Effect of the incorporation of chromating reagents in an acrylic lacquer applied on galvanised steel, galvanneal and galfan during the UVCON test

This paper analyses the effect of the exposure to humidity and UV radiation (UVCON test) of an organic coating (lacquer) containing chromating reagents applied on galvanised steel, galvanneal and galfan substrates. An attempt is made to establish possible relationships between the degradation of the lacquered surface during its exposure and the presence or absence of alloying elements on the original zinc-based substrate. With the galvanised steel and galfan coatings, 60 days of exposure to the UVCON test are necessary for a slight reduction in the lacquer content to be observed. Removal of the lacquer leaves the Cr(III) formed on the surface of the zinc-based substrate exposed. With the galfan coating, the chromate layer grows on the initial aluminium oxide film. With the galvanneal coating, in contrast to the lacquered galvanised steel and galfan, 15 days are sufficient for detachment of the lacquer to be observed. Longer exposure times lead to an appreciable increase in the percentage of lacquer removed. Surprisingly, the chromate layer remains on the surface of the galvanneal coating, preventing its corrosion, even after 60 days of exposure. XPS data suggest the presence of CrOC type bonds at the interface, due to the oxidation of the lacquer surface associated with the reduction of Cr(VI) to Cr(III), which contribute significantly to the increase in lacquer adhesion.