Water-immersion conditions are found in a variety of industrial applications. Circulating water systems, tanks and other critical infrastructure made of metallic substrates are typically protected from corrosion by immersion-grade coatings designed to act as barriers to prevent corrosion. For asset owners and maintenance staff, the ability of coatings to provide continued protection from corrosion or erosion is crucial to their industrial processes, as a coating failure can lead to a host of issues, up to and including forced shutdowns.
Broadly speaking, industrial coatings used in water-immersion conditions fail due to the following: Osmotic Blistering, Microbially Induced Corrosion, and Poor Surface Preparation/ Application/Specification. In this series of blogs, we will explore each of these categories, detailing how each of these impacts the protective coatings, how these issues can be avoided and inspection techniques to identify these problems.
Causes of Osmotic Blistering
Immersion-grade coatings are generally organic polymers with functional inorganic fillers and pigments. A reasonable assumption is that an immersion-grade coating is designed to stop water from permeating through it, thus preventing moisture from reaching the metallic substrate and allowing corrosion to set in. Unfortunately, all commonly used organic polymers have some degree of water permeability. This means they are all semi-permeable and over time, the naturally occurring force of osmosis will ensure that moisture will reach the substrate and, being unable to permeate through the non-permeable metallic substrate, cause a blister to form in the coating.
Chlorides or Other Ions on the Metal Surface
It is a common misconception that blistering is caused by a failure of the coating to adhere to the substrate. In truth, osmotic blistering can most often be traced back to some factor that accelerates the rate of permeation through the coating. The most common accelerating factor is the presence of chlorides or other ions on the metal surface although high purity water and elevated temperature service are also common factors. The removal of existing chlorides and/or other ions is critical in reducing the driving force of osmosis. While chlorides are the most common, sulfates, sulfites and iron oxide can also embed themselves in the surface of the substrate. Testing for chlorides and other common ions, with strict limits as well as cleaning procedures for high ion areas should be a part of all immersion-grade coating specifications, even for new assets being coated.
Water Purity
Water purity can also increase the risk of osmotic blistering in immersion conditions. Areas that have water with very low ion levels, or even areas with extremely clean, fresh water will experience accelerated water permeation rates. Coatings containing condensate or deionized water are especially prone to blistering in immersion conditions. Prevention, or delaying, of osmotic blistering can be accomplished by using extremely permeation-resistant systems or increasing the overall film thickness of the coatings. Consider that even glass-flake filled epoxies and vinyl esters, which are among the more permeation-resistant industrial coatings, are generally installed at 40 mils minimum thickness to provide a long-term solution in deionized water service.
Cold Wall Effect
The temperature of the water is also important to consider. When the temperature inside an immersive environment is above ambient and the pipeline or tank is not insulated, something known as the Cold Wall Effect will occur. Moisture will migrate towards areas of lower temperatures, and so, when the temperature of the water inside the pipeline or tank is higher than the temperature of the substrate on the other side of the coating, condensation will occur and build up between the substrate and the coating, building up osmotic pressure and increasing the rate of osmotic blistering. The greater the difference in temperature, the greater the risk factor. A difference of just 30°C can increase the permeation rate through a coating by a factor of 10. Blistering due to the cold wall effect is most common on the divider plate in condensers, the outlet water boxes and the warm side of circulating water lines. Similar to high purity water, a standard immersion-grade coating will not withstand Cold Wall service conditions, making extremely permeation-resistant coating systems required for this service. For reference, common coating systems for divider plates are generally applied at over 125 mils in total thickness to provide a long-term solution.
Initial stages of osmotic blistering forms on thin spots in the coating in only three years of service in the circulating water lines at a gas-fired power generation facility
Severe blistering on an immersion-grade coating in a waterbox at a gas-fired power generation facility
Widespread blistering in a structural element within a cooling tower. Corrosion is clearly occurring behind the coating system and weeping out through cracks in the blisters.
Key to Limiting Corrosion Associated with Osmotic Blistering
The key to preventing asset failures associated with osmotic blistering is a routine inspection program for all your water-immersion coatings. The inspection should focus on the highest risk areas like those that have any of the accelerating factors listed in this article. Many of these factors are unknown until after a failure is found, so a comprehensive inspection program should be established, as the repair options are significantly less expensive if coating failures are found prior to extensive substrate corrosion. Finally, a detailed inspection plan must include some cleaning of the surface to facilitate inspection as well as an inspector with in-depth knowledge of failures associated with water immersion coatings.
Repair Solutions for Osmotic Blistering
Advanced FRP Systems provides assistance with coating assessments and inspections, failure analysis, and a wide range of streamlined, custom solutions depending on the severity of the failure. Whether the inspection is done in-house or in conjunction with us, you can count on us to thoroughly diagnose your corrosion problems and build the right custom solution or preventative measure.
Our expert teams provide solutions for severe corrosion that range from structural reinforcement with high strength composites to a thick film coating system. If blistering is found in the early stages, we may be able to selectively remove damaged areas and salvage large portions of your previous coating. If you’re planning a coating project for water immersion, or repairing osmotic blistering, reach out for a free expert consultation from Advanced FRP and learn how the proper solution can cost as little as 10-15% more than a standard coating while providing double the service life.
Coating Failures in Water-Immersion Conditions: Osmotic Blistering
Water-immersion conditions are found in a variety of industrial applications. Circulating water systems, tanks and other critical infrastructure made of metallic substrates are typically protected from corrosion by immersion-grade coatings designed to act as barriers to prevent corrosion. For asset owners and maintenance staff, the ability of coatings to provide continued protection from corrosion or erosion is crucial to their industrial processes, as a coating failure can lead to a host of issues, up to and including forced shutdowns.
Broadly speaking, industrial coatings used in water-immersion conditions fail due to the following: Osmotic Blistering, Microbially Induced Corrosion, and Poor Surface Preparation/ Application/Specification. In this series of blogs, we will explore each of these categories, detailing how each of these impacts the protective coatings, how these issues can be avoided and inspection techniques to identify these problems.
Causes of Osmotic Blistering
Immersion-grade coatings are generally organic polymers with functional inorganic fillers and pigments. A reasonable assumption is that an immersion-grade coating is designed to stop water from permeating through it, thus preventing moisture from reaching the metallic substrate and allowing corrosion to set in. Unfortunately, all commonly used organic polymers have some degree of water permeability. This means they are all semi-permeable and over time, the naturally occurring force of osmosis will ensure that moisture will reach the substrate and, being unable to permeate through the non-permeable metallic substrate, cause a blister to form in the coating.
Chlorides or Other Ions on the Metal Surface
It is a common misconception that blistering is caused by a failure of the coating to adhere to the substrate. In truth, osmotic blistering can most often be traced back to some factor that accelerates the rate of permeation through the coating. The most common accelerating factor is the presence of chlorides or other ions on the metal surface although high purity water and elevated temperature service are also common factors. The removal of existing chlorides and/or other ions is critical in reducing the driving force of osmosis. While chlorides are the most common, sulfates, sulfites and iron oxide can also embed themselves in the surface of the substrate. Testing for chlorides and other common ions, with strict limits as well as cleaning procedures for high ion areas should be a part of all immersion-grade coating specifications, even for new assets being coated.
Water Purity
Water purity can also increase the risk of osmotic blistering in immersion conditions. Areas that have water with very low ion levels, or even areas with extremely clean, fresh water will experience accelerated water permeation rates. Coatings containing condensate or deionized water are especially prone to blistering in immersion conditions. Prevention, or delaying, of osmotic blistering can be accomplished by using extremely permeation-resistant systems or increasing the overall film thickness of the coatings. Consider that even glass-flake filled epoxies and vinyl esters, which are among the more permeation-resistant industrial coatings, are generally installed at 40 mils minimum thickness to provide a long-term solution in deionized water service.
Cold Wall Effect
The temperature of the water is also important to consider. When the temperature inside an immersive environment is above ambient and the pipeline or tank is not insulated, something known as the Cold Wall Effect will occur. Moisture will migrate towards areas of lower temperatures, and so, when the temperature of the water inside the pipeline or tank is higher than the temperature of the substrate on the other side of the coating, condensation will occur and build up between the substrate and the coating, building up osmotic pressure and increasing the rate of osmotic blistering. The greater the difference in temperature, the greater the risk factor. A difference of just 30°C can increase the permeation rate through a coating by a factor of 10. Blistering due to the cold wall effect is most common on the divider plate in condensers, the outlet water boxes and the warm side of circulating water lines. Similar to high purity water, a standard immersion-grade coating will not withstand Cold Wall service conditions, making extremely permeation-resistant coating systems required for this service. For reference, common coating systems for divider plates are generally applied at over 125 mils in total thickness to provide a long-term solution.
Key to Limiting Corrosion Associated with Osmotic Blistering
The key to preventing asset failures associated with osmotic blistering is a routine inspection program for all your water-immersion coatings. The inspection should focus on the highest risk areas like those that have any of the accelerating factors listed in this article. Many of these factors are unknown until after a failure is found, so a comprehensive inspection program should be established, as the repair options are significantly less expensive if coating failures are found prior to extensive substrate corrosion. Finally, a detailed inspection plan must include some cleaning of the surface to facilitate inspection as well as an inspector with in-depth knowledge of failures associated with water immersion coatings.
Repair Solutions for Osmotic Blistering
Advanced FRP Systems provides assistance with coating assessments and inspections, failure analysis, and a wide range of streamlined, custom solutions depending on the severity of the failure. Whether the inspection is done in-house or in conjunction with us, you can count on us to thoroughly diagnose your corrosion problems and build the right custom solution or preventative measure.
Our expert teams provide solutions for severe corrosion that range from structural reinforcement with high strength composites to a thick film coating system. If blistering is found in the early stages, we may be able to selectively remove damaged areas and salvage large portions of your previous coating. If you’re planning a coating project for water immersion, or repairing osmotic blistering, reach out for a free expert consultation from Advanced FRP and learn how the proper solution can cost as little as 10-15% more than a standard coating while providing double the service life.