Paintability of Traditional and Chromium(VI)-free Conversion Layers on Galvanized Steels

Continuous hot dip galvanized steel samples have been treated with traditional chromium(VI)-containing passivator and environment friendly rare-earth metal containing cerium-salt and lanthanum-salt based solutions using different immersion times. The treated surfaces have been examined by scanning electron microscope equipped with an energy dispersive X-ray analyser. After the pre-treatment the samples have been spray-painted with epoxy-based organic paint. The adhesion was tested with bending and cross-cut test. Thereafter salt spray test was performed on duplex systems. After 48 hours samples have been taken out from the corrosion chamber in order to perform electrochemical impedance measurements for 24 hours. Based oil the results the most promising passivation among the chromium(VI)-free solutions has been found to be the cerium-salt based treatment. The adhesion of the organic coating was better on cerium-chloride treated substrate than on chromated one, while the anti corrosion properties of the two duplex systems were comparable.