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Anodizing Aluminum

I have not yet done any anodizing as of this writing, but it has been on my list of ’things I gotta try’ for a long time.

Recently there has been some good discussion on the 7x group, so I thought it would be a good idea to capture the info here for future reference. (Hmmm… guess I should do this for the powder paint thread, too.) – Aha, Dave Audette has some info on powder coating paint.

Anodizing Links:

The thread really got started with this post by Bruce Simpson (msg 53307) in response to a question about putting a nice finish on an aluminum gear shift knob:

You can get the nicely frosted look by dipping the part in sodium hydroxide (household lye or caustic soda) for a short period of time (dependent on the concentration of the solution).

However, if you want the finish to be scratch and wear-resistant (and avoid ending up with the type of black hand you sometimes get from handling raw aluminum, you really should anodize it.

Anodizing is a very simple process involving the use of little more than some diluted battery acid and a 12V charger (although it doesn’t work on all aluminum alloys) and well worth doing.

As for the shiny rings – you could turn them before anodizing or afterwards. They will be shinier if you add them afterwards but then they will tend to tarnish somewhat over time – however, an occasional polish will fix that.

I suggest you have a play with the process on some scrap and see how it turns out. You need to use one of the 1000 or 6000 series alloys for best results. Avoid 2000 or 7000 series – they require a more complicated (and dangerous) process to anodize.

For this type of application I’d go for 6061 aluminum. It’s cheap and readily available with pretty good machining properties.

Then James McGee posted this message 53334, which got the discussion rolling.

> Anyone know of a web site that explains home anodizing in detail?

Steve (mysticZ28.org) posted this info:

> In addition to the other reply, there are a couple of places that sell kits to do it yourself. Simple clear anodizing is surprisingly easy, zap it in acid and pitch it in boiling water.

> > Adding color is where it starts to get hard. Fortunately I like shiny stuff, so I’ll be doing most of my personal ano in clear over polished aluminum.

Here’s some links to a couple of places that sell the stuff:

> www.caswellplating.com - Lots of good stuff here beyond anodizing.

> www.hobbyplating.

com - Room temp ano sealer, which sounds promising for cosmetic work.

More from Steve:

> It’s one of those things that can be very hard or very easy, depending on what you do. A simple clear ano job to protect the metal is easy, just zap and boil.

> > Cosmetic work that, for example, fades from one color to another is pretty tough until you figure out how to do it. Just remember that any tiny spec of contamination (oil, dirt, even a fingerprint) will show up.

> > I’m going to take advantage of that by building the ano layer, sticking my thumb on it, then dying and sealing. Should make a thumbprint appear perfectly.

From Rod B.:

> Doing color is as easy as doing clear; just add dye to the water. I use plain old liquid Rit dye in the boiling water bath and it works just fine. > > I have pieces done in black over a year ago that look every bit as good as the commercial dye.

I’ve also done red and green, and they look great also. I mix the solution to about double that recommended on the bottle of dye.

> On reading my reply, I realized that ‘mixing to double’ is a poor description.

I mix the dye using half the water specified. There, that should make sense

From Rick Kruger:

> Rit dye does work, although you can get inconsistent results. I used if at first, but one batch came out with a green cast to it. Some speculation put the cause at pore size in the anodize layer selectively taking up dye particles.

Probably only an issue with black.

> I switched to Caswell black dye and have never had any problems like that since.

From Brian Metz:

> I also had a problem with Rit. Part came out splotchy and greenish/yellowish ick, but still corrosion-free after a year outside.

More from Steve:

> Every time I’ve used Rit for black I’ve gotten an almost bronze color. Most of the time I use Rit for the rest of the colors, though. Fades aren’t too tough, just time consuming.

> > Dye it the dark color, then slowly lower the part into a bucket of bleach until the area that will be the light cover is completely submerged, then slowly pull it back out and dunk it in the light color.

> > Seal it and it’s done. I’ve done splash by flinging dark colors at light colors, but it never came out very good. Now that I’ve found some room temp sealer I have higher hopes

From Bruce Simpson:

> I just use those disposable nitrile gloves you can buy in packs of 12 or more. Since you’re working with an acid, it’s not a good idea to use your bare hands anyway.

> Dyeing is a very simple addition to the basic process and I’ve had great success using regular fabric dyes to get impressive gold and red colorations that have yet to fade even when the parts anodized are operated at high temperatures for prolonged periods.

> The accepted way of dyeing is to take the work from the anodizing tank, rinse in cold distilled water, immerse in the dye bath for several minutes (with gentle agitation) then remove and seal in steam or boiling water.

> That’s the system I use and it works great. I would be worried that putting the dye into the boiling water sealing bath might cause the pores of the anodized layer to be closed before the full dye-uptake has been achieved.

> I know that when I take the work from the dye bath I steam it for a couple of minutes and then, when I put it into the boiling water, there is very little leaching of dye from the work.

Whether this is because the steam has already sealed the work or whether the boiling water causes a very rapid sealing I don’t know.

> I use Dylon brand – they come in little plastic pottles about an inch in diameter with an aluminum tear-off lid and are available in hot or cold water versions.

I’ve used both.

From Rod B:

> The majority of my work is in black and the aluminum has always been 6061. It has always come out like a commercial job, a deep neutral black. Perhaps the technique I use makes the difference.

> I anodize in the acid bath for 120 minutes, keeping the bath at 68-70F with ice cubes in a water jacket (a bigger tub). > > I always use the liquid Rit, and mix it using half the specified amount of water for a really concentrated solution.

> > I then heat the solution on the stove and submerge the work into the solution when the temperature is about 90F. > > I then raise the dye bath temp and hold it at 130-140F for 30 minutes.

> > I then raise the temp until it just starts to simmer, then pull the piece and rinse it in a boiling water bath for a few seconds, then steam it in a vegetable steamer for 30 minutes.

> > I’ve never had a bad job.