Copper alloys erosion-corrosion

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. 

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. 

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. 

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

Table 1.29 tabulates most known examples of erosion corrosion problems occuring in aqueous systems. Historically, erosion corrosion first became a problem with the copper alloy (70%Cu 29%Zn l%Sn) condensers of naval shipsErosion corrosion of copper alloys has been an ongoing problem since then. The other major problem areas are (a) power plants where steels are exposed to water or water/steam mixtures in the temperature range 90°-280°C (b) the oil and gas industry where steels are exposed to various liquid, gas, and sometimes solids combinations containing carbon dioxide. 

Briefly the important developments in copper alloys with respect to their erosion corrosion behaviour in seawater have been ... 

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). 

Piping or plumbing systems made of copper alloys are susceptible to erosion-corrosion in unfavorable fluid flow conditions. Erosion-corrosion can occur when erosive action of the flowing stream removes the protective copper oxide film from the metal surface, and thus exposing the bare metal surface to a corrosive environment (44). 

Copper-nickel alloys are used in tubings and coils of heater and air-conditioning systems because of their high thermal conductivity in heating and cooling applications. Copper-nickel alloys such as 70/30 Cu/Ni and 90/10 Cu/Ni have sufficient erosion-corrosion resistance in water compared to pure copper. 

Reasonably, the corrosion form is typical at relatively high velocities between the material surface and flie fluid, and it is particularly intensive in cases of two-phase or multiphase flow, i.e. hquid-gas and liquid-solid particle flow. Components often liable to erosion corrosion are propellers, pumps, turbine parts, valves, heat exchanger tubes, nozzles, bends, and equipment exposed to liquid sputter or jets. Most sensitive materials are those normally protected by corrosion products with inferior strength and adhesion to flie substrate, e.g. lead, copper and its alloys, steel, and under some conditions aluminium/aluminium alloys. Stainless steel, titanium... 

Table 7.5 does not give any clear information about critical velocities, but it indicates that such thresholds exist for the copper alloys in the velocity range represented in the table (1.2-8.2 m/s). More specifically, both Figure 7.46 and Table 7.6 show examples of critical velocities for erosion corrosion. The values are not absolute they depend on the composition of the environment, the temperature, geometrical conditions, the exposure history, the exact composition and treatment of the material etc. In connection with Figure 7.46 it can be mentioned that austenitic stainless steels show excellent resistance to erosion corrosion in pure liquid flow at high velocities, while some ferritic [7.42] and ferritic-austenitic steels are attacked less than the austenitic ones if the liquid carries solid particles. The data in Table 7.6 originate from work by Efird [7.43], who interpret his results as follows for each alloy in a certain environment, there exists a critical shear stress between the liquid and the material surface. When this shear stress is exceeded, surface films are removed and the corrosion rate increases markedly. 

Other researchers have stated that erosion corrosion of copper alloys is controlled by mass transport, i.e. diffusion of copper ions away from the metal surface [7.44]. Their opinion is that the shear stresses at the actual flow conditions are too low for tearing off particles from the surface films. A combination of chemical-electrochemical and mechanical effects has also been proposed. 

Syrett BC. Erosion corrosion of copper-nickel alloys in seawater and other aqueous environments - A literature review. Corrosion, 32(6), June 1976. 

Hodgkiess, T. and Vassiliou, G. (2003) Erosion Corrosion of Copper—10% Nickel Alloy Revisited, JCSE, Vol. 6, Paper C038, pp. 1-A. 

Bronzes are copper-tin alloys which upon prolonged contact with the atmosphere form a dark patina that is much appreciated in the art world. In presence of certain pollutants such as chloride the dark patinas eventually turn to green. Aluminum-containing bronze forms surface films containing AI2O3 which improves the resistance to erosion corrosion compared to copper or brass. 

Copper alloys (Admiralty, copper-nickel) and austenitic stainless steels are the most commonly used materials for feedwater heater tubing based upon their resistance to general and localized corrosion, erosion-corrosion, and SCC, and adequate heat transfer performance [1,2]. Carbon and low-alloy steels are most often used for the shells of such heaters for economy and availability. 

Soft and low-strength metals such as copper, aluminum, and lead are especially susceptible to erosion corrosion, as are the metals and alloys that are inherently less corrosion resistant, such as carbon steels. 

The addition of a minor element can also improve the resistance of copper-nickel alloys to erosion-corrosion. The effect of iron content on the corrosion and impingement resistance of 90/ 10 copper-nickel is maximized with the addition of about 2 percent... 

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