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Fight Corrosion With Plastics
Condensed from
Flight Corrosion with Plastic, by P. Khaladkar, DuPont Engineering, Chemical
Engineering, October 1995.
Rust and corrosion are common problems in process systems which combine to deteriorate the performance and contaminate equipment. Additionally, processors are running plants longer and using higher temperatures and throughput to meet increased production demands and reduce operating costs but often at the cost of increased corrosion severity. One approach to reduce corrosion of steel equipment that is applicable in many cases is the use of plastics, elastomers and composites. These applications include:
The most important differences between designs with metals and polymeric materials are the lower temperature limits of the latter along with high creep rates. Furthermore, the properties of polymers may also change with time resulting from absorption, permeation and other aging phenomena. Service experience with these materials is usually the best and most reliable predictor of success. However, due to the variables involved in polymeric materials, fabrication methods and use conditions, detailed written specifications are also needed for each of these areas. Another way to increase the probability of success with polymeric materials is the use of laboratory testing followed by in-plant tests where possible.
For barrier coatings and linings, the use of exposure tests (ASTM C868) is common. Also, attention to correct surface preparation is a must in obtaining good service performance. This includes the use of blend grinding of weld joints, rounding of sharp corners and preparation of "white metal surfaces" (SSPC SP-5) with a good anchor pattern free from contamination. Thin film coatings such as epoxies and phenolics are also used but require careful selection for chemical compatibility. For thicker linings, materials such as PVC, vinyl esters, fluorocarbons and polyethylene (PE) are very useful.
Polymers such as PVC, CPVC, PE, polypropylene (PP) and fluoropolymers can also be utilized to fabricate a range of components including fittings, piping and tanks. Composite structures and equipment made from a range of fiber reinforced plastics (FRP) offer high corrosion resistance and reduced maintenance costs. Again, specifications are very important when using FRP equipment with designs performed per ASME RTP-1 (low pressure) and/or Section X of the ASME Boiler and Pressure Vessel Code (high pressure) which includes workmanship standards and acceptance criteria. Piping should be made to ASME B-31.3. To increase chemical resistance of FRP equipment, the innermost layer is usually composed of around 90 percent chemically resistant polymeric resin with only 10 percent fiber.
Failures of polymeric materials can be a valuable source of information that can be used in the design of future equipment. The type of failure should be fully characterized. Common modes of deterioration include:
Additional information such as loss of adhesion and discoloration can also be useful in determining the influence of permeation or absorption.
Condensed from Flight Corrosion with Plastic, by P. Khaladkar, DuPont Engineering, Chemical Engineering, October 1995.
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