First, this is only meant to apply to aluminum. Other metals, such as titanium, niobium, and possibly magnesium and others, can also be anodized.
Aluminum oxidizes very quickly, and rapidly forms an aluminum oxide coating that inhibits further oxidization. This coating is useless as is, in terms of protecting the metal, because it is so thin. A thicker coating can be produced by immersing the part in an electrolytic solution and passing an electrical current through it, similar to electroplating. The resulting film is nearly colorless, and can be easily dyed because it is very porous at the molecular level. Then, by placing the part in boiling water, the film's pores can be sealed; the oxide changes from one form to another as a result.
To be more specific, parts should be very clean and grease-free. Commercial plants will first clean and etch the surface in a caustic solution, such as lye (sodium hydroxide), followed by a thorough wash.
The parts are placed in an acid solution, such as 15-25% sulphuric acid, and connected to the positive source of a power supply (use only aluminum hardware to make the connection; no copper in the solution!). The part(s) comprise the anode, and the cathode is lead (lead sheet, or the whole tank might be lead). Appropriate current is applied, e.g. 1.5 amps per decimeter (3.4 square inches). This lasts 15-25 minutes if no dying is planned, or 45-60 minutes for dying.
Dying is the next step, if desired. Since the pores are extremely small, many common dyes will not work. Some wool dyes are known to work, or you can purchase commercial anodizing dyes from an industrial supplier. Typically this involves immersion in the dye solution, which may have to be heated to be effective. (Steve Rayner reports that telescope builders like a black anodizing dye called Nigrosin Biological Stain, which is water soluble and mixed 1 teaspoon full to a quart of water; his bottle is labeled Aldrich 19,828-5 and cost around $35.00 Canadian, for 25 grams).
Sealing is then done by putting the part in boiling water, which changes the film from gamma aluminum oxide a hydrated form called boehmite. Boil for about 20 minutes.
Another anodizing process involves using chromic acid. This is not suitable for alloys with more than 5% copper. The film is thinner, but very durable. The very thin film is also a benefit when very close tolerances must be maintained. However, because chromic acid is a very nasty chemical, its use on an amateur basis is discouraged, and its commercial use may be closely regulated.
Anodizing is sensitive to the type of alloy. For example, alloys for die casting have a lot of silicon, which makes it pour and mold well, but it makes anodizing almost impossible.
By all means, find a few references and read them carefully before attempting anodization. If at all possible, try it out on scrap pieces before attempting it on your masterpiece. Be sure to flush the part completely to remove remaining acid.
An alternative is to use a commercial firm to do the anodizing. Some net folks report they can be a bit rough to deal with, as they may be more used to dealing with large jobs and may not appreciate a home machinist with a single 5-inch part. Look in the phone book under anodizing, plating, or electroplating.
Some have reported using a lye solution to treat the surface, which yields a fairly uniform fuzzy surface, that may be acceptable as a final surface without further treatment. Don't use a strong solution or the aluminum will disappear too fast; do this in a well ventilated area, as hydrogen gas is emitted, stay away from flames/sparks, and remember that lye will eat people as well as aluminum!
The resulting surface can be dyed, or painted with a chromate primer. (One tablespoon of lye per pint of water has been suggested).
Another alternative is a product called Aluminum Black from Birchwood Casey, which is often advertised in model railroading magazines, and may also bevia gun stores (it is in the Brownells catalog). Brownells also lists a nickel plating solution for aluminum, though it appears non-trivial to use.
See also the ArtMetal WWW pages athttp://wuarchive.wustl.edu/edu/arts/metal/ArtMetal.html for information on anodizing for artists (more specifically, http://wuarchive.wustl.edu/edu/arts/metal/TOC/finishes/anodize.html)
Another interesting site ishttp://super.zippo.com/~anobrite which is an anodizing shop with lots of information. They have a few do-it-yourself publications on anodizing, etching, silkscreening, marbelizing, etc. They also sell the chemistry.
1.Aluminum (Volume 3): Fabrication and Finishing Kent Van
Horn, Editor American Society for
Metals, 1967 Library of Congress #66-16222 813 pages
2.Electroplating for the Amateur, by L. Warburton. Model &
Allied Publications. Available via
3.The Surface Treatment and Finishing of Aluminum and its Alloys, Edited by S. Wernick, R.
Pinner, and P.G. Sheasby. Published 1987 by ASM International, Metals Park, Ohio. 2 volumes.
4.ARTISTS ANODIZING ALUMINUM- the sulphuric acid process, by David LaPlantz. ISBN:
0-942002-03-2, apparently published by Press de LaPlantz
Inc. Box 220 Bayside, CA USA
95524; It's about $20 US. There is also a book and tape
for aboout $45. Both should be
available from Rio Grande Supply (see vendor section).
The book is extremely thorough and
contains several methods of anodizing with lots of
pictures and references. It is intended for
small individual pieces, at home in the garage or basement.
5.Metal Finishing - Guidebook and Directory. Metals and
Plastics Publications, Inc. One University
Plaza, Hackensack, NJ 07601
6.Anodizing Aluminum, by Harold Hoffman. Available from H &
H Publihing, 7174 Hoffman Road,
San Angelo, Texas, USA 76905, or Centaur Forge. This
book seems fairly complete, with
7.Anodizing Aluminum in the Amateur Workshop, Ham Radio
Magazine, January 1979, pages
62-69, by David W. Hembling. Sadly, this magazine is out
of business, but should be in larger
libraries, or available via inter-library loan. This
article lists several other references, including
addresses for dye makers.
8.Passivating Aluminum Alloys, in 73 Magazine, September 1965, pages 74-80, by Robert A.
9.Reynolds Aluminum, years ago, published "Finishes For
Aluminum", now available as a reprint
from Lindsay Publications, P.O. Box 12 Bradley, Il
60915-0012 (Don't call, write). It covers
many different finishes including anodizing.
10.Some jewelry-making suppliers deal with anodizing.
11.The magazines Strictly IC and Model Engineer allegedly
have discussed this, but I don't have
12.Argus Workshop Practice Series, number 11, by J Poyner. 13.Sandoz Chemicals Corp (now apparently Clarion Corp) supplies dyes for aluminum anodizing.
Their phone number is (704) 331-7000.
14.Kepro in Fenton, MO sells black aluminum anodizing dye
for their aluminum anodized nameplate
kit. They sell it in small quantities. The phone number is (800) 325-3878 or (314) 343-1630.
15.How to Anodize Aluminum, Popular Science February 1963, pages 144-146.
A long time ago, I came to this newsgroup asking for answers on how to anodize aluminum, and nobody really had advice. Since then, of course, I've seen lots of suggestions. In any case, at that point I promised to post if I had any results, and never posted except to mention my success. Well, while there are probably better methods out there, this is what worked for me, and has continued to work for me. I hope this is helpful. BTW, it has been a while, and Sandoz chemical is now called Clarion corp. Their proprietary anodal ms-1 is basically a weak nickel acetate solution. As to where to get sulphuric acid, I wouldn't know, since I work at a university where such things are readily available.
I hope this helps, and thanks to those who gave me advice along the way.
Oh yeah, and be careful . . . sulphuric acid is dangerous (I have "bad lab practice" reminders in more than one pair of shortsnot that I recommend wearing shorts in the lab . . .) and eats an awful lot of otherwise sturdy materials. Also, beware of the high currents. I, of course, take no responsibility for anything (and that seems to be my problem in general).
Sulfuric acid at 13% by volume, at a temperature of 3 degrees C. Note that the temperature is not overly important, so long as it is close. The current density should be ~15 amps/square foot. The aluminum piece should stay in the bath for at least 2 hours, with four hours being optimum.
As an alternative to hard anodization, leave all conditions the same except for the temperature, which should be 70-75 degrees C. The difference in the two procedures is the thickness and quality of the anodic layer. Hard anodization leave the material more absorptive.
The cathode, or negative lead, should be lead, with the anode to catho proportion being about 1:1.
For dyeing the object, there are two options:
Rit is a standard wool dye, which can be purchased at any grocery store. This method is both inexpensive and offers a wide variety of colors. The drawbacks are that Rit, an organic dye, is not light fast, and will fade in direct sunlight.
With Rit, simply prepare a bath of about 50ml/litre at about 70 degrees celsius. One hour minimum is required, with the preferable duration matching the anodization time.
Black MLW can be purchased from Sandoz Chemical (my contact was Ron Rupple (214) 423-1674, the sales rep for texas. The company is based at (704)
331-7000.) The price is between $15/lb and
$60/lb (don't have an exact quote, but this is the range of
prices for all of their black dyes). Small
test quantities can be obtained.
Black MLW is favorable because it is a) light fast in the visible (being inorganic), and b) a rather flat black. It's drawbacks are expense (a minimum order is 5 lbs, which is an awful lot of dye), and requires special working procedures (the dust is hazardous to the lungs).
Black MLW is, however, simpler, and more convenient, to use.
The temperature should be between
51 and 60 degrees C. The duration being 5-10 minutes depending on quality of blackness desired.
The concentration (keep this in mind before ordering 5+ pounds) is 10 grams/litre.
The sealing process hydrates the porous anodic layer, closing it, protecting it, and sealing in the dye.
For most applications, sealing is desirable.
Supposedly, this process can be done with boiling water, but I've only managed the following.
1) Simply leave the piece in the dye bath, and boil the dye bath for one hour. This will require removal of the excess dye afterwards and is not generally recommended.
2) 2% anodal MS-1 (again from sandoz chemical (though, nickel acetate should work as a substitute). The temperature of the bath should be between 70 and 87 degrees C, with a duration of 5-20 minutes (depending on porosity and thickness of coating . . . longer never hurts).
Note that after each step, the object should be rinsed with deionized water to avoid contamination. I did this all with distilled water, though deionized water should still work.
Important information about the piece. The purity of the aluminum is important. The higher the silicon content, the less likely the process is too work. Also, the product should be newly machined (for a shinier, more professional finish) or sandblasted (for a flatter, though less even, finish). All oil, dirt, glass beads, etc, must be removed prior to the anodization process. This can be done by thorough cleaning with acetone and methonal (soap is dangerous, since phosphates in the solution tend to negate the anodization process). For better cleaning, immerse the part in 1 tsp/200 litres NaOH (be careful). For small parts this is undesirable, since the NaOH actually removes aluminum.
After this bath, a smutty brown finish is obtained. To remove, rinse the part, and soak in a 10% nitric acid solution until a satin finish is obtained.
While aluminum anodization is typically clear (depending on the alloy and the process used (there are various other processes using different acids, the most common being chromic and oxalic), copper, at least via this process, anodizes black.
The desired voltage is 6 volts across the circuit. The bath should be 120 grams/ litre of NaOH. The temperature MUST be between 82 and 99 degrees C or the process will not even begin. The duration of the anodization process is 30 seconds to 3 minutes, depending again on the quality of black desired . . . more time, flatter black. The cathode is Steel in a 1:1 ratio. Again the piece must be dirt/ grease free.
This process will also work for brass, though it is shinier.
Also, different colors may appear under the
black, depending on the brass.
For both of these metals, remember that they oxidize quickly in air (which is not the same as the anodized layer . . . though in aluminum, this layer is also protective). Therefore, they should be anodized as soon as possible after machining and preparing.
There have been several postings about blackening aluminum recently. I wrote an article for Live Steam magazine back about 1977 on the subject and have modified it a little since then. I do use it myself and thought that it would be of interest to the newsgroup. So: BLACKENING ALUMINUM by JEROME KIMBERLIN There are several ways to color aluminum black and among them are black anodizing and paint. You could rub dirt into the aluminum surface, I suppose, but of all the methods, I think chemical coloring is the superior method. It is certainly cheaper, faster, and home use allows the model engineer greater flexibility in the timing of his decoration of models in progress.
Surface preparation of parts to be colored black is all important as any irregularities are not covered by this finish. Paint does build up and fill in scratches and other voids. Castings, however, should look like castings if the prototype used castings, so surface finish is always adjustable to the builders idea.
The point here is to emphasize that this blackening technique will not cover up mistakes.
You will need three chemicals. These are: Nitric Acid, Copper Nitrate, and Potassium Permanganate.
You will also need some good quality water - either distilled or deionized. I will give the dimensions of the mixture in both metric and English units so that both types of measures are accommodated:
Take: water 3 quarts 750ml
Add Acid ½ oz 5ml
Add Copper 3 oz 25gm
Add Permanganate 1 oz 10gm
Add Water to make 1 gal 1 liter
Obviously you will have to make up more or less solution to fill the container you will use to color aluminum parts and the parts to be colored should be completely covered by the solution. You should use a glass or plastic container. A metal container will poison the solution prematurely.
At 75 degrees F (24 C) temperature, the blackening process will take about 15 minutes using a fresh solution. If it takes longer it means the solution is deficient in one of the components. Usually, copper nitrate and nitric acid need be added.
Aluminum is a strange metal to most of us. While we cannot
see it, the surface of a newly machined
or cleaned piece of aluminum combines with oxygen in the air
to form a self protecting coating of
aluminum oxide. This happens within minutes. If this surface
continues to grow (get thicker) the
blackening solution described here will not work
satisfactorily. Thus, the piece to be colored should be cleaned just before immersing into the coloring solution. In my experience, glass bead blasting is a superior way to clean the aluminum surface and the choice of bead size determines surface finish.
Once the bead blasting has been accomplished, the beads can be washed off with hot water and the aluminum piece immersed in the blackening solution. I recommend that the time between blasting (cleaning) and immersion in the blackening solution be less than two hours. I once waited five hours and was disappointed in the results. Once the blackening process has been completed, wash off the workpiece with tap water, drain and spray with WD-40 or other water displacing oil.
There are a number of ways to clean aluminum satisfactorily. It is possible to simply sand the surface clean, or scrub it clean with an abrasive. One can also chem clean aluminum by degreasing the workpiece then dipping it into lye (Draino, for instance) for a few minutes or seconds as required, then rinsing. The shape of the workpiece and the model engineer's facilities often dictate what method of surface preparation will be used.
Model engineers wishing to use this solution to blacken aluminum castings or other parts should be aware that the chemical components may be hazardous. While the solution itself is not particularly dangerous it can make your hands purple, so use rubber or plastic gloves. Potassium Permanganate is classified as an oxidizer even though dilute solutions of it are used throughout the world to sterilize vegetables used in salads, etc. Concentrated nitric acid is just plain bad. The technique for using it is to pour out a little in a glass container and then use an eye dropper to transfer the liquid to a measuring container when the volume wanted is small, such as that described here. Nitric acid also turns your hands yellow, hurts, and removes fingerprints. A good way to avoid eye damage is to wear a face shield such as the one you should be wearing when working in front of your grinder.
I have received a few requests to explain the process of anodizing at home. I have had very good success so I'll pass the method I use along for others. I'll leave the source of supply of chemicals and materials up to you because I did a lot of searching and then begged most of them and even I will be in trouble getting more when I run out of them.
The first thing to do is go the local hardware supplier and get a cheap range hood ($35) they have a nice light too - nice to see what your doing. I mounted mine on a bench mounted stand and put a piece of flexible dryer vent hose on it to point it out the basement window. The fumes are pretty nasty - probably sulfuric acid vapor - and even a little of it will cause coughing and lung irritation for some time. There isn't much given off and with the vent there is no problem and no sign of any smells outside the hood area. I also use a respirator if I need to get my head in close for a view of the process or I hold my breath.
I intend(ed) to improve on the equipment used but since it worked so well with the quick prototype setup I have, I left well enough alone for now.
I purchased a Rubbermaid plastic contained about 18" long about 10" wide and about 8" deep. I then went to my scrap metal dealer and got some sheet lead about 1/16 thick and cut it into long strips.
One strip goes over the edge at the long end of the
container, down inside, across the bottom and
back up and over the edge on the other end. This bit over
the edge helps to hold the lead in the
container and it's handy for clipping the power connection
to (large battery clips). There is a second
strip making a (flat) ring around the inside of the
container below the proposed liquid line. The ring and the strip are soldered together at both ends for electrical continuity. I used a big 150 watt iron used for stained glass work for that operation and any regular solder. Don't use regular fasteners.
Everything must be aluminum or lead (so they tell me).
I will add that I have no idea if all of this lead is. A single plate at one end may be all that is necessary. I just thought it a good idea that current can flow from all directions to the part which will be hanging in the bath to avoid "shadows". I have also heard that aluminum is also acceptable as a cathode. Aluminum may be easier to work with, the lead is very heavy and floppy and you have to be careful when you turn the contained over or the lead tends to sag. Anyway that's what I have and I'm sticking with it.
I just happen to know someone in the plating business, so obtaining sulfuric acid was no problem. I also think you can get it from Auto supply houses - they may have it around for putting into car batteries.
NEVER ADD WATER TO ACID
ADD ACID TO WATER
ALWAYS USE THE PROPER SAFETY EQUIPMENT, ETC. ETC.
ALWAYS STORE ACID IN AN APPROVED CONTAINER.
Wear old clothes - no matter how careful you think you've been you will find a few holes in you pants after you do the next laundry.
Make a solution of acid that has a specific gravity of 1100. You can use a regular car battery hydrometer to measure this. It's not too critical. This would be approx. the concentration in a discharged battery. I don't recommend using old car battery acid as it probably has other chemical in it that may mess up the anodizing. Make enough of this to fill the Rubbermaid container to a safe level.
So far I'm just using a regular 12 volt car battery charger as a power supply. Although the one I'm using is one of these automatic jobbies that shuts off when the battery is charged, I don't recommend this type because it tends to be fooled by the anodizing setup and shuts off, or quickly clicks on and off and is generally a pain in the butt. Just use a regular type charger - maybe one that has a 3amp and 10amp range.
For good measure I put a very large capacitor of about 20,000 micro farads across the output - this obtained from a local electronics surplus house. Just make sure the cap is rated for at least 25 volts.
I'm not sure this is necessary and I had good success without it too but I do know that battery chargers are not a filtered supply and I just thought getting rid of the ripple might make things better.
The negative lead of the supply and capacitor goes directly to a clip connected to the lead liner in the container.
In series with the positive lead I put a 25 watt rheostat of about 2 ohms to control the current.
Some people say a piece of resistance wire can be used here. I had the rheostat - so I used it. The system works without it but I found that if the current to the parts in the container gets over more than a couple of amps I was having trouble with the electrical connection going bad and the connecting leads were anodizing and the part wasn't. Since the battery charger isn't variable the resistor was necessary. It does get very hot though so be careful.
An anodized surface is non conductive and if the connection goes bad between the part and the wire used to suspend it then the wire anodizes and insulates itself from the part. This is the single biggest problem I have with my set up.
As I mentioned the parts to be anodized must be suspended in the acid solution so as not to touch the lead at the sides of the container. I just used a simple wood stick across the length of the container with a few screws spaced along its length to wind the wire to and to give a point to clip the positive power lead.
The more polished the piece is BEFORE you anodize it the nicer it looks after. I usually shine the pieces up until I can see my face in them. Anodizing will not cover any sins.
THE WIRE USED TO SUSPEND THE PARTS MUST BE ALUMINUM. I
understand copper or steel will contaminate the solution and the part to be anodized. For this I purchased a roll of regular aluminum wire feed welding machine wire available at any welding supply or home depot. It seems to work, I couldn't find anything else although it may be the source of some of my connection problems.
It is of a very hard alloy of aluminum and would rather break than form to the part. But I use it anyway.
Use a good long length of the wire and wrap it very tightly through any available holes in the part. Be aware that at the point the wire touches you won't get any anodizing - hence no color if you dye it, so don't wrap the wire around the part proper. Double up the wire and twist it tight with pliers. Like I said before this electrical connection is my biggest problem. If anyone has any way to improve this situation let me know.
You should wear rubber gloves from now on as you should not touch the part else you get a greasy spot that won't anodize well.
First you must clean, clean, clean the parts to be anodized. I use laundry soap because it cuts grease and is a bit abrasive. I then put the piece in a plastic container and with a brush, use a bit of caustic cleaner (again obtained from my plating shop friend). I think a very mild lye or caustic soda solution would work here. The parts tend to bubble a bit with white foam, but it DOES NOT etch the surface and I don't leave it on more than a few seconds. This step may not be necessary if you scrub the part well with soap and water. The purpose is to be sure there is no oil on the part. Rinse the parts thoroughly. I then like to leave them submerged in water because they dry in the air and sometimes look a bit white - something that LOOKS like it may affect the anodizing.
The parts are then suspended from the wood stick and screw by the wire in the acid so as to be completely covered but not touching the sides of the container.
Connect the positive of the supply - from the rheostat and capacitor - to the screw and wire holding the part(s).
I'll assume one part is being anodized here because the description is easier.
Do some calculation of total surface area in square FEET of the part - DON'T FORGET ALL SURFACES - inside and out, edges etc.
Turn on the power and measure the current. I usually find it to be around 3 amps or so. If it goes way high change the rheostat setting or select a lower output from the battery charger. Like I said before the electrical connections from the wire to the part are the biggest problem and I find that if the current gets much over 3 amps the connection fails and the wire - not the part- anodizes - actually it corrodes away to nothing and the part falls off. The part should soon begin to have small bubbles all over. If not, and the wire does, you have a bad connection. I usually have the least problem with leaving the current to each part at about 2-3 amps. It takes quite a bit longer to get the job done but with less headaches. Jiggle the part occasionally so you don't have gas bubbles lingering and shielding the parts from the acid solution.
The rule of thumb I have been using is to apply 900 amp/minutes per square foot of area. Sounds like a lot but for a 1.5 inch diameter by 4 inch long piece of bar (I hope I get this right)
3.14 x 1.5 x 4 = 18.8 square inches for the sides plus 3.14 x .75^2 = 1.8 square inches for the ends. equals = 20.6 square inches total area There are 144 square inches in a square foot so...
20.6/144 = .143 square feet of surface
The part is drawing 3 amps so 900/3 = 300 amp minutes per square foot required.
300 x .143 = 42.9 minutes. (sounds about right)
This is only a rough guide. In reality I start with this and then watch the part. If the part was nice and shiny when it started you can watch the part as it anodizes and I have found that the part turns ever so slightly a milky lemony yellow color. Definitely not as bright and shiny as it started. This would be the aluminum oxide layer buildup. If I hadn't told you this you might not notice, its very slight. But I have found that if my time estimates are approaching and the part looks this way it always seems to work out well and takes the dye nicely. Not scientific but seems to work.
After the anodizing bath I immediately put the parts in a bucket of water or they can stay in the acid with the power off - but don't let them dry. I just like to use the same fume hood while I do the dyeing so I have to remove the acid container and empty it out etc. Keep em wet.
Sorry, I won't recommend a source for the dyes but I got
mine as samples from a dye supplier. Again
I found this guy through my plating friend, although I found
a lead on my own by calling chemical
companies and asking for their reps. You can try your local
anodizer but for some reason they guard
their suppliers - maybe because they know the procedure is
simple and they don't want you to take
business away by doing it yourself. Sandoz chemicals makes a
black dye, maybe they can put you on
to your local rep and you can go from there. I don't want
everybody bugging my supplier for samples
you'll mess up my credit for more :-)
An interesting story - before I had the bright idea for aluminum wire (described below) I called a local anodizer to see if I could beg or buy some aluminum wire from him. He took two days to call me back and then quoted me a price of $30 for about ten feet of wire. I said aluminum not gold !! I kept looking.
In any case find a supplier of dyes and ask for samples. The dyes are powder and I got a small amount of black, red, green, blue, gold, bronze, and yellow. Some of them are tremendously expensive and you only need about three tablespoons to make a gallon of dye, so you don't need to buy a pound for several hundred dollars that would last a thousand years of home use. Black is about $50 per pound - red is a couple of hundred (I was told). The dye solution is re-usable and apparently you just add a bit more powder when say your black turns a bit blue. I haven't yet.
Mix the dye according to specs. The solution is supposed to have a particular PH and I got this with some litmus paper strips and found that it just meant adding a couple of tablespoons of household vinegar to the gallon of dye. It MIGHT be necessary to be accurate here but I DOUBT IT.
I use an old two gallon stainless cooking pot and a one element hot plate for the dying step. DO EVEN THINK OF DOING THIS IN THE WIFE'S KITCHEN. The dye is very nasty stuff and is IMPOSSIBLE to get out of anything it touches. Stainless is the name of the game here.
My dye instructions say the dye should be at 130 degrees (I think). The important thing is not to heat it anywhere near boiling or the pores in the oxide layer of the part will seal up and the dye will not take. The part almost immediately takes the black dye I have although I believe about 20 minutes is recommended in the solution with the part being stirred to keep fresh dye moving around.
After dyeing the parts, the parts should be put into clean boiling water and boiled for about another ½ hour. Keep the water boiling. This seals the part. I have heard on this list that there is something that should be in the water to aid the sealing process. Maybe someone could repeat this. I don't use anything and I have good results. My black parts come out looking like they have been professionally black lacquered. In fact much better that I've seen from a lot of commercial places.
I haven't quite determined how much parts should be machined undersized in order to maintain fit. I do know that I had a piece of tubing that was machined to fit inside another and I had a nice sliding fit before anodizing and they wouldn't fit afterward. I had a hell of a time hand sanding he one piece down to fit (600 paper). The surface was VERY HARD, but I did manage it and the part is still relatively shiny. I probably only took off about ¼ thou, I don't know.
That's about it - give it a try. It's a pretty forgiving process.
I'm sure I'll hear about all the stuff I'm doing wrong from some of the commercial anodizers out there. It was more of an experiment for me - one that I'll keep cause it works so well.
Just be careful eh !! (Canadian)
If it comes out poorly, is
there a way to reverse the process? As long as i've not sealed
it, does it come off with some other chemical?
Even after the part has been dyed the whole mess can be removed with a caustic (soda) solution (or lye I suppose), with a corresponding dulling and etching of the finish.
[he also wrote, responding to a similar question]:
Anodizing is VERY EASILY stripped off using a mild Caustic solution - aka household lye. Start weak and watch for the dye colour to disappear. Watch carefully, the caustic also eats aluminum quite nicely. Flush well with water and wash with dish soap. The part may be a milky or stained colour after but the re-anodizing process will (in my experience) return it to an even finish again. You could polish the part first as well with 0000 steel wool and/or 600 wet/dry paper.
[and one more question/answer]
When you are anodizing Al you have to attach the peice to the
power supply. Does this leave a blemish on the article.
You must use aluminum wire or some sort of aluminum clip.
Never use steel, copper or anything
else. It will disintigrate quickly.
Assuming you use aluminum wire, the blemish is very small and usually un-noticeable. Have a look at some commercially anodized piece you may have around. The marks are there somewhere and may only be the size of a pin prick. There is always more than one because the wire is usually wrapped in and around the piece. The size of the blemish is only as large as the contact point between the wire and the piece. Since the wire is round and it usually touches the piece on an edge or something, the point of contact is actually SMALLER than you think. In a lot of cases there is usually a hole in the piece and if you put the wire through the hole the point of contact will be on the edge of the hole. If a screw is normally in the hole it will hide the mark.
I have read about Alligator clips, Wrapping the peice in wire,
drilling and tapping a hole to attach to. What is the best
method for not leaving any marks or blemishes.
In almost everything I have built so far I usually have a threaded hole somewhere. I find that the best and simplest connection method is to FORCE thread the aluminum wire into the threaded hole. In this way the blemish is large but is then inside the hole and not visible when assembled.
You must understand that the aluminum wire I have is probably # 14 guage and is VERY soft aluminum. So soft that it can be stretched and broken so it threads easily into 6061 T6 tapped holes without damaging them. Sometimes doubling the wire over first is necessary to make it large enough to thread in.
Failing that, the wire can be wrapped and twisted but the connection MUST be VERY tight and if you jiggle the part while it is hanging on the wire and you feel it move the connection is not tight.
assuming I want to anodize a square block of alloy with no where
convenient to clip to
This type of shape is difficult. Just do your best to wrap the wire around it - perhaps in several directions and twist the wire tight. The blemishes will only be at points of contact.
As a point of interest - in applications where VERY large numbers of small parts need to anodized and blemishes are not a big concern the parts are piled into an aluminum cage and a weight is put on the top of the pile. The parts are then all connected to each other tightly by multiple points of contact where they touch. They all anodize just fine except where they touch. After dyeing the parts are examined. If there are any with unacceptably large blemishes they are simply stripped of their anodizing with caustic solution and re-anodized with the next batch.
And the stupid statement required to cover myself... If you try this and something gets messed up, or someone gets hurt, you are on your own. Deal with it, you can't blame it on anyone else.