Evaluation of the corrosion protection performance of automotive coatings

Reliable and fast evaluation of the corrosion protection performance of automotive coatings is of significant interest of manufacturers and users of coatings. For most of known laboratory tests, the evaluation of coatings bases on visual inspection, which is however not objective, not sensitive enough and allows no insight into the mechanism of the coating degradation. In this work, weathered samples of automotive coating systems were evaluated, using new detection methods. The reversibility of water uptake was chosen for the evaluation of the coating degradation. With the electrochemical impedance spectroscopy (EIS), it was possible to detect the stage of a coating degradation under a hydro-thermal cyclic loading. For the evaluation of the corrosion protective properties of electrophoreticaly deposited primers (EDP) in a multilayer coating system, upper layers were locally injured by drilling a small hole down to the EDP layer and the system was exposed to a hydro-thermal cycling. A microscopic analysis of the injured place provides information about the corrosion protection with EDP. For the detection of the photodegradation of EDP, special cross sections of a coating system were prepared to allow the FT-IR analysis. Based on IR spectra, a photooxidative index POI was calculated and used for the evaluation of EDP. The delamination resistance of coating systems was tested by drilling a small hole in the coating down to the metal substrate and by detection of this place with Scaning Kelvin Probe (SKP) after a hydro-thermal cyclic loading. SKP scans allowed a discrimination of coating systems due to their delamination resistance.