Sherwin-Williams Liquid epoxy coatings offer cost-effective alternative

After cathodic protection, protective coatings are the next line of defense for the many thousands of miles of pipeline around the globe. Pipeline coating selection depends upon technical criteria such as operating conditions, environment, and laboratory certification of performance. Other factors include practical considerations such as cost, availability, scheduling, ease of application, field touch-up and past performance.

Two types of coatings, fusion-bonded epoxy (FBE) and liquid epoxy coatings are used in above-ground, below-grade (buried) and underwater applications. FBE has been the preferred coating for buried steel pipelines over the past several decades. Liquid epoxies are more typically applied in the field for touch-up and maintenance. The two technologies deliver similar performance.

Because of significant technical advances in liquid epoxies and overall global demand for steel pipe, the liquid epoxies have lately come under consideration for new applications. Curing times have decreased to as little as two to five hours, and return-to-service is much faster than a decade ago. Backfilling can be achieved within 24 to 36 hours. Global demand for steel pipe has put a strain on FBE fabrication facilities, many of which are running at full capacity. Some steel pipe manufacturers have turned to liquid epoxies to reduce backlogs.

Pipeline owners have noticed the advantages liquid epoxies offer. Cost competitiveness (discussed later in this article) is one advantage, but owners have also expressed a desire that liquid epoxies provide enhanced impact or abrasion resistance. There is also concern among owners that the high temperatures involved in applying FBE coatings may affect the metallurgy of newer high-strength steels in use on large pipe projects.

Coatings criteria
Several factors must be considered regarding any coating for steel pipe. For below grade, these include soil resistivity, pH, presence of chlorides and sulfates, degree of aeration or oxygen concentration, and resistance to microbiologically induced corrosion (MIC). In the most aggressive of these conditions, FBE, liquid amine epoxy or novolac epoxy technologies are usually warranted.

To be considered, a coating should pass several laboratory testing protocols set by standards organizations such as ASTM (American Society for Testing and Materials), NACE International (National Association of Corrosion Engineers), ISO (International Standards Organization) and CSA (Canadian Standards Organization). In North America, these include ASTM G8, ASTM G42, ASTM G80, ASTM G95 and CSA Z245.20. The ASTM standards focus primarily on cathodic disbondment (CD) testing, while the CSA standard is a more comprehensive evaluation that includes testing for CD resistance, impact resistance and evaluation of adhesion and flexibility characteristics. These factors are discussed in detail below.

CD resistance is arguably the most important characteristic test for pipeline coatings. The array of CD testing standards assess a coating’s resistance to an applied electrical current, simulating field conditions of impressed cathodic protection, or conditions of “stray current” emanating from other cathodically-protected pipelines or assets. There are multiple CD testing protocols, but when run under similar conditions of time and temperature, the tests will produce comparable results. Ultimately, these standards measure the extent of film delamination in a coating resulting from an applied current.

Impact testing is used to determine a coating’s ability to resist mechanical damage during shipping, handling and installation. It is expressed as the energy required to rupture a coating from a falling weight. ASTM G14, the “Standard Test Method for Impact Resistance of Pipeline Coatings” (aka the “Falling Weight Test), defines the testing methodology. The height of each impact and the dropped weight are used to calculate the force of impact.

Adhesion to the substrate is another important coating characteristic. Coatings exhibit good resistance to corrosion, delamination and cathodic disbondment when they adhere tightly to the substrate. ASTM D3359, the “Standard Test Method for Measuring Adhesion by Tape Test X-Cut,” measures adhesion on a qualitative basis. However, adhesion is more typically measured quantitatively, in accordance with ASTM D4541, the “Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.” Quantitative testing is preferred when comparing coatings for pipeline service.

Flexibility, or “bend,” testing evaluates a coating’s ability to withstand flexing or bending at a prescribed arc of deformation. Regardless of the test standard, a coating will pass only if no cracking has occurred in the film. Bend testing is significant for shop-applied coatings as it simulates flexing or bending experienced by the coated pipe when it is transported and installed.

Weathering tests are performed to evaluate a coating’s resistance to atmospheric conditions such as UV light, freeze/thaw cycles and condensation. While these tests are more significant for above-ground installations, weathering is an important consideration for coated pipe that will be stored outdoors prior to buried installation. Adverse affects of outdoor exposure for extended periods can lead to premature coating failure and corrosion. ASTM D5894 is the preferred standard for evaluation of corrosion weathering.

Fusion bonded epoxy
Fusion bonded epoxy coating is a completely automated process that became the preferred coating for buried steel pipeline in the 1970s. It has performed well in the field and is used primarily to coat new pipe. FBE technology is rarely, if ever, used to repair coatings in the field. It is an “in-house” process that requires elaborate equipment, including a high temperature furnace, and uses high levels of energy. Any field repair or touch-up is typically achieved using liquid epoxies.

FBE is applied in a fabricator’s facility on straight pipe sections after they are abrasion blasted to promote adhesion. Spools and elbows typically cannot be accommodated in this process, since most FBE coating lines are not designed to coat these components. If the furnaces can accommodate these intricate shapes, concerns exist that the FBE spray pattern may not coat the surfaces evenly. Liquid epoxy coatings are usually used for these components.

Straight pipe sections are heated to temperatures between 438º F and 475º F and are then transferred into a coating bay. Sections are sprayed with a powdered coating which transforms to a liquid film and bonds to the pipe’s exterior. The pipe section then moves into a cooling bay, and the coating cures rapidly, typically in less than five minutes.

A potential issue arises should the pipe need to have an internal surface coating. Pipelines transporting alternative fuels often require a smooth internal surface for efficient transport, resulting in the need to coat the interior. Internal coatings are typically less than 3 mils thick and are usually applied before the external surface coating. FBE coatings are applicable to external surfaces only. Therefore, the internal coating must be able to withstand temperatures up to 475º F.

FBE-coated pipe can also be damaged during shipping by the mechanical means used during loading, offloading and welding. Rehabbing or repairing FBE coated pipeline with liquid epoxy coatings prior to installation is a common occurrence in the industry.

Liquid epoxy coatings
Until the late 1990s, liquid-applied epoxies were cumbersome, at best, to apply correctly and cure properly, even in the field. But rapid curing technologies and fast return to service, coupled with excellent CD resistance, chemical abrasion and impact resistance, as well as UV protection, have provided both steel pipe manufacturers and pipeline owners with an effective, reliable alternative to FBE.

The liquid epoxy coating process for new pipe may be automated, similar to an FBE application, but does not require the high cost equipment or the high energy expenditure. In a recent project involving a large refinery expansion, FBE was the specified coating for all below-grade pipelines within the refinery. Due to high demand within FBE facilities, the chosen supplier was not able to meet the project schedule. A liquid epoxy coating was selected as an alternative, allowing the project to proceed on schedule and under budget.

Another example where liquid epoxy may be an alternative to FBE is in the instance of a project that utilizes multiple diameters of straight-run pipe. It may be more economical to use liquid epoxy over FBE due to cost savings of having to re-tool an FBE line to accommodate multiple sizes.

Liquid epoxies are typically used to rehabilitate FBE and asphalt wrap/coal tar epoxy coated pipelines in the field. When FBE is encountered in rehabilitation projects, the FBE coating is either completely removed or receives an SPC-SP7 brush blast prior to being refinished with a liquid epoxy coating.

The process for field application of liquid epoxy coatings varies from project to project. Corrosion data is uploaded from pigging devices, and the lengths and sections of pipeline to be recoated are determined. The scope of the project is a major component in determining equipment needs and which coating technology to employ. Most coatings manufacturers have products readily available in small cartridge guns, small one-gallon kits, five-gallon kits or larger multiple drum units; selection depends on the project’s scope.

Liquid epoxy application
For either new pipeline projects or repair projects in the field, there are several optimal application methods, all of which depend on the size of the project.

Coating rehabilitation projects that involve sections of pipeline under 20 feet in length are best achieved through the use of cartridge/caulk gun applicators, which use dual component/static mixer guns. The unit is similar to a basic caulking gun. The use of small cartridges reduces waste, requires minimal clean up and eliminates the need to mix material by hand. Cartridge sizes vary between manufacturers.

Projects that involve steel pipe sections 20 to 100 feet in length should employ coating with a cartridge gun/air assisted spray applicator. This system uses an air regulator and pressure gauge to control atomization and spray pattern, and allows the applicator to achieve the specified mil thickness in a uniform pattern. The device also reduces waste, eliminates hand mixing and requires minimal cleanup.

Projects that involve steel pipeline sections over 100 feet in length benefit from plural component spray equipment. This technology has been employed in the tank lining industry for several years and is becoming commonplace for pipeline coatings. The process is cost effective because large sections of pipe can be coated quickly and is highly mobile. The plural component application stores a resin and curing agent in separate tanks. Heaters are used to raise the component’s temperature between 100 and 130°F. The components come in contact only when they are sent to a mixing chamber just prior to being sprayed onto the substrate at a prescribed pressure, where they react to form a solid coating.

Applicator experience is vitally important to successful liquid epoxy coating. Most manufacturers require that certification and training be completed prior to being employed for application in the field. Currently NACE is developing a Task Group, TG #337, to study the quality field application of liquid epoxy coatings.

Cost comparison
Application of liquid epoxy coatings on new pipe in a shop facility negates the need to ship pipe from the manufacturer to an FBE fab shop. Shipping and handling costs add up quickly when shipping hundreds of miles of pipe for FBE coating. It also eliminates energy costs for heating both the pipe and powder. Finally, the applied material cost of FBE powder is usually higher than liquid epoxies.

Unused powder can, however, be collected and reused in an FBE application, while any unused liquid epoxies cannot be used in the future due to pot life issues. In general, the anticipated shop-applied square foot price for liquid epoxies range from $4-$6 per square foot. Prices can fluctuate, and small quantity projects will increase costs. In general, shop application of FBE coatings range from $10-$15 per square foot.

Costs to apply liquid epoxy coatings in the field are typically in the range of $6-$10 per square foot. Costs depend on the quantity of pipe, length and diameter of pipe, accessibility of pipeline right of way (remote, waterway or mountainous), specified abrasive, number of required mobilizations, material prices, specified dry film thickness, labor rate, per diem and schedule demand.

Conclusion
The performance requirements and laboratory testing of both fusion bonded epoxy and liquid epoxy coatings indicate that both perform exceptionally well for buried pipeline service. While most specifications call for FBE coatings for new pipelines, liquid epoxy coatings continue to come under consideration more often due to speed of delivery, availability and lower costs. Both coatings will play a long-term role in the pipeline industry. n

The authors
Justin Hair, Sherwin-Williams, Tulsa, Oklahoma, is a petrochemical business development manager in the U.S. Midwest. He holds a BS in marketing and is also a NACE Certified Coating Inspector. Hair also has extensive experience within the industrial paint contracting sector, with a particular emphasis in regards to external buried pipeline rehabilitation.

Yasir Idlibi, Sherwin-Williams, Calgary, Alberta, is a petrochemical business development manager in Western Canada and Alaska. He holds a Ph.D. in Chemistry with emphasis on protective coatings and corrosion. Dr. Idlibi has experience in coatings and corrosion testing.