Understanding the pitting mechanism of super ferritic stainless steel in bromide solutions: The role of Ti/Nb–Mo precipitates with a core–shell structure

The core–shell structure of Ti/Nb precipitates in UNS S44660 was revealed by focused ion beam–transmission electron microscopy (FIB–TEM). In the bromide solution, the Nb in the shell is preferentially and selectively eluted, leaving a micro-crevice. The combined effect of the strong anodic adsorption of Br − and the large strain field in the shell promotes the free dissolution of residual Mo. Micro-crevices surround the Ti-rich nucleus and expand into pits. Consequently, pitting preferentially initiates at the Ti/Nb precipitates, which was verified by scratching electrode tests and immersion tests in bromide solutions. • Core–shell structure of Ti/Nb-Mo precipitates in S44660 is revealed. • Ti-rich phase acts as the core and Nb-Mo-rich nanoprecipitate acts as the shell. • The shell has the largest lattice distortion and strain field distribution. • The shell dissolves preferentially in NaBr, leading to pitting initiation. • Pitting mechanism of S44660 in bromide solutions is proposed.