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Test for Chromate Conversion Coating on Aluminized Steel

We are looking for a suitable method for identifying whether aluminised steel has been treated with a chromate conversion coating. We have discovered that the Chromate interferes with the degreasing / application of high temperature paint [affil links] to aluminised steel, causing the coating to fail in service.

Allan Nicholls
Quality Manager - Auckland,, New Zealand

First of two simultaneous responses -- 2006

Get yourself some 1,5 diphenylcarbohydrazide [affil links]. If you have an analytical lab that does your waste water monitoring, they will probably give you a couple of grams. Mix 1 gram with about 50 grams reagent grade sodium chloride, well.

Now, take your part, and put it in some 0.1N sodium hydroxide [on eBay or Amazon] solution. Use just enough to cover the part. This will take the chromating off, along with a bit of the Al surface. Now, remove the part, and add 1:1 sulfuric acid to this solution to a pH of about 1. Then, add a scoop of your 1, 5 etc mixture. A red color indicates the presence of hexavalent Cr.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

Second of two simultaneous responses -- 2006

Allan -

There are several ways to check for that kind of coating. X-ray fluorescence can look for the Cr but you have to have a piece of your aluminized steel that is non-chromated to compare with since your base metal may have Cr in it. Another way is to use FTIR. This will work if the coating is like an Alodine 1200 which will display a CN band. Finally, there are chemical spot tests for these kinds of coatings, but can you tell which particular one you are dealing with?


Terry Tomt
- Auburn, Washington

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