BLACK ANODIZE COLOR PROBLEM

A. Jeff, we have had similar problems with A-356 castings. The program that wants the Black Dye insist on the casting coming out black. The problem with castings is the porosity and the silicon content. First I would recommend improving the surface characteristics of the part by glass bead blasting or vibratory deburr, then use a nitric/hydrofluoric mixture as your desmut. If you prefer not to use that as your desmut you can take a brush and hand clean the remaining silicon/smut off the parts.

Best I can do to help,

Dave

A. Dear Jeff ,

the problem to me sounds like a rinsing problem , I would suggest that you agitate vigorously ( with Air ) the rinse between anodising & Black Dye .

The reason is that if the pH of the Dye varies wildly from the parameters set by your supplier then a loss of absorbency will occur with the resultant "Grey" appearance & the production of a "Smutty" appearance .

I would suggest that it would pay you to dump your Rinse after anodising , your dye solution & Final Boiling water seal, as they are probably all contaminated by now .

tell me what eventuates

regards

A. The way to get the casting black is to modify the surface of the casting -- increase aluminum content in the surface. How can you do it? --

First you need a good caustic etch to remove the contaminated outer layer, than treat in nitric- hydrofluoric acid mix: 10 - 20% of concentrated hydrofluoric acid in nitric (no water!). The parts surface will lose the silica from the alloy and will become almost pure aluminum. The only problem now - it is way too porous to be anodized. Rinse and dry the parts very well. You are going to have to compact the surface layer by blasting it with fresh clean glass beads. After glass bead blast you shall avoid any etch or strong deoxidize: you do not want to destroy the aluminum shall you have just created. Use a mild non-etching alkaline clean and very quickly dip in nitric acid (50%) prior to anodize. Your anodizing cycle seems to be OK. I would strongly recommend to use nickel acetate or some proprietary seal, otherwise you risk getting parts back after a few months due to a significant colour change.

Hope this helps.
Max

A. Dear sir: firstly check the metal composition; second, decrease the time of anodizing; thirdly, heat the black dye and check pH after. Then try it; I think colour is ok?