The basic principle of cathodic protection is that a metal dissolution is reduced through the application of a cathodic current. Cathodic protection basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential, bringing the metal closer to an immune state. When dissimilar metals are placed in an electrolyte (and are connected by a metallic return path) a current will flow from the metal with the higher potential, through the electrolyte, to the metal with the lower potential. The cathodic protection reaction will not work in air, because air is not an effective electrolyte.
The introduction of the sacrificial magnesium anode produces a galvanic cell in which the magnesium works as an anode and provides a flux of electrons to the structure, which imparts a cathodic (negative) charge to the structure to suppress corrosion. The cathode is protected because the anode will be corroded more quickly than the relatively cathodic metal structure.
The least active metal is called the cathode, and the most active, the anode. When the anode supplies current, it will gradually dissolve into ions in the electrolyte, and at the same time produce electrons, which the cathode will receive through the metallic connection with the anode. The result is that the cathode will be negatively polarized and hence be protected against corrosion.
The purpose of the sacrificial anode is not to solve electrolysis, rather it is to buy us time and be able to monitor and measure it to see if we have electrolysis present.
Typically, when it is present, there is an imbalance in the supply of electricity to the building, or usually within a mile of the building. Often it is at a substation where the polarity coming in is out of balance, so it is running to ground. Since the pool is a great battery, often the electricity will come to the pool. The pool essentially acts as a grounding instrument.
The life expectancy of the anodes varies according to resistivity of water, quality of the applied coating and other factors. In most instances, it appears that one anode will last at least three to five seasons.
Galvanic sacrificial anode protection is remaining out of the Master Coordination Documents: Consultant Coordination Document & Design Considerations Document (October 2010).
After consulting with a corrosion engineer and an electrical engineer, no clear direction could be determined for whether or not Counsilman Hunsaker should be specifying or providing information on sacrificial anodes to the project design team. It is agreed, however, that sacrificial anodes have value when a project is in close proximity to a potential stray current source (i.e. Power Station, Building Transformer, etc.) or if the swimming pool is utilizing a saline chlorine generation sanitization system. The anode must be properly bonded to the pool system and have access for maintenance to be effective. Specifying of the sacrificial anode should be the responsibility of the Architect or the Building Electrical Engineer. The anode should only be used when absolutely necessary and should be evaluated on a case by case basis.
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