The co-occurrence of solid deposits and sulfate-reducing bacteria (SRB) poses significant risks of sudden localized failure for copper–nickel alloy infrastructure in complex marine settings. This study systematically investigates the synergistic corrosion mechanisms of B10 alloy beneath four representative deposits with distinct physicochemical properties: insulating silica sand (SiO₂), electroconductive magnetite (Fe₃O₄), semiconductive cuprous sulfide (Cu₂S), and adsorptive kaolin clay. Corrosion kinetics and complex interfacial electrochemical processes were monitored over long-term exposure using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and Tafel polarization. Furthermore, the severity of localized attack and denickelification was quantified via 3D laser confocal microscopy, SEM, and EDS analysis. The results demonstrate that the presence of deposits acts as a critical diffusion barrier, entrapping aggressive biogenic sulfides and triggering severe localized acidification and occluded cell effects at the metal/deposit interface. Crucially, conductive deposits such as Fe₃O₄ and Cu₂S were found to significantly accelerate cathodic depolarization by increasing the effective cathodic surface area and promoting robust galvanic coupling between the covered and bare metal regions. Meanwhile, kaolin clay exhibited a pronounced "synergistic protection" for SRB colonies by effectively adsorbing released Cu²⁺ ions, thereby mitigating their inherent biocidal effect and fostering robust biofilm development beneath the deposit layer. This research delineates how different deposit types modulate interfacial charge transfer, metabolic byproduct accumulation, and ion migration to accelerate the degradation of Cu-Ni alloys. These findings provide a comprehensive theoretical framework and practical data support for the durability assessment and corrosion mitigation of marine engineering materials under multispecies fouling conditions.