The aim of this review is to communicate some essential knowledge of the underlying mechanism of the corrosion of structural containment alloys during molten salt reactor operation in the context of prospective online monitoring in future MSR installations. The formation of metal halide species and the progression of their concentration in the molten salt do reflect containment corrosion, tracing the depletion of alloying metals at the alloy salt interface will assure safe conditions during reactor operation. Even though the progress of alloying metal halides concentrations in the molten salt do strongly understate actual corrosion rates, their prospective 1st order kinetics followed by near-linearly increase is attributed to homogeneous matrix corrosion. The service life of the structural containment alloy is derived from homogeneous matrix corrosion and near-surface void formation but less so from intergranular cracking (IGC) and pitting corrosion. Online monitoring of corrosion species is of particular interest for molten chloride systems since besides the expected formation of chromium chloride species CrCl2 and CrCl3, other metal chloride species such as FeCl2, FeCl3, MoCl2, MnCl2 and NiCl2 will form, depending on the selected structural alloy. The metal chloride concentrations should follow, after an incubation period of about 10,000 hours, a linear projection with a positive slope and a steady increase of < 1 ppm per day. During the incubation period, metal concentration show 1st order kinetics and increasing linearly with time1/2. Ideally, a linear increase reflects homogeneous matrix corrosion, while a sharp increase in the metal chloride concentration could set a warning flag for potential material failure within the projected service life, e.g. as result of intergranular cracking or pitting corrosion. Continuous monitoring of metal chloride concentrations can therefore provide direct information about the mechanism of the ongoing corrosion scenario and offer valuable information for a timely warning of prospective material failure.

1. Introduction and Background
2. Chloride Salt Systems
3. Stainless-Steel Containment and StructuralMaterials
    3.1 Nickel-based Mo-Cr Hastelloy N Family
    3.2 Ferritic-Martensitic Stainless Steel
    3.3 Austenitic Stainless Steels SS-304 andSS-316
4. Thermodynamics of Corrosion Mechanismof Stainless Steel in Halide SaltMelts
5. Kinetics and Mechanism of StructuralSteel Corrosion in Halide Salt Melts
    5.1 Chromium Self Diffusivity in Ferritic/martensiticand Austenitic Stainless Steels
6. Experimental Corrosion Data on StructuralAlloys for MSR Application
7. Corrosion Monitoring in Future MSRDesigns
8. Irradiation-induced Alteration of CorrosionProperties
9. Conclusion
Acknowlegements
REFERENCES

• Thomas Hartmann(Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, Washington 99354, USA) Corresponding author
• Patricia Paviet(Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, Washington 99354, USA)