CATHODIC PROTECTION

Cathodic protection is a widely used and accepted form of corrosion prevention. The goal of cathodic protection is to reduce the deterioration of a metal exposed to an aqueous electrolyte by lessening the thermodynamic driving force for corrosion. A properly maintained cathodic protection system can effectively eliminate metal dissolution and provide a long-term solution to many corrosion problems. The two cathodic protection systems that are most important to the marine industry are (1) sacrificial anode and (2) Impressed Current Cathodic Protection (ICCP). Figure 1 is an excerpt from Naval Ships Technical Manual Chapter 633 and lists many of the advantages and disadvantages of these two methods for naval ships.



Figure 1: Adapted from Naval Ships Technical Manual: S9086-VF-STM-010/CH-633 p 13


It is important to note that sacrificial anode and ICCP systems present an effective means of corrosion protection only when they are submerged in a conductive fluid medium. Anodes do not work in air due to its limited charge-carrying ability.

Sacrificial Anodes

This type of cathodic protection involves the coupling of an active metal to a structure for which corrosion protection is desired. In this system, the active metal corrodes preferentially and provides protection for the structure. In other words, one metal is sacrificed to protect the other.

Overview
Two dissimilar metals or alloys joined together in an electrolyte form what is called a galvanic couple or galvanic cell. Any metal or alloy, when submerged in a conductive electrolyte, has its own unique corrosion potential (open circuit potential). Figure 2 provides a listing of various alloys and their potentials in seawater. This chart, known as a galvanic series, provides meaning to the terms active and noble in reference to metals. The further down and to the left a metal appears in the series, the more active or prone to corrosion it is. The most noble, or corrosion resistant, metals appear at the top of the series.


Figure 2: Galvanic series of metals and alloys in seawater. Potential values are shown as ranges. Adapted from Naval Ships Technical Manual: S9086-VF-STM-010/CH-633 p 6 and D. Jones Principles and Prevention of Corrosion 2nd Ed. page 170.

When an electrical connection is established between two dissimilar metals in a conductive environment, electrons will flow from the more negative (active) surface to the more positive (noble) surface. The electrons that flow to the noble metal drive it to more negative potentials (cathodic polarization). This current flow and polarization correspond to an electron surplus that reduces the rate at which the noble metal corrodes. As an example, a zinc block welded to a steel hull forms a galvanic couple in seawater. As shown in Figure 2, zinc is more active than steel. The anodic reactions for steel and zinc in seawater are:

Fe ® Fe2+ + 2e- (1)

Zn ® Zn2+ + 2e- (2)

If an excess of electrons is provided to the steel surface where reaction (1) is taking place, a driving force for the reverse reaction will be present. As a result, the oxidation of metal will be slowed. The source of these electrons, in this case, is the corrosion of the zinc block, reaction (2), attached to the steel surface.

In summary, when a galvanic couple is immersed in an electrolyte, corrosion will take place on the surface of the more active metal. The more noble metal in the couple, acting as a cathode, will be protected from corrosion. In the presented case of zinc coupled with steel in seawater, zinc will corrode preferentially and provide protection to the steel.

Equipment
Sacrificial anodes need be nothing more than a block of metal electrically connected to the surface to be protected. There are, however, a few electrochemical properties to consider when creating a successful sacrificial anode system. The corrosion potential of the anode must be active (negative) enough to drive current through the electrolyte. The resistance of the electrolyte and the separation between anode and protected structure play a role in the effectiveness of the system. The higher the resistance and separation between anode and structure, the more active the anode must be. Periodic replacement of sacrificial anodes is necessary due to continuous consumption by corrosion. The cost of replacing anodes, however, pales in comparison to the potential costs of corrosion damage.

Zinc anodes are often used in marine applications and are effective at reducing corrosion of steel structures in seawater. Zinc anodes, while effective, tend to be consumed rapidly. Aluminum anodes have been developed for longer service life than zinc in seawater. The corrosion rate of the aluminum-protected steel may be higher than the cases where zinc is used but for many applications is still acceptable.

Navy Applications
On the majority of U.S. Navy ships, the primary source of underwater hull corrosion protection is anti-corrosive coating systems. Impressed current cathodic protection systems serve as the primary back-up to the coating system. Sacrificial anodes are also installed on the underwater hull and in sea chests as a supplement to coating systems and ICCP systems.

The Navy also uses zinc sacrificial anodes or “zincs” in several other applications. Zincs are used inside ballast tanks, bilges, heat exchangers, collection, holding and transfer (CHT) tanks, and in various machinery.



Figure 3: Inside of ballast tank protected by zinc sacrificial anode.

Zinc anodes must conform to MIL-A-18001 and MIL-A-18001 Amendment 1. Aluminum anodes must conform to MIL-A-24779/QPL-24779.

Impressed Current Cathodic Protection Method

Another effective method of cathodic protection involves the direct application of current from an external power source to surfaces prone to corrosion rather than by using sacrificial anodes previously discussed.

Overview
The basic principle of the two methods is the same. In both cases, the vulnerable metal is supplied with a surplus of electrons. The excess electrons reduce the potential of the metal (cathodic polarization) and tend to drive the anodic corrosion reaction in reverse. This results in a reduced corrosion rate. The general form of the anodic reactions discussed above is:

M ® Mn+ + ne-

In the above equation, M represents the atomic symbol of some metal and n is the number of electrons involved in the reaction. Excess electrons supplied to the surface slow this reaction. Both sacrificial anode and impressed current cathodic protection techniques operate on the same basic principal; the source of current, however, differs. Impressed current systems use an external power supply (DC) such as a battery or rectifier to supply the current necessary to provide cathodic protection.

Equipment
Most ICCP systems are designed to control the potential of the metal at a fixed value where anodic dissolution of the metal effectively does not occur. This is known as controlled-potential cathodic protection and is the method used most extensively for ship hulls and seawater applications. A controlled-potential system consists of the following basic components: a power supply, a transformer-rectifier (to convert the AC signal to DC), anodes, reference electrodes, and a dielectric shield. The dielectric shield prevents shorting of the anode current to the hull adjacent to the anode, allowing wider current distribution.



Figure 4: Diagram of basic ICCP system. Taken from Naval Ships Technical Manual: S9086-VF-STM-010/CH-633 p 18.


Unlike sacrificial anodes, impressed current anodes are designed to be resistant to corrosion. Desirable properties include low resistance to current flow, physical toughness, low rate of consumption, and low cost of production. Platinum is an ideal candidate for impressed current anodes because consumption is almost non-existent; however, it is cost-prohibitive. Using platinum-coated titanium rather than solid platinum for the anodes can reduce cost. The Navy specifies exclusive use of platinized niobium impressed current anodes for its ships.

The US Navy designs ICCP systems at the Naval Research Laboratory in Key West, FL. The methods used by the Navy ensure optimum anode to reference cell configuration and location to provide the best protection to the ship.

Dielectric Shield Coating Requirements
ICCP system anodes are surrounded by thick shielding material (often referred to as “capastic coating” or “capastic epoxy”) consisting of a high-solids epoxy with high dielectric strength. As noted above, this shielding prevents shorting of the anode current to the hull near the anode and aids in wider current distribution to the hull. The dielectric shield includes an inner and outer shield and covers an area about 13 x 16 feet around a 4-foot anode or 13 x 20 feet around an 8-foot anode. The shield is topcoated with anti-corrosive and anti-fouling coatings. The shielding deteriorates over time and eventually requires replacement. Dielectric shield material is not covered by a military specification.

Repair Requirements
The Standard Specification for Ship Repair and Alteration Committee (SSRAC) is responsible for providing technically and contractually sound standards for the Navy’s ship repair and alteration community. The NAVSEA Standard Item applicable to surface ship preservation is Standard Item 009-32, “Cleaning and Painting Requirements.” It contains cleanliness, surface preparation, and coating application requirements, along with complete system application instructions for each product (number of coats, coating thickness per coat, etc.) approved for cathodic protection system preservation.

All current NAVSEA Standard Items may be found here.

New Construction Ships
New construction ships are painted in accordance with the specific Ship Specification for that class of ship.

NAVSEA Point of Contact

Andrew Seelinger
Naval Sea Systems Command, SEA 05P23
1333 Isaac Hull Ave., SE
Washington Navy Yard
Washington, DC 20376
Phone: (202) 781-3670
Email: andrew.seelinger@navy.mil

References

  • D. Jones, Principles and Prevention of Corrosion, Prentice Hall, New Jersey (1996)
  • V. G. DeGiorgi, E. Hogan, and S. A. Wimmer, “New Horizons in Cathodic Protection Design,” U.S. Naval Research Laboratory. Online. http://www.nrl.navy.mil/content.php?P=04REVIEW51. (2003)
  • A.R. Parks, E.D. Thomas, and K.E. Lucas, “Verification of Physical Scale Modeling with Shipboard Trials,” Corrosion 90, Paper 370, National Association of Corrosion Engineers (1990).
  • K.E. Lucas, E.D. Thomas, A.I. Kaznoff, and E.A. Hogan, “Design of Impressed Current Cathodic Protection Systems for the U.S. Navy,” Designing Cathodic Protection Systems for Marine Structures and Vehicles, American Society for Testing and Materials, Special Technical Publication (STP) 1370, H.P. Hack (ed.), 17-33 (1999).
  • R.A. Adey and S.M. Niku, “Computer Modeling of Corrosion Using the Boundary Element Method,” Computer Modeling in Corrosion, American Society for Testing and Materials, Special Technical Publication (STP) 1154, R.S. Munn (ed.), 248-264 (1992).
  • V. G. DeGiorgi, E. Hogan, K.E. Lucas, and S. A. Wimmer, “Computational Modeling of Shipboard ICCP Systems,” J. Corrosion Sci. and Eng. (http: www2.umist.ac.uk/corrosion/JCSE/), 4, Paper 3 (2003).
  • V.G. DeGiorgi, A. Kee, K.E. Lucas, and E.D. Thomas, “Examination of Modeling Assumptions for Impressed Current Cathodic Protection Systems,” in Proceedings of the Corrosion ’99 Research Topical Symposium, Cathodic Protection: Modeling and Experiment, National Association of Corrosion Engineers, 1-16 (1999).
  • V.G. DeGiorgi, E.D. Thomas, and K.E. Lucas, “Scale Effects and Verification of Modeling of Ship Cathodic Protection Systems,” Eng. Anal. Bound. Elements 22, 41-49 (1998).
  • NSTM Chapter 633: Cathodic Protection
  • U.S. Navy Underwater Safety Handbook, Chapter 19 (Cathodic Protection Systems)