Page 334 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 323 Wednesday, October 18, 2006 7:01 AM
Corrosion Control and Anti-fouling Systems 323
applicable to production schemes; flame cleaning is much less effective;
and hand cleaning gives the worst results.

TEMPORARY PAINT PROTECTION DURING BUILDING After the
steel is blast cleaned it may be several months before it is built into the ship
and finally painted. It is desirable to protect the material against rusting in
this period as the final paint will offer the best protection when applied
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over perfectly clean steel.
The formulation of a prefabrication primer for immediate application
after blasting must meet a number of requirements. It should dry rapidly to
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
permit handling of the plates within a few minutes and working the plates
within a day or so. It should be non-toxic, and it should not produce harmful
porosity in welds nor give off obnoxious fumes during welding or cutting.
It must also be compatible with any subsequent paint finishes to be applied.
Satisfactory formulations are available, for example a primer consisting of
zinc dust in an epoxy resin.

PAINT SYSTEMS ON SHIPS The paint system applied to any part of a
ship will be dictated by the environment to which that part of the structure
is exposed. Traditionally the painting of the external ship structure was
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divided into three regions.

(a) Below the water-line where the plates are continually immersed in
sea water.
(b) The water-line or boot topping region where immersion is intermit-
tent and a lot of abrasion occurs.
(c) The topsides and superstructure exposed to an atmosphere laden
with salt spray, and subject to damage through cargo handling.

However now that tougher paints are used for the ship’s bottom the dis-
tinction between regions need not be so well defined, one scheme covering
the bottom and water-line regions.
Internally by far the greatest problem is the provision of coatings for var-
ious liquid cargo and salt water ballast tanks.

(a) Below the Water-line The ship’s bottom has priming coats of corrosion-
inhibiting paint applied which are followed by an anti-fouling paint. Paints
used for steels immersed in sea water are required to resist alkaline conditions.
The reason for this is that an iron alloy immersed in a sodium chloride solu-
tion having the necessary supply of dissolved oxygen gives rise to corrosion
cells with caustic soda produced at the cathodes. Further the paint should
have a good electrical resistance so that the flow of corrosion currents
between the steel and sea water is limited. These requirements make the
standard non-marine structural steel primer red lead in linseed oil unsuitable