| 1. |
This manual provides guidance for the selection, design, installation, operation, and maintenance of cathodic protection systems (CPSs) used to supplement paint systems for corrosion control on civil works hydraulic structures. It also discusses possible solutions to some of the problems with CPSs that may be encountered at existing projects. |
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| 2. |
USACE uses CPSs in combination with protective coatings to mitigate corrosion of hydraulic structures immersed in fresh, brackish, or salt water. Protective coatings alone generally cannot offer complete corrosion protection because they usually contain some pinholes, scratches, and connected porosity, and over time these imperfections become increasingly permeable. As coatings degrade with time, these imperfections, commonly known as holidays, have a profound effect on overall coating integrity because of underfilm corrosion. CPSs, when used in conjunction with protective coatings, have been effective in controlling corrosion. CPSs consist of anodes that pass a protective current to the structure through the electrolyte environment. CPSs can be one of two types, sacrificial anode or impressed current anode. Hybrid CPSs installed on structures can include both types of anodes to provide protective current. |
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| 3. |
Test equipment should consist of a fresh and calibrated coppercopper- sulfate reference cell, a submersible connection, cabling suitable for immersion use, and a high-impedance voltmeter capable of measuring polarized potentials with the CPS on. Sensitivity should be more than 5 meg-ohms per volt. The reference electrode should be placed in the electrolyte adjacent to and within 200 mm to the face of the gate at each test point. All tests should be supervised by an NACE-certified corrosion specialist, senior corrosion technologist, or cathodic protection specialist, a licensed corrosion engineer, or a Corps of Engineers representative assigned and qualified to do this work. |
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| 4. |
Corrosion occurs on all metallic structures that are not adequately protected. The cost of replacing a structure which may have been destroyed or weakened due to excessive corrosion is substantial but avoidable, and means should be taken to consistently prevent or mitigate this added cost through cathodic protection. In addition to preparing and applying protective coatings to the surface of a structure, corrosion protection can be provided by applying a protective electric current to the structure surface which is immersed and in contact with an electrolyte. In the presence of certain other metals contacting the electrolyte near the structure, this technique transforms the structure into a cathodic electrode. A properly selected and designed cathodic protection system can prevent surface corrosion of the structure, or drastically reduce the rate at which it occurs. |
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| 5. |
Sacrificial CPS. This type of system helps reduce surface corrosion of a metallic structure immersed in an electrolyte by coupling a less noble metal with the structure. Sacrificial CPSs work through the sacrifice of an anodic metal, i.e., one that has a negative electrochemical potential relative to the protected ferrous structure, to prevent deterioration of the structure through corrosion. Sacrificial anodes for fresh water applications typically are composed of zinc- or magnesium-based alloys. In the past, installation of sacrificial anodes has often been done on an ad hoc basis, relying largely on the installer’s individual knowledge and experience.
However, recent research on sacrificial anode materials has provided an improved engineering basis for designing civil works applications of these systems.
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| 6. |
CPS Selection. When selecting which type of system to use, the designer should consider the size of the structure to be protected and past project experience in operating and maintaining both types of systems. |
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| 7. |
For existing structures, a current requirement test should be made to accurately assess the overall system design. The designer should become familiar with the availability and suitability of types of commercially manufactured anodes which would satisfy the system requirements for cathodic protection. |
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| 8. |
Services of corrosion engineer. The construction contractor should be required to obtain the services of a licensed corrosion engineer to supervise the installation and testing of the CPS. The term "corrosion engineer" refers to a person who has knowledge of the physical sciences and the principles of engineering and mathematics, acquired by professional education and related practical experience, and who is qualified to engage in the practice of corrosion control on metallic structures. Such person may be a licensed professional corrosion engineer or may be certified as being qualified by NACE International if such licensing or certification includes suitable cathodic protection experience. |
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| 9. |
In Page 74, figure D-1; Skin Plate example: Because the Skin Plate will usually require multiple anodes distributed uniformly both vertically and horizontally, the design procedure is somewhat different than it is for the chamber anode configuration. In this case: |
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Use the total square footage of the submerged skin plate surface (133 m2) and divide by the number of anodes required to protect the skin plate (30 anodes) = 133 m2/30 anodes = 4.43 m2/anode. |
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We can use an arbitrary number for the anodes and their distance apart. |
| 10. |
While the slab and disk galvanic anodes previously described in this manual are generally preferred for civil works structures due to their inherent ruggedness and ease of installation, occasionally the elongated shape of the anodes described in this section may provide design solutions for some structures in higher resistivity environments. Their elongated shape may provide better current distribution in some structure configurations and will usually deliver higher current output for the same weight of material. |
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| 11. |
In Page 77; figure E-1 shows magnesium anode cross sections showing galvanized steel core wire at center. |
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| 12. |
Locating anodes is simply a geometric process of distributing the anodes uniformly on each structure element to achieve good current distribution. |
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| 13. |
Construction. Installation of a CPS by a construction contractor should be accomplished under the supervision of an NACE-certified corrosion specialist, senior corrosion technologist, or cathodic protection specialist or a licensed corrosion engineer. |
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| 14. |
Zebra mussel guidance. In areas with potential for zebra mussel infestations, the CPS components may be at risk of failure or disruption. Design considerations in preventing these infestations should be included. For control strategies, refer to Zebra Mussel Research Technical Note ZMR-3-05, compiled by the Zebra Mussel Research Program at Waterways Experiment Station, Vicksburg, MS. |
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| 15. |
Design. For existing structures, a current requirement test should be made to accurately assess the overall system design. The designer should become familiar with the availability and suitability of types of commercially manufactured anodes which would satisfy the system requirements for cathodic protection. |
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| 16. |
Impressed current CPS. This type of system uses direct current applied to an anode system from an external power source to drive the structure surface to an electrical state that is cathodic in relation to other metals in the electrolyte. A number of impressed current anode materials and geometries are used. Materials include mixed metal oxides, precious metals (e.g., platinum-clad titanium, niobium), and high-silicon chrome-bearing cast iron. The most common geometries are slab or button anodes, rods, and strings. Any anode mounted on the structure must be isolated with a dielectric shield to assure effective current distribution. |
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| 17. |
CPS Selection. When selecting which type of system to use, the designer should consider the size of the structure to be protected and past project experience in operating and maintaining both types of systems. Early in the selection process, if practical, it is useful to perform a current requirement test to help define the total amount of electrical current needed to protect the structure (see PROSPECT Corrosion Control course handbook [009, 2003-01 et seq]). For large structures with significant expanses of bare or poorly coated metal, where the total current requirement tends to be very high, a properly maintained impressed current system can provide 10 to 30 years of effective corrosion protection. Where current requirements are lower and the structure’s protective coatings are well maintained, sacrificial anode systems can be very effective. Improved modern coating systems and maintenance practices today allow for a wider use of sacrificial CPSs on large civil works structures than was the case in the past. For both
types of systems, preliminary design estimations and comparisons of costs, current output, and overall design life should give an adequate indication of which system is preferable for the specific application. Other factors such as future maintenance needs, reliability, accessibility, and impact on operations may also warrant consideration. |
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