This investigation was conducted to explore the importance of local crack tip electrochemical processes in precipitation-hardened NiCrFe alloys driven by galvanic couples between grain boundary precipitates and the local matrix. The electrochemical behavior of γ' [Ni3(AI,Ti)] has been determined as a function of titanium concentration, temperature, and solution pH. The electrochemical behavior of NiCrFe solid solution alloys has been investigated as a function of Cr content for a series of 10Fe-(variable)Cr (6 to 18%)-(bal)Ni alloys, temperature, and pH. The investigation was conducted in neutral and pH 3 solutions over the temperature range of 25 to 300 C.
For the γ'[Ni3(AI,Ti)] system, peak activity occurs over a narrow temperature range near 100 C, which correlates well with observed cracking susceptibility of alloy X-750 in water. Behavior of this system was also found to be a strong function of titanium concentration. In all cases, the Ni3(AI,Ti) phase was active with respect to the matrix.
It is proposed that this galvanic effect can result in extremely local anodic and cathodic process, which can promote accelerated cracking resulting from stress corrosion cracking (SCC), hydrogen assisted cracking (HAC), or a combination of the two. The rate-controlling process is suggested to be a strong function of grain boundary phase composition. A schematic model is presented to describe the role of these effects on the embrittlement process.
