Take the oxygen absorption corrosion (the most common) of steel (mainly composed of iron and carbon) as an example. Iron (Fe) and carbon (C) form a primary battery, in which iron is the negative electrode and undergoes oxidation reaction; The carbon electrode is the positive electrode, and the reduction reaction takes place. Electrolyte solution is generally an aqueous solution of some salts (such as sodium chloride).

Negative electrode: Fe-2e- = Fe2+

Positive electrode: O2+4e-+2H2O = 4OH-

Total: 2Fe+O2+2H2O = 2Fe(OH)2

According to the condition of the primary battery

Protection methods are: 1. Separate metal from electrolyte solution (painting, baking blue, etc.). 2. Sacrifice active metals such as galvanized iron pipes, so that zinc will be corroded first and iron will not be corroded. 3. The external power supply method says that the metal to be protected is connected to the positive pole of the power supply (the oxidation reaction is to lose electrons, and the lost electrons are given by the power supply, so that the metal will not lose electrons and will not be corroded).

exceed ....

Then the reaction 4Fe(OH)2+O2+2h2o = 4Fe(OH)3 occurs.