If the anodic and cathodic reactions occur at different
points,
local corrosion occurs, i.e. deeper attack at cer-
tain points. The attacks on materials which can be pas-
sivated, such as stainless steel and aluminum are gen-
erally localized. There are different types of local cor-
rosion. The most common types of attack on stainless
steels and aluminum are pitting corrosion and crevice
corrosion.
In addition to these local attacks, attack can be caused
by galvanic corrosion or stray currents. In areas where
rapid water flow occurs, damage cause by cavitation
can also occur.
If we ignore attacks related to material defects, the fol-
lowing types of corrosion can occur:
- General corrosion.
- Pitting.
- Crevice corrosion.
- Galvanic corrosion.
- Stray current corrosion.
- Cavitation.
A brief description of each type of corrosion is given
below.
General corrosion
General corrosion is the most common type of corro-
sion. This results in even attack across all or large parts
of the surface.
In seawater, mild steel and bronze are subject to gen-
eral corrosion, but not stainless steel. In stationary
seawater, the corrosion rate of mild steel is about 0.1
mm/year (0.3 mm/year at the waterline) unless the
steel is protected by cathodic protection. Bronze is ini-
tially attacked at a rate of 0.05 mm/year, but after some
time the corrosion rate falls to a low level, since the
corrosion products (black, brown) have a protective
effect. Green/blue corrosion products are a sign of
higher corrosion rates and that the protective layer has
not been developed.
Aluminum can be subject to a certain amount of gen-
eral corrosion in rapidly flowing water, but not in sta-
tionary water.
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