Cathodic Protection for Marine Structures

Why Marine CP Matters

Seawater is a highly conductive electrolyte, so corrosion can be severe. Marine structures are also affected by oxygen gradients, biofouling, tidal changes, temperature, salinity, coatings, deposits, and mechanical damage.

Marine CP systems must account for submerged zones, mudline conditions, splash zones, atmospheric exposure, coatings, and anode accessibility.

Reference electrode selection is critical. Marine potentials are commonly measured using silver-silver chloride reference electrodes, not copper-copper sulfate electrodes used in soil applications.

Core Concept

Seawater as electrolyte

Seawater generally has lower resistance than many soils, allowing CP current to distribute differently than in buried pipeline or tank applications.

Galvanic marine systems

Zinc and aluminum anodes are commonly used in marine environments. Anode selection depends on salinity, structure material, current demand, design life, and environmental conditions.

Impressed current marine systems

Impressed current systems may be used on large marine structures, ship hulls, offshore assets, docks, sheet piles, or structures requiring adjustable output.

Reference electrodes

Silver-silver chloride reference electrodes are commonly used for seawater measurements. Reported potentials must identify the reference electrode type and test condition.

Current distribution

Current distribution is affected by geometry, coatings, anode placement, water depth, mud burial, shielding, tidal exposure, and structural continuity.

Field Application

Marine CP surveys may include underwater potential measurements, diver or ROV inspections, anode condition surveys, coating inspections, current output checks, and continuity testing.

Marine anodes should be inspected for consumption, damage, attachment integrity, and remaining life. Impressed current systems should also be checked for power supply output, anode condition, and reference electrode feedback if used.

CP performance should be evaluated by zone. Submerged areas, buried mudline areas, splash zones, and atmospheric zones do not behave the same way.

Common Mistakes

1. Using soil-based assumptions in seawater.
   Why it is wrong: Seawater conductivity, reference electrodes, anode materials, and current distribution differ from soil applications.
2. Reporting potentials without reference electrode type.
   Why it is wrong: Marine readings are commonly reported versus silver-silver chloride, which is not numerically interchangeable with CSE.
3. Ignoring splash-zone corrosion.
   Why it is wrong: CP is most effective where electrolyte contact permits current flow; splash zones may require coating or other controls.
4. Assuming anodes protect shielded areas.
   Why it is wrong: Geometry, deposits, mud, or structural shielding can block current access.
5. Ignoring anode attachment condition.
   Why it is wrong: A detached or electrically disconnected anode cannot provide intended protection.

Field Example

A steel dock pile has aluminum anodes installed below the waterline. Underwater potential readings are acceptable near the anodes but less protective near the mudline.

The difference may be caused by current distribution, coating condition, mud shielding, anode placement, or changes in electrolyte conditions near the bottom.

Practice Questions

1. Which reference electrode is commonly used in seawater CP measurements?
2. Why are marine potentials not automatically comparable to CSE soil potentials?
3. What are two common marine galvanic anode materials?
4. Why can splash zones be difficult to protect by CP alone?
5. Why should marine structures be evaluated by exposure zone?

Related Pages

• Reference Electrodes
• Galvanic Anodes
• Impressed Current Anodes
• Cathodic Protection Criteria
• CP Survey Methods