Passivation Processes for Zinc, Zinc Alloys and Cadmium Electrodeposits.
The purpose of passivation processes is to protect zinc, zinc alloys or cadmium plated surfaces from corrosion, so that their main task of sacrificial protection to steel is not reduced.
There are several main causes of corrosion. In conditions of high humidity and high temperature particularly where there is condensation, zinc, zinc alloys and cadmium form a white corrosion product on their surfaces called” white rust”. Passivated coatings increase the resistance of zinc, zinc alloys and cadmium against this type of corrosion and thus extend considerably the life of the base metal. Passivated coatings also form a perfect bonding layer for subsequent applied paint schemes.
Historically, passivates were based on hexavalent chrome processes and are still used in some markets today. However, these passivates have been superseded by trivalent chrome, based products. These new generation products comply to REACH and ELV requirements and have been adopted by the automotive and other industries.