Do-it-yourself anodising

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.

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.

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.

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

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.

Regards,

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.

Regards,

A. A final sulfuric anodising electrolyte density of 1.7 seems too high to me. I've read typical mix should be battery acid ⇦this on eBay or Amazon [affil links] (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?

A. Hi Rajiv. Hi John. You guys just 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 ⇦this on eBay or Amazon [affil links] . Professionals dilute Electronic Grade 98% sulfuric acid rather than battery acid ⇦this on eBay or Amazon [affil links] . You can't reliably go by specific gravity because dissolved aluminum throws it off. Good luck.

Regards,

Yes sorry you are correct I should have read the post fully from the outset. 16% as you say is stated as the sulfuric to water electrolyte ratio by weight. I do use Electronic grade sulfuric 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.

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.

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.

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.

Regards,

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.

A. Hi Amber. I don't think that will be practical. Although it is relatively easy to remove trace contaminants from the makeup water via de-ionization (aka demineralization), once you've added the sulfuric acid and dissolved aluminum from the anodizing process, you probably have no practical way of removing the contaminants. But please tell us what contaminant you suspect or wish to deal with.

Regards,

A. Hi again Amber. You are certainly welcome to research your idea or to experiment with it, but I have personally never heard of anyone cleaning contamination out of an anodizing bath in this way. Most of us cannot afford the time to re-invent a new method of anodizing with azelaic & boric acid in solution, and then develop a technology based upon it, thus the famous quotation often attributed to Newton: "If I have seen further it is by standing on the shoulders of Giants."

So if you want to get on with practical anodizing rather than engaging in raw research, I think you must start with standard methods. You'll need to devote hundreds of hours to the subject, and no one will teach you one-on-one in that fashion, so you must somehow obtain books on anodizing or resign yourself to trying to learn on-line by becoming highly proficient with search engines. We have hundreds of threads about anodizing on-line on this one site alone, and dozens about brightening aluminum before or after anodizing. Books are a much better way to go, but if you absolutely can't find them in Pakistan's libraries, you can start with our "Introduction to Anodizing of Aluminum". Best of luck.

Regards,

A. Hi again. Please search the site for "aluminum bright dip", read the threads, and after that I will try to answer your question. Presumably by then you will have a vessel on hand, built of type 316L stainless steel, and double walled for containment for when the inner tank eventually leaks? You will have this tank and at least its first rinse completely contained in a ventilated "garage" with proper exhaust ventilation and fume scrubbing. You will have an automated hoist carrying the parts to it, and a plan for under what conditions people can safely enter that area. And you will have researched dealing with NOx fumes.

Bright dipping is difficult and risk-prone; only a small percentage of highly experienced anodizing shops perform it. I have only seen three such installations in my whole career and been involved in the design of only one in a whole lifetime of work as a plating & anodizing equipment engineer. The idea that a person who says they can get no hands-on training in any of this wants to start a bright-dipping operation is quite frightening to me.

Have you considered trying to get a work visa and a job for a year in an anodizing shop which does bright dipping to learn this process, or to offer your anodizing services to customers who do not require bright dipping? Your life is priceless but possibly at risk if you try to offer bright dipping services with no knowledge of them. Good luck.

Regards,