Cathodic Protection Knowledge Area  

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 and Oversight (DASD) [Materiel Readiness]

Updated: 07-29-2019

 

INTRODUCTION

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

Figure 1 Schematic Illustration of a GCP System
Photo credit: Thomas Tehada, P.E., NAVFAC EXWC. Adapted from UFC 3-570-01 Cathodic Protection, with Change 1, January 2019

Figure 1 Schematic Illustration of a GCP System

Figure 2 Schematic Illustration of an ICCP System
Photo credit: Thomas Tehada, P.E., NAVFAC EXWC. Adapted from UFC 3-570-01 Cathodic Protection, with Change 1, January 2019

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:

  1.  –850mVCSE potential criteria with current applied;
  2.  –850mVCSE polarized potential; and
  3.  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:

  1.  Providing sufficient, continuous current density to all parts of structure to acceptable criteria;
  2.  Minimizing interference effects to other structures;
  3.  Providing operational flexibility for expected changes in environmental, protective coating, and system service life;
  4.  Adhering to applicable codes and standards to ensure public and operational personne's' safety;
  5.  Providing CP system design life that coincides with the protected system's service life; and
  6.  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.

Discrete zinc anode (wrapped in green mesh) in concrete deck repair. The device with the yellow lead wire is a permanent reference electrode.

Discrete zinc anode (wrapped in green mesh) in concrete deck repair. The device with the yellow lead wire is a permanent reference electrode.
Photo credit: 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; and

Other systems that may employ CP:

  • Systems with hazardous products;
  • Potable water distribution systems;
  • Compressed air distribution systems;
  • Sewage lift stations; and
  • Concrete reinforcing steel.
Periodic Performance Testing - Rectifier Output Current of ICCP System

Periodic Performance Testing - Rectifier Output Current of ICCP System.
Photo credit: 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.

Consulting A Cathodic Protection Subject Matter Expert (SME)

The CP SME provides invaluable consultation skills developed from years of experience assessing corrosion prevention requirements and many environments. The SME can translate local conditions into CP solutions that provide immediate and long–term benefits to the installation and its SRM bottom line costs. Typical consulting services provided by the CP SME include:

  • Problem assessment
  • Failure analysis
  • Environmental Severity impacts
  • System and material selection
  • Review and assessment of contractor deliverables
  • Quality assurance
  • System design
  • SRM analysis and recommendations (system inspection and testing assistance)
  • Hazards analysis
  • Assessment of new technologies
  • Process analysis

Relevant Codes and Standards

Department of Defense

Unified Facilities Criteria (UFC)

Unified Facilities Guide Specifications (UFGS)

U.S. Army Corps of Engineers - Public Works Technical Bulletin (PWTB)

National Association of Corrosion Engineers (NACE) and Society for Protective Coatings (SSPC)

ADDITIONAL RESOURCES

Organizations

Assistance

Obtaining SME Assistance for DoD Installations

Federal Facility Criteria: 
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