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Rectifier size (current, voltage) for Chromic Acid Anodizing

I am looking into setting up my own anodising plant to support some major Airbus contracts we currently have. The anodising tank will be 11 feet long, 3 feet wide and 6 feet deep, our components are ABM 3-1029 T7651. The bath temp will be 40 °C, and the voltage will be max 50 volts. The sequence is 0 - 40 volts in 10 mins, hold for 20 mins then up to 50 volts for 5 mins. What size rectifier will we need?

Andrew Jones
- Cardiff, South Wales, Wales, UK.

I have never heard of anodizing at 40C. At 4C or 40F is more likely, but that is approaching hard anodizing requirements. At that temp, I have no idea of the required amperage, but conventional anodizing is about 15 amps per sq ft of surface area of the parts and has nothing to do with the size of the tanks. Remember that you have to include any of the exposed rack in you calculations.

I would add 10 to 20 % to the calculated load, just to cover future possibilities. Auto current control is a nice feature to have as is auto voltage control. Have you considered heating and cooling requirements.It generates a lot of heat.

James Watts
- Navarre, Florida


Sounds/looks like chromic acid anodising (the Bengough Stuart cycle) by the look of your voltage ramping, figures I have for the typical current density requirements for this process are 3 to 20 amps per square foot depending upon the type of aluminium.(Pure AL 3 ASF, Dural 6 ASF, Cast High Si Al 20 ASF).

Make sure you use appropriate Al jigs, as you will find using Ti jigs the current will pull as much as your rectifier can supply, as the Ti jigs do not build up an insulating film in chromic anodising.

Are you also aware of the new IPPC regulations concerning processing facilities containing >30 cubic meters of processing solution. If you fall within the scope of these regs you will have to be permitted PRIOR to starting any installation, and by starting I mean breaking the ground.


Richard Guise
- Lowestoft, U.K.

Uh-oh. I think your 3rd paragraph completely changed the subject, Richard :-)

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

simultaneous replies

Well, I guess that the memory is really failing. I totally forgot about chromic anodizing. It does sound like Richard might have dropped a big Oh-Oh.

James Watts
- Navarre, Florida

Surely Andrew's tank is only 11ft X 6ft X 3 ft, which is 198 cubic feet. When I went to school, this was equivalent to only 6.6 cubic meters (give or take a bit), so he is well below the statutory 30 cubic metres, unless he has multiple tanks.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

In the USA, some agencies total all of the process tanks, excluding the rinses. He will have to look into the agencies interpretation.

James Watts
- Navarre, Florida


Yes you're right Trevor about the volume, but I would have thought he would have needed more than one tank to build an aerospace chromic acid anodising line for airbus parts. It may be that the aggregate volume of his tanks (for solutions that are relevant, certain pre & post treatments are included within the calculation) will be less than 30 m3, in which case he can go ahead. But surely forewarned is forearmed, at least he has the opportunity to investigate the legislation now if he wasn't aware of it to see if he does fall within it's remit. There is talk of UK industry representation by a number of the bigger players in an effort to reduce the volume threshold, something about a level playing field. Whether this will come to anything I don't know.

Anyway I'm back to preparing my permit application !


Richard Guise
- UK

Thanks to everyone who replied to my question. It was interesting to read about the legislation, I am in the process of contacting our local government authority to find out if they have any objections but are there any other authorities I need to contact before we decide to go ahead with the installation.

Andrew Jones
- Wales

I guess you could start with the Environment Agency they are guys policing IPPC, if you don't fall within it good luck to you. You may also research the following, Local Water Authority if you intend to discharge industrial effluent you will need permitting. Local Council to check that your planning permission allows you to anodise (different building category to general engineering I believe). Checkout the COMAH (Control of major accident hazard) regulations they might not apply, again they go on aggregate volume of solutions and types of hazard associated with the solutions. As chromic acid anodising is an electrolytic chromium process you will have to instigate health screening on employees working with it checking for chrome ulcers etc HSE can advise. Probably for a more detailed advice contact the Surface Engineering Association in Birmingham. If you have no experience in the field it may be best to look at employing a consultant.

Best of luck,

Richard Guise
- UK

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