Section 2 Sacrificial anodes

2.1.1 Sacrificial anodes intended for installation on units are to be manufactured in accordance with the requirements of this Section.

2.1.2 Plans showing anode nominal dimensions, tolerances and fabrication details are to be submitted for approval prior to manufacture.

2.1.3 Approval for the manufacture of anodes is not required although the anodes should preferably be type approved in accordance with Lloyd’s Register’s (LR’s) List of Type Approval Equipment.

2.1.4 The works should have a quality management system certified by a recognised third-party certification body. However, alternative arrangements may be accepted provided they ensure a consistent quality for the anodes.

2.2.1 The anode materials are to be approved alloys of zinc or aluminium. A closed-circuit potential more negative than –1,00 volt (Ag/AgCl reference electrode) for Zinc anodes and -1,05 volt (Ag/AgCl reference electrode) for Aluminium anodes shall be achieved in seawater at ambient temperature up to 30°C. Magnesium-based anodes may be used for short-term temporary protection of materials not susceptible to hydrogen embrittlement, see also Pt 8, Ch 2, 2.13 Anode installation 2.13.12. Anode materials and anode designs specified in BS EN 13173 or BS EN 12495 are also permitted.

2.3.1 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.

2.3.2 The steel inserts are to have sufficient strength to withstand all external forces that they may normally encounter such as wave, wind, ice loading and operating conditions.

2.3.3 The anodes are to be sufficiently rigid to avoid vibration in the anode support.

2.3.4 The steel inserts are to be of weldable structural steel bar, section or pipe with a carbon equivalent not greater than 0,45 per cent determined using the following formula:

Carbon equivalent,

Rimming steel is not permitted.

2.3.5 Requirements for welded fabrication and non-destructive testing are to be in accordance with Ch 13 Requirements for Welded Construction of the Rules for the Manufacture, Testing and Certification of Materials (hereinafter referred to as the Rules for Materials).

2.3.6 The steel insert is to be degreased if necessary and blast cleaned to a standard equivalent to ISO 8501-1 Sa 21/2 with a minimum surface profile of 50 μm. This standard of cleanliness is to be maintained up until the time of castings. For zinc anodes, blast cleaning may be followed by galvanising or by an approved zinc plating process.

2.4.1 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. Typical chemical compositions for Al-Zn-In type and zinc type anodes which are known to perform well in many conditions are provided below. Other compositions may be used if testing demonstrates that the required electrochemical properties can be achieved. Any testing shall be submitted to LR for approval.

2.5.1 Generally anodes are to be supplied in the as-cast condition although certain aluminium anodes may be heat treated in accordance with the approved specification.

2.5.2 Where heat treatment is carried out it is to be in properly constructed furnaces which are efficiently maintained and have adequate means for the control and recording of temperature. The furnace dimensions are to be such as to allow the whole item to be uniformly heated to the necessary temperature.

2.6.1 The manufacturer is to adopt a system of identification of the anodes to enable the material to be traced back to its original cast.

2.6.3 Where the anodes are heat treated they are also to be marked with the appropriate heat treatment batch number.

2.7.1 All anodes are to be cleaned and adequately prepared for inspection. The surfaces are not to be hammered, peened or treated in any way which may obscure defects. However, any flash or other protrusions should be removed prior to inspection.

2.7.2 Anodes are to be inspected prior to the application of any coating which may be applied to the underside of the anode or to the exposed steelwork.

2.7.3 The surface should be free of any significant slag or dross or anything that may be considered detrimental to the satisfactory performance of the anodes.

2.7.4 Shrinkage depressions should not exceed the smaller of 10 per cent of the nominal depth of the anode or 50 per cent of the depth to the anode insert.

2.7.6 Cold shuts or surface laps should not exceed a depth of 10 mm or extend over a total length equivalent to more than three times the width of the anode. All material is to be completely bonded to the bulk material.

2.8.1 The accuracy and verification of dimensions is the responsibility of the manufacturer unless otherwise agreed.

2.8.2 The diameter of cylindrical anodes should be within ±5 per cent of the nominal diameter.

2.8.4 The maximum deviation from straightness should not exceed two per cent of the length.

2.8.5 The steel insert should be within ±5 per cent of the nominal position in anode width and length and within 10 per cent of the nominal position in depth. Some anodes may have the insert close to one surface, in which case a closer tolerance may be more appropriate.

2.8.6 Except where previously agreed, the anode insert fixing dimensions are to be within ±1 per cent of the nominal dimensions or 15 mm, whichever is the smaller.

2.8.7 Anode nominal dimensions, tolerances and fabrication details are to be shown on manufacturing plans prepared by the manufacturer and submitted for approval, see Pt 8, Ch 1, 3.3 Sacrificial anode systems.

2.9.1 Anodes are to be weighed and individual anodes should be within ±5 per cent of the nominal weight for anodes less than 50 kg or ±3 per cent of the nominal weight for anodes 50 kg and over.

2.9.2 No negative tolerance is permitted on the total contract weight and the positive tolerance should be limited to two per cent of the nominal contract weight.

2.10.1 It will be necessary for the manufacturer to demonstrate that there is a satisfactory bond between anode material and the steel insert and that there are no significant internal defects. This may be carried out by sectioning of an anode selected at random from the batch or by other approved means.

2.10.2 Where sectioning is carried out, at least one anode or at least 0,5 per cent of each production run is to be sectioned transversely at 25 per cent, 33 per cent and 50 per cent of the nominal length of the anode or at other agreed locations for a particular anode design.

2.10.3 The cut surfaces are to be essentially free from slag or dross.

2.10.4 Small isolated gas holes and porosity may be accepted provided their surface area is not greater than two per cent of the section.

2.10.5 No section is to show more than 10 per cent lack of bond between the insert and the anode material.

2.11.1 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.

2.12.1 The manufacturer is to provide copies of the Material Certificate or shipping statement for all acceptable anodes.

2.12.3 The manufacturer is to confirm that the tests have been carried out with satisfactory results in accordance with the approved specification and the Rules.

2.13.1 The location and means of attachment of anodes are to be submitted for approval.

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

2.13.3 Where bracelet anodes are proposed the tightness of the anodes is not to rely on the anode material being in direct contact with the structure.

2.13.4 The location and attachment of anodes are to take account of the stresses in the members concerned. Anodes are not to be directly attached to the shell plating of main hull columns or primary bracings.

2.13.5 The anode supports may be welded directly to the structure in low stress regions provided they are not attached in way of butts, seams, nodes or any stress raisers. They are not to be attached to separate members which are capable of relative movement.

2.13.6 The attachment of all anodes to primary bracing members and nodes is to be submitted for approval. Anodes are not to be welded directly to the structure and the supports are to be welded to small doubler plates which are attached by continuous welds to the structure.

2.13.7 All welding is to be carried out by qualified welders using a qualified welding procedure in accordance with Ch 12 Welding Qualificationsand Ch 13 Requirements for Welded Constructionof the Rules for Materials.

2.13.8 The welds are to be examined using magnetic particle inspection or other acceptable means of non-destructive testing in accordance with Ch 13 Requirements for Welded Construction of the Rules for Materials.

2.13.9 Anodes attached to studs ‘fired’ into the structure are not permitted.

2.13.10 The anodes are to be located on the structure to ensure rapid polarisation of highly stressed areas such as node welds and with due regard to a possible reduction in throwing power in re-entrant angles.

2.13.11 Anodes should not be located in positions where they may be damaged by craft coming alongside.

2.13.12 Magnesium anodes are not to 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, see Pt 8, Ch 2, 2.2 Anode materials 2.2.1.