Black anodizing problem - purple not black

A. Hi Joshua. The short answer is that your anodizing is not thick enough for good black color. If all went well, your thickness is about 0.3 mil whereas most people think 0.5 mil is the minimum for black color, and anodizing to 0.8 mil is not uncommon.

On a matte surface you possibly may have got by with it because the light bounced around a bit in the peaks and valleys and got absorbed before being reflected, but even then 0.3 is not thick enough. I am also not convinced that the parts were fully clean; you might try a scrub brush ⇦ this on eBay or Amazon [affil links] and powdered pumice ⇦ this on eBay or Amazon [affil links] .

Luck & Regards,

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. Hi. I confess to being largely unfamiliar with hobby anodizing, and low current density anodizing. I've never even heard the term "PAR" (peak anodizing resistance) and had to look it up on a hobby anodizing site.

But it is true that at a given temperature the acid can dissolve the anodizing as rapidly as it forms at a given voltage; so it may be the case, although I'm not claiming that it is, that you can't get over 0.3 mils with LCD. But hard anodizing up to 2 mils is possible at higher current and voltage on "easy" substrates like type 6061. Are you sure the temperature hadn't risen and was still 18 °C at the end of your run?

Luck & Regards,

A. Try direct chemical coloring of aluminium (no need for anodizing).

Black for aluminum:

use boiling solution of 20 g ammonium molybdate and 5 g of sodium thiosulphate ⇦ this on eBay or Amazon [affil links] in a liter of water, immerse the objects, rinse, dry, wax or lacquer after the development of color.

According to Wikipedia article on chemical coloring of metals:
https://en.wikipedia.org/wiki/Chemical_coloring_of_metals#Examples

Hope it helps and good luck!

A. If nothing has changed elsewhere (and you are dyeing to saturation), the dyeing problem could be due to: weak solution, improper pH, impurities clogging pores (e.g., high sulphates), high temperature causing premature sealing, or imbalance in a multi-component dye. Send a sample to your dye supplier for analysis; I know of a case where a gold dye addition was recommended to cancel a blue-ish tinge in a black dye (and it worked!).

A. This is a very common problem.

When the anodizing thickness is low the blue components of the black dye are absorbed preferentially. So just increase your thickness.

Also common, is the fact that people who use so-called computerized controls do not understand that you must have the correct surface area for the computer to apply enough amperage.

If you are doing a 2000 series alloy, you MUST have 21 volts (plus 1 more if titanium racks) to get 12 amps per square foot. So override the computer and raise the voltage to 22, then all the components of the black dye will absorb.

A. The answers you have already received are accurate. One thing I would add other than the causes mentioned is the pH of the seal after the anodize and dye process. A lot of proprietary sealers have specific pH ranges. If the seal is not maintained within these parameters, I have seen it cause the condition you describe. Typically, manufacturers of the sealants recommend checking and adjusting pH on a daily basis as a means of control.

A. 6061 should anodize nicely and have no problem with dyeing black. 3 thoughts come to mind.
#1. Inadequate coating thickness
#2. Weak dye concentration
#3. Dye Ph is out of spec.

A. Hi Mike. The only times I've personally seen this it was the result of the anodizing being too thin, as mentioned by Robert Probert. But in your case, with heavy thickness verified, and pH of the seal tank verified, I wonder about Ken Vlach's idea that the dye tank itself is causing partial sealing before the dye has been fully absorbed? Have you verified that the temperature is not too high?

If you search this site for "black anodize" you'll find a number of threads which might offer further ideas. Good luck.

Regards,

A. Varying shades on the same rack has to be (1) contact reliability or (2) air agitation displacing solution (which again is thickness).

? Hi Robert. But if the problem is lack of thickness how do we account for Mike measuring it as .007"^^.0007"?

Regards,

A. Hi Mike,
If that black you're running is the ubiquitous Reliant HBL (specs sound about right), 1.5 years is fine if you've got your concentration kept up, since is a single-component dye and won't age-shift like a mixed dye in my experience, and anyway if you've got a mix of shades on one rack it's not a tank shifting issue. And anyway that dye is DEAD STABLE. The last time I changed my bath was in October of 2014- it's still black as night. I also run it stronger than recommended, but that's neither here nor there in terms of my response to your issue, which is that I'm wondering whether you've got good, even rinsing coming out of the ano tanks, and whether you precondition parts for dye with a nitric dip (about 5-6% vv from 42 °Baumé, not too long or they'll take on a scorched appearance, immersion time depends on concentration and circulation)? Do you find yourself having to run elevated buffer in your dye (this is totally fine per the manufacturer) or really keep beating the pH of the dye tank up constantly with ammonia? The theory here is that Sulfuric is way more 'sticky' than Nitric, and the post-anodize nitric dip will ensure that the parts are squeaky clean to accept dye, and minimize Sulfuric drag-in. It's also a lifesaver if you have issues with blind holes bleeding out after seal. Of course you still have to rinse the Nitric off really well before going into the dye, but it rinses MUCH cleaner, and with MUCH less effort!
If you're already doing this, then yeah I'm gonna go with lost contact :)

A. Sorry, is that coloring bath electrolytic bath as tin bath or electroless coloring? Also if I were you I would share pics which describe the problem. I have some answers but let me know the electrolytic or electroless bath?

Sincerely

Mike,
When you mention varying shades of purple rather than black, is each piece the same color purple throughout the part, or is it a gradient with the middle of the affected parts being lighter than the ends? I have seen both of these cases on my line in the past.

If you have a gradient within each part, then I would highly recommend checking your connections on the racks themselves; including taking off the bolts/connectors and giving them a good scrub to make sure you're working with clean surfaces. If that were the case you would probably see a variation in film thickness within the part. Also make sure the connection from the work bar to the anodizer blocks is very solid and clean. A gradient would mean you're not getting enough power to your parts.

If every part is the same shade of light through the part then I would still recommend to service the rack to eliminate that as a potential source. Personally every single time I see this light black (purple) condition the part is always loose on the rack- regardless of the film thickness measurement. Every time. It does not make sense to me why the thickness is adequate even on a loose rack, but it is what it is.

Also of note, is the position of the light pieces on the work bar consistent? Or do they seem random? I have had instances where a poor rinse (when sprays do not penetrate the work load toward the inside of the rack) affects the colorability of the inside portion of the work load. If this is the case I would increase the immersion time of your pre-Dye DI rinse to give your spray rinses a helping hand. If you don't have spray rinses at all make sure your agitation does not have any dead spots in which gunk could hide in the pores and prevent dyeing.

Best of luck.