Abstract
A comparison is made between potentiostatic (PS) and galvanostatic (GS) polarization processes for studying the degradation mechanism in polymer-coated metals. The PS process integrates effects over the measured system (the coating itself, the coating/metal interface, and the substrate metals), making it difficult to derive information for the coating and substrate separately because the polymer coating still retains a high insulation to prevent the representation of undercoating impedance in the early exposure time. In order to study the coating/metal interface effect, the GS process was applied to furnish a constant DC current- +0.15μA or -0.15μA -on the measured system from the beginning of exposure. The correlation between coating degradation and interfacial adhesion can be evaluated by the visual observation over of coating blister, SEM surface photographs, and X-ray diffraction.
The degradation processes occurring between Zn plated steel and the steel with Cu-Ti added. Coating blisters on Zn plated steel are mainly due to the general corrosion behavior of the anodic area. On the other hand, in the case of the steel with Cu-Ti added, interfacial adhesion would deteriorate as a result of cathodic reduction reaction, and be controlled by the diffusion behavior of the reactants.
