20 Cents Belgian Congo [решено]

This coin is listed as Copper-Nickel.

My coin just lost the Nickel and left visible the copper, right?

Over time, after exposure to the weather, copper-nickel gradually turns black.

Цитата: "Cerulean"​Over time, after exposure to the weather, copper-nickel gradually turns black.

​That is it, it is NOT possible to 'unmix' the Cu-Ni alloy. Maybe in a laboratory or something, but not under normal circumstances.

Where does he state something of unmixing?
The color mostly comes from copper-oxides.

Here and article about Cu-Ni alloys and corrosion (seawater is just faster than sweaty hands and land environment, or being in he ground) LINK

Цитата: "Idolenz"​Where does he state something of unmixing?
​The color mostly comes from copper-oxides.
​
​Here and article about Cu-Ni alloys and corrosion (seawater is just faster than sweaty hands and land environment, or being in he ground) LINK

​He asked if his coin lost the nickel-part and if the copper is remaining, this implicates 'unmixing' or not? I know it is poor english what I use, but I guess the intention is clear.

Цитата: "yvon"​

Цитата: "Idolenz"​Where does he state something of unmixing?
​​The color mostly comes from copper-oxides.
​​
​​Here and article about Cu-Ni alloys and corrosion (seawater is just faster than sweaty hands and land environment, or being in he ground) LINK

​​He asked if his coin lost the nickel-part and if the copper is remaining, this implicates 'unmixing' or not? I know it is poor english what I use, but I guess the intention is clear.

Most copper-nickel coins are a homogeneous alloy, but some (USA, Thailand, South Africa) are made in layers. I think the OP was considering the omission or removal of such a layer, which almost never happens.