Extended exposure to seawater results in the erosion of the structural high-strength steels utilized in marine equipment, primarily due to the infiltration of hydrogen. Consequently, this erosion leads to a decrease in the mechanical properties of the material. In this investigation, the mechanical responses of Q690 structural high-strength steel specimens were investigated by considering various hydrogen charging parameters, such as the current density, charging duration, and solution concentration values. The findings highlighted the significant impacts of electrochemical hydrogen charging parameters on the mechanical behaviors of Q690 steel samples. Specifically, a linear relationship was observed between the mechanical properties and the hydrogen charging current densities, while the associations with the charging duration and solution concentration were nonlinear. Additionally, the fracture morphology under various hydrogen charging parameters was analyzed and discussed. The results demonstrate that the mechanical properties of the material degrade with increasing hydrogen charging parameters, with tensile strength and yield stress decreasing by approximately 2–4%, and elongation after fracture reducing by about 20%. The findings also reveal that macroscopic fractures exhibit significant necking in uncharged conditions. As hydrogen charging parameters increase, macroscopic necking gradually diminishes, the number of microscopic dimples decreases, and the material ultimately transitions to a fully brittle fracture.

Remora, which is well known for it's 'hitchhiking' behavior, can attach to diverse marine animals and travel with them for a long distance with low energy consumption due to its special disc. In this letter, inspired by the unique 'hitchhiking' behavior, a new prototype of a robotic remora with good maneuverability, reliable adhesion system and robust motion control strategy is designed to apply the 'hitchhiking' behavior to engineered system. In the design of the mechanics, a robotic remora with wire-driven propulsion mechanism and pectoral fins diving mechanism is developed to realize the decoupled planar and vertical motion. Besides, a stable adhesion system with considerable adhesive force( $\sim$ 274N), low pre-load demand and perception ability is developed. In the aspect of motion control, considering the underactuated characteristics, highly nonlinear and model uncertainty of the robotic remora dynamics, a planar controller consisted of a line-of-sight guidance law and an active disturbance rejection heading controller, and a proportional-integral-derivative based depth controller are adopted. The combination of the two controllers enable the robotic fish to achieve autonomous motion in three-dimensional space, providing strong support for achieving the 'hitchhiking' behavior. Extensive experiments, including the coordination between the robotic remora and underwater vehicle, have been conducted to verify the reliability of the designed robotic remora and the corresponding control strategy.