1 Atmospheric relative humidity

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 air saturated water vapor content at the same temperature, expressed as a percentage.

The relative humidity determines whether the thickness of the water film and the water film are formed on the metal and the time of retention. The higher the relative humidity,

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

2 temperature

In general, the rate of reaction increases as the temperature increases. The action of metal and oxygen and moisture in humid air reaches above the critical temperature, causing condensation and corrosion on the metal surface due to temperature changes. When the relative humidity is lower than the critical temperature of the metal, the effect of temperature on atmospheric corrosion is small; when the relative humidity reaches the critical temperature of the metal, the influence of temperature is obvious, the temperature increases, and the reaction speed increases.

3 Oxygen effects

Without oxygen, atmospheric corrosion of metals does not occur. In some special cases, the concentration of oxygen on the metal surface is different, resulting in a special form of differentially charged batteries. For example, on the overlapping surface, that is, when the metal surface is in close contact with the other surface, the edge of the contact surface is not rusted, but there is cloud-like shadow or rust at the edge. This rust product is often gray or black (iron oxide) on steel.

4 Effects of pollutants in the atmosphere

In addition to oxygen and water vapor, the atmosphere contains a variety of pollutants. Such as carbon dioxide, nitrogen oxides, carbon dioxide and other gases and solid dust. These contaminants will condense on the metal surface together with the water mist in the air. Dissolving in water produces the following results: various non-metal oxides dissolve in water to become acid, which will destroy the passivation film on the metal surface; various electrolytes increase the water film. 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 make dozens of iron atoms into oxides).