Study of degradation mechanisms of a protective lacquer film formulated with phosphating reagents applied on galvanised steel, galvanneal and galfan in exposure to UV/condensation test

This paper analyses the effect of exposure to humidity and UV radiation (UVCON test) of an organic coating (lacquer) containing phosphating reagents applied on galvanised steel, galvanneal and galfan substrates. An attempt is made to establish possible relationships between the degradation of the lacquered surface due to the effects of its exposure and the presence or absence of alloying elements on the original zinc substrate. After 15 days of exposure to the UVCON test, XPS has revealed an important reduction in the carbon content on the surface of the material, accompanied by an increase in the zinc, phosphorus and titanium contents. This change in surface composition suggests the rapid removal of a very considerable percentage of the lacquered coating. On the galvanised steel and galvanneal coatings, sufficiently long exposure times to the UVCON test (60 days) have led to the elimination of the zinc phosphate layer and its replacement by a layer of zinc carbonate. Surprisingly, the zinc phosphate layer has remained on the surface of the galfan coating (95%Zn–5%Al) even after 60 days of exposure. The attack of this substrate in exposure to the UVCON test seems to be preferentially focused on its aluminium-rich phases.