Galvanic, or sacrificial anode, systems are attractive in that there is no external control, so they are simpler than impressed current systems. They usually consist of zinc applied to the concrete surface or embedded within it and directly connected to the reinforcing steel. Zinc based alloys are also used and other metals such as aluminium have been trialed too. The principles of galvanic cathodic protection are the same as for impressed current CP, except that the anode is a less noble metal than the steel to be protected and is consumed preferentially, generating the cathodic protection current. The potential difference between anode and cathode is a function of the environment and the relative electrode potentials of the anode and cathode materials. The current is a function of the potential difference and the electrical resistance. As the voltage and current cannot be controlled, the level of protection cannot be guaranteed and a low resistance environment is required.
Initially, most successful and extensive application of galvanic CP was to the splash and tidal zones of prestressed concrete piles and epoxy coated reinforcing steel in substructures supporting bridges in the Florida Keys. These were either thermal sprayed zinc or zinc sheets clamped or grouted onto the surface of the concrete. This system is now widely used internationally. In the last decade a commercial version of a sacrificial anode for installation in patch repairs has been developed to suppress the incipient anode effect as discussed under patch repairs above. This applied local cathodic protection or incipient anode suppression and ins internationally successful along side a similar product that is inserted into cored holes in unrepaired concrete. Another development is the application of a zinc sheet to the concrete surface with an adhesive gel on it.
The simplicity of galvanic CP is very attractive. It requires only the installation of the anode and a direct connection to the steel. In Florida, one method was to clean up the steel exposed by corrosion damage and to arc spray zinc across the steel and the concrete, directly connecting them. This procedure provided an added advantage in the cases where the steel had been epoxy coated as it made direct electrical connections. The low current and voltage meant that the risk of accelerating corrosion in isolated bars was lower than for an impressed current system.
The current flows and voltages in galvanic CP systems are lower than in impressed current systems. Galvanic systems are therefore more readily useable with prestressing because the risk of hydrogen evolution and subsequent embrittlement is lower. The lack of control and the limited voltage and current generated in galvanic systems can be a major limitation, which is why most early galvanic systems were used in splash and tidal zone applications.
The anodes are consumed and therefore must be replaced regularly, about every 5 to 10 years for thermal sprayed zinc in a splash/tidal situation. The “jacket” and embedded anodes last far longer. Galvanic anodes have frequently been found to work well initially and then the current drops, probably due to rising internal resistance at the anode/concrete interface.
There are variations on the thermal sprayed zinc system with the use of a chemical agent to increase the moisture around the anode and hence the effectiveness of the galvanic CP system. There is also a proprietary Aluminium/Zinc/Indium system that claims to deliver higher current densities.
More information can be found in the latest edition of my book. Details for ordering are on the home page of the website. There is also more information in Corrosion Prevention Association Technical Note No 6 “The principles and practice of galvanic cathodic protection to reinforced concrete structures” which can be downloaded here.
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