Cupro-nickel alloy

Plain tubes (either as solid wall or duplex) are available in carbon steel, carbon alloy steels, stainless steels, copper, brass and alloys, cupro-nickel, nickel, monel, tantalum, carbon, glass, and other special materials. Usually there is no great problem in selecting an available tube material. However, when its assembly into the tubesheet along with the resulting fabrication problems are considered, the selection of the tube alone is only part of a coordinated design. Plain-tube mechanical data and dimensions are given in Tables 10-3 and 10-4. 

Table 21. Conductivity and H04 (Hard) Temper Tensile Properties of Cupro—Nickel Alloys...

The cupro-nickel alloys (5-30% of nickel) are perhaps the best of all for strength and resistance to corrosion. Table 3.20 gives typical properties. 

Table 3.20. Mechanical Properties of Annealed Cupro-Nickel Alloys [30]...

Mechanical properties of aimealed cupro-nickel alloys. 83... 

The non-ferrous alloys include the misleadingly named nickel silver (or German silver) which contains 10-30% Ni, 55-65% Cu and the rest Zn when electroplated with silver (electroplated nickel silver) it is familiar as EPNS tableware. Monel (68% Ni, 32% Cu, traces of Mn and Fe) is used in apparatus for handling corrosive materials such as F2 cupro-nickels (up to 80% Cu) are used for silver coinage Nichrome (60% Ni, 40% Cr), which has a very small temperature coefficient of electrical resistance, and Invar, which has a very small coefficient of expansion are other well-known Ni alloys. Electroplated nickel is an ideal undercoat for electroplated chromium, and smaller amounts of nickel are used as catalysts in the hydrogenation of unsaturated vegetable oils and in storage batteries such as the Ni/Fe batteries. 

The results of tests on copper alloys have been given by Tracy , Thompson , Mattsson and Holm and Scholes and Jacob , the first two of these investigations being made under the aegis of the American Society for Testing and Materials. The tests of Tracy, and Scholes and Jacob were both for periods up to 20 years in those of Thompson, and Mattsson and Holm specimens have been removed after 2 years and 7 years and further specimens remain exposed for removal after 20 years. The numbers of materials tested are given in Table 4.11 they included brasses, nickel silvers, cupro-nickels, beryllium coppers and various bronzes. Mattsson and Holm tested 14 alloys in the form of rod in addition to the sheet materials, the results for which are given in Table 4.11. 

In addition to nickel alloys, nickel also forms an important alloying element in stainless steels and in cast irons, in both of which it confers additional corrosion resistance and improved mechanical and engineering properties, and in Fe-Ni alloys for obtaining controlled physical and magnetic properties (see Chapter 3). With non-ferrous metals nickel also forms important types of alloys, especially with copper, i.e. cupro-nickels and nickel silvers these are dealt with in Section 4.2. 

Some duplex alloys have even better pitting resistance than type 316 and should be considered in severely pitting media. Titanium is virtually immune to chloride pitting and cupro-nickel alloys are used for condensers where sea-water is the coolant high pitting resistance in this duty is claimed for Cu-25Ni-20Cr-4-5Mo. 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

A similar method of test was used at the International Nickel Company s Corrosion Laboratory at North Carolina. The specimen discs are mounted on insulated vertical spindles and submerged in sea-water, which is supplied continuously to the tank in which the specimens are immersed. The maximum peripheral speed of the spinning disc is about 760cms , and the characteristic pattern of attack is shown in Fig. 19.3a. Studies of variation of depth of attack with velocity indicate that at low velocities (up to about 450 cm s ) alloys such as Admiralty brass, Cu-lONi and cupro-nickel alloys containing iron maintain their protective film with a consequent small and similar depth of attack for the diflferent alloys. At higher velocities the rate increases due to breakdown of the film. 

The thermal conductivity of cupro-nickel alloys = 50 W/m K and, from Table 9.16, scale resistances will be taken as 0.00020 m2K/W for the water and 0.00018 m2K/W for the organic. 

The cupro-nickel alloys (70 per cent Cu) have a good resistance to corrosion-erosion and are used for heat-exchanger tubes, particularly where sea water is used as a coolant. 

Alloying with palladium (0.15 per cent) significantly improves the corrosion resistance, particularly to HC1. Titanium is being increasingly used for heat exchangers, for both shell and tube, and plate exchangers replacing cupro-nickel for use with sea water. 

Further improvements in bullets were made betw 1900 and WWI. They consisted in sharpening the nose(pointed-nose bullets), bevelling the base(boat-tailing, tapering) and replacement of steel and cupro-nickel in the jacket by gilding metal and allied alloys in order to eliminate excessive bore erosion and metal-fouling... 

Copper-Base Alloys. There is a wide range of copper-base alloys that have given good service in sea water. Admiralty brass, 70 Cu-29 Zn-1 Sn, plus an inhibitor such as arsenic, has found wide use as condenser tubes in marine-based plants using sea water for cooling. While it is not so resistant as the cupro-nickels, it often seems to be preferred because of the lower initial cost. 

Cupro-Nickels. At the present time, most marine operators agree that the cupro-nickels are the most useful material of construction for sea-water plants. Since World War II, the 90 Cu-10 Ni alloy modified with about 1.5% of iron has become well established. The lower nickel content results in a cost advantage over the 70 Cu-30 Ni alloy. Alloys containing 70 Cu-30 Ni or 80 Cu-20 Ni, each with added iron, are preferred by some designers to withstand more severe conditions. 

The cupro-nickels have been widely accepted as the best available alloy for condenser tubes. For handling hot sea water, there has been some favorable experience with cupro-nickel for pumps and heat exchangers. 

Cupro-nickels are recommended for consideration in evaporator service, especially if sea water is to be handled at 200° F. or higher. Monel should give good service. There is some interest in using either Type 316 stainless steel or Alloy 20. Because of the danger of stress corrosion, precautions must be taken with these latter two materials. Hastelloy C is said to be very resistant to hot sea water. 

A large number of copper-base and nickel-base alloys (such as cupro-nickels, Monel, and aluminum brass) have been used in sea-water service with success. Special materials such as Hastelloy C, Illium, and titanium are available for extremely corrosive situations. The evidence, so far, indicates titanium to be outstanding and to rank above other commercially available metals in corrosion resistance under conditions involving high temperature, velocity, and other adverse environmental conditions. 

Various alloys of nickel and copper are of great industrial importance. The two metals mix in all proportions, the nickel reducing the conductivity and increasing the hardness of the copper. The alloys are variously known as cupro-nickel, nickel bronze, and nickel brass, whilst certain of them have special names such as monel metal, Benedict metal, nickel or German silver, etc. 

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