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Last updated: May 31, 2018

What Does Wetness Mean?

Wetness refers to the percentage of water in steam. It is also used to describe the presence of a water film on heating surface interiors. Wetness can result from ingression of moisture from rain, fog, dew, mist and water, and it can cause corrosion, specifically atmospheric corrosion.

The concept of wetness (time of wetness) is extensively used in atmospheric corrosion disciplines for corrosion prognostics and environmental corrosivity classification. A parallel concept is leaf wetness, which used in environmental science.


Corrosionpedia Explains Wetness

The period required for the formation of a surface layer of moisture on a metal or alloy in favorable atmospheric conditions is known as time of wetness (TOW). The TOW of a corroding surface is a key parameter, directly determining the duration of the electrochemical corrosion processes. This moisture film is extremely important from the point of view of the chemical mechanisms of the corrosion process. Therefore, wetness duration is important in corrosion rate.

For example, atmospheric corrosion of zinc is roughly proportional to the time of wetness in a particular location, providing that the nature and quantity of environmental pollutants does not change. Increasing temperature usually reduces the time of wetness, lowering the corrosion rate.

Wetness can also cause surface corrosion on galvanization when steel sheets become wet while tightly bundled (in coils, or in lifts of blanked sheets/roll-formed panels), and then not immediately separated and allowed to dry. The continual wetness prevents the formation of a protective passive film on the zinc surface. The result is a stained, discolored sheet for which it is virtually impossible to return to its original shiny metallic appearance.

Wetness also causes corrosion of metals (such as fasteners) embedded in timber when the timber moisture content exceeds 17%. The increased time of wetness is a major contributor to the accelerated corrosion of metallic coated steel when placed in contact with wet timber.


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