Clear Anodized 6061 -- Black Pitting in Bore

A. Good morning Lee

Yes I have had that same experience. I would first like to say that a picture might be of some help. It is not an uncommon defect so to speak to find these little black specks in a bore which is a larger hole or even smaller holes. I have seen at least two reasons for this The first, that black could be tiny particles of aluminun (chips) that get embedded into the wall of the bore during the drilling process. The don't really pick up any anodize but rather are there just to be a nuisance. You may accasionally see the same result when an actual part loses contact during the anodic process. It comes out looking black. Stripping and reprocessing is not generally of any help in this case because to try to etch until the particle is gone would end up rendering that part useless as you would blow up the dimensions.

The second cause is pitting or porosity in the bore itself. Aluminum many times looks nice and pretty on the outside but depending on the manufacturing process, the machining process exposes what is lurking beneath the surface. Stray sulfuric gets lodged in the tiny pits and starts to do its thing. One reason is because it does not always rinse out as cleanly as you would like. A help in identifying which is the culprit would be a strong magnifying glass, a lupa which is a jeweler's eye piece or a microscope. ( the lupa and myself were on a first name basis. It became a great friend to me.)

Unfortunately there is not much you can do about it. Everybody always wants to blame the anodizer unfortunately but that is not always the case.