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topic 11605

Chromate Conversion Coatings on Zinc and Zinc Alloys


 

Q. Is there an ASTM spec that addresses chromate conversion coatings on Zinc and Zinc Alloys (as there is for aluminum alloys ASTM B449-93 [link by ed. to spec at TechStreet])? I have been able to locate a MIL spec, MIL-C-17711B but surprisingly nothing under ASTM.

Secondly, why are there typically no thickness specifications for chromate conversion coatings as opposed to coating mass in grams/sq meter? Isn't the thickness measurable by eddy-current testing? Salt spray testing is appropriate for initial qualification but is not reasonable for verification lot sampling on incoming product. What method is typically used in the industry for production lot sampling to verify adequate coating is present?

Darin L. Moore
- Smithfield, North Carolina


 

A. Take a look at ASTM B201-80 [link by ed. to spec at TechStreet], Darin. That's as close as the ASTM gets to a chromate conversion coating specification for zinc, as you'll note in ASTM B633 [link by ed. to spec at TechStreet] it says "the film treatment shall be in accordance with Practice B 201." B 201 refers to salt spray, to rubbing with a gum eraser for abrasion resistance testing, and a chemical spot test for presence of clear coatings. Perhaps you can modify the spot test to suit your needs.

We don't coat zinc, so I can't comment on industry tests. But we do conversion coat aluminum alloys, and since we do the coating in house we control the baths, do monthly salt spray and tape test checks, and accept parts on that basis.

Good luck!

lee gearhart
Lee Gearhart
metallurgist



(2007)

Q. i am a worker in a small zinc plating shop .and we buy chromate conversion coating in 10 kilo package. after several dipping parts in solution it becomes weak and unusable .i want to know the complete ingredients and the percentage of them and the pH value and the exact temperature should be used . yours sincerely peenory

peenory piany
- Tehran Iran


February 2007

A. Hello, Peenory. In Western countries only proprietary chromate conversion coatings are used these days for many reasons. Some of those reasons are the need to move to hex-free formulations for consumer safety and to comply with RoHS requirements, to eliminate the cyanide traditionally used in older formulations, to comply with Qualified Product Lists in specifications, to insure ISO compliance, and so that there is an advanced laboratory that can issue technical data sheets specifying the optimal temperature and pH for the particular formulation.

So no one will tell you the secret formulations that they worked so hard to develop. If you want to make your own chromates to older formulas, the best source is expired patents. Good luck.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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