Where the
world gathers for metal finishing
Q&As since 1989


-----

White Spots on Chromate Conversion Coating

We plate mild steel components with a chromate conversion coating. On one particular component white spots start to appear 3 to 4 days after plating. Could this be zinc that is corroding? Can anyone please help in this regard?

John Zaaijman
proximity fuzes - Alberton, Gauteng, South Africa
2003

---

Yes, there is a good likelihood that this white material is the corrosion products of the zinc plating. Are you sure that you get the parts fully dry?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

You did not say which type of chromate you are using. If it is a clear or blue bright film, you may have rapid breaching of the chromate film. If the passivate is yellow or green, I would doubt that white spots would be zinc.

It is possible that your rinse tanks could be in need of being renewed. If the soluble compounds in the rinse get too high, you could see white spots quite rapidly. They would be a fine white salt and not a zinc compound.

You might also want to remember that it is far better to "cure" the chromate. That means a slow and warm drying period, instead of a very hot curing atmosphere will allow the chromate film to form more uniformly and give the maximum protection.

Ed Budman [dec]
- Pennsylvania
With deep sadness we advise that our good friend Ed passed away Nov. 24, 2018
2003

I have seen white spots form if the chromate dip is done in cold solution due to air bubbles formed on the surface. If this is the case, a hot water (65 C) dip before chromating could help.

Mandar Sunthankar
- Fort Collins, Colorado
2003

Thanks for the suggestions. We are currently doing gold chromate conversion. The problem is that the white spots only appear on one type of component on not the other types. This would mean that it is not the tanks that is the problem, but the type of material. The material that causes the spots comes from another supplier. The thought is that the materiel is porous. Is there any way to minimise the effects, if this is the case ?

John Zaaijman
proximity fuzes - Alberton, South Africa
2003

At what magnification do you see these spots? Can you give any further description of these spots?

Tim Purdy
automotive - Greenfield, Indiana, USA
2003

These spots can be seen with the naked eye. They are very small in the region of 0.2mm in diameter.

John Zaaijman
proximity fuzes - Alberton, South Africa
2003

---

I can't say that vacuum impregnation will be a practical solution, but if you can't get the parts rinsed well enough to stop this staining, it's certainly worth looking into.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey