Corrosion of Copper and Copper Alloys

COPPER AND COPPER ALLOYS are widely used in many environments and applications because of their excellent corrosion resistance, which is coupled with combinations of other desirable properties, such as superior electrical and thermal conductivity, ease of fabricating and joining, wide range of attainable mechanical properties, and resistance to biofouling.

Copper corrodes at negligible rates in unpolluted air, water, and deaerated nonoxidizing acids. Copper alloy artifacts have been found in nearly pristine condition after having been buried in the earth for thousands of years, and copper roofing in rural atmospheres has been found to corrode at rates of less than 0.4 mm in 200 years.

Copper alloys resist many saline solutions, alkaline solutions, and organic chemicals. However, copper is susceptible to more rapid attack in oxidizing acids, oxidizing heavy-metal salts, sulfur, ammonia (NH₃), and some sulfur and
NH₃ compounds.

Copper and copper alloys provide superior service in many of the applications included in the following general classifications:

1. Applications requiring resistance to atmospheric exposure, such as roofing and other architectural uses, hardware, building fronts, grille work, hand rails, lock bodies, doorknobs, and kick plates
2. Freshwater supply lines and plumbing fittings, for which superior resistance to corrosion by various types of waters and soils is important
3. Marine applications - most often freshwater and seawater supply lines, heat exchangers, condensers, shafting, valve stems, and marine hardware - in which resistance to seawater, hydrated salt deposits, and biofouling from marine organisms is important
4. Heat exchangers and condensers in marine service, steam power plants, and chemical process applications, as well as liquid-to-gas or gas-to-gas heat exchangers in which either process stream may contain a corrosive contaminant
5. Industrial and chemical plant process equipment involving exposure to a wide variety of organic and inorganic chemicals
6. Electrical wiring, hardware, and connectors; printed circuit boards; and electronic applications that require demanding combinations of electrical, thermal, and mechanical properties, such as semiconductor packages, lead frames, and connectors

Brasses (C 20500 - C 28580) are basically copper-zinc alloys and are the most widely used group of copper alloys. The resistance of brasses 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 Brasses (C 40400 - C 49800; C 90200 - C 94500). Tin additions significantly increase the corrosion resistance of some brasses, especially resistance to dezincification.

Cast brasses 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 Brasses (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 Nickels (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 tubes 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 (CF₃COOH), 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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