Determination of the Phosphate Quality of Automotive Organic Coatings on Different Steel Substrates by Electrochemical Impedance Spectroscopy

Various steel substrates which were processed under different pretreatment conditions and coated with automotive paint systems, were investigated by electrochemical impedance spectroscopy (EIS). The impedances of damaged and undamaged organic coatings on cold rolled steel were measured in an aqueous sodium chloride solution. These results were compared with two conventional accelerated exposure tests, the High Temperature Chipping Corrosion Test (HTCCT), and the modified Volvo outdoor exposure test. The chipped organic coatings on cold rolled steel, electrogalvanized zinc, ZnFe electrogalvanized, and ZnFe hot dip galvanized were then investigated by EIS. An inhomogeneous three-dimensional surface model was used to analyze the EIS data and characterize the corrosion protection of the paint/phosphate/metal system. The fraction of the total test area which had undergone paint delamination and corrosion was calculated from the model results. Agreement with the paint loss results from HTCCT and Volvo tests was obtained. The EIS is shown to be a useful and reliable tool to evaluate phosphate quality, and the corrosion performance of painted automotive sheet steels.