Discoloration of Black Anodized Aluminum Parts

A. All of the answers you have obtained thus far have merit. However, I believe one or two important factors have been missed.

In order to obtain a good, lightfast black color, it is imperative to anodize to at least 0.6 - 0.7 mil oxide thickness. This gives you enough "pore capacity" to hold sufficient organic dye to minimize the UV effects on the dye.

It is also very important to dye the parts to "full adsorbancy", that is ... allow the part to remain in the dye solution until the pores will not adsorb any more dyestuff. For good black color, this can be as long as 30 minutes, but usually 20 minutes is sufficient.

After this has been done, a good nickel containing seal is imperative to minimize leaching of the dye and to adequately seal the part.

If you have any additional questions please let me know.

A. Hi, Ciaran.

It sounds like the anodizing is too thin in spots if not everywhere. The dye only works when absorbed into the pores of the anodized film; if it is too thin you will not get saturated colors.

Regards,

A. The dye manufacturers have a rating system for dye stability. Find out which one your anodizer is using and check to see for yourself on what its relative rating is.
Black organic dyes do not like fluorescent lights at all.
There are some metal based ones that will last longer. Finally, you can use the two step system for architectural anodizing that can obtain some dark colors, but tend to have lighter tones. Black is not really possible, but some anodizers can come close.

A. Hi, M.

My first guess is that the parts in question are a different alloy or use a different fabrication method which causes the anodizing to be thinner. Please look over Charlie Grubbs response about the need for thick anodized coatings for good, stable, saturated blacks. Find out from your vendor, or through your own testing, how thick the anodized coating is on the parts that fade. Certain alloys, like 2xxx series, require significantly higher voltage and processing time to acquire the same thicknesses as 6xxx alloys.

It's usually best to address all readers because these threads stay on line as reference information for years; I don't know B. Kling, but he could have retired a decade ago :-)

Regards,

A. Hi Alan. I can't say for absolutely positively, but I strongly doubt it because the dye is sealed into the anodizing pores, that is, the top of the pores are sealed off.

Regards,

A. Hi Charles. Photos can sometimes not convey things correctly, but it looks to me like your cover is purple rather than black, indicating lack of saturation due to the anodizing being too thin. As long as the anodizing is still too thin, it may not be sufficiently insulative to divert the current and steer it to the low current density areas (which is the bottom of the grooves, which you have painted, if I understood correctly).

But I don't think that's really the problem with the edges. Perhaps they are not truly clean, or perhaps the current density was too high and these edges burned.

Grayness when etching is to be expected. Aluminum alloys contain other materials like copper and silicon which are left behind when you etch away the aluminum. Good luck.

Regards,

A. The MLW that I am familiar with is a good black dye. It is possible that you are using a knock-off product.

All black dye will fade/turn purple in strong UV conditions.

The best that you can do is to have the anodize a bit thicker and use a very good seal after the dye.

You also mention switching to an ink. I am not familiar with this type of product.

For superior UV resistance, you would have to switch to architectural anodizing which involves plating with one of two metal salts after anodizing. Unfortunately, true black is not an option in this process and it obviously costs more.

A. Hi. Another, but costly, solution would be an add-on to James' reply: overdyeing, which means the electrolytic coloring with the metal salt that he speaks of, followed by conventional organic dyeing.

Regards,

A. Possibly.
All dyed anodize will fade if exposed to constant sunlight and certain fluorescent lights. Some lab cleaners can be a kiss of death for some parts.

We used a different Sanodal black dye with good luck.

I think that one key might be a thicker coat of anodize so it can absorb more dye which equals longer fade time. We used a very heavy coat because it did not really cost that much more and the rejects were few (and they were mostly caused by the operators).

Another thing to try if you are allowed is to give it a light coat of black lacquer. A couple of vendors support this site.

Finally, 2xxx and 7xxx aluminum is difficult to anodize as well as some of the others. Temper does make a difference, but we were not exposed to it that much.

A. For Milspec A-8625, it lists the dye required for compliance. The deep black MLW is the only one that complies. We have currently got testers in full Australian sun with monthly intervals over the past 16 months out of 7075, 6061 and 5005. Even at 16 months there is very little fade or discolour (no noticable change unless next to standard). Minimum layer thickness I would use for high UV exposure would be 20 µm. If your anodiser is good at their job, this shouldn't be an issue if you specify your requirement.