Copper
How patina is formed on copper?
Firstly, the copper surface oxidizes in air, forming a brown layer of copper oxide. Then, under the action of carbonic acid contained in the air, a water-insoluble copper carbonate is formed, that is, a patina of green color.
Why is there no copper-protecting patina in air polluted by industry??
As a result of the action of air containing sulfur compounds in industrial areas instead of water-insoluble copper carbonates (patina) water-soluble black copper sulfates are formed. Therefore, sulfates do not constitute a protective layer for copper.
Why should the patina be removed from the copper before coating??
The patina on copper is a substrate with insufficient adhesion for the coating.