Although the anticorrosion coating combination of fusion-bonded epoxy (FBE) with abrasion-resistant overcoat (ARO) is well known within the oil and gas industry, ARO coating systems are not typically chosen by specifiers in the municipal water industry. However, a recent water pipeline project was successfully lined internally with cement mortar after applying both FBE and ARO and showed the advantages of using this combination to protect municipal water piping from corrosion.
FBE is an epoxy-based thermosetting powder coating widely applied to steel pipe, steel reinforcing bars for concrete, tanks, and a wide variety of piping connections (valves and fittings). The “fusion-bonded” designation comes from the chemical cross-linking that occurs during the application process, which differs from the application of standard paints and coatings. FBE is applied by preheating the substrate, typically over 205 °C (401 °F) and spraying the coating powder onto the surface. Small parts and fittings can be dipped into FBE powder that has been fluidized by air. When applied as a base layer in a multi-coating system, FBE is typically referred to as the primary corrosion protection layer.
ARO is a thermosetting powder coating applied over a compatible FBE during the initial coating application process to protect the FBE coating during storage, handling, and installation. The FBE layer is first applied electrostatically to the pipe surface and then the ARO is applied in the same fashion in a second spray booth immediately afterward. The dual coating is then water quenched to cool and set. When applied, the ARO becomes integrally bonded to the corrosion-resistant FBE base layer and forms a tough outer layer that is resistant to gouging, impact, abrasion, and penetration. ARO enhances the overall coating system’s adhesion performance and is useful in situations where pipe must be pulled through a bore hole, and where handling, installation, and rocky soils during backfill can damage the primary coating.
Coatings must be applicable for use with drinking water. FBE and ARO coatings contain no solvents and are based on 100% solids technology. When designed for contact with potable water, FBE and ARO coatings meet or exceed many applicable standards for potable water from the American Water Works Association (AWWA), International Organization for Standardization (ISO), American National Standards Institute (ANSI), and NSF, including NSF/ANSI 61.3.
Typically, water transmission pipelines are internally lined first, and then externally coated to minimize handling damage to the external coating. The most common lining used for water transmission pipelines is cement mortar. The centrifugal cement lining process distributes the mortar throughout the length of the pipe through a retractable lance while the pipe is spinning at a relatively low speed. After the mortar is applied, the cement lining is smoothed and compacted by spinning the pipe at a higher speed (typically 550 to 650 rpm) and vibrating the pipe to expel the water and produce a dense cement mortar lining that is in intimate contact with the pipe surface. Historically, the major obstacle to externally coating cement mortar-lined water pipelines with FBE was that the pipe had to be lined first. The concern was that handling and heating of the pipe during the FBE application may damage the cement mortar lining.
For this South Dakota, water pipeline project, technical staff carefully reviewed the coating process and proposed coating the external pipe surface with a dual layer of FBE/ARO first, then lining the pipe with cement mortar. This plan would only work if the ARO protected the FBE during the cement lining process as the lining process plus the pipe-handling equipment could damage the FBE and lead to excessive repairs.
The water pipeline owner and the project’s engineering team were interested in this innovative approach for their Treated Water Pipeline Segment 1 project because of the proven success of FBE coatings in the oil and gas market, plus their low cathodic protection current requirements, high adhesion, abrasion resistance, physical strength, and non-shielding characteristics. An intact, well-bonded coating with good corrosion protection properties was critical as the pipeline route was located in extremely corrosive soil conditions, with very low soil resistivity (<500 Ω-cm) and high chloride content (>4,500 ppm).
While several standards cover FBE pipeline coatings, none of them were directly applicable to the proposed dual layer FBE/ARO coating. The pipe plant’s quality control department proposed, and the engineering team utilized, an evaluation of the FBE/ARO coating per the Canadian Standards Association (CSA) criteria for FBE,7 which was much more rigorous than evaluations normally used in the water industry for other types of coatings in terms of the number and types of quality control and performance testing required and the testing acceptance criteria.
Pipe plant quality control testing included salt contamination and surface profile tests, visual inspection, and a clear tape test to determine the cleanliness of the pipe surface. Visual and physical inspection of the applied coating included a high-voltage holiday test, dry film thickness test, durometer (hardness) test, weight drop test, bend test, 24-h water soak test, 24-h cathodic disbondment test, backside contamination test, and backside and side porosity test. The results showed the FBE/ARO coating met the minimum CSA testing criteria, and remained in excellent, undamaged condition during the cement mortar lining and handling processes. When pull-off adhesion was tested per ASTM 4541,8 no substrate-type failures, intercoat delamination, or separation of the FBE from the ARO layer were observed. Some of the adhesion tests went to almost 6,000 psi (41 MPa) before the glue failed, which confirmed that the FBE/ARO coating system was tightly bonded to the pipe surface and to itself.
Once the application trial proved to be successful, the remaining 12 mi (19 km) of 24-in (610-mm) diameter steel pipe was coated by the pipe manufacturer in its plant. The nominal applied thickness of the FBE was 16 to 20 mils (405 to 510 μm), while the nominal thickness of the ARO was 20 to 25 mils (510 to 635 μm). Minimal to no damage on the coated pipe during transportation and installation was reported except for the few times when the pipe was mishandled.
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Originally appeared on materialsperformance.com as “Fusion-bonded epoxy coating protects water pipeline from Corrosion” by Dustin Traylor, global functional product manager/technical services manager with Axalta Coating Systems, Philadelphia, Pennsylvania, USA.