R. Posner

Wet- and Corrosive De-Adhesion Processes of Water-Borne Epoxy Film Coated Steel I. Interface Potentials and Characteristics of Ion Transport Processes

Steel substrates were coated with a water-borne epoxy polymer layer and investigated by in situ scanning Kelvin probe (SKP) experiments, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and peel tests. After ion transport processes along the polymer/substrate interface were initialized at an electrolyte covered coating defect, nonspecific potential profiles were detected with the SKP. An identification of the front position of the electrolyte was not possible in air of high relative humidity, because wet de-adhesion and hydrolysis processes determined the local interface potential. Although the established mechanistic model for cathodic delamination solely predicts a cation transport in humid air, anions of the defect electrolyte were also verifiable at the epoxy/steel interface. It is discussed which forces rather than ion diffusion were additionally effective and contributed to the overall driving forces for the ion transport in this case. The oxidation state of the resulting substrate structure was investigated to further analyze the epoxy/steel interface degradation. These results are compared to the initial surface condition of the uncoated steel surface. They are also compared to the surface condition after oxygen reduction induced electrolyte spreading proceeded along the uncoated steel substrate.

UV-Curable Polyester Acrylate Coatings: Barrier Properties and Ion Transport Kinetics Along Polymer/Metal Interfaces

Zinc and iron substrates were coated with polyester acrylate films. The samples were cured by an exposure to different doses of ultraviolet (UV) radiation energy at different radiant fluxes of the UV light source. Infrared spectroscopy was used to estimate the resulting degree of macromolecular cross-linking. Its effect on the barrier properties of the coating was investigated by Electro-chemical Impedance Spectroscopy (EIS). A reduced water uptake and a reduced swelling of the coating as well as a decelerated diffusion of water through the polymer bulk were detected on polyester acrylate coated zinc samples that were exposed to increasing UV energy doses at low UV light intensity. With the Scanning Kelvin Probe (SKP), a similar tendency was detected for the resistance of polyester acrylate/iron oxide/iron interfaces against cathodic delamination. In contrast, curing with large energy doses at high UV intensity led to decelerated ion transport kinetics along the interface, but did not further improve the barrier properties of the coating. This indicated that SKP data gained on Fe substrates and EIS data recorded on Zn substrates can be correlated if UV curing does not result in strong tension within the polyester acrylate films.

Wet- and Corrosive De-Adhesion Processes of Water-Borne Epoxy Film Coated Steel II. The Influence of -Glycidoxypropyltrimethoxysilane as an Adhesion Promoting Additive

Low carbon steel substrates were covered with a γ-glycidoxypropyltrimethoxysilane (γ-GPS) modified epoxy layer. Electrochemical impedance spectroscopy and attenuated total reflection infrared spectroscopy were used to investigate the barrier properties of the coating. The resistance of the samples to ion transport processes along the epoxy/steel interface was determined by in situ scanning Kelvin probe (SKP) measurements of the interface potential. X-ray photoelectron spectroscopy studies helped to analyze the resulting ion distribution on the substrate surface. The application of γ-GPS resulted in a reduction of the interfacial water activity and temporarily stabilized the polymer/steel interface. In contrast to the experiments with the unmodified epoxy coating, interfacial ion transport processes were verifiable with the SKP. After some days of sample exposure to humid air, the stabilizing effect of γ-GPS diminished and SKP potential profiles had to be recorded in dry atmosphere to identify the electrolyte front position, zones of cation and anion separation, and areas of local corrosion damage at the interface. This approach seems to be generally promising to analyze the degradation state of polymer/oxide/metal interfaces after long-term exposures in humid air.