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Zinc and clear passivate problem

Hi, I work for an engineering firm that uses zinc & clear passivate on some of our products. We have recently been contacted by one of our customers who has come across a high level of corrosion which has occured on a product that has been in service for only 18 months. the product is used around water and is showing what appears to be almost powder type deposits on the surface. any ideas why is it occurring? Could it be harmful/damaging?

Mark Greaves
buyer - Plymouth, Devon, UK
February 25, 2008

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This sounds like normal corrosion of a finish that can't hold up to those exposure conditions, Mark. After the chromate protection gives out, the zinc starts corroding with white rust. But that is not to say that the coating on the particular component in question was not defective; it is not uncommon to over-dry the chromate or dry it before it has set, resulting in a powdery chromate deposit.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February , 2008

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Mark, take also under an account that so called clear passivation (clear means usually bluish trivalent chromium based one) must be modern advanced process to reach corrosion protection requirements. For hexavalent chromium based finishing has been prohibited in importand branches recently, many customers have changed their finishing into trivalent. Can we believe that all of them make it deliberately and correctly?

Janusz LABEDZ
- Warsaw, Poland
February 26, 2008

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April 17, 2008

Hi Mark,

This is normal corrosion of the Zn deposit. A Zn and clear chromate finish is the least corrosion resistant finish available. A clear finish is not really meant to be used in any humid, wet or salty environments. There are much more corrosion resistant finishes available for these environments such as Zn yellow, Olive or Zn Alloys.

Regards

Jonathan Timms
- Hong Kong