Zinc phosphate coating is crystalline heavy coating gray in colour. While iron phosphate coating is amorphous conversion coating that ranges in color from iridescent blue to gray. Iron phosphate coating gives minimum sludge formation and hence smoother finish than zinc phosphate. Apart from this the bonding characteristics are also good. Iron phosphate processes are much easier to operate than zinc phosphate processes and require fewer process stages (zinc phosphate coating requires a predip in activation chemical prior to phosphating to have micro crystalline grain structure), but iron phosphates do not provide the degree of corrosion protection imparted by zinc phosphates. Iron phosphate systems are therefore used for a range of products requiring a durable finish that are not exposed to severely corrosive environments. Zinc phosphating processes have been developed to provide exceptional painted part durability in corrosive requirements. Typical industries using zinc phosphate processes include automotive, appliance and truck and bus.
After a metal surface receives a conversion coating, the surface is water rinsed to remove unreacted conversion coating chemicals and a post-treatment is applied. The post-treatment can provide a two- to ten-fold increase in corrosion resistance and humidity resistance when compared to conversion coatings without final rinses. Post-treatments are generally based on chromic acid.