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How to Anodize
by John and Chad
An anodized rip drive for a paintball gun hopper - click for larger image
What is Anodizing?
Anodizing (or Anodising if you're from England) is the process of creating an artificial oxide layer on the surface of aluminum. This oxide layer is also known as Aluminum Oxide (Al2O3) or Alumina. This layer has several important mechanical properties. Alumina is a transparent ceramic, not a metal. This ceramic is much harder than aluminum, so it's very scratch resistant. It's also very non-conductive (both thermally and electrically). Unlike steel, which can rust away to nothing, the layer of oxidization on aluminum helps prevent further corrosion of the surface. High salinity or caustic environments can still speed up corrosion.
The down side of anodizing is that it makes the surface layer much more brittle. This lowers fatigue life and promotes crack propogation. Luckily, most anodized parts are mostly cosmetic or aren't employed in high fatigue environments.
The anodizing process is very famous because it allows you to introduce dyes and other substances such as minerals into the oxide layer. When the ceramic layer is formed, there are small pockets or pores left in the layer. During the sealing part of the anodizing process, these pores are closed up. If dyes are introduced before the sealing process, they are locked in.
Ok, so enough of the technical mumbo-jumbo, how do I anodize something?
In order to anodize parts in your home, you will need a DC power supply, a thermometer, some battery acid, several containers, access to a stove or some heaters, some dye, aluminum wire, some aluminum plate, latex gloves, and preferably some sealant. I'd also recommend safety glasses and an apron of some sort.
The process I discuss here is known as normal semi-hard anodizing or type 2 anodizing. There are other types, but they require much higher currents and much closer temperature regulation. Next let me say that anodizing can be very dangerous! It involves electricity, the handling of acid, and in some cases toxic commercial dyes . The fumes given off during the anodizing process can also be harmful if inhaled. That being said, there are thousands of people who do anodizing in their house or garage, all it takes is some caution and knowledge of what you are doing.
Step 1: Finding and setting up a power supply
ATX computer power supply
You will need a DC power supply, capable of putting out 12v at a large amperage. Some people buy small rectifiers to use, but I recommend finding an old computer power supply. With the number of old computers people/companies get rid of on a daily basis, they are easy to find, and best of all, free! When looking at computer power supplies, almost all of them have a label on the side which tells you how much amperage they are capable of outputing at 12v. Mine are rated for 8 amps.
There are several types of computer power supplies. Note that some older models can't be used because they have proprietary interfaces with the motherboards and need two way communication to work. Most ATX style power supplies will work though. In order to use the ATX power supply, you will need to connect the green wire to ground (a black wire). This tells the power suppy to turn on it's outputs. After connecting the green wire to a black wire, plug in the power supply. The fan should turn on. If it does, congratulations, your power supply is working. The yellow wires are 12v, the black wires are ground, the red wires are 5v, and the rest of the wires are negative voltages. The only ones you will use are a yellow wire and black wire.
Power supply connector with 12v and ground wires connected.
It is very convenient to make alligator clip wire for both 12v and ground that are a couple feet long to plug into the connector on the power supply. This makes it quick to set things up.
Alligator clips connected to the anodizing bath
It's very imporant when anodizing to make sure that you aren't trying to do a part that's too big. If you do, the part will pull too high of an amperage and blow your power supply. We will discuss how to calculate this amperage draw later on. However, to prevent amperage overdraw, Chad started using a PWM controller (pulse width modulator) that he bought for $15 off ebay in order to handle large parts. The PWM controller can be used in combination with a resistor or potentiometer in order to keep the amperage draw within a certain value (it's a good idea to use a volt meter to verify the amperage ouptut). The PWM controller can be seen below, covered in electrical tape to prevent accidental shorting.
PWM controller used to limit amperage draw
Step 2: Setting up your anodizing bucket/tub
Now that you have a power supply, you need to set up your bucket. This bucket is going to hold acid, so it needs to be made out of a non-corrosive substance, (IE plastic). Most people use 5-10 gallon plastic buckets, like the ones you can get from Home Depot or any other hardware store.
Inside this bucket, you will need to make an aluminum cathode. The cathode is your negative/ground plate. It is important to have a cathode that's at least 3x the surface area of the part you are anodizing, and it should encompass all sides of the part to promote even anodizing on all sides. One way to make a cathode would be to wrap aluminum wire all around the inner wall of the bucket. In our case, we decided to go with something a little more sturdy and permanent. It's made from 1/4" aluminum plate and aluminum flat stock.
Custom made cathode |
Cathode sitting in the bucket |
Now the bucket gets filled with sulphuric acid solution in a 10-15% by volume ratio. A common source for sulphuric acid is car/motorcycle battery acid. Most of the time, this acid comes in a 50% solution, so you will need to mix it with 3-5x as much water depending on the desired ratio. You will want to use de-ionized water (not tap water), if the water contains any chlorides it can ruin the finish.
If you want to get more fancy and do things like the commercial big boys, you will need to purchase pure Sulphuric Acid (B.P.C) sp.gr. 1.84. This gets mixed at 16 fl oz for a 10% solution, or 24 fl oz for a 15% solution. And the maximum bath chloride content for commercial grade anodizing should be kept below .002 % (200 p.p.m. as sodium chloride).
This is very important to remember. Always add acid to water, not water to acid. When acid and water are mixed, they create heat. If you add acid to water, the heat gets absorbed by all of the water. When you add water to acid, the heat gets absorbed by the smaller amount of water that's hitting the acid. This water can boil on contact, causing acid to splash everywhere. You will want enough solution for the bucket to filled enough to complete submerse any parts you are hanging in it. The more solution you that you have in the bucket requires more energy to raise the temperature of the acid during anodizing. While anodizing the bath should be held between 20-25 degrees Celsius (68-77 degrees Farenheit). It may be necessary to pack some ice on the outside of the bucket during anodizing to maintain this temperature (or you could build a heat exchanger).
It's not a bad idea to add a fish tank bubbler to the anodizing bucket. It helps keep the acid mixed and helps provide consistent results. Also, while anodizing aluminum will dissolve off of the cathode. Some dissolved aluminum in the solution increases efficiency, but once it reaches 2oz of dissolved aluminum per gallon, the brightness of the finish starts to suffer. This means it's a good idea to replace your acid on a regular basis.
Step 3: Part Preparation
A freshly machined part in need of prep (notice the tool marks)
This is quite possibly the most important step in achieving a nice finish. Anodizing does not cover up any flaws in the material. Any scratches, knicks, tool marks, or dull spots will show up in the finished surface. In order to get a mirror like shine from your finished part, it needs to go into the anodizing bath with a mirror like shine. The picture below shows you what happens if you don't polish the part.
Anodized part with no surface preparation
It's recommended to sandblast parts if you want a dull matt finish, but make sure you use a glass media and cover the surface in nice even passes, otherwise the finish will appear cloudy and uneven.
If you need to polish parts, be prepared to use a lot of elbow grease. If you cut corners, it will show. Start by using a coarse grit sand paper and sand back and forth in one direction only. Slowly work your way up from the coarse to the fine criss crossing the sanding direction each time and polishing until you can no longer see the lines left by the previous grit of paper. A good set of sanding papers would be to go from a 60 to 80 to 150 to 220 to 320 and finish up with a 600 grit wet sand paper. After this step, a mop style polishing wheel and some polshing compound will give you a true mirror finish.
When you are done polishing or sandblasting the part, it's very important to keep it clean. You should degrease the part and rinse it thoroughly. Be sure not to touch the part without gloves on or else oil will transfer from your hands to the part, which can leave marks on your finish.
Step 4: Hang the part and anodize
The hardest part (surface prep) should now be over. The part you are anodizing will now become your Anode (hence the name anodizing). The anode gets connected to the positive 12v power supply, but don't do it yet. At this stage, harmful fumes are going to be created, so make sure to do this outside or in a well ventilated area. I use a fume hood.
You want to use the aluminum wire to hang your part in the acid bath. It's very important that the wire makes a very tight and rigid contact with your part. If you can wiggle the wire around on/in the part, it's not tight enough. Use a die to put threads on the wire and thread it into a hole, or bend the wire and shove it into a hole so that the wire provides some spring force on the walls of the hole. It's important to note however that where the aluminum wire makes contact, your part will not be anodized and won't take dye. It's important to try and rack the part in places that aren't seen, such as inside of holes.
Note: The anode and cathode are well seperated
If the wire makes poor contact, the wire will anodize first and prevent further electrical connection to your part, or the part won't get enough electricity and it will barely anodize. The only other material you can use to hang a part is titanium. If you use any other materials, they will dissolve in your acid bath, ruinning it. It's also good to note that the wire will anodize with the part and will no longer be usable afterwards since it will no longer be electrically conductive. Meanwhile, titanium can be re-used over and over, but it looses some conductivity with each use. If you use titanium, you will need to have larger contact areas than with aluminum in order to make up for the loss of conductivity.
Use the wire to hang your part fully submerged inside the acid bath. Make sure that the part and wire make no contact with your cathode or else you will short out your power supply.
Part hanging in the bath, connected using aluminum box tubing instead of wire
Now that your part is hanging, turn on your power supply. The anode and cathode should start bubbling as gasses are given off. Do not breath these gases. The anodizing process will also create heat. It's important to try and keep the temperature at 68-77 degree farenheit, so use ice on the outside of the bucket as necessary to keep the temperature as close to 68 as possible. If it's winter and you are doing this outside, you might need to use a fish tank heater to keep it close to 68.
Part bubbling while anodizing
How long do I leave it in the anodizing bath?
The anodizing layer will build up at a rate based on the sulphuric acid solution you used. If you are using a 10% solution, it will take 3.6 minutes for every micron (.00004 in) of anodzing layer to form. For a 15% solution, it will take 2.4 minutes per micron. These times are based on a 15 amp per square foot of surface area on the part. The thicker your anodizing layer, the harder it will be to scratch. There is a maximum layer thickness that keeps you from making your aluminum part into a ceramic however. The anodizing process no longer adds any thickness after about 25 microns, or .001". The voltage and amperage requirements to make it thicker skyrocket at this point.
Maximum thickness will be achieved by anodizing for 60 minutes at a 15% solution, or for 90 minutes at a 10% solution.
Last Step: Rinse, Dye, Seal!
Once your time is up, turn off your power supply so that you don't accidentally touch the part to the cathode while removing it. Take your part out of the bucket and swirl it around inside of a container of cold de-ionized water. This will serve to rinse off any remaining acid without sealing up any of the pores before dying.
Dye (left) and sealant (right) heated up and waiting for the part
Now you need to transfer your part into the dye bath. The dye bath should be held at 130-150 degrees Farenheit. The only dye that I have used is RIT clothing dye from the grocery store and commercial dye available from Caswell. Both have given me great results, although the clothing dye isn't supposed to hold up as well under long term ultraviolet exposure. Check the part after a couple seconds to see how quickly it is taking color. Some dyes take completely in 10-15 seconds, others can take up to 5 minutes before you'll achieve the deepest color.
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Checking the part's dye absorption
Once you've reached your desired color, if you are using a sealant, transfer the part to your sealant and leave in the sealant bath for 5 minutes. The sealant should be a boiling nickel acetate or cobalt acetate solution. These sealants add a lot of colorfastness and weather resistance. Then transfer the part to boiling water or a steamer for final sealing.
Part being transfered to the sealant
If you don't want to use a sealant, the part can be transfered right from the dye to the steamer or boiling water, but a lot of color leaching will occur, and it will take much longer to seal. Leave the part in the boiling water or steamer for 30 minutes if sealant is used, and hour if it wasnt used. When you take the part out, if it feels sticky to the touch, it isn't fully sealed.
Part in the steamer
As a finishing touch, you can add a little extra shine by rubbing some WD-40 on the part.
The finished part (with poor surface prep)
Disposing of Acid:
It's obviously not a good idea to pour acid down the drain or pour it out in your neighbor's garden. A trick I've heard of is to put baking soda in your solution to neutralize the acid. A PH indicator, such as the litmus ones available from fish stores is suitable for testing the PH level before disposal.
Additional Reading:
If you would like to learn more about the technical side of anodizing, I suggest reading The Canning Handbook on Electroplating, 22nd edition. They go into great detail about the different anodizing processes, detailed techniques, masking methods, electro and chemical polishing of aluminum, and much much more.