Formation of Al2O3 diffusion barrier in cold-sprayed NiCoCrAlY/Ni multi-layered coatings on 304SS substrate

- Al2O3 diffusion barrier formed on 304SS with cold-sprayed Ni(O)/NiCoCrAlY coatings.
- Al2O3 was achieved by the oxidation of Al from NiCoCrAlY with O in Ni(O) during annealing.
- Alumina layer grows following a parabolic law at a temperature range of 800-1000 °C.
- Al2O3 is thermally stable and effective for inhibiting elemental inter-diffusion.

This work deals with a double-layer coating for the corrosion protection of stainless steel (SS) structures used in molten fluoride salt reactors for energy production. It aims to mitigate the dissolution of chromium from stainless steel structures into the molten salts. On 304SS substrates, three coating layers consisting of NiCoCrAlY, Ni(O), and the nickel layer on the top were deposited by cold spraying. The Ni(O) coating was made by using a powder containing active oxygen and prepared by mechanical-alloyed pure Ni powders in ambient atmosphere. The Al2O3 diffusion barrier was evolved by annealing the cold-sprayed coating system in argon at temperatures ranging from 800 to 1000 °C at the interface between NiCoCrAlY and Ni(O) layers. During the annealing, the Al2O3 layer formed with a growth following a parabolic law, resulting from the reaction of the oxygen in the Ni (O) layer with the aluminium in the NiCoCrAlY layer. The coating test conducted at 900 °C for 100 h in argon showed that the Al2O3 layer changed little during annealing and could effectively prevent the diffusion of Fe and Cr from the SS substrate to the upper Ni layer.