Editor's note: Learn more about pipeline coatings in this new Materials Performance special feature, “Science in Action.” After you’ve read the MP article about a field-applied liquid epoxy coating for use on pipelines, explore its application, which is presented in several related CoatingsPro Magazine features listed at the end of the article.
We’ve come a long way since the old days, when a coating job involved handing some paint to the cheapest laborers and sending them off. Now all steps of a coating procedure must be documented to ensure a quality end result. Pipeline surface preparation and coating application are becoming more advanced and necessitate skilled professionals that require, in many cases, product-specific training. The operator qualifications and generic coating application certifications often do not cover the nuances of the various products on the market.
Due to the mandated inspections of pipelines, there are more and more in-service pipe condition evaluations where the results justify replacement with pipe that has a mill-applied coating or removal of the existing coating and a pipe recoat in the field. The coating used after inspection and/or repair must be designed for the service environment and able to be field-applied and interface with various existing coatings under less than perfect conditions.
Pipeline operators must carefully choose coatings that will meet pipeline operating and soil environment conditions, while providing ease of application.
This discussion begins with the application of 100% solids coatings at a pipe coating facility. The coating most commonly applied is fusion-bonded epoxy (FBE); however, in some instances, a 100% solids liquid epoxy may be specified. As with all coatings, the surface must be cleaned, profiled, and coated in accordance with the coating manufacturer’s specifications.
Many pipeline operators will hire a third-party inspector to verify and document that all procedures are properly followed. The surface preparation in a coating facility is fairly consistent provided the abrasive blast medium, which is normally recycled, is properly inspected and maintained at the correct sieve size to achieve the appropriate profile. FBE coating is normally considered to be consistent. Typical dry film thickness (DFT) for FBE ranges from 12 to 18 mils (305 to 457 µm); however, field verification of film thickness has discovered DFTs of 6 mils (152 µm) or less. This highlights the need for a thorough inspection before the pipe leaves the plant. Coating plant application of liquid epoxies, because of their higher DFTs, seems to stay more in range (Figure 1).
When a coating plant is required to apply an additional layer of FBE or other coating as an abrasion-resistant overcoat (ARO), care must be given to follow the manufacturer’s specifications for surface preparation of the parent coating. Most, if not all, coatings applied as an ARO require a surface profile on the parent coating. There is a belief that an additional layer of FBE can be added to existing FBE without any surface preparation if it is applied within a few hours of the initial application. I have found no data to substantiate that claim. This practice may result in the overcoat not adhering to the primary coat and failing due to the absence of an anchor profile that can be adhered to. FBE, unlike 100% solids epoxy, is in a liquid state only upon application to the heated pipe. Therefore, no chemical bonding can occur between FBE layers. It can be compared to trying to paint wax paper.
The question then arises; is field surface preparation as consistent as coating mill surface preparation? A well-trained crew, proper equipment, and good inspectors are able to achieve coating manufacturers’ requirements on a regular basis. Field surface preparation has one major hurdle: weather! A coating mill is a controlled environment, whereas field surface preparation and coating application must sometimes alter the environment to meet the requirements, including temperature and humidity.
An additional element that field application of 100% solids liquid coatings must address is interfacing with existing coatings, such as mill-applied FBE. Coatings such as coal tar or three-layer polyethylene (PE) contain wax to some degree. Liquid epoxies do not readily adhere to coatings containing waxes unless they are correctly prepared. There are procedures that will assist with increasing the adhesion; however, the area where the liquid epoxy is applied to coal tar may be susceptible to disbonding. This could allow moisture to get between the overlap areas and possibly under an existing coating. This can result in a corrosive environment in contact with the pipe metal that is shielded from cathodic protection (CP) current. A procedure that is followed by many applicators and operators is to apply tape, starting ~2 in (51 mm) onto the coal tar or PE coating, and tightly wrap it around the pipe with about a 50% overlap to a point on the epoxy coating ~2 in from the interface. This will essentially seal the interface and stop the ingress of moisture.
There is a common misconception that an applicator must use the same coating—sometimes the same color—that was previously applied at the mill or in the field so that they are compatible. One hundred percent solids coatings are just what the name means: no aromatic solvents. Since there are no solvents, compatibility is not an issue for 100% solids coatings. All they need is a surface profile they can adhere to.
If an applicator has more than one type or kit size of epoxy coatings on site, it should always be verified that comparable components and kit sizes are used. Intermixing the components of different manufacturers may result in coating failure, as will mixing a 1-L activator into a 2-L base.
Once the hurdles of surface preparation and interfacing have been overcome, the coating must be successfully applied to the pipe.
Consistency of the film thickness for field coatings has always been an issue. Again, a correctly trained applicator using appropriate application and testing equipment should be able to maintain variations within 10 mils (254 µm). Most liquid epoxy field coatings are applied at a specified film thickness ranging from 20 to 50 mils (508 to 1,270 µm).
The most consistent method of field-applying liquid epoxy on weld joints is an automated spray ring. This method is typically followed by a manual plural-component spray (Figure 2). The manual plural-component spray procedure is also used in the field to coat complete pipe sections, turns, fittings, etc.
A new liquid epoxy application procedure is the sprayable cartridge. This method employs a pneumatic caulking gun. The dispenser is powered by an appropriately sized air compressor. The compressor provides plunger air and air at the tip of a static mixer to disperse heated liquid epoxy (Figure 3). The finished surface ranges from slightly bumpy to smooth. The finished product appearance, as with all application methods, is dependent upon the applicators’ proficiency. A primary concern with this method is that the air must be well conditioned to remove moisture. Moisture in the air stream at the static mixer will become entrained in the coating and cause failures.
The method of field application used most consistently is application with brush or roller. While labor intensive, this method requires less equipment and the simplest training for the applicator (Figure 4).
The coating is applied by either pouring it on the pipe followed by brushing or rolling, or by dipping a brush into the coating and applying.
One condition that often arises is a pipe section that must be coated while in service, and pipe temperature and ambient conditions affect the substrate to be coated (i.e., they cause sweating).
Most pipeline coatings are designed to be applied to clean, dry, properly prepared pipe that is above a certain temperature and dew point so moisture does not condense on the surface.
If these conditions cannot be met, the operator must find a coating that can be field-applied to a sweating surface while still meeting all other requirements for buried or immersed service, such as adhesion, resistance to operating temperature, soil stress resistance, and dielectric strength.
Application of such a coating requires strict adherence to the manufacturer’s application procedure, including surface profile and cleanliness. The applicator must ensure that the moisture is totally displaced or encapsulated, which is dependent upon the formulation of the coating. When correctly applied, this type of coating can save hours of waiting for a pipe to dry.
As discussed earlier, dry air also is essential for abrasive blasting. Contractors spend hours tearing down blast equipment that has become clogged or frozen due to moisture entrained in the air stream. A properly sized conditioning device will save time and money (Figure 5).
One last issue is the lingering belief among some applicators and inspectors that a pipe must be heated with a torch to remove the moisture from the metal. The metal contains no moisture other than what may form as dew on the surface. Some people still believe that there is moisture “in” the steel. The moisture that applicators see “coming out of the steel” is simply the moisture that is a by-product of combustion condensing on the surface. This condensation causes the flash rust that is observed. It is because of this flash rust that applicators are trained to brush blast any steel surfaces that have had direct flame contact after initial blasting for coating application.
A pipeline operator must choose a coating that will meet the pipeline operating parameters; be suitable for the environment where it will be used; and in the case of field application, require the least effort and complexity. Applicators should be trained in product-specific application procedures. Inspectors also should be trained regarding the specific coating they are inspecting. All should work together to ensure our infrastructure functions as it should and is safe.