Deterioration resistance

The low crystallinity (high degree of branching) and the polar properties of EVA copolymers, resulting from the vinyl acetate content, determine its lower chemical resistance compared to PE-LD, especially under the attack of acids. Increasing vinyl acetate content and increasing temperature deteriorate resistance of EVA. 

A.-H.J. Ai-Tayyib and M.M. Ai-Zahrani, Use of poiypropyiene fibers to enhance deterioration resistance of concrete surface skin subjected to cyciic wet/dry sea water exposure. Ad Mater. J. 87,1990, 363-370. 

The properties of SAN resins depend on their acrylonittile content. Both melt viscosity and hardness increase with increasing acrylonittile level. Unnotched impact and flexural strengths depict dramatic maxima at ca 87.5 mol % (78 wt %) acrylonitrile (8). With increasing acrylonitrile content, copolymers show continuous improvements in barrier properties and chemical and uv resistance, but thermal stabiUty deteriorates (9). The glass-transition... 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Elastomeric materials, which provide relatively low practical static deflections and have relatively high natural frequencies, are used only to isolate higher frequencies. The volume compressibiUty of elastomeric materials is relatively low, therefore the shape of the elastomeric isolator must be taken into account, and space must be provided for lateral expansion. Because of their inherent resistance to chemical and environmental deterioration, neoprene and other synthetic materials often can be used in severe environments where natural materials would deteriorate. 

Usually, copper surfaces are mated when joints must be periodically discoimected because copper offers low resistance and good wear. Junctions between copper and aluminum bus bars are improved by using a copper—aluminum transition joint that is welded to the aluminum member. Deterioration of aluminum shunt connections by arcing is eliminated when a transition joint is welded to both the primary bar and the shunting bar. 

Sihcone-based coatings are well suited for high temperature and high speed appHcations. They are flexible, tough, and resistant to thermal and oxidative deterioration. They have good surface resistance and are fungus- and flame resistant. However, they possess a high coefficient of thermal expansion and have poor adhesion. 

Plasticizer esters are relatively iaert, thermally stable Hquids with high flash points and low volatihty. Consequently they can be stored safely ia mild steel storage tanks or dmms for extended periods of time. Exposure to high temperatures for extended periods, as encountered ia dmms ia hot climates, is not recommended since it may lead to a deterioration in product quaUty with respect to color, odor, and electrical resistance. 

Many antioxidants ia these classes are volatile to some extent at elevated temperatures and almost all antioxidants are readily extracted from their vulcanizates by the proper solvent. These disadvantages have become more pronounced as performance requirements for mbber products have been iacreased. Higher operating temperatures and the need for improved oxidation resistance under conditions of repeated extraction have accelerated the search for new techniques for polymer stabilization. Carpet backiag, seals, gaskets, and hose are some examples where high temperatures and/or solvent extraction can combine to deplete a mbber product of its antioxidant and thus lead to its oxidative deterioration faster (38,40). 

For waterproofing, sodium silicate concentrations below 30% are adequate concentrations between 35 and 70% are used for strength improvement. Grouts having 35 vol % or higher silicate resist deterioration on freeze—thaw or wet—dry cycles. Water permeability of sands can be reduced from 10 to 10 cm/s. Unconfined compressive strengths of stabilized sand can vary from 103 to 4130 kPa (15—600 psi) the normal range is between 690 and 1380 kPa. 

Sulfur concretes are used in many specialty areas where Porfland cement concretes are not completely satisfactory. Because SC can be formulated to resist deterioration and failure from mineral acid and salt solutions, it is used for constmction of tanks, electrolytic cells, thickeners, industrial flooring, pipe, and others. In addition, SC is under investigation for many other prospective uses (58,59) (see Cement). 

Wood is widely used as a stmctural material in the chemical industry because it is resistant to a large variety of chemicals. Its resistance to mild acids is far superior to that of steel but not as good as some of the more expensive acid-resistant alloys. Wood tanks used to store cold, dilute acid have a relatively long service life. However, increasing concentration or temperature causes the wood tank to deteriorate rapidly (6). 

Heat resistance is improved markedly by resin impregnation. A block of impreg, subjected to forty-five 1-h exposures at 204°C, showed no apparent loss in properties, although an untreated sample showed signs of deterioration after three 1-h exposures (71). 

This boron- and carbon-doped SiC exhibits excellent strength and stiffness, extreme hardness, and thermal and chemical resistance. The strength of this system is not affected by temperatures up to I650°C. Creep is virtually nonexistent up to I400°C. CycHc durabiUty testing conducted at I370°C in air showed no deterioration of strength after 3500 h (94). 

Most recent studies (69) on elevated temperature performance of carbon fiber-based composites show that the oxidation resistance and elevated temperature mechanical properties of carbon fiber reinforced composites are complex and not always direcdy related to the oxidation resistance of the fiber. To some extent, the matrix acts as a protective barrier limiting the diffusion of oxygen to the encased fibers. It is therefore critical to maintain interfacial bonding between the fiber and the matrix, and limit any microcracking that may serve as a diffusion path for oxygen intmsion. Since interfacial performance typically deteriorates with higher modulus carbon fibers it is important to balance fiber oxidative stabiHty with interfacial performance. 

The selection of material to resist deterioration in seivdce is outside the scope of the B31.3 code (see Sec. 23). Experience has, however, resulted in the following material considerations extracted from the code with the permission of the pubhsher, the American Society of Mechanical Engineers, NewY ork. 

Vessels for high-temperature serviee may be beyond the temperature hmits of the stress tables in the ASME Codes. Sec tion TII, Division 1, makes provision for construction of pressure vessels up to 650°C (1200°F) for carbon and low-alloy steel and up to 815°C (1500°F) for stainless steels (300 series). If a vessel is required for temperatures above these values and above 103 kPa (15 Ibf/in"), it would be necessaiy, in a code state, to get permission from the state authorities to build it as a special project. Above 815°C (1500°F), even the 300 series stainless steels are weak, and creep rates increase rapidly. If the metal which resists the pressure operates at these temperatures, the vessel pressure and size will be limited. The vessel must also be expendable because its life will be short. Long exposure to high temperature may cause the metal to deteriorate and become brittle. Sometimes, however, economics favor this type of operation. 

Biological Corrosion The metabohc activity of microorganisms can either directly or indirectly cause deterioration of a metal by corrosion processes. Such activity can (1) produce a corrosive environment, (2) create electrolytic-concentration cells on the metal surface, (3) alter the resistance of surface films, (4) have an influence on the rate of anodic or cathodic reaction, and (5) alter the environment composition. 

Corrosion resistance of stainless steel is reduced in deaerated solutions. This behavior is opposite to the behavior of iron, low-alloy steel, and most nonferrous metals in oxygenated waters. Stainless steels exhibit very low corrosion rates in oxidizing media until the solution oxidizing power becomes great enough to breach the protective oxide locally. The solution pH alone does not control attack (see Chap. 4, Underdeposit Corrosion ). The presence of chloride and other strong depassivating chemicals deteriorates corrosion resistance. 

The electrolytes are non-corrosive and the electrodes do not corrode with time. This feature is of special significance when compared with an ordinary liquid resistance starter used commonly for slip-ring motors. Electrolytes do not deteriorate and therefore do not require replacement. The evaporated liquid can be replenished with drinking water when the level of the electrolyte falls as a result of evaporation. In Europe such starters have been used for over 15-20 years. Electrolyte switching is a costlier proposition compared to direct on-line or star/delta switching due to additional shorting contactor and timer, and the cost of electrolyte, its tank and thermostatic control etc. The cost may. 

A d.c. insulation resistance test or polarization index reveals only the surface condition of the insulation and does not allow a realistic assessment of internal condition. Loss tangent values are true reflections of the insulation condition to detect moisture content, voids, cracks or general deterioration. The tan 5 versus test voltage curve may be drawn and compared with the original curve provided by the manufacturer, and inferences drawn regarding the condition of the insulation. The different starting tan lvalues will reveal the condition of the insulation in terms of amount of contamination, as noted in Table 10.4 (See lEE, Vol. 127, May 1980). 

Corrosion-resistance test This can be done with the help of a salt spray test. The test piece is suspended in a salt spray chamber (Figure A13.6) for. seven days in 100% relative humidity (IS 101 and IS 11864). After the test, the surface should have no signs of deterioration or corrosion. 

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