Introduction
The use of Fiber Reinforced Polymer (FRP) for vessels and piping in the chemical processing industry (CPI) started in the 1950s. As experience with the behavior of the material system grew, standards and codes were developed for design and construction that aligned with standards and codes for equipment made with metallic materials. Several features were incorporated to create reliable operation.
One of the design features that evolved to significantly improve the reliability of FRP equipment is to incorporate a corrosion-resistant barrier onto the surface of the FRP that is to be exposed to corrosive chemical conditions – usually the inner surface of pipes, tanks or process vessels. The purpose of the corrosion-resistant barrier (“Corrosion Barrier”) is to protect the FRP used for structural support (structural FRP) from damage by the operating environment.
When the use of corrosion barriers was introduced, many owner-operators determined that a key to FRP reliability was to properly maintain the corrosion barriers. The principle behind this is to monitor the condition of the corrosion barrier. This approach almost always requires an outage and confined space entry.
This article describes the construction of corrosion-resistant FRP and the practices used for inspecting corrosion barriers, starting from the original visual inspection, to microscopic evaluation of sections through cutouts, and then to an advanced ultrasonic method that yields good correlation to destructive analytic results. Advanced ultrasonic techniques provide additional results that can be directly related to an ASTM standard[1] that is used globally to provide quantitative performance of FRP in corrosion service.
FRP Construction
Fiber reinforced polymers are used in many corrosive applications because the polymers provide superior corrosion protection to many metal alloys and they also protect the fiber reinforcements that provide structural properties. The surfaces and areas of FRP that will be exposed to corrosive service conditions are covered with a corrosion-resistant barrier.
The corrosion-resistant barrier is normally composed of layers of reinforced thermosetting polymer or a thermoplastic sheet. A thermosetting polymer is a polymer that is applied in liquid form with curing agents added that react with the polymer to form bonds between the polymer chains, known as cross-linking. Examples of thermosetting polymers include epoxy, vinyl ester, and polyester resins. A thermoplastic polymer is a polymer that can be deformed by some combination of heat and stress. Example thermoplastic materials include: polypropylene; polyvinyl chloride; polyethylene; polyvinylidene fluoride; and many others. Figure 1 shows typical configurations.
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