Energy for the sacrificial anode system is provided by the difference in energy level between the anode and the structure being protected.  Typically, magnesium anodes are utilized to protect steel structures.   Referring back to the galvanic series chart under the section on galvanic corrosion, it is indicated that the magnesium anode would corrode preferentially when connected to steel.  There is approximately a 1.0 volt difference between these two materials.

A sacrificial anode installation is also a DC circuit with positive current supplied from the anodes and a return negative current supplied from the structure.  Again, Ohms law applies: E=IR, where E is the driving voltage of the circuit or approximately 1.0 volts, difference between magnesium and steel, I is the current magnitude that results from the resistance of the circuit R.  Proper system design seeks to minimize the resistance of the circuit through anode groundbed design.  

Sacrificial anode systems do not provide as much energy output as the impressed current design.  Therefore, they would not function properly in many applications where cathodic protection would be required.  Sacrificial systems require that the structure is coated with a tightly adhered coating system, and is electrically isolated from all other metallic structures and system components.

Electrical Continuity

In order to get effective protective current distribution, the pipe must be electrically continuous and its internal resistance must be very low.  This data is obtained in the field investigation phase of the study. 

For the majority of riveted steel, lock-bar steel and welded steel piping systems, electrical continuity is established by mechanical metal to metal connections.  Cast iron lead joints and ductile iron joints are often not electrically continuous.  Should it be determined that adequate electrical continuity does not exist, electrical bonding would be required to provide adequate pipe resistance for application of cathodic protection.

Electrically continuous pipe can be subject to long line galvanic cells.  Electrically continuous pipe can also gather current along a long length, from a foreign pipeline.  These factors would indicate that discontinuous pipe would be desirable.  However, in order to mitigate the effects of long line galvanic cell, and current flow due to foreign pipelines, cathodic protection would be used.  Cathodic protection relies on the piping being electrically continuous over its length, so that anodes in one location can be utilized to protect the entire length.

In areas affected by stray current activity, electrical continuity may be desirable to enable control and mitigation of any corrosive effects.  While it may seem that stray current operations are not near the project area, it is not uncommon for stray currents to travel along piping and other grounding systems for miles between DC traction substations.

It may be desirable to maintain electrical continuity during pipe upgrade operations to ensure that the stray current activities are not interrupted and then cause corrosion problems.  Should the electrical properties of a piping network change i.e. by the installation or removal of isolation joints, valves or other appurtenances, severe stray current corrosion could occur.

Electrical Isolation

Electrical isolation of the pipeline or structure from other nearby structures is important to contain the spread of the impressed current effects, and provide a distinct structure for the cathodic protection system. Electrical isolation may also be required to control stray current from outside sources and to control galvanic corrosion exposure

Electrical isolation is provided between adjacent structures, materials of construction and AC electrical ground.  Establishing isolation requires the installation of insulating flange components or electrically insulating mechanical couplings.

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