External barrier coatings

Carbon and low-alloy steels are probably the most commonly used materials for pipes handling water, petroleum products, and some chemicals. Reference 1 provides a summaiy of the different specifications used for pipelines. Steel tends to corrode by both pitting and uniform surface deterioration [2]. Steel must usually be protected from corrosion both on the inside from the material being carried and on the outside from corrosion by the atmosphere, soil, or water that surrounds the pipe. External corrosion protection is provided by material selection, selective backfill, barrier coatings, stray current control, and cathodic protection. Internal corrosion protection can be provided by inhibitors, coatings, design process control, and materials selection. 

In this context plasma is a gas containing free electrons, ions, and neutral particles and may be created by applying external excitation to the gas. When used to polymerize ethylene a plasma can give coatings of higher molecular weight, cross-linked and without pores—and provide relatively impermeable barrier layers on blow mouldings. 

The choice of permeation barrier materials selected here for coatings is reasonable based on permeation resistance, but external coatings of ceramics on metals are difficult to perfect since many metals have high thermal expansion coefficients and most ceramics have low ones. This causes large thermal stresses in the coatings to develop, which leads to defect formation in the coatings and lowers permeation resistance. A better technique, which will be discussed in the next section, relies on the formation of intrinsic oxide films on the surface of the metals, either by direct oxidation or by alloying followed by oxidation." ... 

Permeation reduction factors of up to 10,000, or 10 , have been realized with the best coatings based on aluminized steels. Ferritic-martensitic steels that were aluminized had the Fe2Al5 phase predominant in the layer sequence, while a 316L steel had FeAlj and FeAl2 as the main aluminide phases. The best permeation barrier resulted from an external alumina film of about 1 micron in thickness grown on the aluminide layers. ... 

Plasma polymer-coated [2B] surfaces showed higher polarization resistance than native and chemically cleaned surfaces. Thus, the corrosion resistance of plasma polymer-coated [2B] was much higher than that of the barrier-type oxides formed after chemical cleaning. Also, as is evident from the higher polarization resistance of the plasma polymers, they are good barriers to water, oxygen, and corrosive species, even under an externally applied potential. The Rp values of the... 

For this reason, encapsulation is used to isolate devices from their external surroundings. Encapsulants comprise a variety of materials viz., metals, inorganic glasses, and conformal organic polymer coatings. Each has the same function, to act as a protective molecular barrier. 

In addition to internal or external coatings, barrier enhancers can be incorporated into the plastic as additives. These can include various metal oxides, glass fibre, mica, etc. Incorporating a foil ply between layers of plastic is a further way of obtaining excellent barrier properties, e.g. multilayer laminated tubes, cold formed blisters, and additional overwraps should not be ignored. 

Certain preservative systems can be lost from certain plastic packs by sorption and solubility in the plastic and, if they are also volatile, by evaporation from the external surface which is in contact with the atmosphere. Reports, particularly as references in textbooks, often suggested that phenol could not be used as a preservative in lowdensity packs. Subsequent work, many years later, established that the solubility of phenol in polyethylene was relatively low and that sufficient phenol for adequate preservative efficacy could be retained by a number of methods, i.e. coating LDPE internally or externally, or enclosing the pack in an additional barrier overwrap (e.g. paper/foil/PE or similar foil-bearing barriers). 

Thin elastomeric coatings or coverings, which also serve as a moisture barrier, act as skin, preventing evaporation and leakage of the electrolyte solution(s). This allows these actuators to be fiiUy operational anywhere. Using a bilayer coating can provide for a compact actuator, where the innermost layer (the layer closer to the EAP) is a conductive layer and serves as the external electrode (Fig. 4.27). 

Of the two most commonly used preform materials, one, alumina, is chemically inert during DMO while the other, silicon carbide, is reactive. We first examine the growth of a DMO composite into an alumina particulate, followed by that into SiC preforms, and finally the DMO infiltration of a suitably coated fibrous preform. Coatings of calcium sulfate with calcium silicate may be used to ensure that oxidative growth ceases at the external boundary of the preform, thereby ensuring near-net-shape fabrication. The mechanism by which these barriers poison the oxidation without impeding the flow of oxygen is not clear at this time. 

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