Resistance to Alkali Attack

Semi-silica compositions The NazO-95% Si02/5% A1203 vertical section (Fig. 9) shows that as Na20 reacts with a semi-silicate refractory, all Si02 phases should disappear, and tridymite, the only stable form of silica, will form very slowly. Albite will form only at temperatures below 1050°C and liquid is present down to 740°C. Semi-silica refractories are not resistant to alkali attack, and may be even less alkali-resistant than high-silica refractories. 

Alkali resistant (AR) glass fibres have been developed with a specific formulation that includes a substantial amount of zirconia (Z1O2). The higher the zirconia content, the better the resistance to alkali attack of the fibres. 

The number of -vinyl groups in a polymer chain can he varied and this will influence the physical properties of the material being created. The larger the number of -vinyl groups, the greater the extent to which a carbon based cross-linked structure is created. Since the carbon based chain is resistant to hydrolysis, the HTV materials have better resistance to alkali attack than comparable RTV materials. 

A different approach was taken by Majumdar [7,8] to develop glass fibres which are resistant to alkali attack. In developing AR glass fibres attention was given to two main aspects ... 

As may be expected of an amorphous polymer in the middle range of the solubility parameter table, poly(methyl methacrylate) is soluble in a number of solvents with similar solubility parameters. Some examples were given in the previous section. The polymer is attacked by mineral acids but is resistant to alkalis, water and most aqueous inorganic salt solutions. A number of organic materials although not solvents may cause crazing and cracking, e.g. aliphatic alcohols. 

Mineral acids attack the nylons but the rate of attack depends on the type of nylon and the nature and concentration of the acid. Nitric acid is generally active at all concentrations. The nylons have very good resistance to alkalis at room temperature. Resistance to all chemicals is more limited at elevated temperatures. 

Resistant to alkalis, most organic solvents Attacked by acids... 

Polymethyl Methacrylate (PMMA), This material has exceptional optical clarity and resistance to outdoor exposure. It is resistant to alkalis, detergents, oils and dilute acids but is attacked by most solvents. Its peculiar property of total internal reflection is useful in advertising signs and some medical applications. 

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. 

Nylons, however, are to some extent subject to deterioration by light. This has been explained on the basis of chain breaking and crosslinking. Nylons are liable to attack by mineral acids but are resistant to alkalies. They are difficult to ignite and are self-extinguishing. 

Lead is not particularly resistant to alkalis, but in some cases the corrosive action of sodium hydroxide and potassium hydroxide can be tolerated (KOH to 50% and up to 60°C, NaOH to 30% and 25°C, 10% and 90°C) The rapid attack of lime solutions is discussed earlier also see Section 19.3). 

Attention must be directed to the fact that alkaline solutions, particularly if concentrated, attack glass. They may be preserved, if required, in polythene bottles, which are resistant to alkali. Furthermore, solutions of the strong bases absorb carbon dioxide from the air. If such solutions are exposed to the atmosphere for any appreciable time they become contaminated with carbonate. This may be prevented by the use of a storage vessel such as is shown in Fig. 3.9 the guard tube should be filled with soda-lime. A short exposure of an alkali hydroxide solution to the air will not, however, introduce any serious error. If such solutions are quickly transferred to a burette and the latter fitted with a soda-lime guard tube, the error due to contamination by carbon dioxide may be neglected. 

Palladium and platinum. These are resistant to chemical attack (Pt more than Pd). Both metals dissolve in fused alkali metal oxides and peroxides. Pt is attacked, at high temperature, by molten alkali and alkaline earth halogenides and by several compounds of B, Si, Pb, P, As, Sb, Bi. Pd dissolves slowly in oxidizing acids Pt is dissolved by aqua regia. 

POM has a coefficient of linear expansion of 10.0 X 10 cm/cm C and high tensile, flexural, and compressive strengths. POM is attacked by adds but is resistant to alkalis and salts at room temperature. 

The attack of water is related to the leaching mechanism described for acids. Table 4 rates glasses based on their resistance to water attack. Low alkali, high alumina, or borosilicate glasses generally have high water durability. 

The enzyme is very sensitive to the secondary structure of the substrate. Native calf thymus DNA is quite resistant to enzymic attack by spleen exonuclease, being split at less than 4% the rate at which alkali-denatured DNA is split (11). Long deoxyribonucleotides (average chain length 20-50), which still have complementary double-stranded structure, are rather resistant to the enzyme (26). Some limited results obtained with synthetic polyribonucleotides (11) are rather puzzling since poly C was found to be completely resistant, whereas poly A, poly I, and poly U were degraded at comparable rates. In the solvent used (0.15 M acetate buffer-0.01 M EDTA, pH 5.0), poly A and poly C are believed to have... 

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

Oxidation produces Si02 and has been studied in detail 112 exhibits high corrosion resistance at 1400°C in presence of 10 5% Na, V, 0.5% S, but corrodes at 900°C in presence 0.005% Na, 0.005% V and 3% S resistant to alkali chloride melts and alkali solutions attacked by alkali metal sulfates and vanadium Attacked by acids and not by alkalis oxidizes at 700°C can be used in reducing conditions resistant to wetting by metals and alloys... 

Several plastics, with high resistance to chemical attack and high temperatures, deserve special mention for process designers of inherently safer plants. For example, tetrafluoroethylene (TFE), commonly called Teflon brand TFE, is practically unaffected by all alkalies and acids except fluorine and chlorine gas at elevated temperatures, and molten metals. It retains its properties at temperatures up to 260°C. Other plastics that have similarly excellent properties (but are different enough that they each have their niche) include chlorotrifluoroethylene (CTFE) Teflon FEP, a copolymer of tetrafluoroethylene and hexafluoropropylene polyvinylidene fluoride (PVF2) (also... 

Asbestos fibers are basically chemically inert, or nearly so. They do not evaporate, dissolve, bum, or undergo significant reactions with most chemicals. In acid and neutral aqueous media, magnesium is lost from the outer bmcite layer of chrysotile. Amphibole fibers are more resistant to acid attack and all varieties of asbestos are resistant to attack by alkalis (Chissick 1985 WHO 1998). Table 4-2 summarizes the physical and chemical properties of the six asbestos minerals. 

A beautiful blue powder. It is easily attacked by acids, but has considerable resistance to alkalis. It is stable in the sun or in hydrogen sulphide, HaS, and withstands heat. 

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