One of the key coating layers that inhibits corrosion on modern automobiles is the pretreatment film. This layer, which is typically a tri-cationic zinc phosphate material, provides both corrosion protection and enhanced paint adhesion to the base metal. Recent tightening of environmental regulations has made the use of this coating more difficult. In response to these pressures, pretreatment suppliers have been developing a new generation of metal pretreatments based on zirconium oxide.
Characterization of these new materials is challenging as the zirconium oxide-based coatings are over ten times thinner than the current zinc phosphate coatings. Methods that are currently employed for studying zinc phosphate films such as coating weight determination by weighing, and scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDS) are not sensitive enough to fully characterize these materials. Therefore, we have employed a combination of surface analysis (Auger electron spectroscopy) and x-ray fluorescence to obtain a more detailed characterization of the zirconium oxide-based pretreatment coatings. These methods allow for an assessment of the variability in coating composition as a function of key parameters such as metal substrate and process variations.
Relatively thick, uniform coatings were found on galvanized substrates while significantly thinner coatings were observed on cold rolled steel (CRS) and aluminum (AL). The coatings formed on CRS and AL had significantly more variability than those deposited on galvanized steel. Coating bath contaminants also affected the coating thickness and corrosion performance on CRS and AL more than they did on galvanized steel.
