Definition - What does Osmotic Blistering mean?
Osmotic blistering is the formation of liquid-filled, dome or spherical-shaped raised areas of coating. An osmotic blister develops if moisture or a low-concentration solution from the environment diffuses or passes through the semi-permeable coating to form a high-concentration solution with the salt or other contamination on the substrate surface.
The variance in concentrations of the liquids on both sides of the semi-permeable coating causes moisture or the low-concentration liquid to continue passing through the semi-permeable coating until equilibrium is achieved. The increasing amount of liquid under the coating pushes it upwards, increasing the size of the osmotic blister.
Some coating materials like polymers are semi-permeable to liquids and gases.
Corrosionpedia explains Osmotic Blistering
When one sees osmotic blistering, poor adhesion of the coating with the substrate resulting from poor surface preparation is usually thought to be the first cause. There are other causes to be considered.
Osmotic blistering is one of the major causes of coating failures. It is therefore important to know its causes or the conditions at which it is most likely to occur, such as any of the following:
Semi-permeable surface coating
Thick polymer and other organic coatings are generally considered as effective barriers to moisture and other liquids. However, all polymers are semi-permeable to liquids and gases, which results in the accumulation of high-concentration solutions that push the coating upwards.
Water dissolved solids (soluble salts, pigments or solvents)
The presence of water soluble solids are due to poor surface preparation before coating or leached out from the coating or substrate itself. Water that passes through the semi-permeable coating forms a high-concentration solution with the solids between the substrate and the inner side of the coating.
The differences in the concentrations of the liquids on both sides of a semi-permeable material will cause the solvent of the less concentrated liquid to pass through the semi- permeable material until equilibrium is attained. The volume of the more concentrated solution increases to push the coating upwards into dome-shaped blisters.
Osmotic blistering can be prevented or reduced by choosing coating material that can resist water or other liquids it will be exposed to, as well as a surface preparation method that will completely remove water soluble solids.