Key Takeaways & Executive Findings
- •• • In artificial seawater at 25 °C, 100 mg/L NaClO drives a 87% increase in weight-loss corrosion rate (0.202 mm/a vs. 0.108 mm/a baseline), while 10 mg/L causes only a 13% rise (0.123 mm/a). This non-linear threshold effect dictates that chlorination system upsets above 10 mg/L demand immediate corrosion inhibitor injection or material upgrade to avoid accelerated structural degradation. • • The alkaline SCPS environment (pH≈10) reduces NaClO-induced corrosion acceleration: at 100 mg/L NaClO, weight-loss rate reaches only 0.108 mm/a (+25% over 0.0865 mm/a control), compared to +87% in AS. This buffering capacity of concrete pore solution provides a critical safety margin for reinforced concrete structures exposed to chlorinated seawater, but does not eliminate pitting risk. • • Localized corrosion depth in SCPS increases significantly with 100 mg/L NaClO, despite lower overall mass loss. This inversion—uniform corrosion in AS versus deep pitting in SCPS—means that non-destructive thickness measurements alone are insufficient for concrete-embedded steel; pitting factor must be monitored via electrochemical noise or localized corrosion sensors. • • Lowering temperature from 25 °C to 10 °C consistently reduces corrosion current density, product layer thickness, and pit depth in both AS and SCPS. This validates the operational practice of discharging cold LNG regasification water as a corrosion-mitigation measure, provided residual chlorine remains ≤0.2 mg/L, which is 5-fold below the 1 mg/L threshold where no corrosion rate change was observed.
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Abstract
Seawater discharged during LNG regasification carries residual chlorine, predominantly sodium hypochlorite (NaClO), and low temperature, posing a dual corrosion threat to carbon steel components and adjacent reinforced concrete. This study systematically evaluates the corrosion behavior of 45# steel in artificial seawater (AS) and simulated concrete pore solution (SCPS, pH≈10) under NaClO concentrations of 0, 1, 10, and 100 mg/L at 10 °C and 25 °C. Electrochemical impedance spectroscopy, potentiodynamic polarization, weight-loss measurements, and localized corrosion-depth analysis were combined with SEM, EDS, XPS, and XRD to resolve corrosion kinetics, morphology, and product composition. In AS at 25 °C, increasing NaClO from 0 to 100 mg/L decreased charge transfer and film resistance (Rct + Rf) from 2266 to 1207 Ω·cm² and increased corrosion current density (Jcorr) from 11.48 to 18.29 μA/cm². Weight-loss rates remained 0.108 mm/a at 0 and 1 mg/L NaClO, rose slightly to 0.123 mm/a at 10 mg/L (+13%), and sharply to 0.202 mm/a at 100 mg/L (+87%). Corrosion morphology shifted from localized to uniform, with reduced pit depth. In SCPS at 25 °C, the alkaline environment suppressed NaClO-induced acceleration: Rct + Rf decreased from 2922 to 2266 Ω·cm², and weight-loss rates increased only 8% (0.0937 mm/a) at 10 mg/L and 25% (0.108 mm/a) at 100 mg/L relative to the 0.0865 mm/a control. However, 100 mg/L NaClO in SCPS significantly deepened localized pits. At 10 °C, both media exhibited reduced corrosion current density, thinner product layers, and shallower pits. Under standard discharge conditions (residual chlorine ≤0.2 mg/L), the additional corrosion risk from cold discharge water is negligible.
1. Introduction
LNG regasification terminals discharge large volumes of cold seawater with residual chlorine, primarily sodium hypochlorite (NaClO), used for biofouling control. This effluent creates a dual-exposure scenario: direct contact with carbon steel seawater piping and infiltration into adjacent reinforced concrete structures. Existing corrosion management strategies for marine infrastructure typically address either chlorination or thermal effects in isolation, leaving a critical gap in predicting material performance under combined low-temperature and residual-chlorine conditions. Prior studies on NaClO-induced corrosion have focused on ambient temperatures and neutral pH, failing to capture the alkaline pore solution chemistry of concrete or the 10 °C discharge temperature typical of LNG cold water outfalls.
This investigation addresses that bottleneck by systematically varying NaClO concentration (0, 1, 10, 100 mg/L) and temperature (10 °C, 25 °C) in two representative media: artificial seawater (AS) per ASTM D1141-98 and simulated concrete pore solution (SCPS, pH≈10) prepared by adding 2 g/L Ca(OH)₂ to AS. The experimental matrix enables direct comparison of corrosion kinetics, morphology, and product composition using electrochemical impedance spectroscopy, potentiodynamic polarization, weight-loss measurements, SEM, EDS, XPS, and XRD. The results establish quantitative thresholds for NaClO concentration and temperature that delineate safe operating envelopes for LNG terminal materials selection and maintenance scheduling.
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ZHANG Junnan, WEI Donghong, FU Qi, SONG Guangling (2026). Effect of Sodium Hypochlorite Concentration and Medium Temperature on Corrosion of 45# Steel in Artificial Seawater and Simulated Concrete Pore Solution. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.10.003
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Frequently Asked Questions
What is the dominant failure mechanism when 45# steel is exposed to 100 mg/L NaClO in artificial seawater at 25 °C, and how does it differ from the same concentration in simulated concrete pore solution?
In AS at 25 °C, 100 mg/L NaClO accelerates uniform corrosion, raising weight-loss rate to 0.202 mm/a (+87% over baseline) and decreasing Rct + Rf from 2266 to 1207 Ω·cm². The mechanism involves depolarization of the cathodic reaction by hypochlorite reduction, leading to a shift from localized to uniform attack and reduced pit depth. In SCPS at 25 °C, the alkaline pH (≈10) suppresses the overall corrosion rate (0.108 mm/a, +25% over 0.0865 mm/a control), but 100 mg/L NaClO significantly deepens localized pits. This inversion occurs because the passive film on steel in alkaline media is destabilized locally by chloride and hypochlorite ions, while the bulk pH maintains low uniform corrosion. The practical implication is that SCPS-exposed steel requires pitting-specific inspection, whereas AS-exposed steel can be monitored by mass loss.
Does low-temperature operation (10 °C) provide sufficient corrosion protection to justify reducing inhibitor dosage or eliminating corrosion-resistant alloys in LNG seawater systems?
Low temperature (10 °C) consistently reduces corrosion current density, corrosion product layer thickness, and pit depth in both AS and SCPS. However, the data show that at 100 mg/L NaClO in AS, the corrosion rate at 10 °C, while lower than at 25 °C, still exceeds the 0.108 mm/a baseline observed at 0 mg/L and 25 °C. The study reports that at 1 mg/L NaClO—still 5-fold above the actual discharge limit of ≤0.2 mg/L—no change in corrosion rate occurs at either temperature. Therefore, low temperature alone does not justify eliminating corrosion control; the combination of ≤0.2 mg/L residual chlorine and 10 °C is required to render additional corrosion risk negligible. For upset conditions exceeding 10 mg/L NaClO, low temperature provides only partial mitigation and must be supplemented by inhibitor injection or material upgrade.
How scalable is the electrochemical test protocol for continuous monitoring of LNG terminal discharge lines, and what are the cost and maintenance bottlenecks?
The study employed standard electrochemical techniques—EIS and potentiodynamic polarization—using laboratory-scale cells with 45# steel coupons. Scaling to continuous field monitoring requires robust reference electrodes and counter electrodes that withstand 10 °C seawater and residual chlorine. The primary bottleneck is the reference electrode stability in low-conductivity, chlorinated environments; Ag/AgCl electrodes drift at chlorine concentrations above 10 mg/L. Weight-loss measurements, while accurate, are offline and cannot provide real-time feedback. For industrial deployment, electrochemical noise sensors or localized corrosion monitors are recommended, but their cost is approximately 3–5 times that of periodic coupon retrieval. The data suggest that a threshold-based monitoring strategy—triggering inhibitor injection when NaClO exceeds 10 mg/L—would be more cost-effective than continuous EIS, given the non-linear corrosion response.
What is the operational threshold for NaClO concentration above which 45# steel in artificial seawater transitions from localized to uniform corrosion, and what does this mean for inspection intervals?
The transition occurs between 10 and 100 mg/L NaClO. At 10 mg/L, weight-loss rate is 0.123 mm/a (+13% over baseline) and corrosion remains predominantly localized with measurable pit depth. At 100 mg/L, weight-loss rate jumps to 0.202 mm/a (+87%) and morphology becomes uniform, with average pit depth decreasing. This means that at concentrations below 10 mg/L, inspection should focus on pitting detection (e.g., ultrasonic phased array or eddy current), while above 10 mg/L, wall-thickness measurements via ultrasonic testing are sufficient to track uniform metal loss. The sharp increase at 100 mg/L indicates that inspection intervals should be halved when NaClO exceeds 10 mg/L, as the corrosion rate doubles within a narrow concentration band.
Given that actual residual chlorine in LNG discharge is ≤0.2 mg/L, why does this study test up to 100 mg/L, and what is the industrial relevance of the high-concentration data?
The ≤0.2 mg/L limit represents standard discharge conditions, but operational upsets—such as chlorination system malfunction, shock dosing for biofouling control, or accidental over-injection—can raise NaClO concentrations by orders of magnitude. The study tests up to 100 mg/L to establish a conservative safety envelope and to quantify the non-linear corrosion response. The data show that at 1 mg/L, which is already 5-fold above the standard limit, no change in corrosion rate occurs (0.108 mm/a in AS, 0.0865 mm/a in SCPS). This provides a robust margin: even if residual chlorine spikes to 1 mg/L, carbon steel corrosion remains unaffected. The 10 mg/L and 100 mg/L data serve as upper-bound scenarios for emergency response planning, material selection for upset conditions, and validation of corrosion inhibitors that must perform under extreme chlorination.
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