Black spots on aluminum

ColinCMR

Are the black spots that appear on aluminum toning or are they most likely carbon spots?

thanks
-Colin

oldshep

Depending on the aluminum alloy used it could be magnesium or manganese, I believe.
Shep

Askari

What Oldshep said. Aluminum always comes in an alloy for coin use and the impurities sometimes cluster. I've also seen a tar-like substance on aluminum and other coins, but I don't have a clue as to how it gets there.

lathmach

As the others have said, it's because of the alloy.
7075 Alloy Aluminum has so much copper in it, it gets a green corrosion. I've seen the black corosion show up more often on 2024 alloy aluminum.
I own a machine shop and machine a lot of aluminum.

Ray

Askari

Thanks for the info!! The amazing questions you can get answers to here!!

So what's the difference between the 2024 and 7075 that the former gets black corrosion?

ColinCMR

Definitely good info, thanks for responding.

I'm also curious about these alloy numbers, some type of ratio?

Askari

Nope, they're "blend" numbers that identify specific commonly used alloys.