Cathodic Protection for Marine Structures

Understanding Cathodic Protection: The Foundation of Marine Structure Longevity

Marine structures operating in the harsh waters of Atlantic Canada face an unrelenting adversary: corrosion. The combination of saltwater, temperature fluctuations, and biological activity creates an environment where unprotected steel can deteriorate at alarming rates—sometimes losing up to 0.5 millimetres of thickness annually. For wharves, jetties, offshore platforms, and vessel hulls operating in Nova Scotia's coastal waters, cathodic protection (CP) represents the most effective defence against this destructive electrochemical process.

Cathodic protection works by converting the entire surface of a metal structure into a cathode, effectively halting the oxidation reactions that cause corrosion. This electrochemical technique has protected marine infrastructure worldwide for over 150 years, and its importance in the Maritime provinces cannot be overstated. With billions of dollars invested in coastal infrastructure throughout Atlantic Canada—from the Port of Halifax to smaller fishing harbours along the Bay of Fundy—proper cathodic protection design and maintenance represents a critical engineering discipline.

The Science Behind Cathodic Protection Systems

To appreciate cathodic protection fully, engineers and facility managers must understand the electrochemical principles at play. When steel is immersed in seawater, it naturally forms a galvanic cell. Different areas of the metal surface become anodic (where metal dissolves) and cathodic (where reduction reactions occur). This differential creates electrical current flow through the electrolyte—in this case, seawater—causing material loss at the anodic sites.

Electrochemical Fundamentals

Seawater serves as an excellent electrolyte due to its high conductivity, typically ranging from 4 to 5 Siemens per metre in Nova Scotia's coastal waters. The chloride ions present accelerate corrosion by breaking down passive oxide films that might otherwise provide some protection. In the cold waters of the North Atlantic, where temperatures can range from -2°C in winter to 18°C in summer, corrosion rates fluctuate seasonally but remain aggressive year-round.

The goal of cathodic protection is to shift the potential of the protected structure to a level where corrosion becomes thermodynamically unfavourable. For carbon steel in seawater, this protective potential typically ranges from -850 millivolts to -1,100 millivolts when measured against a silver/silver chloride reference electrode. Maintaining this potential window is crucial—too little protection allows corrosion to continue, while excessive polarisation can cause hydrogen embrittlement or coating disbondment.

Types of Cathodic Protection Systems

Two primary methods exist for achieving cathodic protection in marine environments:

• Galvanic (Sacrificial) Anode Systems: These systems utilise metals more electrochemically active than steel—typically zinc, aluminium, or magnesium alloys—to provide protective current. The anodes corrode preferentially, "sacrificing" themselves to protect the structure. This approach requires no external power source and offers simplicity in design and installation.

• Impressed Current Cathodic Protection (ICCP): These systems use an external power source (typically a transformer-rectifier) to force current from relatively inert anodes to the protected structure. ICCP systems can protect larger areas and provide adjustable output but require ongoing power supply and more sophisticated monitoring.

Design Considerations for Maritime Environments

Designing cathodic protection systems for marine structures in Atlantic Canada presents unique challenges that demand careful engineering analysis. The region's environmental conditions, tidal ranges, and structural diversity require customised solutions rather than generic approaches.

Environmental Factors in Nova Scotia Waters

The Bay of Fundy, home to the world's highest tides, presents exceptional design challenges. Tidal ranges exceeding 16 metres mean that portions of marine structures experience alternating immersion and atmospheric exposure. The splash and tidal zones—areas of intermittent wetting—often exhibit the highest corrosion rates, sometimes three to five times greater than fully submerged zones. Cathodic protection is most effective in continuously submerged areas, necessitating supplementary protection strategies for upper zones.

Water temperature significantly affects both corrosion rates and cathodic protection current requirements. Colder waters generally have higher dissolved oxygen content, increasing corrosion rates, while simultaneously reducing anode output efficiency. Design engineers must account for seasonal variations, typically using average annual temperatures for calculations while ensuring adequate protection during peak demand periods.

Current Density Requirements

The amount of protective current required depends on numerous factors, including coating condition, water velocity, and temperature. For bare steel in North Atlantic waters, initial current densities of 150 to 200 milliamperes per square metre are commonly specified. Once polarisation films develop, maintenance current densities typically reduce to 90 to 120 milliamperes per square metre.

Coated structures require substantially less current—often only 5 to 20 milliamperes per square metre—making high-quality coating application an excellent complement to cathodic protection. This combination approach, known as "belt and braces" protection, offers the most economical long-term solution for major marine infrastructure.

Galvanic Anode Systems: Design and Application

For many marine structures throughout the Maritimes, galvanic anode systems provide an optimal solution due to their simplicity, reliability, and minimal maintenance requirements. These self-regulating systems automatically adjust their output based on environmental conditions and structure potential.

Anode Material Selection

The choice between zinc, aluminium, and magnesium anodes depends on application requirements:

• Zinc Anodes: The traditional choice for seawater applications, zinc anodes offer reliable performance and predictable consumption rates of approximately 11.2 kilograms per ampere-year. Their driving potential of roughly 250 millivolts (versus steel) suits most marine applications.

• Aluminium Anodes: Offering higher energy capacity (approximately 2,800 ampere-hours per kilogram versus 780 for zinc), aluminium anodes have become increasingly popular for offshore applications. Their lighter weight reduces structural loads, and they perform well in both seawater and brackish conditions.

• Magnesium Anodes: With the highest driving potential (approximately 550 millivolts versus steel), magnesium anodes suit low-conductivity environments such as harbours with freshwater influence. However, their high consumption rate limits their use in full seawater applications.

Anode Configuration and Placement

Proper anode distribution ensures uniform protection across the entire structure. Design engineers must consider current attenuation—the reduction in protective current with distance from anodes—when determining spacing. For typical wharf structures, anodes are often installed at 3 to 5-metre intervals along steel sheet piling, with additional anodes concentrated near complex geometries or areas of high current demand.

Anode attachment methods significantly impact system longevity. Direct welding provides excellent electrical continuity but creates heat-affected zones that may accelerate localised corrosion. Bolted connections offer easier replacement but require careful attention to electrical continuity and mechanical security against wave action and ice loading—a particular concern in Maritime harbours.

Impressed Current Systems: When Greater Control Is Needed

For larger marine structures or those with complex protection requirements, impressed current cathodic protection systems offer advantages that justify their increased complexity and cost. Major port facilities, offshore installations, and long-span bridges throughout Atlantic Canada commonly employ ICCP systems.

System Components and Design

A typical ICCP system comprises several key elements:

• Transformer-Rectifier Units: These convert AC power to controlled DC output, typically ranging from 10 to 100 amperes at voltages up to 50 volts. Modern units incorporate automatic potential control, adjusting output to maintain target protection potentials.

• Anode Groundbeds: Impressed current anodes—commonly high-silicon cast iron, mixed metal oxide-coated titanium, or polymer-based conductive materials—are configured to provide uniform current distribution. Anode life often exceeds 25 years with proper design.

• Reference Electrodes: Permanent reference electrodes (silver/silver chloride or zinc) enable continuous potential monitoring and system control. Strategic placement ensures representative measurements across the protected structure.

• Cabling and Junction Boxes: Marine-grade cabling and connection points must withstand constant immersion, UV exposure, and mechanical stress. Proper cable sizing prevents excessive voltage drop, which can compromise system performance.

Monitoring and Control

Modern ICCP systems increasingly incorporate remote monitoring capabilities, allowing operators to track protection levels, adjust outputs, and receive alarm notifications without site visits. For facilities in remote areas of coastal Nova Scotia, this capability significantly reduces operational costs while improving protection reliability.

Inspection, Monitoring, and Maintenance

Even the best-designed cathodic protection system requires regular inspection and maintenance to ensure continued effectiveness. Canadian standards, including CSA S478 and relevant portions of NACE/AMPP standards, provide guidance on inspection frequencies and acceptance criteria.

Potential Surveys

Regular potential surveys verify that protection levels remain within acceptable ranges. For submerged structures, divers or remotely operated vehicles (ROVs) equipped with reference electrodes collect measurements at predetermined survey points. Survey frequency typically ranges from annual inspections for critical structures to every three to five years for lower-risk installations.

Close-interval potential surveys, recording measurements at 0.5 to 1-metre intervals, identify areas of under-protection or over-protection that spot checks might miss. These detailed surveys are particularly valuable following system installation or significant modifications.

Anode Inspection and Replacement

Galvanic anodes require periodic visual inspection to assess consumption rates and remaining life. Anodes are typically replaced when 85% consumed or when potential surveys indicate inadequate protection. Maintaining accurate installation records and consumption tracking enables proactive replacement scheduling, avoiding protection gaps.

For ICCP systems, anode groundbed resistance measurements indicate degradation over time. Increasing resistance suggests anode consumption or cable deterioration, prompting investigation and corrective action.

Integration with Protective Coatings and Structural Design

Cathodic protection works most effectively as part of an integrated corrosion management strategy. Modern marine engineering practice recognises that coatings, cathodic protection, corrosion allowances, and materials selection must be coordinated from the earliest design stages.

Coating Compatibility

Not all coatings perform equally well with cathodic protection. High-quality, holiday-free epoxy coatings provide excellent cathodic protection compatibility, dramatically reducing current requirements while preventing coating disbondment at holidays (coating defects). Coating systems must be specified with cathodic protection in mind, avoiding products that may saponify or lose adhesion under cathodic polarisation.

Design Life Considerations

Marine structures in Atlantic Canada are increasingly designed for extended service lives—often 50 to 75 years or longer. Achieving these lifespans requires cathodic protection systems designed for the full structure life, including provisions for anode replacement, system upgrades, and changing operational conditions.

Partner with Experienced Marine Engineering Professionals

Effective cathodic protection for marine structures demands expertise in electrochemistry, marine engineering, and local environmental conditions. From initial feasibility assessments through detailed design, installation oversight, and ongoing monitoring programme development, every phase requires careful engineering attention.

Sangster Engineering Ltd., based in Amherst, Nova Scotia, brings decades of experience to marine engineering projects throughout Atlantic Canada. Our team understands the unique challenges facing marine infrastructure in Maritime waters—from the extreme tides of the Bay of Fundy to the ice conditions affecting northern harbours. We provide comprehensive cathodic protection services, including condition assessments of existing systems, new system design, specification development, installation inspection, and monitoring programme implementation.

Whether you are developing a new marine facility, extending the life of existing infrastructure, or troubleshooting protection deficiencies, our engineers deliver practical, cost-effective solutions tailored to your specific requirements. Contact Sangster Engineering Ltd. today to discuss your cathodic protection needs and discover how professional engineering expertise can protect your marine infrastructure investment for decades to come.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.