Salinity-driven shifts in calcareous deposits formed under cathodic protection and influenced by marine biocalcifying...

Calcareous deposits formed under cathodic protection (CP) may be influenced by microbial activity and salinity, particularly in harbours subjected to tidal mixing and freshwater inputs. This study investigates the impact of the biocalcifying marine bacterium Pseudoalteromonas sp. CD10 on calcareous deposit formation on carbon steel under CP, with a particular attention to salinity effects. Experiments were conducted in natural and artificial seawater at two NaCl concentrations (100 and 400 mM), representative of low-tide and seawater conditions. Bacterial growth, pH evolution, electrochemical behaviour, and mineral deposits were characterised using culture-based methods, chronoamperometry, Raman spectroscopy and SEM-EDX. In natural seawater, bacterial activity did not induce significant bacterial growth, whereas CP promoted typical calcareous deposits composed of aragonite, calcite, monohydrocalcite and brucite. Calcite and monohydrocalcite, stabilised in the presence of bacterial biofilms, suggest a microbial influence on deposit mineralogy. Under free corrosion, iron oxyhydroxides dominated, suggesting no protective effect of bacterial activity. Although in solid media, the reduction of salinity enhanced bacterial CaCO₃ production, it is not the case in liquid media under CP. Overall, salinity was the main factor controlling electrochemical behaviour and carbonate polymorph selection, while bacterial activity mainly influenced deposit composition and morphology rather than total mineral deposition.