Thermally sprayed anode
Keywords: Corrosion protection, Thermal spray, Concrete
A new modified form of cathodic protection system for reinforced concrete has been developed by J.E. Bennett Consultants, Inc (Chardon, Ohio). The method is based on the use of a conductive metal, thermally applied to the exposed concrete surface, to form a porous anode. A lithium salt solution at the interface of anode and concrete is reported to enhance current flow, partially by functioning as a humectant, absorbing moisture from the atmosphere to provide an electrolyte at the anode-concrete interface.
Reviewing the current state of corrosion protection for concrete, Bennett comments that exposure to seawater, set accelerators, and de-icing salt, common agents in the environment of reinforced concrete structures, introduces chloride ions that are absorbed by the concrete. The chloride interferes with the passivating effect of the concrete that otherwise protects steel rebar from corrosion.
The products of corrosion of the steel occupy 2.5 to 4 times the volume of the original steel and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. A recent US Federal Highway Administration report states that of the nation's 577,000 bridges, 226,000, or 39 per cent, were classified as deficient, and 134,000, or 23 per cent, as structurally deficient. Structurally deficient bridges are closed, or restricted to fight vehicles only, unless they are immediately rehabilitated.
The new invention is intended to overcome what Bennett considers to be previous weaknesses in the technology of thermally sprayed zinc used as a sacrificial or impressed current CP anode. In explanation Bennett states that when used as a sacrificial anode system, a direct electrical connection is made between the anode and the reinforcing steel, and current flows spontaneously. The amount of current is uncontrolled, however, and is sometimes insufficient to meet CP needs of the system. When current is supplied by a power supply in an impressed current system, resistance gradually increases over time. After several years of service, the voltage requirement may exceed the capacity of the power supply.
The central innovation of the invention is a technique to establish what Bennett terms controlled resistivity by means of a system to increase current delivery from the anode. After thermally applying a sufficiently generous layer of zinc or aluminium alloy to the exposed concrete surface in a manner that will form an effective planar anode that also is permeable, a lithium salt solution is applied to the concrete. The solution migrates through the pores of the permeable anode to the interface between anode and concrete. The lithium salt functions as an enhancer for the current and also functions as a humectant,absorbing moisture from the atmosphere to provide an electrolyte at the interface. Together, the two effects substantially increase current delivery from the anode. Preferably enough lithium salt solution is applied to the external surface to position at the interface at least 10g of lithium salt dry basis, per m2 of anode.
