A decrease in the anode-to-earth resistance.
Retains moisture thereby maintains a lower resistance.
b. Impressed Current Cathodic Protection.
(1) Whereas galvanic mode installations are self-powered, impressed current installations use a
separate source of direct (D-C) current. The current from this source is impressed on the circuit between the
structure to be protected and the ground bed.
(2) An A-C (alternating current (120, 240, or 440 volts)), is connected to a "rectifier" that changes
the power to D-C (direct current). An insulated cable connects the rectifier to the ground bed (anode). Current
flows from the ground bed (anode through the earth (electrolyte) to the metal structure, then along it to the cable
connection on the metal structure. Finally, the current moves along the cable connection between the metal
structure and the rectifier to complete the circuit.
(3) Rectifiers commonly used have a fixed output, that are adjusted to suit a specific requirement.
For cases where current output changes are required, there are automatically controlled rectifiers.
(4) Advantages of impressed current installations.
(a) Wide range of D-C voltage and current output capacities. (This provides great flexibility in
system design.)
(b) Single installations which will protect much larger structures. (This provides protection to
structures that a galvanic anode installation cannot)
(5) Disadvantages of Impressed Current Installations.
(a) Greater maintenance required.
(b) Dependence on availability of a dependable power supply.
(c) Continuing energy cost for A-C current
12.
Drainage (Mitigation) Bonds.
This type of corrosion control method is particularly applicable to stray current corrosion problems.
a. Stray current travels along the metal structure until a point of exit is reached; at this point corrosion
accrues.
b. The drainage/mitigation bonds are insulated cables attached to the metal structure and provide a path
for the stray current to return to its source without damage to the structure.
16-9
QM5200
Previous Page
