Specialized alloys

Lead and its alloys are generally melted, handled, and refined in cast-iron, cast-steel, welded-steel, or spun-steel melting ketdes without fear of contamination by iron (qv). Normal melting procedures require no dux cover for lead. Special reactive metal alloys require special alloying elements, duxes, or covers to prevent dross formation and loss of the alloying elements. 

Specialized alloys are used for high temperature appHcations on turbine blades, furnace parts, thermocouples, etc. These coatings can be as simple as iron—silicon—chromium or as exotic as chromium—aluminum—hafnium (36,41,52). 

Liquids. Liquids usually are moved through pipelines (qv) by pumps. Special alloys, plastic pipe and liners, glass, and ceramics are widely employed in the chemical industry for transport of corrosive hquids. Care is required in making the connections, to prevent exposure of unprotected metal such as flanges and bolts to the corrosive material inside the piping. 

Using supercritical water is not without its drawbacks, two of which are the high pressures and temperatures involved. Another difficulty is the extreme corrosive nature of water at supercritical conditions. If halogenated organics are treated, special alloy reactors are requited. 

Special Alloys. AHoys of tin with the rater metals, such as niobium, titanium, and 2kconium, have been developed. The single-phase alloy Nb Sn [12035-04-0] has the highest transition temperature of any known superconductor (18 K) and appears to keep its superconductivity in magnetic... 

Materials of Construction A wide variety of materials is available for tanks, as indicated earlier. Most mechanisms are made of steel however, submerged parts may be made of wood, stainless steel, rubber-covered or coated steel, or special alloys. 

Stainless Steel There are more than 70 standard types of stainless steel and many special alloys. These steels are produced in the wrought form (AISI types) and as cast alloys [Alloy Casting Institute (ACI) types]. Gener y, all are iron-based, with 12 to 30 percent chromium, 0 to 22 percent nickel, and minor amounts of carbon, niobium (columbium), copper, molybdenum, selenium, tantalum, and titanium. These alloys are veiy popular in the process industries. They are heat- and corrosion-resistant, noncontaminating, and easily fabricated into complex shapes. 

Good Good 100 7.3 Special alloy with good resistance to sulfuric, phosphoric, and fatty acids resistant to clilorides in some environments... 

CrNiMo stainless steels and Cr-rich special alloys Seawater (25°C) Cl" containing media <0.0 <-0.3 (in general more positive UfQ values determine) U becomes more negative with increasing Cl" concentration and temperature [4, 57-59]... 

Besides the use of anodic polarization with impressed current to achieve passivation, raising the cathodic partial current density by special alloying elements and the use of oxidizing inhibitors (and/or passivators) to assist the formation of passive films can be included in the anodic protection method [1-3]. 

Many applications of novolacs are found in the electronics industry. Examples include microchip module packaging, circuit board adhesives, and photoresists for microchip etching. These applications are very sensitive to trace metal contamination. Therefore the applicable novolacs have stringent metal-content specifications, often in the low ppb range. Low level restrictions may also be applied to free phenol, acid, moisture, and other monomers. There is often a strong interaction between the monomers and catalysts chosen and attainment of low metals levels. These requirements, in combination with the high temperature requirements mentioned above, often dictate special materials be used for reactor vessel construction. Whereas many resoles can be processed in mild steel reactors, novolacs require special alloys (e.g. Inconel ), titanium, or glass for contact surfaces. These materials are very expensive and most have associated maintenance problems as well. 

Fluorinated polymers stand out sharply against other construction materials for their excellent corrosion resistance and high-temperature stability. In this respect they are not only superior to other plastics but also to platinum, gold, glass, enamel and special alloys. The fluorinated plastics used in process plants are polytetrafluorethylene (PTFE), fluorinated ethylene/ propylene (FEP), polytrifiuoromonochlorethylene (PTFCE) and polyvinyl fluoride (PVF). They are much more expensive than other polymers and so are only economical in special situations [59]. 

Special alloy materials are required for corrosion control and hieh temperatures. 

Placing the fluid through the tubes is a consideration when special alloy materials aie needed for corrosion control, because the materials would be needed only on the tubes. If the corrosive material is in the shell, both the tubes and the shell would need to be protected with special alloy. It the fluid is at high pressure, it should be put in tubes because tubes can contain high pressure much more cheaply as they are much smaller in diameter than the shell. The low-pressure fluid would be in the shell. If the fluid contains vapor and non-condensable gases, heat transfer will be greater if it is placed in the tubes. If the fluid is scale forming it should be in the tubes, which can be reamed out. 

Long-life special alloy valves have chromium-plated stems, hardened shrink-fit valve seats, and replaceable guides. 

About one-third of the copper used is secondary copper (i.e. scrap) but the annual production of new metal is nearly 8 million tonnes, the chief sources (1993) being Chile (22%), the USA (20%), the former Soviet Union (9%), Canada and China (7.5% each) and Zambia (5%). The major use is as an electrical conductor but it is also widely employed in coinage alloys as well as the traditional bronze (Cu plus 7-10% Sn), brass (Cu-Zn), and special alloys such as Monel (Ni-Cu). 

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

The requirements of nuclear energy application fostered an interest in special alloys for service in high temperature aqueous environments, but their utilisation has not been widespread. Encouraging results have been reported for alloys of 2Ni-0-5Fe and 1-2Ni-l 8Fe . ... 

Corrosion Properties of Special Alloys and Rare Metals, Jacob and Korres G.m.b.H, 473 Ahlen, W. G. 

Today boiler vessels are usually fabricated from special boiler plate and firebox steels of varying thickness, while their auxiliaries (supplementary equipment) and appurtenances (boiler accessories and instruments, especially those employed for safety reasons) may be produced from any of several different constructional metals, alloys, and other materials, including cast iron, copper alloys, stainless steels, and so forth. Tubes and tube plates may be variously constructed of carbon steel, low-alloy steels, or special alloy steels, with each design providing for particular required levels of thermal and mechanical stress and corrosion resistance. The overall boiler plant system may have a life expectancy in excess of 50 to 60 years, although individual components may need to be replaced periodically during this period. 

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