The electrical current which an anode discharges is controlled by Ohm's law; that is:
I=E/R
I= Current flow in amps
E= Difference in potential between the anode and cathode in volts
R= Total circuit resistance in ohms
Initially current will be high because the difference in potential between the anode and cathode are high, but as the potential difference decreases due to the effect of the current flow onto the cathode, current gradually decreases due to the polarization of the cathode. The circuit resistance includes both the water path and the metal path, including any cable in the circuit. The dominant value here is the resistance of the anode to the seawater.
For most applications the metal resistance is so small compared to the water resistance that it can be ignored. (Not true for sleds, or long pipelines protected from both ends). In general, long thin anodes have lower resistance than short fat anodes. They will discharge more current, but will not last as long.
Therefore a cathodic protection designer must size the anodes so that they have the right shape and surface area to discharge enough current to protect the structure and enough weight to last the desired lifetime when discharging this current. As a general rule of thumb:
Length of the anode determines how much current the anode can produce, and consequently how many square feet of steel can be protected.
Cross Section (Weight) determines how long the anode can sustain this level of protection.
by: Richard Baxter, Jim Britton
Source: http://www.cathodicprotection101.com/
E= Difference in potential between the anode and cathode in volts
R= Total circuit resistance in ohms
Initially current will be high because the difference in potential between the anode and cathode are high, but as the potential difference decreases due to the effect of the current flow onto the cathode, current gradually decreases due to the polarization of the cathode. The circuit resistance includes both the water path and the metal path, including any cable in the circuit. The dominant value here is the resistance of the anode to the seawater.
For most applications the metal resistance is so small compared to the water resistance that it can be ignored. (Not true for sleds, or long pipelines protected from both ends). In general, long thin anodes have lower resistance than short fat anodes. They will discharge more current, but will not last as long.
Therefore a cathodic protection designer must size the anodes so that they have the right shape and surface area to discharge enough current to protect the structure and enough weight to last the desired lifetime when discharging this current. As a general rule of thumb:
Length of the anode determines how much current the anode can produce, and consequently how many square feet of steel can be protected.
Cross Section (Weight) determines how long the anode can sustain this level of protection.
by: Richard Baxter, Jim Britton
Source: http://www.cathodicprotection101.com/
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