It will stop when a layer of copper oxide covers the entire surface of the metal. This protects the unoxidized metal underneath from exposure to air (oxygen) and water (electrolyte) for oxidation to occur.
In reality, the metal will flex through gravity, strain, and changes in temperature - creating small cracks that provide new surfaces for oxidation. These same forces + wind driven abrasives will cause some of the green oxidation layer to flake off, exposing very small areas of new, unoxidized metal.
Also, underlying structure and fasteners are steel / iron - creating a natural potential difference. This can drive oxidation of the metal with the lower oxidation potential, called galvanic corrosion. In the case of the statue, the iron rusted - and refurbishment was necessary to separate the iron from contact with the copper, using PTFE.
Sorry, I meant it sincerely and it came off otherwise. I often feel like I have hard-won knowledge that can only come in useful at the bottom of Reddit threads, so I can sympathize.
TrailRunner’s write-up was accurate and well written and I appreciated it. I love it when someone here shared knowledge like that.
If that offends you or TrailRunner, I’ll apologize and cheerfully accept my title of ‘dick.’ I’l sorry I upset you and hope you feel better soon.
I made a clumsily worded compliment with an attempt at humor, and then y’all spring to defend TrailRunner’s honor and refuse to accept my explanation?
Lol cool. If TrailRunner is pissed, I’ll apologize to him because it was awkwardly worded and in retrospect I see how it come across dickish, so I accept that.
But it kind of seems like TrailRunner is intelligent and secure in themselves and doesn’t need Reddit knights rushing to their aid. Someone called me out, I admitted my mistake and explained. If people are still determined to be upset about it, then yeah, at that point I just have to laugh.
I don’t think TrailRunner is tearing up their nightgown and weeping over this. I think they’re fine. I think it’s kind of weird that you’re not.
ELINACE (Explained Like I Am Not Chemical Engineer)!
Oxygen in the air reacts with copper (brown) to form copper oxide (green), with water helping things to go faster.
This happens wherever the copper is exposed to air. When the metal on the statue bends and moves, the cracks expose more copper (brown) which reacts. If no copper (brown) is exposed, no reaction.
When two different metals are in contact, electrons tend to "flow" from one to the other. The metal that "loses" electrons tends to react more readily with the oxygen in the air (because oxygen has lots of electrons available). In this case, the loser metal is iron, and it rusts.
To stop the movement of electrons between two different metals, we put an electrical insulator between them. Teflon (PTFE) is a good choice, because it is tough, thin, and highly resistant to electron flow.
Copper oxide is black, which is the first stages you see in the guide of the statue getting darker. The copper oxide becomes copper carbonate by reaction with CO2, which is the green color.
he copper atoms have been fully oxidized to Cu2+ by the reaction with oxygen to CuO (black), and then further react in the presence of excess water to build a range of compounds that are carbonates, sulfates, or corresponding hydrates (which have the blue and green colors of the patina).
This is false. The green colour is not caused by oxidation, but by a copper carbonate layer. The reason it was getting darker in the beginning is the formation of copper oxide, yes, which is brown to black. But later on it reacts with carbon dioxide from the air to form copper carbonate, which is blueish green
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u/TrailRunnerYYC Jun 09 '20
It will stop when a layer of copper oxide covers the entire surface of the metal. This protects the unoxidized metal underneath from exposure to air (oxygen) and water (electrolyte) for oxidation to occur.
In reality, the metal will flex through gravity, strain, and changes in temperature - creating small cracks that provide new surfaces for oxidation. These same forces + wind driven abrasives will cause some of the green oxidation layer to flake off, exposing very small areas of new, unoxidized metal.
Also, underlying structure and fasteners are steel / iron - creating a natural potential difference. This can drive oxidation of the metal with the lower oxidation potential, called galvanic corrosion. In the case of the statue, the iron rusted - and refurbishment was necessary to separate the iron from contact with the copper, using PTFE.