Alloys and Alloying Elements

There are three main types of copper alloys: copper-tin (bronze), copper-zinc (brass), and copper-nickel (cupro-nickels). Each of these main alloys may be alloyed with additional elements, which in some cases provides increased corrosion resistance and improved material properties. The following sections briefly describe the corrosion characteristics of some common copper alloys.

Pure Copper and High-Copper alloys

Pure copper is accepted as having greater than 99% copper, while high-copper alloys have greater than 96% copper. Both pure copper and high-copper alloys have excellent resistance to corrosion, especially in seawater. They are also highly resistant to microbiological-influenced corrosion, as copper is toxic to microorganisms. They are, however, susceptible to erosioncorrosion.

Bronze

Alloying tin (Sn) with copper improves the resistance to corrosion in fresh water and seawater environments. Hence, bronze has an excellent corrosion resistance in fresh water and in contaminated water, as well as a very good resistance to corrosion in marine environments. Furthermore, alloys that contain approximately 8 to 10% tin have a good resistance to attack by impingement, which is a form of erosion corrosion. Bronze has moderate resistance to pitting corrosion. Moreover, the addition of tin to copper pushes copper more toward the cathodic end of the galvanic series, further protecting it from galvanic corrosion.

Aluminum is added in 5-12% to Cu-Ni-Fe-Si-Sn systems to make aluminum bronze alloys, which show improvements in general corrosion resistance and exhibit excellent resistance to impingement attack (erosion corrosion) and high temperature corrosion. With an aluminum content of less than 8%, aluminum bronze alloys have an excellent resistance to pitting. Aluminum bronze alloys can be used in nonoxidizing mineral acids, organic acids, neutral saline solutions, alkalis, seawater, brackish water and fresh water without being significantly susceptible to corrosion. They are not generally suitable, however, for use in nitric acid, metallic salts, humidified chlorinated hydrocarbons and ammonia.

Phosphorous is added to copper-tin alloys to provide enhanced resistance to nonoxidizing acids (except HCl) and flowing seawater. These phosphor bronze alloys also have superior resistance to SCC compared to brass. The addition of silicon can make bronze susceptible to pitting, as well as embrittlement in the presence of high-pressure steam environments.

Brass

Brass is a copper alloy with a significant zinc content. The content of zinc can be as great as about 40%, but corrosion by selective leaching (dezincification) can be significant when the content is more than 15%. The effect of zinc content on the susceptibility of brass to pitting and dezincification is shown in Figure 35. Copper alloys that have more than 85% copper are resistant to dezincification, but may also be more susceptible to corrosive attack by impingement. Low concentrations of zinc in brass leads to a very good resistance to pitting. The addition of zinc to copper moves it further down the galvanic series toward the anodic end, and therefore, it is more susceptible to galvanic corrosion. High zinc content can also lead to a greater susceptibility to SCC. Brasses with 20-40% Zn, for example, are highly susceptible to SCC, while brass alloys with less than 15% Zn are highly resistant to SCC. For marine environments, brasses with a copper content between 65 and 85% are the most resistant to corrosion. Copper-zinc alloys have a good resistance to corrosion in fresh water environments. The one type of brass that is considered to have the best corrosion resistance in fresh water is red brass (85% Cu, 15% Zn).

Alloying brass compounds with additional elements can enhance the corrosion resistance. The addition 1% Sn, for example, improves the resistance to dezincification in 70 Cu-30 Zn alloys; this alloy is called admiralty brass. (The addition of 0.75% Sn to 60 Cu-40Zn produces the alloy called Naval brass.) Alloying nickel with brass produces nickel-silver alloys, which have a good resistance to fresh water corrosion, are resistant to dezincification, and significantly improves corrosion resistance in salt water. The addition of Pb, Te, Be, Cr, or Mn to brass has no significant affect on its corrosion resistance.

Al addition (2%) added to 76 Cu-22 Zn produces aluminum brass, which has improved corrosion resistance. These alloys exhibit improved resistance to impingement attack in seawater flowing at high velocities, but are still susceptible to dezincification. The addition of arsenic, phosphorous or antimony can be used to increase the resistance of aluminum brass, admiralty brass or naval brass to dezincification. Arsenic added to aluminum brass in an amount of approximately 0.10%, for example, improves dezincification resistance.

Copper-Nickel

Copper-nickel has a resistance to fresh water, contaminated water, and marine environments that is similar to that of bronze. It is also more noble than pure copper on the galvanic series, and therefore, less susceptible to galvanic corrosion. Copper-nickel alloys with a composition of 70% Cu and 30% Ni have the best resistance to corrosion in aqueous and acidic environments, in addition to having a very good resistance to SCC and impingement attacks. Copper-nickel alloys have a moderate resistance to pitting. Cu with 10% Ni also has a very good resistance to impingement attack and SCC. Copper-nickel alloys have a moderate resistance to pitting, although some specific alloys have an excellent resistance to pitting in seawater (e.g. alloys C70600 and C71500). Copper-nickel alloys with additions of Fe are usually very resistant to SCC. Cu-18Ni-17Zn and Cu-18Ni-27Zn exhibit good corrosion resistance in freshwater and seawater, and a good resistance to dezincification. Some copper-nickel alloys, however, are susceptible to crevice corrosion in seawater.

Other Alloying Elements

Copper-silicon alloys have a greater resistance to SCC than brass, in general. Copper-beryllium alloys are the only copper alloys that have shown a susceptibility to pitting in atmospheric environments. Additions of phosphorous in amounts greater than 0.04% can lead to serious SCC. Additions of aluminum result in a more anodic metal compared to pure copper, in terms of galvanic corrosion.