Effects of electric double layer and zone-dependent exposure conditions on corrosion development of steel pipe piles in marine environments

The splash zone is the most corrosion-critical region of steel pipe piles (SPPs) in marine environments, as cyclic wetting and drying, high oxygen availability, and repeated salt deposition jointly accelerate electrochemical deterioration. This study develops an experimentally validated, reliability-based numerical framework to investigate the role of the electric double layer (EDL) in governing the corrosion behaviour of SPPs along their lengths and its impact on their residual service life. Open-circuit potential monitoring (OCP) and electrochemical impedance spectroscopy (EIS) are used to characterise the corrosion tendency and interfacial response of steel specimens under simulated marine exposure conditions. By explicitly incorporating EDL-induced ion redistribution into a zone-dependent corrosion modelling framework, the coupled multi-physics model developed in the present study integrates Nernst-Planck ion transport, electrochemical reaction kinetics, and interfacial EDL effects to predict corrosion development across different marine zones. The results demonstrate that explicit inclusion of the EDL is essential for accurately capturing near-surface chloride enrichment and corrosion evolution, as the local chloride concentration adjacent to the steel pile surface can exceed three times the concentration in the bulk seawater. The experimental results show that, after four weeks of seawater exposure under simulated atmospheric, splash, and immersion conditions, the corrosion depth in the splash zone is more than four times that in the immersion zone, while the atmospheric-zone corrosion depth is approximately three times that in the immersion zone. The validated model is subsequently applied to assess the long-term service-life reliability of SPPs protected by different coating systems, showing that thermally sprayed aluminium (TSA) provides more than 10 additional years of protection compared with epoxy coatings. The proposed EDL-based, experimentally validated corrosion modelling framework integrates microscale ion transport and electrochemical process with zone-dependent corrosion behaviour of SPPs, enabling more reliable assessment of their residual service life.