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Hardcoat vs. regular anodize of aluminum

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Q. I need to know the difference between Regular anodizing vs. Hardcoat anodizing. Are the chemicals interchangeable, etc.?

Thank you.

Gary W. Schooley


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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.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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

Raymond Hendrix
H&H Equipment

Troy, Tennessee



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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.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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Q. You mentioned additives that allow hard coat to be run at higher temperatures. What are they, where do you get them, and how high a temperature will they let you run at? Is any other special equipment needed?

Terry Burgess
Consolidated Metal Technologies


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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. Companies like Metalast that have a excellent additive will not sell it alone and it takes special equipment to operate it and it is a licensed product.

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

James Watts
- Navarre, Florida


December 2014

A. Hi Terry. These additives may involve glycolic and/or oxalic acid but, as James says, they're usually proprietary additives rather than generic acids.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

Pinner & Sheasby
Surface Treatment & Finishing of Aluminium and Its Alloys


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Q. Dear Sirs :

We are about to initiate an important aluminum operation in Peru (South America) and the only missing piece in our project is the anodizing plant. We haven't being able to contact directly manufacturers of these plants or second hand sellers and we would highly appreciate if you could recommend us where to turn for this information.

As extra information, we would be processing 300 tons a month.
Thank you in advance for your help.

Manuel Tirado C.
- Peru


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A. Dear Manuel Tirado C.

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

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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adv.

Dear Manuel Tirado C.
H&H Equipment is a supplier of new and used anodize equipment.
I have 30 years of experience in anodizing. We can supply you with equipment and arrange for installation.

If I can be of further assistance, email me.

Raymond Hendrix
H&H Equipment

Troy, Tennessee




October 13, 2014

Q. Hi,
I have a regular design of anodized valve housing.
Now if I go for hard anodised process is there any changes in tolerances or dimensions? Any more twisting or warpages in housing that can happen in this new process?

Pankaj Singh
- GGN , HA , India


December 2014

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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