Anodizing

Anodizing Aluminum
by Bryan Pryor
 
This website comes from the the rambling mind of Bryan, and some of his knowledge on the subject of small-scale anodizing.  This is an update of my older anodizing page that can be found here. The main goal is to provide information on how to set up a small scale anodizing line that provides professional results.  My method may not be the cheapest, but it will truly give you spectacular results.  Read carefully through this entire page before you begin anything, as this is not a step-by-step process but an overview.
 
Example of a DIY Fade Anodize
It all started back in the college days, when a friend decided he needed to anodize a paintball gun.  I had found a magazine article claiming that with some battery acid, RIT clothing dye, and a battery charger it was possible to achieve a good quality anodized finish at home for a fraction of the cost.  As you may guess where this is going, it didn’t work so well. 
 
However, after a couple of years of reasearch, trial and error, and way too many hours of polishing aluminum, I finally found a method of anodizing that works well for the small scale anodizer.
 
 
My basic method could be described as a modified version of the low current density method  proposed (and sold) by Caswell Plating.  In the low current density method, Caswell recommends using a constant current source and a low concentration sulfuric acid bath.  I tried this method, and it worked, but I was still not 100% happy with the results.  By using a constant current density of around 12 amps per square foot (ASF or amps/ft^2), and a higher concentration sulfuric acid bath, I could achieve the same or better results in about half the time.
 
I must give credit to Ron Newman, who has had an anodizing site up for quite some time.  It was his original site that prompted me to even attempt anodizing at home.  Although I can’t say I agree 100% with his method of using a battery charger as the power supply (and yes, I have tried it), I do agree with most of his other information.  I have bought some dye from Ron and I was very pleased with the service I received from him.  I would recommend doing business with him.
 
Let’s get started with the basics:  What will you need to set up your own cheap, inexpensive do it yourself (DIY) home anodizing line?  This will be for type II anodizing (type 2 anodizing).  That is the type of anodizing that takes dye well and is typically used mainly for cosmetic purposes, but also adds a hard aluminum oxide coating to the part that is very corrosion resistant as well as wear resistant and has electrical insulating properties.  I’m getting ahead of myself, though.  Back to the concept at hand:  What do you need to set up your own anodizing kit?
 
Items needed:
 
  • Tanks to hold sulfuric acid (anodizing bath, or anodizing solution)
  • Aluminum parts to anodize
  • Power supply.  Should be capable of constant current, have a high enough current capacity, and go to at least 20 volts
  • Sulfuric acid (battery acid) to make acid bath
  • Aluminum wire or titanium wire to hold parts
  • Distilled water.  It’s cheap, so buy lots of it.
  • Anodizing dye. Don’t bother with the RIT clothing dye, it doesn’t work well.  Buy good anodizing dyes, they’re not expensive, either.
  • Degreaser.  Aluminum Parts must be surgically clean before anodizing, or the color won’t be consistent.
  • Desmut (optional)
  • Sealer (optional)
  • Lye, also known as sodium hydroxide (NaOH). Used to strip existing anodizing off parts.
  • Baking Soda. Keep a large amount of this on hand to neutralize acid in case of accident.
  • Rubber gloves.  I prefer nitrile because they are tough and do not tear as easily as latex gloves.  You will want these because you do not want to be touching parts with your hands after they have been cleaned, as the oils in your skin are enough to leave bare spots on an anodized finish.
 
Basic DIY Anodizing Kit
Basic DIY Anodizing Kit
 
 
Anodizing Power Supply
One of my Anodizing Power Supplies – 5 Amps
 
Here’s an example of a larger constant current power supply at Amazon:
Getting started:
 
Basic Steps:
 
1.  Clean the aluminum parts.
2.  Degrease the aluminum parts.
3.  De-Smut the aluminum parts.
4.  Anodize in the acid bath at 12 amps/square foot for 45 minutes. Keep temp at 70-72 degrees (F).
5.  Remove parts from acid bath and rinse with distilled water.
6.  Dye parts (if desired) in warm dye, ~100 – 140 degrees (F).
7.  Seal parts by placing in boiling distilled water for 20-30 minutes.
 
It should be assumed that the parts are thoroughly rinsed in distilled water between each step.
 
A typical DIY setup for beginning anodizing is shown on the right.  A 5-gallon bucket will work for small parts, the large plastic storage totes offer a variety of shapes and sizes available for larger parts.  In my experience, these have worked well.  These will be the tanks that will be storing your sulfuric acid anodizing bath, so take caution to ensure they will not leak.  Sulfuric acid is a corrosive chemical.
 
Cleaning the Parts
 
This is likely the most critical, yet overlooked, step in anodizing.  The parts must be surgically clean before anodizing to ensure good results.   I prefer a multi-step process to ensure good results.  First, put your rubber/nitrile gloves on and wash your hands thoroughly.  This ensures the gloves are clean on the outside.  Then, go through a 3-step process to clean and de-grease the parts.  Step 1:  Clean the parts with hot water and dishwashing detergent.  Depending on the geometry, a brush might be helpful at this point.  After the dishwashing detergent, the parts should be visually clean.  Next, clean the parts with a degreaser such as Simple Green.  Finally, use an anodizing specific degreaser, usually heated, to get the level of cleanliness needed.  I have had good success with the SP cleaner available from Caswell.  At this point, water should not bead up on the surface of the part, but form an even sheet over the entire part.  If you see water beading up, that shows the presence of grease & oil, and the cleaning process should be repeated.
 
Now it is time for the Desmut.  Dip the parts in the Desmut tank for 1-4 minutes.  The Desmut solution can dull the finish on your parts, so if you are going for a mirror-finish, keep it under 2 minutes.  What is the purpose of the Desmut step, you might ask?  To remove impurities from the surface of the aluminum part.  For instance, some alloys have high levels of copper, which do not anodize well without a Desmut step.  For consitency, I desmut every time.  Once you are quite familiar with the process, you may experiment with differing times in the desmut tank. As always, rinse well after Desmutting.
 
Anodizing
 
Finally – it’s time to anodize!!!  By this point you should have prepared your acid bath by mixing 1 part battery acid to 1 part distilled water.  Remember “Acid into water is the way you oughta.”  That’s right, slowly pour the acid into the water, don’t pour the water into the acid.  You can find sulfuric acid at any auto parts store by buying battery acid.  With this concentration of battery acid, you will need to anodize at 12 amps/square foot using a constant current power supply.  Constant current density anodizing is one of the easiest ways to ensure consistent results.
 
FAQ:
 
Where do I buy the sulfuric acid/battery acid? I have found it available at most good auto parts retailers.
How do I mix the acid? Acid into Water
What ratio should I mix the acid at?  I prefer somwhere around 1:1 for 12 ASF.
What type of container should I put the acid into?  I have had good success with 5 gallon buckets as well as Rubbermaid style plastic storage bins.
 
What current density should I use?  You can choose either 6 ASF or 12 ASF.  Mix the electrolyte to match your current density.
What is the 720 rule?
 
anodize, anodizing, home anodizing, diy anodizing, how to anodize, learning anodizing
Let’s first get some nomenclature out of the way.  I say anodizing.  Some call it anodising.  Just depends on which side of the pond you come from.  Some may call it aluminum anodizing, others may say aluminium anodising.   I kind of like the sound when a european says aluminium, but I grew up saying aluminum.  However, looking at the periodic table of elements, I believe the Europeans have it right when they say aluminium.  So, for the sake of variety, I may say aluminium anodizing at some point.  Since we’ll be using sulfuric acid, we should get that out of the way, too.  Sulfuric acid is H2SO4, commonly called battery acid by non-chemists.  It may also be called sulphuric acid.  At some point we may be  talking about titanium racking systems, too, but I don’t think there’s a large variety in the ways to spell titanium.  Current Density & constant current anodizing are important.  voltage controlled anodizing may work, as well as battery charger anodizing, but I’m not going to recemmend those.

21 thoughts on “Anodizing

  1. Thanks for this page…exactly the info I was looking for. Do you happen to have any pointers to anodizing graphics using some sort of overlay or screen? I don’t know the correct terminology for it. Anodizing all one color is great but I’d love to anodize graphics like logos and stuff, too.

  2. Hello, can you tell me where to find an Aluminum Anodizer Kit. Building one can’t do that best to buy. Thanks!

  3. Hey thanks for the great tips! One question though, you call for a current density of 12A per square foot and your power supply looks to be rated for only 5A (given the model number), do you simply use a very small container? I’ve tried to find higher current precision power supplies and have only come across high current regulated models. Thanks!

    • Hi Shaun, I have both a 5 amp and a 20 amp power supply. Caswell Plating used to sell a decent priced 20 amp constant current/constant voltage power supply. You can also find these on eBay for a bit less. With a 20 amp PS, you can get close to 2 square feet at 12 ASF or just over 3 square feet if you use a low current density method like Caswell’s 6 ASF. You’re not going to anodize a car bumper or anything large like that, though. :)

  4. You talk about rinsing after the Desmut step but not after anodizing. I assume you would rinse after all steps. Could you use a shared rinse tank?

  5. Hi Nick, you are right. You should rinse after anodizing as well. I used a different tank to rinse after anodizing. This allows you to recapture the acid, let the rinse water evaporate off, and put it back into your anodizing tank. If you use a single rinse tank for all rinse steps, you will have to dispose of more waste.

  6. Hello,
    I have just bought a small sailboat that has some anodized parts which are rather faided – originally black but now nearly white/metal in some parts. Could you tell me if such parts can be anodized again and if so how should I prepare the parts for the process?
    Since it is to be used in a marine environment, should it be type 2 or type 3 anodized?
    Thanks an advance,
    Kenneth

    • Hi Kenneth,
      You can strip the old anodized layer off using Sodium Hydroxide. The parts can be re-anodized. In the future, make sure the dye you are using is UV resistant to prevent fading. If you are interested in the maximum corrosion protection and color-fast nature, you could also powder-coat over the anodized finish. This will give you a whole other layer of protection, but will change the look from an anodized aluminum surface to a painted surface.

  7. Good,concise info on aluminium anodizing. Easy to comprehend and to rhe point. ..thanks.. SAL SANGILLO, MSME (retired).

  8. Pingback: Deburring aluminum?

    • Hi Brad, 5 amps should work for most small pieces. Even if you’re anodizing at 12 Amps/Square foot, you should still be able to anodize 60 square inches at a time. At 6 ASF you could do double that.

  9. Hey Brad Great Site man,

    Im going to be attempting or want to anodize my ar15 lower I machined since there is noone available for months to send it off to have it done I figured hey I’d love to try it myself.

    Anyways, I want to keep things simple and to the point. Im not going to be anodizing anything else just the lower receiver and I’m positive a 5 gallon bucket would suffice.

    What setup would you recommend? I was looking at your Power Supply in the pic you provided and found it on Amazon, so how many buckets would i use and is there a way you could give a step by step in more detail with that basic setup on how you’d anodize a lower receiver just using 5 gallon buckets?

    Thanks
    Brandon

  10. Hi Bryan !
    Thanks for sharing. I have 2 questions:
    1) you say “mixing 1 part battery acid to 1 part distilled water” and then in FAQ you say “I prefer somewhere around 1:2 for 12 ASF”. So what’s the truth ?
    2) I would like to anodize AL panels. If you are talking about anodizing 1 sq.foot, does it mean workpiece 1 x 1 foot big or half of it ? (both sides summed together) ? I hope you understand me :)

    • Hi Enriqe, thanks for the question. 1:1 is what I have used for best results with 12 Amps/ft^2 (ASF). I’ll correct my notes above.

      For the second question, you need to count the surface area of both sides. Actually, you need to count all 6 sides, but the 4 edges will have a pretty small surface area if they’re just thin pieces of sheet metal, such as 0.032″ thick.

  11. Hi Bryan,
    I’ve been experimenting with powder coating anodised aluminium parts in an oven at 200 C for 10 minutes. Is it possible that the anodising would craze slightly at that temp?
    cheers,
    Tim.

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