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Low tech hard anodising

 

I run a small anodising shop doing sulfuric, (incl. colour), and Chromic acid anodising. I need to start doing hard anodising, (parts up to 1.6 square meters surface area). I understand that it is possible to hard anodise at room temperature using 80g/L oxalic acid + 50g/L formic acid (or) 250g/L sodium sodium bisulphate monohydrate + 100g/L citric acid ⇦this on eBay or Amazon [affil links] , either at around 55 amp/square ft. Can somebody confirm this, as I find the cost of a suitable refrigeration plant prohibitive at this stage.

Alternatively, if I have to hard anodise at around freezing point, what sulfuric acid concentration should I use for best results and what is the lowest current density I will get away with.

Willem Kleinschmidt
- Cape Town, South Africa

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Willem,

There are commercially available additives that one can add to their existing bath that will allow you to "hard" anodize at around 50-60 degrees F. Although, these coatings are not as hard as ones run at a lower temp. The minimum current density you would need to use would be 24-26 ASF

Marc Green
anodizer - Boise, Idaho

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Oops, I forgot to address your concentration question. I'm sure that subject may be debatable, but I, personally, wouldnt go above 200 gr/litre.

Marc Green
anodizer - Boise, Idaho

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Willem,

First of all there are a lot of processes out there claiming Hard anodize. Generally they are something better than type II and close to true Hard anodize. You have to know what your customer really is looking for, if it is true hard anodize than best approach is 32 f. Also I have heard that oxalic is a carcinogen and most shops have gone away from it. (Please contact me for more information.)

Drew Nosti, CEF
Anodize USA

Ladson, South Carolina