Section 4 Cathodic Protection (CP)

4.1.1 Where the design requires application of cathodic protection systems, either by impressed current or sacrificial means, this Section gives guidance on satisfying this requirement.

4.1.2 The type of cathodic protection of the structure required is to be considered in terms of the following zones:

1. Submerged zone. That part of the hull below the maximum design operating draught.

2. Splash zone. That part of the hull above the submerged zone and subject to wet and dry conditions.

3. Atmospheric zone. That part of the hull above the splash zone.

Cathodic protection inside tanks is considered separately (see Section Ch 15, 4.8 Cathodic protection in tanks).

4.1.3 The cathodic protection system for the submerged areas should be capable of polarising the steelwork to a sufficient level in order to minimise corrosion. This may be achieved using an impressed current system, sacrificial anodes or a combination of both.

4.1.4 All parts of the submerged structure should be electrically continuous, and where considered necessary, appropriate bonding devices and straps should be fitted across such items as propeller shafts, thrusters, stabilisers and rudders, pipe work, etc. Where bonding straps are not fitted then a supplementary cathodic protection system should be considered. The influence of any connecting structures, such as docking structures, on the efficiency of the cathodic protection system should be evaluated.

4.1.5 The cathodic protection system is to be capable of polarising the steel structure to potentials measured with respect to a silver/silver chloride/seawater (Ag/AgCl) reference electrode to within the range -0,80 to -1,20 volts for open seawater conditions. An average potential range from -0,90 to -0,95 volts vs. Ag/AgCl may be considered ideal. The electrode potential for steels which have surfaces operating above 25^oC should be 1 mV (0,001 V) more negative for each degree above 25^oC.

4.1.6 Potentials more negative than -1,20 volts vs. Ag/AgCI must be avoided in order to minimise any damage to the coating system.

4.1.7  In applications where high strength steels (tensile strengths in excess of 700 N/mm²) are used in the submerged zone, potentials should be limited to -0,95 volts vs. Ag/AgCl in order to reduce the possibility of hydrogen embrittlement.

4.1.8 High strength fastening materials should be avoided due to the possible effects of hydrogen absorption. If the use of high strength fasteners is unavoidable, then the hardness of such bolting materials should be limited to a maximum of 300 HV.

4.1.9 When selecting protection systems the following items are normally considered:

1. A surface area breakdown for all areas to be protected, including appendages.

2. The assumed resistivity of the seawater.

3. All current densities used for design purposes.

4. The type and location of any reference electrodes and their methods of attachment.

5. Full details of any coatings used and the areas to which they are to be applied.

6. Details of the shaft grounding.

7. The electrical bonding.

4.2.1 Where the design specifies cathodic protection, the typical requirements of the hull and hull appendages and openings are given in Table 15.4.1 Typical corrosion protection requirements for the hull steelwork.

4.3.1 When designing an ICCP system, the following items are to be considered in addition to those identified in Ch 15, 4.1 General principles 4.1.9:

1. The anode composition and, where applicable, the thickness of the plated surface together with consumption and life data.

2. Anode resistance, limiting potential and current output.

3. System maximum operating voltage.

4. Details of the anode and reference electrode cofferdam construction, including the specification of the steel used and welding details, and the standards and specifications which are applicable.

5. Size, shape and composition of the dielectric shields.

6. Diagram of the wiring system used for the impressed current and monitoring systems including details of cable sizes, junction boxes, joints (if any), type of insulation and normal working current in circuits, as well as the capacity, type and make of the protected devices.

7. Details of glands and size of steel conduits (if applicable).

8. The locations of the anodes and reference electrodes.

9. The coating manufacturer should confirm that the hull coating is compatible with an impressed current cathodic protection system.

4.3.2 All impressed current anodes and reference electrodes should be capable of being replaced easily by divers.

4.3.3 Impressed current anode materials are to consist of lead/silver alloy or platinum over such substrates as titanium, niobium, tantalum or mixed metal oxide coated titanium.

4.3.4 The design and installation of electrical equipment and cables are to be in accordance with the requirements of the relevant Rules.

4.3.5 All equipment is to be suitable for its intended location

4.3.6 The arrangements for glands, where cables pass through shell boundaries, are to include a small cofferdam.

4.3.7 Cables which pass through ballast tanks are to be enclosed in a steel tube of at least 10 mm thickness.

4.3.8 Cables to anodes are not to pass through tanks intended for the storage of low flash point products, including but not limited to, oils.

4.3.9 Cables which pass through the cofferdams of storage tanks which may contain low flash point products are to be enclosed in a steel tube of at least 10 mm thickness.

4.3.10 Cable and insulating material shall be resistant to chlorides, hydrocarbons and any other chemicals with which they may come into contact.

4.3.11 The electrical connection between the anode cable and the anode body is to be watertight, and mechanically and electrically sound.

4.3.12 Where electrical power is derived from a rectified a.c. source, adequate protection is to be provided to trip the supply in the event of:

1. A fault between the input or high voltage windings of the transformer (i.e. main voltage) and the d.c. output of the associated rectifier; or

2. The ripple on the rectified d.c. exceeding 5 per cent.

4.3.13 Suitable dielectric shields are to be fitted in order to avoid negative potentials of -1,70 volts (vs. Ag/AgCl) and below.

4.3.14 Where protection is primarily by an impressed current cathodic protection system, sufficient sacrificial anodes are to be fitted that are capable of polarising the critical regions of the structure from the time of initial immersion until full commissioning of the impressed current system.

4.4.1 The use of ICCP for the corrosion protection of aluminium has a significantly higher risk of failure than ICCP used for the protection of steel vessels. However, applications for its use in fast craft such as fast ferries, sailing yachts and water jet tunnels must be considered because traditional zinc sacrificial anode cathodic protection is unsuitable for aluminium structures.

4.4.2 The accepted potential limits for the corrosion protection of aluminium hulls in clean, undiluted and aerated sea water using ICCP are as follows:

1. Positive limit: -0,90 V (vs. Ag/AgCl)

2. Negative limit: -1,15 V (vs. Ag/AgCl)

4.4.3 In addition to the information required for design as given in Ch 15, 4.1 General principles 4.1.9 and Ch 15, 4.3 Impressed Current Cathodic Protection (ICCP) 4.3.1, the ICCP system as applied to aluminium hulls has the following additional considerations:

1. The corrosion protection potential range must be strictly controlled and alarmed with the overprotection limit setting not to be exceeded.

2. The ICCP is to operate within the following potential limits:

   Under-protection limit: -0,85 V (vs. Ag/AgCl)

   Overprotection limit: -1,15 V (vs. Ag/AgCl)

3. The monitoring and control reference electrodes are to be positioned to register the maximum and minimum limits on the hull. In practice this usually means that a reference electrode should be placed near the propellers and on the periphery of the anode dielectric shield.

4. The anode dielectric shield is to be fabricated from plastic unless an alternative coating system can be proven to maintain adhesion and dielectric properties for at least the duration of the dry dock maintenance period under the maximum operating anode voltage.

5. The power/control and monitoring unit shall have a dielectric shield breakdown detection system capable of detecting water ingress behind the shield and current leakage through the shield. The unit should be able to detect and shut down current output automatically as well as registering an alarm. Each dielectric shield shall have an in-built sensor to detect water leakage behind the shield and current leakage through the shield.

6. The bare marine grade aluminium (series 5000) has very low corrosion rates in open seawater and will require a current density of less than 1 mA/m² to fully protect it. Therefore the most suitable location of the anodes will differ from that of a steel hulled vessel. Any requirement for current density of coated aluminium can be ignored provided that the full current demand requirement for the metal underwater appendages has been taken into account.

7. The anodes shall be located near the appendages where the current demand is at its greatest.

4.4.4 Special additional consideration needs to be undertaken to prevent bi-metallic corrosion at areas vulnerable to such attack, e.g. water-lubricated sterntubes and the underwater hull penetrations such as water cooling suctions (intakes), overboard discharges and exhaust discharges. Such considerations include:

1. For hull penetrations, the sea valves are to be electrically isolated from hull and pipework, and the penetration depth (depth from the hull) should be less than four times the internal diameter of the penetration.

2. For sterntubes, the wetted areas of the sterntube should be lined with a plastic tube or other sufficiently insulating material, and the shaft should have an effective passive shaft grounding system.

4.5.1 Sacrificial anodes intended for installation on Classed structures are to be manufactured in accordance with the requirements of this Section.

4.5.2 For offshore structures, plans showing anode nominal dimensions, tolerances and installation details are to be submitted for approval prior to manufacture.

4.5.3 For ships, anode design and details are to be agreed between the shipyard and owners.

4.5.4 When calculating the number and location of sacrificial anodes, the following items are normally considered in addition to those identified in Ch 15, 4.1 General principles 4.1.9:

1. The design life of the system in years.

2. Anode type, material and minimum design capacity of anode material, in Ah/kg.

3. The dimensions of anodes, including details of the inserts and their locations.

4. The nett and gross weight of the anodes, in kilograms.

5. The means of attachment.

6. The location of the anodes.

7. Calculation of anodic resistance (in ohms) in the as-installed condition and when consumed to their design utilisation factor.

8. Closed circuit potential of the anode material, in volts.

9. Computer modelling or supporting calculations.

10. The anode design utilisation factor.

4.5.5 The anode materials are to be approved alloys of zinc or aluminium with a closed circuit potential of at least -1,00 volt (vs. Ag/AgCI). Magnesium-based anodes may be used for short-term temporary protection of materials which are not susceptible to hydrogen embrittlement.

4.5.6 The anode material is to be cast around a steel insert so designed as to retain the anode material even when it is consumed to its design utilisation factor. The steel inserts are to have sufficient strength to withstand all external forces that they may encounter such as wave, wind, and ice loadings in the vessel or structure’s normal operating conditions.

4.5.7 The anodes are to be sufficiently rigid to avoid vibration in the anode support. The steel inserts are to be of weldable, fully killed structural steel bar, section or pipe with a carbon equivalent not greater than 0,45 per cent determined using the following formula:

1. Carbon equivalent C_eq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

4.6.1 The definition of the hybrid system is that it is an impressed current cathodic protection system, supplemented by sacrificial anodes.

4.6.2 If sacrificial anodes are installed on the submerged parts of a vessel or structure where impressed current is the primary cathodic protection system, then the cathodic protection should be treated as a single hybrid system and the Rules stated below are applied. The exception to this would be if the hull of a vessel is purposely electrically isolated from any of the appendages. In this case the hull and its appendages can be independently protected with either system, provided that sufficient measures are taken to prevent interference and stray current damage.

4.6.3 The design of the hybrid system should be integrated, with the impressed current and sacrificial anodes positioned so that the sacrificial anodes do not interfere with the performance of the impressed current cathodic protection system. The distances of the sacrificial anodes from the ICCP reference electrodes should not be less than three metres.

4.6.4 Where the hybrid system is installed on a vessel, the impressed current portion of the hybrid system should be designed in accordance with Section 4.3. The number, size and location of the sacrificial anodes should be sufficient to protect the aft section of the ship, without the impressed current, for a design life of one year.

4.6.5 For applications where there is a risk of anode passivation or poor performance, only zinc sacrificial anodes should be used.

4.7.1 Offshore structures with a sacrificial anode cathodic protection system should be monitored regularly to confirm the least negative potential from the system.

4.7.2 For ships over 300 m, a permanent monitoring system is to be installed on the bow of the hull protected by an all-aft impressed current cathodic protection system. Consideration is to be given to the use of sacrificial anodes in way of known high potential zones.

4.7.3 For offshore structures, details of the monitoring system should be submitted for review.

4.8.1 Impressed current cathodic protection systems are not to be fitted in any tank.

4.8.2 Particular attention is to be given to the locations of anodes in tanks which can contain explosive or other inflammable vapour, both in relation to the structural arrangements and openings of the tanks.

4.8.3 Aluminium and aluminium alloy anodes are permitted in tanks which may contain explosive or flammable vapour, or in tanks adjacent to tanks which may contain explosive or flammable vapour, but only at locations where the potential energy of the anode does not exceed 275 J. The weight of the anode is to be taken as the weight at the time of fitting, including any inserts and fitting devices. The height of the anode is, in general, to be measured from the bottom of the tank to the centre of the anode. Where the anode is located on a horizontal surface (such as bulkhead stringer) not less than 1 m wide, provided with an upstanding flange or face plate projecting not less than 75 mm above the horizontal surface, the height of the anode can be measured above that surface.

4.8.4 Aluminium anodes are not to be located under tank hatches or other openings unless protected by adjacent structure.

4.8.5 Magnesium or magnesium alloy anodes are not permitted in tanks which can contain explosive or flammable vapour, or in tanks adjacent to which can contain explosive or flammable vapour. Where permitted for other tanks, adequate venting must be provided.

4.8.6 Anodes fitted internally should preferably be attached to stiffeners, or aligned in way of stiffeners on plane bulkhead plating. Where they are welded to asymmetrical stiffeners, they are to be connected to the web with the welding at least 25 mm away from the edge of the web.

4.8.7 In the case of stiffeners or girders with symmetrical face plates, the connection may be made to the web or to the centreline of the mild steel face plate but well clear of the free edges. Where higher tensile steel face plates are fitted, the anodes are to be attached to the webs.

4.8.8 Anodes are not to be attached directly to the shell plating of main hulls, columns or primary bracings.

4.8.9 For guidance on the design of sacrificial anode systems in tanks, see Ch 15, 4.5 Sacrificial anodes.

4.9.1 Surveys of electrical potential in way of the submerged areas of the external hull or structure should be carried out at regular intervals.

4.9.2 Should the results of any potential survey measured with respect to a Ag/AgCl reference cell indicate values more positive than -0,8 volts for aerobic conditions or -0,9 volts for anaerobic conditions then remedial action should be carried out at the earliest opportunity.

4.10.1 Replacement and retrofitting of sacrificial anodes would preferably be carried out during dry docking. Alternatively, these operations could be carried out whilst afloat, if suitable arrangements can be made.

4.10.2 Where it is proposed to fit additional anodes, design approval is required in accordance with Ch 15, 4.5 Sacrificial anodes 4.5.2 or Ch 15, 4.5 Sacrificial anodes 4.5.3.

4.10.3 Where it is necessary to weld anodes to the structure only approved welding procedures and consumables are to be used.

4.10.4 The welding procedure is to be qualified under fully representative conditions in accordance with the requirements of Ch 12 Welding Qualifications.