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Hygroscopic Corrosion

A material which gains moisture from the atmosphere as the relative humidity increases is said to be hygroscopic. The more hygroscopic a material is, the more moisture it will pick up during periods of high humidity.

The moisture content of a surface contaminant can be defined as the percentage weight of water in relation to the dry weight of the surface contaminant. Surface contaminants in which moisture can be present can be classified in two categories: hygroscopic and non hygroscopic. Examples of hygroscopic materials are salts, vegetal fibers, most metal oxides, many polymers, etc. Examples of non hygroscopic surface contaminants are metal powders, glass granules, etc.

Regarding the moisture content of a surface contaminant, the static equilibrium is defined as a set of conditions under which the surface contaminant does not exchange any moisture with its environment. Under conditions of static equilibrium, the moisture content of a hygroscopic surface contaminant depends on the nature of the surface contaminant and also on the two following factors:

  1. The partial pressure of water vapor in the immediate environment of the surface contaminant

  2. The temperature of the surface contaminant

If the moisture content of a surface contaminant is not dependent on both these factors, then the surface contaminant is not hygroscopic.

Hygroscopic surface contaminants may absorb water in different ways: sorption with formation of a hydrate, binding by surface energy, diffusion of water molecules in the material structure, capillary condensation, formation of a solution, etc. Depending on the absorption process, water is bound to the surface contaminant with more or less strength. Moisture content can include both an immobilized part (e.g. water of hydration) and an active part.

Some definitions: (reference)