1.Relative humidity of the atmosphere

Absolute humidity: the weight of water vapor per unit volume of air (g/m3)

Relative humidity: The ratio of the water vapor content in the air to the saturated water vapor content of the air at the same temperature, expressed as a percentage.

The relative humidity determines whether or not a water film is formed on the metal, and the thickness of the water film and the retention time. The higher the relative humidity,

The greater the humidity in the air, the faster the metal corrosion. The critical relative humidity of steel is 70%.


Generally, the speed of chemical reactions increases when the temperature rises. The metal interacts with the oxygen and moisture in the moist air to reach above the critical temperature, and the temperature changes cause condensation on the metal surface, resulting in corrosion. When the relative humidity is lower than the critical humidity of the metal, the influence of temperature on atmospheric corrosion is small; when the relative humidity reaches the critical humidity of the metal, the influence of temperature is very obvious, the temperature increases, and the reaction speed increases.

3.The influence of oxygen

Without oxygen, atmospheric corrosion of metals will not occur. In some special cases, the concentration of oxygen on the metal surface will be different, which will produce a special form of differential gas battery. For example: on the overlapping surface, that is, when the metal surface is in close contact with another surface, the edge of the contact surface will not rust, but there will be cloud-like shadows or rust away from the edge. The corrosion products on steel are often gray or black (iron oxide).

4.The impact of pollutants in the atmosphere

In addition to oxygen and water vapor, the atmosphere also contains various pollutants. Such as carbon dioxide, nitrogen oxides, carbon dioxide and other gases and solid dust. These pollutants will condense on the metal surface together with the water mist in the air, and dissolve in water to produce the following results: various non-metal oxides dissolve in water and become acids, which will destroy the passivation film on the metal surface; various electrolytes increase the water film The conductivity; reduce the critical relative humidity of the metal. (For example, when the atmosphere contains 0.01% SO2, the critical humidity can be reduced from 70% to 50%. In a humid atmosphere, one SO2 molecule can turn dozens of iron atoms into oxides).