Hardcoat vs. regular anodize of aluminum

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

Also available in Spanish

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A. Hi Gary. In anodizing, DC electricity converts the aluminum metal on the surface of the part to non-conductive aluminum oxide; simultaneously the acid in the electrolyte is dissolving the coating. The process is thus self-limiting -- because the oxide impedes current flow and when it reaches a certain thickness the current flow cannot build a coating any faster than the acid dissolves it.

So, in simplest terms, hard anodizing involves a higher voltage rectifier and lower temperatures (of about 28 °F rather than 68 °F) which allows more current to flow for a heavier build, while reducing the acid attack on the coating.

They both employ sulfuric acid of about 10 to 12 percent concentration by volume.

There can be proprietary extra ingredients that are added and which purportedly build a better coating or allow the hard coat process to operate at higher temperatures. Generally hardcoat is considered to be 0.002" thick whereas "regular" anodizing would usually be between a tenth and a third of that.

A. I have 30 years of experience in anodizing and I have found 16-18% Sulfuric concentration works best.

Thanks, Raymond, I'm sure you're correct. My 10-12 percent, while perhaps approximately correct by volume, was not really the correct answer, and I appreciate your catching the error. Some published values in the reference books include, by weight, 15 percent and 185 g/l.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1998

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A. Terry,

There are several companies that offer an additive. Check out the list in the Metal Finishing Guidebook or similar reference. It does take a higher voltage and it does not produce a true hardcoat, but one with some of the properties such as a little less hardness, a little less wear. In some cases this will suffice for real hardcoat.

Most allow you to "hardcoat" in the 45 °F range vs. 32 °F. Some are higher.

A. Hi Terry. These additives may involve glycolic and/or oxalic acid but, as James says, they're usually offered as proprietary additives rather than purchased as commodity chemicals. As James notes on another posting, these pseudo hardcoats, operating at a temperature in between MIL-A-8625 / MIL-PRF-8625 [on DLA] Type 2 and Type 3 coatings are sometimes facetiously called Type 2-1/2 :-)

Regards,

A. Dear Manuel Tirado C.

In our Equipment Directory at www.finishing.com/equipment you will see some suppliers listed. You may also wish to consider to retaining an anodizing consultant if you don't have significant in-house experience. Good luck.

A. Kevin.

I agree with you, a well sealed Type 2 coating should be sufficient. I would suggest sending a sample of your work out for salt spray testing, and assuming you pass the Mil Spec, show your customer the results. Sealing is the key here. Your parts must be well sealed to stand up to constant salt spray and sunlight,; and even then, after a long period of time, your parts may eventually fade. It sounds like the best coating would be electrolytic coloring. I'm sure there are others in here that are more knowledgable on that process than myself.

A. Reporting from the International Hard Anodizers Association's Technical Symposium in Barcelona, Spain.

Hard coating is a thicker, denser version of commercial sulfuric anodizing. It is better in wear, corrosion, and insulation protection. If you want maximum protection it should be specified. However, the added costs should be considered. Regular anodize is used successfully in a lot of applications.

A. Type II is conventional room temperature sulfuric anodize and in is never referred to as "hard anodize." Type III is also done in a sulfuric acid electrolyte, however, it usually involved lower temperature, a bath additive to inhibit dissolution of the aluminum oxide back into the sulfuric acid, and/or higher current density and voltage. The result is a HARDER, DENSER (and thicker) oxide layer for Type III coatings. My guess would be that someone made an error in calling out Type II.

A. Milt is 100% accurate, as he comes from a leading high reliability anodizing shop that would never call Type II "hardcoat". That said, in the real world of all sorts of job shops, there is no fine line definition between Hard coat and soft coat. The Mil Spec says 0.002 inch "unless otherwise specified". Picatinny Arsenal has some purchase orders out calling hard coat: 0.001" thick, dyed, and sealed. The Mil Spec does not even mention "hardness" - they talk about abrasion resistance. Hardness, which is pore size, varies with temperature, concentration, and current density, and OH, alloy too. Five shops in the same Florida county will give you five different pore sizes, five different colors, and five different thicknesses.

A. I typically see this mistake on many customers callouts every year, describe to your anodizer the nature of the application (what is the part application) and decide whether or not a functional, or a decorative finish is desired.

If the anodizing is for part identification, maybe type 2 is appropriate, if the coating is intended to provide wear resistance, or corrosion resistance, type 3 would be more appropriate.

Thanks Ryan. Your posting is right on, but offers an opportunity to present an example of Robert's about dyeing & sealing "hardcoat". To clearly distinguish between live rounds and dummy rounds, a military installation might want to dye them different colors and seal them even when the thickness of anodizing they require & specify exceeds Type II anodizing.

Luck & Regards,

A. Hi Mike. Just to minimize crosstalk and make sure that we are all using the same language and understandings in discussing your question, are we conceding that the most basic difference between conventional anodizing and hardcoating is the thickness? (Hardcoating being approx. .002" thick, whereas conventional anodizing might range from perhaps .0002" for many uses through maybe .0007" for architectural use). In other words, hardcoating is about 3 to 10 times as thick as conventional anodizing.

Please try to tell us what you are up to, as people & their problems are interesting, but dry & abstract discussions about parameters aren't much fun. Thanks!

Regards,

A. The .002" coating will certainly give you much better corrosion resistance IF...and I mean IF, it is properly sealed. Your typical 15 min mid-temp nickel acetate seal that one could use on a type 2, class 2 coating would not be adequate on a typical hardcoat subjected to a corrosive environment. An old-fashioned 2-3 min/micron sealing in deionized water @ 200-212 °F, held within a tight ph range, is a far superior seal for corrosive environments than your standard mid-temp nickel acetate seal.

I believe it was either Products Finishing, or Metal Finishing magazine that had a fairly solid article comparing different types of sealing, and the resulting effects on the coating properties. It was quite some time (years) ago, so the article could be hard to locate.

Bottom line here Mike, is that there are things an anodizer can do not only to the coating itself by tweaking processing parameters, but also with the sealing techniques, that can greatly enhance the corrosion resistance properties of the coating. You can cite all the articles you'd like, but if you're not following the same processes as detailed (or not detailed, as the case may be) in the articles, it really does little good.

A. Hi Pankaj. Hard anodizing is usually considered to be 0.002" thick, which is probably at least 3 times thicker than your present anodizing, so there will be significant dimensional changes. I doubt that it will cause twisting or warpages in most parts, but there could be very thin parts where it might be a problem. Good luck.

Regards,

A. Hi Luke. I think you are correct that Type III anodizing will be more corrosion resistant than Type II -- for one thing it is much thicker, 0.002" rather than the 0.0003"-0.0006" you were probably using before -- but see Marc's important note about proper sealing.

But I don't think you need to worry about "cold anodizing". Although I'm not familiar with that particular slang, Type III doubtless IS the "cold anodizing" someone is telling you about. Type II operates at about 68-72 °F but type III operates at about 28-32 ° F.

As a brief explanation, two opposing things are happening during anodizing, an electrically insulating oxide film is being grown on the parts while the sulfuric acid is simultaneously attacking and dissolving that film. To get the thick anodized film associated with Type III anodizing in the face of that insulating film tending to stop further anodizing, plus the acid attacking it, requires higher voltage rectifiers than Type II, plus keeping the sulfuric acid cold so it is less corrosive.

Luck & Regards,

A. Hi Luke! If you haven't run Type III "cold anodizing" also referred to as "hardcoat anodizing" due to its excellent abrasion resistance ... beyond Ted's advice, the additional pointers I have are to take extra care in ensuring that any fasteners on your racks are both snug, and VERY clean prior to starting work. If you re-use racks, as I assume most of us do, not only should you verify stripping, but take the extra time to disassemble and inspect them and sand/grind contact points. Use virgin bolts for reassembly. Any stray anodic coating that increases resistance can cause mid-run conductivity failures and burning due to the increased current.

Another tool is the surfactant "anti burn" additives that can be added to the plating bath. This is not a substitute for good rack maintenance, but will help protect the load. They're very expensive, but at less than 1% of total bath volume worth the money.

A. Hi again. I just took a quick look through MIL-A-8625 to refresh my memory, and it does not require that Type III anodizing be done cold, although some people, perhaps a bit facetiously, call it "Type 2-1/2" when it's not done cold :-)

There is also no corrosion resistance required of hardcoat unless sealed. And although the spec says it's 0.002" thick, it also says "unless otherwise specified". And there is an abrasion resistance spec in section 3.7.2.2, but it doesn't apply after sealing. So it's pretty hard to say exactly what hardcoat is & isn't.

But because you are suffering an abrasion-corrosion problem you are currently trying to fix, I'd probably suggest that you do an abrasion & accelerated corrosion test on what you're getting now, plus find a supplier who does actual cold hardcoating and do an abrasion & corrosion test on that rather than conjecture.

Luck & Regards,

A. Checking in as someone who facetiously calls it "type 2-1/2"! :)
We like it for a certain product range (think recreational outdoor products) that are more desirable with a slightly improved abrasion resistance, but have to be dyed (organic azo dyes) to jewel tones. The lighter colored anodic layer, when compared to "cold" tyIII, allows this. 32 °F/0 ° "cold" hardcoat is a deep charcoal color and to shamelessly misquote Henry Ford, "You can dye it any color, as long as it's black".