Definition - What does Patina mean?
Corrosionpedia explains Patina
Patina is a form of rust, which can be written as Fe2O3.nH2O. From the chemical point of view, the coatings of the materials such as oxides, sulfides, carbonates, and other elements in the coatings of the materials react with the weathering agents such as acid, oxygen, rain, sulfur-bearing compounds, and carbon dioxides to produce the resulting patina. Accumulated patina on the surface of objects can sometimes cause poor electricity conduction as well as electrical shocks.
The most common form of patina is that which is found on the surface of coins, especially after subjected to the agents of weathering. Archaeologists' studies show that patinas can protect a material from the agents of weathering for up to a century before the object becomes fully decayed. Patina can be seen as a decay-prevention method that materials themselves can implement.