What is Anodizing?
Anodizing a process that creates a very hard, protective, and sometimes decorative finish to aluminum.
Aluminum is a wonderful material and is often used for the valve seat and intake section of pulsejet engines. Its excellent thermal conductivity and ability to cushion the impact of reed valves as they slam shut make it a good choice for these components – but its lack of hardness also means that it’s easily damaged.
In this picture you can see the effect of hardened reed valves repeatedly slamming into a plain aluminum valve plate. You can see how the repeated impacts have damaged the surface of the valve seat and this can affect the ability of the valves to seal properly.
By anodizing the valve plate, a thin protective layer of aluminum oxide can be created which will significantly reduce the amount of damage that occurs.
This protective layer is extremely hard – almost as hard as diamond in fact – but it is so thin that it doesn’t adversely affect the valve seat’s ability to absorb the heavy impacts of the valves – thus you get the best of both worlds.
Choosing an Aluminum Alloy
Note that only some aluminum alloys can be anodized successfully. Some grades, such as the copper-alloys of the 2000 series (2024, etc) which are often considered “high-strength” are not suitable.
The easiest aluminum to anodize are the “pure” alloys of the 1000 series (1050, 1100, etc) – but these are very weak and not suitable for such things as valve-plates where there are significant physical stresses involved.
On the PJ8 and PJ15 engines, I use 6061 grade aluminum tempered to a T-5 level. This alloy is a considered a medium to strong structural alloy and anodizes very well indeed.
How To Anodize Aluminum
The anodizing process is deceptively simple and uses readily available chemicals. Providing you exercise reasonable care, it’s also a very safe process with no adverse environmental impact.
Materials:
To anodize aluminum you’ll need the following materials:
1. Fresh sulfuric acid. This can be obtained from your local automotive battery supplier and is very cheap. Don’t be tempted to use acid decanted from an old battery – it will be
contaminated and produce very poor results.
2. Distilled water. If you don’t have a distiller, this can also be obtained from your local battery supplier. You’ll need to dilute the acid with 2 parts of water for each part of acid.
3. A plastic tub to hold the acid solution and the items to be anodized. I used a container designed for kitchen use – this had the added advantage that it came with a secure clip-on lid so I can store the acid solution in this container between anodizing sessions.
4. A lead or pure aluminum plate to act as the cathode. The purity of this plate is fairly important – if it’s alloyed with some other metal then there could be leaching into the acid solution with resultant contamination. Use 1050 or 1100 grade aluminum or lead from a known source (although some people have reported good results using lead flashing torn from old roofing iron).
5. Some aluminum wire or thin rod. A good source of this is aluminum welding wire as used in MIG welders. Alternatively you can cut a thin strip of aluminum from a sheet of the metal and use that. This will be used to hang the item in the solution and provide an electrical connection. Remember that this piece of metal will also be anodized and if there isn’t a firm, watertight connection to the workpiece, the oxide layer created may break the circuit and stop the current from reaching it.
Remember that at the point where the wire connects to the workpiece, anodizing will not occur. Make sure that the connection point is not in a position where this lack of
anodizing will cause an unsightly blemish if looks are important.
You’ll also need a source of 12-24V DC electrical power. Since the anodizing process can draw several amps of current, you’ll need something more powerful than a simple wall-wart type of supply. I use a car battery (or two) that I recharge between sessions.
To connect everything up you’ll want to use several clip-leads (lengths of wire with a crocodile clip on each end) and be sure there’s a fuse in the circuit close to the power supply or battery terminals. If the item being anodized accidentally touches the lead/aluminum plate then a short-circuit will occur and, without a fuse, this could cause a fire or even an explosion.
Setting up Your Anodizing Tank
Because anodizing uses an acid solution and creates large amounts of potentially explosive hydrogen gas you should set up your equipment in a safe, well-ventilated location where it’s not going to be knocked or in the vicinity of any type of naked flame.
First, cut and bend your cathode plate (aluminum or lead sheet) so that it runs around the inside edge of your plastic container.
To get an even and “all-over” layer of anodizing, you need to have your item surrounded by the cathode plate.
Place the item to be anodized into the container and make sure that plenty of space (at least a couple of inches) remains between it and the cathode plate. This is to make sure that you don’t accidentally touch the two together and produce a dangerous short-circuit.
The amount of acid solution you need to mix up depends pretty much on the size of the object you’re planning to anodize and the volume of your plastic container.
To avoid having a “bald-patch” on the bottom surface, you need to suspend the item to be
anodized in the acid solution so that it’s not touching the bottom of the container so allow extra liquid for this.
Don’t’ forget however, that when you lower your item into the solution, the level will rise. If you are planning to anodize a larger item, the level may rise significantly so factor this in so as to ensure you don’t overflow the container.
Workpiece Preparation
It can’t be emphasized just how important it is that the workpiece is scrupulously clean prior to anodizing.
Any grease, oil or fingerprints will be magnified by the process and produce unsightly blemishes in the final finish.
To this end, the following steps are useful in ensuring the cleanliness of the workpiece:
1. wash the workpiece in a weak solution of detergent in hot water
2. rinse thoroughly with hot water as any detergent residue will also cause problems.
3. Dip the workpiece into a warm (120 deg F) bath of 5% sodium hydroxide (lye) solution for about one minute. This will remove any remaining oil or grease and provide a very
consistent surface.
4. Rinse thoroughly with hot water as in step 2 (above)
I find that wearing a pair of cheap latex or nitride gloves when performing the above steps allows the workpiece to be held without leaving further fingerprints on the cleaned surface.
It’s also advisable in light of the fact that the sodium hydroxide solution is extremely corrosive to your skin.
Performing The Anodizing
Having set up your anodizing tank and prepared your workpiece, the item to be anodized is lowered into the acid so that it is completely immersed.
It is then connected to the positive side of your battery or power supply.
The aluminum cathode plate is connected to the negative side.
Within a minute or so you should see a steady stream of bubbles rising from the workpiece – if not, you have a problem.
The most common reason that the workpiece fails to bubble is because the electrical connection to it is inadequate.
Once the anodizing process is underway you can go and read a book for 20-60 minutes, depending on the size of your item and the depth of anodizing required.
You can briefly remove the workpiece from the tank from time to time to see how the process is going. Depending on the alloy involved, it will turn a dull gray or very light yellow color as the oxide layer forms on the surface.
Using the anodizing setup described above, most workpieces will have a health layer of oxide on them within 60 minutes of processing – at which stage they can be removed and rinsed thoroughly in COLD water.
It is important that you continue to avoid direct contact with the surface or contamination with any form of dirt, oil or grease.
Dying the Workpiece
One of the great things about anodizing is that it not only creates a tough, hard layer that protects the metal from wear but it also allows you to impart a color to it.
The reason you can dye a piece of anodized aluminum is because the oxide layer formed during the process is quite porous. The millions of tiny oxide crystals act like a sponge and soak up any dye that has a fine particle structure.
Of course the dying step is optional and if you’d like to simply retain the gray color of plain anodized aluminum you can move right on to the “Fixing” stage.
The choice of dye is critical to successfully imparting a nice rich color to the anodized layer.
I’ve had great results from the Dylon range of cold water dyes. These are the little plastic pots with an aluminum lid that can be bought from most fabric stores. They’re dirt cheap and one little container produces enough dye to last a very long time.
So far, I’ve tried the gold, red and blue colors with good results but others have reported that the greens also work well.
To dye your workpiece you should remove it from the anodizing tank, rinse well in COLD water (it is important that the water is not warm or hot) and then imerse it in the dye solution.
I mix one little pot of Dylon to almost a pint of water (500ml) and this concentration works fine.
Leave it in the dye for about 10 minutes or so to allow the color to get deep into the crystaline structure of the oxide layer.
Fixing the Anodizing
This is a critical step that physically and chemically changes the structure of the oxide layer produced by the anodizing process. If you have dyed your workpiece, it effectively locks the dye in place so it won’t leach out.
To perform the fixing you need to hold the anodized (and optionally dyed) workpiece over a source of steam for at least 10 minutes.
I find that a pot of vigorously boiling water on the stove works fine.
Rotate the work so that the steam comes into contact with all the anodized surfaces.
It’s normal for some of the dye to leach out at this stage and color turn the boiling water – but most of it will stay in place.
After 10 minutes of steaming, the workpiece can be immersed in the boiling water to finish the job. Leave it boiling for another 10 minutes.
Once you’ve removed the item from the boiling water the anodizing is complete.
Adding Luster
You will notice that your newly anodized (and optionally dyed) piece of aluminum probably looks quite dull when it dries. This is normal.
The anodizing itself produces a rough surface that imparts a matte finish rather than a shiny one.
To make the surface shine and add luster all you have to do is either rub in some oil or polish of some kind.
With some polishes you may get a small amount of the dye coming off on the polishing cloth but it’s nothing to worry about.
The result should be a nice shiny workpiece with a deep luster.
Hard Anodizing
The anodizing process described here is often referred to as “decorative anodizing” because the thickness of the layer produced is quite low (about 0.0001 to 0.0005 inches) and therefore only provides a limited amount of protection to the underlying metal.
Another process known as “hard anodizing” can produce layers of up to 0.005 inches which are extremely hard and protective.
Performing hard anodizing is a little more complex than the above process however and involves the use of a chilled acid solution (not necessarily sulfuric) and much higher voltages/currents.
While it is possible to perform hard anodizing in a home workshop, there are safety issues involved since the amount of explosive gas produced and the risk of electrical shock are far higher.
For most purposes, the simple anodizing process described here will provide more than enough protection.