Page 330 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 319 Wednesday, October 18, 2006 7:01 AM
Corrosion Control and Anti-fouling Systems 319
there is a variety of isocyanate reactions, both one-pack and two-
pack polyurethane paints are available. These paints have many good
properties; toughness, hardness, gloss, abrasion resistance, as
well as chemical and weather resistance. Polyurethanes are not
used under water on steel ships, only on superstructures, etc., but they
are very popular on yachts where their good gloss is appreciated.
(v) Vinyl resins Vinyl resins are obtained by the polymerization of
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organic compounds containing the vinyl group. The solids content
of these paints is low; therefore the dry film is thin, and more
coats are required than for most paints. As vinyl resin paints have
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
poor adhesion to bare steel surfaces they are generally applied
over a pretreatment primer. Vinyl paint systems are among the
most effective for the underwater protection of steel.

(f) Zinc-rich paints Paints containing metallic zinc as a pigment in suffi-
cient quantity to ensure electrical conductivity through the dry paint
film to the steel are capable of protecting the steel cathodically. The
pigment content of the dry paint film should be greater than 90 per
cent, the vehicle being an epoxy resin, chlorinated rubber, or similar
medium.
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CORROSION PROTECTION BY MEANS OF PAINTS It is often assu-
med that all paint coatings prevent attack on the metal covered simply by
excluding the corrosive agency, whether air or water. This is often the main
and sometimes only form of protection; however there are many paints
which afford protection even though they present a porous surface or con-
tain various discontinuities.
For example certain pigments in paints confer protection on steel even
where it is exposed at a discontinuity. If the reactions at the anode and cath-
ode of the corrosion cell which form positive and negative ions respectively,
are inhibited, protection is afforded. Good examples of pigments of this
type are red lead and zinc chromate, red lead being an anodic inhibitor,
and zinc chromate a cathodic inhibitor. A second mode of protection occurs
at gaps where the paint is richly pigmented with a metal anodic to the basis
metal. Zinc dust is a commercially available pigment which fulfils this
requirement for coating steel in a salt water environment. The zinc dust is
the sacrificial anode with respect to the steel.

Anti-fouling Systems

The immersed hull and fittings of a ship at sea, particularly in coastal
waters, are subject to algae, barnacle, mussel and other shellfish growth