Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
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M AT E R I A L S S E L E C T I O N & D E S I G N Preventing Corrosion under Insulation with Fiber-Reinforced Polymer Laminates FIguRE 3 (a) Representation of a pipe wall and (b) representation of pipe wall and composite repair. areas. It can occur in similar and dissimilar metals, and may be caused by the retention of water in the crevice, which creates differential aeration and chloride concentration cells. Pitting corrosion is very similar to crevice corrosion, and relies heavily on a local holiday in the passive layer to act as an initiation point. Metals that rely on the formation of passive layers for protection, such as stainless steels (SS), are highly susceptible to pitting and crevice corrosion. Corrosion Prevention Stainless Steel Regrettably, many people believe that the solution to all corrosion problems is to use SS.2 While ferritic SS (400 series) is highly resistant to SCC, its resistance tends to drop with elevated temperatures, making it less attractive for some processes. The 400 series steels are a better choice than the 300 series (austenitic) steels. It has been extensively documented in the literature that chlorides attack the protective passive layer of 300 series steels, and can quickly cause pitting and crevice corrosion, and hot, pressurized pipe is vulnerable to SCC. Fiber-Reinforced Polymer Laminates Clearly, the best way to avoid CUI is to prevent or minimize the conditions that lead to the attacks described previously. My experience has shown that the application of a highly chemically resistant novalac epoxy fber-reinforced polymer 76 MATERIALS PERFORMANCE May 2013 (FRP) will protect existing piping and restore lost structural integrity. Figures 1(a) and (b) illustrate a piping tee that experienced pitting and crevice corrosion, and was successfully repaired using FRP. ASME3 has developed guideline qualifcation standards and equations to calculate the required laminate thickness to restore a pipe to its original integrity. Designing FiberglassReinforced Polymer Repairs In many cases, FRPs can effectively restore pipeline strength that has been lost to CUI. FRPs are strong, and in many cases, the tensile strength of glass or carbon fbers exceed that of steel. Steel's stress-strain curve allows for signifcant yielding (strain) before failure should the stress stray beyond yield and into the plastic deformation zone. Composite materials, however, do not allow for any plastic deformation once limiting strain values are attained. The amount of preexisting strain at the time of installation affects the overall ability of the FRP to add additional strength (Figure 2). If we take the available strength of steel to be at the 0.2% offset, the limiting stress can be found at s=eE, where E is Young's modulus and e is the strain limit (0.002 for this example). If the modulus of steel is taken as 29E6 psi, the strength before yield is 0.002 × 29E6 psi = 58,000 psi, less any factors of safety called for in the design. Again referencing Figure 2, observe the two cases where the FRP is applied: 1) with the pipe unloaded and 2) with the pipe under load. In both cases, the limiting case is the yield point of steel. When the FRP is applied, the FRP assumes a portion of the load, but only above the point at which the FRP was installed. If the FRP is applied while the pipe is loaded under pressure, then the FRP acts only to assist when the pressure rises above that point. When dealing with composites, strain has to be considered too. Figures 3(a) and (b) present another representation of the pipe. In this representation, the pressure force is acting to "pull" the pipe apart, where the spring stiffness of the pipe wall is what is keeping it together. Figure 3(b) shows two springs in parallel. In this case, the force is divided between two springs and the amount of "stretch" on each spring is less. The load shared between the two springs is dependent on the individual spring stiffness. If one of the two springs is weaker, it would still have to stretch, but the other spring would be doing more of the work. In a composite repair, the stiffness is proportional to the modulus. Steel is fairly stiff (high modulus). FRP material is not as stiff, and has a lower modulus, about one-third that of steel. Therefore, to replace the steel "spring" with a composite "spring," it needs to be three times as large. Properties of FiberglassReinforced Polymer Repairs FRP repairs do not require hot-work permits. Many industrial facilities operate for years between scheduled maintenance shutdowns, and establishing safe plant conditions for hot-work may be extremely diffcult to establish between shutdowns. Depending on the severity of the CUI, specialized no-spark pneumatic surface NACE International, Vol. 52, No. 5