Resistance to chemical attack

For the development and application of plastic materials, proof of sufficient resistance to media load represents an important goal and a decisive parameter. In various branches of industry, such as chemical equipment construction, the packaging industry, building construction, medical technology, and microelectronics, medial behavior alone is often in fact decisive for the actual use of plastic articles. The physical and/or chemical interactions initiated by media contact generally lead to less serviceability and often premature failure of components and protective coatings [162], 

However, the high resistance of plastics to media that often destroy metallic materials very quickly has led to their increasing substitution by plastics in numerous important application areas. Yet plastics, too, are less resistant to certain media and, depending on their chemical composition and physical structure, are subject to certain changes that can have both a reversible and an irreversible character [13]. 

Questions as to the chemical resistance of a material appear at first glance to be easily answered by the classifications resistant , limited resistance and not resistant . In fact, many of the complex processes initiated by exposure of plastics to chemicals have not yet been completely explained. 

Depending on the type of effect, we distinguish two groups of media  

The effects of an aggressive medium on plastics can be subdivided into the following sub-steps  

Refractory materials are exposed to a wide variety of environments in their applications. Many of these environments can attack the materials. An environment could be in a molten state such as with molten metal and slag, both of which are highly corrosive. Hence, it is necessary to characterize the corrosion resistance of refractories in the particular enviroiunent in which they are going to be used. 

Available methods can only compare different materials on the basis of their corrosion resistance. Their actual behavior cannot be determined because the service conditions cannot be simulated in laboratory tests. For example, service temperature tends to vary with time. Laboratory tests are carried out at a constant temperature. Hence, the effect of thermal gradients is not considered in laboratory tests. 

Static or dynamic methods are used to find the attack of molten media. The test for slag resistance is a static test. There are three types of tests for slag resistance. 

In the second method, ground brick and slag are mixed in various proportions. Refractoriness of each of these mixtures is determined. A composition versus refractoriness graph is drawn for each brick for different slag as well as for each slag and different brick combination. Based on these graphs, proper brick-slag combination is selected. 

In the third type of test, individual bricks are dipped in slag or hot slag is sprayed onto walls built into specially designed furnaces. Field trials can also be performed by incorporating test bricks into working equipment. 

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Silver has little tendency to formally lose more than one electron its chemistry is therefore almost entirely restricted to the + 1 oxidation state. Silver itself is resistant to chemical attack, though aqueous cyanide ion slowly attacks it, as does sulphur or a sulphide (to give black Ag S). hence the tarnishing of silver by the atmosphere or other sulphur-containing materials. It dissolves in concentrated nitric acid to give a solution of silver(I) nitrate. AgNOj. 

Metallic gold, which is found free in nature, has always been valued for its nobility, i.e. its resistance to chemical attack. This property is to be expected from its position in the electrochemical series. It... 

Chemical Properties. Molybdenum has good resistance to chemical attack by mineral acids, provided that oxidizing agents ate not present. The metal also offers excellent resistance to attack by several liquid metals. The approximate temperature limits for molybdenum to be considered for long-time service while in contact with various metals in the hquid state ate as follows ... 

Isophthahc (y -phthahc) acid [121 -91 -5] (IPA) is selected to enhance thermal endurance as well as to produce stronger, more resiUent cross-linked plastics that demonstrate improved resistance to chemical attack. TerephthaUc (p-phthaUc) acid [100-21-0] (TA) provides somewhat similar properties as isophthahc acid but is only used in selective formulations due to the limited solubiUty of these polyester polymers in styrene [100-42-5] (see Phthalic acid AND OTHERBENZENEPOLYCARBOXYLIC ACIDS). 

Resistance to Chemicals. Different species of wood vary in their resistance to chemical attack. The significant properties are beheved to be inherent to the wood stmcture, which governs the rate of ingress of the chemical and the composition of the cell wall, which affects the rate of action at the point of contact (56). 

Resistance to chemical attack is generally improved by resin impregnation, which protects the underlying wood and reduces movement of Hquid into the wood. Resistance to acids can be obtained by impregnating with phenoHc resin and to alkaHes by impregnating with furfural resin (see Phenolic RESINs). 

Chlorinated paraffins are relatively inert and exhibit excellent resistance to chemical attack and are hydrolytically stable. They are soluble in chlorinated solvents, aromatic hydrocarbons, esters, ketones, and ethers but only moderately soluble in ahphatic hydrocarbons and virtually insoluble in water and lower alcohols. 

Tin—Nickel. AHoy deposits having 65% fin have been commercially plated siace about 1951 (135). The 65% fin alloy exhibits good resistance to chemical attack, staining, and atmospheric corrosion, especially when plated copper or bron2e undercoats are used. This alloy has a low coefficient of friction. Deposits are solderable, hard (650—710 HV ), act as etch resists, and find use ia pfinted circuit boards, watch parts, and as a substitute for chromium ia some apphcafions. The rose-pink color of 65% fin is attractive. In marine exposure, tin—nickel is about equal to nickel—chromium deposits, but has been found to be superior ia some iadustfial exposure sites. Chromium topcoats iacrease the protection further. Tia-nickel deposits are bfitde and difficult to strip from steel. Temperature of deposits should be kept below 300°C. 

Resistance to Chemical Attack. The resistance to alkah and acid attack is evaluated on the basis of loss in weight, loss in gloss, or reduced cleanabihty of the surface. 

Nickel and Nickel Alloys A wide range of ferrous and nonfer-rous nickel and nickel-bearing alloys are available. They are usually selected because of their improved resistance to chemical attack or their superior resistance to the effects of high temperature. In general terms their cost and corrosion resistance are somewhat a func tion of their nickel content. The 300 Series stainless steels are the most generally used. Some other frequently used alloys are hsted in Table 10-35 together with their nominal compositions. For metallurgical and corrosion resistance data, see Sec. 28. 

Minimum resistance to filtrate flow (i.e., high produc tion rate) Resistance to chemical attack... 

Polyethylene, in essence a high molecular weight alkane (paraffin), would be expected to have a good resistance to chemical attack and this is found to be the case. 

In addition to the four compounds discussed above, the final Portland cement may contain gypsum, alkali sulfates, magnesia, free lime and other components. These do not significantly affect the properties of the set cement, but they can influence rates of hydration, resistance to chemical attack and slurry properties. 

Compared with platinum, rhodium and iridium, palladium has much less resistance to chemical attack. Its theoretical corrosion diagram is depicted in Fig. 6.5, from which it may be seen that the metal is stable in the presence... 

Crucibles and basins required for special purposes are often fabricated from various metals, amongst which platinum holds pride of place by virtue of its general resistance to chemical attack. 

A recent development is the introduction of ZGS (Zirconia Grain Stabilised) platinum. This is produced by the addition of a small amount of zirconia (zirconium(IV) oxide) to molten platinum, which leads to modification of the microstructure of the solid material with increased hot strength and greater resistance to chemical attack. Whereas the recommended operating temperature for pure platinum is 1400 °C, the ZGS material can be used up to 1650 °C. 

Hafnium nitride (HfN) is a refractory which is resistant to chemical attack. It is produced by CVD mostly on an experimental basis. Its properties and characteristics are summarized in Table 10.4. 

Among elastomers, artificial rubbers have replaced natural rabber for many uses because of their high resistance to chemical attack by ozone, an atmospheric pollutant. When ozone reacts with polymer chains, it breaks CUCn bonds and introduces additional cross-linking. Breaking 7r bonds causes the rabber to sofien, and cross-linking makes it more brittle. Both changes eventually lead to rupture of the polymer structure. 

Because CFCs are highly resistant to chemical attack, they are stable in the lower atmosphere, where they can exist for up to 100 years. This stability gives CFCs time to diffuse up through the troposphere and into the stratosphere. There, CFCs absorb short-wavelength ultraviolet light from the sun that breaks carbon-chlorine... 

High-alumina cement is a rapid-hardening cement made from bauxite and limestone. It is comparatively resistant to chemical attack. Milling retards the setting of aluminous cement [1582]. On the other hand, setting accelerators such as lithium carbonate increase their effect by this treatment. 

The solvent reservoir is a storage container made of a saterial resistant to chemical attack by the mobile diase. In its simplest fora a glass jug, solvent bottle or Erlenmeyer flask with a cap and a flexible hose connection to the pump is adequate. The PTFE connecting hose is terminated on the solvent side with a 2 micrometer pore size filter to prevent suspended particle matter from reaching the pump. In more sophisticated instruments the solvent reservoir may also be Cjquipped for solvent degassing, as... 

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