In CORROSION 2017 paper no. 9061, “Internal Lining Damage Investigation of 24-inch Jet Fuel Pipelines,” NACE International members Mushaid Nauman and David Eyre discuss an investigation into the damage of the epoxy lining on two 24-in (610-mm) diameter pipelines for a new fuel hydrant system. The pipelines, constructed as part of an airport expansion project, were externally coated with three-layer polyethylene (3LPE) and internally lined with epoxy. The lining was applied to ensure a clean pipeline so contamination of the aviation fuel could be avoided. It was not intended to be an anticorrosion barrier coating. Since heat shrink sleeves were to be applied at the joints, the girth welds were not lined during construction. The lining was comprised of a two-coat amine-cured, modified epoxy coating with high solids content in both the primer and top coat (65% and 63% by volume, respectively). The dry film thickness (DFT) for both coats was in the range of 100 to150 µm. The material data sheet for the coating material indicated the epoxy could withstand 232 °C (450 °F) wet dry under insulation for short periods, and cautioned that excessive humidity or condensation on the surface during curing could interfere with the cure.
During construction, a leak in a nearby 36-in (914-mm) irrigation line caused water to flood an open trench that contained two of the fuel hydrant system pipelines—83 of the project’s 469 pipe segments were flooded. The flooding caused mud, water, and debris to accumulate in 472 m (1,549 ft) of pipe. The trench was drained and the flooded pipelines were cleaned by flushing with water, air blowing, and drying. After the cleaning operations, the pipelines were inspected internally with a robotic crawler.
The robotic inspection revealed lining damage on 35 of the flooded pipe segments. The damage included topcoat removal, bubbles and blisters, rust, and staining, which varied at different locations within the pipelines. Rust had formed on exposed metal at the welded joints, and at some locations the lining had peeled off adjacent to the joints. Initially, the cause of the lining damage was thought to be caused by the ingress of water and mud, followed by the cleaning operations. Testing and analysis were implemented, however, to determine the actual cause of the lining damage.
The mechanisms investigated as the possible cause of the internal lining damage were overheating of internal weld joints, osmotic blistering, and amine blush. Tests done during the investigation included an amine blush test, weld joint test, aviation fuel soak test, water soak test, DFT test, cross cut adhesion test, pull-off test, as well as a general visual inspection. The results indicated the cause of blistering and delamination damage in the flooded and flushed pipelines was osmotic blistering due to presence of water soluble material at the interface between the primer and top coat.
According to the authors, blisters are local regions where the coating has lost adherence from the substrate and allows water to accumulate and corrosion to begin. Three main constituents must be present for osmotic blistering to occur: water (liquid and/or vapor), salts or water-soluble (hygroscopic) organics, and a semi-permeable membrane. Paint films are semi-permeable membranes that are permeable to water but impermeable to dissolved solids. The main driver of osmotic blistering is the water-soluble material at the interface between the coating and the substrate or between coats. The water-soluble materials that cause the osmotic gradient are generally either inorganic salts, corrosion products, or retained solvents in the coating. When in contact with an aqueous environment, the water-soluble contaminant dissolves and creates a solution with a different concentration than the bulk environment. Water accumulates and the blister is formed.
The authors note that osmotic blistering has been related to chlorides, sulfates, and other inorganic soluble often found on substrates or beneath the top coat. These materials can result from environmental contamination or depassivating salts such as chlorides and sulfates. Water extractable materials accidentally or deliberately entrained in the coating can also cause problems.
In the case of the fuel hydrant system pipelines, the paint film on the internal pipe surface was in contact with an aqueous environment (i.e., the flood water). This water was absorbed by the top coat and then transferred to the top coat/primer interface. Here, the water encountered the water-soluble material on the primer surface, which was likely present due to exposure of the primer to moisture before the epoxy phenolic resin and amine hardener had fully reacted. This indicates that the problem was caused by poor procedures during application of the top coat. If the water-soluble materials were present on the substrate beneath the primer, blistering and delamination would have occurred at substrate/primer interface. Records at the plant that applied the lining indicated that conditions were not favorable over the entire lining application period. Very low temperatures and high humidity conditions were observed.
Osmotic blistering is not expected if the line is exposed only to aviation fuel, even with the presence of soluble material on the primer surface. Jet fuel transfer through the coating to any significant degree is unlikely because jet fuel molecules are larger than water molecules and are unlikely to be capable of penetrating through the epoxy phenolic amine matrix in the coating. The flood identified the issue of water-soluble materials on the primer surface; however, the source for entrainment of these contaminants on the primer surface has not been confirmed.