by Robert J. Evans Jr., P.E. (AFCEC) and Lean–Miguel San Pedro (NAVFAC) and Thomas Tehada, P.E. (NAVFAC) for the Director, Corrosion Policy & Oversight (DCPO), (DASD) [Materiel Readiness]
Updated: 10-07-2021
INTRODUCTION
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Cathodic Protection (CP) is a technique used for prevention of corrosion by making a metal, which would ordinarily behave like an anode and corrode, instead behave like a cathode and reduce or eliminate corrosion attack. It is the application of the electrochemical corrosion cell with beneficial results. CP is generally accomplished using two methods: sacrificial anodes (providing galvanic CP (GCP)) (see Figure 1) and direct current (providing impressed–current CP (ICCP) (see Figure 2). CP System anodes transfer the location of the corrosion attack from the structure being protected to the anodes itself (another material specifically selected to make the CP circuit function).
For GCP system anodes to supply current, its electrochemical potential difference must be more electronegative than the structure protected. Sufficient potential difference must exist between them to overcome the circuit resistance to provide adequate current to achieve structure protection. Since GCP anodes provide protective current through the electrochemical corrosion process, they must also be low cost and have sufficient current capacity to be cost effective.
Figure 1: Schematic Illustration of a GCP System
Source: Thomas Tehada, P.E., NAVFAC EXWC. Adapted from UFC 3-570-01 Cathodic Protection
Figure 2: Schematic Illustration of an ICCP System
Source: Thomas Tehada, P.E., NAVFAC EXWC. Adapted from UFC 3-570-01 Cathodic Protection
The most common GCP anodes are aluminum, magnesium, and zinc. In ICCP, a direct current from a power source through ICCP anodes is applied to the structure being protected to prevent the electrochemical mechanism of corrosion prior to its attack. ICCP system uses semi–inert anodes to supply protective current. These anodes exhibit relatively noble electrochemical potentials; therefore, to produce charged flow in the direction to cathodically protect a steel structure, it is necessary to connect an external power source. The power source must overcome galvanic potential difference between ICCP anodes and protected structure before it can even supply the first increments of protection to the structure. We often call that potential the power source must first overcome, back voltage. The most common ICCP anodes are graphite, high silicon cast iron, platinum, mixed metal oxide and polymer. Electrical energy used up in the transfer of charge across the respective cathode and anode/electrolyte interfaces causes this change in potential and we call it, polarization, which is critical in determination of meeting adequate protection criteria.
DESCRIPTION
CP systems must be carefully designed, correctly installed, inspected for proper function, certified to deliver adequate protection and maintained. Current NACE standards recognize adequate protection through three primary criteria for steel exposed to soil environments:
- –850mVCSE potential criteria with current applied
- –850mVCSE polarized potential
- 100mV polarization shift criteria
Aeration (oxygen), agitation (velocity), temperature, pH, surface area, and time affect polarization, potential measurements, and criteria validity. When designing or evaluating a CP system, the fundamental CP design objectives include:
- Providing sufficient, continuous current density to all parts of structure to acceptable criteria
- Minimizing interference effects to other structures
- Providing operational flexibility for expected changes in environmental, protective coating, and system service life
- Adhering to applicable codes and standards to ensure public and operational personnel's safety
- Providing CP system design life that coincides with the protected system's service life
- Providing testing and monitoring facilities to ensure CP system performance meet industry criteria, standards, and regulations
We ultimately confirm the effectiveness of a CP system by whether it controls corrosion or not. Determining corrosion rate directly is generally not a simple process. In most instances, we rely upon indirect methods of assessing CP system. The main method is the measurement of the structure to electrolyte potential as compared to the selected criteria. We also measure system currents as an additional performance parameter. We routinely measure structure to electrolyte potentials for compliance with these criteria and ensure meeting any regulatory standards. We must also take into account any of those factors that affect polarization, potential measurements, and criteria validity.
Photo 1: Discrete zinc anode (wrapped in green mesh) in concrete deck repair. The device with the yellow lead wire is a permanent reference electrode.
Source: UFC 3-570-01 Cathodic Protection
Cathodic protection is utilized to protect critical infrastructure, much of which would have calamitous results due to failure:
- Natural gas piping and distribution systems
- Liquid fuel piping
- Oxygen piping
- Fire mains and underground fire protection piping
- Ductile iron pressurized piping under floor slab (slab on grade)
- Underground heat distribution and chill water piping in metallic conduit
- Steel sheet pile seawalls, pier support, fender piles, and other submerged steel structures
- Underground, ground level and elevated storage tank systems
Other systems that may employ CP:
- Systems with hazardous products
- Potable water distribution systems
- Compressed air distribution systems
- Sewage lift stations
- Concrete reinforcing steel
Photo 2: Periodic Performance Testing—Rectifier Output Current of ICCP System.
Source: Steve Geusic, P.E, LEIDOS
A properly designed, operated, and maintained CP system can extend the life of a structure indefinitely with the replacement of CP components. The annual maintenance and periodic repair costs are far less than major repairs to or replacement of the structures themselves. The reduction of potential liability from premature failure of utilities such as gas line explosions and jet fuel leaks is enormous. Cost avoidance caused by the leakage that results in fines, environmental cleanup, remediation and disposal of contaminated soil along with the associated negative public image is an overarching consideration in selecting and sustaining a functioning and effective CP solution. For more information, review the Cathodic Protection Basics training module. The CPC Checklists Tool provides several design and field inspection reminders to ensure that CP meets requirements over the life cycle.
Relevant Codes and Standards
Department of Defense
Unified Facilities Criteria (UFC)
- UFC 1-200-01 DoD Building Code
- UFC 3-570-01 Cathodic Protection
- UFC 3-570-06 Operation and Maintenance: Cathodic Protection Systems
Unified Facilities Guide Specifications (UFGS)
- UFGS 26 42 13 Galvanic (Sacrificial) Anode Cathodic Protection (GACP) System
- UFGS 26 42 15 Cathodic Protection System for the Interior of Steel Water Tanks
- UFGS 26 42 17 Impressed Current Cathodic Protection (ICCP) System
- UFGS 26 42 19.10 Cathodic Protection Systems (Impressed Current) For Lock Miter Gates
U.S. Army Corps of Engineers - Public Works Technical Bulletin (PWTB)
- PWTB 420-49-29 Operation And Maintenance Of Cathodic Protection Systems
- PWTB 420-49-37 Cathodic Protection Anode Selection
Association for Materials Protection and Performance (AMPP) [National Association of Corrosion Engineers (NACE) and Society for Protective Coatings (SSPC)]
- SP0169 Control of External Corrosion on Underground or Submerged Metallic Piping Systems
- SP0285 Corrosion Control of Underground Storage Tank Systems by Cathodic Protection
- SP0388 Impressed Current Cathodic Protection of Internal Submerged Surfaces of Carbon Steel Water Storage Tanks
- SP0193 External Cathodic Protection of On-Grade Carbon Steel Storage Tank Bottoms
- SP0196 Galvanic Anode Cathodic Protection of Internal Submerged Surfaces of Steel Water Storage Tanks
ADDITIONAL RESOURCES
Organizations
- (AMPP) NACE Cathodic Protection Program provides multiple courses and certifications on cathodic protection and its technology, including, but not limited to:
- Cathodic Protection Tester (CP1), covering basic corrosion science and taking measurements from rectifiers and portable reference electrodes
- Cathodic Protection Technician (CP2), covering in–depth corrosion science and how to interpret CP–test readings for adequate CP and for determining problems with CP systems
- Cathodic Protection Technologist (CP3), covering troubleshooting and design calculations for CP systems
- Cathodic Protection Specialist (CP4), covering CP–system design
- (AMPP) SSPC (The Society for Protective Coatings) Training and Certification
- STI (Steel Tank Institute) Training and Certification
Assistance
DoD Installations Organizations
- Office of the Deputy Assistant Secretary of Defense (Construction)
- Engineer Research and Development Center, Construction Engineering Research Laboratory (ERDC-CERL)
- Air Force Civil Engineer Center (AFCEC)
- Naval Facilities Engineering and Expeditionary Warfare Center (NAVFAC EXWC)
