Letter 11401

Aluminum anodising 

I have a problem with aluminium anodising. When I need to anodise a piece who has a bigger surface than usually, I have the problem of white powder in the surface.

Pedro Miguel Oliveira
- V. N. Famalic, o, PORTUGAL

You need enough power to deliver a minimum of 20 amps per square foot of surface area. If you don't have that, you need a bigger power supply. If you do have it, I'm not understanding what makes the larger parts a problem.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

White powder normally means that the temperature is too high. Try checking that. The problem may be the larger than normal part is pulling more than normal current heating the vat too much. Try turning the set temperature down on the chiller.

Martin Trigg
surface treatment shop - Stroud, Glos, England

Hi Pedro,

From what I understand, the white powder could be the result of, as Ted Mooney rightfully pointed out, a shortage in your power supply.

During the anodizing process, you have two competing reactions:

a) An electrochemical reaction, the one intended, is the oxidation of aluminum to form the aluminum oxide

b) A chemical reaction, taking place at the surface of that oxide layer and which consists in the chemical attack of the oxide film by the acid.

In order to grow a good oxide layer with the known properties, one must have a certain equilibrium between these 2 reactions. In your particular case, it seems that while the chemical reaction is carried out at its normal speed, the electrochemical reaction is a bit slower than normal. You have to always remember, that the electrochemical reaction is completely dependent on the current density at that electrode:

i (Amps/unit surface) = I (amps) / S (unit surface area).

Consequently you are dissolving faster than building the anodic layer. This tends to soften the outer face of the film and make it chalky, hence the white powder.

Hope this would help.

Dr. Hocine Djellab
Verdun Anodizing Inc. - Verdun, Quebec, CANADA

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