Accelerated Internal Corrosion in a Fresh Water Carbon Steel Pipeline Due to Intermittent Seawater Intrusion: Failure Analysis and Integrity Implications

Internal corrosion in carbon steel fresh water distribution systems is generally limited under chemically stable operating conditions. However, operational deviations can significantly alter electrochemical behavior and accelerate localized degradation. This study investigates severe internal corrosion identified in a carbon steel pipeline nominally designated for fresh water service onboard an oil tanker during dry docking. Operational records indicated intermittent short-term seawater use through a secondary bypass line during periods of fresh water generation (FWG) unit breakdown. These contingency operations introduced saline water into sections of the fresh water distribution network not designed for chloride exposure. Macroscopic examination revealed extensive tuberculation, severe hydraulic restriction, and deep under-deposit pitting concentrated at flange interfaces and low-flow regions. Cross-sectional evaluation confirmed substantial reduction of effective flow area and localized wall penetration beneath consolidated corrosion products. The degradation morphology is attributed to chloride-assisted destabilization of corrosion product films, differential aeration beneath deposits, and potential microbiologically influenced corrosion following marine contamination. The findings demonstrate that intermittent saline use in fresh water systems during operational contingency events can produce corrosion severity substantially exceeding that expected under stable fresh water conditions. A risk-based integrity management framework is proposed, emphasizing operational segregation, post-event chemical stabilization, targeted inspection of high-risk regions, and evaluation of appropriate internal protection strategies.