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Testing for cure of Chromate on Zinc
Q. Over the past several months we have been facing numerous issues with Zinc plating (cloudiness after powder coating, Chromate delamination, and blistering). We have remedied most of these issues by working with our current suppliers to ensure their processes remain in control. Now that we are at a point where we are consistently receiving and producing good parts again, we would like to get ahead of the game by establishing a set of tests to validate the quality of the plating at incoming inspection.
One of the tests/parameters we would like to establish is the level of moisture in the Chromate that would be considered "cured". I understand that the curing of a Chromate conversion coating is somewhat subjective, but we feel that 24 hr-48 hrs of aging achieves what we feel is sufficiently cured to resume processing. However, during the more humid months, this may change, therefore establishing a quantitative level of moisture in the Chromate would be a more scientific, and hopefully foolproof approach.
Does anyone know of a way to test for the amount of moisture in a Chromate conversion coating? If so, how? And, what is required to do so?
- San Jose, California
June 11, 2012
A. Sir:
A key feature of hex chrome conversion coating on zinc is the pH of the chromate solution. This pH must be near 4.2.
Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA
Galvanizing Consultant - Hot Springs, South Dakota, USA
A. Hi Jason.
Few electroplating shops use hexavalent chomates anymore due to RoHS restrictions; virtually all use trivalent chromates (yes, 'trivalent chromate' is an oxymoron, but it's widely used).
All trivalent chromates are highly proprietary and often rather different one from the other (thin film vs. thick film, no topcoat vs. various types of topcoat) . Despite the shops' desire to withhold some info as trade knowledge, I think you have a right to know what brand and type of chromate they are applying, and to secure a technical data sheet from the supplier which will give some quality control and troubleshooting info. Sorry, I don't know how to determine percentage of moisture, but I'm not convinced that it is among the critical control parameters anyway. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 12, 2012
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