Aluminun dark in color after anodiziation.

[scocklin]

I just finished an anodizing run on an unknown aluminum alloy. I thought the run went perfect but when the metal emerged from the electrolyte bath it was dark in color almost like a coat of smut covering the outside.

I prepped the piece by first stripping it and than polishing it with black and than brown polishing compound. I degreased it and preformed the water break test (passed) rinsed and than de-oxided and de-smutted.

The surface area was 46 sq inches. I was using 6A as a current density. I set my CC PS to 1.9 volts. My voltage readings for the run where:

0 min. 8.0 V
10 min. 14.2 V
20 min. 15.6 V
30 min. 16.2 V
40 min. 16.4 V
50 min. 16.4 V
60 min. 16.3 V
70 min. 16.3 V
80 min. 16.1 V
90 min. 16.0 V

Electrolyte temperature at start was 67 degrees and at the end was 71.1 degrees.

The polished look is completely gone. It holds the dye but only looks good with the darker colored dyes. Any Ideas on what I did wrong? Thanks again for your help.

[M_D]

The voltage looks a tad high to me, for that current density (CD). Our tank temperature is usually a little higher, like 72º or so, which lessens the voltage for a given CD some. But, even at a true 10 amps CD to the part, 16 volts is close to what we expect to see.

There are other factor besides tank temperature, such as how close to the cathodes the part is, and acid concentration (and aluminum content) that vary the voltage some, so you can't say a little difference voltage is absolutely wrong. But I still think it may be a clue that something was off a bit.

Could be the connection, that can change the voltage more than the other factors. Not that I doubt your calculation on surface area, but if you calculate it higher than it actually is, you will be applying a higher CD and the volts will be higher also. If you are too far off, it is possible to burn the part.

One other thing, excessive oxide can cause small dark spots that make the part look darker. How did you "de-oxidize" it? Did you have to buff it heavily?

Not knowing the alloy presents some difficulty in trouble shooting also, was this a casting?

The way the voltage peaked at 40 minutes may show it would have been done about then or a little latter.

With as little information as I know about your job, I'm mostly speculating about what may have happened.

[lastone75]

voltage is a little high. was it yellow in color? i have the same issues if the voltage is to high and i leave the parts in the tank for long.

[Fibergeek]

I too suspect connections, how is the cathode connection? This is the one to the negative terminal of the PS, the cathode is in close proximity to the acid electrolyte and gets cruddy fast. M_D (and I) clean this connection regularly.

I also suspect inadequate removal of the native oxide. This can be particularly bad if the part has had long exposure to a marine environment, and has had its anodized finish (if it even had one) long since worn away.

Are the polishing compounds completely removed? You will come to appreciate the thoroughness this actually requires. This stuff can be tough to get off, especially the grease based ones.

And then there's alloy, too bad you don't know what it is, some of the old ones can be a real PITA, but not impossible.

This may not be your fault at all. But it will be a valuable lesson, actually a blessing in disguise.

(edited to add)
Perhaps M_D can find the time to give you some metal prep tips? He's better at this than I am. He will require more detailed information from you however for this to be effective.[/quote]

(edited again, I've been working too hard)
Lastone75 also spotted the voltage issue.

[scocklin]

I stripped and re-prepped the piece for another run today. Yes I am sure it is cast aluminum now. My cathodes are soldered together with 6 gauge copper wire and I have applied heat shrink over the solder connections. I have a piece of bare copper wire coming off the cathode that I connect my negative battery clamp from my power supply to. All of the connections are made outside the tank below the lip. I had my tank very close to an open window last night probably leading to the cool electrolyte. I’m using Caswell de-smut/de-oxidizer. After polishing the piece I pre wash it with simple green than soak it in Caswell de-greaser at a temperature of 180 degrees for 5 to 10 minutes.

One thing I am a bit confused on is MD said “The way the voltage peaked at 40 minutes may show it would have been done about then or a little latter” I have been recording my voltages way past PAR for most of my runs. Looking back at my notes PAR seems to be coming in between 40 and 60 minutes. 90 minutes has been the time limit on all of my runs. I read in Fibergeek’s post “Anodization Curves, 1 of 3 that dissolution allows more time to open up the pores before the desired coating thickness is achieved”

Does this mean that dissolution begins before PAR and once PAR is accomplished the piece being anodized is complete?

I Ran the piece through again today with the same results as yesterday but this time I used the medium LCD of 4.5 A/f2. My voltages where as follows:

0 min. 8.5V
10 min. 10.6
20 min. 11.3
30 min. 11.6
40 min. 11.8
50 min. 11.8
60 min. 11.8
70 min. 11.9
80 min. 11.8
90 min. 11.8

One thing I noticed between the two runs, last nights being at 6A density and today being at 4.5 A density, when I calculated ohms for each voltage and compared them to last night results the resistance is just about the same for both runs. Is this a coincidence?

The piece takes dye with no problems but loses the polished finish I gave it during the prep phase. It comes out dark in color not glossy. Is this a result of cast aluminum?

I would appreciate any metal prepping tips that MD could give me. Thanks again to all.

[M_D]

I suspected the piece was cast, but didn't know for certain. That may be why the voltage looks high, I can't honestly say I know though. It's also a good possiblity why it is dark.

As far as the comment about it being "done" at PAR, that may or may not be true. With all of the variables, sometimes you have to go by what works. In some cases, going longer may improve the pore structure for dying, and in some it may make it worse. I was thinking you may have "burned" the part(s), and thus my comment about it may have anodized too long. Usually in our operation we can go quite a long time past PAR, and the voltage stays pretty flat. But we usually use Ti racking, not always though. That changes things also.

There are various ways to remove oxide, both mechanically and chemically. Fibergeek bead blasts alot of his parts. A good buffing should remove most if not all of it, it would depend on how thick it was and how it was buffed. The oxide is pretty close to impossible to spot visually.

There are acid de-oxidizers, and alki de-oxidezers such as lye (sodium hydroxide). If you leave the part in too long, it will etch, or dull a polished surface. If the part it freshly machine or otherwise mechanically cleaned, further de-oxidizing usually isn't needed. The de-smut is best top remove smut, and not oxide, I have found. It works to slow for de-oxidizing, and may not provide the quality of surface needed.

I like the cleaner from Caswell, but when it is that hot it will etch the parts when left in that long. You may have dulled them some then. Also, since it is cast, it may have a tendancy too dull in the anodizing process. You said previously you did one batch and it turned out, I don't know if it was polished and stayed that way or not. When doing troublesome alloys, sometimes it is a fine line line between "perfection" and failure, so perhaps you weren't too far off.

We do quite a bit of anodizing on as machined parts, and they turn out with a pretty good gloss. We also have done some polished parts, and they stay nice. On anything where you want a good gloss, you need to be careful not to push the time in the hot cleaner, etch/de-oxidizer if used, and anodized coating thickness.

[Fibergeek]

One thing I noticed between the two runs, last nights being at 6A density and today being at 4.5 A density, when I calculated ohms for each voltage and compared them to last night results the resistance is just about the same for both runs. Is this a coincidence?

No. You are not measuring the voltage and current with sufficient resolution for the small surface area parts you are dealing with, you can't see what is going on. I did most of the curves using 0.1 sq.ft. samples. The resolution and accuracy of my equipment is 3 decimal places for both voltage and current. This provides 1 mV (0.001 volt) and 1 mA resolution. The meters on your power supply are not capable of even two decimal places reliably. You aren't using a $3K power supply.

This is the same reason why we are recommending that you use the 720 Rule and not PAR, you don't need real accurate equipment with the 720 Rule.

The purpose of the three curves is to determine and illustrate what is really going on in anodization. For any practical anodization purpose, you don't need to be doing this. You do need to understand the behavior of these alloys and the trends they show under various anodizing conditions, which is why the curves are there.

You don't need to be using (but you can if you want) such heavy wire, it isn't providing any advantage, 6 AWG will support about 100 amps.

[Fibergeek]

Does this mean that dissolution begins before PAR and once PAR is accomplished the piece being anodized is complete?

Dissolution begins immediately, and if it didn't you would not be able to grow the columnar pore structure anodizing depends on. When dissolution becomes excessive the trouble starts.

Don't worry about PAR, I was wrong when I wrote that in assuming that everyone would have the level of instrumentation that I have. A pretty dumb assumption now that I think about it.

Use the 720 Rule.

[scocklin]

I chemically stripped, polished, cleaned and re-anodized the cast aluminum part again and watched it change from shinny to dull inside the anodize tank. I now assume that this is a charateristic of this type of cast aluminum.

Where can I find out more information on types of aluminum and how they are affected by the anodization process?

Thanks again for the help.

[Fibergeek]

Wernick, Pinner, and Sheasby's book, that I recommend to you previously would still be your best source. Most everything else is at one extreme or the other; either folksy anecdotes or super technical metallurgy. The former is a waste of time, and the latter I can't understand.