Stress assisted oxidative failure of Inconel 601 for thermal energy storage

- Fracture behavior of Inconel 601 against carbonate molten salt was studied.
- EBSD analysis revealed partial recovery with stress relieving in the sample.
- TEM-EDS examination showed de-alloying among grain boundaries.
- Stress assisted oxidation was found as the main reason for intergranular failure.

Degradation of an Inconel 601 superalloy subjected to a eutectic mixture of carbonate salt at 450 °C was studied using advanced microscopy and microanalysis techniques. On the outside of the vessel, long term exposure to elevated temperature led to intergranular stress corrosion cracking via a stress assisted oxidation process. Elemental analysis verified diffusion controlled de-alloying of Ni and Cr from grain boundaries. Transmission electron microscopy showed that a (CrFeNiAl)3O4 oxide phase was formed at grain boundaries as a result of oxygen penetration. Intergranular cracking appeared due to the presence of residual stresses and microstructural heterogeneities facilitated the crack branching.

Failure of Inconel 601 crucible containing a eutectic mixture of lithium carbonate, potassium carbonate and sodium carbonate at 450 °C: (a) intergranular morphology of crack, (b) and (c) TEM-EDS analysis of a grain boundary along a crack tip showing a de-alloyed zone surrounding the grain boundary and the subsequent formation of an oxide phase.291