What Coat to Wear
Advances in Damage Resistant Pipeline Coating Technology
By Dr. Jennifer K. Pratt, Meghan Mallozzi and Dr. Andrew D’Souza
Today, the use of a durable external coating system in combination with cathodic protection is well accepted as the most effective means of corrosion prevention for oil and gas pipelines. Many factors go into the coating selection decision, including things like cost, damage resistance, pipeline terrain, backfill, soil conditions, joint coating solutions, quality assurance, logistics, electrical-resistance stability, non-shielding cathodic protection (CP), pipeline operating temperature, stress-corrosion cracking and lifetime cost among others. This article will review some of the most common coating solutions used in the market today to provide resistance to damage during transportation and installation.
Figure 1: Micrograph of a traditional 2LFBE coating at 10,000X.
Single Layer FBE (1LFBE)
Fusion-bonded epoxy (FBE) is a one part powdered epoxy coating that is sprayed onto the hot pipe where it melts, flows and cures to give a corrosion resistant coating. The first pipe coated with FBE was placed into service in 1960. FBE is now the most commonly used pipeline coating in North America for new installations.
There are a number of properties that make it a good choice for many pipeline installations: 1) excellent adhesion to steel; 2) good chemical resistance; 3) non-shielding to CP — fails friendly; 4) no reported cases of stress-corrosion cracking (SCC) of pipe coated with FBE; 5) installation friendly; 6) excellent penetration resistance, good abrasion and gouge resistance; 7) good impact resistance (impact damage is limited to the point of contact, damage is easily seen and damage is easily repaired); and 8) good flexibility.
As with all coatings systems, improper surface preparation, improper cleaning or a contaminated substrate can result in a loss of bond and/or blistering. However, FBE does not shield cathodic protection current, so even with adhesion loss, the pipe can still be protected by CP. The major concern with the use of 1LFBE is transportation and handling damage. This is especially important in countries with inadequate infrastructure and long haulage routes to pipeline installation sites. Any damage that does occur can be easily repaired by application of a liquid epoxy resin. The thickness of the coating can be increased to help provide added absorption of impact and abrasion. However, as the thickness of the coating increases, the flexibility performance decreases.
Figure 2: Micrograph of the more flexible 2LFBE at 10,000X.
Dual Layer FBE (2LFBE)
The use of two layers of fusion bonded epoxy can significantly improve damage resistance in comparison with 1LFBE. The first layer allows the system to retain the properties of a standalone FBE coating while the top layer can provide additional mechanical damage resistance from impact or gouging during handling, transportation and construction. The first abrasion resistant outer (ARO) coating for directional drill pipeline installation was introduced in 1998. In 2002, the first major pipeline was coated with a dual layer coating system from end to end, including the girth welds. Since then, 2LFBE has been used on multiple projects in India and Australia. Formulation design allowing for improved gouge and impact resistance typically result in a slight reduction in flexibility. This reduction in flexibility can have a negative influence for use in reel barge or cold-weather installation.
Three Layer Polyolefin (3LPO)
The use of three layer polyolefin coatings for protection of pipelines was introduced in Europe around 1980. These coating systems comprise: 1) epoxy primer coating (can be FBE or two-part liquid epoxy); 2) polyolefin-adhesive (or tie) layer (can be either an acid copolymer or grafted polyolefin); and 3) polyolefin topcoat (can be either polyethylene or polypropylene).
In some applications, such as offshore installations, additional layers are added to provide thermal insulation, provide additional weight or introduce a frictional surface.
The epoxy primer is the foundation of the system. Proper selection and application of the epoxy primer is crucial to overall performance of the system. Many things should be considered during the selection process including: 1) projected operating temperature, 2) humidity of the environment, 3) permeability of corrosive species, and 4) adhesion performance. As previously mentioned, excellent surface preparation is crucial to overall coating performance for any coating system.
The adhesive or tie layer provides a means of bonding the epoxy primer layer to the polyolefin topcoat. The first adhesives used for this application were copolymers of polyethylene or polypropylene co-reacted with functional groups such as acetates, acrylates, or organic anhydrides. Further developments in the field led to the introduction of maleic-anhydride grafted adhesives.
The polyolefin topcoat provides mechanical protection and reduces coating damage. It also slows the ingress of water to the FBE layer and the substrate below due to its excellent barrier properties. There are many types of polyolefin materials that are available including low-density polyethylene (LDPE), linear-low density polyethylene (LLDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), bimodal HDPE and polypropylene.
The 3LPO systems are well accepted and are regarded as the coating of choice by many in the industry when damage resistance is required. Unfortunately, going back to 2001, there have been numerous concerns expressed with three-layer coatings in service today. Problems and concerns that have been reported include: 1) potential for SCC, 2) cracking and splitting of the polyolefin coating (loss of bond between FBE and steel, and adhesive and FBE), 3) adhesion failure and corrosion at the girth weld area, 4) loss of bond between FBE and steel layer, 5) shielding from CP and potential corrosion and 6) disbonding at cutback.
Improved Dual Layer FBE System
Typical 2LFBE systems applied at a thickness appropriate to provide proper damage resistance have flexibility ranges between 1.5 degrees/pipe diameter (PD) and 2.5 degrees/PD depending on the application conditions. A new concept first published in 2008 addresses this compromise between damage resistance and flexibility by providing a 2LFBE system that creates a unique morphology within the coating that limits crack propagation.
The differences in morphology are observed at the microscopic level. A micrograph of a standard 2LFBE system is shown in Figure 1 while a micrograph of the flexible 2LFBE system is shown in Figure 2. The flexible 2LFBE system has cold temperature flexibility close to that of a 1LFBE even at high thickness without sacrificing impact and gouge resistance. This improved 2LFBE coating system has the ability to reach 4 degrees/PD at minus 30 degrees Celsius and 3 degrees/PD at minus 50 C even at a total thickness of 50 mils.
Figure 3: Steel samples shown in this photo were coated with a 2LFBE system, the top with a standard 2LFBE and the bottom with the newly developed, more flexible 2LFBE. Both samples were cooled to minus 30 C and bent at 4 degrees.
Figure 3 illustrates the difference in performance between the standard 2LFBE and the flexible 2LFBE. Both samples in the photo were coated at a total thickness of 30 mils under the same conditions and both were bent at 4 degrees/PD at minus 30 C. The bar at the bottom of the photo was coated with the flexible 2LFBE system while the bar on the top was coated with a standard 2LFBE product. A more flexible 2LFBE system gives pipeline end users another solution to protect their pipeline in adverse geotechnical and service conditions.
Conclusions
There is a need in the market place for FBE coatings that are more resistant to mechanical damage during handling and installation of pipe. One approach to provide more resistance to mechanical damage is to increase the thickness of the overall coating. However, as the thickness of the coating increases, the coating has a greater tendency to crack under impact. Another conventional approach is to use dual layer fusion bonded epoxy coatings (2LFBE) with an abrasion resistant overcoat (ARO) having higher filler loadings to increase the damage resistance of the coatings. However, higher filler loadings can dramatically decrease the flexibility of the FBE coating. Thus, damage resistant dual layer coatings currently available require a compromise between toughness and flexibility.
This new flexible 2LFBE system has the ability to reach 4 degrees/PD at minus 30 C and 3 degrees/PD at minus 50 C even at a thickness of 50 mils. Gouge and impact resistance performance are equivalent to a standard 2LFBE system. This improved dual-layer coating gives the pipeline end user another solution to protect their pipeline in difficult geological conditions.
(Author’s note: The experiments referenced in this article were conducted by researchers at 3M in both Austin, Texas, and St. Paul, Minn. The test methodology and test results described in this report are, to the best of our knowledge, accurate. 3M makes no representations or warranties regarding the test methodology or the test data itself. As a matter of customer service, 3M will work with customers to resolve any related challenges encountered in the future. Ultimately, it is the responsibility of the customer to determine whether or not additional testing is required and whether or not the final product is suitable for use in the customer’s specific application(s).)
Dr. Jennifer Pratt works in product development in 3M’s Infrastructure Protection Division. Her area of research is in FBE coatings used in multiple market segments, including oil and gas. She has a doctoral degree in organic chemistry from the University of Illinois.
Dr. Andrew D’Souza works in research and development in 3M’s Safety Security and Protection Services Lab. His areas of research are in FBE coatings and inorganic stay clean coatings. He has a doctoral degree in materials science and engineering from the Pennsylvania State University.
Meghan Mallozzi obtained a bachelor’s degree in Chemical Engineering from the University of Colorado and an MBA from the University of Texas. She joined 3M in 2004 with the Corrosion Protection Products Division as a Product Development Engineer with a focus in the areas of nano-composite chemistry and epoxy/polyolefin interpenetrating coatings. In 2011, she accepted a position in the Electrical Solutions Division to manage the Industrial Interconnect Portfolio.
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