Optimization of commercial zirconic acid based pretreatment on hot-dip galvanized and Galfan coated steel

Abstract The influence of a commercial zirconic acid based pretreatment has been investigated on hot-dip galvanized and Galfan coated steel. The studied pretreatment system includes two alkaline cleaning steps, an alkaline oxidation and a zirconic acid based after rinse. The optimum conditions for the after rinse were tested by varying the rinse parameters such as treatment time, temperature, solution concentration and pre- and postdrying of panels. The pretreated panels were characterized by using electron spectroscopy for chemical analysis (ESCA), glow discharge optical emission spectroscopy (GD-OES) and contact angle measurements. Pretreated panels were also clear coated and corrosion resistance of the coated panels was tested. The results show that, for the two lowest solution concentrations, time and temperature are the most important treatment parameters. They affect the amount of zirconium adsorbed in the conversion coating, but are less important for the highest solution concentration. The highest level of zirconium on the samples is obtained with the intermediate concentration, in combination with the longest treatment time and highest temperature. According to peak positions in the ESCA spectra, the Zr adsorbed is mainly in the form of ZrO 2 . GD-OES depth profiles show that the zirconic acid based pretreatment has reacted to different depths in the studied zinc coated steel strip materials. The trend in the corrosion resistance is similar for both substrates. According to contact angle measurements the initial pretreated surface is wetting and hydrophilic in nature. The pretreated surface was also found to age rapidly.