Anodize refuses to dye

Letter 105

Anodize refuses to dye

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Hi. I've been trying my hand at anodizing and dyeing aluminum, and have encountered a problem. The techique is essentially that found in an associated Web page: Degrease in sodium hydroxide, rinse, anodize in 20% sulphuric acid with current density 0.4 amps per square inch, rinse, soak in dye (Nigrosin Bio Stain, black).

The problem is, no color is absorbed; none, absolutely none.

It appears that an oxide layer has formed, but it is hard to tell how thick it is. It seems to be uniform over the entire surface of the parts. The materials treated were 5056 and 2024 aluminum alloys; all wires were pure aluminum. Time in the anodizing bath was 1 hour. I started with the dye bath at room temperature, then after about 20 minutes slowly raised its temperature to boiling (took about 30 more minutes). Any suggestions as to what is (or is not) happening here would be greatly appreciated.

Dr. Christopher G. Paine -
JPL, Pasadena, CA

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I'm not a practicing anodizer but I think the sequence should be: 1) clean/etch, 2) deoxidize, 3) desmut, 4) anodize, 5) dye, 6) seal. Step 1 is surface clean and finish, while step 2 should remove the existing oxide layer. Step 3 removes the inclusions such as copper. I may be wrong in one or more details, but I'm pretty sure that you have to remove the existing oxide layer before you can form a new one.

Bill Vins
microwave & cable assemblies
Mesa (what a place-a), Arizona

Surface Finishing and Treatment of Aluminum and its Alloys, Wernick, Pinner & Sheasby

Aluminum How-To, Robert Probert

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Suggest you try cleaning the item in a mild detergent ( up to 150 deg. F. ) to remove any organic contamination prior to "degreasing" in sodium hydroxide. The sodium hydroxide attacks uncontaminated areas while removing any organic films that may be present causing a selective attack that will create an etched pattern. We obtain black coatings on 2,000 series in 30 minutes and in 50 minutes for the 5,000 series. Our parameters are typical of most anodizing baths for aluminum: 15% sulfuric acid, 68-72 deg. F., and 12 amps/sq ft.. Solution agitation is important to dissipate the substantial heat generated.

If either the temperature, time, and or concentration of solution is exceeded, or there is insufficient agitation to dissipate the heat, the solvent action of the solution will attack the coating as it is being created. This could partially or totally re-disolve the coating. These conditions will provide a "Standard" anodize coating as opposed to a "Hard anodize" coating. The conditions you are using are more toward the "hard anodize" type coating, but they still do not sound correct for a proper film to form. .4 amps/ sq. in. ( 57 amps/ sq. ft. ) exceeds our "hard anodize" method of 36 amps/sq. ft.. The 20 % concentration is o.k. but the temperature should be at 32-40 deg. F. These conditions will create a periodic drop in voltage as the coating builds - then an increase in voltage is necessary to maintain the 36 amps/ sq. ft. It is common to start at 20 volts d.c. and end at 40-50 volts over a period of 30-50 minutes.

Even if you acquire a proper "hard" anodize coating, the density is much greater than "Standard" anodize coatings. The porosity of "Standard anodize" coatings is what makes them easier to dye. We have difficulty dyeing "Hard anodize" coatings for this reason. Last, if you have a created a coating at all, I believe it is being attacked by the extreme temperatures. A properly formed coating will be non-conductive and have a color ranging form satin-aluminum below .0004 tk. to dark bronze and black ( depending on alloy ) at .003 tk.. One more thing, you might want to check the terminal connections - are you sure the item to be coated is "positive"? - and a proper cathode material is used ( aluminum or lead is recommended).

DLTATGE

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