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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:
- Barrier linings and coatings.
- Self supported equipment and structures from
plastics and composites.
- Use of Seals, gaskets, adhesives and caulks.
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:
- Spalling.
- Pinholes.
- Blistering.
- Cracking.
- Delamination.
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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