Copper-base alloys corrosion

Schussler, M. and Napolitan, D. S., Selective Dissolution from Copper-Based Alloys, Corrosion, Vol. 12, 1956, p. 107t. 

Serre, J. and Lawreys, J., "Selective Dissolution from Copper-Based Alloys," Corrosion Science, Vol. 2, 1965, p. 135. 

Corrosion. Copper-base alloys are seriously corroded by sodium thiosulfate (22) and ammonium thiosulfate [7783-18-8] (23). Corrosion rates exceed 10 kg/(m yr) at 100°C. High siUcon cast iron has reasonable corrosion resistance to thiosulfates, with a corrosion rate <4.4 kg/(m yr)) at 100°C. The preferred material of constmction for pumps, piping, reactors, and storage tanks is austenitic stainless steels such as 304, 316, or Alloy 20. The corrosion rate for stainless steels is <440 g/(m yr) at 100°C (see also Corrosion and corrosion control). 

Most metals are subject to erosion-corrosion in some specific environment. Soft metals, such as copper and some copper-base alloys, are especially susceptible. Erosion-corrosion is accelerated by, and frequently involves, a dilute dispersion of hard particles or gas bubbles entrained in the fluid. 

Use of inhibitors. Because corrosion is such a vital aspect of the erosion-corrosion process, inhibitors that will reduce corrosion under conditions of high fluid velocity have been a cost-effective method of dealing with erosion-corrosion. For example, injection of ferrous sulfate either intermittently or continuously has been successful in inhibiting erosion-corrosion, especially with copper-base alloys. 

When possible, avoid coupling materials having widely dissimilar galvanic potentials. If this cannot he avoided, make use of favorable area ratios by giving the active metal a large exposed area relative to the noble metal. For example, copper or copper-based alloy tubes may be joined to a steel tube sheet. Because of the favorable area ratio in this case, a relatively inexpensive steel tube sheet may be intentionally substituted for a bronze or a brass tube sheet if thickness specifications allow for a small amount of galvanic corrosion of the steel. 

Similarly, graphitically corroded cast iron (see Chap. 17) can assume a potential approximately equivalent to graphite, thus inducing galvanic corrosion of components of steel, uncorroded cast iron, and copper-based alloys. Hence, special precautions must be exercised when dealing with graphitically corroded pump impellers and pump casings (see Cautions in Chap. 17). 

If conditions are such as to require a duplex tube, it is quite likely that a plain end detail for the tube will not be satisfactory. Grooved or serrated joints are recommended for this type of tube, and the ends should be flared or beaded. Table 10-8 gives recommended flare or bell radii for copper-based alloys. Also see Table 10-8A. In service where galvanic corrosion or other corrosive action may take place on the outside material used in the tube, a ferrule of inside tube... 

In contrast, the selective dissolution or leaching-out by corrosion of one component of a single-phase alloy is of considerable practical importance. The most common example of this phenomenon, which is also referred to as parting , is dezincification, i.e. the selective removal of zinc from brass (see Section 1.6). Similar phenomena are observed in other binary copper-base alloys, notably Cu-Al, as well as in other alloy systems. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

The effect of alloying additions on the marine corrosion properties of non-ferrous metals can be very significant, and for copper-based alloys has been comprehensively reviewed by Bradley... 

Many of the alloys of copper are more resistant to corrosion than is copper itself, owing to the incorporation either of relatively corrosion-resistant metals such as nickel or tin, or of metals such as aluminium or beryllium that would be expected to assist in the formation of protective oxide films. Several of the copper alloys are liable to undergo a selective type of corrosion in certain circumstances, the most notable example being the dezincification of brasses. Some alloys again are liable to suffer stress corrosion by the combined effects of internal or applied stresses and the corrosive effects of certain specific environments. The most widely known example of this is the season cracking of brasses. In general brasses are the least corrosion-resistant of the commonly used copper-base alloys. 

Titanium in contact with other metals In most environments the potentials of passive titanium. Monel and stainless steel, are similar, so that galvanic effects are not likely to occur when these metals are connected. On the other hand, titanium usually functions as an efficient cathode, and thus while contact with dissimilar metals is not likely to lead to any significant attack upon titanium, there may well be adverse galvanic effects upon the other metal. The extent and degree of such galvanic attack will depend upon the relative areas of the titanium and the other metal where the area of the second metal is small in relation to that of titanium severe corrosion of the former will occur, while less corrosion will be evident where the proportions are reversedMetals such as stainless steel, which, like titanium, polarise easily, are much less affected in these circumstances than copper-base alloys and mild steel. 

Copper-base alloys will corrode in aerated conditions. It is, therefore, sometimes appropriate to consider cathodic protection. It becomes particularly relevant when the flow rates are high or when the design of an item causes the copper to be an anode in a galvanic cell (e.g. a copper alloy tube plate in a titanium-tubed heat exchanger). Corrosion can be controlled by polarisation to approximately — 0-6V (vs. CU/CUSO4) and may be achieved using soft iron sacrificial anodes. 

Both metals are applied to copper-base alloys, stainless steels and titanium to stop bimetallic corrosion at contacts between these metals and aluminium and magnesium alloys, and their application to non-stainless steel can serve this purpose as well as protecting the steel. In spite of their different potentials, zinc and cadmium appear to be equally effective for this purpose, even for contacts with magnesium alloys Choice between the two metals will therefore be made on the other grounds previously discussed. 

The hardness of such coatings may reach a maximum of about 400 Hy as compared with approximately 50 Hy for a soft gold deposit. A series of corrosion studies in industrial and marine atmospheres by Baker" has indicated that the protective value of hard gold coatings is comparable with that of the pure metal, and that a thickness of only 0-002 5 mm gives good protection to copper base alloys during exposure for six months. 

Both iron- and copper-based alloys are corroded more easily on either side of the neutral pH band. In low pH conditions e.g. due to carbon dioxide, the acidic environments attack the alloys readily, causing damage both at the points of initial corrosion and perhaps, consequentially, further along the system, by screening the surface with corrosion products and permitting the development of differential aeration cells. 

Azole compounds such as benzotriazole, benzimidazole, indazole and imidazoles are efficient anti-corrosion agents for copper and copper-base alloys [1-10]. Many experimental techniques [11-15] have been used to study the corrosion inhibition mechanisms, however, the mechanisms are still not well understood. It is believed that the complex formation between copper and nitrogen atoms would inhibit oxygen adsorption on copper surface [16-20]. 

For several years, additions of Be to commercial copper- and nickel-based alloys have enabled these materials to be precipitation-hardened to strengths approaching those of heat-treated steels. Yet Cu-Be alloys retain the corrosion resistance, electrical and thermal conductivities, and spark resistance of copper-based alloys. 

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. 

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. 

Rebak, R.B., Corrosion of Non-Ferrous Alloys, Part I Nickel-, Cobalt-, Copper-, Zirconium-and Titanium-Base Alloys, Corrosion and Environmental Degradation, Vol. II, Wiley-VCH, Weinheim, p. 69, 2000. 

Austenitic SS s are also used in freshwater. However, because of cost their use is limited mainly to applications in which copper-zinc alloys are unsuitable, as in tubes in which the process side is incompatible with copper base alloys. To avoid pitting, type 304 (UNS S30400) SS is normally limited to services in which the chloride ion concentration is at a maximum of 100 ppm, and type 316 SS is limited to services in which the chloride ion is a maximum of 500 ppm. The relative pitting and crevice corrosion resistance of SS alloys can be approximated by the following equation ... 

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. 

Both austenitic and super SS s have excellent resistance to erosion-corrosion in velocities up to 85 ft/s (26 m/s). Usually, copper base alloys are not considered because of poor resistance to hydrogen sulfide/10 poor resistance to erosion, and low strength. Prevention of corrosion by coatings is usually impractical in production equipment because of limited life, as described previously, and because the coating can be blown off by sudden depressurization when the operating pressure is above -650 psi (4,480 kPa). 

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