Copper and its alloys are widely used in marine and offshore structures, such as heat exchangers and tubing systems, due to their excellent durability and corrosion resistance. However, they remain susceptible to microbiologically influenced corrosion (MIC). Despite advances in understanding MIC mechanisms, a significant challenge originates from a simplified perspective of microbial interactions within biofilms, as many existing models continue to attribute MIC primarily to sulfate-reducing bacteria (SRB), overlooking the complex multispecies interactions that characterize biofilms under real-world conditions. In this regard, this study investigated the corrosion behaviour and temporal succession of total and metabolically active microbial communities driving MIC on copper-nickel (CuNi) 90/10 and titanium (Ti, control) metals exposed to natural seawater over a 16-week period. Metal coupons were collected at specified intervals for analysis. Electrochemical behaviour was assessed using open-circuit potential, linear polarization resistance, and electrochemical impedance spectroscopy. Biofilm morphology, corrosion products, and pitting morphology were characterized by scanning electron microscopy, Raman spectroscopy, and optical profilometry. Next-generation sequencing (NGS) of 16S/18S rRNA gene amplicons from genomic and complementary DNA was used to characterize total and active microbial communities, while quantitative PCR quantified functional genes associated with copper resistance and biofilm formation. CuNi initially exhibited increasing corrosion resistance for 12 weeks but showed signs of corrosion at 16 weeks, as indicated by a significant decrease in polarization resistance and charge transfer resistance. In contrast, Ti demonstrated enhanced corrosion resistance, with increasing polarization and charge transfer resistances over time. Raman spectroscopy confirmed the presence of unprotective cupric oxide (CuO) and hydroxide (Cu(OH)2) on CuNi, while Ti maintained a stable protective titanium oxide (TiO2). Pitting analysis revealed several pits on CuNi but absent on Ti. NGS identified Halomonas and Marinobacter, genera associated with biofilm formation and copper resistance, as early colonizers on CuNi, and showed an increasing species richness and diversity over time. Conversely, Ti showed a decline in species richness and diversity, with higher abundances of Aestuariibacter, Methylophaga, and predatory bacterial genera among early colonizers. SRBs and acid-producing bacteria (APB) showed a declining trend on CuNi initially, but increased from 12 weeks, indicating increased susceptibility to MIC, while these groups remained low or undetectable on Ti. Overall, this study demonstrates the significant role of early microbial colonisers and community succession in driving MIC and highlights the importance of moving beyond single-species SRB models to integrate multispecies community structures and their interactions in future frameworks for accurate identification of MIC mechanisms.
