As global opportunities and uses for steel pipes expand, so do the technologies that are used to protect them. Because of their constant exposure to natural elements and other corrosive agents, application of pipeline coatings are essential to ensuring that pipelines are safer and longer-lasting.
From the old days of field-applied asphalt and coal tar coats, various industries can now leverage on the availability of state-of-the-art coatings like SplashTRON®, a yellow elastomeric coating developed in the 1960s that’s specifically designed for splash zone areas. Because it is used in areas where the constantly changing water levels threaten pipe integrity, it not only features excellent thermal insulation, but also has strong bio-fouling resistance, able to withstand weathering, fungus, scale, and nautical environments.
In the oil and gas industries, newer field-applied and plant-applied coatings also exist. Because plant-applied coatings are generally cheaper and longer lasting, they have become a preferred alternative for many projects where pipelines are closer to the right-of-way or the pipe mill. Another advantage of plant-applied coatings is its resilience to varying weather conditions, making application more convenient.
Types of plant-applied coatings
In use since the 1960s, fusion-bonded epoxy (FBE) coatings are solid powder coatings made of epoxy resins, pigments, hardeners, and stabilizers that stick to steel surfaces after heat application to create an anti-corrosive coating.
Prior to coating, the pipes are preheated using gas burners or induction up to around 245 °F, then shot to ensure a cleaner, more adhesive surface. This coating type can either be a single, thin layer or dual layer. The latter involves re-application of FBE as topcoat to prevent mechanical tearing such as boring or drilling.
Three-layer polyolefin coatings
Three-layer coatings consist of a primer (FBE or liquid epoxy), an adhesive intermediate layer (copolymer), and a topcoat (polyolefin). In use since the 1980s, this type of coating is especially suited to pipelines with high electrical resistance.
Like FBE coatings, the first coat for three-layer coatings requires pre-heating the pipes. The FBE and copolymer layers are sprayed on (the tie layer can also be applied via side extrusion), followed by a final topcoat. Often, the topcoat used is polyethylene, although polypropylene is preferred if the pipeline is to be exposed to great service temperatures.
Hot tape or heat-shrinkable tape coatings
Specifically designed for off-shore pipelines installed in high-temperature environments (up to 250 °F), heat-shrinkable tapes use thick, polyethylene-based backing coating with thermoplastic adhesive. The pipe is preheated to melt and bond the adhesive to the steel surface. The polyethylene backing is heated using a propane torch, melting it onto the pipeline surface. After cooling, the adhesive is attached to the pipe before burying it into the ground.
Polymeric tape coatings can be applied in both hot and cold conditions. Unlike other plant-applied coatings, cold-applied polymeric tapes do not require heat for the adhesive to stick to the surface. The application of tension on the tape offers protection from air and water.
Some of the most popular uses for cold-applied polymeric tapes include rehabilitation of pipelines in oil and gas industries, as well as prevention of corrosion in steel and iron pipes in water supply systems. The pipes on which the coats are applied usually accommodate volatile organic compounds, so primers have to pass environmental compliance protocols with regards to such emissions.
Such polymeric tape systems can be single- or multi-layer that utilize primers, followed by an adhesive, which often consists of butyl rubber or synthetic resins. The middle layer is polyethylene or polypropylene. A UV-resistant outer layer is applied to protect against sunlight.
Hot-applied tapes also utilize three layers, starting from a thermally activated primer containing anti-corrosive additives. This is followed by a thermoplastic butyl adhesive then a thermoplastic high-density polyethylene top layer. This system is preferred for applications where the likelihood of mechanical damage is high.