Characterization of Zirconium Oxide-Based Pretreatment Coatings Part 2 – Challenges in Coating Aluminum Body Panels

The pretreatment of aluminum sheet material in preparation for further paint application can be challenging due to the presence of a thick oxide layer. The composition of the oxide layer is primarily aluminum oxide, but it may also contain magnesium that is typically dispersed unevenly throughout the oxide layer. Zinc-phosphate systems remove much of the oxide layer on aluminum, but questions remain on the extent of removal of the oxide layer by zirconium oxide-based pretreatments and how these oxide layers may affect the zirconium oxide-based pretreatment deposition on aluminum.

Several methods have been used to characterize the coating of zirconium oxide-based pretreatments on aluminum. Scanning electron microscopy at very high magnification reveals a coating on aluminum that is significantly different in morphology than the same coating chemistry on steel substrates. Auger electron spectroscopy allows for further analysis of the coating, revealing a significant amount of inhomogeneity in the coating and often the presence of oxide layers under the coating. Although X-ray fluorescence has been a useful tool to characterize the coating on steel substrates, the technique is challenging on aluminum due to small amounts of zirconium in the aluminum substrate. This paper will discuss the variability in the pretreatment coating technology, the effects of oxides, aged material, and contamination on coating deposition and corrosion behavior.