Corrosion Resistance in Various Environments

Atmospheric Environments

With the exception of instances where ammonia (NH3), sulfur compounds (H2SO4) or certain other chemical agents are present, copper and its alloys generally exhibit an excellent resistance to corrosion in atmospheric environments, including clean (rural), polluted (industrial), marine and tropical. Copper and its alloys are therefore suitable for long-term use in atmospheric environments. Table 33 provides the corrosion rates of certain copper alloys in various atmospheric environments.

Water Environments

In fresh water environments, copper tends to form a protective coating on the surface, and is typically very resistant to corrosion in such environments. The corrosion rate is slightly higher in soft water or water having a significant amount of dissolved CO2. Marine environments typically pose little threat to copper and most copper alloys, although at high flow velocities in seawater copper is very susceptible to erosion corrosion. Copper and its alloys are also very resistant to biofouling.

Copper is generally resistant to corrosion in steam environments. If there is a significant concentration of CO2, oxygen or ammonia in the steam, however, the copper is more susceptible to corrosion.

Acids/Alkalines

Copper does not usually corrode in the presence of acids unless there are oxidizing agents (e.g. oxygen, HNO3) available. For instance, copper and sulfuric acid do not react unless oxygen is present. Hence, it is susceptible to oxidizing acids, in addition to oxidizing heavy metal salts, sulfur, and ammonia. Exposure to environments containing ammonia can result in rapid and severe attack in the form of uniform corrosion or SCC. However copper is resistant to neutral solutions and solutions with a pH slightly on the alkaline side. The most threatening environments are ammonia, cyanide solutions, oxidizing salts and acids, or salts and acids in oxidizing conditions. Table 34 provides the uniform corrosion rate of copper in three different acids.

Soil

Copper is generally very resistant to corrosion in soil, and copper-tin (bronze) alloys are especially resistant to corrosion in soil. The presence of organic compounds, ammonium compounds, sulfates, or cinder, however, adversely affects the corrosion resistance of copper. Figure 36 shows the rate of uniform corrosion for copper in four different types of soils over a long period of time.