Section 2 Cathodic Protection

2.1.1 The cathodic protection system should be capable of polarizing the steelwork to a sufficient level in order to minimize corrosion. This may be achieved using sacrificial anodes with or without an impressed current system.

2.1.2 All parts of the structure should be electrically continuous and where considered necessary, appropriate bonding straps should be fitted.

2.2.1 Sacrificial anodes should be manufactured in accordance with the requirements of this sub-section.

2.2.2 Drawings showing anode nominal dimensions, tolerances and fabrication details are to be submitted for approval prior to commencement of casting.

2.2.3 Approval for the manufacture of anodes is not required although the anodes should be type approved. The works should have a quality management system certified by a recognized third party certification body. However, alternative arrangements may be accepted provided they ensure a consistent quality for the anodes.

2.2.4 The anode materials are to be approved alloys of zinc, aluminium or magnesium with a closed circuit potential of at least minus 1,00 volt (Ag/AgCl sea water reference electrode).

2.2.5 The anode material is to be cast around a steel insert designed so as to retain the anode material even when it is consumed to its design utilization factor. The steel inserts are to have sufficient strength to withstand all external forces that they may normally encounter. The anodes are to be sufficiently rigid to avoid vibration in the anode support. The steel inserts are to be of weldable structural steel bar, section or pipe with a carbon equivalent (CE) not greater than 0,45 per cent determined using the following formula:

2.2.6 Where the steel insert is a welded fabrication, it will be necessary for welding procedures and welders to be approved. Adequate examination of production welds is to be carried out by appropriate non-destructive testing.

2.2.7 The steel insert is to be degreased if necessary and blast cleaned to a standard equivalent to Sa 2,5 of the Swedish Standard SIS 055900, with a minimum surface profile of 50 μm. This standard of cleanliness is to be maintained up until the time of casting. For zinc anodes, blast cleaning may be followed by galvanizing or by an approved zinc plating process.

2.2.8 The chemical composition of the heat is to be determined prior to casting. No alloying additions are to be made following chemical analysis without further analysis. For heats greater than 1 tonne, a further sample is to be analysed at the end of the cast. All anodes cast are to comply with the approved specification.

2.3.1 Electrochemical performance testing is to be carried out by the manufacturer in accordance with previously approved procedures designed to demonstrate batch consistency of the as cast electrochemical properties.

2.4.2 The manufacturer must confirm that the tests have been carried out with satisfactory results in accordance with the approved specification and these Rules.

2.5.1 The location and means of attachment of anodes is to be submitted for approval.

2.5.2 The anodes are to be attached to the structure in such a manner that they remain secure throughout the service life.

2.5.3 The location and attachment of anodes must take account of the stresses in the members concerned. The anode supports may be welded directly to the structure in low stress regions with long fatigue lives provided they are not attached in way of butts, seams, nodes or any other stress raisers. They are not to be attached to separate members which are capable of relative movement.

2.5.4 All welding is to be carried out by qualified welders using a qualified welding procedure.

2.5.5 The welds are to be examined using magnetic particle inspection or other acceptable means of non-destructive testing.

2.5.6 Attachment to studs fired into the structure is not permitted.

2.5.7 Anodes should not be located in positions where mechanical damage may be experienced.

2.5.8 Magnesium anodes should not be used in way of higher tensile steel or coatings which may be damaged by the high negative potentials unless suitable dielectric shields are fitted.

2.6.2 Impressed current anode materials may be of lead-silver alloy or platinum over such substrates as titanium, niobium or tantalum. Alternative materials may also be considered for specific applications.

2.6.3 The design and installation of electrical equipment and cables is to be in accordance with the requirements of Pt 6 Electrical Installations and Control Engineering Systems of these rules.

2.6.4 All equipment is to be suitable for its intended location. Cables to anodes are not to be led through tanks intended for the containment of low flash point fuels.

2.6.5 Cable and insulating material should be resistant to chloride, hydrocarbons and any other chemicals with which they may come in contact.

2.6.6 The electrical connection between the anode cable and the anode body must be watertight and mechanically and electrically sound.

2.6.7 Where impressed current cathodic protection systems are filled (for external hull protection only) and where cables pass though the shell, it is essential for the cables to pass through a small cofferdam.

2.6.9 Suitable dielectric shields should be fitted in order to avoid high negative potentials.

2.7.1 A permanent monitoring system is to be installed on structures protected by an impressed current cathodic protection system.

2.7.2 Zinc sea water or Ag/AgCI reference electrodes should be used.

2.7.3 The location and attachment of the reference electrodes must take account of the stresses in the members concerned and they should not be attached in highly stressed areas or in way of butts, seams, nodes or any other stress raisers.

2.7.4 The location of the reference electrodes should be such as to enable the performance of the cathodic protection system to be adequately monitored.

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

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

2.8.3 Aluminium and aluminium alloy anodes are permitted in tanks that may contain explosive or flammable vapour but only at locations where the potential energy of the anode does not exceed 275J (28kgfm). The weight of the anode is to be taken as the weight at the time of installation, including any inserts and fitting devices. The height is to be taken as the distance from the bottom of the tank to the centre of the anode but exception to this may be given where the anodes are located on wide horizontal surfaces from which they cannot fall. Where the anode is located on, or closely above, a horizontal surface (such as a bulkhead girder) 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 may be measured above that surface.

2.8.4 Magnesium alloy anodes should not be attached to high tensile steel.

2.8.5 Anodes filled internally should preferably be attached to stiffeners, or aligned in way of stiffeners on 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. In the case of stiffeners or girders with symmetrical face plates, the connection may be made to the web or the centreline of the mild steel plate but well clear of the free edges. Where higher tensile steel face plates are fitted the anodes should be attached to the webs.

2.9.1 Potential surveys are to be carried out at agreed intervals on permanently submerged steelwork.

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