Page 182 - Introduction to Naval Architecture

168 STRENGTH

Table 7.5

If the difference exceeds about 0.25 volts, significant corrosion
of the metal with the higher potential can be expected.
(5) Stress corrosion. The combined action of corrosion and stress can
cause accelerated deterioration of the steel and cracking. The
cracks grow at a negligible rate below a certain stress intensity
depending upon the metal composition and structure, the
environment, temperature and strain rate. Above this threshold
level the rate of crack propagation increases rapidly with stress
intensity. Environment is important. The rate of crack propaga-
tion in normal wet air can be an order of magnitude higher than
in a vacuum.

Protection against corrosion

Protective coatings
Painting can provide protection all the while the paint film is intact. If
it becomes removed in a local area that area can become seriously
pitted. Classification societies specify a comprehensive range of
protective coatings for a ship's structure depending upon the spaces
18
concerned. Typical corrosion rates for different ship types against age
of ship are presented in Figure 7.22.
Cathodic protection
Two methods of protecting a ship's hull are commonly used under the
term cathodic protection. The first, a passive system, uses a sacrificial
anode placed near the area to be protected. Typically this might be a
piece of zinc or magnesium. The corrosion is concentrated on the
anode. A more effective system, an active one, is to impress a current
upon the area concerned, depressing the potential to a value below any