Question

How do acids corrode copper?

Answer 1

Acid does not corrode copper. Here's what causes the corrosion.

Explanation:

Copper corrodes by two mechanisms:

• oxidation
• galvanic action

Oxidation

Initially, copper atoms react with air to form pink copper(I) oxide (`"Cu"_2"O"`).

This gradually oxidizes further to form black copper(II) oxide (`"CuO"`).

In the presence of moisture, the blackish layer slowly reacts with `"SO"_2` and `"CO"_2` from the air to eventually form a blue-green layer that is a mixture of 3 minerals:

• a green, hydrated copper carbonate, `"Cu"_2"CO"_3("OH")_2`
• a blue, hydrated copper carbonate ,`"Cu"_3("CO"_3)_2("OH")_2`
• a green, hydrated copper sulfate, `"Cu"_4"SO"_4("OH")_6`

The Statue of Liberty has changed its appearance since it was new.

Galvanic action

Copper pipes in homes and in the ground usually corrode by galvanic action.

(From www.nachi.org)

This copper pipe suspended by an iron strap in a moist environment has slowly corroded over the years.

Copper pipe buried in acid soils usually corrodes by galvanic action.

Differences in oxygen concentration along the length of the pipe can form concentration cells, causing oxidation of the pipe.

`1×["O"_2 + 2"H"_2"O" + 4"e"^"–" → 4"OH"^"-"]`
`2×["Cu" → "Cu"^"2+" + "2e"^"-"]`
`"2Cu + O"_2 + 2"H"_2"O" → "2Cu"^"2+" + 4"OH"^"-"`