What Does Permeation Mean?
Permeation is the rate of molecular diffusion of gases, vapor and fluids through a solid material membrane. It is inversely proportional to the subsurface concentration. It is affected by the membrane thickness, increases with temperature, and is independent of pressure. It is used in the application of gas separation. It is used in petroleum and petrochemical industries to determine the effects of hydrogen in high-strength steel.
Corrosionpedia Explains Permeation
In steel, hydrogen permeation is associated with surface reaction, corrosion, electrochemical processes or cathodic protection. The accumulation of hydrogen in the bulk of an alloy, areas of high stress or trapping sites induce cracks, which in turn compromises the mechanical integrity of the structure. But permeation depends on the atmospheric environment.
The permeation flux for a thin membrane is constant and governed only by the charging flux crossing the entry face. On the other hand, in thick membranes, the diffusion flux is inversely proportional to the subsurface concentration. Permeation in metals causes embrittlement and blistering. The permeation through coatings is a result of the diffusion and solubility of water or another fluid that can trigger corrosion of the substrate.
Permeation in metals is measured by using sensors to develop permeation current density in conjunction with the weight loss or gain of the substrate through a wet-dry cycle experiment. The permeability coefficient (P) is directly proportional to the concentration of diffusing substances (C) and inversely proportional to the solubility coefficient (S).