Critical Humidity

Last updated: February 14, 2020

What Does Critical Humidity Mean?

Critical humidity is the humidity of a system's atmosphere above which a crystal of a water-soluble salt always absorbs moisture from the atmosphere and below which it always releases moisture into the atmosphere.

In metallurgy, it is the atmospheric humidity above which the corrosion rate increases rapidly for a particular metal. Critical humidity has an effect on atmospheric corrosion, especially damp corrosion.

Critical humidity level is a variable that depends on:

  • Nature of the corroding material
  • Presence of atmospheric pollutants
  • Tendency of corrosion products and surface deposits to absorb moisture

Corrosionpedia Explains Critical Humidity

Critical humidity is the relative humidity above which the atmospheric corrosion rate of some metals increases sharply. It is the humidity level at which water does not evaporate or get absorbed from the atmosphere due to equilibrium between the water-vapor pressure of the substance and the partial pressure of the water vapor in the atmosphere.

A fundamental requirement for the atmospheric corrosion processes is the presence of a thin-film electrolyte that can form on metallic surfaces when exposed to a critical level of humidity. While this film is almost invisible, the corrosive contaminants it contains are known to reach relatively high concentrations, especially under conditions of alternate wetting and drying.

Damp moisture films are created at a certain critical humidity level (largely by the adsorption of water molecules), while wet films are associated with dew, ocean spray, rainwater and other forms of water splashing. This moisture film is extremely important from the point of view of the chemical mechanisms of the corrosion process. Apart from the primary critical humidity (associated with clean surfaces), secondary and even tertiary critical humidity levels may be observed where the corrosion rate increases abruptly. Hygroscopic corrosion products and capillary condensation of moisture in corrosion products are thought to account for these effects.


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