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Anodized 7075 Aluminum is Blotchy with Only Some Dyes

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July 29, 2021

Q. Hi, I'm from Argentina and I'm trying to anodize some sprockets made of 7075 T6 Aluminium for cometic purpose. The process that I'm doing is:
1. Machining the sprocket.
2. Clean with degreaser (I have a ultrasound that I could use but I haven't implemented it yet.
3. Immerse the sprocket in caustic soda (18%) until it takes a dark color all over the piece (if i don't do this the color after the anodizing is not uniform)
4. Immerse the piece in hydrochloric acid (20%) 10 second
5. Anodize in sulfuric acid (18%), for a sprocket of 7 Dm2 I use 10 Amp 18 Volt for 120 minutes at 18 °C, Use cathode of aluminium double the size of the sprocket. I don't use agitation (I need help with this too)
between steps I clean with water.
6. Put the sprocket in aniline for cloth 30 min at 50 °C
7. Put the piece in boiling water for 10 minutes.

61302-2 61302-1

The problem that I have is that I don't get consistent results or I have problems with some colors, for example the blue one, I sent a picture of one problem that I have

Julio Vacca
- Buenos Aires - Argentina

probertEthumb Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert
(We've sold 750+ copies without a return request)

August 2021

A. Hi cousin Julio. I don't know what you have access to and what you don't have access to; and I don't know what you can afford and what you can't afford. I don't know if you are a hobbyist with a one-time interest to anodize your bicycle parts, or if you are trying to run a professional anodizing service. But the inconsistencies you suffer are the result of compromises and not doing things quite right in numerous places -- so there is probably no one thing you can change which will give you consistency :-(

2. I don't know whether or not you get the parts truly clean. A waterbreak-free surface after an alkaline cleaner isn't proof of a clean surface but a waterbreak would be proof of a dirty surface.
3. This step is probably okay, but should be done for a fixed amount of time and at a fixed temperature.
4. HCl is not a proper desmut/deox chemical.
5. 18% sounds too high. Can we assume you mix this from electronic grade rather than using battery acid? Is 7 dm2 the surface area of one side or both sides put together? Can we assume that you slowly ramp up to 18 volts? 120 minutes sounds terribly long. 20 °C would be better. Good agitation either with a pump or air is absolutely necessary. The parts will get warm from the electrical power without it and anodize inconsistently.
6. Fabric dye is somewhat similar to anodizing dye, but not the same thing.
7. If you are anodizing for 120 minutes, then 10 minutes of sealing is not even half enough.

I think you need to see a good anodizing book, and your national library has at least one.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

August 7, 2021

A. Hi Julio

it would be to your benefit to use a more standard etch time rather than to etch until ... this may be the very source of your splotchy appearance. 7075 tends to freckle as etch time is increased. 7075 material can be etched for 10 seconds maximum to stay away from the splotchy appearance that you are referring to. There is normally no freckling appearance with a light etch. Also I have to agree, anodizing at 18% seems rather high. 14% seems much more sane.
One more thought... all is not lost. The freckling can normally be removed with a scotch brite pad or steel wool. Light to moderate pressure is all that is required while rubbing the part.

Philip J. Verzal
supervisor - Cicero, IL USA

August 9, 2021

Q. Hi. I am trying to start a motorcycle accessories manufacturing venture and I wanted to do the anodizing on my own. I am willing to make the necessary investment to obtain the expected results, here in Argentina it is not so easy to get some things but we are looking for it, so any suggestion will be welcome
2. OK one thing I have to improve is that
3. OK
4. What do you recommend? Nitric acid is hard to get here
5. i use battery acid;
7 DM2 is the total area.
I'm not ramping voltage
With less time I don't have better results.
I'm going to put in some agitation....

P.S.: Etch time is the time in the time in caustic soda right?
P.P.S.: I almost forgot: I already requested a book in the library, but it's closed for covid :-(

Julio Vacca [returning]
- Buenos Aires - Argentina

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pdf is currently available from academia.edu

August 2021

A. Hi again Julio.

4. Robert Probert knows a lot more about anodizing than I do, and his book seems to suggest that proprietary tri-acid salts are the thing to use if you can get them.
5. Battery acid is not ideal because it may contain contaminants including lead; see if you can get any specs on it.

Since you're treating 7 DM2 with a 10-Amp rectifier, the available current density is 1.43 A/dm2 or 13.3 ASF, and that's okay. Depending on your rectifier you may not strictly need 'ramp control'; if your rectifier cannot and will not exceed the amperage rating you set, that will suffice, because what you're trying to avoid is current gone wild when the aluminum is bare, causing burning.

Sometimes the cleaner tank is largely caustic soda, but it's not a great idea to try to use a single tank as both the cleaner and the caustic etch.

While you are waiting you get the library book you're seeking, you can consult the anodizing chapter of the Metal Finishing Guidebook, available online.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

November 29, 2021

Almost everything you are doing is incorrect, let us start here:

1- Parts must be waterbreak-free cleaned
2- 7000 series is loaded with zinc so only etch for a short time 1-2 min
3- HCl is not a good de-oxidizer, switch to nitric or a proprietary tri-acid.
4- ASF should be 20-24 ASF and for around 60 min

drew nosti
Drew Nosti, CEF
Anodize USA
supporting advertiser
Ladson, South Carolina

December 28, 2021

A. Hi Julio,
By using HCl in your process you are introducing a chemical that is a well known antagonist to good anodizing results. Any amount of chloride dragged into the anodizing bath is going to be detrimental. Even the amount left in city supplied drinking water is often too much! A normal desmut of sulfuric and nitric acid with a bit of ferric sulfate is better, but of course you still need to minimize dragthrough.

You should not have chloride in your process at all. If you can test your anodize tank for contamination, it should be under 10ppm free chloride for ideal results. Its possible it can be higher and still work, and hopefully someone more experienced can give an upper limit, but I only know what I hold to aerospace spec for a dual use Al/Ti tank, and it's 10. But please choose a different desmut solution.

Rachel Mackintosh
Lab Rat and some other things - Greenfield, Vermont

December 29, 2021

A. The above from Mr Mooney covers all but one other possibility. Electrical contact reliability influences thickness and color.
Contact must be "tight" and "wide enough to deliver the current".

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

"The Metallurgy of Anodizing Aluminum"
by Runge & Kaufman
from Abe Books

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August 8, 2021

Q. I just received a shipment of parts from my anodizing provider who usually provides great results. But this time the parts aren't really black and are kind of splotchy and grey. Is this a bad finish or just something that can be cleaned off? I've tried wiping it off but that's not really helping. Wiping it with oil kinda helps but I feel like that's just covering it up. I've included a photo of the part up against a properly finished part for contrast:


Can anyone tell me what I am looking at here and how I might be able to clean/fix the issue? Thanks

Dan Haga
- Baltimore MD

August 16, 2021

A. I suspect these products's duration time in sealing bath might be much more then before. So as if problem's source is sealing bath. I don't suppose any problem in electrocoloring due to splotchy and grey surface. If you had said "brownish appearance" or "too long coloring time" then I would say problem is in electrocoloring bath. Please ask your vendor which type sealing bath is being used? My guess is cold sealing.

Alaattin Tuna
Anodizing Supervisor - TURKEY,sakarya

"Anodizing and Coloring of Aluminum Alloys"
by S. Kawai
from Abe Books

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August 2021

A. Hi Dan. Electrocoloring is one method of coloring anodized parts, and widely used in architectural anodizing where outstanding resistance to UV 'bleaching' is required. Alaattin is obviously well experienced with it.

However, I see no reason so far to assume that these parts are electrocolored; I would presume they are dyed by immersion in a tank of warm black anodizing dye. Obviously it would be a good idea to ask your vendor for the details on what he is doing for coloring & sealing before we go off on tangents :-)

You are showing us two different parts; the splotchy one looks to me like it may be a diecasting, whereas the good one looks like it might be a machined extrusion. Sorry to say that if this is the case, expecting a diecasting to come out a uniform jet black like an extrusion might be unrealistic. You need to also determine the fabrication methods and alloys.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

August 16, 2021

A. This looks like a bead blasting pattern to me, so that would be my first guess, the "light black" parts would probably look darker right after anodizing and prior to dying due to the silicone contamination during blasting, and because of it those areas would absorb less dye compared to less contaminated areas (which look darker black after dying)

Strip the part and re-anodize, if it comes out very even - you know it is the blasting process that has caused the issue

Too high pressure during bead blasting could be the root cause, it makes blasting quicker, but it also smashes the beads into dust which then gets embedded in the surface and causes this problem, reduce the pressure for glass, or even better - switch to stainless steel beads

Janis Ziemelis
- Riga, Latvia

September 30, 2021

A. Agreed that this problem looks like it has origins in the blast process. Source: we do a LOT of blasted black dyed hardcoat firearms components. I've seen this in person.

Aluminum oxide blast media, untainted by other metals, is your friend... that defect looks like what we got when someone had the bright idea to use glass bead for a hot minute. The beauty of alox is that the normal pre-anodize processes can clean it off the substrate really effectively, leaving a substrate that will anodize evenly and take dye in a uniform manner.

Rachel Mackintosh
Lab Rat and some other things - Greenfield, Vermont

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