Electrochemical electrolyte spreading studies of the protective properties of ultra-thin films on zinc galvanized steel

Reactive electrolyte spreading along the surfaces of different conversion films on zinc galvanized steel in humid air was monitored visually and with a height-regulated scanning Kelvin Probe. Electrochemical impedance spectroscopy and current density-potential curves revealed that decelerated spreading kinetics are connected with increasing pore resistances of the pre-treatment layers and decreasing oxygen reduction current densities in the electron transfer controlled potential region. After a few days the progress ranking of electrolyte spreading along uncoated conversion films reflected the progress tendencies of cathodic delamination observed on epoxy coated conversion layers after long-time exposure to the same corrosive environment. Such correlation was not discovered for pre-treatment films that do not provide relevant electrochemical barrier properties. The results suggest that oxygen reduction driven electrolyte wetting is an option for accelerated performance testing of anticorrosive ultra-thin films on metal substrates that can be subject to cathodic delamination.