Correlation Between the State of Atomic Order in Selected (Ni–Mo–Cr)-Based Alloys and Their Corrosion Resistance

A study has been conducted to determine the effect of the state of atomic order in Hastelloy alloys C-276, C-4, and C-22 on their corrosion properties. It is shown that alloy C-22 outperforms alloys C-276 and C-44 in terms of resistance to aqueous oxidizing and reducing media in the annealed condition with face-centered cubic structure containing short-range order. This behavior could be related to the optimized combination of Cr and Mo in alloy C-22 with the smaller addition of W. However, the three alloys are found to be resistant to stress-corrosion cracking despite the combination of short-range order and relatively low stacking fault energy. Long-range ordering to a Pt₂Mo-type superlattice with body-centered tetragonal structure is found to enhance the resistance to aqueous corrosion with alloy C-22 still outperforming alloys C-276 and C-4. However, the three alloys become highly susceptible to stress-corrosion cracking. The results are correlated with the ordered microstructure and its effect on the deformation behavior.