Environmental fate of sacrificial zinc anodes and influence of a biofilm

Laboratory experiments were performed to determine the environmental fate of zinc compounds formed during sacrificial dissolution of zinc anodes in seawater and to further determine the role of biofilms in mobilizing/immobilizing zinc corrosion products. Experimental conditions simulated cathodic protection and overprotection (40x normal) of ship ballast tanks with an anode to cathode ratio of 0.091. Anodic dissolution and percentage of soluble zinc increased with the level of protection. For normal cathodic protection in abiotic seawater, approximately 3% of the sacrificed zinc was dissolved in solution. Additionally, weight loss and percent soluble products increased dramatically in the presence of an agar film, but were unchanged due to the presence of natural marine biofilms. Under overprotection conditions, weight loss did not vary in the presence of surface films. Solid zinc corrosion products were precipitated, suspended in solution, and attached to electrode surfaces. Experimental data were compared with zinc concentrations measured in ballast tanks for two United States Navy surface ships.