oxygen on the change in the passive film structure of the metal substrate was observed. The thickening of the metal oxide film during the corrosive delamination of the polyurethane film indicated that the de-bonding mechanism is based on the ingression of water in the interface leading to an alkaline interfacial pH in combination with the interfacial oxygen reduction. Within the frame of the project a new Height Regulated Scanning Kelvin Probe Blister Test (HR-SKP-BT) was designed based on a commercial height-Regulated Scanning Kelvin Probe and a custom made electrochemical blister test. It enables the combination of the application of a mechanical load during the corrosive delamination of a polymer film. Based on initial studies of Wapner and Posner et al. at the MPI für Eisenforschung in Düsseldorf a new setup was designed at the TMC in Paderborn. Within this project we mainly addressed the understanding of the influence of the applied mechanical load and the water activity on the corrosive delamination based on the combination of the experimental studies with theoretical calculations (finite element methods) in the cooperating groups within the AiF/DFG-BestKleb-program (Prof. Diebels, Dr. Brede). We could show that the delamination kinetics at moderate blister pressures is determined by the interfacial degradation of adhesive interactions which are due to oxygen reduction reactions at the front of delamination. Only when the interfacial forces are lowered to a critical value by these electrochemical processes the mechanical force applied by the blister leads to a macroscopic de-adhesion. The acceleration of the delamination process with increasing pressure of the blister and increasing water activity in the adhesive/metal interphase is assigned to the promotion of the cathodic oxygen reduction reaction in the water containing interface and the increase in the energy release during de-adhesion. The aim of back-side Raman spectroscopic studies of delamination processes was to complement electrochemical SKP data with spectroscopic information. Moreover, the Raman studies should allow for an analysis of the de-adhesion reactions not only for free-standing adhesive films but for complete adhesive joints. An electrochemical in-situ cell was designed allowing the cathodic delamination process to be accelerated while the back-side imaging Raman analysis was employed to follow the interfacial and interphasial degradation mechanisms. ITO was used as working electrode and single MICA sheets as the second impermeable substrate which were then joined using an epoxy adhesive. Based on this approach we could detect a molecular degradation of the epoxy film in the delamination area with a spatial resolution of few microns which is superior to the possible spatial resolution of the SKP-approach. The effect of alkaline and acidic pretreatment steps on the surface chemical composition and adhesion properties of ZnMgAl-alloy coated steel was investigated by means of spectroscopic methods, Scanning Kelvin Probe (SKP) and peel test measurements. The spectroscopic results indicate that the surface film composition can be adjusted by the wet-chemical treatment. To study the corresponding surface adhesive properties, the samples were coated with an epoxy amine adhesive. Peel tests under humid conditions indicated an increased interaction between the acidic pre-treated surface and the adhesive. The results of the SKP analysis show that the acidic cleaned substrates have the highest resistance to delamination which can be explained by the shift of the interfacial electrode potential.
Publications
- Adhesion and corrosive delamination of epoxy films on chemically etched ZnMgAl-alloy coatings, Materials and Corrosion 2016
K. Pohl, O. Ozcan, M. Voigt and G. Grundmeier
(See online at https://doi.org/10.1002/maco.201608968) - Ageing mechanisms of polyurethane adhesive/steel interfaces, International Journal of Adhesion and Adhesives, 2016
J. Weiss, M. Voigt, C. Kunze, J. E. Huacuja Sánchez, W. Possart and G. Grundmeier
(See online at https://doi.org/10.1016/j.ijadhadh.2016.06.009) - Scanning Kelvin Probe Blister Test Measurements of Adhesive Delamination – Bridging the gap between experiment and theory. International Journal of Adhesion and Adhesives, Volume 73, March 2017, Pages 8-15
R. Grothe, Chen-Ni Liu, M. Baumert, O. Hesebeck, G. Grundmeier
(See online at https://doi.org/10.1016/j.ijadhadh.2016.11.006)
