Effects of Copper Alloy Chemical Compositions on Corrosion

Copper and high-copper alloys (C10100 - C19600; C 80100 - C 82800) have similar corrosion resistance. They have excellent resistance to seawater corrosion and biofouling, but are susceptible to erosion-corrosion at high water velocities. The high-copper alloys are primarily used in applications that require enhanced mechanical performance, often at slightly elevated temperature, with good thermal or electrical conductivity. Processing for increased strength in the high-copper alloys generally improves their resistance to erosion-corrosion.

Brass (C 20500 - C 28580) are basically copper-zinc alloys and are the most widely used group of copper alloys. The resistance of brass to corrosion by aqueous solutions does not change markedly as long as the zinc content does not exceed about 15%. Above 15% Zn, dezincification may occur.

Susceptibility to stress-corrosion cracking (SCC) is significantly affected by zinc content; alloys that contain more zinc are more susceptible. Resistance increases substantially as zinc content decreases from 15% to 0%. Stress-corrosion cracking is practically unknown in commercial copper. Elements such as lead, tellurium, beryllium, chromium, phosphorus, and manganese have little or no effect on the corrosion resistance of coppers and binary copper-zinc alloys. These elements are added to enhance such mechanical properties as machinability, strength, and hardness.

Tin Brass (C 40400 - C 49800; C 90200 - C 94500). Tin additions significantly increase the corrosion resistance of some brass, especially resistance to dezincification.

Cast brass for marine applications are also modified by the addition of tin, lead, and, sometimes, nickel. This group of alloys is known by various names, including composition bronze, ounce metal, and valve metal.

Aluminum Brass (C66400-C69900). An important constituent of the corrosion film on a brass that contains few percents of aluminum in addition to copper and zinc is aluminum oxide (A1203), which markedly increases resistance to impingement attack in turbulent high-velocity saline water.

Phosphor Bronzes (C 50100 - C 52400). Addition of tin and phosphorus to copper produces good resistance to flowing seawater and to most nonoxidizing acids except hydrochloric HCl. Alloys containing 8 to 10% Sn have high resistance to impingement attack. Phosphor bronzes are much less susceptible to SCC than brasses and are similar to copper in resistance to sulfur attack. Tin bronzes-alloys of copper and tin-tend to be used primarily in the cast form, in which they are modified by further alloy additions of lead, zinc, and nickel.

Copper Nickel (C 70000 - C 79900; C 96200 - C 96800). Alloy C71500 (Cu-30Ni) has the best general resistance to aqueous corrosion of all the commercially important copper alloys, but C70600 (Cu-3ONi) is often selected because it offers good resistance at lower cost. Both of these alloys, although well suited to applications in the chemical industry, have been most extensively used for condenser tubes and heat exchanger tube in recirculating steam systems. They are superior to coppers and to other copper alloys in resisting acid solutions and are highly resistant to SCC and impingement corrosion.

Nickel Silvers (C 73200 - C 79900; C 97300 - C 97800). The two most common nickel silvers are C75200 (65Cu-18Ni-17Zn) and C77000 (55Cu-18Ni-27Zn). They have good resistance to corrosion in both fresh and salt waters. Primarily because their relatively high nickel contents inhibit dezincification, C75200 and C77000 are usually much more resistant to corrosion in saline solutions than brasses of similar copper content.

Copper-silicon alloys (C 64700 - C66100; C 87300 - C 87900) generally have the same corrosion resistance as copper, but they have higher mechanical properties and superior weldability. These alloys appear to be much more resistant to SCC than the common brasses. Silicon bronzes are susceptible to embrittlement by high pressure steam and should be tested for suitability in the service environment before being specified for components to be used at elevated temperature.

Aluminum bronzes (C 60600 - C 64400; C 95200 - C 95810) containing 5 to 12% Al have excellent resistance to impingement corrosion and high-temperature oxidation. Aluminum bronzes are used for beater bars and for blades in wood pulp machines because of their ability to withstand mechanical abrasion and chemical attack by sulfite solutions.

In the most of practical commercial applications, the corrosion characteristics of aluminum bronzes are primarily related to aluminum content. Alloys with up to 8% Al normally have completely face-centered cubic structures and a good resistance to corrosion attack. As aluminum con tent increases above 8%, a-b duplex structures appear.

Depending on specific environmental conditions, b phase or eutectoid structure in aluminum bronze can be selectively attacked by a mechanism similar to the dezincification of brasses. Proper quench-and-temper treatment of duplex alloys, such as C62400 and C95400, produces a tempered (b structure with reprecipitated acicular a crystals, a combination that is often superior in corrosion resistance to the normal annealed structures.

Nickel-aluminum bronzes are more complex in structure with the introduction of the K phase. Nickel appears to alter the corrosion characteristics of the b phase to provide greater resistance to dealloying and cavitation-erosion in most liquids.

Aluminum bronzes are generally suitable for service in nonoxidizing mineral acids, such as phosphoric H3PO4, sulfuric H2SO4, and HCl; organic acids, such as lactic, acetic CH3COOH, or oxalic; neutral saline solutions, such as sodium chloride NaCI or potassium chloride (KCl); alkalies, such as sodium hydroxide NaOH, potassium hydroxide (KOH), and anhydrous ammonium hydroxide (NH4OH); and various natural waters including sea, brackish, and potable waters. Environments to be avoided include nitric acid HNO3; some metallic salts, such as ferric chloride (FeCl3) and chromic acid (H2CrO4); moist chlorinated hydrocarbons; and moist HN3. Aeration can result in accelerated corrosion in many media that appear to be compatible. 

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