Koji Nagata

Stress Corrosion Cracking of Copper Alloys in Pure Steam and Water at High Temperatures

Several instances have been experienced in which copper alloy tubes used for feed water heater failed by stress corrosion cracking (SCC). An experimental study has been made to investigate the phenomenon of these failures using an autoclave in which the stressed and non stressed specimens were exposed to pure water and steam at high temperatures. The results obtained were as follows. 1) Cu-Zn-AI alloys showed different corrosion morphology according to the alloy composition in degassed steam at 150 to 300C. Namely, the low zinc alloys were liable to suffer intergranular corrosion, which developed to crack under applied tensile stress of 3 to 10kg/mm2. This phenomenon is considered to be the intergranular SCC in a broad sense. On the other hand the high zinc alloys had a tendency to suffer layer type corrosion and to be immune to the intergranular SCC. They, however, were deteriorated by creep rupture owing to intergranular cavitation under applied tensile stress of 3 to 10kg/mm2. 2) Cupronickel alloys, 90/10 and 70/30, suffered intergranular corrosion in pure water and steam at 300 to 350C. In the autoclave tests, intergranular SCC could be easily reproduced under applied tensile stresses of 15 to 20kg/mm2, independent of alloy composition, temper of specimen, steam temperature, and oxygen concentration in water. On the other hand, Monel metal was not failed under the experimental conditions of the pure water and tensile stresses up to 38kg/mm2. 3) It may be conclude that the SCC of copper alloys in high temperature pure water and steam is similar to the stress accelerated intergranular corrosion in a strict sense, which may be associated with the equilibrium grain boundary segregation of active metals.