Hydrogen embrittlement and hydrogen-induced crack initiation in additively manufactured metals: A critical review on mechanical and cyclic loading

Understanding the impact of hydrogen embrittlement (HE) on the mechanical properties of additively manufactured (AM) metals is of utmost importance for industries utilizing these materials, including critical hydrogen transportation and storage applications. This comprehensive review paper explores the effects of HE on AM alloys, emphasizing the crucial role of microstructure and its influence on HE and hydrogen-induced crack initiation (HICI) and propagation processes. Recent studies indicate that the HE in AM metals may deviate from that observed in conventionally manufactured (CM) metals. The unique characteristics of AM processes may introduce additional factors that affect the complex hydrogen-materials interactions and HE. The hydrogen accumulation at phase interfaces and local reaching of the critical hydrogen concentration represents the primary reason for HICI in AM metals. The specific microstructure of AM and interfaces between phases in the microstructure present crucial factors that influence the HE of AM metals. The interface between phases, which serves as a material structure discontinuity and a location for misfit energy within the structure, can play a critical role in the drop of the HE resistance of certain materials (e.g., martensite/austenite interface in stainless steels, ferrite/perlite interface in low carbon steels, alpha/beta interface in titanium alloys, gamma '/ gamma '' interface in nickel-based alloys, etc.). Titanium and nickel alloys demonstrate comparable microstructural features concerning HE due to the laminar phase structure that develops during heat treatment and the secondary phase allotropy in both metals. However, stainless steels, such as SS316 and SS304, follow a distinct mechanism where austenite to martensite transformation predominantly governs hydrogen embrittlement. It is noteworthy that the effect of hydrogen embrittlement in additively manufactured metals seems to be less pronounced compared to CM metals. A comprehensive investigation of HE mechanisms and their interaction with microstructure according to the HELP + HEDE model can provide valuable insights into the susceptibility of AM metals to HE and HICI. This review underscores the need for continued investigation to ensure the reliable performance of AM metal components exposed to hydrogen and HE in various industrial applications. Also, it provides an in-depth understanding of hydrogen embrittlement in AM metals, providing recommendations for the design, development, and safety introduction of new additively manufactured alloys in hydrogen-based energy solutions. Finally, a perspective on future necessary experiments for exploring the influence of porosity in AM metals on HE, hydrogen-induced crack initiation, and other hydrogen damage mechanisms, including its interaction with microstructure, is given.