What does aluminium ingot blacken because of temperature? Rain? Aluminium oxide? Iron salt?
Oxidized by oxygen; alumina
The humidity is contacted with the air and is oxidized. The aluminum ingot also contains trace amounts of other metal elements, which are also oxidized to black
Is the oxidation of alumina. The color is not necessarily to generate the first white alumina, then gray green, finally is black. Because there is water vapor. Laboratory control conditions, can certainly get white alumina. In reality are black.