Knowledge / Shipyards and production

Cathodic protection on ships: current, anode life and potential measurement

Distinguish galvanic anodes, impressed current, local protective potential, coating effects and anode capacity.

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Cathodic protection reduces corrosion by changing the electrochemical behaviour of immersed metal. Success is not established merely by finding anodes or observing current at a control panel: suitable protection must reach the relevant parts of the structure. How much current an anode can deliver, how long it can sustain that current and where the current reaches are separate questions.

Galvanic and impressed-current systems use different energy sources

A galvanic system consumes a more active metal electrically connected to the structure. An impressed-current cathodic protection system, or ICCP, uses a regulated direct-current source to supply purpose-designed anodes. Cathwell’s technical explanation describes this distinction. Both depend on a functioning circuit through the electrolyte and the metallic connections.

A component electrically isolated from the hull may therefore remain outside the hull system’s protection even when the panel appears normal. Conversely, an unintended metallic connection can add another area’s current demand. Define the protected boundary through electrical continuity and immersed surfaces, rather than appearance. Showing the anodes, their connections and the protected materials together makes diagnosis more meaningful.

Coating condition changes current demand

Coatings and cathodic protection complement each other. An effective coating reduces metal exposed to the electrolyte; damage and ageing can increase demand. Cathwell’s explanation of anode life connects the required anode quantity with exposed area and intended service duration. The question here is how coating condition changes the electrical problem, rather than how surface preparation is inspected.

Explicitly calculating bare area and applying a coating-breakdown factor are alternative ways to represent exposure. Calculating an already bare area and reducing it again by a breakdown factor can underestimate demand. Mean current determines cumulative consumption, whereas demand in the most onerous condition concerns instantaneous capability. Satisfying one does not establish the other.

A current-demand example with explicit dimensions

Consider a hypothetical zone with 2 000 m² of wetted surface, a coating-breakdown factor f = 0.05 and an assumed bare-metal current density j = 0.10 A/m². The simple model I = A f j gives an effective exposed area of 100 m² and a current demand of 10 A. These are teaching assumptions, not values selected from a classification design table.

With the same assumed j but f = 0.15, the exposed area becomes 300 m² and demand becomes 30 A. Tripling current does not mean tripling the protection at every point: this is only the total requirement from the selected area model. If water conditions, speed, material, protective potential or zone distribution change, holding j constant may itself be inappropriate.

Separate anode mass from current-delivery capability

For a mean demand of 10 A over two years, taking 365 days per year, the required charge is 10 × 2 × 365 × 24 = 175 200 A·h. With a hypothetical electrochemical capacity of 2 000 A·h/kg and utilisation factor of 0.85, the required net anode-alloy mass is 175 200 / (2 000 × 0.85) ≈ 103.1 kg. A steel insert does not contribute to this alloy capacity.

This is only a capacity calculation. Anode shape, resistance, driving potential and end-of-life geometry can limit delivery of the required current. One large anode and distributed anodes of the same total mass need not protect every location equally. Without verified material capacity, utilisation and environmental conditions, the illustrative 103.1 kg cannot become an equipment-selection instruction.

Potential is always relative to a reference

A record of −0.85 V is incomplete without its reference electrode. The ABS guidance dated December 2017 discusses marine reference electrodes, their placement and voltage drop through water during measurement. Salinity and temperature can also affect interpretation. Structure-to-electrolyte potential is a different quantity from the voltage supplying the anodes.

Suppose readings made with the same electrode at the same point under comparable conditions are −0.82 V and −0.90 V. The difference is −0.08 V, or −80 mV. This is a more negative reading; it does not by itself establish better or acceptable protection. Raw values obtained with different reference types cannot be compared without a valid conversion and the relevant conditions. Record the reference and position as carefully as the sign.

Total current does not establish local protection

DNV’s marine-corrosion modelling overview describes identifying underprotected and overprotected areas on a structure. This supports treating a single total-current value as insufficient evidence for the entire surface. Geometry, shielding and current paths enter the assessment; the name of a modelling tool is not proof that a particular model is valid.

In a useful comparison, measurement positions are identified on the hull, with draught, speed or harbour condition, water environment and ICCP outputs recorded together. A change after dry docking might be consistent with the exposed-area effect of renewed coating, but a disconnected bond or faulty reference also needs consideration. The explanation should fit panel readings and independent local observations.

More negative is not always better

Excessively negative potential can introduce hydrogen-related problems in susceptible materials and coating disbondment. Cathwell’s explanation of protection levels therefore distinguishes suitable protection from overprotection. Moving a controller setpoint in the negative direction without material- and environment-specific limits may improve one region while adversely affecting another.

Cathodic protection must also be distinguished from marine-growth control. The explanation of electrolytic antifouling describes a different function, limiting settlement of marine organisms. An anode-like component does not identify the service it provides: its purpose, material and circuit connections must be understood.

Which evidence supports a useful conclusion?

Anode-consumption observations, protective potentials, coating condition and thickness loss provide complementary evidence. An anode showing little consumption might be long-lived, or disconnected. Rapid consumption might indicate high demand, unsuitable material or an unexpected current path. A consistent explanation of the circuit is more useful than assigning a cause from one symptom.

The assessment should identify the material- and reference-specific criterion, measurement uncertainty and actual surface coverage. Current-demand and capacity examples explain why separate checks are needed. An actual anode arrangement, controller adjustment or diving operation requires the vessel’s specific design and safe working arrangements.

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