Corrosion of metals can be classified as either corrosion in aqueous solutions or as direct oxidation at high temperatures. Both are electrochemical in nature.

Aqueous Corrosion

An aqueous corrosion cell consists of an anode where metal ions go into solution and electrons are produced, a cathode where electrons are consumed, an aqueous solution between the anode and cathode, and an external electrical connection between the anode and cathode (Figure 23.1). The anode reaction can be written M → Mn+ + ne−, where M stands for a metal. There are several possible cathode reactions:

1. Mn+ + ne− → M (This can occur only if there is a high concentration of Mn+ ions in solution.)

2. 2H+ + 2e− → H2 (This can occur only in an acid solution.)

3. O2 + 2H2O + 4e− → 4(OH)− (This is the most common cathode reaction. Note that it requires dissolved O2.)

4. O2+ 4H+ + 4e− → 2H2O (This occurs in acidic solutions and there must be O2 in an acid solution.)

The anode and cathode reactions must occur at the same rate. The corrosion rate is often limited by the cathode reaction.

The material of the anode is more active (less noble) than the material of the cathode. The electromotive series (Table 23.1) lists the relative activity of common metals in one-molar solutions of their own salts. The most noble (least reactive) metals appear at the top and most reactive at the bottom.