How does copper’s patina protect the metal underneath?

When copper is exposed to the atmosphere over long periods, it undergoes a chemical reaction that results in the formation of a thin, green surface coating known as a patina. This patina is a mixture of compounds, including hydroxocarbonate and hydroxosulfate.1

Unlike the porous rust that forms on iron—which promotes further degradation—the patina that develops on copper is a compact, adherent layer. This stable film acts as a protective barrier that limits further environmental attack on the underlying metal, effectively shielding it from continued corrosion.2