Corrosion-resistance Corrosive fluoropolymers

Materials of Construction. Glass has excellent corrosion-resistance to wet or dry bromine. Lead is very usefiil for bromine service if water is less than 70 ppm. The bromine corrosion rate increases with concentrations of water and organics. Tantalum and niobium have excellent corrosion-resistance to wet or dry bromine. Nickel has usefiil resistance for dry bromine but is rapidly attacked by wet bromine. The fluoropolymers Kynar, Halar, and Teflon are highly resistant to bromine but are somewhat permeable. The rate depends on temperature, pressure, and stmcture (density) of fluoropolymer (63). 

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

Materials that come in contact with wet halogens must be corrosion-resistant. Glass, ceramics, tantalum, and fluoropolymers are suitable materials. Granite has been used in steaming-out towers. 

Hydrogen fluoride is very corrosive to many metals, glass, and other materials. Many plastics are resistant to HF and fluorinated polymers such as Kel-F, Teflon, Teflon-PFA, and others are inert to HF. A variety of useful equipment for handling and studying HF can be easily prepared from commercially available fluoropolymers and fluoropolymer components. Copper, Monel, and nickel are excellent metals for handling HF at higher temperatures. Stainless steels are satisfactory at lower temperatures. 

With the exception of two fluoropolymers, PVF and PTFE, the rest of the resins described in this entry can be processed by standard melt-processing techniques, such as injection, transfer and blow molding, extrusion, and rotational molding. Process equipment for fluoropolymers must be made from corrosion resistant alloys because of the corrosive compound that may be produced when fluoropolymers are heated above their melting points. Higher melt viscosity of these resins may require more powder and higher pressure rating equipment. 

Corrosion resistant fluoropolymer coatings are currently marketed by W. L. Gore Associates, Inc. (Fluoroshield) and Pfaudler. 

Another dynamic seal type used is the poly(tetrafluoroethylene) (PTFE) seal utilized on applications with PV values that are not as stringent as those of clutch systems. PTFE is a fluoropolymer that has many other applications besides automotive. DuPont has the trade name Teflon for this material. Other usages include nonstick cookware, lubricants, bearings, bushings, gears, and plumbing materials. It is useful as a resistant material to corrosive and reactive chemicals. It is an extremely nonreactive material that was discovered by accident by Roy Plunkett when he... 

Selection of fluoropolymers is an integral part of the overall material selection process. This implies that all the available materials such metals, ceramics, and plastics are considered candidates for an application. The end user then considers these materials against established criteria such as required life, mean time between inspection (MTBI), ease of fabrication, frequency of inspection, extent of maintenance and, of course, capital cost. More often than not it is the initial capital cost, rather than the life cycle cost of equipment, that affects the decision made during the material selection step. However, the most important piece of data is the corrosion resistance of a material in the medium under consideration over the life of the equipment. This information is available in a different format for plastics than for metals. A comparison is appropriate. 

Selecting Fluoropolymers for Corrosion Control Table 4.5. Chemical Resistance of Filled PTFE Compounds ]... 

Products of the degradation of molten fluoropolymers are highly corrosive, often containing hydrofluoric acid. The parts of the machine that come in contact with molten fluoropol5miers must be constructed from corrosion-resistant metals that are significantly more expensive than lower grades of steel. Corrosion of process surfaces can result in the contamination of the finished product and deterioration of its physical properties. 

Monoaxially and biaxially oriented films of fluoropolymer are made by melt extrusion of the resin into flat webs or tubes. The main function of orientation is to enhance the mechanical properties of the film such as tensile break strength and tear resistance. The decision to orient is usually made according to the requirements of the end use for mechanical properties. All process surfaces that contact molten fluoropolymers must be corrosion resistant because of the formation of corrosive compounds such as HF and HCl from the high-temperature degradation of these plastics. 

All parts that contact the fluoropolymer melt must be corrosion resistant. 

Synthetic rubbers, EPM/EPDM, nitrile, polychloroprene (neoprene), epichlorohydrin, and polyacrylate have good oil resistance, heat stability, and chemical resistance. Fluoropolymers are used in oil and gas wells 20,000 ft (6096 m) deep. These depths can have pressures of 20,000 Ib/in (137.5 MPa) which cause extrusion failures of down-hole seals by forcing the rubber part out of its retaining gland. TFE/propy-lene jackets protect down-hole assemblies which consist of stainless steel tubes that deliver corrosion-resistant fluid into the well. 

Corrosive wear also occurs in non-filled polymers well-known examples are fluoro-polymers and chlorine containing polymers, such as HPFA and PVC. Fluoropolymers have a tendency to form hydrofluoric acid at high temperatures in combination with air and moisture. PVC tends to generate hydrochloric acids at elevated temperatures. The corrosion problem is generally more severe with rigid PVC than with flexible PVC. With polymers like these, the metal parts in contact with the polymer should be made out of a corrosion-resistant metal, such as Hastelloy, 17-4 PH, 15-5 PH, etc. 

The versatile new fluoropolymer found use in other applications where tough and abrasion resistant properties were important. For example, corrosion engineers and cathodic protection service companies found that PVDF insulation solved many of the corrosion problems associated with underground anode-bed installations and down-well jacketing to protect the instrumentation cable in deep wells. 

Nonetheless, it can be seen that a range of fillers and polymeric matrices were studied and different ways were used for the production of composite bipolar plates. Graphite and carbon black as fillers are to be found in nearly every study, and accordingly, some polymers are preferred to be used for composite bipolar plates. The reasons for both the fillers and the matrices are obvious. Graphite and carbon black have outstanding chemical stability against corrosion when compared with metallic fillers they achieve an adequate conductivity and are obtainable at a reasonable price. In case of the matrices the chemical corrosion resistance is also a main criterion, and polyolefin materials, fluoropolymers, polyphenylene sulfide (PPS), and phenolic resins are particularly favored. 

Fluoropolymers have excellent heat, chemical and corrosion resistance. The most common is polytetrafluoroethylene (PTFE), often known by the Dupont trademark, Teflon . Other tradenames include Dyneon (3M) and Fluon (Asahi Glass). The invention of PTFE is often used as an example of serendipity, but it was actually a combination of serendipity, curiosity, and hard work. Roy Plunkett was working on experimental refrigerants when a cylinder that had been filled withtetrafluoroethylene (TFE) gas did not deliver gas when the valve was opened. Often, when something does not occur as planned, people discard the results and move on, but Plunkett was curious. When the cylinder was cut open, a white lubricious solid was discovered. Further investigation revealed the solid to be a polymer of tetrafluoroethylene [28]. 

In 1939, Roy Plunkett (1911-1994) at Dupont discovered the polymerization of TFE to give PTFE the chemically and thermally most resistant fluoropolymer ever made (Scheme 2.4) [33]. However, similar to Schloffer s and Scherer s experience with the same material, Plunkett would not obtain any royalties from Dupont as the company would not work on PTFE for another four years for the same reasons [32]. It was only in 1943 when the Manhattan project created a demand for corrosion-resistant liners and gasket for reactors and valves to handle highly corrosive UEg [34]. Then, people at Dupont remembered the highly hydrophobic and chemically inert material. This is when PTFE came into play again and its small-scale production started. At the end of the 1940s, PTFE was produced on a small scale for the civilian market under the brand name Teflon . 

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