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topic 946

Do-it-yourself anodising

A discussion started in 1997 but continuing through 2018


Q. I commenced my internet "life" specifically to find out the pros and cons of anodising. I build and race my own drag racing vehicles and as a consequence make many brackets and parts from aluminium to keep weight to a minimum. Where I live in Queensland in the northern part of Australia puts me about 1200 kilometers from the nearest anodiser, hence my new found "love" of anodising.

Surface Treatment & amadeya42 of Aluminium and Its Alloys
Wernick, Pinner & Sheasby

I have anodised most of the aluminium bits and pieces on my car with a good deal of success, so much so, friends have asked me to do their parts, which brings me to the point of this letter.

While I have been happy to do my own bits, I need a few questions answered before I'd be confident enough to put friends parts in a bath of acid.

Currently I am using a bath with about 30 liters of electrolyte, and some sample dyes from Clariant (Australia). I anodise at about .1amp / square inch for about 40 mins. I agitate the solution with a fish tank air pump and maintain the temp at about 25 °C - 30 °C.
I mix the dyes as per manufactures instructions, and leave the work in the dye till the desired color is achieved. I then boil work in clean water for 10 mins. and at least till now, I end up with a nice piece of colored aluminium.

My concerns are:

I realise that these questions are of a somewhat basic nature, but I would be grateful if you could clear up some of my concerns. I have tried to purchase some of the references that are listed in amadeya42.com, but alas, no go, at least in Australia.

I must admit that since I started my foray into anodising, I never had more fun, but I am still "spooked" by the chemistry side and the dangers or problems that may exist with some of these "potions".


Rory MClaughlin
- Australia


A. I have 30 years experience in anodizing and will try to help answer some of your questions and you are welcome to pose other questions. I may be able to be of help or not.

Sulfuric ratio ideal is 16-18%
98% with 1.85 S.G.
electrolyte coating should not wear off
most organic dyes are not good for outdoor exposure, they will fade in sunlight
Dyes do not present a hazard to user
Chemical brightening prior to anodizing is called a bright dip
Ideal solution of 80-85% phosphoric acid and 3% nitric acid
Specific gravity should be approx. 1.710 to 1.720
Temperature range 200-220 °F
Sodium bicarb should not have any lasting effects on finish.

This should give you a very bright part.

Hope this is of some help.

Raymond Hendrix Troy, Tennessee


A. One other consideration for you regarding anodizing parts for your friends race cars is the affect of anodizing on structural components --
-- anodizing significantly decreases fatigue strength, sometimes by up to 50%. This means that anodized structural components will be much more susceptible to fracture over time than components without anodizing. This may or may not be a factor in the components that you are anodizing, but is something to keep in mind.

larry hanke
Larry Hanke
materials testing laboratory
Minneapolis, Minnesota


A. I am the project chemist at ACA. As to your questions:

As you treat aluminum parts, whether it is cleaning, etching, bright dipping, or anodizing, aluminum is constantly being removed from the surface. The aluminum that is removed from your part(s) stays in the bath and can affect the quality of the anodic coating, especially in the etch bath and bright dip. (BTW, premixed bright dip solutions are available [in the US, anyhow] from Albright & Wilson). Aluminum in the drag-out from the anodize bath to the dye bath can kill your dye, even if you keep the pH corrected. Also, stored organic dyes can support life (read: MOLD), so watch out.

Gerald Janssen
-coil anodizing

What is sodium bicarb?

May 19, 2014

Q. Dear All,

Can you advise what is sodium bicarb?

Best regards,

Alex Au
- Hong Kong China

probert book
Aluminum How-To

by Robert Probert
$89 New
The Chromating - Anodizing - Hardcoating Handbook

May 2014

A. Hi Alex. "Sodium bicarb" is just a slang abbreviation for sodium bicarbonate (NaHC03), aka baking soda. It is a very mild alkaline often used to neutralize acid. As you can read above, sodium bicarbonate is sometimes used for that purpose in the anodizing process.


Ted Mooney, amadeya42.com Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

May 23, 2014

Q. Hi Raymond,

Is the ideal solution of 80-85% phosphoric acid and 3% nitric acid for brighten up? Is this a process just before anodizing?

Best regards,

Alex Au
- Hong Kong China

May 2014

A. Hi Alex. Raymond's posting was from a long time ago and he's probably not monitoring this thread but, yes, he is speaking about a chemical polishing solution there. It is used to brighten aluminum before anodizing in limited applications like the manufacture of aluminum reflectors. It is not a part of the process in most anodizing shops. This is a very nasty process, so be sure to read up on "aluminum bright dipping" and to visit at least one shop that does it before considering implementing it yourself. Good luck.


Ted Mooney, amadeya42.com
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

September 28, 2015

thumbs up signIt is 2015 and I am grateful for this info. I am making an acid bath, 98% Sulphuric Acid and DI water but I was unsure of the mixture. I bought a hydrometer so I will gauge adding acid until I reach the 1.7 gravity.
Thanks folks!

Rajiv Ram
- doral, florida usa

January 1, 2016

A. A final sulphuric anodising electrolyte density of 1.7 seems too high to me. I've read typical mix should be battery acid (sg of 1.265) cut 50/50 with DI water, so density is going to end up around 1.13 or am I being stupid?

John Stokes
engineering - abergavenny, Gwent, wales

February 2016

A. Hi Rajiv. Hi John. You guys need to take your time and read the page a little more slowly ... Ray's 1.7 S.G. was referring to the chemical bright dip process, not the anodizing process :-)

For anodizing he suggested 16-18% sulfuric acid (by weight), and his 16% by weight is quite widely accepted. John, I assume you are a hobbyist from your reference to battery acid. Professionals use Electronic Grade 98% sulfuric acid rather than battery acid. You can't reliably go by specific gravity because dissolved aluminum throws it off. Good luck.


Ted Mooney, amadeya42.com
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

February 4, 2016

thumbs up signYes sorry you are correct I should have read the post fully from the outset. 16% as you say is stated as the sulphic to water electrolyte ratio by weight. I do use Electronic grade Sulphuric Acid concentrate with a density of 98% I was referring to an article that based it's starting mix using battery acid that is already supplied with a lower density initial density.

John Stokes [returning]
skirrid engineering - abergavenny, Gwent, wales

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Storage of Anodizing Acids

February 4, 2016

Q. Getting started Anodizing and I am wanting to know how to store the various chemicals. Not a fulltime hobby so leaving in the tank is not an option. I also understand that what may work for short term, may not be suitable for longer storage. Obviously the original container would be preferred, but as I will be diluting, my storage volume will not be the same.

My Battery Acid came in a plastic container. Can the diluted Sulfuric Acid anodizing bath be stored in a standard 'Home Depot' 5 Gal bucket? If not, suggestion for ~5 Gal storage.

My 67% Nitric Acid also came in plastic and instructions to transfer to glass asap. The website stated that concentrations over 30% must be stored in glass, but makes no mention for concentrations under 30%. Can 10% Nitric acid be stored in the 'Home Depot' bucket? I've searched but can find no references to Nitric Acid storage at lower concentrations. This is the one that I am most concerned about.

Frustrated because I just gave away all my 5-6 gal carboys (was beer making).

I do not foresee an issue with Dyes in the 5 gal buckets. Let me know if I am wrong on this point.


Sterling R [last name deleted for privacy by Editor]
Getting Started with Aluminum Anodizing - Oklahoma City, Oklahoma

February 9, 2016

A. Hello Sterling, I have only used polypropylene tanks or buckets for storage of acids. With the acid concentrations that you described a white polypro with a lid should be fine.

Mark Baker
Process Engineering - Phoenix, Arizona USA

February 13, 2016

A. You would be just fine storing either or both of the chemicals you mentioned in a 5 gal Home Depo plastic bucket. If you happen to splash some on the metal handle, you'll want to rinse that off, as it will corrode.

Marc Green
Marc Green
anodizer - Idaho

Is it possible to anodize heavy duty aluminum foil?

February 10, 2018

Q. First I would like to say thanks to everyone who contributes their knowledge and experience here. I have no background in amadeya42 of any kind, and have only recently began researching these processes. I've been reading many posts & responses on this forum and have learned so much. I will be attempting amateur anodization with a 12V supply rated for 20A continuous. For the current project I'm working on, I'm curious to know if it is possible to anodize heavy duty aluminum kitchen foil, approximately a 12" square foot.

Here's what I've learned from y'all:
The 720 rule yields a result of a 1 mil coating for a square foot, taking 36 minutes at 20 ASF. It would most likely be Type II or 'decorative' anodizing with Sulfuric Acid, as Chromic seems to be unusual for home brew anodizing.

Here's what has me scratching my head.

Household 'heavy duty' Aluminum foil is on average 0.94 mils thick, but the above calculation is for a 1 mil thick anodized coating. Could I simply reduce the time the foil is submerged to achieve a very thin coating?

Intuitively it seems plausible, but then why is there a clear distinction of coating thickness ranges between all three Types?

Additionally, the military spec (not sure if that's what it's called) states that for Type II a 67/33% ratio thing occurs, which confuses me a bit more about how that plays a role with these processes a thicknesses of materials to be anodized.

I am genuinely interested in conceptually and practically understanding this process, and what the military standards of differentiating processes of anodizing have to do with what is actually possible, say Type II anodizing thin foil.

Again, I truly appreciate the time y'all dedicate to helping the community and even new learners like me. Thanks!

Nathan Martinez
Student designing a product - Dallas, Texas, USA

February 2018

A. Hi Nathan. Please start with our "Introduction to Anodizing" if you haven't yet seen it.

Although the 720 Rule is called a Rule, it probably really ought to be called the 720 Relationship because you are right that it's just about the relationship between thickness vs. time & amperage; it's not a "rule" saying that you should anodize to a full .001" thickness; in fact, regular Type II anodizing is almost never that thick. It's usually more like .0002" thick, although it might be .0004-.0005" for fully saturated black color and .0007" for architectural applications.

You mention a 67/33 'ratio', but 50/50 is more commonly quoted, and what that means is that the anodized film is generated from the aluminum of the article itself, such that some base metal is consumed in making the anodizing. If you do .0002" anodizing, you'll consume about .0001" of the aluminum. So if you anodize only one side of the foil to .0002" thick anodizing, it will only be about .0001" thicker after anodizing.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

February 13, 2018

A. Nathan

A 12" x 12" square will yield 2 sq ft of surface area; 2 sides.

At 1 mil thick you will need multiple contact points to keep from burning/arcing. There is little material to carry the current. And you will lose the ability to carry current as the aluminum is replaced by anodize.

After anodize the foil will be quite brittle. Handle with care.

Willie Alexander
- Green Mountain Falls, Colorado

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