Water copper-based alloys

Essentially all industrial metals are susceptible to SCC in some specific environment. Of the metals commonly used in cooling water systems, copper-based alloys and stainless steels are most frequently affected. Common specific corrodents causing SCC in these and other heat exchanger metals are listed in Table 9.1. 

Parker, J. G. and Roscow J. A., Method for the Assessment of the Quality of Surface Films Formed on the Cooling Water Side of Copper-Based Alloy Condenser Tubes , Br. Corros. J., 16, 2, 107-110(1981)... 

Aluminum and Aluminum Alloys. Aluminum can be employed in sea water as a resistant material of construction. Experiments at Fort Bel voir, Virginia, and elsewhere, indicate that by proper corrosion-control practices, aluminum can be used for an entire plant which processes sea water. The sea water entering the plant should be free of all metallic ions, especially copper or nickel. It is essential, in such a plant, that no copper-base alloys be used at all and that galvanic couples to most other metals be avoided. 

Stainless steel generally withstands polluted sea water and polluted brackish water better than copper-base alloys. Substituting an austenitic stainless screen for silicon-bronze trash racks has resulted in greatly improved service at a west coast power plant. Normally stainless steel screens, because of the crevices involved (where the wires cross), are not recommended for use in sea water. This alteration of the usual corrosion mechanism, presumably related to the hydrogen sulfide content of polluted sea water, needs to be studied. 

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. 

Titanium. Unlike other metals, titanium normally does not pit, is not susceptible to stress corrosion, is free from local corrosion under fouling organisms, is free from impingement and cavitation attack at velocities which attack copper-base alloys, and is not susceptible to sulfide attack in contaminated sea water. Experiments with water velocities at 20 to 50 feet per second show no attack on titanium. 

If sea water is first deaerated, only a small amount of corrosion inhibitor, if any, probably would be needed to prevent attack on steel, or copper-base alloys. For aluminum, oxygen is needed to promote a protective film. 

Incidentally, the small amount of iron introduced into sea water by such corrosion, or by intentional chemical addition, is considered beneficial by some authorities for promoting protective films on copper-base alloys. Reduced attack can also be accomplished by flaring the tube ends to facilitate streamline flow. It is essential that the cross-over area in the head or channel be larger than the cross-sectional area of the tubes to reduce turbulence. Munro (6) recommends 125% for the cross-over area in water boxes for sea-water service. Also from the standpoint of turbulence, side entry is preferred to axial entry at the front end of the condenser. 

In certain chemical plants, the process solution being cooled is under pressure or is very corrosive. It is found expedient in some cases to put the low-pressure sea water on the shell side of the heat exchanger. Under these conditions, the steel shell will suffer more rapid attack because of galvanic coupling to the copper-base alloy tubing. However, only the outer tubes are seen by the shell in this couple. Nevertheless, this represents a large cathode. 

Carbon steel is the predominant construction material for carbonate and amine solution containers. Corrosion in the overhead lines (hydrogen sulfide or carbon dioxide plus water from the regenerator) is prevented by adding corrosion inhibitors. Although amine carry-over can act as a corrosion inhibitor in the overhead line, SCC of carbon steel has occurred when amine added as a corrosion inhibitor became concentrated. Copper and copper base alloys should be avoided in amine service and are questionable in carbonate seivice. Nickel or cobalt base alloys (e.g., Monel00 400 and Inconel 600) except for Stellite01 should be avoided in carbonate service. Monel 400 should be avoided in amine service if UCC Amine Guard02 corrosion inhibitor is used. 

Oxygen is the common cathodic reduction species found in water, which is responsible for continued corrosive attack on some engineering materials, such as low carbon steel. However, passive engineering alloys utilize the oxygen to form thin, tenacious, and adherent protective oxide films. Some common alloys with protective films are stainless steels, nickel alloys, copper-base alloys and aluminum alloys. The oxygen concentration at ambient temperatures and atmospheric pressure is approximately 6-8 mg/L. An increase in temperature decreases oxygen solubility, whereas an increase in pressure increases oxygen solubility. 

Titanium is immune to corrosion in all natural waters, including highly polluted seawater, at temperatures up to the boiling point. Titanium has replaced copper-based alloys that were corroding in the presence of sulfides, as well as stainless steels that were suffering from pitting and SCC caused by chlorides. 

AL 29-4C is d f srritic stainless steel developed specifically for power plant surface condenser tubing. AL29-4C has excellent resistance to brackish, polluted or high chloride waters, e.g., seawater. Consistent with its composition, AL 29-4C performs similar to austenitic stainless steels in a variety of other environments. Of particular importance is the excellent resistance of AL 29-4C to condenser environments where ammonia, other noncondensables and sulfides attack copper base alloys. 

The metal or alloy must have a proven compatibility to the corrosive environment. For instance, stainless steel (SS) 316 with 2% Mo is a better material for seawater service than SS 304 without molybdenum. Brass, bronze and copper based alloys are highly desirable for salt water transportation, however, they are vulnerable for an environment containing ammonia frequently encountered in agriculture. A good design to prevent corrosion must be compatible with the corrosive environment. Following is a summary of the effect of major contributors to corrosive environments. 

The corrosive action of dilute hydrochloric acid at the temperatures encountered in distillation equipment cannot be satisfactorily withstood by any common materials. However, the copper-base alloys have been widely used for hydrochloric acid and brine corrosion. Although conditions are not the same in all plants, the use of copper-base alloy tubes for condensers and water-cooled exchangers appears to be increasing. One refiner has been using the following kinds of tubes ... 

Ammonia is unsuitable as a neutralizer if copper-base alloys are used, because if used in excess it may destroy the metal faster than acid corrosion. The other neutralizers appear to produce protective films so that corrosion halts after an initial action. If scale-forming water is used, admiralty-metal tubes must be cleaned frequently, because corrosion occurs mainly beneath breaks or porous spots in the scale. The zinc in the brass is dissolved and is replaced by spongy copper. ... 

Corrosion due to acid and that due to sulfide can never be completely segregated. Thus Table 9-4 on the corrosion of copper-base alloys shows attack by sulfur as well as attack by water, meaning that copper-base alloys were used at points where acid or brine corrosion required their use but that failure occurred mainly by sulfide corrosion. 

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. 

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