Seawater discharged during the gasification process at LNG terminals is usually characterized by low temperature and residual chlorine, mainly form of sodium hypochlorite (NaClO). This type of effluent may directly contact carbon steel components in marine environments or infiltrate adjacent reinforced concrete structures. However, the combined effect of NaClO concentration and temperature on the corrosion behavior of steel has not yet been systematically clarified. This article studies the corrosion of 45# steel under varying NaClO concentration (0, 1, 10, 100 mg/L) and temperature (10 and 25 ℃) in artificial seawater (AS) and simulated concrete pore solution (SCPS, pH≈10). The AS is prepared according to ASTM D1141-98, and SCPS is obtained by adding 2 g/L Ca(OH)2 to AS. EIS, potentiodynamic polarization, weight-loss measurements, and localized corrosion-depth analysis, are employed to evaluate the electrochemical response and corrosion rate of the samples. In addition, surface morphology, corrosion products, and microstructural characterization are investigated by SEM, EDS, XPS, and XRD. The results show that in AS at 25 ℃, when NaClO concentration increases from 0 to 100 mg/L, the charge transfer resistance and corrosion product film resistance (Rct + Rf) decrease from 2 266 to 1 207 Ω·cm2, while the corrosion current density (Jcorr) increases from 11.48 to 18.29 μA/cm2. The weight-loss corrosion rate remains 0.108 mm/a at 0 and 1 mg/L NaClO. It increases slightly to 0.123 mm/a at 10 mg/L, and then increases sharply to 0.202 mm/a at 100 mg/L. This corresponds to increases of 13% and 87% at 10 and 100 mg/L, respectively. With increasing NaClO concentration, the corrosion morphology in AS changes from localized corrosion to more uniform corrosion, and the average pit depth decreases. In contrast, SCPS at 25 ℃, the alkaline environment significantly inhibits NaClO-induced corrosion. The Rct + Rf decreases from 2 922 to 2 266 Ω·cm2 as NaClO concentration increases, and Jcorr increases less significantly than in AS. The corrosion rate in SCPS is 0.086 5 mm/a (0 mg/L), unchanged at 1 mg/L, rises to 0.093 7 mm/a at 10 mg/L (8% increase), and reaches 0.108 mm/a at 100 mg/L (25% increase), respectively. Unlike in AS, the localized corrosion depth in SCPS increases significantly with NaClO concentration, indicating that high NaClO concentration induces deeper pitting in alkaline media. Temperature also plays an important role in controlling the corrosion process. When temperature is reduced from 25 to 10 ℃, in both AS and SCPS, the Nyquist arc radius increases, the fitted Rct + Rf values rises, and Jcorr decreases. Meanwhile, the corrosion rate and pit depth both are effectively suppressed. EDS results show that the chlorine content in the corrosion products increases after adding 100 mg/L NaClO in both media. XPS and XRD analyses further confirm that the main corrosion products are FeOOH, Fe2O3, CaCO3, and residual NaCl. The addition of NaClO does not change the surface corrosion product composition. The mechanism is as follows: in near-neutral AS, NaClO hydrolyzes to form strongly oxidizing HClO, which participates in cathodic reduction, accelerating cathodic depolarization and anodic dissolution. In alkaline SCPS (pH≈10), the equilibrium shifts toward less oxidizing ClO-, so corrosion acceleration is weakened. However, local acidification at anodic sites can regenerate HClO from ClO-, explaining why high NaClO still causes deep pitting in SCPS. Low temperature (10 ℃) suppresses NaClO decomposition, slows electrode kinetics, and reduces ion diffusion, leading to lower corrosion rates and shallower pits. Overall, this study provides a clearer understanding of the corrosion behavior of 45# steel under the combined effects of NaClO concentration and temperature. The pH-dependent oxidizing ability of chlorine species, the transition from uniform to pitting corrosion depending on the medium, and the inhibitory effect of low temperature are the main factors governing the corrosion response. These results provide a theoretical and experimental basis for anti-corrosion design and maintenance of steel and reinforced concrete structures in LNG terminals and marine engineering.
Key words
corrosion /
carbon steel /
seawater /
temperature difference /
sodium hypochlorite
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Funding
The National Natural Science Foundation of China (52250710159, 51731008, 51671163); The China Postdoctoral Science Foundation (2024M751292)
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