Why is corrosion more aggressive in marine environments?

Seawater is an extremely conductive medium as an electrolyte. Its high chloride (salt) content and low electrical resistance lead to the rapid formation of galvanic cells on steel surfaces and the easy flow of corrosion currents. Therefore, corrosion in marine structures progresses significantly faster and more extensively compared to many terrestrial environments.

Chloride ions trigger pitting corrosion by locally disrupting protective oxide layers that may form on the steel surface. In addition, biofouling, caused by variations in oxygen access and local chemistry, further complicates corrosion behavior.

Wave motion, current and tidal changes combine wetting-drying cycles with mechanical wear. These dynamic conditions cause the corrosion rate to vary at different depths and regions of the structure, requiring protection to be designed according to these zones.

How does cathodic protection work in seawater?

Cathodic protection (CP) is a method that suppresses corrosion reactions by electrochemically bringing the metal structure to be protected into a cathodic state. When the structure surface is brought to a sufficiently negative potential, the process leading to the dissolution of the metal (anodic reaction) is significantly slowed down.

In seawater, cathodic protection is typically applied as a complementary barrier, often in conjunction with protective coatings. While the coating provides the first line of defense against corrosion, the cathodic protection takes on the responsibility of protecting the steel in areas of defects and damage within the coating. This dual approach is a widely accepted engineering practice in marine structures.

Standard Family: EN 12473, EN 12495, EN 13173, EN 16222 and ISO 15589-2

Cathodic protection in seawater is addressed within a family of complementary standards. Below the framework standard, which defines general principles, are application standards specific to the building type.

  • EN 12473 — General principles of cathodic protection in seawater: basic concepts of CP design, reference electrodes and secondary effects in seawater, brackish water and marine mud environments.
  • EN 12495 — Fixed offshore steel structures: application of cathodic protection for platforms and fixed offshore constructions.
  • EN 13173 — Floating offshore steel structures: principles for cathodic protection of floating offshore units.
  • EN 16222 — Ship hulls: Cathodic protection design of the hull's outer surface and determination of anode requirements.
  • ISO 15589-2 — Offshore pipelines: CP design, installation, operation and monitoring requirements for submerged pipelines and rises.

Two Methods: Galvanic (Sacrificial) Anode and Impulsed Current (ICCP)

Cathodic protection in seawater is provided by two main methods. The choice of method is determined by an engineering assessment based on factors such as structure size, design life, accessibility, current requirement, and operating conditions.

  • Galvanic (sacrificial) anode system: Metals more active (more negative) than steel are used as sacrificial anodes. The anode dissolves over time, providing a protective current to the structure. Zinc and aluminum-based alloys are commonly preferred in seawater; magnesium is more often used in high-resistance environments. The system is simple, robust, and does not require an external power source.
  • Impressed Current Cathodic Protection (ICCP): A controlled protective current is applied to the structure via an external DC source and insoluble/partially soluble anodes (e.g., platinum-plated titanium). Since the current can be adjusted according to the conditions, it provides flexibility for large structures and high current requirements.

Prevalence of Sacrificial Anodes in Seawater and Comparison of Methods

The high conductivity of seawater creates a favorable environment for galvanic anodes to generate sufficient protective current. Therefore, sacrificial anode systems are a very common solution in fixed offshore structures, underwater pipelines, and submerged steel components.

Galvanic systems are advantageous due to their low maintenance requirements and lack of need for an external power source. Impulse current systems, on the other hand, stand out because the current can be adjusted according to variable conditions (water conductivity, ship speed, coating age, etc.) and large surfaces can be protected with a small number of anodes. For example, ICCP is preferred in large ship hulls, while sacrificial anode solutions are frequently applied in smaller boats and more confined structures.

Protective Potential Criterion and Reference Electrode

The adequacy of cathodic protection is evaluated based on the measured potential of the structure surface. In seawater applications, potential measurement is conceptually expressed relative to a silver/silver chloride/seawater (Ag/AgCl/seawater) reference electrode. This electrode is widely used as a stable and reliable reference in marine environments.

The aim of the protection assessment is to demonstrate that a sufficient negative protection potential has been achieved in all relevant areas of the structure and that secondary effects due to excessive negative potentials (e.g., cladding degradation or hydrogen-related risks) have been avoided. This information does not provide application-specific numerical criteria (mV values); precise thresholds are determined by the qualified engineer within the framework of relevant standards and project conditions.

Marine Protected Areas

A marine structure is divided into zones exhibiting different corrosion behaviors depending on the water level. Cathodic protection is only effective in submerged (continuously in contact with the electrolyte) zones; zones outside the water are addressed by coatings and other methods.

  • Atmospheric zone: The area above water level, exposed to salty air. Cathodic protection is not effective here; protection relies primarily on coating.
  • Splash zone: One of the most aggressive and challenging areas, constantly wetted by waves and spray. Usually requires additional coating and protection measures.
  • Tidal zone: The area that is submerged and exposed alternately with the tides; wetting-drying cycles intensify corrosion.
  • Full immersion zone: The area that remains continuously submerged. This is the primary area where cathodic protection is effectively applied.
  • Mud/bottom zone: Portions embedded in seabed mud are evaluated separately due to varying conductivity and oxygen conditions.

Monitoring, Auditing and Enforcement Sectors

The effectiveness of cathodic protection systems must be monitored even after installation. Potential measurements, anode status, and current values are periodically tracked to verify that the system continues to meet its design objectives. Monitoring is critical for the sustainability of protection over the long design life of marine structures.

Cathodic protection in seawater is relevant to numerous sectors, including maritime and ship management, offshore energy facilities (offshore oil and gas and offshore wind), port and pier infrastructure, and underwater pipeline systems. The safety, service life, and integrity of these structures depend on a properly designed and monitored cathodic protection system.

AES Accreditation Scope

AES (aesinn.com) is a Type A electrical inspection organization accredited by TÜRKAK. AES's accreditation scope covers only the "6.1 Electrical Installation" area. The content on marine cathodic protection on this page is intended to provide general information on the relevant engineering subject and is not part of AES's accredited inspection scope.

Information Note

AES is NOT an accredited inspection/certification body in this context.

This content is for informational purposes only and is subject to the technical assessment of a qualified engineer. The design, implementation, and inspection of cathodic protection in marine structures and seawater must be carried out by a qualified engineer, taking into account the current versions of the relevant standards and the specific conditions of the project.

Note: This page focuses solely on marine/seawater cathodic protection. Buried onshore pipelines and reinforced concrete structures are covered on separate pages.

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Frequently Asked Questions

What is the difference between EN 12473, EN 12495, and EN 13173?

EN 12473 is the framework standard defining the general principles of cathodic protection in seawater. EN 12495 is the application standard for fixed offshore steel structures (platforms), while EN 13173 is for floating offshore steel structures. General principles are addressed in EN 12473; structure-specific requirements are covered in the respective application standards.

For seawater applications, should a galvanic anode or an ICCP (International Carbon Filter) be preferred?

Both are used in seawater. Due to the high conductivity of seawater, galvanic (sacrificial) anode systems are very common and do not require an external power source. ICCP, on the other hand, stands out because of its ability to adjust the current according to varying conditions and to protect large surfaces with a small number of anodes. The choice is made by qualified engineers based on an assessment of the structure size, lifespan, and operating conditions.

Which sacrificial anode metals are used in seawater?

Zinc and aluminum-based alloys are commonly used in seawater applications. Magnesium is preferred in high-resistance environments. The appropriate anode selection depends on the environmental conditions and design requirements.

How is the conservation potential of seawater measured?

Potential measurements in seawater are conceptually expressed relative to a silver/silver chloride/seawater (Ag/AgCl) reference electrode. This electrode is a stable and reliable reference in the marine environment. Precise numerical thresholds are determined by a qualified engineer within the framework of relevant standards and project conditions.

Why are splash and tidal zones treated separately?

Splash and tidal zones are among the most aggressive corrosion zones due to wetting-drying cycles and high oxygen access. Cathodic protection is only effective in areas that are continuously submerged; these intermediate zones are usually addressed with additional coatings and special protection measures.

Does AES certify marine cathodic protection systems?

No. AES is not an accredited inspection/certification body in this scope, and AES's accreditation only covers area 6.1 Electrical Installation. This content is for informational purposes only and is subject to technical evaluation by an authorized engineer.