Electrochemistry of Multiphase Nickel-Base Alloy in Aqueous Systems

A study has been conducted to evaluate the effect of galvanic coupling between precipitated phases, matrix, and near grain boundary chemistries in Ni-base alloys (Ni--Cr--Fe, X750 and 718). Corrosion potential, potentiodynamic polarization behavior, galvanic effects, and passivation kinetics have been determined for grain boundary gamma ', Laves phase, and Ni--Cr--Fe matrices for a wide range of Cr contents in aqueous systems over the temperature range 30-288 deg C. Electrochemical activity of gamma ', Laves phase, and sensitized chemistries is significantly higher than the matrix and varies with composition and temperature. Galvanic couples, gamma '/matrix, and Laves/matrix in a simulated crack tip environment exhibit higher current density than a sensitized chemistry (6.3 wt.% Cr)/matrix couple. The results of the study show that the electrochemical relationships between grain boundary phases and their couples with the matrix can provide for an extremely local (i.e. at the crack tip) source of anodic and cathodic processes. A correlation between grain boundary gamma '(Ni sub 3 Al)/matrix galvanic coupling and environmentally assisted intergranular cracking in alloy X-750 has been made. At low temperatures, approx 100 deg C, hydrogen embrittlement is proposed as the dominant mechanism. At high temperature, it is suggested that both H embrittlement and stress corrosion cracking are viable mechanisms for crack propagation. A schematic model is presented which describes the relationship between embrittlement mechanisms and local electrochemistry at the crack tip. Graphs. 25 ref.--AA