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DIY Home Anodizing

Before you think I'm an expert, I should state up front that I've never actually anodized anything. I was interested in anodizing (I thought), but didn't know anything about it, so I did some research on-line to see if it was a feasible DIY-type project. It was. It's surprising simple, once you break it down to its basic steps. Unfortunately, anodizing is limited to aluminum and the project I had in mind involved steel, so it looks like what I really need to learn something about is electro-plating... another project, looks like!

I think any discussion of anodizing would start with a couple of basic questions. Such as...What IS anodizing, actually?

Anodizing is a electro-chemical process that builds up a (hopefully) carefully controlled layer of aluminum hydroxide on the surface of the metal, aluminum hydroxide being much harder and more durable than just plain aluminum. The process is accomplished by the use of an electrolytic bath.

And... Why would you want to anodize something?

Aluminum is a soft metal and the surface is very succeptable to dents and scratches. It also oxidizes quickly. Anodizing makes the surface very hard and durable, giving it an almost chrome-like finish. Also you have the option of adding a color that becomes a part of (or is sealed into) the surface. And, anodizing dramatically slows the oxidation process.

There are several methods of anodizing aluminum - all of them basically the same - but commercial anodizing is a much more rigid process; using higher currents, stronger dyes and chemicals and a much tighter control over the entire process, especially the temperatures of the chemicals used in the various steps.

Looking at just the process shows how simple anodizing actually is:

If you plan to try anodizing as a home project, here are most of the things you'll need:

Enough background. Here is the entire anodizing process broken down into step by step detail for the home anodizer:

  1. Preparation

    The piece of aluminum to be anodized should be carefully finished. Anodizing won't hide anything, so care should be taken at this stage to insure that the surface is as smooth and defect-free as you can make it. Or that you want it to be, at any rate.

    This is also the time to ensure that the chemical solutions you will need later are prepared - especially the sulfuric acid solution. It generates quite a lot of heat when it's mixed and you'll want that to cool off as much as possible before using it.

  2. Cleaning

    The surface of the aluminum piece needs to be completely clean, especially of oil or grease. The cleaning agent can be as simple as soap and water (which will do a poor but, possibly, acceptable job) to the use of stronger chemicals. For home use, alcohol is probably the most likely to be on hand, followed by carburetor or brake cleaners (good de-greasers) or acetone, more commonly known as paint thinner. A better option is to use a lye (caustic soda) solution to clean and etch the part, but the best solution is to use a nitric acid solution to both clean and etch the part. Cleaning is mandatory, etching makes the anodizing process work faster. Unless your part has just been machined, it will have some oxidation and may even be partially anodized. Either the lye solution or the nitric acid solution will remove the oxidation and will work at removing anodizing - they will remove it with time. You'll leave the part in these solutions from 5 minutes (for a shiny finish) to possibly 30 minutes (for a matte finish). Rubber gloves should be worn from the cleaning stage to the end of the process - you don't want to touch the piece to be anodized with your bare hands until the process is finished.

    *SAFETY ALERT!*

      Both lye (caustic soda or sodium hydroxide) and nitric acid are nasty chemicals to work with. You should have a pail of water nearby that has a good bit of bicarbonate of soda added to it, ready to neuturalize any acid dripped, splashed or spilled. Any time you are diluting acid with water, remember the most important and basic rule: AAA! Always Add Acid! NEVER add water to acid - ALWAYS add the acid to the water! And as a corollary, add the acid very slowly, mixing well (stirring) as you go. These chemicals should only be used in plastic or glass tanks - never metal.

    **

     If you use the lye solution, do it outside if possible. Placing aluminum into caustic acid will cause it to fizz, bubble and give off strong fumes. A strong lye solution would be about 1:6, that is 1 part lye to 6 parts distilled water. The caustic soda solution can be stored and re-used many times.

    **

     If you use the nitric acid solution, your solution should be 1 to 2 ounces of nitric acid to one gallon of distilled water. Be sure your nitric acid is clearly labeled and is safely stored in an unbreakable container - it is a very strong acid. The nitric acid solution also can be stored and re-used many times.

    After cleaning, the aluminum should be rinsed in cold water prior anodizing. Don't use hot water - you want to keep everything as cool as possible until the final step.

  3. Anodizing

    The actual anodizing process takes several steps of preparation. You'll need to mix the sulfuric acid with distilled water to come up with a solution that is somewhere between 10% and 70% sulfuric acid. Some folks think the stronger solution works best, others get good results with the weakest solution. I think I'd start off with maybe a 20% solution and see how that works. You can always add more acid... This mixing process should be done well ahead of time, as mixing the acid with distilled water will create a good bit of heat. And because too much heat will seal the surface of aluminum, the anodizing process should be carried out at the lowest temperature you can easily obtain.

    The anodizing vat should be of plastic, rubber or glass and must be deep enough that the entire workpiece can be submerged in the acid solution. For safety's sake it should never be more than 2/3 full.

    In the container you'll need a cathode to hook the negative lead from your power source (the part to be anodized will be the anode, that is, connected to the positive lead of your power source). You can use sheet aluminum, sheet lead, or you can make a cathode using aluminum wire and aluminum foil. Use aluminum wire to make a paddle shape and cover the paddle with aluminum foil. Think of something shaped like a tennis raquet, sort of, with the handle bent up to stick out of the acid. The handle is where you'll connect the negative lead from your power source. You'd like the cathode to have at least twice the surface area as the part you are going to anodize. This should be completely submerged in the acid solution - usually sitting on the bottom of your anodizing vat - and you must be very careful that your workpiece never makes contact with it when power is applied, or you'll short out your power source. The only two metals that should ever be in the acid solution are aluminum and lead - any other metal will contaminate the solution. So be sure that your electrical connections are outside the acid solution. The anode and cathode should be aluminum - wire, welding rod, or whatever - where they are in the anodizing solution.

    The power source is cause for another short digression. What you'll need is a stable source for - roughly - 12 volts D.C. at a fairly high amperage - say, up to 20 amps, or so, for large pieces. A battery charger is probably the most commonly used item. It has the power and voltage you need, plus it usually has a meter where you can monitor the amperage you're putting through the solution. One guy built his own power supply, using a step-down transformer and a heavy bridge rectifier, with an ammeter added. That's probably beyond most folks {smile}. He also used a computer power supply for his first experiments. That would work. It won't put out a lot of current, but would do the job over time. Another guy used two 55 amp-hour car batteries hooked in parallel. Car batteries are what he had on hand, but deep cycle batteries (golf cart, marine or RV) would be better. There was some discussion of the current needed to get the job done - one figure being 1.5 amps needed per 3.5 square inches of surface area, thus a 6 inch square piece would require something like 20 amps. I dunno. It seems like people were having success using much less current - probably just taking longer. A 250 watt computer power supply will only deliver about 9 to 9.5 amps at 12 volts, but one person I read about seemed to do quite well anodizing using just this source. I guess I'd go with what's easily available, and upgrade when (and if) needed.

    Connect the positive lead of your power source to the part to be anodized via something aluminum (wire, welding rod, strut - whatever, just make sure you have a good electrical connection), and turn on the power. You should notice some small bubbles coming up from the cathode and the part you are anodizing. That's cause for another...

    *Safety Alert!*

       Some of those little bubbles are hydrogen gas - extremely flamable! (rember the Hindenberg?) - and the others are oxygen, just what flamable gasses need to get really dangerous! So take extreme care that there is no source of ingition anywhere near your anodizing process.

    How long should it take to anodize the part? Well... I got a couple of different answers. The consensus seems to be about 45 minutes, up to 70 minutes. One of the sources said that the part would no longer conduct electricity when it was anodized, so when the current dropped to zero, the anodizing was complete. I'd say this whole anodizing thing is an experiment anyway, so try 45 minutes (and see what the current reading is at that time) and go ahead and finish your part. If it works well, then that's right. If not, try something else. Your acid bath will last through quite a few anodizing jobs before it becomes so contaminated that it doesn't work quite so well any more, so I'd experiment with some scrap (or unimportant, anyway) pieces of aluminum before I anodized something that was important to me.

    The anodizing process will create heat, and that's something you want to control at this stage. The cooler your acid solution stays, the more effecient the anodizing process is. If you plan on anodizing multiple parts, plan on letting the solution cool off a bit between runs. Even a fan blowing across the solution helps. I saw one picture of a guy using a little room air conditioner to cool his acid solution.

    When the anodizing is complete, turn off the power and remove the piece from the acid solution, in that order! Rinse well in cool water. A precaution you should take when removing the part from the acid is to always keep a rag soaked in a baking soda/water solution under the part so it doesn't drip acid on anything. Or, if you do drip some acid, immediately clean up the drip with your baking soda/water. You should have a bucket of a baking soda/water solution handy when you're working with any kind of acid.

  4. Coloring

    Dip your part in your chosen dye solution for between 1 and 15 minutes. Leave the part in until it becomes just a little darker that you want it to be; it will lighten up a bit in the sealing process. The coloring process seems to work best using warm dye - not real hot, but somewhere between warm and hot - probably something under 100 degrees Fahrenheit (or 50 C.). This is probably another area for the home anodizer to experiment. Your dye mixture can also be used over and over again, so keep it out of sunlight and seal it between uses.

  5. Sealing

    You need to boil the part in water for 30 minutes, or so, to seal the surface. One of the sources suggested that a ceramic coated pan works best. Place the part in the water just before it comes to a boil and bring it to a simmering boil within a few minutes. Boiling it longer seems better than trying to shorten the sealing (curing) time. There are anodizing sealers on the market, but the home anodizers I read didn't seem to think they needed it. One source suspicioned that it might be possible that conmmercial dyes needed the commercial sealers more than his amateur clothing dyes did.

    Rub the part with a soft white rag. You may get a small amount of color (dye) from the part, but that should stop pretty quickly.

So that's it - a quick overview of the home anodizing process. My final thoughts on this, after doing the research and writing this article, are:

  1. The process seems pretty forgiving. The voltage used doesn't seem critical, nor the current drawn. It seems like with less current the time might stretch out a little, but the anodizing would still take place.
  2. Also, the strength of the sulfuric acid solution seems to be a really wide variable, with users reccommending anywhere from a 10% solution to a 70% solution. With any acid being more dangerous when it is more concentrated, I think I'd start with the 10% solution and work up from there.
  3. And finally, this doesn't look like it's a one-shot deal. It looks to me like this is something you'd want to do fairly frequently, otherwise the set-up and expense would be too great. And disposing of the used chemicals in an environmentally friendly way could be a problem. If you can see where you would anodize things fairly often in the future, then it would make sense to develop this as a hobby. Otherwise, well.... how hard is it to paint aluminum?

Here are my weasel-words about what I've written... I've tried to be accurate in what I've written here, but if you try to do anything based on what you read here, you're on your on. I'm taking no responsibity for any actions you might take or any accidents you might have. Anodizing deals with potentially dangerous chemicals, gasses and processes, so you need to learn all you can about the process before you take your first step. Google "home anodizing" and you'll get plenty of more sources for study.

And lastly, if you see something wrong in what I've written here, email me and I'll certainly try to correct it.

My sources for this article were:

Anodizing at Home, by Jim Bowes - an article which appears in the WarPig website. Nice overview article about home anodizing, with a focus on anodizing parts of paintball guns.

Then DIY Home Aluminum Anodizing, by Tom How. A nicely done, fairly detailed article on home anodizing. Because he's in the U.K., some words and most products are a little strange to those of us on this side of the pond, but the article is lucid, well organized and written. Best of the articles I read.

And How To - Home Anodizing, an article in "PbNation - Paintball's Home Page". Good overview of home anodizing with some nice details.

Finally, a source of commercial supplies: dyes, some chemicals, the tools and equipment needed and even anodizing kits. Caswell Inc., located in Lyons, NY, the supplier of choice of at least one of the articles I read.