Osmotic blistering is a chemical phenomenon where two substances attempt to reach equilibrium through a semi-permeable membrane.
For steel substrates: The presence of soluble salts (particularly sulfates and chlorides) at the metal/paint interface is known to have a detrimental effect on the integrity of most paint systems including fluorocoatings.
When a paint coating is applied on a metallic surface contaminated with soluble salts, an osmotic blistering process takes place (Figure 8.10).
Osmosis is the spontaneous net movement of solvent molecules (water) through a semipermeable membrane (coating film) into a region of higher solute concentration (the salt contaminated substrate).
As the soluble substance dissolves under the paint layer, the pressure caused by the increase in volume can exert a greater force than the paint adhesion and cohesion forces, giving rise to the formation of a blister; the process called osmotic blistering.