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Dichromate Seal Finish is Greenish

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I have a problem receiving aluminum parts that should be black but actually have a greenish tint to them. Our application is such that the parts must be black...no greenish tint allowed. My finish process is Anodize TY 2, CL 2 Black per fed std 595, 37038 and dichromate sealed per Mil-A-8625 [link is to spec at TechStreet]. Each plater seems to have a diffent reason for the problem but sometimes parts are black, sometimes green tint. How can I achieve a consistent black finish? Is my finish callout sufficient?

Thanks!

Scott Higgins
mfgr - Orlando, Florida, USA

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A semi educated guess is the black out put is a metal based dye and the green output is an organic based dye that the chromate is reacting with. Switch to a nickel based seal and you should have good results and a black finish with both dyes, plus you have eliminated a source of hex chrome which makes many tree huggers happy.

James Watts
- Navarre, Florida

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Dichromate Seals are ALWAYS yellow-greenish tint if properly applied. Dichromate Seals leave the bst corrosion protection. Dichromate Seals should never be used on dyed work: not just because it changes the color of the dye but also because Dichromate Seals are for maximum corrosion protection and ALL dye slightly lowers corrosion protection. So why would you want to put on Black dye to lower corrosion protection and then discolor it with dichromate which raises corrosion protection? Or just shooting yourself in the foot !

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

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Hi, read Mil-a-8625f section 6.11 it will explain duplex seal nickel seal then dichomate. also 3.5 last sentence.

John J. Crowell
- New Port Beach, California