Requirements for Initiation of Intergranular SCC in DHP-Copper Tubes

Abstract In an artificial heating circuit differently manufactured DHP-copper tubes (deoxygenated high residual phosphorus, UNS C12200) have been investigated with respect to susceptibility to SCC in 1 M potassium nitrite solution at 75 °C. DHP-copper tubes were industrially manufactured by stretch reduction milling and drawing respectively. Experiments were done under different levels of tangential residual stresses and longitudinal external stresses. Threshold stress level for SCC initiation has been determined. Besides stress level and manufacture process of tubes the investigated parameters were the applied potential and mechanical properties of tubes. Time to fracture has been measured and cracked specimens were investigated by SEM. Results show that threshold stress level for SCC initiation is about 25 MPa and independent from manufacturing process. Cracking is always intergranular. No transgranular cracking was obtained. Even hard DHP-copper with an UTS (ultimate tensile strength) of almost 400 MPa and a heavily cold worked microstructure with elongated grains failed intergranular. Time for SCC initiation increases with strength of material. Potential has only little influence on cracking behavior in the range of -200 and 100 mVSCE. Free corrosion potential of copper tubes under investigated conditions is about -70 mVSCE. More negative potentials yield to longer cracking times, more positive potentials to a decrease of cracking time due to additional anodic dissolution of copper. SCC susceptibility is slightly higher for annealed copper tubes when compared to semi-hard and much higher when compared to hard qualities. This is caused mainly by changes of microstructure. Elongated grains of hard copper tubes result in longer cracking times since cracking has to take place perpendicular to elongated grains. An explanation for occurrence of inter- and transgranular SCC of pure copper is given.