Steel structures such as ship hulls are constantly exposed to seawater in the harsh marine environment, making them highly vulnerable to corrosion. For shipowners, ensuring the structural integrity of their vessels is not only about extending service life but also about enhancing safety and minimizing operational costs. Cathodic Protection (CP) is one of the most effective methods for preventing the corrosion of metal surfaces, especially in aggressive environments like oceans and seas.
Let’s explore the technical aspects of Cathodic Protection, its types, and how CARELL can provide advanced solutions to protect your fleet.
What is Cathodic Protection?
Cathodic Protection is an electrochemical process that prevents corrosion by converting the ship’s hull into the cathode of an electrical circuit. The idea is to reduce the potential difference between the ship’s metal hull (which acts as a conductor) and its surrounding environment. This is done by supplying electrons to the hull, thereby inhibiting the electrochemical reaction that leads to rust and corrosion.
Cathodic Protection can be applied in two main ways:
- Sacrificial Anode Cathodic Protection (SACP)
- Impressed Current Cathodic Protection (ICCP)
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Sacrificial Anode Cathodic Protection (SACP)
Sacrificial Anode Cathodic Protection operates on the principle of galvanic corrosion, where a more reactive (anodic) metal corrodes instead of the protected steel structure. Common metals such as zinc, aluminum, and magnesium are chosen as sacrificial anodes because they have a more negative electrochemical potential compared to steel, allowing them to corrode preferentially and thus shield the steel from oxidation.
The sacrificial anode, being more reactive than the steel hull, corrodes in place of the hull. During the oxidation of the anode, electrons are released and flow to the steel hull, preventing the hull from corroding.
Key Components
Sacrificial anodes are generally cast in standard shapes such as blocks or cylinders and are attached to submerged parts of the ship, including the hull, rudders, and propeller shafts.
Advantages
This system operates passively, requiring no external power source, and is a cost-effective solution for small to medium vessels.
Replacement Cycle
Sacrificial anodes degrade over time and must be regularly inspected. They are replaced when their weight has significantly decreased, typically after 1 to 3 years, depending on environmental conditions. It is commonly exchanged when the vessel is under Dry Dock project services or in some case during IWS (In Water Survey) in case this is urgent.
Pictures: Anodes installed on Hull Area, inside Sea Chests and Bow Thruster ducts
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Impressed Current Cathodic Protection (ICCP)
Impressed Current Cathodic Protection (ICCP) is a more advanced, active CP system that utilizes an external power source, such as a rectifier, to supply direct current (DC) to the ship’s hull, preventing corrosion effectively
How It Works:
ICCP systems use inert anodes, often made from materials like platinum, titanium, or mixed metal oxides, which are strategically positioned around the hull. Unlike sacrificial anodes, these inert anodes do not corrode. Instead, they deliver a controlled DC current to the hull, counteracting the electrochemical process that would otherwise lead to corrosion. This controlled current maintains the hull’s potential at a level that inhibits oxidation, thereby protecting the steel structure.
Anodes
Reference Anodes
Reference anodes (or reference electrodes) are sensors that measure the electrochemical potential of the hull in real time. They do not emit current like main anodes but instead monitor the hull’s existing electrical state to ensure the ICCP system is operating correctly.
Main (Protective) Anodes
Main anodes are the current-emitting components of the ICCP system. Their purpose is to deliver a controlled direct current (DC) to the hull, providing the active protection required to prevent corrosion.
Power Supply
The ICCP system is powered by the ship’s onboard electrical system, where rectifiers convert alternating current (AC) from the ship’s power supply to the necessary direct current (DC). The DC current is then distributed to the anodes. The power requirements are automatically regulated according to various conditions, such as vessel size, seawater salinity, and hull cleanliness. The system adjusts the current to respond to these factors dynamically, ensuring consistent protection levels despite varying environmental conditions.
Monitoring and Control
ICCP systems feature reference electrodes placed around the hull, which monitor the vessel’s potential (or electrical state) in real-time. These electrodes feed data back to a control panel, which continually adjusts the system output to maintain the optimal protective current. This real-time monitoring and feedback loop ensures the hull’s potential stays within the desired range, preventing both under-protection (leading to corrosion) and over-protection (which can cause paint damage or hydrogen embrittlement). Operators can use the control panel to check system health, adjust parameters, and receive alerts if any abnormalities are detected.
Advantages
ICCP systems offer a long-term, low-maintenance solution, capable of protecting larger vessels and offshore structures. They also help reduce fuel costs by keeping the hull smoother, reducing drag over time.
Pictures: Replacement of a reference anode. The old anode is removed and the hull surface around it is prepared to bear metal condition for proper application of the new putty. New conductive stock is applied to the area, followed by painting as per customer’s agreed paint spec.
Key Considerations
The effectiveness of both SACP (Sacrificial Anode Cathodic Protection) and ICCP (Impressed Current Cathodic Protection) systems relies on the current density, measured in milliamps per square meter, required to protect the hull. In more aggressive seawater environments, characterized by higher salinity, temperature, and flow rate, a greater current density is needed.
Marine growth, such as barnacles and other fouling organisms, can accumulate on the hull, reducing the efficiency of the CP system. To maintain effectiveness, regular hull cleaning is recommended to ensure that anodes remain exposed to seawater.
The primary goal of Cathodic Protection is to polarize the metal surface, reducing its corrosion potential. In sacrificial systems, this is achieved through the natural flow of electrons, while ICCP systems allow for current flow adjustments to maintain continuous polarization.
Environmental considerations are also important. SACP systems may impact the marine environment by releasing metallic ions from sacrificial anodes, so selecting the appropriate anode material and monitoring its usage are essential.
Conclusion
Cathodic Protection is a critical component in the maintenance and longevity of ships. By implementing either Sacrificial Anode Cathodic Protection or Impressed Current Cathodic Protection, shipowners can significantly reduce the risks associated with hull corrosion. When properly maintained, these systems can extend the vessels’ lifespan, lower operational costs, and ensure safe and efficient performance at sea.
How CARELL Can Support Your Cathodic Protection Needs
At CARELL, we offer comprehensive Cathodic Protection solutions tailored to the specific needs of your vessels. We supply and install high-quality zinc, aluminum, and magnesium sacrificial anodes to ensure optimal protection, depending on the environmental conditions your ship operates in. Our team also designs and installs advanced ICCP systems, which feature real-time monitoring and automatic current adjustments, providing continuous and reliable protection for larger vessels and offshore structures.
Our experts conduct detailed hull inspections to measure the remaining life of sacrificial anodes and assess ICCP system performance. We provide timely replacement of anodes and ensure ICCP systems are operating within the required potential ranges. Additionally, with years of maritime industry experience, our team offers corrosion control consultation, advising on best practices to protect your fleet from corrosion and offering customized solutions that meet international standards.
Whether you require a sacrificial anode replacement or an ICCP system upgrade, CARELL has the expertise to deliver durable and effective corrosion protection, safeguarding your assets from the harsh marine environment.
CARELL is committed to providing the most effective and reliable Cathodic Protection solutions for the maritime industry. With our expert services, shipowners can rest assured that their vessels will remain protected against corrosion, even in the most challenging marine environments.
