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

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 degrees 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 NiCoCrAIY layer. The coating test conducted at 900 degrees 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. (C) 2016 Elsevier B.V. All rights reserved.