Sprayed-Zinc Sacrificial Anodes for Reinforced Concrete in Marine Service

Abstract

The use of sacrificial anodes consisting of sprayed zinc on the surface of concrete containing corroding reinforcing steel bars was examined in field tests and laboratory experiments. The anodes were sprayed directly onto the external surface of spalled regions of marine substructure elements. Electrical contact between the steel and zinc was achieved efficiently by spraying directly over exposed steel. Field tests were conducted on the substructure of bridges in the Florida Keys and in Tampa Bay. The installations were performed economically and were functional as much as 5 y following placement. Protective current densities reaching 1 mA/ft2 (1 μA/cm2) of the reinforcing steel were achieved. Tests with reinforcing steel probes and with entire reinforcement assemblies showed steel polarization decays (upon temporary disconnection) typically exceeding 100 mV even after several years of service. Laboratory tests revealed concrete resistivity in the marine substructure conditions of interest did not represent a main limiting factor in performance of the galvanic anodes. However, absence of direct wetting of the anode surface could result in long-term loss of adequate current delivery, even when the concrete was in contact with air of 85% relative humidity (RH). Periodic water contact (by seawater mist, splash, or weather exposure) was considered a necessary factor for long-term anode performance. The method was considered a competitive alternative to impressed-current cathodic protection (CP) and a considerable improvement over simple gunite repair of corrosion-damaged substructure concrete.