In some specific service environments, NiTi shape memory alloys (SMAs) are inevitably in contact with hydrogen, which can change their mechanical properties and service performance. In this work, the effect of hydrogen charging on the deformation behavior of the superelastic NiTi SMA helical springs is investigated experimentally and theoretically. In the experimental aspect, the ex-situ
electrochemical hydrogen charging is performed for the superelastic NiTi SMA helical springs with two different spring indexes (8.5 and 11.7). Then, a series of tension-unloading tests under different loading amplitudes are performed for the superelastic NiTi SMA helical springs with and without hydrogen charging. Experimental results show that the hydrogen can significantly affect the hardening behavior of NiTi SMA springs and cause more decrease in the critical force of martensite transformation. In the theoretical aspect, a diffusional-mechanically coupled constitutive model is constructed in the framework of irreversible thermodynamics based on the experimental results. In this model, the strains associated with elasticity, martensite transformation and hydrogen expansion are taken into account, and the effect of the hydrogen concentration field on the martensitic transformation is considered. The thermodynamic driving force of martensite transformation is derived from the newly established Helmholtz free energy. The evolution of the hydrogen concentration field is obtained by combining the mass conservation equation and Fick’s law. To accurately describe the deformation behavior of the superelastic NiTi SMA helical springs with and without hydrogen charging, a semi-analytical model of helical spring is developed by simultaneously considering the torsion and bending deformation modes, and the coupling effect between the deformation and hydrogen diffusion. The proposed theoretical model is verified by comparing the predicted results with the experimental ones. It is found that the proposed model is able to predict the deformation behaviors of the superelastic NiTi SMA helical springs under the hydrogen-rich environment in a reasonable manner.