Black anodize turning bronze color

A. Heat can make black anodize parts fade to bronze or gold in a hurry. A poor seal will accelerate the process.

A. I suspect that the parts were poorly sealed...poorly sealed parts will fade even if exposed only to fluorescent light. Another, but less likely culprit, would be inadequate coating thickness, or perhaps the dye used had very poor lightfastness. You didn't say how long it took for these parts to fade, but I'd bet my lunch on the bad seal.

A. 1. The first thing that comes to mind is inadequate sealing.
2. The second thing is iron contamination in the rinse water and/or the seal.
3. The third thing is copper in the anodizing solution from previous 2000 series alloy work where the copper is so high in concentration that it occludes in the pores later to bleed due to, again, inadequate sealing.
4. Also, some very cheap black dyes are very fast to fade, ask the vendor what the light fastness number is on the black dye he is using, it should be 7 or 8.

My thanks to all of the responders to my question of black anodize fading to a bronze color. You all seem to have a consensus that poor sealing is the culprit. I will be taking this up with the anodizer.

Thanks again for your help.

A. No problem, Lou.. its nice to get a "Thank You" every once in awhile! Send your anodizer a coupon of the same alloy as your parts, have him run it with his next load, then ask him to preform a dye-stain test. If he's worth his salt, he'll know how to preform the test, and it will pass.

A. As to the discoloration on your black anodized parts, what type of process did the parts go thru prior to anodizing? If you get any oxide impregnated into your part, that will cause surface rust to start to bleed thru the anodize. Maybe someone used some type of oxide sandpaper to deburr the parts?

A. Yes I've seen it plenty! It's the lack of a quality seal (or possibly no seal at all!). The mil spec is ambiguous when it comes to Class 2, Type III dyed hardcoat. The spec mandates "unless otherwise stated, Type III coatings shall be unsealed." Now anodizing house supplies unsealed dyed anodic coatings, however, where is it otherwise stated to seal the dyed hardcoat. Our shops take on it is that sealing is implicitly stated by indicating the parts are to be dyed.ASTM B136, anodic seal integrity test will quickly tell you if the parts are sealed or not - if you drop a little nitric on it and it turns instantly white, it's not sealed! The safest thing for an OEM or designer when calling out Type III, Class 2 coatings is to specify "nickel acetate or equivalent seal required."

P.S. Of course I might be wrong! If your nitrided parts are prematurely rusting, is it possible that your parts are in an overly aggressive corrosive environment that no coating can withstand. I recently had a relative spend a couple of weeks in China, Shanghai in specific, and he came home with a terrible cough he blamed on the air there. It took him a month for his throat to recover.

A. Suggestion. Do a seal test and re-post.

First off, thank you very much everyone for their valuable input, it's very much appreciated.

In reference to the ASTM B136 seal test. Unfortunately, as I understand it, this type of test is beyond my facilities and capabilities. However, the light test will be very easy for me to perform in a controlled manner and I will undertake this immediately. As soon as I have some results I'll post them up here.

Thanks again.

Kevin

A. OK, so you cannot do the ASTM test with nitric acid, then at least dip a cotton ball inacetone ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! or alcohol, and wipe it across the black dyed surface - if it picks up black color, then the pores are NOT sealed.

Kevin, Robert is most likely correct in that the parts probably weren't sealed well. A couple of other causes could be that the black dye used in the anodizing process did not have good light-fastness properties, or that the coating was not thick enough to allow the dye to absorb deeply into the pores.

Keep in mind that dyed anodizing is not the best coating for continuous exposure to light, and it will eventually fade, even if it is properly applied, and sealed. But certainly you should expect much better results than what you're currently seeing.

Regardless, you are an unhappy customer whose purchase is not performing up to par. As with anything else you'd buy that didn't perform as expected, I'd return the product, and ask for a refund, or a replacement. It would be up to the manufacturer to stand behind their product. Obviously some do, and some don't. If they don't, I guess you're stuck, and chalk it up to a lesson learned not to buy equipment from that manufacturer again.

A. Hi Kevin. Although your acetone rub test is not as conclusive an indicator of proper sealing as more formal tests, yes the sealing sounds okay, and the dye may be the problem. Can you determine the manufacturer, type of dye, and lightfastness rating?

Regards,

A. Anthony

The blemish looks like chemistry dried on it during a tank transfer, or the part was not completely submerged during the dye or seal step.

Hi Willie,

Thank you very much for your advice, we will reduce the transfer time between tanks and see it any improvement...

...

Hi! Willie, the problem already solved.

Thanks once again for your advice.
Best regards,

A. I would suspect it's the UV exposure, which can lead back to a poor seal, inadequate anodizing thickness, or choosing a black dye that does not have good light fastness (albeit most of the black colors do). If it's extreme UV exposure, then dyed anodizing may not have been the appropriate choice in coatings.

A. Hi Sean. I'm slightly confused about who "you" and "they" are. Sounds like the ultimate customer orders the finished parts from you and you order the anodizing and chem-film services from others? But what sounds strange then is that the parts were rejected by your customer and sent back to the anodizing/chem-film shop for rework without you having a chance to see what was originally allegedly wrong with them, and now after re-work they are worse?

Since you sound like you'd like an intro to the technology, let's go through what is most common practice: hard anodized (Type III) parts are usually neither dyed or sealed; they are (depending on the alloy) charcoal or "blackish" in color as a natural consequence of the alloying materials in them. And they are not sealed because that softens them. But this is not to say that they can't be sealed and dyed like Type II parts, and even on this page there is a discussion of that approach. I am quite certain you have dyed parts.

You say the holes are chem-filmed rather than anodized, so re-anodizing should not affect the tolerance unless I am misunderstanding. You mask the chem-filmed holes and then proceed to re-anodize. Topic 33933, "Masking/stripping when chromate conversion and anodizing on same part" should give you a good introduction to what is involved in partially anodizing and partially chem-filming/chromate conversion coating (either can be done first). Whether re-anodizing, dying, and sealing actually has a real chance of working, however, I can't say because I'm a book-knowledge guy, not an anodizer. Hopefully one of our many readers who are experienced anodizers will weigh in :-)

Regards,

A. Sean

1] Like Ted I am not sure of the sequence of events, but I believe you are close.

2] If the purchase order specified 'hard black anodize', the parts may or may not have been sealed, depending on the shop's practice. Your description of the IPA wipe, and discoloration later on suggests they were not sealed.

If unsealed, the coating is capable of absorbing contaminants that in your case, make restoration of the original coating unlikely. If the anodized surfaces/dimensions have tolerance of > ±0.002", I would think the close tolerance bores could be plugged/masked, the exposed anodize stripped, and then re-anodized. A refinishing step to restore surface finish and appearance might be desirable between the stripping and re-anodizing.

3] For determining if the anodize is properly sealed, you could perform the test described in ASTM B136. The test takes about 5 minutes to perform.