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Four factors that cause stress corrosion cracking are sufficient stress, a suitable corrodent, a susceptible alloy, and time. The nature and sources of both applied and residual stress are discussed. The sources of residual stress that are peculiar to the brass industry are described. The copper (Cu) metals are listed in order of their relative susceptibility to stress corrosion cracking. Zinc (Zn) in Cu contributes to the probability of its cracking; nickel (Ni) in Cu metals tends to inhibit it. Stress corrosion cracking is not instantaneous, so time is a factor in its occurrence. Various methods for the prevention of stress corrosion cracking are discussed. Some examples of stress corrosion cracking in service are described. Probability is discussed in relation to its role of predicting the stress corrosion cracking of Cu metals. Testing of Cu metals for their resistance to stress corrosion cracking may employ loops or U-bends. Applied stress may be effected by dead weight loading or by the use of springs. Corrodents may be chemical solutions, natural environments (atmosphere, sea water) or exposure to service environments. The metallographic appearance of stress corrosion cracks in Cu alloys is described as are the crack paths in brass and in other metals.
Liquid metal embrittlement is similar to stress corrosion cracking but is thought not to be identical. The similarities and differences are discussed. Mercury (Hg) is used to test for the presence of residual stress in brass, however Hg is an extremely dangerous metal in contact with any Cu alloy. The possible sources of Hg are mentioned. A chiller which failed by Hg cracking is described. Other liquid metals may cause cracking of Cu alloys including low melting alloys which are used in tube bending.
stress corrosion cracking, copper alloys, external stresses, residual stresses, brasses, ammonia, embrittlement, failure, plastic deformation, transgranular corrosion, intergranular corrosion
Senior research metallurgist, Research and Technical Center, Anaconda American Brass Co., Waterbury, Conn.