Letter 16013

Quality test for chromate conversion on aluminium [California] 

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If I'm using aluminium mounting plate with chromate conversion finish, what is the common/best methods to test the quality of coating? Thickness measurement? Humidity test? Temperature shock? What is the failure symptom or indication from those tests?

Lim, Chee Beng
Agilent Technologies - Rohnert Park, CA, USA.

The most common tests for validation of conversion coatings are destructive. If you have test coupons or spare parts from the same run then the tests should give you enough information to make a good call regarding the quality of the coating. However, the final decision is also dependant upon what your purpose for the coating is and what your standards are. If you only require paint adhesion then a failing salt spray is almost inconsequential.

The first test is salt spray per ASTM B-117 [link is to info about spec at TechStreet] for 168 hours, the coating should exhibit zero corrosion (~4 pits per 30 sq. inches is acceptable).

The second test is an adhesion scribed wet tape test per FTMS 141 Method, 6301. There should be zero paint removal.

The third test is a coating weight test, a good film will have a weight of about 40-50 mg/sq. ft. This can be checked by weighing to four decimal places, etching the coating off in a 50% HNO3 bath within 3 hours of when the part was coated, and reweighing. I know this test can be performed with a different chemical makeup if the three hour limit has been exceeded, but I do not know that makeup.

Other than those tests, inspection is mostly visual. There should be no cracks, voids, or bare areas. The coloration should be a light yellow through irridescent to golden brown (a dull brown is generally indicative of a thick coating and is undesirable) and very even. A water-break free test (after cleaning with a mild solvent) and a powder test should also be performed.

Hopefully this answers your question! Let me know if not.

Benjamin J. Curto
Cygnus, Inc. - Ponderay, Idaho, USA

I am setting up a chromate conversion coating system for aluminum and Fed Test Method Std. 141D Method 6301.3 is called out as one test to perform. The roller that is to be used is a 4.5 pound rubber covered roll. Does anybody know where to get that roller?

Thanks,

Dirk Aubram
- Patchogue, NY, USA

The roller for the Wet Tape Test may be obtained from the Gardener Co., Part number PA-1411, $185.

Anthony R Fragala
Radar Mfg - Andover, MA, USA

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