When Does Osmotic Blistering Occur?
Osmotic blistering occurs when moisture penetrates through the surface of a coating, is trapped beneath the layer of paint or epoxy and creates blisters. It is not an uncommon phenomenon, but the mechanism can be somewhat complex. Generally, blisters form where pressure builds after moisture accumulates at certain locations within a coating film. Blistering is a result of osmosis; non-osmotic bubbling also occurs, but it’s caused by a different mechanism.
Paints and epoxies form semi-permeable membranes that can resist water, but ultimately water might be able to penetrate the film. Blistering is frequently encountered when coatings are applied to carbon steel and other metals that are then submerged or exposed to high humidity. Several forces come into play, including:
- Temperature differences along the length of the coated surface
- Salts that contaminate the substrate
- Coatings in contact with water-soluble solvents that become entrapped in the film
Osmotic blistering can occur when the temperature of an immersed metal substrate is below that of the surrounding liquid. The temperature gradient causes warm water molecules to penetrate the film coating and then condense on contact with the colder substrate. Pressure builds as the water accumulates, reaching a critical value that forms blisters in the coating. One way to slow down or eliminate this source of osmotic blistering is to thermally insulate the water tank or vessel surrounding the coated substrate.
Salts such as nitrates, sulfates and chlorides dissolve in water, but are not apparent unless uncovered by analytic tests. If salted water comes into contact with a metal substrate, a concentration gradient of salt ions develops across the film layer. The resulting osmotic pressure drives water across the semi-permeable membrane and creates a high-pressure accumulation of water molecules. Pressures can exceed 15,000 psi, depending on the salt concentration. Eventually, the pressure overwhelms the adhesive bond between the substrate and coating, causing blistering to commence.
Water-soluble solvents can cause blistering in a manner similar to that of the dissolved salts. The solvents might be more noticeable because some emit an odor, but analysis is needed for positive identification, often through samples of liquid trapped within blister bubbles. Gas chromatography and mass spectroscopy are two common identification methods.
Interestingly, a good-quality coating, such as an aliphatic urethane, might develop osmotic blisters that remain intact because the film has sufficient structural integrity. In those cases, the need for immediate remediation must be carefully assessed, since the coating may continue to function for the remainder of its predicted service life.
In a future article, we’ll examine the other common mechanism of coating failure: bubbling.