Effect of Underlayer Metallic Coating Composition on Phosphating and the Corrosion Performance for Automobile Applications

Abstract

Automobile coating system consists of a metallic underlayer followed by a phosphate coating and, lastly, multilayer organic coating. In this work, the effect of the underlying metallic coatings, namely, a Mg-Al-Zn alloy coating (Magizinc [MZ]) and a conventional galvanized Zn coating on the phosphate coatings formed thereon, and its corrosion performance was investigated. The corrosion resistance offered by the phosphate coating formed on the MZ coating was higher than the phosphate coating on the galvanized Zn coating (a reference coating used in the study) in NaCl solution, as revealed by potentiodynamic polarization, electrochemical impedance spectroscopy, and salt-fog tests. In-depth characterization of the phosphate coatings was performed using scanning electron microscopy and glow discharge optical emission spectroscopy. It was revealed that the phosphate crystals formed on the MZ coating were more fine-grained, compact, and crack-free as compared to that formed on the galvanized coating and contained Mg aiding 4 to 10 times increase in the corrosion resistance as determined by the electrochemical studies. However, it only improved marginally against the appearance of red rust in a salt-fog test over the unphosphated MZ coating. The phosphate coating on MZ marginally improved the adhesion of an epoxy primer coating applied on the phosphated MZ coating and significantly (>3.5 times longer exposure) retarded the deterioration of the epoxy primer coating in the salt-fog environment in comparison with the similar studies performed on the phosphated conventional galvanized zinc coating. Notably, phosphating the MZ coating caused a 10 times reduction in the H pickup compared to that in the galvanized coating under identical phosphating conditions, suggesting the former coating lowered the propensity for hydrogen embrittlement in the steel.