Coating and interface degradation of coated steel, Part 1: Field exposure

Abstract

The cathodic delamination (CD) rate of organic coatings on steel can be easily measured with a Scanning Kelvin Probe (SKP). In this paper, the measurement of CD by SKP (CD-SKP) was used for the first time to assess the interface stability after field exposure of polymer-coated steel. The uncoated region used as defect to initiate the CD in the humid SKP chamber was covered during field exposure. The CD rates of the field-exposed samples were compared to unexposed samples to evaluate the interface degradation. The CD-SKP assessment was related to the visual inspection of corrosion products through the transparent coating and electrochemical impedance spectroscopy (EIS) results. The CD rate measured after field exposure corresponded to the site severity that was estimated by the visual inspection. The EIS results were not always reproducible owing to the small exposed area, but overall the coating protection measurements were in accordance with the CD-SKP results. However, CD-SKP was more sensitive and faster than EIS. Furthermore, CD-SKP measurements exhibited clear evidence of improvement of interface stability during the initial stages of field exposure, which was followed by degradation of the interface. The CD-SKP technique can be used in practice to shorten the field exposure time of coated steel and provide early detection of the onset of interface degradation.