ANODES GALVANIC

Magnesium . Magnesium is the most commonly used sacrificial anode material for the protection of buried structures . Magnesium anodes are also used for the protection of the interiors of water tanks and heaters , heat exchangers and condensers , and waterfront structures . Magnesium anodes are available as castings and extrusion weighing from 1 to 200 pounds , and in a wide variety of shapes ( refer to para . 7.8.1.4 ) . Two anode compositions are commonly used . They are standard alloy and a " high potential " alloy .

Composition . The composition of both the standard alloy and high potential magnesium alloy are given below :
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Element Standard High Potential

Aluminum 5.3 – 6.7 % 0.1 % max
Manganese 0.15 % min 0.5 – 1.3 %
Zinc 2.5 – 3.5 % -----
Copper 0.02 % max 0.02 % max
Silicon 0.1 % max ------
Iron 0.003 % max 0.03 % max
Nickel 0.002 % min 0.001 % max
Other metals 0.3 % max 0.3 % max total
0.05 % max each
Magnesium Remainder Remainder

Anode Efficiency . The theoretical efficiency of magnesium is 1,000 ampere hours per pound or 8.8 1bs / A yr . The efficiency of magnesium alloys used for cathodic protection seldom exceeds 65 percent of this theoretical value due to self consumption . The efficiency of both the standard alloy and high potential alloy magnesium alloys is dependent on the current densities on their surfaces. The efficiency of the standard alloy is higher than the efficiency of the high potential alloy . Thus , the high potential alloy should only be used when its higher driving potential is required ( usually in soil resistivities above 12,000 ohm – cm ) . For design purposes , 50 percent is used for the efficiency of both types .


-Aluminium Anode
Aluminum anodes are sometimes used for the protection of the interior of water storage tanks . They are consumed at a fairly high rate of approximately 9 1bs / A yr in most applications . The main advantages of using aluminum anodes in the protection of water storage tanks is their low cost , light weight , and lack of water contamination from the products of deterioration of the anodes . They are commonly used when seasonal icing of and are replaced each spring . HSCBCT and graphite anodes are more commonly used in water tanks and , when installed on a floating raft , can be made resistant to icing conditions

Zinc Anode

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