Question

What is the chemical formula for corrosion in copper and silver?

Answer 1

Corrosion

• Corrosion is a natural process that converts a refined metal to a more chemically stable state, such as oxide, hydroxide, or sulfide.
• For example, when iron rusts, it loses its toughness, and the rusty iron crumbles readily.

Corrosion in copper

• When copper metal is exposed to the environment, it combines with oxygen in the air to generate copper (I) oxide $\left({\mathrm{Cu}}_2\mathrm{O}\right)$, which appears red.
• The reaction is as follows:

$2\mathrm{Cu}\left(\mathrm{s}\right)+\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)$

• ${\mathrm{Cu}}_2\mathrm{O}$ is oxidized further to produce black-colored copper (II) oxide $\left(\mathrm{CuO}\right)$.
• The reaction is as follows:

${\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)+\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{CuO}\left(\mathrm{s}\right)$

• $\mathrm{CuO}$ interacts with carbon dioxide $\left({\mathrm{CO}}_2\right)$, sulfur trioxide $\left({\mathrm{SO}}_3\right)$and water $\left({\mathrm{H}}_2\mathrm{O}\right)$ in the environment to generate blue ${\mathrm{Cu}}_2{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)$ (Malachite) and green ${\mathrm{Cu}}_4{\mathrm{SO}}_4{\left(\mathrm{OH}\right)}_6\left(\mathrm{s}\right)$ (Brochantite).
• This is why copper is turning bluish-green in color.

Corrosion in silver

• Silver becomes silver sulfide $\left({\mathrm{Ag}}_2\mathrm{S}\right)$ when it reacts with sulfur and sulfur compounds in the air.
• Exposed silver interacts with Hydrogen Sulfide $\left({\mathrm{H}}_2\mathrm{S}\right)$ in the atmosphere to generate ${\mathrm{Ag}}_2\mathrm{S}$ with the hydrogen gas $\left({\mathrm{H}}_2\right)$.
• The reaction is as follows:

$2\mathrm{Ag}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{Ag}}_2\mathrm{S}\left(\mathrm{s}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$