Perspective review on factors that influence the stress corrosion of Ti alloys for deep-sea applications

This paper reviews the current state of knowledge and advances on the stress-corrosion cracking (SCC) of Ti alloys subject to harsh corrosive environments in the deep sea, and presents the knowledge gaps and future directions. A comprehensive review of classifications and applications of Ti alloys for deep-sea engineering indicates that the near-α and α + β Ti alloys with high strength and great weldability are the primary selection for deep-sea equipment. The role of residual stress, microstructure types, alloying elements and corrosive environmental factors on SCC performance of Ti alloys are also summarised. It is revealed that the Ti alloys with Widmanstatten structure show the lowest SCC susceptibility, and alloying of Nb, Mo and Al elements plays a positive role in the boost corrosion resistance of passive film. Synergistic effects of environmental deep-sea factors include high hydrostatic pressure, low dissolved oxygen content, low temperature and decreasing pH levels intensify the SCC of Ti alloys by inducing local dissolution of the passive film and facilitating hydrogen-induced cracking at crack tip. The study also highlights future research requirements in SCC of Ti alloys in deep sea: including the set-up of unified and suitable methods of in-situ and simulated experiments, modeling and predicting of SCC behaviour in real situations, and exploring practical protective strategies specifically. These findings provide a reference for further SCC mechanisms research and promote the microstructure optimisation and performance improvement of the advanced Ti alloy-based material systems for deep-sea engineering.