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Help on hard anodizing?

Hi! I've being doing colored anodizing at home for some time with good results,Now,Before anybody tell me this is a danger thing to do at home,I've bought a plating manual and I Know the safety rules,and the parts that I'm doing are small like 100 sq. in. or less.

What I would like to know is:How the process of hard anodizing differs from cosmetic anodizing,what temperature,acid concentration,amps/sq. ft., and if there is something that I can add to the acid solution to get a harder anodic film without change temperature, acid concentration or amps/sqf

Thank you all!

Alvaro Fogassa
Bridgeport, Connecticut

There are two major differences, temperature and electrical. Temperature-32F ± 1 degree, two for less quality work.This requires an excellent chilling system.

Electricity, normally requires 40 volts minimum with more modern units using a minimum of 60 volts. This will normally will require a slightly larger amperage capability or the ramp up time will be longer.

There are lots of additives that will give you a poor mans hard anodize. This is not identical to all properties of a good hard anodize, but some people find that the product is acceptable for their use. If you are not a business with a waste disposal permit, most companies will not sell to you because of the cradle to the grave liabilities. Glycolic acid is a common additive for several older formulations.

James Watts
- Navarre, Florida

Thank's James! this is not the first time you help me. one more question, do you know a inexpensive way to keep the solution cold? I use a 5 gallon container as a tank.

I'm using about 15amps/sqf,should I raise it to 25/sq.ft.?

I'm using 15% acid and I've notice that if I lower the acid concentration to 10% I can get a harder film.
Thank's again.

Alvaro Fogassa
Bridgeport, Connecticut

First of two simultaneous responses--

Try to find a used lab chiller. This will run around $500.00 if you are fortunate enough to find one.

You could make a coil out of seamless 316 SS tubing to go into the acid tank. Use a copper coil in a chilling tank. connect with plastic tubing and a small cheap pump. Use dry ice and methyl alcohol in your chilling tank. This goes to about -40. The alcohol is infinitely reusable. Use propylene glycol rather than ethylene glycol since if you spill it, it is non hazardous. EPA gets upset about antifreeze spills. You will have to control the temp of your anodize tank with a temp controller of manually adjust the flow with a valve to maintain temp. A crude pain! Do not put any more Dry Ice in the bucket than to keep some in it. Saves a buck or two. Dry ice will last for days in a taped styrofoam cooler inside a cardboard box that is also taped.

Additional thoughts, take the SS coil out of the bucket and rinse it off. Sulfuric acid attacks SS above 65F.

No, you can not put the dry ice in the acid. Temp is uncontrollable and it will form a lot of carbonates, changing the conductivity of the solution.

No, you can not use it in a double boiler manner, as plastic is a poor conductor of cold and gets worse as the plastic gets thicker.

James Watts
- Navarre, Florida

Second of two simultaneous responses--

For a good 2 mil low temperature hardcoat, 40VDC is too low. Hardcoating is normally done at 35 to 40 asf and can require up to 75VDC to maintain that current density throughout the process. At these high power levels, racking becomes much more critical.

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