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Equation:

A primary cell is formed and oxygen absorption corrosion occurs:

Negative electrode (Fe) : 2Fe - 4e⁻ = 2Fe²⁺

Positive electrode (C) : O₂ + 4e⁻ + 4H₂O = 4OH⁻

This is a relatively typical example in electrochemical corrosion. When steel is exposed to a humid environment, a thin layer of water film forms on its surface. This water film dissolves gases such as carbon dioxide, sulfur dioxide, and hydrogen sulfide from the atmosphere, causing it to contain a certain amount of hydrogen ions. As a result, a thin film of electrolyte solution is formed on the surface of the steel, which, together with the iron and a small amount of carbon in the steel, forms an original battery.

Because hydrogen gas is constantly released during the corrosion process, it is called hydrogen evolution corrosion, which is a type of corrosion caused in an acidic environment. If the water film adsorbed on the surface of steel is weakly acidic or neutral but contains a certain amount of dissolved oxygen, oxygen absorption corrosion will occur at this time.

Electrochemical corrosion occurs when a metal and an electrolyte form two electrodes, creating a corrosion galvanic cell. For instance, iron and oxygen. Since the electrode potential of iron is always lower than that of oxygen, iron is the negative electrode and gets corroded. The characteristic is that on the surface where oxygen corrosion occurs, many small bulges of different diameters will form, followed by black powder-like ulcerative corrosion pits.