Corrosion off Copper and Copper Alloys

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Introduction to 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.

Corrosion Resistance Properties of Copper

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.

Applications of Copper Alloys

Copper and copper alloys provide superior service in many applications including:

• Architectural applications requiring resistance to atmospheric exposure, such as roofing, building fronts, grille work, hand rails, lock bodies, doorknobs, and kick plates
• Freshwater supply lines and plumbing fittings, where superior resistance to corrosion by various types of waters and soils is important
• Marine applications including freshwater and seawater supply lines, heat exchangers, condensers, shafting, valve stems, and marine hardware, which require resistance to seawater, hydrated salt deposits, and biofouling from marine organisms
• 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 where either process stream may contain corrosive contaminants
• Industrial and chemical plant process equipment involving exposure to various organic and inorganic chemicals
• Electrical wiring, hardware, connectors, printed circuit boards, and electronic applications requiring demanding combinations of electrical, thermal, and mechanical properties, such as semiconductor packages, lead frames, and connectors

Effects of Alloy Composition on Corrosion Resistance

Coppers and High-Copper Alloys

Coppers and high-copper alloys (C10100-C19600; C80100-C82800) have similar corrosion resistance. They demonstrate 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 requiring 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.

Brasses

Brasses (C20500-C28580), essentially copper-zinc alloys, are the most widely used group of copper alloys. The resistance of brasses to corrosion by aqueous solutions remains relatively consistent as long as the zinc content does not exceed about 15%. Above 15% zinc, dezincification may occur.

Susceptibility to stress-corrosion cracking (SCC) is significantly affected by zinc content; alloys with higher zinc content 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 mechanical properties like machinability, strength, and hardness.

Tin Brasses

Tin brasses (C40400-C49800; C90200-C94500) benefit from tin additions that significantly increase corrosion resistance, especially against dezincification. Cast brasses for marine applications are often modified by adding tin, lead, and sometimes nickel. This group of alloys is known by various names, including composition bronze, ounce metal, and valve metal.

Aluminum Brasses

Aluminum brasses (C66400-C69900) contain a few percent of aluminum in addition to copper and zinc. An important constituent of the corrosion film on these alloys is aluminum oxide (Al₂O₃), which markedly increases resistance to impingement attack in turbulent high-velocity saline water.

Phosphor Bronzes

Phosphor bronzes (C50100-C52400), created by adding tin and phosphorus to copper, offer good resistance to flowing seawater and most nonoxidizing acids except hydrochloric acid (HCl). Alloys containing 8-10% tin 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 cast form, modified by further alloy additions of lead, zinc, and nickel.

Copper Nickels

Copper nickels (C70000-C79900; C96200-C96800) include alloy C71500 (Cu-30Ni), which has the best general resistance to aqueous corrosion among commercially important copper alloys. However, C70600 (Cu-10Ni) is often selected because it offers good resistance at lower cost. Both alloys, though well-suited for chemical industry applications, are most extensively used for condenser and heat-exchanger tubes in recirculating steam systems. They are superior to coppers and other copper alloys in resisting acid solutions and are highly resistant to SCC and impingement corrosion.

Nickel Silvers

Nickel silvers (C73200-C79900; C97300-C97800) include the common alloys C75200 (65Cu-18Ni-17Zn) and C77000 (55Cu-18Ni-27Zn). They demonstrate good resistance to corrosion in both fresh and salt waters. Due to their relatively high nickel contents inhibiting dezincification, C75200 and C77000 are usually much more resistant to corrosion in saline solutions than brasses with similar copper content.

Copper-Silicon Alloys

Copper-silicon alloys (C64700-C66100; C87300-C87900) generally have the same corrosion resistance as copper but with higher mechanical properties and superior weldability. These alloys appear much more resistant to SCC than 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 used at elevated temperatures.

Aluminum Bronzes

Aluminum bronzes (C60600-C64400; C95200-C95810) containing 5-12% aluminum have excellent resistance to impingement corrosion and high-temperature oxidation. These alloys are used for beater bars and blades in wood pulp machines because of their ability to withstand mechanical abrasion and chemical attack by sulfite solutions.

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

Depending on specific environmental conditions, β 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 β structure with reprecipitated acicular α crystals, a combination that is often superior in corrosion resistance to the normal annealed structures.

Nickel-aluminum bronzes have more complex structures with the introduction of the κ phase. Nickel appears to alter the corrosion characteristics of the β 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 hydrochloric (HCl) acids; organic acids such as lactic, acetic (CH₃COOH), or oxalic acids; neutral saline solutions such as sodium chloride (NaCl) 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 NH₃. Aeration can result in accelerated corrosion in many media that appear to be compatible.