Cupronickel alloys

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

For firewater, steel pipes are used but corrosion products can block sprinklers. Cement asbestos pipes are utilized but pressure limitations restrict their use. For critical applications, including offshore oil installations, cupronickel alloys and even duplex stainless steels are used. Fire-retardant grades of fiber-reinforced plastics are now available. 

High velocities of aqueous solutions impinging on copper and brass tubes produce impingement on the metal or alloy. The aluminum brass and cupronickel alloys have great resistance to flow-induced attack up to a well-defined maximum for the flow rate, beyond which the film on the metal surface will be disrupted. Admiralty brass and aluminum brass have lower values for maximum velocity of flow than cupronickels. Admiralty brass and aluminum brass are preferred to cupronickels for use in media containing sulfide species. Coatings have been developed for cupronickel and aluminum brass condenser tubes for land-based and marine systems. 

Constantan /kon-stan-tan/ (Trademark) A copper-nickel (cupronickel) alloy containing 45% nickel. It has a high electrical resistivity and very low temperature coefficient of resistance and is therefore used in thermocouples and resistors. 

The electrolytic separation of copper and nickel from cupronickel alloy can be achieved by controlling the pH and electrolytic conditions. Copper is determined in strongly acidic solution at a potential not exceeding 4V (generally 2-3 V) as above this potential nickel may also plate out. The solution should contain a mixture of sulfuric and nitric acids for complete deposition. However, a high concentration of acid is not recommended as it may lead to incomplete deposition of copper or the deposit may not adhere satisfactorily to the cathode. 

Nickel metal reacts only slowly with fluorine gas due to the self-formation of a thin protective passivating layer of nickel fluoride (NiF ). Therefore nickel and cupronickel alloys such as Monel 400 and K-500 are used extensively for handling fluorine gas, anhydrous hydrogen fluoride, and hydrofluoric add. Nickel is extensively used in coinage but is more important either as pure metal or in the form of alloys for its many domestic and industrial applications. 

Cupronickels (Ni-Cu) In this category the most common cupronickel alloys are the Monel 400 and Monel K-500. The Ni-Cu alloys differ from nickel 200 and 201 because their strength and hardness can be increased by age hardening. Ni-Cu alloys exhibit higher corrosion resistance than commercially pure nickel, especially to sulfuric and hydrofluoric acids, and chloride brines. Handling of waters, including seawater and brackish water, is the major application of these two alloys in the CPI (e.g., desaUnation plants). In addition, Monel 400 and K-500 are immune to chloride-ion stress-corrosion cracking, which is often considered in their selection. 

The final group of copper alloys are the copper-nickel (cupronickels) alloys. They exhibit the best resistance to corrosion, impingement, and SCC of all the copper alloys. They are among the best alloys for seawater service and are immune to season cracking. Dilute hydrochloric, phosphoric, and sulfuric acids can be handled. They are almost as resistant as Monel to caustic soda. 

Alloy selection is not made fiom only consideration of strength and conductivity. For example, the cupronickels have about the same strength as do copper—2inc brasses, and also have much lower conductivity. However, the corrosion resistance of the cupronickels far exceeds that of brass and is worth the higher cost if needed in the appHcation. Similar trade-offs exist between these properties and formabiUty, softening resistance, and other properties. 

Cupronickels (10 to 30 percent Ni) have become very important as copper alloys. They have the highest corrosion resistance of all copper alloys and find apphcation as heat-exchanger tubing. Resistance to seawater is particularly outstanding. 

Many shell-and-tube condensers use copper alloy tubes, such as admiralty brasses (those containing small concentrations of arsenic, phosphorus, or antimony are called inhibited grades), aluminum brasses, and cupronickel austenitic stainless steel and titanium are also often used. Utility surface condensers have used and continue to use these alloys routinely. Titanium is gaining wider acceptance for use in sea water and severe service environments but often is rejected based on perceived economic disadvantages. 

Certain conditions, ultimately dictated by economics, make the substitution of more resistant materials a wise choice. Stainless steels (not sensitized) of any grade or composition do not form tubercles in oxygenated water neither do brasses, cupronickels, titanium, or aluminum. However, each of these alloys may suffer other problems that would preclude their use in a specific environment. 

Denickelification generally produces less wastage in cupronickels than dezincification in brasses. Wastage decreases as nickel content increases, becoming very slight in alloys containing 30% or more nickel. 

Copper alloys include brasses (Cu-Zn alloys), bronzes (Cu-Sn alloys), cupronickels (Cu-Ni alloys) and nickel-silvers (Cu-Sn-Ni-Pb alloys). 

As you can see from the tables in Chapter 1, few metals are used in their pure state -they nearly always have other elements added to them which turn them into alloys and give them better mechanical properties. The alloying elements will always dissolve in the basic metal to form solid solutions, although the solubility can vary between <0.01% and 100% depending on the combinations of elements we choose. As examples, the iron in a carbon steel can only dissolve 0.007% carbon at room temperature the copper in brass can dissolve more than 30% zinc and the copper-nickel system - the basis of the monels and the cupronickels - has complete solid solubility. 

Gold, Monel, Inconel, nickel-moly- bdenum alloys Cupronickels silver solder. Copper Steel... 

Corrosion in these areas is sometimes effectively controlled by cathodic protection with zinc- or aluminium-alloy sacrificial anodes in the form of a ring fixed in good electrical contact with the steel adjacent to the non-ferrous component. This often proves only partially successful, however, and it also presents a possible danger since the corrosion of the anode may allow pieces to become detached which can damage the main circulating-pump impeller. Cladding by corrosion-resistant overlays such as cupronickel or nickel-base alloys may be an effective solution in difficult installational circumstances. 

These galvanic corrosion processes take place when one or more elemental constituents of an alloy is leached, often leaving a weak, porous structure, although the component dimensions often are unchanged. Dealloying particularly affects equipment constructed of cupronickels, bronzes, brasses, and gunmetal, such as FW heaters, strainers, valves, and pump impellers. 

Certain pre-boiler cupronickels, such as 70 30 alloy tubes, subjected to high temperature, stress, and stop-start operation may suffer oxygen corrosion-induced dealloying followed by exfoliation corrosion, in which oxidized sheets peel away from the solid metal. 

Calculate the relative number of atoms of each element contained in each of the following alloys (a) coinage cupronickel, which is 25% Ni by mass in copper (b) a type of pewter that is about 7% antimony and 3% copper by mass in tin. 

Nickel, Ni, is also used in alloys. It is a hard, silver-white metal used mainly for the production of stainless steel and for alloying with copper to produce cupronickels, the alloys used for nickel coins (which are about 25% Ni and 75% Cu). Nickel is also used in nicad batteries and as a catalyst, especially for the addition of hydrogen to organic compounds, as in the hydrogenation of vegetable oils (Section 18.6). 

Copper and Alloys Copper and its alloys are widely used in chemical processing, particularly when heat and electrical conductivity are important factors. The thermal conductivity of copper is twice that of aluminum and 90 percent that of silver. A large number of copper alloys are available, including brasses (Cu-Zn), bronzes (Cu-Sn), cupronickels (Cu-Ni), and age-hardenable alloys such as copper beryllium (Cu-Be) and copper nickel tin (Cu-Ni-Sn). 

These alloys are available as wrought or cast alloys. The principal wrought copper alloys are the brasses, leaded brasses, phosphor bronzes, aluminum bronzes, silicon bronzes, bciylhum coppers, cupronickels. and nickel silvers. The major cast copper alloys include the red and yellow brasses, manganese, tin, aluminum, and silicon bronzes, beryllium coppers, and nickel silvers. The chemical compositions range widely. For example, a leaded brass will contain 60% copper, 36 to 40% zinc, and lead up to 4% a beryllium copper is nearly all copper, containing 2.1% beryllium, 0.5% cobalt, or nickel, or in another formulation, 0.65% beryllium, and 2.5% cobalt. 

The nickel silvers generally are classified as brasses. Cupronickels fall more into basic copper-nickel alloys. Possible minor ingredients are manganese, iron, and zinc. These alloys can be used for severe drawing, spinning, and stamping operations because Ihey do not harden readily. They also are extensively used for condenser lubes and plates, heat exchangers, and other process equipment. 

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