From JPCL April 2020
Fig. 1: Rail tank cars hauling highly corrosive shale crude oil can benefit from the proactive application of protective linings to mitigate corrosion potential and enhance safety. PHOTOS COURTESY OF THE AUTHOR
Rail is one of the primary methods for moving crude oil in North America due to a lack of pipeline infrastructure. As crude oil production increased in the region and shale oil extraction blossomed in inland areas, more oil has been shipped by rail over longer distances to refineries and processors. This increased hauling need, combined with a variety of other factors, has created new implications for lining rail tank car interiors.
Historically, the interiors of railcars that hold and transport crude oil have not been coated for corrosion protection due to the typically non-corrosive nature of the oil. However, the fluids extracted from fracking and shale oil production have become more acidic with higher concentrations of impurities. The fluids also have a high moisture content due to the fracking process. This combination creates an environment inside tank cars that’s ripe for corrosion. As a result, owners and operators of railcars transporting crude oil are observing a surprising increase in premature corrosion inside cars, potentially shortening the life of these critical storage and transport vessels, not to mention increasing safety risks.
Due to the increased corrosion potential from today’s extracted products, railcar owners and operators are beginning to recognize the merits—and necessity—of coating rail tank cars to prolong their service lives. They are now lining previously unlined crude oil cars that have been in service and show signs of pitting and corrosion. In addition, they are specifying linings for new tank cars to proactively address corrosion prevention.
To successfully combat corrosion and enable long service lives for rail tank cars transporting today’s impure crude oil, railcar owners and operators must specify suitable lining materials designed to withstand the most aggressive contents a car may carry. This includes the introduction of hydrochloric acid that can wreak havoc on uncoated steel inside tank cars, particularly during backhauls.
Considering the growing market trend for hauling more aggressive contents, owners, operators and applicators can take steps to ensure linings are applied properly—and to specifications—to prolong lining service lives and reduce the total cost of tank car ownership.
The advent of fracking introduced a variety of aggressive chemicals and moisture to extracted crude oil, which traditionally had a very low corrosion potential. With traditional oil extraction methods, water could be present in crude oil, but typically just in trace amounts that would have difficulty contributing to corrosion—thus explaining why tank cars were rarely lined previously.
With the elevated moisture content, impurities and additives found in today's extracted shale crude oil, unlined tank cars face an aggressive corrosion environment. Significant amounts of water are used during hydraulic fracturing, which means more moisture and oxygen—two key requirements for corrosion—are being trapped inside tank cars during their long journeys from extraction sites to processors and back. Various chemicals from the fracking process are also trapped inside cars, causing a series of reactions that can increase corrosion potential. For example, chloride salts mixed with hydrocarbons will break down to hydrogen chloride gas at elevated temperatures. As temperatures drop, that gas can react with moisture to produce highly corrosive hydrochloric acid.1
Still, the risk of rampant corrosion inside an unlined tank car filled with crude oil remains relatively low, even with high moisture and impurity levels. That’s because most of the steel is immersed in fluid that will only lightly contribute to corrosion. However, the metal in the void space above the crude oil line faces a greater threat, as vapors collect in this space and can create a high concentration of hydrochloric acid in the right conditions. This issue is exacerbated when cars are emptied and backhauled to drilling locations. The emptied cars are often not cleaned and therefore contain toxic remnants of the previously stored contents, as well as moisture, until they’re refilled. Most of the steel is therefore now exposed to this aggressive environment, which can result in severe pitting corrosion.
The corrosive conditions inside empty tank cars can become even worse when cars face ambient heat from outside and operate in the humid and coastal environments surrounding many refinery locations. In addition, tank cars often sit for days at offloading sites before being transported back to drill sites, providing even more time for corrosion to proliferate inside a car before it is filled again. Adding to that exposure timeline, fracking sites are often located inland far from processing facilities. A car may run back and forth between North Dakota and the Gulf Coast, for example, facing corrosion potential the whole way, especially on the backhaul.
Given the increased corrosive contents of crude oil extracted from shale fracking operations, tank cars face a much greater risk of corroding and failing inspections—or worse, developing a leak that results in a spill, fire or explosion. These risks even prompted the industry to adopt a new, safer tank car design—the DOT-117 car (TC-117 in Canada)—to mitigate risks in 2015 (Fig. 2). To further enhance safety today, it would behoove railcar owners and operators to adopt the practice of proactively lining tank cars intended for shale oil service to combat corrosion.
Fig. 2: Although newer DOT-117 cars (TC-117 in Canada) cars feature several integral safety measures in their design, their safety can be further enhanced by lining the cars with a 100%-solids novolac epoxy to mitigate corrosion.
Beyond safety, lining tank cars also holds cost savings advantages. It’s possible that an unlined tank car carrying highly corrosive shale crude oil may only be able to make a handful of trips between drilling sites and refineries before its steel becomes too corroded to enable an additional transport. At that time, the car will need to be pulled out of service, blasted and lined to enable restored service. Thereafter, the tank should hold up for a long time, provided the lining was applied properly. If that tank was lined at the outset of its operational service, it could remain in service for much longer before requiring initial maintenance.
In this scenario, the unlined car carries a much higher cost of operation due to the early anticipated repairs. Because that unlined tank car will eventually require a lining—and that requirement may come quickly—it makes sense for railcar operators to consider proactively lining cars to accommodate the transport of corrosive crude oil.
When it comes to lining cars, applicators need to follow different steps depending on whether the car has been in service or if it is a new build.
Steel loses strength as pitting corrosion continues. But, if found early enough, pitting corrosion can be resolved with intervention. Cars taken out of service to be lined need to be thoroughly inspected for corrosion followed by abrasive blasting to the SSPC-SP 10/NACE No. 2, Near-White Metal Blast Cleaning standard. Any pitted areas should then be filled, for example, with a conventional novolac epoxy at 4–8 mils dry film thickness. Such fillers have lower solids contents than most epoxies, so they can readily flow into pits to build up any voids and restore the metal to its original surface plane.
Next, applicators will apply a full coat of a 100%-solids novolac epoxy at about 12–15 mils DFT. New tank cars obviously will not have pitting corrosion to contend with and can therefore be coated with a single coat of a 100%-solids novolac epoxy, typically at 12–16 mils DFT following an SSPC-SP 10 surface preparation, but possibly up to 22 mils DFT for added protection. This single, monolithic coat should provide continuity of the lining around the entire tank interior.
An ultra-high-solids novolac amine epoxy tank lining has been developed as an optimal lining system to combat corrosion inside tank cars holding corrosive shale oil and byproducts. Such coatings are typically designed for immersion service in oil tanks, refined fuel storage tanks and more for the storage and processing of crude oil at elevated temperatures. Most formulations on the market have a lengthy track record of service, providing peace of mind for operators. In addition, newer formulations relying on existing chemistries are enabling more successful, long-term applications, backed by the track records of their predecessors.
To deliver long-term service, specified tank car linings should have high-build, edge-retentive properties to ensure sufficient film thicknesses and adherence on sharp corners and edges. They should also be able to withstand the elevated temperatures inside steam-jacketed tank cars, as well as the heat cycling that commonly occurs when loading and offloading crude, which may involve cycling from ambient temperatures to 325 F (160 C) and back. The coating should also be densely crosslinked for superior corrosion and chemical resistance, as well as provide superior pit-filling capabilities for maintenance applications.
Fig. 3: Railcar owners and operators may consider proactively lining tank cars with a 100%-solids novolac epoxy to avoid the early maintenance expense of lining a corroded car that has been in service for a short period.
To save application costs and enhance shop throughput, the selected lining should be able to be applied direct-to-metal in a single high-build coat, even on complex geometries. This creates major time savings compared to older technologies that require two coats to achieve the right build. Some formulations also offer single-leg spraying capabilities, which can further decrease application costs.
After specifying the optimal tank car lining, as with any coating system, it is important for applicators to achieve a successful initial application, as that influences the service life and total cost of ownership of the car. They should follow proper application instructions, which may include heating the material per guidelines for either plural- or single-leg application, using the right spray gun tip size and setting the spray pressure properly for application. As guidance, applicators should always use the minimum amount of pressure to atomize the material and achieve a proper fan pattern.
To further ensure linings are applied properly, owners and applicators can specify lining systems that feature optically activated pigments embedded in the coating. This option allows applicators to easily check for pinholes as small as 0.25 mils, holidays, uniform coverage and proper film thickness while coatings are wet or dry. Because railcar linings are typically applied DTM in a single coat, the OAPs are included in the lining material. After spraying the inside of a tank car, applicators can shine an eye-safe ultraviolet (UV) light on the coating. The fully coated areas will fluoresce and illuminate brightly, and areas that do not illuminate at all or show darker areas indicate pinholes or areas of insufficient film build.
If used in a two-coat system as shown in Figure 4, the OAPs are included in the intermediate coat. Therefore, when applicators shine an eye-safe UV light on the coatings, the properly coated areas will remain dark and any voids or thin areas will shine pink. By inspecting OAP-embedded coatings while they’re still wet, applicators can proactively touch up any deficient areas, helping to reduce or possibly eliminate the number of required touchups following holiday testing.
Fig. 4: In the two-coat system shown here, optically activated pigments in the intermediate lining coat make the pinhole and thin areas appear pink (right) when using an eye-safe UV light for inspections. When inspecting single-coat linings featuring OAPs, dark areas will indicate coating deficiencies, as the rest of the coated areas will fluoresce.
Specifiers should also ensure the selected lining is rated for the contents the car will be hauling. However, determining those contents can be difficult, as many oil producers do not disclose the composition of their fracking chemicals. This deficiency leaves railcar owners without some critical knowledge about coating compatibility, which can cause specification confusion. When in doubt, it may be advisable to consider linings with glass flake reinforcement to be able to handle the most aggressive chemicals anticipated from fracking operations.
Given the increasingly corrosive contents of today’s extracted shale crude oil, tank car linings have practically become a necessity to mitigate corrosion and ensure safety. Railcar owners and operators have two options to meet this challenge. They can use unlined cars at first and eventually line them to ensure continued service after pitting corrosion develops. Or, they can line new cars proactively and mitigate corrosion worries at the outset. Either option is valid when the safety of operators and the public is at the forefront. The decision on when to line will likely come down to weighing the risks against total cost of ownership considerations.
Michael Manetta is Global Market Director—Rail, Marine and Power Generation for Sherwin-Williams Protective & Marine. He has 16 years of experience in the coatings industry with several sales and marketing roles within multiple end use segments.
Tagged categories: Coating Application; Features; Linings; Oil and Gas; Railcars; Sherwin-Williams
