Resistance to Acid Attack

Refractories used in chimney linings, incinerators, and kiln linings should have sufficient resistance to acid attack. ISO 8890 1988 gives the standard 

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The sulfate process is based on the attack of activated beryl by sulfuric acid to form water-soluble Be and A1 sulfates and insoluble silica. A flow diagram summarizing this process is given in Scheme 1. Activated beryl must be used since the natural ore is resistant to acid attack except by HF, which is prohibitively expensive. High-grade beryl ore (> 10% BeO) is normally activated by a heat-treatment process... 

Despite the failing of the dental silicate cement under acid conditions it is more resistant to acid attack than all other dental cements with the notable exception of the glass polyalkenoate cement (Norman, Swartz Phillips, 1959 Walls, McCabe Murray, 1985 Beech Bandyopadhyay, 1983 Kuhn, Setchell Teo, 1984 Wilson et al., 1986a). These studies have been confirmed by in vivo observations (Norman et al., 1969). A clinical study carried out by Robinson (1971) over many years showed that when carefully prepared and placed, the dental silicate cement was capable of giving good performance. Many of the failures of this material must be attributed to faulty preparation. 

High-temperature/low-pressure inorganic digestions are an area of application that has benefited from recent advances in vessel and sensor design. The inert properties of Teflon and its resistance to acid attack make it the material of choice for microwave pressure-vessel construction. Improved commercial systems offer additional safety precautions and improved facilities for pressure and/or temperature control. Also, the distribution of microwave radiation inside the oven cavity is fairly homogeneous. Low-pressure systems allow decomposition temperatures of about 180 °C. However, for many matrices, such temperatures are not sufficient to guarantee the complete ashing of thermoresistant sample components. 

The incorporation of fluoride in place of hydroxyl groups is chemically straightforward [59,60] and, as we have seen, results in a substance of greater resistance to acid attack. This is partly due to the greater electronegativity of fluorine, which means that the electrostatic attraction between Ca + and F is greater than that between Ca + and OH. As a result, the fluoridated apatite lattice is more stable than hydroxyapatite [61-63]. It is also more crystalline [64]. 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

Nickel Thiopyrophosphite, Ni2P2S6, may be obtained5 by heating nickel for forty hours with sulphur and phosphorus in sealed tubes. It forms brilliant grey hexagonal crystals of density 2-4. They are very resistant to acid attack, even to nitrohydroehloric acid. 

A compound of selenium and iridium, but of indefinite composition, is obtained by heating the two elements together at red heat in sealed tubes. The product is black, and very resistant to acid attack.5... 

Preparation of Pure Platinum.—Commercial platinum usually contains some 2 per cent, of iridium, and for many purposes this is no disadvantage as the alloy is very durable and resistant to acid attack. 

Osmium yields alloys with platinum characterised by hardness and tensile strength. They are very resistant to acid attack, and offer high electrical resistance. The alloys usually contain from 1 up to 10 per cent, of osmium.7... 

The addition of B2O3 to a silicate glass markedly reduces the thermal expansion coefficient and consequently enhances the resistance to thermal shock. At the same time, the chemical durability, especially the resistance to acid attack, is greatly improved. 

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. 

In the event the surface does offer chemical resistance by virtue of slower reaction rates, resistance to acid attack, etc., the gas concentration will not be zero at the surface and the dry deposition rate will be reduced accordingly. Thus structures made or coated with such materials will show a different relationship between calculated deposition velocities, as presented here, and the actual operational values, which may be governed more by the material properties and less by aerodynamic resistance. This would be an important property to establish, by testing over a range of aerodynamic conditions. 

Resistant to acids attacked by basic substances at high temperature Least compatible with BeO, MgO Vessels for metallic and alloy melts... 

In soapstone samples, a minor resistance to acid attack is associated with a larger magnesium and/or iron content in the stone. 

To prevent corrosion of double-hulled tanks, coatings with corrosion resistance at temperatures of 70-90 C and resistance to MIC as well as resistance to acid attack in the ullage were found desirable. Because of the flexing of HT steel, the coating needs to be flexible and does not become brittle and break off over time as the ship flexes. The coating should last for 20 years. Solvent-free epoxies satisfied these requirements. 

The healthy tooth is covered with a thin layer of enamel (Fig. 1.1), the hardest substance in the human body [1,2]. It is highly mineralized, containing distinct crystalline phases of the calcium phosphate mineral hydroxyapatite Caj (PO ) (OH)j. It also contains traces of the protein collagen, which has important effects on the mechanical properties of the enamel. Specifically, the protein causes the enamel to be tougher than it would otherwise be, and hence more resistant to crack propagation through it [4]. It also reduces the permeability of the enamel, thus increasing its resistance to acid attack [4]. 

Several possible calcium and aluminium phosphates exist and they differ in their solubility in aqueous media and also in their resistance to acid attack. Among the factors determining which of these products are formed are powderiliquid ratio of the cement and concentration of the phosphoric acid solution. As a result, this material was easy to prepare in a soluble or acid-sensitive state. Incorrect metering of the powder to liquid components increased the solubility of the set cement, and leaving the bottle of phosphoric acid solution open to the air led to uptake of moisture from the atmosphere, with a corresponding reduction in acid concentration. This resulted in an increase in the proportion of more soluble metal salts in the set cement. These factors combined to make the dental silicate difficult to use in the clinic and gave the material a reputation for unreliability [8]. 

The ability to exchange ions with the surroundings also applies to the solid tooth. As will be described in more detail in Section 5.9, at the interface between the glass-ionomer cement and the tooth surface, an ion-exchange layer develops, due to diffusion of ions from each surface. The result is a distinctive structure that can be viewed under the scanning electron microscope, and which results from the mobility of key ions [120]. This ion-rich layer seems to be very resistant to acid attack, and secondary caries is very rarely observed around glass-ionomer restorations. 

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