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topic 5980

Sulfuric Vs. Hard Anodize


Q. I am currently in a debate with a customer who insists our product supplied to him needs to be hard anodized. I do not think the environment (outdoor marine/saltwater) surrounding his part suggests using a hard anodize - I think a sulfuric anodize is sufficient.

Can you provide me with technical data or anything else to convince my customer that a sulfuric anodize will stand up to the marine environment. Or if I am wrong, can you explain to me why a hard anodize would be preferred?

Kevin Hess
- Dayton, Ohio, USA

TUTORIAL for readers trying to learn from these discussions, but needing a bit of intro first --

Conventional anodizing of aluminum involves immersing the parts into a bath of dilute sulfuric acid and subjecting them to anodic current, which converts some of the aluminum on the surface to aluminum oxide/hydroxide/boehmite. This surface is hard, non-conductive, attractive, semi-transparent, & corrosion resistant, and includes millions of small open "pores" that look like miniature drill holes extending almost down to the aluminum surface. After anodizing, the component is often dipped into a dye tank so the pores can absorb colorful dyes, and then into a "seal" tank of boiling hot water or other liquid which swells the anodizing to close off the pores and seal in the dye (and seal out dirt).

"Hard anodizing" or "hardcoat anodizing" or "hardcoating" is essentially the same process, but involves lower bath temperature, higher voltage, and longer immersion times to build a much thicker anodized layer. Because wear-resistance is often paramount, hardcoating is usually not "sealed" because that would somewhat soften the surface.

Mil spec MIL-A-8625 is often used to specify anodizing and mentions three types: Type 1 (a somewhat different process utilizing chromic acid instead of sulfuric), Type 2 ('conventional' sulfuric acid anodizing), and Type 3 (hardcoating).

Organic dyes are usually used for coloring, but are subject to fading; Marc makes an allusion to 'electrolytic coloring' whereby inorganic salts are used to deposit metals in the pores to impart more fade resistance.


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.

Marc Green
Marc Green
anodizer - Idaho


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.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
supporting advertiser
Wadsworth, Ohio

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The Technology of Anodizing Aluminum
from Abe Books


October 31, 2013

Q. Chris,

You stated above the following: "Hard coating ... is better in wear, corrosion, and insulation protection."

The literature I have found agrees that hard coat is definitely harder and more insulating than conventional anodizing.

However, I am having a hard time verifying the assertion that hard coat is more corrosion resistant than conventional anodizing with any published studies that I can find.

I assume that the required testing would need to control two of the three factors that Brace =>

lists as influencing corrosion resistance (aluminum alloy and sealing quality). The third factor of film thickness would obviously vary between the groups.

Do you (or anyone else) know of a study/reference that addresses the relative corrosion resistance of conventional vs. hard coat anodizing given the groups are of the same alloy and seal quality?

Thanks in advance for your help!

Mike Eye
- Alexandria, Minnesota, USA

November 2013

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 our regular readers keep reminding us that they find people & their problems fun, but they are less interested in volunteering their time on dry & abstract discussions about parameters. Thanks!


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

November 8, 2013

Q. Ted,

You had stated above, the following question: "are we conceding that the most basic difference between conventional anodizing and hardcoating is the thickness?"

Yes, I agree with that.

Per your request, the back story is as follows:

I have a customer that currently contracts our finishing operation for anodizing services. The substrate is usually a 6000 series and the thicknesses are usually in the 0.0004" - 0.0007" range of sulfuric acid anodizing and sealed in nickel acetate based sealer.

That customer is looking to improve the corrosion resistance of these parts.

They would believe that the thicker nature of the hard coat (and yes, it would be sealed) would yield better performance against corrosion.

My basic question above was trying to address if this customer could justify the extra expense of hard coat vs type II anodizing by knowing that the parts would better resist corrosion.

I know that at thin deposits of 0.0002" - 0.0004", the corrosion resistance is not very good. But at thicknesses of 0.0004" - 0.0007" (which we are normally at due to the parts being dyed), I understand that the corrosion resistance is better.

The $64,000 question is would a hard coated and sealed part do significantly better than a type II anodized and dyed part anodized to 0.0006" thick?

I am mostly interested in finding a study or reference from a paper or book that has addressed something like this scenario. I would love to be able to show my customer data that addresses this question directly.

If nothing exists, perhaps I will anodize some test samples and send them out for testing. Maybe that will be a good subject for a short paper at the next AAC or NASF conference.

Thanks in advance for your time and advice.

Mike Eye
- Alexandria, Minnesota

November 13, 2013

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.

Marc Green
Marc Green
anodizer - Idaho

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