Protective current generated by galvanic anodes depends upon the inherent potential difference between the anodes and the structure to be protected. Thus, if the structure is made of iron or steel, any metal that is more active in the electromotive force series can theoretically be used as anode material. In practice, the materials generally used for galvanic anodes are zinc and magnesium.
Although aluminum is also a material which is more active than iron, it has not yet proved to be an effective galvanic anode material for underground use because of the polarization films which build up on the aluminum surface as it corrodes, thereby ceasing the generation of protective current. In recent years, some alloys of aluminum have been used successfully in seawater applications and work is progressing on alloys that may prove to be effective in other applications.
It should be noted that galvanic anodes consume themselves in the process of generating protective currents. The rate of consumption is dependent upon the magnitude of current generated as well as the material from which the anode is made. For example, the theoretical consumption rate of zinc is 23.5 lbs. per ampere year and that of magnesium is 8.7 lbs. per ampere year. In actual practice, not all of the metal is consumed in generating current that is useful for cathodic protection. Some of the metal is consumed in self-corrosion. Zinc is approximately 90% efficient and magnesium is approximately 50% efficient. Therefore, the actual pounds consumed per ampere year of protective current are 26 and 17 lbs. for zinc and magnesium respectively.
In underground applications, these anodes are normally surrounded with a special backfill. The backfill is usually a mixture of gypsum, Bentonite and sodium sulfate. This special backfill serves a number of purposes. First, it provides a uniform environment for the anode, thereby making the corrosion of the anode uniform; second, the backfill decreases the anode-to earth resistance; third, it retains moisture and thereby maintains a lower resistance; and fourth, it acts as a depolarizing agent.
Although aluminum is also a material which is more active than iron, it has not yet proved to be an effective galvanic anode material for underground use because of the polarization films which build up on the aluminum surface as it corrodes, thereby ceasing the generation of protective current. In recent years, some alloys of aluminum have been used successfully in seawater applications and work is progressing on alloys that may prove to be effective in other applications.
It should be noted that galvanic anodes consume themselves in the process of generating protective currents. The rate of consumption is dependent upon the magnitude of current generated as well as the material from which the anode is made. For example, the theoretical consumption rate of zinc is 23.5 lbs. per ampere year and that of magnesium is 8.7 lbs. per ampere year. In actual practice, not all of the metal is consumed in generating current that is useful for cathodic protection. Some of the metal is consumed in self-corrosion. Zinc is approximately 90% efficient and magnesium is approximately 50% efficient. Therefore, the actual pounds consumed per ampere year of protective current are 26 and 17 lbs. for zinc and magnesium respectively.
In underground applications, these anodes are normally surrounded with a special backfill. The backfill is usually a mixture of gypsum, Bentonite and sodium sulfate. This special backfill serves a number of purposes. First, it provides a uniform environment for the anode, thereby making the corrosion of the anode uniform; second, the backfill decreases the anode-to earth resistance; third, it retains moisture and thereby maintains a lower resistance; and fourth, it acts as a depolarizing agent.
Source:www.bushman.cc/pdf/corrosion_theory.pdf
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