Page 333 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 322 Wednesday, October 18, 2006 7:01 AM
322 Ship Construction
It is particularly important before painting new steel that any millscale
should be removed. Millscale is a thin layer of iron oxides which forms on
the steel surface during hot rolling of the plates and sections. Not only does the
non-uniform millscale set up corrosion cells as illustrated previously, but it may
also come away from the surface removing any paint film applied over it.
The most common methods employed to prepare steel surfaces for paint-
ing are:
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Blast cleaning
Pickling
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Flame cleaning
Preparation by hand

(a) Blast cleaning is the most efficient method for preparing the surface
and is in common use in all large shipyards. Following the blast cleaning it
is desirable to brush the surface, and apply a coat of priming paint as soon
as possible since the metal is liable to rust rapidly.
There are two main types of blasting equipment available, an impeller
wheel plant where the abrasive is thrown at high velocity against the metal
surface, and a nozzle type where a jet of abrasive impinges on the metal
ึ
surface. The latter type should preferably be fitted with vacuum recovery
equipment, rather than allow the spent abrasive and dust to be discharged
to atmosphere, as is often the case in ship repair work. Impeller wheel
plants which are self-contained and collect the dust and re-circulate the
clean abrasive are generally fitted within the shipbuilding shops.
Cast iron and steel grit, or steel shot which is preferred, may be used for
the abrasive, but non-metallic abrasives are also available. The use of sand
is prohibited in the United Kingdom because the fine dust produced may
cause silicosis.
(b) Pickling involves the immersion of the metal in an acid solution,
usually hydrochloric or sulphuric acid in order to remove the millscale and
rust from the surface. After immersion in these acids the metal will require
a thorough hot water rinse. It is preferable that the treatment is followed by
application of a priming coat.
(c)Using an oxy-acetylene flame the millscale and rust may be removed
from a steel surface. The process does not entirely remove the millscale and
rust, but it can be quite useful for cleaning plates under inclement weather
conditions, the flame drying out the plate.
(d) Hand cleaning by various forms of wire brush is often not very satis-
factory, and would only be used where the millscale has been loosened by
weathering, i.e. exposure to atmosphere over a long period.
Blast cleaning is preferred for best results and economy in shipbuilding, it
is essential prior to application of high performance paint systems used
today. Pickling which also gives good results can be expensive and less