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Originally Published: JPCL, January 2019
Have you ever wondered how all those products upon which we rely in our everyday personal and professional lives are delivered throughout North America? American highways are clogged with trucks, trailers and transporters of all sorts moving packaged and bulk goods throughout the country. Barges and pipelines move countless millions of gallons and pounds of materials every day without fail. And the railroads, with its transportation infrastructure of over 2,000,000 pieces of rolling stock are moving autos, lumber, plastics and bulk liquids by the millions of tons every year.
Tank cars comprise over 20 percent of the rail rolling stock in North America. More than 400,000 tank cars ply the rails throughout the continent, delivering raw materials, intermediate chemicals and finished products for both industrial use and direct or indirect human consumption.
A vast network of independent service facilities located throughout the U.S. and North America maintain the equipment railroads and private fleet owners use to move all these products from point of origin to your front door.
These cars are made of mild steel, not exotic materials that are impervious to corrosion, so how is it that they stay intact and keep the products being hauled pure and safe? The answer: with coatings and linings not only applied by skilled technicians to exacting standards, but also inspected consistently and maintained accordingly.
Interior protective coatings and linings are applied to either maintain the purity of the product being transported or to protect the railcar from corrosion. A wide range of coatings may be used as the protective barrier depending upon the severity of the service environment. Epoxies, epoxy phenolics, novolacs, zinc and baked phenolics are typical spray-applied coating types used. For any interior coating system to perform at a high level, the surface preparation and application processes must be closely followed, with attention to detail paid at every step.
Where the commodity is known to be corrosive, progressively thicker-film coatings are used, such as vinyl esters applied at 40 mils or thicker. For severely aggressive and corrosive services such as strong acids (HCl, HF, and H3PO4), sheet rubber linings are also used. Title 49 of the Code of Federal Regulation Section 180, Subpart F governs the inspection and maintenance of tank cars. By this Subpart, interior coatings must receive a periodic inspection or requalification when the commodity is known to cause a corrosion rate of greater than 2.5 mils per year on mild steel. Most liquid commodities shipped by rail fall into this category, including chlorides, sulfides, fertilizer solutions, other oxidizing agents and crude oil from the tar sands and shale fields.
For commodities considered non-corrosive according to this standard, such as glycols, corn sweeteners, solvents and phenol, no periodic interior coating inspection is required. The owner of the coating may choose to periodically inspect the coating and make repairs as needed based on their own product purity requirements. The owner of the coating must prove their commodities to be non-corrosive if a product purity claim is made. If requalification is required, the owner must be able to document the analysis used in establishing the requalification cycle.
Without specified periodic maintenance cycles, coating systems for these non-corrosive commodities need to be all the more reliable in order to prevent premature failure.
A case in point is baked phenolics in phenol service. Baked phenolics are normally used for product purity where corrosion is not a concern. Phenol is not corrosive to mild steel, but will be discolored by contact with bare steel. Color and clarity of phenol is critical in its applications as a starting chemical for plastics, explosives and some medicines such as aspirin. Phenol is also an excellent antiseptic and is used in household cleaner and disinfectants.
Very few sprayable coatings are resistant to phenol, but baked phenolics have proven over many decades of use to be outstanding performers, given that surface preparation and application protocols adhere to the proper specifications.
Tank cars are considered a permit-required confined space according to 26 CFR 1926, and therefore, before each shift begins, a confined space entry permit must be present, and the interior environment must be monitored for proper air quality.
Railcar relining begins with the cleaning and decontamination process. Depending on the previous service and the degree of coating failure (if any), a thorough cleaning must be performed to remove any contaminants from the substrate and to achieve a neutral pH on the surface. Although a railcar may appear clean, the previous service may have permeated the substrate or become engrained in such a way that the substance must be removed to prevent contamination of the blast media, which would lead to cross-contamination of other railcars, and to remove substances that can promote osmotic blistering or interfere with adhesion of the coating to the substrate. The surface should be tested for contaminants such as oils and non-visible soluble salts, especially if an acidic or alkaline environment exists.
If additional cleaning or neutralization is required, the appropriate method will be repeated before the final blast is performed. This may include prebaking, steam injection, hot water wash or chemical wash/cleaning. Prebaking involves heating the railcar until the surface temperature reaches 450 F and holding it for four-to-six hours to remove any volatile compounds that have a relatively high decomposition temperature. Steaming is used for water-soluble substances that can be washed from the surface relatively easily. Where highly acidic or alkaline services are involved, the surface is neutralized and steam-cleaned. The nature of the previous service will dictate the appropriate method for decontamination.
The generally accepted specification for abrasive blast-cleaning is SSPC-SP 5/NACE No. 1, “White Metal Blast Cleaning.” Depending on the film thickness to be applied, the blast profile may be specified between 1.5 and 4.5 mils. Recyclable metallic abrasives are the common media used in the railcar repair industry for surface preparation. Steel grit, not shot, of a size and hardness to assure the necessary depth with a dense angular profile should be used.
After interior blast, the car should be cleaned of all debris, dust and loose abrasive by vacuum and hand-cleaning. With the internal environment controlled to prevent condensing moisture to form and maintain at least 60 F substrate temperature, baked phenolics are then applied in two or three spray coats to achieve a final dry-film thickness of 5-to-8 mils.
Phenolics are thermosetting coatings, so each coat must be baked to a substrate temperature of at least 225 F. Following the application and intermediate baking of the final coat, a thorough inspection of the coating should be performed to verify that proper film thickness has been achieved and that the coating is holiday-free prior to the final baking stage.
Final baking requires gradually raising the steel temperature at least 375 F and holding it there for at least 90 minutes or until a minimum color change is achieved, which can best be described as caramel to milk chocolate brown. All baked phenolic coatings darken during final baking, although coatings from different manufacturers have differing degrees of color change. The final bake process takes 10-to-12 hours in most railcars, but larger cars or other factors may require a longer bake cycle. The complete process from start to finish can take five-to-six days, including cleaning and decontamination.
Generally speaking, the service life of an interior coating is five-to-10 years, with repairs to the coating possibly extending that usable life of the applied system. A well-applied coating system can last far longer in some commodities.
One example is a railcar serviced recently, in which the 24-year-old dark brown phenolic coating was found to be free of any defects with the minor exception of damage at the manway opening, which is typical and unavoidable. Applied in 1994 to these exacting specifications, the coating and the railcar have been through at least two routine inspection events required by regulation. In each case the interior coating was found to be in perfect condition (Fig. 1).
Fig. 1: A baked phenolic lining applied to the interior of a phenol-carrying tank car has protected the underlying substrate from both corrosion and chemical attack for 24 years, passing two different inspection cycles.
When properly maintained and loaded over the course of its service life, an interior lining can provide many years of protection both to the railcar and the products being hauled, as evidenced by the nearly quarter-century and still counting for this baked phenolic coating.
By: John Myers, Carboline Company