Comparison of different commercial pretreatment methods for hot-dip galvanized and Galfan coated steel

The performance of different pretreatment methods for hot-dip galvanized (HDG) and Galfan coated steel has been studied by treating both substrates with three different commercial pretreatment systems. Conventional chromate based pretreatment, zinc phosphating pretreatment and zirconium based pretreatment systems were studied. The chemically pretreated substrates were characterized by using electron spectroscopy for chemical analysis (ESCA), glow discharge optical emission spectroscopy (GD-OES) and atomic force microscopy. The pretreated surfaces were coated with a polyurethane clear coat and the corrosion resistance of the coated panels was tested by a prohesion test. ESCA results show that chromium from the chromate final rinses in the conventional chromate based and the zinc phosphating pretreatment is adsorbed of equal amount. ESCA results also show that there is a difference in surface composition of the zirconic acid rinsed samples prepared with two different techniques. According to the GD-OES results the galvanized steel strip materials react to a different degree in the chromate and zirconium based pretreatments. According to the prohesion test results of the clear coated pretreated HDG samples the zirconic acid rinse pretreatment is similar in corrosion performance to the two other pretreatments studied. The prohesion test result also show that the untreated Galfan coated steel strip has better corrosion resistance than the untreated HDG steel strip.