A 17 pound magnesium anode can produce 1 amp of d.c. current for 1 year; therefore, if it produced 1/10 amp, it would last 10 years compared to 3-1/2 years for a 5 pound anode. A 17 pound anode placed in 1,000 ohm-cm clay would generate 170 M.A. of current and would last only 6 years.
If the clay's resistivity is 2,000 ohm-cm, it would last 11 years. In 10,000 ohm-cm dry sand, it would last 52 years. A normal current output is under 100 M.A. However, this engineer has seen the natural current output of a magnesium anode reach 200 milliamps (M.A.) in wet, alkali rich clay. The current output is dependent on the resistivity of the soil or backfill in which the tank is installed. Clay or highly mineralized earth is very corrosive and has a low soil resistivity value of 1,000 to 2,000 ohm-cm. Conversely, dry sand is not corrosive and would have a soil resistivity value over 10,000 ohm-cm. In order to calculate an anode life, one has to measure the soil resistivity, know the weight of the anode, know the alloy composition of the magnesium and factor in the loss of magnesium due to self corrosion. Finally, when an anode is 85% consumed, it becomes too small to be effective. Because soil conditions change with time, one can hope for the best but should plan for the worst.
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