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Copper Alloys: Corrosion
Copper Alloys: Corrosion$ J Brock, Olin Corporation, New Haven, CT, USA OS Zaroog, Universiti Tenaga Nasional, Selangor, Malaysia r 2017 Elsevier Inc. All rights reserved.
1 2 3 4 5 6 7 8 References
1
Introduction Dealloying and Stress Corrosion Atmospheric Exposure Soil Water Oxidation Hydrogen Embrittlement Summary
1 1 2 2 2 3 3 3 4
Introduction
The protective nature of the oxide films formed in the neutral pH range, together with other properties of copper and its alloys, such as their ease of fabrication, strength, conductivity, and relatively low cost, results in copper alloys being used successfully in many applications where corrosion resistance is required. Thus, for example, their good resistance to corrosion in soil results in copper alloy tubes being widely used for underground water lines, while their resistance to corrosion in potable, brackish, and cooling tower waters, as well as seawater, results in their extensive use for plumbing tube and for tubes in heat exchangers and power utility condensers. Also, copper and its alloys extensively used in marine piping application, heat exchanger, tubing and condensers; moreover copper used to coat carbon steel piping as well as in art applications [1–6]. Copper is cathodic with respect to hydrogen ion present in nonoxidizing acids. Consequently, in the absence of oxygen or other oxidizing agents, copper will not be attacked by acids. Such conditions are not often realized and consequently the dissolution of copper can proceed by the reaction: Cu2 Cu2þ þ 2e
½1
this anodic reaction being supported by a cathodic reaction such as the reduction of dissolved oxygen: O2 þ 2H2 O þ 4e -4OH
½2
In more neutral solutions the anodic reaction results in the formation of cuprous oxide or hydroxide: Cu2 O þ 2Hþ þ 2e-2Cu þ H2 O
½3
In the pH range 4–12, the oxide films so formed generally grow with a decreasing rate to an essentially constant thickness. Once formed, the films protect the metal from further attack. In the pH range 4–12, the oxide is insoluble and acts as a protective layer to prevent further oxidation. For corrosion to occur, either copper ions or anions of the corrodant must migrate through the protective films. Cuprous oxide is a p-type semiconductor, where there is a insufficiency in Cu þ ions, and permits the transport of ions through it. Accordingly, protection of the metal is not complete. The cation vacancies move inside the protection layer while electron move outside through the structure. Cation hole is a driven for cuprous ion diffusion where a greater number of vacancies are at the oxide interface than at the metal [7]. By doping the oxide with higher valency cations than copper ion, the resistance of the film to ion migration, and hence the protection afforded by the film to the underlying metal, is increased. Such doping occurs naturally on alloys containing elements such as aluminum or nickel, so that the films formed on them are more protective than those formed on pure copper. In alkaline solutions at pH values above 12, the anodic reaction results in the formation of soluble cuprite ions þ Cu þ 2H2 O-CuO2 2 þ 4H þ 4e
½4
Another way of looking at the above is to say that at pH values below 4 or higher than 12, the oxide film is soluble and in these solutions, in the presence of an oxidizing agent, copper will corrode. ☆ Change History: February 2016. O.S. Zaroog added Abstract,Keywords; expanded text with additional review materials; added equation numbers; and updated the list of references.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02892-7
1
2
Copper Alloys: Corrosion
Table 1 Relative stress corrosion susceptibility of representative copper alloys in ammonia environments
2
Alloy
Susceptibility index
Cu–30%Zn Cu–29%Zn–12%Ni Cu–15%Zn Cu–3%Al–2%Si–0.4%Co Cu–17%Zn–18%Ni Cu–5%Sn–0.2%P ETP Cu Cu–3%Si–2%Sn–0.1%Cr Cu–10%Ni–1%Fe
1000 300 200 50 40 20 0 0 0
Dealloying and Stress Corrosion
Dealloying is an electrochemical reaction when one alloy element with less noble is selectively removed from the alloy leaving behind a porous copper substrate with no mechanical strength [8]. Very specific to some copper alloys, however, is their tendency to undergo dealloying or stress corrosion cracking. Such attack can be rapid and mechanically detrimental. This is most typically found with copper alloys containing zinc, but also occurs in alloys containing manganese and, to a lesser extent, aluminum or nickel. In the case of the brasses, the process is termed dezincification and occurs in alloys with greater than 15% zinc. In alpha brasses, dezincification results in a uniform layer of porous copper. In two-phase brasses, the beta phase is attacked preferentially leaving discrete plugs of dealloyed metal. This process is termed plug-type dezincification. Certain alloying elements, arsenic, antimony, and phosphorus, inhibit layer-type but not plug-type dezincification. In alloys with around 30% zinc, they are usually present at levels of 0.02–0.1%. Certain copper alloys are very susceptible to stress corrosion cracking. These include the brasses and copper–manganese alloys. The susceptibility to stress corrosion increases with increasing level of the reactive element and with increasing stress level. This corrosion phenomenon is usually associated with the presence of ammonia or its compounds although cracking has also been reported in solutions containing citrates, tartrates, or nitrites. Table 1 lists the relative tendency of alloys to undergo stress corrosion cracking, with 1000 being the highest and 0 being immune.
3
Atmospheric Exposure
Copper and its alloys corrode at rates usually less than 0.001 inch year1 in the atmosphere. The rates are generally higher in industrial locations, particularly if these include sulfur-containing gases. In many instances corrosion is accompanied by the formation of green corrosion products which form a patina over the alloy surface. These coatings are made use of in architectural considerations, as for example on the Statue of Liberty.
4
Soil
Copper has remarkably good corrosion resistance to many types of soils. Some copper service lines removed from Billings, Montana after 70 years of exposure showed no visible attack on the soil side. Whether attack will occur depends on factors such as the presence of sulfides, to which the metal is much less resistant, and on extraneous issues such as the presence of fertilizers.
5
Water
The corrosion resistance of copper alloys leads to their widespread use for water handling. Domestic water lines are usually made of phosphorus deoxidized copper. Only in very infrequently encountered waters with specific chemistry is corrosion a problem in domestic waters. Here the corrosion takes the form of pits, which can rapidly lead to perforation of the tube wall. Copper has good resistance to corrosion in seawater. Table 2 lists the weight loss of several copper alloys during immersion in seawater. In this environment, copper–nickel alloys are the most resistant. As well as providing good corrosion resistance, all copper alloys share the property of resisting marine growths such as barnacles. Copper and its alloys find wide use in condensers in the power utility industry where the coolant may be fresh, brackish, recirculated cooling tower water or seawater. In these applications, admiralty brass and copper nickel alloys find the widest use.
Copper Alloys: Corrosion
Table 2
3
Weight loss of copper alloys following immersion in seawater at Daytona Beach
Alloy
C11000 C19400 C23000 C26000 C70600 C77000
Composition
99.9%Cu 2.4%Fe, 0.13%Zn, 0.04%P 15%Zn 30%Zn 10%Ni, 1.4%Fe 27%Zn, 18%Ni
Weight loss (mg cm2) 60 days
156 days
365 days
10.6 9.6 8.9 17.2 3.6 9.7
14.6 11.0 10.2 15.6 3.4 14.2
19.7 16.0 19.1 21.9 5.1 –
A common copper–nickel alloy is C706 with 10% nickel and 1.4% iron, the iron adding resistance to erosion–corrosion. This phenomenon is essentially due to the removal at high local water velocities of the protective oxide layers on the alloy. Further resistance to erosion–corrosion is provided by additions of manganese or chromium.
6
Oxidation
At temperatures up to 100 1C, pure copper oxidizes in air to produce films consisting mostly of cupric oxide, CuO. The oxide thickness increases linearly with the logarithm of time. At increasing temperatures the oxide film grows according to a parabolic or cubic rate law and is duplex in nature. The outer layer is cupric oxide and the inner layer cuprous oxide, Cu2O. The relative amount of cuprous oxide increases with increasing temperature. The oxidation behavior of copper alloys depends on the alloying element. Those more noble than copper, such as silver, gold, platinum, and palladium, do not change the oxide composition. The scale contains metallic inclusions of the noble metals, and the metal substrate adjacent to the oxide is enriched in the noble element. Oxidation of copper alloys containing more reactive elements depends on the nature and amount of the element, the temperature, and the relative rates of diffusion of oxygen and the alloying element in the alloy substrate. Dilute solid solution alloys oxidize in air to form outer layers of CuO and Cu2O with oxides of the baser element beneath. The oxide of the reactive element may be present as a continuous layer oralternatively, as particulate in a zone of the alloy adjacent to the cuprous oxide. Reactive elements such as aluminum, beryllium, and zinc, which have a high affinity for oxygen, have critical concentrations of around 4, 2, and 15 wt.%, respectively, at which they form a continuous protective film of their oxides at high temperatures. The formation of such protective films is also favored when oxidation is conducted in reducing gases, which preclude the formation of oxides of copper.
7
Hydrogen Embrittlement
At elevated temperatures, hydrogen can diffuse into copper and react with any internally precipitated oxides of copper, forming water vapor under high pressure. This can promote severe cracking when the metal is subsequently deformed at low temperature. Such an embrittlement phenomenon is most common in cast tough pitch copper. It has been observed at temperatures as low as 400 1C, the severity of attack increasing with increasing temperature. To prevent the inclusion of copper oxide particulate, which can lead to hydrogen embrittlement, copper is deoxidized by small additions of phosphorus, calcium, or magnesium before casting. These scavenge the oxygen forming their oxides which cannot be reduced by hydrogen.
8
Summary
As described in the preceding sections, copper and its alloys have an excellent resistance to corrosion during outdoor exposure and in a wide variety of waters. This resistance, combined with other desirable properties, such as thermal and electrical conductivity, ease of fabrication, and low cost, results in their wide commercial use in such environments. The resistance to corrosion stems from the films of copper oxide which form in what can generally be described as neutral solutions. For this reason they are also used in many other applications where such conditions are met. Thus, the metal and its alloys find use in chemical plant process equipment involving a wide variety of organic and inorganic chemicals. In considering copper and its alloys for such applications, the judicious selection of the appropriate copper alloy, based on an understanding of the nature of the process streams, will prevent local failure of the equipment by phenomena such as dealloying or stress corrosion cracking.
4
Copper Alloys: Corrosion
References [1] [2] [3] [4] [5] [6] [7] [8]
Tuthill, A.H., 1987. Guidelines for the use of copper alloys in seawater. Mat. Perf. 26, 12–22. Davis, J.R. (Ed.), 2001. Copper and Copper Alloys, ASM Specialty Handbook. Materials Park, OH: ASM International. King, F., Liljab, C., Vähänenc, M., 2013. Progress in the understanding of the long-term corrosion behaviour of copper canisters. J. Nucl. Mater. 438 (1–3), 228–237. Brock, A.J., 2001. Corrosion of copper and its alloys. In: Jones, R.H. (Ed.), Environmental Effects on Engineered Materials. Boca Raton, FL: CRC Press. Scott, D.A., 2002. Copper and Bronze in Art: Corrosion, Colorants, Conservation. Los Angeles, CA: Getty Publications. MacLeod, I.D., 1981. Bronze disease: An electrochemical explanation. Bull. Inst. Conserv. Cultural Mater. 7, 16–26. Trethewey, K.R., Chamberlain, J., 1995. Corrosion for Science and Engineering. Harlow: Longman. Ding, Y., Kim, Y.J., Erlebacher, J., 2004. Nanoporous Gold Leaf: “Ancient Technology”/Advanced Material. Adv. Mater. 16, 1897–1900.
1 2 3 4 5 6 7 8 References
1
Introduction Dealloying and Stress Corrosion Atmospheric Exposure Soil Water Oxidation Hydrogen Embrittlement Summary
1 1 2 2 2 3 3 3 4
Introduction
The protective nature of the oxide films formed in the neutral pH range, together with other properties of copper and its alloys, such as their ease of fabrication, strength, conductivity, and relatively low cost, results in copper alloys being used successfully in many applications where corrosion resistance is required. Thus, for example, their good resistance to corrosion in soil results in copper alloy tubes being widely used for underground water lines, while their resistance to corrosion in potable, brackish, and cooling tower waters, as well as seawater, results in their extensive use for plumbing tube and for tubes in heat exchangers and power utility condensers. Also, copper and its alloys extensively used in marine piping application, heat exchanger, tubing and condensers; moreover copper used to coat carbon steel piping as well as in art applications [1–6]. Copper is cathodic with respect to hydrogen ion present in nonoxidizing acids. Consequently, in the absence of oxygen or other oxidizing agents, copper will not be attacked by acids. Such conditions are not often realized and consequently the dissolution of copper can proceed by the reaction: Cu2 Cu2þ þ 2e
½1
this anodic reaction being supported by a cathodic reaction such as the reduction of dissolved oxygen: O2 þ 2H2 O þ 4e -4OH
½2
In more neutral solutions the anodic reaction results in the formation of cuprous oxide or hydroxide: Cu2 O þ 2Hþ þ 2e-2Cu þ H2 O
½3
In the pH range 4–12, the oxide films so formed generally grow with a decreasing rate to an essentially constant thickness. Once formed, the films protect the metal from further attack. In the pH range 4–12, the oxide is insoluble and acts as a protective layer to prevent further oxidation. For corrosion to occur, either copper ions or anions of the corrodant must migrate through the protective films. Cuprous oxide is a p-type semiconductor, where there is a insufficiency in Cu þ ions, and permits the transport of ions through it. Accordingly, protection of the metal is not complete. The cation vacancies move inside the protection layer while electron move outside through the structure. Cation hole is a driven for cuprous ion diffusion where a greater number of vacancies are at the oxide interface than at the metal [7]. By doping the oxide with higher valency cations than copper ion, the resistance of the film to ion migration, and hence the protection afforded by the film to the underlying metal, is increased. Such doping occurs naturally on alloys containing elements such as aluminum or nickel, so that the films formed on them are more protective than those formed on pure copper. In alkaline solutions at pH values above 12, the anodic reaction results in the formation of soluble cuprite ions þ Cu þ 2H2 O-CuO2 2 þ 4H þ 4e
½4
Another way of looking at the above is to say that at pH values below 4 or higher than 12, the oxide film is soluble and in these solutions, in the presence of an oxidizing agent, copper will corrode. ☆ Change History: February 2016. O.S. Zaroog added Abstract,Keywords; expanded text with additional review materials; added equation numbers; and updated the list of references.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02892-7
1
2
Copper Alloys: Corrosion
Table 1 Relative stress corrosion susceptibility of representative copper alloys in ammonia environments
2
Alloy
Susceptibility index
Cu–30%Zn Cu–29%Zn–12%Ni Cu–15%Zn Cu–3%Al–2%Si–0.4%Co Cu–17%Zn–18%Ni Cu–5%Sn–0.2%P ETP Cu Cu–3%Si–2%Sn–0.1%Cr Cu–10%Ni–1%Fe
1000 300 200 50 40 20 0 0 0
Dealloying and Stress Corrosion
Dealloying is an electrochemical reaction when one alloy element with less noble is selectively removed from the alloy leaving behind a porous copper substrate with no mechanical strength [8]. Very specific to some copper alloys, however, is their tendency to undergo dealloying or stress corrosion cracking. Such attack can be rapid and mechanically detrimental. This is most typically found with copper alloys containing zinc, but also occurs in alloys containing manganese and, to a lesser extent, aluminum or nickel. In the case of the brasses, the process is termed dezincification and occurs in alloys with greater than 15% zinc. In alpha brasses, dezincification results in a uniform layer of porous copper. In two-phase brasses, the beta phase is attacked preferentially leaving discrete plugs of dealloyed metal. This process is termed plug-type dezincification. Certain alloying elements, arsenic, antimony, and phosphorus, inhibit layer-type but not plug-type dezincification. In alloys with around 30% zinc, they are usually present at levels of 0.02–0.1%. Certain copper alloys are very susceptible to stress corrosion cracking. These include the brasses and copper–manganese alloys. The susceptibility to stress corrosion increases with increasing level of the reactive element and with increasing stress level. This corrosion phenomenon is usually associated with the presence of ammonia or its compounds although cracking has also been reported in solutions containing citrates, tartrates, or nitrites. Table 1 lists the relative tendency of alloys to undergo stress corrosion cracking, with 1000 being the highest and 0 being immune.
3
Atmospheric Exposure
Copper and its alloys corrode at rates usually less than 0.001 inch year1 in the atmosphere. The rates are generally higher in industrial locations, particularly if these include sulfur-containing gases. In many instances corrosion is accompanied by the formation of green corrosion products which form a patina over the alloy surface. These coatings are made use of in architectural considerations, as for example on the Statue of Liberty.
4
Soil
Copper has remarkably good corrosion resistance to many types of soils. Some copper service lines removed from Billings, Montana after 70 years of exposure showed no visible attack on the soil side. Whether attack will occur depends on factors such as the presence of sulfides, to which the metal is much less resistant, and on extraneous issues such as the presence of fertilizers.
5
Water
The corrosion resistance of copper alloys leads to their widespread use for water handling. Domestic water lines are usually made of phosphorus deoxidized copper. Only in very infrequently encountered waters with specific chemistry is corrosion a problem in domestic waters. Here the corrosion takes the form of pits, which can rapidly lead to perforation of the tube wall. Copper has good resistance to corrosion in seawater. Table 2 lists the weight loss of several copper alloys during immersion in seawater. In this environment, copper–nickel alloys are the most resistant. As well as providing good corrosion resistance, all copper alloys share the property of resisting marine growths such as barnacles. Copper and its alloys find wide use in condensers in the power utility industry where the coolant may be fresh, brackish, recirculated cooling tower water or seawater. In these applications, admiralty brass and copper nickel alloys find the widest use.
Copper Alloys: Corrosion
Table 2
3
Weight loss of copper alloys following immersion in seawater at Daytona Beach
Alloy
C11000 C19400 C23000 C26000 C70600 C77000
Composition
99.9%Cu 2.4%Fe, 0.13%Zn, 0.04%P 15%Zn 30%Zn 10%Ni, 1.4%Fe 27%Zn, 18%Ni
Weight loss (mg cm2) 60 days
156 days
365 days
10.6 9.6 8.9 17.2 3.6 9.7
14.6 11.0 10.2 15.6 3.4 14.2
19.7 16.0 19.1 21.9 5.1 –
A common copper–nickel alloy is C706 with 10% nickel and 1.4% iron, the iron adding resistance to erosion–corrosion. This phenomenon is essentially due to the removal at high local water velocities of the protective oxide layers on the alloy. Further resistance to erosion–corrosion is provided by additions of manganese or chromium.
6
Oxidation
At temperatures up to 100 1C, pure copper oxidizes in air to produce films consisting mostly of cupric oxide, CuO. The oxide thickness increases linearly with the logarithm of time. At increasing temperatures the oxide film grows according to a parabolic or cubic rate law and is duplex in nature. The outer layer is cupric oxide and the inner layer cuprous oxide, Cu2O. The relative amount of cuprous oxide increases with increasing temperature. The oxidation behavior of copper alloys depends on the alloying element. Those more noble than copper, such as silver, gold, platinum, and palladium, do not change the oxide composition. The scale contains metallic inclusions of the noble metals, and the metal substrate adjacent to the oxide is enriched in the noble element. Oxidation of copper alloys containing more reactive elements depends on the nature and amount of the element, the temperature, and the relative rates of diffusion of oxygen and the alloying element in the alloy substrate. Dilute solid solution alloys oxidize in air to form outer layers of CuO and Cu2O with oxides of the baser element beneath. The oxide of the reactive element may be present as a continuous layer oralternatively, as particulate in a zone of the alloy adjacent to the cuprous oxide. Reactive elements such as aluminum, beryllium, and zinc, which have a high affinity for oxygen, have critical concentrations of around 4, 2, and 15 wt.%, respectively, at which they form a continuous protective film of their oxides at high temperatures. The formation of such protective films is also favored when oxidation is conducted in reducing gases, which preclude the formation of oxides of copper.
7
Hydrogen Embrittlement
At elevated temperatures, hydrogen can diffuse into copper and react with any internally precipitated oxides of copper, forming water vapor under high pressure. This can promote severe cracking when the metal is subsequently deformed at low temperature. Such an embrittlement phenomenon is most common in cast tough pitch copper. It has been observed at temperatures as low as 400 1C, the severity of attack increasing with increasing temperature. To prevent the inclusion of copper oxide particulate, which can lead to hydrogen embrittlement, copper is deoxidized by small additions of phosphorus, calcium, or magnesium before casting. These scavenge the oxygen forming their oxides which cannot be reduced by hydrogen.
8
Summary
As described in the preceding sections, copper and its alloys have an excellent resistance to corrosion during outdoor exposure and in a wide variety of waters. This resistance, combined with other desirable properties, such as thermal and electrical conductivity, ease of fabrication, and low cost, results in their wide commercial use in such environments. The resistance to corrosion stems from the films of copper oxide which form in what can generally be described as neutral solutions. For this reason they are also used in many other applications where such conditions are met. Thus, the metal and its alloys find use in chemical plant process equipment involving a wide variety of organic and inorganic chemicals. In considering copper and its alloys for such applications, the judicious selection of the appropriate copper alloy, based on an understanding of the nature of the process streams, will prevent local failure of the equipment by phenomena such as dealloying or stress corrosion cracking.
4
Copper Alloys: Corrosion
References [1] [2] [3] [4] [5] [6] [7] [8]
Tuthill, A.H., 1987. Guidelines for the use of copper alloys in seawater. Mat. Perf. 26, 12–22. Davis, J.R. (Ed.), 2001. Copper and Copper Alloys, ASM Specialty Handbook. Materials Park, OH: ASM International. King, F., Liljab, C., Vähänenc, M., 2013. Progress in the understanding of the long-term corrosion behaviour of copper canisters. J. Nucl. Mater. 438 (1–3), 228–237. Brock, A.J., 2001. Corrosion of copper and its alloys. In: Jones, R.H. (Ed.), Environmental Effects on Engineered Materials. Boca Raton, FL: CRC Press. Scott, D.A., 2002. Copper and Bronze in Art: Corrosion, Colorants, Conservation. Los Angeles, CA: Getty Publications. MacLeod, I.D., 1981. Bronze disease: An electrochemical explanation. Bull. Inst. Conserv. Cultural Mater. 7, 16–26. Trethewey, K.R., Chamberlain, J., 1995. Corrosion for Science and Engineering. Harlow: Longman. Ding, Y., Kim, Y.J., Erlebacher, J., 2004. Nanoporous Gold Leaf: “Ancient Technology”/Advanced Material. Adv. Mater. 16, 1897–1900.