Evolution of surface state of 70/30 cupronickel tubes with stable protective films under cathodic polarisation

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.