When the outer sheath of submarine cables is damaged, the degradation of cross-linked polyethylene (XLPE) insulation by anions in seawater becomes a critical factor affecting cable service life. This study investigates 500 kV three-core XLPE insulation and systematically reveals the differential and synergistic degradation mechanisms of major seawater anions (Cl⁻, SO₄²⁻, HCO₃⁻). Accelerated aging tests at 90 °C were conducted using solution systems simulating both single-ion and composite environments, combined with electrical performance evaluation, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Results show that seawater causes significantly greater deterioration of resistivity, breakdown strength, and molecular structure than any single-ion solution. Mechanistic analysis demonstrates that Cl⁻ induces nucleophilic substitution, SO₄²⁻ promotes oxidative chain scission, and HCO₃⁻ facilitates hydrolysis via pH regulation, while their coexistence produces nonlinear synergistic effects through oxidative reactions, electrochemical coupling, and ion transport. This work provides the first systematic comparison of individual and combined anion effects on XLPE, offering new mechanistic insights and quantitative evidence for understanding multi-ion degradation, with implications for insulation material design, protective strategies, and service life prediction of submarine cables.

The long-term reliability of XLPE submarine cable insulation is influenced by progressive thermal degradation during operation and subsequent seawater ingress caused by external damage. Although thermal aging and seawater exposure have been widely investigated individually, the influence of the prior thermal-aging state on the subsequent seawater-induced degradation behavior of XLPE remains insufficiently understood. In this study, XLPE insulation specimens prepared from the same commercial compound used for 500 kV submarine cables were subjected to sequential accelerated aging consisting of controlled thermal pre-aging followed by simulated seawater exposure. Broadband dielectric spectroscopy, AC breakdown testing with two-parameter Weibull analysis, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR) were employed to investigate the evolution of electrical properties, surface morphology, and molecular structure. The results demonstrate that seawater-induced electrical deterioration strongly depends on the initial thermal-aging state of XLPE. Increasing thermal pre-aging duration resulted in progressively higher relative permittivity and dielectric loss, together with reduced characteristic breakdown strength after subsequent seawater exposure. Under the most severe condition of 1440 h thermal pre-aging followed by 672 h seawater exposure, the power–frequency relative permittivity increased by 32.1%, while the characteristic breakdown strength decreased by more than one-third compared with the initial state. SEM observations revealed that thermally pre-aged specimens developed accelerated surface damage during seawater exposure, including pores, cracks, corrosion pits, and honeycomb-like structures. FTIR analysis further indicated molecular-chain degradation and increased hydroxyl-related species during sequential aging. These results suggest that thermal-aging-induced molecular oxidation, polar-group formation, and microstructural defects enhance water and ion penetration pathways, thereby increasing the susceptibility of XLPE insulation to subsequent seawater-induced degradation. This study provides material-level experimental evidence for understanding sequential aging processes in submarine cable insulation and highlights the importance of considering historical thermal damage in future condition assessment and lifetime evaluation models. Since accelerated laboratory conditions were adopted, the results should be interpreted as comparative degradation characteristics rather than direct predictions of field-service lifetime.