Corrosion of painted galvanized steel pretreated with Zr-based thin films

Licentiatavhandling, 2015

For decades, phosphating has been considered state of the art for pretreating car bodies in order to promote corrosion resistance and paint adhesion. Although it offers excellent corrosion protection, the phosphating process has drawbacks. These drawbacks include limits on how much aluminium can be used in a car body, high process costs due to heating the process baths, and high costs for wastewater treatment. The phosphating process also utilizes toxic metals, such as nickel, which are on the REACH list. Since the early 2000’s, commercial alternatives to phosphate pretreatment have been available. These technologies are based on zirconium chemistry and have addressed the drawbacks with the phosphating process. There is limited knowledge regarding the paint delamination and corrosion behavior of automotive materials pretreated with these replacement technologies. In this work, the behavior of these zirconia-based technologies has been studied and compared to traditional phosphating. Utilizing an accelerated corrosion exposure regime, both short and long term exposures in atmospheric conditions have been used. Hot dipped galvanized (HDG) steel was used as a substrate. The resulting corrosion products have been analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). It was found that the paint delamination mechanism did not differ between the different pretreatment technologies when using the current exposure method. The main corrosion product found was simonkolleite (Zn5Cl2(OH)8•H2O). It was also found that the scribe was the area of cathodic activity and that the zinc was dissolved anodically.