Color Anodizing

The exact steps any home anodizer is going to take are dictated by time, available resources, attention to detail and various other factors. Here is my quick guide to home aluminium anodizing - but don't blame me if it doesn't work.

• Mix up 10 to 20% Sulphuric Acid solution with pure distilled water. Enough to fill whatever container you are going to use about 2/3 full. Leave to cool. This mixture can be used many hundreds of times for anodizing runs. It will eventually pick up impurities any become less effective. Remember, never add water to acid, always add acid to water so it doesn't fizz and bite back! Do not let any extra water, caustic soda, sodium bicarbonate or similiar near the acid bath.
• Prepare your aluminium piece. Finish is everything - anodizing does not hide a poor finish. Clean it up with 1200 paper and maybe polish.
• Cover your working area in something disposable. Putting the anodizing bath on a big sheet of glass is a good idea - keeps any splashed acid off the worktop. Make sure the bucket of sodium barcarbonate solution is handy for dipping things in. I suggest getting a big (ie several kilos) carton of bicarbonate from a catering suppler or cash and carry. If you do spill a serious amount of acid, its nice to have some alkali handy to neutralise it.
• Fizz the aluminium in caustic soda solution until it looks a nice grey colour. If the aluminium is already anodized, it is possible to remove the anodized layer by leaving it in the caustic soda bath for longer. I've not read of the correct strength of the caustic soda bath for preparing the metal. An eggcup or two of caustic soda granules in a pint of warm water works for me.
• Suspend the aluminium part in the acid so it is completely immersed using some kind of aluminium wire or aluminium strut. The only metals allowed in the bath are aluminium and lead. Make sure you get a good electrical connection. Bear in mind that any parts where the suspending wire touches the part it will not be anodized, and will not take up the dye. Twist a bit of wire into a tapped hole or something. Make sure that you don't touch the part. Grease from finger prints can leave a mark on the finished item. Get some good gloves.
• Place a Lead cathode in the bath. This should have a surface area of at least twice that of the aluminium part. Don't let it touch the aluminium part at the anode.
• Attach the positive connection of your power supply to the aluminium anode and the negative connection to the lead cathode.
• Run the power at 12 volts for about 45 minutes. The cathode will fizz a lot, the anode will also show some small bubbles. The acid will heat up. If you are not sure its working, use an ammeter to see whats going on. You should not allow the acid to become warm - ideally it wants to stay at 20C. Let the acid cool between anodizing runs, or rig up a cooler. Remember only lead or aluminium in the tank. Even a fan blowing on the tank helps. If you think about it, 12v at, say 2 amps, acts like a 24 watt header, and thats before the heat created by the reaction.
• There is a lot of words written about what current to anodize with. Apparently you are supposed to anodize at between 4 and 12 amps per square foot of anode surface area. With most parts its almost impossible to estimate the surface area. After etching in the caustic soda, you'll throw your calculations out even further. For my purposes I just run the whole thing at 12 volts and let it draw as much current.
• Remove aluminium part from the acid and wash in distilled water. Try not to drip acid from the part over the kitchen whilst moving to the water. If you must walk around the house with bits of aluminium covered in acid, hold a bowl of bicarbonate underneath.
• Dip the part in the chosen dye for between 1 and 15 minutes depending on how much colour you want. Heating the dye will increase the speed of colour uptake, however no hotter than 50C or you will start to seal the layer. Experiment is the key! With the Dylon dyes I normally mix them up with about a litre of warm water and use that. The dye mix can be used over and over again. Keep the dye mix out of sunlight.
• Boil the part in distilled water for 30 minutes to seal the surface. Some of the dye will leak out into the water before the surface is sealed, but its not too much of a problem. You might want to hold the part in hot steam for a while before you put it in the water. Start the water at about 95C and bring it to a simmering boil over the course of a few minutes. You can buy anodizing sealers to add to the water, but I've not needed this. I have an unconfirmed suspicion that commerical anodizing dyes need a special sealer.
• Give it a good rub with a very soft white cloth. Sometimes a get a bit of colour coming off the sealed part, but this stops after a few moments rubbing. I find a good long boil reduces this problem.

This pages contains some small comments to help you on your path to home anodising - just some small remarks that I've noticed during my anodizing aluminium experiments.

• The caustic soda phase is very critical. A minute or so in the bath is required. Leaving it a bit longer gives a very matt finish to the final aluminium part: Which is a Good Thing (TM) if you are anodising an aluminium optical part.
• The thing to remember with the caustic soda bath is that it is dissolving the aluminium. Leave it too long and it the part gets smaller. This is generally noticed when two closely fitting parts no longer closely fit. Also, after 10 minutes in the bath, tapped holes will no longer hold their correct sized screws.
• You can remove anodising with a few extra minutes in the caustic soda bath. I find that a mug of caustic soda granules in 2 litres of hot water will strip off the anodising in about 5 minutes. The caustic soda solution ends up looking very yucky. Also see the point above.
• After the caustic soda bath phase, it is very important to clean the part. Because aluminium is normally in an alloy with lots of other metals, these metals will smut the surface. You can buy de-smutting solution if you like. However, I find a good wash works wonders. After the caustic soda bath, fill the sink with hot water with lots of washing up liquid. With rubber gloves on, scrub the part with a toothbrush until it is an nice even finish. Don't use a "green scrubby thing" or a Brilo pad - this will scratch the surface (which you might want, so hey!). This scrubbing is VERY important to the final finish.
• Getting a good electrical connection is vital. It is worth spending some time making something out of aluminium that has a threaded section to screw into tapped holes in your aluminium part. If you are anodising several things at once, you'll find the one with the poorest connection doesn't anodise well
• Don't use steel screws to hold the part in the acid - steel in the acid will dissolve!!!
• Don't use those pretty coloured aluminium bolts sold for bicycles either. These are anodised... I supposed you could caustic soda them in the bath first and then use them... I haven't tried.
• Anodised aluminium does not conduct electricity well. If you have used a bit of scrap aluminium to hold a part in the acid bath, clean off the anodising with a file before you use it again.
• I find about 1 hour in the acid bath gives a good anodized layer.
• Be very very careful not to touch a freshly anodised part with your fingers. The grease will ruin the final part. You end up with a nice anodised fingerprint on the part.
• If you use steel bulldog/alligator clips to attach the power to the acid bath, be sure to dip them in the bicarb solution before you put them away, other wise they will slowly dissolve.
• Boil mercilessly. Boiling for 45 minutes is not going to harm the part. This ensures a good seal on the anodising.
• It doesn't matter if the part being anodised touchs the plastic sides of the acid bath.
• It does matter if it touchs the lead cathode.
• Make the cathode as big as you reasonably can. Fold the lead around on itself several times to get a good surface area.
• After a few sessions the acid bath will have a lot of muck at the bottom. This doesn't effect the anodising qualities.
• If you snap a hardened steel tap off in your job, anodise it anyway. When the anodising is finished, you will find the broken end of the tap that was embedded in the aluminium part has dissolved. Don't do this too often otherwise you'll end up with very mucky acid.
• With larger parts, the acid bath will fizz lots. It is best to do the anodising outside.
• You are going to use a lot of distilled water. Battery topup water can be purchased in Halfords in gallon containers, but it is about 3 quid. This gets expensive. I've yet to figure out a way of getting hold of lots cheaply. I suppose I could make my own water distiller.
• Don't keep the caustic soda in the same box/drawer/cupboard as the acid bath.
• Do put a warning label on the box to say it contains nasty stuff.
• Don't leave lead cathodes where animals are going to lick them.
• Shutting your pet cat in the living room during anodising sessions is a)A really good idea b)Really pisses the cat off. This also applies to small children.

DIY Home aluminum Anodizing for a Hobby

DIY Information on home anodising (or anodizing!) aluminium (or aluminum!) using simple methods and inexpensive dyes.

Disclaimer - None of the following is garrenteed to be strictly accurate. However it is garranteed to be extremely harmful to your eyes, fingers and other extremities if you don't take care with chemicals. Take care and don't blame me if it all goes badly wrong.

Spelling - anodise or anodize - I am British. Many people are American. I spell in British English. Aluminium Anodising. Other countries spell this Aluminum Anodizing. I am going to stick to Aluminium Anodising. I do wonder if Google knows the difference. I also say Sulphuric, not Sulfuric. However, I might spell it another way just for variety!

Anodizing Aluminum - Introduction.

Aluminium alloys are a good choice of metal for home machining. I normally use aluminium on my mini-lathe to make telescope parts, camera adapters and other useful bits and pieces. There is an unfortunate drawback to using aluminium in this way. The main advantage and disadvantage of aluminium is its relative softness to other metals such as steel. This soft nature makes it much easier to machine on a mini-lathe than steel, but, once you have completed your part, it is very susceptible to small dents and surface scratches.

Chemically speaking, aluminium is an extremely reactive metal. We are familiar with the action of the environment on iron. It oxidises quite readily to produce iron oxide - or rust as it's more commonly known. Aluminium is more reactive than iron, and, as such, will rust more readily. However, with aluminium a rough layer of aluminium oxide forms which strongly inhibits further oxidation. This is why aluminium is known for its resilience in corrosive environments - eg car radiators or boats. After a period of time the aluminium "rusts" - slowly producing a white powdery coat.

Thankfully for the aluminum industry there is a technique which overcomes the inherent softness of the metal and protects against further oxidation. Using chemical processes it is possible to build a carefully controlled layer of aluminium hydroxide on the surface of the metal which is extremely hard - much harder and more durable than the rough natural oxidised layer that normally forms. The process involves suspending the aluminium in and acid bath and passing an electric current through the it. This is known as anodizing - so called because the aluminium part forms the anode in the electrolysis bath.

The other advantage of anodizing aluminium is the potential of coloured anodizing dye. Coloured dye seeps into the microscopic pores of the anodized layer and colours it. The layer is then sealed and the colour appears as part of the aluminium metal itself. This can be used to great artistic effect, producing aluminium parts with strikingly tasteless colours to adorn your motor bike or, more topically, telescope. With optical applications, black is the most popular anodizing colour to reduce internal reflections. Typically of course, black is supposed to be one of the most difficult colours to achieve and commercial black anodizing dye fetches the highest price.

Anodizing aluminium (or indeed anodizing aluminum) is a fairly simple process, and providing you can lay your hands on the correct chemicals (ie sulfuric acid) it is fairly straightforward to do simple diy anodizing aluminium in the home. I would limit yourself to anodizing aluminium in fairly small amounts - if you want to anodize a large object you need a lot of electrical power and a lot of acid, so if you want to do an entire bike frame in one go, I suggest you contact an aluminium anodizing company.

Some three years ago I took to wondering if it would not be practical for the home workshop enthusiast to renew those faded or damaged anodised parts which contribute so much to the good looks of a completed project and or new parts.
Since practical advice seems to be rather difficult to come by, I read a few books, followed some experimentation, achieved much discovery of an artful process, and Success !!!

The process itself, though chemically complex, is rather simple.
First it involves the transformation of the surface aluminium oxide to
aluminium hydroxide (anodise), then to a hydroxide monohydrate.
An interesting property of hydroxide is its ability to absorb dyes into the microscopic porosity's of its surface. After impregnating, or dyed with a colour medium, the surface is then "sealed" into a monohydrate and the surface becomes very hard and resistant to wear.

Method And Materials

The process requires the use of either chromic or sulphuric acid in the
anodising electrolysis bath. I have limited my use to the sulphuric process
because of the ready availability of battery acid and the ability of the
process to absorb a wide variety of dye .The acid used is "1270 SElead
acid(, battery electrolyte obtainable wherever you buy new car batteries.
Cut this 50/50 with distilled water to obtain the anodising solution.

NOTE Always wear eye protection and rubber gloves when working with acid.

CAUTION Always add the acid to the water never the reverse.

Distilled water is recommended because the use of ordinary tap water invariably contains some minerals which will cause smutty deposits on the work and generally not contribute to consistent results. The acid anodising solution needs to be stored and used in a suitably sized plastic, or glass, open ended container.
A lead strip cathode plate(s) is required. The lead plate(s) should be about twice the surface area of the largest workpiece (anode) to be anodised.
I use a variable DC power supply (2 to 30 V surplus unit) which I find ideal but any charged car battery will do the job. I find that most small parts require around 10 V to maintain the required current density through the bath; so 12 V should suffice for most work.
An ammeter reading from 0 to 3 AMPS (for 20 square inch, maximum) Is a must, as well as a heavy duty rheostat in series with the supply and the anodising tank.
The resistance of the rheostat will, of course, vary with the size of the work
contemplated, but it can be calculated from the required
current density of 145 to 175 milliamperes (ma) per square inch of the anode
workpiece. The workpiece surface area must be calculated In order to set the anodising current. And, a surplus wirewound rheostat (variable resistor) of a few hundred ohms will do.

Let's assume that we want to anodise a propeller spinner, or flywheel, the surface of which, is calculated to be 2.5 sq. inches . The anodising current density required will be:

Minimum: 0.145 X 2.5 = 0.362 Amps
Maximum: 0.175 X 2.5 = 0.437 Amps

So, the current must be between 362 and 437 ma. In anodising this part I would
maintain my adjustment at 4oo ma.
The part to be anodised must be chemically clean. No effort must be spared buffing and cleaning prior to complete degreasing in hot water using a strong detergent. At this stage handle the part with rubber gloves or not at all.
Use only the aluminium contact strip fixed to the part. Rinse well and you're ready to anodise.

Process Notes

A good anodised coating thickness will be built up after 40 minutes at the
calculated current density. Keep a running check on the current reading as this will tend to vary during the process.
Too low a current setting will result in a surface that will have difficulty
absorbing the colour dye.
Too high a current setting will result in overheating the tank solution and a porous finish which will leach out the dye during fixing.
A good anodised surface will have a slightly milky appearance when ready for fixing; or colour dyed and fixed.
Copper , brass. or iron will contaminate the tank and degrade the process.
Use only lead or aluminium contact strips. I use wooden clothes pegs to set my workpiece height in the tank.
Few, if any, parts you make will be made from pure aluminium. Most will be made from aluminium alloys which contain varying proportions of copper, manganese, silicon, and sometimes, other elements in the mix. These alloys have an effect on the ultimate colour shade obtainable with a given dye and process. If colour shade repeatability is required, the same alloy, process times, and temperatures must be carefully duplicated. Previously anodised parts must first be strip-cleaned in a strong Akali to remove all traces of prior anodising oxides.

COLOUR DYES

Organic dyes are usually used because of their great variety and depth of colours. Industrial dyes can be obtained, but only in too large a quantity for our purposes. Coloured artist Inks are generally suitable, as are food colour
Dyes with varying results. A yellow dye gives the "gold" anodised look because of the translucence of the anodic coating and the metallic reflection.
Some writing Inks are also suitable such as Skip or Carter, which gives a great "black".
The trick is to find a colour with a pigment size small enough to enter the microscopic anodic oxide coating and be sealed there. Experimentation and patience are both recommended.

COLOUR PROCESS

Dyes may be used hot, usually 150 F, or at room temperature. The dye and the effect required will determine the choice. I usually use mine at room temperature and an immersion time of between one minute to 15 minutes, depending on the depth of shade required. Agitation is required. A "coarse" dye will just accumulate on the surface and will wash off during fixing.

FIXING PROCESS

Fixing is done in plain old H2O (near boiling) for about 20 minutes.
Preferably, use distilled water to avoid those nasty mineral deposits
on our nice parts - temperature: 200 F.
A certain amount of dye will leach out into the water before the surface seals. It is best to avoid actively boiling water since this agitation will accelerate the colour loss.
Chemical additives for the fixing bath are available, but I haven't found any to recommend.
To keep colour loss at a minimum I have found that rotating the part in
steam for ten minutes before total immersion does a considerable job in reducing leaching probably by closing the pores and sealing the dye before washing it out.
The finished part is buffed with a clean cloth to remove any smutty deposits.
A little wax brings out the colour.

Reproduced from other Net Resources

In general, anodizing is done for Aluminum (Al). Al has a very high affinity to bond with oxygen (O) forming oxide film, Alumina (Al2O3) which covers the metal surface in the open air. In some application, a particular treatment should be implemented to increase the corrosion resistance of the material, besides developing the surface appearance such as the color or texture of the material surface.

Alumina is very hard (800 VHN) and also resistive in a corrosive environment. The anodizing process produces Al2O3 film on the surface of the Al material when DC (direct current) with particular voltage is connected in an acid electrolyte. In this case, Al becomes the anode, and the other metal (ex: Pb) becomes the cathode.

To do a complete anodizing process, some equipment will be necessary:
1. Coating bath
A place for anodizing itself occurs. This will be a place to accommodate the electrolyte. The electrolyte is corrosive that should be a consideration for choosing the material of coating bath.
2. Rectifier
Function as power source. Requirement for a good rectifier is the one which able to keep a constant current density as long as the anodizing process occurs.
3. Cathode
Cathode needs conductive material to flow the electron to the work piece to ensure the chemical reaction to happen. The material must not solve in the electrolyte.
4. Hanger
Hanger is used to hang the work piece and metal.
5. Connection cable
Connection cable connects the cathode and anode to the rectifier with a particular value current and voltage.

There are several kinds of electrolyte can be used in anodizing process:
1. Sulphate Acid
Mostly used for the product coloring purpose
2. Chromate Acid
The using of this electrolyte will increase corrosion resistance. Mostly used as the base layer for painting application. Aircraft and ship industries use this kind of process
3. Oxalic Acid
Specifically is done to increase the wear resistance because the thickness of the oxide film produced can reach a thickness until 60μm. This process is also done for the decorative purpose
4. Phosphate Acid
This anodizing is usually done for the preparation of the surface in adhesive bonding process
Different electrolyte will produce the different structure of the Alumina film too; in Borate Acid and Nitrate Acid, there will be produced a very thin and non-porous Alumina film, but in Sulphate Acid and Chromate Acid, there will be barrier and porous layers. Lacombe and Beaujard had studied the morphology of the porous layer, which appears like fiber, consists of hexagonal prisms. Each prism has a hole / porous in its central point.

Several compounds of Alumina:

Type of Alumina Crystal Structure
α Alumina / corundum (most common) Rhombohedra
β Alumina Hexagonal
γ Alumina Cubic
δ Alumina Rhombohedra

Anodizing is a process to produce oxide film (thin oxide layer) from metal and its alloy by means of electrolysis reaction in a proper electrolyte (solvent).
Mechanism of electrolysis reaction:
1. Electrolyte will dissociate to positive ion (cation) and negative ion (anion) in water
2. Electrical current will affect the ion to migrate to the proper electrode direction, where cation (+) will migrate to anode (-), and anion (-) will migrate to cathode (+)
3. In each electrode, particular chemical reaction will occur, where there will be chemical bonding between electrolyte ion with ion from the reduction and oxidation of the metal electrodes
In anodizing process, the work piece is placed as the anode (positive electrode) while in electroplating, the work piece is placed as the cathode (negative electrode).

There are several materials which can be anodized such as Al, Nb (Niobium), Ta (Tantalum), Ti (Titanium), W (Tungsten), Zr (Zirconium), Mg (Manganese), Cu (Cooper), Ge (Germanium), In (Indium). Note that the key word of the anodizing process is the oxide film, which have a great role of the quality increasing of the surface of material.