Electrochemical polarization and stress corrosion cracking of alloy 690 in 5-M chloride solutions at 25°C

An experimental, potential-pH diagram was constructed for the nickel-based Alloy 690 (UNS N06690) in 25degreesC concentrated (5 M) sodium chloride (NaCl) solution, using an electrochemical hysteresis method. The domains of immunity, general corrosion, passivation, and pitting in 5-M NaCl solutions were defined. At PH > 4, the passive region subdivided into areas of perfect passivation, imperfect passivation, and Pitting. After anodic polarization, the surface of each specimen was carefully examined metallographically. Pitting corrosion was observed over the entire PH range investigated (0.3 to 8.52), but general corrosion predominated at lower PH values (<3). On the other hand, the mechanical properties, such as ultimate tensile strength (UTS), strain at fracture, and the reduction area (RA) measured by the slow, strain rate test (SSRT), decreased significantly at PH < 3. The SSRT results are consistent with fractography and side-view observations of the tested specimens by scanning electron microscopy (SEM).