Spots on Aluminum After Alkaline Cleaning

Question:

As of late, I have been experiencing some intermittent spotting on mainly 7000 and 2000 series alloys after immersion in our alkaline cleaner . The company’s lab reports tell me the blackish/whitish spots are actually small particles of silicates and phosphates that are plating onto the aluminum surface. Prior to this lab report, I was thinking I may have a galvanic corrosion issue. As a job shop, we do clean multiple types of aluminum in the same load. I have been asked to dump my bath and it replace with a new product. This fix does little to satisfy me. Our tank solution is circulated through a filter. However, it would not surprise me if there were small particles that circulate through the tank that pass through the filter. If this is the case, I suspect there must be some type of electrical current present that causes the particles to attach themselves to our parts. I do not want to spend money on a replacement product until I am convinced that we have no other problems present. Do you have any suggestions? P.C.

Answer:

It sounds like there is some information missing. I believe that both silicate and phosphates would show up as white residues on your parts. If that were the case, I would suspect that the rinsing processes need to be improved. However, since you indicate that there are black spots present, I wonder if they could be silicon from the base alloy. If the cleaner is not silicated, it is almost guaranteed to etch the aluminum, in which case some of the residual smut would be from silicon present in the alloy.

Replacing the etching cleaner with an inhibited (silicated) cleaner may extend the life of it, although you would have to judge if the cleaning was sufficient for any downstream processing you or your customer is doing with the parts.

Regarding the current bath, it may be possible to switch to a finer filter to collect this residue, although more frequent filter changing would likely be necessary. I also don’t think that electrical current is playing a significant role in collecting these particles on your parts unless the parts are purposely cathodically charged (you did not mention that this was an electrocleaning bath).