Page 328 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 317 Wednesday, October 18, 2006 7:01 AM
Corrosion Control and Anti-fouling Systems 317
A similar impressed current system employs a consumable anode in the
form of an aluminium wire up to 45 metres long which is trailed behind the
ship whilst at sea. No protection is provided in port.
Although the initial cost is high, these systems are claimed to be more
flexible, to have a longer life, to reduce significantly hull maintenance, and
to weigh less than the sacrificial anode systems.
Care is required in their use in port alongside ships or other unprotected
--- ใช้เพื่อการศึกษาเท่านั้น---
steel structures.
PROTECTIVE COATINGS (PAINTS) Paints intended to protect against
film. The drying may take place through one of the following processes. ี
corrosion consist of pigment dispersed in a liquid referred to as the ‘vehi-
งานห้องสมุด ศูนย์ฝกพาณิชย์นาว
cle’. When spread out thinly the vehicle changes in time to an adherent dry
(a) When the vehicle consists of solid resinous material dissolved in a
volatile solvent, the latter evaporates after application of the paint, leaving
a dry film.
(b) A liquid like linseed oil as a constituent of the vehicle may produce a
dry paint film by reacting chemically with the surrounding air.
(c) A chemical reaction may occur between the constituents of the vehi-
cle after application, to produce a dry paint film. The reactive ingredients
ึ
may be separated in two containers (‘two-pack paints’) and mixed before
application. Alternatively ingredients which only react at higher temperatures
may be selected, or the reactants may be diluted with a solvent so that the
reaction occurs only slowly in the can.
Corrosion-inhibiting paints for application to steel have the following
vehicle types:

(a) Bitumen or pitch Simple solutions of bitumen or pitch are available
in solvent naphtha or white spirit. The bitumen or pitch may also be
blended by heat with other materials to form a vehicle.
(b) Oil based These consist mainly of vegetable drying oils, such as linseed
oil and tung oil. To accelerate the drying by the natural reaction with
oxygen, driers are added.
(c) Oleo-resinous The vehicle incorporates natural or artificial resins
into drying oils. Some of these resins may react with the oil to give a
faster drying vehicle. Other resins do not react with the oil but heat is
applied to dissolve the resin and cause the oil to body.
(d) Alkyd resin These vehicles provide a further improvement in the
drying time and film forming properties of drying oils. The name
alkyd arises from the ingredients, alcohols and acids. Alkyds need
not be made from oil, as an oil-fatty acid or an oil-free acid may be
used.
(Note. Vehicle types (b) and (d) are not suitable for underwater
service, and only certain kinds of (c) are suitable for such service.)