Dark Gray Spots in Clear Anodizing

Question:

I am having a problem with surface imperfections in the anodizing on a new product of ours which we machine from 7075.  The imperfections look like small dark gray spots in the clear anodizing.  Under a microscope they look like corroded areas which have actually eaten away the machine marks in spots.  Any advice would be greatly appreciated. R.Z.

Answer:

It’s hard to say exactly what caused the spots on your 7075 parts, but there are most likely two possibilities.  It could be “metallurgical blistering” which sometimes occurs on 7075.  I’ve written about this in the past so I won’t take time to explain again how this happens. (See the August, 2000 issue of Products Finishing).  Perhaps you’ve read what I and others have written about this.  It’s a metallurgical problem which you can’t do anything about except to try more parts, usually a different mill batch gives the best chance for successfully eliminating this condition.

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The other likely candidate is electrolytic corrosion which may be occurring somewhere in the process.  This can be caused by “stray electrical currents” in the anodizing line and it can occur in pretty much any tank.  This condition can be eliminated, albeit sometimes with great difficulty.  Here are some things to look for.  If the anodizing line has only plastic tanks, the probability of this condition occurring is minimized.  If tanks are metal, the load (bar or rack with parts on it) must always be insulated electrically from each tank.  Also, each tank must be grounded—to a water line is best, but it could be a ground rod or a building column, too.  A metal water line is best.  If there is an overhead hoist there must be an electrical isolation “device” somewhere between the part of the hoist that is “connected” to the building and the load of parts.

If the parts are dyed (especially black) there could be an electrolytic condition in the dye tank or excessive solution contamination in the dye, especially black dye.

Thoroughly inspect the load of parts after each tank as it is being processed.  This will enable you to find the place where the condition is occurring.  This is very important.  Probably a good indication of metallurgical blistering is if you see the condition only when the parts come out of the anodizing tank.

Let me know what happens, please.  Good luck.