# Chemical reaction of bronze tarnishing

Can anyone explain to me the chemical process of bronze tarnishing? According to Wikipedia:

Bronzes are typically ductile alloys, considerably less brittle than cast iron. Typically bronze oxidizes only superficially; once a copper oxide (eventually becoming copper carbonate) layer is formed, the underlying metal is protected from further corrosion.

Does this mean the tarnish that we see in the picture below is created by copper oxide turning into copper carbonate? If so, how do you write out that equation? What is being oxidized and what is being reduced?

• I don't see much tarnish in the picture, if any at all. Well, yes, a little bit of CuO, maybe. Nov 11 '20 at 9:37
• $\ce{2 CuO(black) + H2O + CO2 -> Cu2(CO3)(OH)2}$ ( found as green mineral malachite ). It can be seen on the old copper roof plating, once shiny metal, now weathered by green coating. Similar compound $\ce{2 CuO + H2O + 2 CO2 -> Cu2(CO3)2(OH)2}$ is found as blue mineral azurite. Nov 11 '20 at 9:51
• @Poutnik thank you for your reply! I’m a total beginner. What does \$\ce mean? Nov 12 '20 at 11:35
• it invokes mhchem extension for Mathjax. It probably stands for "chemical equation". See MathJax Nov 12 '20 at 12:39
• If you are seeing dollar signs, it means your phone does not know how to format the math and chemistry. Try to look at the question using a computer instead. Nov 20 '20 at 15:07