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Black staining of zinc dichromate

 

Black staining of zinc dichromate.

We have a 'can' shaped component that is coated with zinc clear dichromate. We see black staining along with white corrosion after a short time in salt spray. This causes some concerns that there may be a problem with the plating process.

We have been told that the black staining is the chromate breaking down. What factors in the process can be adjusted to eliminate, reduce or prolong the appearance of the black staining?

Keith Robertson
- Chatham, On, Canada

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Even if you have .0002 to .0003 zinc thickness, and a hexavalent clear chromate applied, you may only get 24-32 hours with no corrosion. That is salt spray ASTM B117 process. The chromate and zinc are sacrificial and doing their job.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan

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Thanks Bill,

So the black spots are caused by a break down of the Chromate? Would normal handling of the coated component lead to localized breakdown of the chromate? What can be done to delay the onset of the black? A thicker Chromate coating?

Keith Robertson
- Chatham, On, Can

Many of our customers are now applying a "clearcoat sealer" immediately after the clear chromate step. Chromate, cold water rinse, sealer. The sealer is not rinsed off. This sealer does give 96 hours minimum in salt spray. It is not a lacquer, and can be non-chrome. A thicker clear chromate would not help. You could consider utilizing a yellow chromate (with hues of red and blue, 96 hours salt spray). Both do cost a little more. Of course yellow changes the appearance of your part.

Bill Hemp
- Grand Rapids, Michigan