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)

• Hygroscopicity: the capacity of a product (e.g. cargo, packaging material) to react to the moisture content of the air by absorbing or releasing water vapor. Of decisive significance for the absorption or release of water vapor is the water content of a product.
• The water content: the percentage of the total mass of a product constituted by water.
• Sorption behavior: the ability of a hygroscopic product to absorb or release water vapor from or into the air until a state of equilibrium is reached.
• Adsorption: the uptake of water vapor.
• Desorption: the water vapor release.
• A sorption isotherm: the graphic representation of the sorption behavior of a substance. It represents the relationship between the water content of a product and the relative humidity of the ambient air (equilibrium) at a particular temperature.
• Dry for shipment: that a product has a water content which does not cause it to suffer any depreciation under normal weather or hold/container conditions when cared for properly.
• Critical water content: a water content which, if exceeded during transport or storage, may cause the onset of depreciative phenomena such as mold, fermentation, rot, self-heating/spontaneous combustion. If the water content falls below the critical value, desiccation losses may occur, such as fragmentation, drying-out, cracking. For safe transport, it is therefore important for the water content of a product to match the required values on acceptance and for this water content to be maintained in transit through the storage climate conditions in the hold/container.