Gasket Corrosion

Stagnant solution in small holes, gasket faces, lap joints, surface deposits, cervices under bolts and rivet heads are all sources of crevice corrosion. It is also called deposit or gasket corrosion. 

Sealed and purged or gasketed, corrosion resistant enclosures should be considered. 

Corrosion between the support device and the pipeline must be avoided. Drainage is provided by the pipeline slope, normally 0.48—0.96 cm /m of length, and gaskets must be nonabsorbent and of a type that does not affect the food product. 

Commercial-grade nitroparaftins are shipped and stored ia ordinary carbon steel. However, wet nitroparaftins containing more than 0.1—0.2% water may become discolored when stored ia steel for long periods, even though corrosion is not excessive. Aluminum and stainless steel are completely resistant to corrosion by wet nitroparaftins. Storage ia contact with lead (qv), copper, or alloys containing these metals should be avoided. Polymeric materials for gaskets, hoses, and connections should be tested for thek suitabiHty before exposure to nitroparaftins. 

Plate and frame coolers using HasteUoy C-276 plates have been used successfuUy. Anodically protected plate coolers are available as weU as plate coolers with plates welded together to minimize gasketing. Another promising development is the introduction of plate coolers made of HasteUoy D205 (105). This aUoy has considerably better corrosion resistance to concentrated sulfuric acid at higher temperatures than does C-276. Because of the close clearance between plates, cooling water for plate coolers must be relatively clean. 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

Lined pipe collapsed at gasket area or ID due to buildup of corrosion products between liner and carbon-steel pipe. 

Casing Joint Gasket. A confined-type nonashestos gasket suitable for corrosive service shall he provided at the casing joint. 

Another way to study corrosion in pipe lines is to install in the line short sections of pipe of the materials to be tested. These test sections should be insulated from each other and from the rest of the piping system by means of nonmetalhc couphngs. It is also good prac-tice to provide insulating gaskets between the ends of the pipe specimens where they meet inside the couplings. Such joints may be sealed with various types of dope or cement. It is desirable in such cases to paint the outside of the specimens so as to confine corrosion to the inner surface. 

Figure 13.64 Red, dezincified gasketed surface of a cast valve throat. Note the partial spalling of the copper corrosion product.

Solvent/fume leak- Maintain integrity of gaskets and seals age from casing, gaskets compatible with process materials joints resulting in loss of heck mating faces for corrosion/unevenness, containment. particularly on clad components ... 

Plate and Frame Composed of metal-formed thin plates separated by gaskets. Compact, easy to clean. Viscous fluids, corrosive fluids slurries. High heat transfer. Not well suited for boiling or condensing limit 350-500°F by gaskets. Used for Liquid-Liquid only not gas-gas. 0.8-1.5... 

Concentration cell corrosion occurs in an environment in which an electrochemical cell is affected by a difference in concentrations in the aqueous medium. The most common form is crevice corrosion. If an oxygen concentration gradient exists (usually at gaskets and lap joints), crevice corrosion often occurs. Larger concentration gradients cause increased corrosion (due to the larger electrical potentials present). 

Figure 2.5. Gasket insertion between pipe llangcs for sealing purposes and to minimize galvanic corrosion between dissimilar piping metals.

In situations in which piping protrudes partitions or bulkheads of dissimilar metals, proper precautions should be taken against galvanic corrosion. Possible solutions include the use of dielectric gaskets or sleeves and the use of plastic adhesive tapes. Examples are illustrated in Figure 2.6. 

Electrical equipment can be mounted in various types of enclosures. A weather-tight enclosure normally has a gasket and does not allow air (and the moisture contained in the air) to enter the enclosure. Offshore, such an enclosure, if properly closed, will help protect the enclosed electrical equipment from, corrosion due to salt water spray. These types of enclosures can be used in Division 2 areas provided they do not enclose arcing, sparking or high temperature devices. 

The construction materials for the plates include most corrosion-resistant metals, usually 304SS, 316SS, titanium, Incoloy 825 , Hastelloy , and others, plus nonmetallic fused graphite, and fluoroplastic Diabon F . Typical gaskets between the plates include nitrile rubber, butyl, and EPDM elastromers, Hypalon and Viton , based on the various manufacturers literature. 

Because systems are normally not designed for use with this type of fluid, certain aspects should be reviewed with the equipment and fluid suppliers before a decision to use such fluids can be taken. These are compatibility with filters, seals, gaskets, hoses, paints and any non-ferrous metals used in the equipment. Condensation corrosion effect on ferrous metals, fluid-mixing equipment needed, control of microbial infection together with overall maintaining and control of fluid dilution and the disposal of waste fluid must also be considered. Provided such attention is paid to these designs and operating features, the cost reductions have proved very beneficial to the overall plant cost effectiveness. 

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