Page 325 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 314 Wednesday, October 18, 2006 7:01 AM
314 Ship Construction
Internal stresses produced by non-uniform cold working are often more
dangerous than applied stresses. For example, localized corrosion is often
evident at cold flanged brackets.

CORROSION/EROSION Erosion is essentially a mechanical action but
it is associated with electro-chemical corrosion in producing two forms of
metal deterioration. Firstly, in what is known as ‘impingement attack’ the
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action is mainly electro-chemical but it is initiated by erosion. Air bubbles
entrained in the flow of water and striking a metal surface may erode
away any protective film that may be present locally. The eroded surface
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becomes anodic to the surrounding surface and corrosion occurs. This
type of attack can occur in most places where there is water flow, but par-
ticularly where features give rise to turbulent flow. Sea water discharges
from the hull are a particular case, the effects being worse if warm water
is discharged.
Cavitation damage is also associated with a rapidly flowing liquid envi-
ronment. At certain regions in the flow (often associated with a velocity
increase resulting from a contraction of the flow stream) the local pressures
drop below that of the absolute vapour pressure. Vapour cavities, that is
areas of partial vacuum, are formed locally, but when the pressure increases
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clear of this region the vapour cavities collapse or ‘implode’. This collapse
occurs with the release of considerable energy, and if it occurs adjacent to a
metal surface damage results. The damage shows itself as pitting which is
thought to be predominantly due to the effects of the mechanical damage.
However it is also considered that electrochemical action may play some
part in the damage after the initial erosion.

CORROSION ALLOWANCE Plate and section scantlings specified for
ships in the rules of classification societies include corrosion additions to
the thickness generally based on a 25 year service life. The corrosion allow-
ance is based on the concept that corrosion occurs on the exposed surface
of the material at a constant rate, no matter how much material lies behind
it. That is if a plate is 8mm or 80mm thick, corrosion will take place at the
same rate, not at a faster rate in the thicker plate.

Corrosion Control

The control of corrosion may be broadly considered in two forms,
cathodic protection and the application of protective coatings, i.e. paints.

CATHODIC PROTECTION Only where metals are immersed in an
electrolyte can the possible onset of corrosion be prevented by cathodic
protection. The fundamental principle of cathodic protection is that the