Cupronickels

Perhaps the most extensive appHcation for conversion-rolled, explosion-bonded clads was for U.S. coinage in the 1960s (34) when over 15,900 metric tons of explosion-clad strip that was suppHed to the U.S. Mint helped alleviate the national silver coin shortage. The triclad composites consist of 70—30 cupronickel/Cu/70—30 cupronickel. 

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. 

Generally, coohng water is of a lower quahty then normal, having also higher mud and silt content. Sometimes even replaceable copper tubes iu shell-and-tube heat exchangers are required. It is advisable to use cupronickel instead of copper tubes (when water is high in chlorides) and to use conservative water side velocities (less then 2 iti/s for copper tubes). 

Yellow brass Admiralty brass Aluminum bronze Red brass Copper Silicon bronze 70-30 cupronickel Nickel (passive)... 

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. 

Figure 2.10 Wastage in a circumferential region where a 90 10 cupronickel main condenser tube passed through a baffle. In places, metal loss was greater than 25% of the nominal wall thickness.

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. 

Below silt accumulations on copper, brass, and cupronickel heat exchangers, a layer of bluish-white copper carbonate often forms (Figs. 4.14 and 4.15). 

Figure 4.7 Small, manganese-rich nodules on a 90 10 cupronickel condenser tube. Note the small pits beneath each nodule. (Courtesy of National Association of Corrosion Engineers Andy Howell Public Service of Colorado. Corrosion 89 Paper No. 197 by H. M. Herro.)...

Figure 4.14 Silt layer on the internal surface of a 90 10 cupronickel condenser tube.

Sample Specifications % in. (2.2 cm) outer diameter, 90 10 cupronickel tube... 

A typical microbiological analysis in a troubled carbon-steel service water system is given in Table 6.2. Table 6.3 shows a similar analysis for a cupronickel utility main condenser that showed no significant corrosion associated with sulfate reducers. When biological counts of sulfate reducers in solid materials scraped from corroded surfaces are more than about 10, significant attack is possible. Counts above 10 are common only in severely attacked systems. 

Because alterations to equipment design can be cumbersome and expensive, a more economical approach may be to change the metallurgy of affected components. Metals used in typical cooling water environments vary in their resistance to erosion-corrosion. Listed in approximate order of increasing resistance to erosion-corrosion, these are copper, brass, aluminum brass, cupronickel, steel, low-chromium steel, stainless steel, and titanium. 

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. 

Specify metals that are more resistant to erosion, such as cupronickel, monel, and stainless steels. Only affected tubes need to be replaced by these metals. 

Sample Specifications 1 in. (2.5 cm) outside diameter, 90 10 cupronickel tubes... 

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. 

Several sections of 90 10 cupronickel condenser tubing were received. The sections were from tubes that had been plugged because of earlier failures. No tube section contained the original tube failure, however. 

Removal of deposits and corrosion products from internal surfaces revealed irregular metal loss. Additionally, surfaces in wasted areas showed patches of elemental copper (later confirmed by energy-dispersive spectroscopy) (Fig. 13.12). These denickelified areas were confined to regions showing metal loss. Microscopic analysis confirmed that dealloying, not just redeposition of copper onto the cupronickel from the acid bath used during deposit removal, had occurred. 

An interesting effect is sometimes observed when cupronickels are galvanically coupled to less noble materials. The corrosion rate of the active metal is increased and the corrosion rate of the cupronickel is diminished, as expected. The diminished corrosion rate of the cupronickel can, however, diminish its fouling resistance since reduced production of copper ions lowers toxicity to copper-ion-sensitive organisms. 

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