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Several factors enter the determination as to how many sacrificial anodes may be required for a given structure and corrosion problem and the manner of distributing them with respect to the location where corrosion is occurring. The anode requirements for a small installation will normally involve the steps taken in the following examples. For cathodic protection of larger structures involving use of six or more anodes or an impressed current (rectifier) system, additional steps must be taken to assure proper functioning of the system, i.e., proper distribution of the anodes, prevention of damage to other buried metal work, design of an economic system, and proper operation and maintenance. (reference)
Determine the galvanic anode requirements for a cathodic protection system of 45.7 m of 0.1 m diameter (0.314 m circumference) coated pipe buried in the ground for a service life of ten years. Assume that the magnesium anodes considered for this application weight 3.6 kg and can provide 1000 Ah per 0.9 kg of metal, i.e. approximately 50% of the faradic capacity.
Required data
Knowledge of the condition of pipe protective coating
Soil resistivity in ohm-centimeters (do not use sacrificial anodes in soil whose resistivity exceeds about 3,000 ohm-centimeters).
Assume a current demand.
Protective current required is equal to area of bare metal to be protected times the required current.
Number of anodes required must be computed.
Data and assumptions for the problem
Pipe surface 5% bare.
Soil resistivity determined as 1,000 ohm-centimeters.
Assume 11 mA/m2 current density demand for bare steel.
For the same soil conditions, determine the sacrificial anode requirement to protect four bare steel transmission tower footings in 3000-ohm-centimeter soil with magnesium anodes now weighting 7.7 kg. Given exposed area of each footing is 9.3 m2 and footing to soil (P/S) reading is 0.85 volt.
