Machining of Aluminium

Black stains are typical when aluminum rubs against aluminum.

Is it possible that the parts are stored, transported together, or rub against each other?

"rust" (ok don't kill me dead)

Consider anodising or a protective tissue when staking or moving.

We make all our parts from aluminium and have seen them as well. They also appear after we "work" the part further to check with slip gauges checking tollerances of gaps and rails.

We don't bother removing them as they are not affecting the metal as such. Do you think your spotting is affecting your products or is it an estetic thing? We do clean and if found that it is a bit of swarf rubbing, it has to be removed.

I can assure you that if it is the same spotting, it does not affect any performance of the metal and we work with very tight tolerances. Machining to 0.03mm and final product to work with ball screws to a repeatability of 2 microns. If those spots were a problem we would have picked it up as each of our products has to pass a microscope test rig with optical feedback for repeatability.

Likely cause is active sulfur in the coolant. This could be from the colant additives, or could be from hydrogen sulfide in the water. This is especially possible in untreated well water.

Sulfur can attack the newly machined surface, as it has not yet created its own protective oxide layer at time of contact.

It doesn't attack the not machined surfaces, because they still have a protective oxide film.

Also could be Chlorine in the coolant.

To clean, I'd try this recipe for removing stains from cookware:

Aluminum Cleaner: 2 tablespoons cream of tartar.
1 quart water.

To clean aluminum cookware, combine ingredients in cookware. Bring
solution to a boil and simmer for 10 minutes. Wash and dry as
usual.

milo "I really think its sulfur."

Anything acid, or vinegary( cream tartar has vinegar ), will attack the aluminium surface thus affecting the tolerances you machined to. This could be undesirable, I would check that first.

It is possible that it is sulfur and if so you would need to get rid of it.

It is a job to tell though where it came from without extensive testing of all ingredients. Sounds like you need to systematically eliminate all possibilities.

Good luck.

thanks everyone.

earlier i tried with acetone, kerosene & petrol to remove the stains but still it doesn't vanish.

some chemical industry people advised me to use SodiummetaSilicate. any idea?....