EXPERT TIP #34 – IMPRESS CURRENT VS. GALVANIC ANODE CP SYSTEMS

The following are considerations when selecting the type of cathodic protection (CP) system for a newly installed, existing buried or submerged structure.

Surface Area of Exposed Metal
The size and cost of most CP systems are controlled by how much CP current is required to effectively protect the exposed metal in contact with the electrolyte (soil or water). Therefore, one of the first considerations when designing or selecting a CP system is the surface area of metal exposed to the electrolyte.

A bare or uncoated structure will require an amount of CP current proportional to the surface area of metal exposed to the electrolyte. Most modern pipelines, as well as other structures, are provided with a coating system.  The coating system is intended to isolate the steel physically and electrically from the electrolyte.  Therefore, if the structure is completely and properly coated, it would be isolated from the soil and no CP would be required. Theoretically, this is correct. However, it is extremely rare that a structure could be buried or submerged without some minor damage or imperfections to the coating system.  Referred to as coating “holidays,” these imperfections result in small areas of the structure metal being in contact with the electrolyte, and where these coating holidays exist, the metal will be vulnerable to corrosion.

Determining the area exposed to the electrolyte can be challenging.  In many instances, tests can be performed on a newly installed structure to precisely determine the “current requirements.”  Otherwise, the surface area is often “calculated” based on many assumptions or, in the absence of these efforts, the CP current requirement is simply estimated.

Regardless, a good coating system can greatly reduce the size of the required CP system, which is detailed below.

Choosing between an impressed current cathodic protection (ICCP) system and a galvanic cathodic protection (GCP) system depends on many factors, including the specific requirements of the structure being protected, environmental conditions, cost considerations and maintenance considerations.

Structure Type and Size

• ICCP: Ideal for larger structures, such as cross-country pipelines, storage tanks and large ships where a continuous and adjustable cathodic protection current output is required.
• GCP: Suited for smaller structures or where obtaining an AC power source is difficult. It is commonly used for smaller underground and above-ground storage tanks, local distribution pipelines and boat hulls.

Current Output

• ICCP: Offers a high range of current output capabilities from a fraction of 1 amp to many hundreds of amperes. It also provides the ability to control the amount of current applied, making it suitable for structures that require specific protection levels.

• GCP: Typically used on structures that can be protected with a relatively low current. Current levels usually range from <0.010 Amperes on the low end to as much as 0.5 Amperes for an anode array in a soil application.  Referred to as a “Galvanic Cell,” the GCP system requires no external power source as the anodes are “self-energized” by the dissimilar voltage between the anode and the structure to be protected.  The total system current is primarily controlled by the size, type and quantity of anodes and the electrolyte (soil or water) resistivity or conductivity. 

To provide a better understanding of the current requirements of a bare vs. coated structure, the following are typical examples:

• Example #1: A bare structure with a surface area of 10,000 ft²
  • The current requirement is calculated at 0.001 Ampere per ft.² = 10.0 Amperes

• Example #2: The exact structure above, but coated, with 0.1% bare steel due to coating holidays. Note: This is a very conservative estimate for a newly coated structure. Now, the exposed surface is 1% of 10,000 ft² or 100 ft². 
  • The current requirement is calculated at 0.001 Ampere per ft.² = 0.10 Amperes

The bare structure would fall into the ICCP category, whereas the coated structure could likely be protected with a GCP system.

Cost Considerations

• ICCP: This type of system typically has higher initial costs due to the need for a power source, a CP rectifier and monitoring equipment.

• • The vast majority of ICCP systems will use available grid AC power, which will result in an open-ended operating cost. 
  • Some ICCP systems operate from solar arrays, but this will require storage batteries for night operation, and the batteries will need to be replaced.
  • Thermoelectric or Stirling generators are also used and also require regular maintenance. 
  • An ICCP system must be regularly monitored to ensure that it is operating properly.  This can be accomplished through on-site testing or “Remote Monitoring” equipment, but ultimately, a person must review the data and determine if the system is operating properly.

• GCP: Generally, it has lower initial costs since it does not require external power sources or rectifiers.

It is important to note that both ICCP & GCP systems require scheduled field testing to ensure that all components and operational levels are as they should be.

Environmental Factors

• ICCP: Requires equipment that must be protected from the elements. Whether protection is provided from weatherproof enclosures or installed inside a building, most of the equipment will be above ground and visible to all. This is sometimes considered as visual pollution with negative aesthetics. Special “Ornamental” enclosures are available to minimize this issue, as they can contain many system components in one enclosure.  Again, an ICCP system must have some external power source, so additional equipment must be used.

• GCP: A significant percentage of GCP systems are one hundred percent below grade; however, in many instances, a pole-mounted test station approximately 3” in diameter and 36” to 48” in height will be installed. Therefore, the “visual pollution” is reduced. Because the GCP is self-powered, no external power is required, so the carbon footprint is limited to whatever it took to produce the anode and test station.

Maintenance Requirements:

• ICCP: Requires regular monitoring and maintenance to ensure the proper functioning of the rectifiers, anodes, cabling, any monitoring equipment and electrical interconnects.

• GCP: Requires minimal maintenance, is typically reliable and has a long service life if designed properly.

The choice between ICCP and GCP depends on a thorough assessment of these factors and any specific regulatory requirements or industry standards that may apply to the protected structure. Consulting with a qualified cathodic protection professional can help make an informed decision based on the project's unique circumstances.