DIY Al anodizing - cause of this trouble

[Steerpike]

Hi
I'm trying to anodize some aluminuim parts that were badly scratched.
I have buffed the surface free of scratches, and got exactly the right shiny finish I want, but when I anodize it (15% H2SO4 ) it tends to go grey and matt.

Is this symptomatic of the wrong concentration of acid? Too high (or low) a current density?.
Or something else?

[acid_k2]

Originally posted by Steerpike
it tends to go grey and matt.

I think it's normal.
Anodization process creates a grey and matt Al oxide on surface.

Is it different from what you expected?

[Steerpike]

My finish is different to the original anodized finish, that I had to sand off (to remove the nicks and scratches).

The freshly sanded surface (done wet with emery paper) looks PERFECT. I was hoping it wouldn't change when anodized.

Maybe I should just reduce the time, and hence the oxide thickness, although the original oxide was thick - it took a lot of effort to remove with emery paper.

[acid_k2]

I suggest 15-20% H2SO4, a current of about 1A/1dm2, or something more, and a voltage of about 15V.
And your parts is probably silver painted after anodization.

There is also another type of anodization (hard anodization) with the acid solution at about 7%, kept at very low temperature, and a lot higher current and voltage. In this case, surface is as hard as stainless steel, and oxidation is a lot thickier (1/10 mm or more). Hard anodization is very difficult to do at home.

[acid_k2]

very useful web link: http://www.focuser.com/atm/anodize/anodize.html

[bulgin]

I had lots of similar troubles using various brands of automotive water papers like Norton or other generic, origin unknown brands.

My troubles ended when I changed the abrasives to 3M. Expensive but worth it. After sanding, you need to degrease it. I use proprietary stuff from Metalquip but I think Handy Andy (the real genuine type), will do the job.

After that, you can 'chemically polish' with phosphoric acid at 100degrees C for no longer than say, two immersions of 20secs and the final preparation with 5mins in a nitric acid bath to de-smut. Wear rubber gloves if you can and only aluminium or titanium should be used to attach the job in the bath.

I of course do not accept any responsibility or claims of whatever kind for the consequences of the information supplied in this post.

Apologies for the rider

bulgin

[Steerpike]

Ah, thanks for the input. I'll try again come the weekend.

What sort of place do you buy phosphoric and nitric acid? Not ordinary hardware stores I suspect.

[bulgin]

Metalquip (but they're here in Sleeptown)

Otherwise an anodising chemical supply firm in Joey's.

bulgin

[Bratislav]

DIY anodized Aluminium will always be 'matt' as that is how aluminium oxide builds up (porous crystalls). The best you can do is after closing the pores (boiling), polish the surface using normal car polishing techniques (e.g. cutting and buffing wax). The slight discoloration can't really be avoided, as oxide has yellowish-gray tinge (that will depend a lot in the alloy used). Make sure you build up a decent layer of oxide, it is possible to cut through it with some more aggressive waxes. Go easy and slow!

[wrenchone]

Given all that bother, I'd just give the piece (with a few others, to make it worthwhile) to a professional anodizer and let him do his thing. I wouldn't rest too easy with all that methy-ethyl badstuff lurking around the house. I'ts expensive and nasty and attracts attention from the wrong sorts of people (the cops may get the notion that you're doing something you're not)... One thing to note is that the final finish in anodizing (unless its clear anodize) involves dying the aluminum oxide layer - that's how you get black, blue, gold, red, army green anodize. Another attractive finish is iridite/chromate, but it's expensive and hard to find, as hexavalent chromium is very bad juju these days.