Section
3 Corrosion protection
3.1 General
3.1.1 All steelwork is to be suitably protected against corrosion by a suitable protective
coating. All coatings are to be in accordance with the requirements of this Section.
3.1.5 Details and recommendations regarding protective coatings are given in LR’s List
of Paints, Resins, Reinforcements and Associated Materials.
3.1.7 Steelwork is to be suitably cleaned and cleared of millscale before the application
of any coating.
3.1.8 Paints or other coatings are to be suitable for the intended purpose in the locations
where they are to be used. Coatings are to be of adequate film thickness, applied in
accordance with the paint manufacturer’s specification.
3.1.9 The paint or coating applied is to be compatible with any previously applied primer.
3.1.10 Paint containing more than 10 per cent aluminum by weight in the dry film should not,
in general, be used in positions where oil or fuel vapours can accumulate, unless it
has been shown by appropriate tests that the paint to be used does not increase the
incendive sparking hazard.
3.1.11 Paints, varnishes and similar preparations having a nitrocellulose or
other highly flammable base are not to be used in accommodation or machinery spaces.
3.2 Prefabrication primers
3.3 External hull protection
3.3.1 External hull surfaces are to be protected from corrosion by the use of appropriate
anti-corrosive measures, which can include the application of suitable coating
systems to all external surfaces.
3.3.4 All equipment is to be suitable for its intended location.
3.3.5 The arrangements for glands, where cables pass through the shell, are to include a
small cofferdam.
3.3.6 Cables which pass through ballast tanks are to be enclosed in a steel tube of at
least 10 mm thickness.
3.3.7 Cables to anodes are not to pass through tanks intended for the storage of low flash
point products; including, but not limited to, oils.
3.3.8 Cables which pass through 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,
see also
Vol 2, Pt 9, Ch 5 Hazardous Areas.
3.4 Internal hull protection
3.4.2 When a cathodic protection system is to be fitted in tanks for the carriage of liquid
with flash point not exceeding 60°C, a plan showing details of the locations and
attachment of anodes is to be submitted. The arrangements will be considered for
safety against fire and explosion aspects only. Impressed current cathodic
protection systems are not permitted in any tank.
3.4.3 Particular attention is to be given to the locations of anodes in relation to the
structural arrangements and openings of the tank.
3.4.4 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 so designed as to
retain the anode even when the latter is wasted.
3.4.5 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 are
acceptable:
- Steel core connected to the structure by continuous welding
of adequate section.
- 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.
- Approved means of mechanical clamping.
3.4.6 Anodes are to be attached to stiffeners, or are to 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.
3.4.7 When locating anodes, care is to be taken to ensure that they do not
cause a stress concentration. 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 shall
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 not be fitted to face plates of higher
tensile steel longitudinals.
3.5 Aluminium and magnesium anodes
3.5.1 Aluminium and aluminium alloy anodes are permitted in tanks which can contain
explosive or flammable vapour, or in tanks adjacent to tanks which can 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 a
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.
3.5.2 Aluminium anodes are not to be located under tank hatches unless
protected by adjacent structure.
3.5.3 Magnesium and magnesium alloy anodes are not permitted in tanks which can contain
explosive or flammable vapour, or in tanks adjacent to tanks which can contain
explosive or flammable vapour. Where permitted for other tanks, adequate venting
must be provided.
3.6 Bimetallic connections
3.6.1 The design shall ensure that the location of all bimetallic connections allows for
regular inspection and maintenance of the joints and penetrations during service.
3.6.2 Where bimetallic connections are made involving dissimilar metals, measures are to be
incorporated to mitigate galvanic corrosion.
3.6.3 Special attention is to be given to the penetrations of and connections to the hull,
bulkheads and decks by piping and equipment where dissimilar materials are involved.
3.7 Deck coverings
3.7.1 The steel deck is to be coated with a suitable material in order to prevent corrosive
action, and the sheathing or composition is to be effectively secured to the
deck.
3.7.2 Deck coverings within accommodation spaces, control stations, stairways and
passageways are to be of a type which will not readily ignite or cause smoke or
toxic or explosive hazards at elevated temperatures in accordance with the
requirements of the International Code for the Application of Fire Test
Procedures or Naval Administration.
3.7.3 Where plated decks are sheathed with wood or an approved composition, reductions in
plate thickness may be allowed.
3.8 Corrosion margin
3.8.1 The scantlings determined from the formulae provided in the Rules assume that the
materials used are selected, manufactured and protected in such a way that there is
negligible loss in strength due to corrosion.
3.8.2 Where steel is not protected against corrosion by painting or other approved means,
the scantlings may require to be further considered.
3.8.4 It is the responsibility of the Owner to specify corrosion margins which meet their
operational requirements. Where the Owner does not specify corrosion margins, it is
recommended that corrosion margins be applied to net scantlings calculated by these
Rules in areas of the ship’s structure where corrosion or wear is expected to occur
in service. The following recommendations are offered as guidance:
- +0,5 mm all hull envelope plating below a line, 1,0 m above the design
waterline
- + 0,5 mm of all plating forming the boundary of tanks
- + 0,5 mm plating forming the lower boundary of wet spaces as appropriate
- +2,0 mm to the keel plate
- + 1,5 mm to flight deck plating.
Consideration should also be given to the addition of a corrosion margin in the
following areas:
- the lower portion of stiffeners located within wet spaces;
- stiffeners located within tanks;
- all tanks containing corrosive fluids and areas where
spillage of corrosive fluids could occur;
- the base of mast structures;
- all uncoated structures.
|