Pathirana, Medhani, Laleh, Majid, Somers, Anthony, Hinton, Bruce, Deacon, Glen B., Junk, Peter C., and Tan, Mike Yongjun (2025) Enhancing the inhibition of localised corrosion on a pre-rusted steel surface by the synergistic effect of rare-earth and inorganic inhibitors. Electrochimica Acta, 541. 147346.

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Abstract

Waterline corrosion of steel, particularly on pre-rusted surfaces, presents a significant challenge in marine and industrial environments due to its localised and aggressive nature. This study investigates the effectiveness of rare-earth metal (REM) carboxylate inhibitors, i.e., lanthanum 4-hydroxycinnamate (La(4OHcin)3·52O), yttrium 3-(4′-methylbenzoyl)propanoate ([Y(mbp)3(H>2O)]), and yttrium 4-hydroxybenzoate ([Y2(4hob)6(H2O)4].2H2O), in mitigating localised corrosion on pre-rusted steel partially immersed in simulated seawater. In-situ galvanic current mapping coupled with local electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation measurements were facilitated using a wire beam electrode (WBE). While these REM inhibitors demonstrated high inhibition efficiency (>90 %) on fresh steel, their efficiency on pre-rusted steel was significantly lower (∼40–49 %), indicating a critical influence of rust layers on inhibitor performance. Efforts to improve inhibitor penetration through the rust layers using smaller-molecule inhibitors showed minimal benefit. However, when REM inhibitors were combined with sodium molybdate, an inorganic anodic inhibitor with buffering capacity, the inhibition efficiency increased substantially, reaching up to 69 %. Local EIS measurements confirmed enhanced inhibitor film resistance and more uniform inhibitor distribution with the mixed inhibitors. These results suggest a synergistic mechanism where REM carboxylates provide cathodic and anodic protection through adsorption and hydroxide precipitation, while sodium molybdate stabilises the environment and reinforces the passive film. This study highlights the limitations of single-component inhibitors for pre-rusted steel surfaces and demonstrates the potential of hybrid organic–inorganic inhibitor systems for effective corrosion mitigation at the waterline.

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