Dual Purpose Anodizing Bath


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When using a sulfuric acid-oxalic acid electrolyte for Type III hardcoat, is it possible to raise the temperature in order to do Type II anodizing in the same tank? M.J.


Yes, but it's not quite that simple. There are come caveats. Here are some other things to think about in this regard:

  1. The success of a dual-purpose bath depends primarily on the bath chemistry. If using the standard Alcoa “Alumilite 225/226” bath (12% H2SO4 + 1% oxalic acid [H2C2O4•2H20]), you will probably be using a slightly higher voltage for Type II than you would in, say, a straight 15-20% anodize bath. Or, for the same voltage used in a higher concentration bath, you will probably have to process the loads a bit longer in the Alumilite 225/226 bath.
  2. Another thing to consider is the temperature. Process voltage is inversely proportional to the bath temperature, given the same bath chemistry. As bath temperature increases, the voltage required to produce a coating of given thickness in the same amount of time decreases. This means that you could possibly do your Type II anodizing at up to 85F (30C) in the “Alumilite” bath without experiencing burning or soft coating. Remember also, if you are dying the parts after Type II anodizing, the coating morphology must be such that the coating will absorb the amount of dyestuff required to obtain a reasonable degree of color intensity and perhaps colorfastness. A variation of higher temperature anodizing would be to raise the concentration of the bath for both Type II and Type III anodizing to as much as 15-20% H2SO4 and 3% oxalic acid (165 to 225 g/liter H2SO4 and 10 to 40 g/liter oxalic). This would give you better dyeability for lower voltage and the same or less anodizing time.
  3. A similar approach to the anodizing chemistry would be to use the Reynolds MAE (Multipurpose Anodizing Electrolyte) bath. This is a bath of 15-25% H2SO4 plus a mixture of glycerin and glycolic acid totaling 3% of the bath by volume. This bath can give as good as, if not better, results than the Alumilite 225/226 chemistry, and for a much wider range of anodizing types. It also allows a wide range of anodizing temperatures (40-85F). It is good, but not foolproof, for anodizing 2024 alloy and can be used for hardcoat, architectural and decorative anodizing. Hence the name “multipurpose electrolyte”.

I hope this is not too confusing, but you can see that the possibilities and variations are many. Feel free to contact me if you need more information.


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