Section 3 Corrosion protection

3.1.1 Where bimetallic connections are made, measures are to be incorporated to preclude galvanic corrosion.

3.2.1 Where a primer is used to coat steel after surface preparation and prior to fabrication, the composition of the coating is to be such that it will have no significant deleterious effect on subsequent welding work and that it is compatible with the paints or other coatings subsequently applied.

3.2.2 When unapproved primers are used tests are to be made as detailed in Pt 3, Ch 2, 3.2 Fabrication primers 3.2.3, Pt 3, Ch 2, 3.2 Fabrication primers 3.2.4, Pt 3, Ch 2, 3.2 Fabrication primers 3.2.5 to determine the influence of the primer coating on the characteristics of welds.

3.2.3 Three butt weld assemblies are to be tested using plate material 20 to 25 mm thick. A `V' preparation is to be used and, prior to welding, the surfaces and edges are to be treated as follows:

1. Assembly 1 - Uncoated

2. Assembly 2 - Coated in accordance with manufacturer's instructions

3. Assembly 3 - Coated to a thickness approximately twice the manufacturer's instructions.

3.2.4 Tests as follows are to be taken from each test assembly:

1. Radiographs. These are to have a sensitivity of better than two per cent of the plate thickness under examination, as shown by an image quality indicator.

2. Photo-macrographs. These may be of actual size and are taken from near each end and from the centre of the weld.

3. Face and reverse bend test. The test specimens are to be bent by pressure or hammer blows round a former diameter equal to three times the plate thickness.

4. Impact tests. These are to be carried out at ambient temperature on three Charpy V-notch test specimens prepared in accordance with Ch 2, 3 Impact tests. The specimens are to be notched at the centreline of the weld perpendicular to the plate surface.

3.2.5 The tests are to be carried out in the presence of an LR Surveyor or with the agreement of the Surveyor by an independent laboratory specializing in such work. A copy of the test report is to be submitted together with radiographs and macrographs.

3.3.1 Particular attention is to be given to the location of anodes, where fitted, in relation to the structural arrangements and openings of the tank.

3.3.2 Anodes are to be of approved design and sufficiently rigid to avoid resonance in the anode support. Steel cores are to be fitted, and these are to be designed to retain the anode even when the latter is wasted.

3.3.3 Anodes are to be attached to the structure in such a way that they remain secure both initially and during service. The following methods of attachment would be acceptable:

1. Steel core connected to the structure by continuous welding of adequate section.

2. Steel core bolted to separate supports, provided that a minimum of two bolts with lock nuts are used at each support. The separate supports are to be connected to the structure by continuous welding of adequate section.

3. Approved means of mechanical clamping.

3.3.4 Anodes are to be attached to stiffeners, or may be aligned in way of stiffeners on plane bulkhead plating, but they are not to be attached to the shell. The two ends are not to be attached to separate members which are capable of relative movement.

3.3.5 Where cores or supports are welded to the main structure, they are to be kept clear of the toes of brackets and similar stress raisers. Where they are welded to asymmetrical stiffeners, they are to be connected to the web with the welding kept 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 to the centreline of the face plate but well clear of the free edges. However, it is recommended that anodes are not fitted to face plates of higher tensile steel longitudinals.

3.4.1 Aluminium anodes are not to be located under tank hatches unless protected by adjacent structure.

3.4.2 Magnesium or magnesium alloy anodes are permitted only in tanks intended solely for water ballast.

3.5.1 Suitable protection of the underwater portion of the pontoon and all submerged steelwork is to be provided.

3.5.2 Where an impressed current cathodic protection system is fitted, plans showing the proposed layout of anodes, reference cells and wiring diagrams are to be submitted.

3.5.3 The arrangement for glands, where cables pass through the shell, are to include a small cofferdam.

3.5.4 Where an *IWS (In-water Survey) notation is to be assigned (see Pt 1, Ch 2, 3.4 Other notations 3.4.1), protection of the underwater portion of the pontoon and/or other underwater steelwork is to be provided by means of a suitable high resistant paint applied in accordance with the manufacturer's requirements. Details of the high resistant paint are to be submitted for information.

3.6.1 At the time of new construction, all ballast spaces are to have an efficient protective coating, epoxy or equivalent, applied in accordance with the paint manufacturer's recommendations. The durability of the coatings could affect the frequency of survey of the spaces and light coloured coatings would assist in improving the effectiveness of subsequent surveys. It is therefore recommended that these aspects be taken into account by those agreeing the specification for the coatings and their application.

3.6.2 For further information and recommendations regarding the coating of ballast spaces see the List of Paints, Resins, Reinforcements and Associated Materials published by LR.