Friday, July 18, 2008

Hard anodizing in warm baths!

So, I developped hard anodizing process that does not require supercooled sulphuric acid. One of the keys are bath additives. The other is current density (with large surface parts, cooling may be needed, the third and most important is to make the process start at the surface. And that one seems to be the hard part.

It is easy to form a hard anodized layer ( a dark brownish-gray thick layer ) once it started forming and this may be the property that one german company uses to form 15 micron hard anodized layer in just one minute and do it selectively! Gramm Technik link here.

Theory behind hard anodizing (type III anodise) - You need higher current density and you need high voltage. Sulphuric acid has high conductivity at high temperatures which will limit your ability to reach any higher voltage in the bath. Also high current baths get warm very quick. So cooling has to be used, ideally to the point that the bath is ready to freeze. That makes 1) the acid attack the formed oxide less and 2) the intrictic conductivity of the bath is lower, it is easiet to reach high voltage.

I simplified the above a bit, but here is what I have done: I decreased the bath conductivity without diluting the 8% sulphuric acid at room temperatures. The ability of the bath to attack the oxide is also lower. Bad points: some reaction may allow appearance of self-catalyzed burning holes on the anode, these may eat the aluminium out without stopping. This can be cured by simply tuning off the current for a second, or by rising the part above the bath surface. Then anodizinfg resumes normally.

What I found out is that the surface of the aluminium is very quickly covered by whitish protective oxide layer after turning the current on. Initial current is high, but quickly falls off and the layer does not seem to grow substantially if at all. That is at 30V voltage! No oxygen bubbles on the anode.

I have to note hat I use only mechanically cleaned and freshly polished alu samples, no chemical cleaning, or HF acid cleaning (or nitric acid). After some time the first oxide will be eaten trough and hard anodised layer will start to form. This happens mostly at the bath-air interface and follows straight down - propapbly some kind of salt created there activates the surface. After that the hard layer spreads to the sides and some aluminium thickness is removed in the process. Anyway, the created layer is not scratchable by a nickel coated metal probe of the multimeter.

Next point of achievement was bringing the cathode closer to the anodized sample and the current went up fivefold to tenfold. I did that on anothers alu sample and the overall thickness went up from 0.3mm to 0.35mm! That means the oxide layer is far thicker than 50 microns! Again, little or NONE oxygen bubbles on the anode.


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