Cathodic protection is an effective strategy for mitigating corrosion in marine engineering applications. This study systematically investigates the evolution of surface states and corrosion resistance of 70/30 cupronickel tubes with pre-formed stable protective films under cathodic polarisation in natural seawater. Electrochemical impedance spectroscopy (EIS) and linear polarisation resistance (LPR) were employed to evaluate the electrochemical behaviour. Key findings reveal that a negative shift in polarisation potential (from −350 to −750 mV) reduces the barrier properties of the pre-existing protective film by nearly one order of magnitude, yet simultaneously accelerates the formation of a dense calcareous deposit layer. Notably, the innovation lies in the quantitative correlation between EIS-derived parameters and deposit growth: charge transfer resistance ( R ct ) increases significantly from 14,260 to 44,790 Ω·cm 2 , while the power index value of constant phase element (CPE) suggests the involvement of O 2 diffusion in the electrochemical processes. The novelty is further highlighted by the identification of aragonite as the dominant phase in the calcareous deposits formed at −550 and −750 mV, which contributes to enhanced surface coverage.
Liu et al. (Mon,) studied this question.