Alloys corrosion-resistant

These alloys are widely used in the chemical process industry. It is always better to use a more corrosion-resistant alloy in spite of its cost as it solves the corrosion problems and saves money in the long run. The relative costs of some alloys used in corrosion control relative to type316L stainless steel as the reference with a ratio 1.0 are given in Table 4.48. 

TABLE 448 Relative Costs of Some Alloys Used in Corrosion Control 

Nickel-chromium-molybdenum alloys are used in reactor vessels in the production of acetic acid. These alloys are cost-effective compared to Ni-Cr stainless steels and have good resistance to oxidizing corrosive media Ni-Mo alloys have good resistance to reducing media. Molybdenum together with the chromium stabilizes the passive film in the presence of chlorides and is particularly effective in increasing resistance to pitting and crevice corrosion. 

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It is extensively used for making stainless steel and other corrosion-resistant alloys such as Invar(R), Monel(R), Inconel(R), and the Hastelloys(R). Tubing made of copper-nickel alloy is extensively used in making desalination plants for converting sea water into fresh water. 

Health and Safety Factors. Unlike fluoroacetic acid, trifluoroacetic acid presents no unusual toxicity problems. However, owing to its strong acidity, its vapors can be irritating to tissue, and the Hquid acid can cause deep bums if allowed to contact the skin. The acid can be safely stored in containers made of glass or common corrosion-resistant alloys and metals such as stainless steel or alurninum. 

Molten ETFE polymers corrode most metals, and special corrosion-resistant alloys ate recommended for long-term processiag equipmeat short-term prototype mas are possible ia standard equipment. 

Aqueous formaldehyde is corrosive to carbon steel, but formaldehyde in the vapor phase is not. AH parts of the manufacturing equipment exposed to hot formaldehyde solutions must be a corrosion-resistant alloy such as type-316 stainless steel. Theoretically, the reactor and upstream equipment can be carbon steel, but in practice alloys are required in this part of the plant to protect the sensitive silver catalyst from metal contamination. 

Hastelloy Corrosion Resistant Alloys, Haynes International, Inc., Kokomo, Ind., 1987 to 1993. 

Oxychlorination of Ethylene or Dichloroethane. Ethylene or dichloroethane can be chlorinated to a mixture of tetrachoroethylene and trichloroethylene in the presence of oxygen and catalysts. The reaction is carried out in a fluidized-bed reactor at 425°C and 138—207 kPa (20—30 psi). The most common catalysts ate mixtures of potassium and cupric chlorides. Conversion to chlotocatbons ranges from 85—90%, with 10—15% lost as carbon monoxide and carbon dioxide (24). Temperature control is critical. Below 425°C, tetrachloroethane becomes the dominant product, 57.3 wt % of cmde product at 330°C (30). Above 480°C, excessive burning and decomposition reactions occur. Product ratios can be controlled but less readily than in the chlorination process. Reaction vessels must be constmcted of corrosion-resistant alloys. 

The two corrosion-resistant alloys presented ia Table 5 rely on chromium and molybdenum for their corrosion resistance. The corrosion properties of IJ1 timet are also enhanced by tungsten. Both alloys are available ia a variety of wrought product forms plates, sheets, bars, tubes, etc. They are also available ia the form of welding (qv) consumables for joining purposes. 

Mechanical Properties. An advantage of the two corrosion-resistant alloys is that they may be strengthened considerably by cold working. MP35N alloy is iatended for use ia the work-hardened or work-hardened and aged condition, and the manufacturers have suppHed considerable data concerning the mechanical properties of the alloy at different levels of cold work. Some of these data are given ia Table 8. 

For most environments quantitative studies have been reported describing the corrosion rate of various materials including a number of corrosion-resistant alloys (30). For example. Table 4 gives weight losses suffered by corrosion-resistant alloys in a solution of 28% phosphoric acid [7664-38-2] 20—22% sulfuric acid [7664-93-9] and 1—15% duoride (36). 

Erosion and Corrosion combined require special consideration. Most of the stainless steels and related corrosion-resistant alloys ow e their surface stability and low rate of corrosion to passive films that develop on the surface either prior to or during exposure to reactive fluids. If conditions change from passive to active, or if the passive film is removed and not promptly reinstated, much higher rates of corrosion may be expected. 

Where erosion by a liquid-borne abrasive is involved, the behavior of a corrosion-resistant alloy will depend largely on the rate at which erosion removes the passive film and the rate at which it reforms. 

Alloy 400 has good mechanical properties and is easy to fabricate in all wrought forms and castings. K-500 is a modified version of this alloy and can be thermally treated and is suitable for items requiring strength, as well as corrosion resistance. Alloy 400 has immunity to stress corrosion cracking and pitting in chlorides and caustic alkali solutions. 

An understanding of the basic principles of the science of metallic corrosion is clearly vital for corrosion control, and as knowledge of the subject advances the application of scientific principle rather than an empirical approach may be used for such purposes as the selection of corrosion inhibitors, formulation of corrosion-resisting alloys, etc. 

The principles of bimetallic corrosion have, in addition, been used in an elegant fashion for the development of highly corrosion-resistant alloys. 

The most satisfactory solution to this problem is to employ a corrosion-resistant alloy, and alloys of the gun-metal type, containing 2-4% zinc, have proved completely satisfactory. The substitution of zinc for phosphorus gives sounder castings and improves the corrosion resistance of the copper-rich matrix. 

Design of Chemical Plant in Wiggin Nickel Alloys, and Wiggin Corrosion Resisting Alloys, Henry Wiggin Co. Ltd. 

Select metal or alloy (or non-metallic material) for the particular environmental conditions prevailing (composition, temperature, velocity, etc.) taking into account mechanical and physical properties, availability, method of fabrication and overall cost of structure. Decide whether or not an expensive corrosion-resistant alloy is more economical than a cheaper metal that requires protection and periodic maintenance. 

PWR steam generators (SGs) are essentially complex shell and tube heat exchangers composed of a variety of corrosion-resisting alloys. The closed loop primary circuit is on the tube side and the steam generating, working fluid or secondary circuit is on the shell side. 

Various newer and more corrosion-resistant alloys are now available for use, including low nickel alloys, such as Inconel 800 (35% Ni, 20% Cr, 45% Fe). Low nickel alloys, in particular those with with less than 0.03% cobalt, are required to limit the development of a cobalt-induced, outer-core radiation field within the SG. 

Bacterial corrosion is often referred to as microbiologically influenced corrosion. MIC involves the initiation or acceleration of corrosion by microorganisms. The metabolic products of microorganisms appear to affect most engineering materials, but the more commonly used corrosion-resistant alloys, such as stainless steels, seem to be particularly susceptible. 

G. Trabanelli, F. Zucchi, and G. Brunoro. Inhibition of corrosion resistant alloys in hot hydrochloric acid solutions. Werkstoffe Korrosion, 39(12) 589-594, December 1988. 

The more expensive, corrosion-resistant, alloys are frequently used as a cladding on carbon steel. If a thick plate is needed for structural strength, as for pressure vessels, the use of clad materials can substantially reduce the cost. 

Hydrogen sulfide promoted corrosion can be a serious problem (150) the best solution is prevention. Corrosion problems can be minimized by choice of the proper grades of steel or corrosion resistant alloys, usually containing chromium or nickel (150, 151) and avoiding generation of H S by sulfate reducing bacteria in situations where H S is not initially present. Cathodic protection of casing is often effective for wells less than 10,000 feet deep (150). 

Cobalt aluminum blue, transparent, 19 412 CobaltCII) aminobenzoate, uses, 7 239t CobaltCII) ammonium sulfate, uses, 7 239t CobaltCII) arsenate, uses, 7 241t Cobalt-base corrosion-resistant alloys,... 

Technetium is one of the few artificially produced elements that has practical industrial applications. One is that a very small amount (55-ppm) added to iron creates a corrosion-resistant alloy metal. This property is shared with many of the other transition metallic elements, but not with other artificially produced elements that have higher atomic numbers and are radioactive. 

Uses/Sources. Corrosion-resistant alloys, electroplating, production of catalysts, nickel-cadmium batteries nickel subsulfide (NisS2) is encountered in the smelting and refining of certain nickel ores and may be formed in petroleum refining from the use of nickel catalysts. 

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