Inconsistent colour change in anodising

A. Hi Ilze,
Your process does not match what would be the normal type of tanks used in an anodising line.
What is your first aqueous degreaser composition?
After that it needs to go in a de-oxidising etch, then rinses, at least depending on the alloy being anodised.
what is the alloy your are trying to anodise?
The parts need to remain wet between each stage on the way to the acid tank.
Also the anodising is controlled by current density relative to the surface area of the parts and jigging, as well as a ramping profile to the final amperage/voltage.
The specifics of the voltage and amperages depend on the type of anodising, hard or soft in general terms, this determines the temperature and composition of sulfuric acid solution.
Any extra information you can provide will better assist in helping you.
Best regards
Mark

A. You are etching before you are clean and that leaves a permanent pattern. Your acid is too high, your voltage is too low.

A. This is very close to setting up your line correctly:
1 clean (non etch cleaner
2- rinse
3- etch (as little as necessary)
4- rinse
5- Deox/desmut (tri acid)
6 rinse x2
7 anodize 16-18 volts-30 min
8- rinse x2
9- block dye
10-rinse x2
11 seal (nickle)

A. If you've been doing this before on the same alloy and customer, there is a possibility that error occurred on the part of the aluminum alloy supplier. The aluminum batch that was prepared might not be homogeneously mixed, or simply the wrong alloy had been supplied. I based this opinion on your observation that it produced a grayish finish which may be indicative of richness in silicon (or magnesium?)

A. What about the acid in the color bath ? It might be a good idea to check the concentration of acid in the color bath.
Thanks

A. Hi, Kieren.

There are many different aluminum alloys -- Which contain different amounts of copper, silicon, magnesium, zinc, etc. And of those metals, only the aluminum portion can be anodized. That means that when an alloy is high copper for example, which might be the case on bike levers, it tends to anodize very dark. Is it possible that your successes were with a different alloy?

Sometimes some improvement is possible by de-smutting (dissolving the copper and silicon on the surface) between cleaning and anodizing, but this may or may not be enough (some castings alloys are not attractively anodizable).The problem is not solely age, but yes, age can cause segregation along grain boundaries and exacerbate the problem of poor looks and poor corrosion resistance. Good luck.

Regards,

A. Hi, Franklin.

Aluminum alloys contain other materials besides aluminum (such as silicon, copper, etc.), and none of these "anodize" -- they just turn into black discolorations and even smuts. Besides changing the alloy or getting better quality aluminum, the single best thing you can do is probably to minimize or eliminate the etching step. (My knowledge of this is mainly book knowledge, and hopefully some real anodizers will chime in).

Regards,

A. Pretty vague question, so I'll assume you're talking about an inconsistent color on one part, not different loads, or different alloys.

Possible reasons for varying shades of color on one part:

Inconsistent coating thickness.

Larger anodic coating pore size on one area, as opposed to another due to localized overheating during the anodizing process (insufficient agitation, too much surface area for the anodizing rack to adequately handle, etc, etc)

Poor rinsing.

Hot/cold areas in dye tank due to lack of solution agitation.

If you're asking about a color change from load to load..part to part...all of the above would apply, as would many other factors.

The first place I'd start would be checking the coating thickness from the light area, to dark area.

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 Faisal. If we are to believe in the law of cause and effect, you don't do the exact same thing to two identical parts and get different results :-)

So what you're actually asking readers to do is to guess what is different despite your claim that nothing is different. Hopefully, you understand that this is difficult and will require a lot of data from you :-)

What alloy are you anodizing? What are these two parts which you claim are identical? What is the actual thickness of the anodizing in both cases. This is non-dyed, natural color aluminum? Are you sure that both parts were thoroughly clean to waterbreak-free standards? Are you etching? Are you desmutting? What is the composition of your anodizing bath and your anodizing conditions? Send good pictures of both parts to mooney@finishing.com for posting here please.

Luck & Regards,

A. Hi Faisal. It seems to me that there are two likely explanations:
1. For some reason the aluminum-colored part did not anodize, and therefore could not accept the dye.
2. For some reason the aluminum-colored part was previously anodized and sealed, and having been sealed it could not accept the dye.

I remember once, many years ago, on a new aluminum wheel plating line startup I was called in to troubleshoot a plating rectifier that would not put out any current. They had put one wheel on the rack to check the rectifier, and the chemical people, equipment people, rectifier serviceman, and I were scratching our heads for an hour. Turned out the wheel had somehow somewhere, even if only accidentally over the months, become anodized. So #2 does happen :-)

Luck & Regards,

A. Everything Ted Mooney said above. In more detail, one minute in the etch, at any reasonable concentration, would not strip old anodizing and would not strip weathered oxide; then you did not deoxidize. Not related to this problem, but if you do any serious anodizing, you need higher voltage, up to 15 for most alloys and up to 21 for the 2000's alloys. Also, electrical contact reliability could have been your problem.

A. Hi Faisal. "Gravity" hooks are often a bad idea for anodizing, because the hooks (except for a tiny sliver where the part is resting) will become anodized and non-conductive. Then a small amount of jostling from handling or agitation will move the part a tiny bit on the hook, to a spot which is anodized and non-conductive.

If you used the same hook for the first and second part, the hook is completely anodized and non-conductive, and that's the problem.

Luck & Regards,

"Trouble in Your Tank: Handbook for Solving plating Problems"
by Larry Durney

on Amazon

(currently UTL
on AbeBooks & eBay)

(affil links)

A. Hi Faisal.
The answers are: Yes, yes, and yes :-)
There is nothing wrong with experimenting and trying to learn. Enjoy it! And a scattergun can be a useful tool when experimenting.

But when your situation changes from experimentation to asking people to help you troubleshoot, you can't say: "I'm supposed to do these three things for proper anodizing -- which one of them can I do and ignore the other two, and still get proper anodizing? And by the way, I doubled my current density" :-)

As Larry Durney reminds us in "Trouble in Your Tank", there is no point even trying to troubleshoot until you decide to "Obey the letter of the law". You need agitation, you don't anodize at 3A/dm^2, and 45 to 52 minutes sounds like quite a long time to me -- what anodizing thickness are you looking for?  :-)

1.5 A/dm2 was working and was in the right ballpark -- what was your reason for simply doubling it?

Luck & Regards,

A. Hi Faisal. Your questions are no problem, and you are correct that for dark, saturated, colors you do need thicker anodizing than for light pastel colors because the greater the anodizing thickness, the more dye that can be absorbed.

Luck & Regards,

A. Hi Faisal,

Yes, a strong enough aerator that gives good agitation all around the tank is in fact a common way to keep the tanks mixed. A CPVC pipe structure drilled with lots of holes and secured to the tank edge is common.

And that brings me to my next advice, relating back to your first picture. I looked at it and thought, "lost contact!" And then I saw your racking.

The advice of these gentlemen is excellent as always and I'll add that once you have good mixing in your tank, a strong, secure rack is even more important to hold the part with good electrical connection all the way from the part back to the rectifier! The bubbles will put pressure on it, and anything less than a rigid rack, clamped securely in position relative to the cathodes and sides of the tank, will allow flopping around, lost contact, uneven current, ... etc.

I would try racking that part using a stiff yet springy v-shaped bent piece of narrow aluminum or titanium plate with the ends bent outwards and cut to a point that will fit in the holes, to contact the bore of the hole where 'rack marks' have the least aesthetic impact on the finished part since I assume a bolt goes through there when installed. This type of pressure racking component is called a "clip", and mounted to a current-carrying frame or "spine" that is in turn secured to something rigid (there are multiple approaches to this) so there is no possibility of movement.

You can look at some of the rack manufacturers' catalogues to get an idea of what I mean. Google "anodizing racks" and you'll find all sorts of images of approaches to racking securely to minimize both movement and rack marks, while maximizing good electrical contact, with examples of various racks for different part configurations.

Don't forget that you have to strip your racks in between uses! The anodic layer is an insulator. You can etch it off in hot caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] and then de-smut it. And check the resistance all the way from the end of the rack assembly to the part surface before starting to anodize. Sometimes the bolts get degraded or don't strip right and you end up with a dead rack ... just take it apart and use fresh bolts and grind the contact surfaces to bare substrate.

Best of luck!

A. Hi, Faisal. I think the basic problem here is that you have no good method or equipment to determine if the article is being anodized correctly and was anodized correctly. Therefore you look at your dye tank, your sealing tank, etc. instead of focusing on the fact that the anodizing is not proceeding properly.

Although the distance from anode to cathode does matter somewhat, it is nowhere near as important as in electroplating because as the surface becomes anodized it becomes electrically insulating, thereby promoting the current flow to other areas of the part or other parts.

If the part that "did not anodize at all" did not have a previously anodized surface preventing further anodizing, then the contact remains the problem.

Luck & Regards,

A. Hi Faisal!
In response to, "can sealing cause bleed out?"
I'm going to guess that though you matched your colors, one part took much longer to strike... And that same part showed more fading. You want to aim for a uniform and repeatable coating thickness to get repeatable behaviour in the dye and seal tanks.
If you have the resources to invest in an eddy current meter to measure build, that is a useful tool. But back to the importance of racking, a rack that has been prepared and tested for resistance to check tgat it can deliver predictable current over the entire run of the load, is the place to start. If your rack-part interface has resistance from incompletely stripped ano layer or smut, it will overheat during the process, and one of the possibilities for what happens next is that the part loses contact from a failure somewhere between the plating solution and the rectifier:)