• With just a layer of Zinc electroplating the parts are still not sufficiently well protected. This is so because the coating can still be easily oxidized. As such an additional layer is needed to protect the coated layer. This is in the form of a chromate layer which is an extremely thin chemical skin.
• The chromate for zinc or zinc alloy plating used to be only of the hexavalent variety (Cr6) which are not environmental friendly.
• Governments are now imposing more stringent environment rules such as the ‘EU Directive’, ELV, ROHS which aim to eliminate the usage of hexavalent chromium
• This and other requirement lead to the development of trivalent Chromate(Cr3) which is more environment friendly and allow under the ROHS & ELV directive.
* Note :
- The concentration of the chemical and dipping time is important.
- Too low a concentration or too short a time will not give the require corrosion protection.
- Too high concentration or too long dipping time will result in poor adhesion.
- Too much of impurities due to extended used will also result in poor protection.
Trivalent Chromate (Cr3)
In UEP most of our products are in Cr3 chromates. The Cr3 chromates are products from our collaborations with our major chemical supplier RICA Marketing & its principal ATOTECH Group in Germany. Products examples are as below:
• The Cr3 chromate for Zinc are
- Cr3 Blue or Cr3 thin film
- Cr3 Iridescent or Cr3 thick film
- Cr3 Black (The zinc black chromate layer is very soft and hence easily scratch or damage from improper handling. Once the layer is damage, corrosion will set in fast. Thus a layer of Black sealant is needed to prevent this from occur).
• By just applying the Cr3 chromate on top of the metal coating might not be sufficient for corrosion protection purposes.In fact, the protection level is inferior to that of the Cr6 chromate except that of zinc alloy plating which have the advantage of having it’s own inherent corrosion protection independent of chromate passivates.
• The reason being Cr3 have got no self -healing properties as compare to Cr6.
• When the chromate layer is scratched, the under layer of metal coating is exposed and open to corrosion.
• In the case of Cr6, the chromate will migrate over the wound and resealed the damage. Cr3 will not be able to do that and hence protection is reduced.
• To overcome this weakness of Cr3 Chromate, a sealer layer is applied over the chromate layer. This sealer will react the same way as the Cr6 chromate.
• With the additional sealer ,the corrosion protection is dramatically increased. The following salt spray test result can be achieved.
|Surface Finish||Thickness||White Rust||Red Rust|
|Zinc + Cr3 Blue||
|72 hours||96 hours|
|Zinc + Cr3 Iridescent||Min 8μm||72 hours||240 hours|
|Zinc + Cr3 Black||Min 8μm||72 hours||240 hours|
|Zinc Ni + Cr3 Iridescent||Min 5μm||200 hours||800 hours|
|Zinc Ni + Cr3 Black||Min 5μm||200 hours||600 hours|