Corrosion Basics: Cathodic Protection in Fixed Seawater Structures
March 16, 2021 •Corrosion CONTROLLED, Corrosion Essentials, CP
Cathodic Protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. CP can be accomplished by sending a current into the structure from an external electrode and polarizing the metallic surface in an electronegative direction. This provides protection to the surface and extends the life of the asset.
Structures such as steel bulkheads, steel piles supporting piers or wharfs, offshore drilling platforms, and other similar structures in seawater may use cathodic protection (CP) to mitigate corrosion. Cathodic protection in seawater environments can be achieved through using sacrificial galvanic anode systems or impressed current systems. Following is an introduction to each.
Galvanic anode cathodic protection systems
Galvanic anode systems in seawater, for the most part, use much heavier anodes than those used in soil. The low-resistivity seawater environment tends to require greater protective current densities and permits greater current outputs from the anodes. Consequently, the greater corrodible mass is needed to provide reasonably long life.
Material options for use in galvanic anode systems
For structures that can be polarized, a low-potential galvanic anode material such as zinc or aluminum is generally preferable to a high-potential material such as magnesium. Magnesium will work perfectly well but may discharge more current than needed. This results in reduced efficiency and shorter service life for the anodes. In some chloride environments, magnesium anodes have a greater tendency than other galvanic materials to self-corrode, which further reduces their service life.
Zinc or suitable aluminum alloy anodes can polarize a steel structure in seawater to within a few millivolts of the characteristic potential of the anode itself. Assume that the stabilized driving voltage between the anode and the polarized structure is 0.050 V, although it can be less. This ability to maintain polarization at a relatively modest current will consume the anodes at an efficient rate.
Compare the typical behavior of magnesium anodes: the structure does not tend to polarize to a potential more negative than ~–1.1 V vs. a silver/silver chloride (Ag/AgCl) electrode (SCE) because the hydrogen overvoltage potential is reached, resulting in the evolution of free hydrogen rather than additional polarization. This means that with a standard magnesium alloy working voltage of ~–1.4 V vs. an SCE electrode, there will be a driving voltage of ~0.3 V. Thus, on a comparable basis, the magnesium will discharge about six times as much current as is actually required to achieve the desired polarization.
Because less electronegative anodes can provide an efficient cathodic protection system, the surplus current from a more powerful anode is, in effect, wasted. However, there is one advantage offered by magnesium anodes in seawater: the greater driving voltage tends to force the more rapid formation of thicker protective calcareous deposits on the structure surface than would be obtained with less powerful anodes.
Special chemical backfills are not needed in the uniform seawater environment because galvanic anodes work satisfactorily without them.
Impressed current cathodic protection systems
Impressed current systems for fixed seawater structures may use suitable anode materials, also without backfill, suspended from the structure being protected or placed on the ocean floor. In the past, various materials such as treated graphite, high-silicon cast iron, platinized titanium, or lead/silver have been used as anodes. However, the introduction of highly efficient, dimensionally stable anodes (DSAs), which are basically mixed metal oxide coatings on titanium, has rendered these other anodes almost obsolete.
Particular attention must be given to the design of the rectifier positioning, header cable distribution system, and anode suspension or placement details. Above-water components are subject to severe marine atmospheric attack, whereas submerged portions must be protected from, or designed to withstand, the mechanical forces exerted by moving seawater as well as by water-carried debris or shipping traffic.
Lay the foundation for a career in this field with NACE Institute certification -- the most specified and recognized validation of cathodic protection theory, practical knowledge, and expertise.
Source: Originally appeared on materialsperformance.com; adapted by Materials Performance from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed.
FREE DOWNLOADS
White Paper: An Action Plan for Reducing Pipeline Failures, Costs with Corrosion in the Water Sector
Special Report: The Future of Corrosion Control, Insights from the Experts
Get Updates
Featured Articles
Categories
- 2024 Olympics (1)
- Abrasive Blasting (1)
- Advanced coating materials (9)
- Advanced Corrosion Control in Oil and Gas Industry (2)
- Advocacy (1)
- AI (2)
- Aircraft (1)
- Alkanization (1)
- AMPP (3)
- AMPP Annual Conference + Expo (1)
- Ampp Chapters (1)
- AMPP logo (1)
- Ampp Membership (1)
- Ampp Standards (1)
- Amusement parks (4)
- Architectural (1)
- Architectural Coatings (1)
- Artificial Intelligence (1)
- Asset integrity (10)
- Asset maintenance (3)
- Asset Protection (1)
- Bim Software (1)
- Biodeterioration of materials (5)
- Biofouling (4)
- Blasting (1)
- Bridges (3)
- Cathodic Protection-CP (15)
- Ceramic epoxies (1)
- Certification (2)
- Chemical Injection (1)
- Civil Engineering (1)
- Coating inspector (1)
- Coating inspector jobs (1)
- Coating inspector program (1)
- Coatings (12)
- Coatings Application (1)
- Coatings failures (2)
- Coatings Industry (2)
- Coatings inspector (1)
- Coatings measurement and inspection (9)
- Coatings Systems (1)
- Cold stress (1)
- Concrete (12)
- Conference and Events (2)
- Corrosion (15)
- Corrosion Basics (5)
- Corrosion Control (13)
- Corrosion Control and Management (22)
- corrosion engineering (1)
- Corrosion Essentials (19)
- Corrosion Prevention (5)
- Corrosion Under Insulation (1)
- cost of corrosion (1)
- Crevice Corrosion (1)
- Cui (1)
- Data Monitoring (1)
- Department of Defense (3)
- Deposition corrosion (1)
- Dissimilar Metal Corrosion (1)
- Dissolved gases (1)
- DoD (3)
- Education (1)
- Energy industry (9)
- entertainment industry (1)
- Epoxy (2)
- Fireproofing (1)
- Flexible coatings (2)
- Flint, Michigan (1)
- Fluoropolymer coating (3)
- Forms of Corrosion (4)
- Freshwater salinization (1)
- Galvanic (1)
- Galvanic Corrosion (3)
- General Corrosion (2)
- Hand tools (1)
- Industrial Application (3)
- Industrial Safety (2)
- Industry Best Practices (1)
- Industry Standards (1)
- Inspection (1)
- integrity management (1)
- Intergranular Corrosion (1)
- Intumescent Coatings (1)
- Machine Learning (1)
- Maintenance (2)
- Maritime Coatings (11)
- Maritime industry (11)
- Master Painters Institute (1)
- Membership (2)
- Membership Benefits (2)
- Michio Kaku (1)
- Microbiological forms (1)
- Microbiologically influenced corrosion-MIC (11)
- Military (2)
- Mineral constituents (1)
- MPI (1)
- Navy (1)
- Non-Destructive Testing (1)
- Oil and Gas (2)
- Oil Fields (1)
- Organic matter (1)
- Oxgen (1)
- Paint and Protective coatings (32)
- Paint specification (1)
- Personal Protective Equipment (3)
- Petrochemical Plant Fireproofing Methods (1)
- Petrochemical Plants (1)
- Pipeline (2)
- Pitting Corrosion (2)
- Pitting Detection (1)
- Power plant (1)
- Power tools (1)
- PPE (3)
- Protective Coatings (5)
- Real-Time Corrosion Monitoring in Oil Fields (1)
- Rebar Corrosion (1)
- Reliability (1)
- Remote monitoring and drones (4)
- Repaint (1)
- Restoration (1)
- ride maintenance (1)
- Road deicers (1)
- Roads and bridges (1)
- Roller coaster (1)
- Rust (1)
- Safety (5)
- Safety Standards (2)
- Salt pollution (1)
- Sensors (1)
- Ship Coatings (9)
- Shiptanks (1)
- Standards (9)
- Standards Committees (1)
- Steel (7)
- Steel Reinforcement (1)
- Stress Corrosion Cracking (1)
- Structural Steel (1)
- Surface Preparation (12)
- Sustainability and corrosion (7)
- Tools (1)
- Turbine (1)
- Types of Corrosion (1)
- Uniform Corrosion (1)
- Water crisis (1)
- Water pipe corrosion (1)
- Water quality (1)
- Water tank coatings (5)
- Water/treatment infrastructure (19)
- Waterway salinity (1)
- Workforce development (1)