Selective Smut On Sealed Parts

Question:

We are producing anodized, electrolytically colored aluminum solar panels. We anodize for 40 min at 18 v to produce a coating thickness of 15–18 microns. We color with stannous sulfate for 10 min and seal in hot DI water with an ammonium acetate buffer for 12–15 min at pH 5.9. After sealing, some of the parts on each load generally are smutty looking. Roughly 25% of the parts are smutty with the rest being good. We have dumped our sealing bath and made up a new one, but the problem still persists. What do you think is the cause of this smut and what can we do about it? A.O.

Answer:

If some of your parts come out of the seal tank with smut and some do not (on the same load) then it is probably not the seal tank that is causing the problem. If it was the chemistry of the seal, all parts would be the same. The bath does not discriminate. You are using plain DI water with ammonium acetate as a pH buffer. Have you considered using a mid temperature nickel acetate seal? It will give a better seal and these proprietary sealing products contain pH buffers and a small amount of surfactants to help reduce smut, as well. Even though I don’t think there is anything wrong with your existing sealing bath, changing the bath to the mid temp NiAc may help.

Chances are the real problem is with the metal. This is usually the case when parts come out good and bad on the same load. It most likely has to do with the temper of the parts. The smutty parts have a temper problem or at least a different temper, which could be the responsibility of the metal producer. Is it possible that you have more than one metal source? Can you have the hardness of the metal tested? The best way to check for temper differences is to check the color of the smut as each load comes out of the caustic etch tank and also after the first caustic rinse. If the smut is different colors, then you have a temper inconsistency. Talk to your metal supplier.

Slight temper differences can sometimes be overcome by desmutting the parts better after caustic etching. Longer in the existing desmutting bath may do the job or you may have to change to a more aggressive desmut bath. What is being used for desmutting now? Usually, if the parts come out of the seal with smut and it is not the fault of the seal, it means the desmutting is inadequate for one reason or another (may be related to above stated temper differences). Probably the least expensive way to approach this is to increase the concentration and/or the temperature of the desmutting bath to see if this eliminates all of the smut. I would want to know what the desmutting bath chemistry is before I recommend any changes in actual chemistry. If you can’t get the existing bath to work, you may want to change to a desmutting product that is more effective and/or discuss the possible metal problem with your metal supplier. If you make processing changes, remember to make only one change at a time.