International Journal of Scientific Research and Engineering Development

A large 300-meter-class LNG carrier with a high block-coefficient hull form developed extensive coating disbondment and pronounced asymmetry in impressed-current cathodic protection (ICCP) performance only 18 months after dry-docking, despite the coating system being rated for a 30-month service interval. During the previous docking, the vertical sides and flat bottom were confirmed to be in good arrival condition, requiring less than 15% localized ISO 8501-1( SA 1 - SA 2.0) spot-treatment and showing no blisters, flaking, delamination, underfilm corrosion, or coating detachment. The existing coating system remained intact, and the measured dry film thickness (DFT) at that time fell within the manufacturer’s specified limits. DFT measurements taken again upon the vessel’s current arrival also remained within the acceptable range, confirming that no excessive film build developed during service. This demonstrates that coating thickness or over-application was not a contributing factor in the observed failures.
Arrival hull-potential measurements revealed a significant imbalance between the port (-870 mV Ag/AgCl) and starboard (-520 mV Ag/AgCl) ICCP zones, indicating that the starboard ICCP circuit had effectively lost polarization. Inspection of the ICCP cofferdam showed epoxy-putty sealing failure that permitted seawater ingress, resulting in deterioration of the dielectric shield and insulation resistance dropping below the threshold required for stable cathodic protection. This produced voltage variation, current starvation, and polarization instability, forcing the starboard hull plating into an electrochemically active condition similar to a juvenile (oxide-free) steel surface. Under these unstable potential conditions, localized alkalinity formation, chloride activity, and differential aeration cells intensified the susceptibility of the coating to cathodic-disbondment stresses. As a result, the most severe coating failures occurred on the starboard vertical sides and flat bottom, presenting as widespread sheet-type disbondment directly correlating with the ICCP-deficient region.
Quantitative assessment using Faraday’s law, Fickian chloride-diffusion modelling, and ITTC-57 hydrodynamic correlations indicated equivalent unprotected corrosion rates of ~0.25-0.30 mm·yr⁻¹, chloride penetration depths of approximately 0.30-0.35 mm at 32 °C, and hydrodynamic shear stresses in the range of 90-110 Pa. These combined in-service effects, together with ICCP interruption and electrochemical asymmetry, fully account for the degradation mechanism observed. The coating failure is therefore attributed to ICCP system imbalance, cofferdam flooding, dielectric-shield failure, and in-service voltage/potential instability, and not to coating thickness, surface preparation quality, or dry-dock application practices.
Remedial measures aligned with NACE SP0575, ASTM G8/G42, and NORSOK M-501 are recommended to re-establish uniform cathodic protection performance and ensure long-term hull integrity.