Copper 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. 

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. 

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. 

Galvanic corrosion may also occur by transport of relatively noble metals, either as particulate or as ions, to the surface of an active metal. For example, ions of copper, perhaps resulting from corrosion or erosion-corrosion at an upstream site, may be carried by cooling water to the surfaces of aluminum, steel, or even stainless steel components. If the ions are reduced and deposit on the component surfaces, localized galvanic corrosion may result. 

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. 

Since the formation nature and breakdown of protective surface films depends on both material and environmental parameters such influences on erosion corrosion will be discussed together. Particular attention will be paid to the copper/seawater and carbon steel/water (steam) systems. 

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

Problems with heating coils Internal coil corrosion Note corrosion debris is green hydrated copper carbonate Cu[11IC03 nH20 red cuprous oxide Cu20 /ntemal coil deposition Acid corrosion from soft water. Pinhole corrosion from 02 and C02. Erosion corrosion over 6 ft/s flow. Hard water scale from hard water. 

A stainless steel pump impeller with an expected lifetime of two years failed in three weeks in a reducing solution. Metals that are soft are readily damaged or worn mechanically examples are copper and lead. Even the noble or precious metals, such as silver, gold, platinum, are subject to erosion-corrosion.16,31... 

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). 

CMP is analogous to the phenomena of erosion corrosion. Normally, during corrosion of a metal, a scale forms which slows further corrosion of the metal by acting as a barrier between the metal and the corrosive medium (Section 4.3). In erosion corrosion, low corrosion rates are accelerated by the removal of this scale via an erosion or wear process. The scale, wear process, and corrosive medium in erosion corrosion are analogous to the surface layer, mechanical abrasion, and slurry chemicals of the CMP process. Thus, in principle, the same electrochemical theories that are used to understand corrosion may be applied to CMP. In this section, we discuss the electrochemical theories that are important in metal CMP. In many instances we shall refer to the electrochemical behavior of copper for illustration. However, these electrochemical principles are applicable to the CMP of all metals. 

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). 

In addition to ductile iron and PVC, copper and lead are used in pipes, and brass in fixtures and connections. Lead is released because of uniform corrosion. Copper is also released because of uniform corrosion, localized-attack cold water pitting, hot water pitting, MIC, corrosion fatigue, and erosion-corrosion. Lead pipes and lead-tin solder exhibit uniform corrosion. Brass corrosion includes erosion-corrosion, impingement corrosion, dezincification, and SCC. The direct health impacts are because of increased copper, lead, and zinc concentrations in the drinking water. Mechanical problems because of corrosion include leaks from perforated pipes, rupture of pipes, and the loss of water pressure because of blockage of pipes by corrosion products. 

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. 

The critical shear stress that causes erosion corrosion of copper in a pipe containing sea water under turbulent flow conditions is taken to be 9.6 N/m. What would be the critical flow velocity in pipes of 5 cm and 20 cm respectively We assume that the friction coefficient is given by the Blasius relation ... 

Water velocity and turbulence can damage protective films and deposits on metal surfaces causing increased corrosion. Soft metals, such as copper, are particularly susceptible to erosion corrosion, but steel and other metals are also susceptible if the water velocity is sufficiently high. The critical velocity for erosion-corrosion of copper in freshwater is about 5 s (1.52 m/s), but this velocity can drop sharply as the chemical corrosivity of the water increases. Suspended solids in the water can increase the erosion characteristics of the water [1]. Deposits can result in accelerated corrosion from the formation of oxygen differential concentration corrosion cells. 

Copper Uniform Pitting Crevice Erosion corrosion Cavitation Galvanic Concentration cell... 

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. 

---