Cr(VI)-Free Post-Treatment Processes for Zinc Zinc Alloys

摘要

The ability of a chromate layer to protect the zinc coating from corrosion is strongly dependent on the thickness of the passivate layer. As a result, yellow chromate coatings are more protective than blue passivate coatings. A new Cr(III) containing passivate solution has been developed that is able to build up a thicker conversion coating than conventional blue passivates. The corrosion protection achieved is even better than for conventional yellow chromate coatings when annealed. This passivate can be applied on zinc, zinc-iron and zinc-nickel deposits. A Cr(VI)-free black passivate has also been developed for zinc-iron. The corrosion protection of the passivated coating can even be improved by deposition of an inorganically based top coat on the passivate layer. The top coat penetrates into the passivate layer, stabilizes it and seals any cracks and pores. At the same time, it increases the surface strength and meets the highest automotive industry corrosion standards.