Origins of the aqueous corrosion and stress corrosion cracking behavior of ductile nickel aluminide

The stress corrosion cracking resistance of ductile nickel aluminide, Ni3Al+B, was evaluated by conducting experiments in solutions with varying pH and ionic concentration. The results demonstrate that the ductility of this material is greatly reduced and the fracture mode changes from ductile transgranular to brittle intergranular cracking when environmental conditions are favorable for hydrogen absorption during the steady state regardless of the solution composition and pH. The results also indicate that, in the absence of cathodic polarization, this material exhibits ductile behavior during free corrosion in solutions of neutral and alkaline pH. Thermodynamic calculations of the activity of aluminum in nickel aluminide indicate that there is sufficient thermodynamic driving force for hydrogen evolution in these environments. Although the presence of surface films and transport through these films prevent this from occurring during steady state free corrosion, the thermodynamic calculations indicate that hydrogen evolution should occur during the transients that follow film rupture in these environments. To evaluate if hydrogen evolution could occur during film rupture and repassivation, nickel aluminide samples were scratched and the resulting potential transient was monitored. The results indicate that the potential drop during the scratch repassivation event will not cause significant hydrogen evolution and absorption. It is postulated that this discrepancy between the thermodynamic calculations and kinetic behavior is due to the ordering of this A3B compound into the Ll2 structure. To test this hypothesis, samples of Ni3Fe, which can be easily ordered and disordered, were tested in the ordered and disordered conditions. The results indicate that ordering significantly alters the repassivation transient that follows scratching in A3B-Ll2 compounds where the more active constituent is the B species.