Sciencemadness Discussion Board - Anodizing Aluminum

Anodizing Aluminum

Punk - 13-4-2007 at 06:36

Just had sweet sucess anodizing/dying some aluminum hotrod parts and was curious if other acids would work to form the aluminum oxide layer other than sulphuric.

%5 acetic [Vingar]?
Phosphoric [pop]?
Hydrochloric?
citric?

Levi - 13-4-2007 at 07:22

I wouldn't try hydrochloric acid as aluminum will dissolve in it. As for the others, I think they would probably work but not as well as sulfuric acid does. Try with vinegar, there are threads on oxidizing iron with it.

[Edited on 13-4-2007 by Levi]

dedalus - 13-4-2007 at 13:08

Phosphoric, oxalic, and chromic acids have been used.

Sauron - 13-4-2007 at 15:10

What you want is Martin Hardcoat.

Most other anodizing just isn't very durable.

dedalus - 13-4-2007 at 15:17

I'm not familiar with that.

I know about hard anodizing. It's sulfuric acid anodizing, with a lower concentration of acid, and a very cold electrolyte. I used to analyze a hard coat bath, and I'd really enjoy sticking my (gloved) hand in the tank to get the sample in the summer heat.

A lot of those other types of anodizing are special purpose. All anodizing makes the surface of the metal harder and more corrosion resistant.

You want to see some really beautiful things made with decorative anodizing, check out this website for a company called PK Selective Plating in Santa Clara, CA. They color electric guitar bodies with this technique.

For dyeing, I think the oxide coating should be a bit less compact and dense than you get with hard coat. I don't think that type takes the dye well.

[Edited on 13-4-2007 by dedalus]

Levi - 13-4-2007 at 15:26

Quote:

Originally posted by dedalus
I'm not familiar with that.

I know about hard anodizing. It's sulfuric acid anodizing, with a lower concentration of acid, and a very cold electrolyte.

[Edited on 13-4-2007 by dedalus]

According to what I've been reading, hard anodizing uses a higher concentration.

Dr.3vil - 13-4-2007 at 15:28

http://www.focuser.com/atm/anodize/anodize.html

this guy sells anodizing kits for home use but his web site contains great general information about dyes and the overall process.

dedalus - 13-4-2007 at 15:56

Quote:

Originally posted by Levi

Quote:

According to what I've been reading, hard anodizing uses a higher concentration.

From the"Metal Finishing Guidebook and Directory":

"Anodizing process conditions have the greatest influence on oxide coating properties. Use of low acid concentrations and temperatures favor less porous, harder oxide coatings, while higher acid concentrations, increased solution temperatures and anodizing times (favoring electrolyte induced oxide dissolution) favor more porous, even powdery oxide coatings."

There are processes that employ chelating agents to complex the dissolved Al at the workpiece/electrolyte interface, that allow one to get hardcoat at higher temperatures. I think this is because they reduce localized heating effects at the interface.

Sauron - 13-4-2007 at 16:14

The process I am talking about certainly uses chillers to keep the temperatures down. That much I remember. Haven't looked at it since I left the States two decades ago.

Levi - 13-4-2007 at 16:30

"The hard anodizing process also uses a sulfuric acid electrolyte, but the bath is maintained at higher sulfuric acid concentration, lower temperature, and higher current density, which increase film crystallinity."
http://www.hohmanplating.com/Processes/Process_Capabilities/...

"The hard anodising process uses sulphuric acid (at a higher concentration than for normal anodising) but it is carried out at a solution temperature close to 0oC with a significantly higher voltage being applied. We have specialist chilling equipment and a modern pulse rectifier to achieve these conditions. The resulting anodic film has more uniform crystalline structure which gives the increased hardness."
http://www.metroplating.co.uk/hard-anodising.php

I guess these companies are doing it wrong?

dedalus - 13-4-2007 at 17:36

Quote:

Originally posted by Levi
"The hard anodizing process also uses a sulfuric acid electrolyte, but the bath is maintained at higher sulfuric acid concentration, lower temperature, and higher current density, which increase film crystallinity."
http://www.hohmanplating.com/Processes/Process_Capabilities/...

"The hard anodising process uses sulphuric acid (at a higher concentration than for normal anodising) but it is carried out at a solution temperature close to 0oC with a significantly higher voltage being applied. We have specialist chilling equipment and a modern pulse rectifier to achieve these conditions. The resulting anodic film has more uniform crystalline structure which gives the increased hardness."
http://www.metroplating.co.uk/hard-anodising.php

I guess these companies are doing it wrong?

The first statement quoted is flat wrong.

The second statement involves pulsed anodizing, which is a different animal.

If I may politely suggest, try reading something in a book, not off the Internet. The Internet is a good start. What I quoted is out of a recognized industry reference book.

Also good is the Electroplating Engineering Handbook, by Lawrence Durney.

[Edited on 13-4-2007 by dedalus]

dedalus - 13-4-2007 at 17:41

Quote:

Originally posted by Sauron
The process I am talking about certainly uses chillers to keep the temperatures down. That much I remember. Haven't looked at it since I left the States two decades ago.

That may be one of those processes that uses additives to make the oxide layer harder at a given temperature.

Punk - 13-4-2007 at 18:11

Im not going for abrasion resistance, just adding color and corrosion resistance.

Im trying to find the cheapest acid to use because $5 for a little jug of watered down sulphuric is keeping me from trying larger items like bike frames and such.

I can get a gallon of 32% hydrochloric for $3 and then dilute it.

Any reason why diluted hcl would not work?

12AX7 - 13-4-2007 at 21:27

HCl attacks aluminum! You're welcome to try it, but it won't work well.

Why not go to a plater and get it done for a small price? Well, if you can't even afford a $5 jug of acid, I suspect you should just paint it with some cheapass $2 cans of spray or something.

Tim

Sauron - 13-4-2007 at 23:27

He's trying to save $2 but will trash his part instead.

Why not try boiling saturated caustic, that does wonders for Al parts. Makes them go away faster than you can say EAT ME.

trolls

Punk - 14-4-2007 at 07:46

12AX7

posted on 14-4-2007 at 12:27 AM

Why not go to a plater and get it done for a small price? Well, if you can't even afford a $5 jug of acid, I suspect you should just paint it with some cheapass $2 cans of spray or something.

Tim

-------------------------------------------------------------------------
Platers dont do anything for a small price as that would be a total waste of there time so they charge quite a bit of $$$.

************************************************

Sauron

posted on 14-4-2007 at 02:27 AM
He's trying to save $2 but will trash his part instead.

--------------------------------------------------------------------------

Actually Im trying to save *alot* more than $2 and no parts will be getting trashed here as I will not try the diluted hcl.

dedalus - 16-4-2007 at 07:17

Quote:

Originally posted by Punk
Im not going for abrasion resistance, just adding color and corrosion resistance.

Im trying to find the cheapest acid to use because $5 for a little jug of watered down sulphuric is keeping me from trying larger items like bike frames and such.

I can get a gallon of 32% hydrochloric for $3 and then dilute it.

Any reason why diluted hcl would not work?

Five bucks? Ye Gods.

You could try drain opener that's based on sulfuric. That's generally 50% Dilute it 1:2 with water, let it cool, and away you go...for starters.

Figure on a substantial "learning curve" before you get stuff to come out looking professional.