1 Relative humidity of the atmosphere
Absolute humidity: 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 a water film is formed on the metal and the thickness and holding time of the water film. 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%.
2 Temperature
Generally, the speed of the reaction of chemical reactions increases when the temperature rises. The interaction between metal and oxygen and moisture in humid air reaches above the critical temperature, and the temperature change causes condensation on the metal surface and produces corrosion. When the relative humidity is lower than the critical humidity of 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 is accelerated.
3 Effects of oxygen
Without oxygen, atmospheric corrosion of metals does not occur, and in some special cases, the concentration of oxygen on the surface of metals is different, which will produce a special form of difference in inflatable batteries. For example, on the overlapping surface, that is, when the metal surface is in close contact with another surface, there is no rust at the edge of the contact surface, but there will be cloud-like shadows or rust at the edge. On steel, this rust product is often gray or black (iron oxide).
4 Effects of pollutants in the atmosphere
In addition to oxygen and water vapor, the atmosphere also contains a variety of 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 the water to produce the following results: various non-metallic oxides dissolve in water to become acids, which will destroy the passivation film on the metal surface; Various electrolytes increase the conductivity of the water film; 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%, and in the humid atmosphere, a SO2 molecule can turn dozens of iron atoms into oxides).
