Corrosion

1. CSWIP PLANT NSPECTOR LEVEL I WEEK 1
Section 11
An introduction to plant corrosion and degradation
Basic types of corrosion:
1. Chemical corrosion 2.Galvanic corrosion 3. Electrolytic corrosion
Chemical corrosion Galvanic corrosion
Electrolytic
corrosion
Crevice corrosion Stress corrosion
Intergranular
corrosion
Erosion corrosion
This is caused by
attack by chemical
compounds in a
material’s
environment. It is
sometimes referred
to as ‘dry’ corrosion
or oxidation.
This is caused by
two or more
dissimilar metals in
contact in
conducting
electrolyte.
For examples:
1. Stainless steel trim
causes anodic
corrosion on carbon-
steel automobile
bodywork.
2. Defective coating
of tin on carbon –steel
increase the corrosion
rate of steel.
The tendency of a
metal to become
anodic or cathodic is
governed by its
position in the
electrochemical
series. This is only
accurate for pure
metals rather than
metallic compounds
and alloys.
Note that the galvanic
action often happens
on a microscopic
scale.
. It is sometimes
referred to as ‘wet or
electrolytic’
corrosion.
It is similar to the
galvanic corrosion in
that it involves a
potential difference
and an electrolyte
but it does not have
to have dissimilar
materials.
For examples:
1. General pitting
2. Pitting of castings
due to galvanic
action between
different parts of the
crystals.
3. Corrosion of
castings due to grain
boundary corrosion.
This occurs between
close-fitting
surfaces.
Crevice faces where
a metal is restricted
from forming a
protective oxide
layer.
Corrosion normally
propagates in the
form of pitting.
For examples:
1. Corrosion in
crevice in seal welds.
2. corrosion in lap
joints used in
fabricated
components and
vessels.
This is caused by a
combination of
corrosive
environment and
tensile loading.
Cracks resulting in
multiple branching
cracks.
For examples:
1. Failure in stainless
steel pipes and
bellows in a
chlorate-rich
environment.
2. Corrosion of
austenitic stainless
steel pressure
vessels.
This is a form of
local anodic attack at
the grain boundaries
of crystals due to
microscopic
difference in the
metal structure and
composition.
For examples:
1. ‘Dezincification’
of brass in sea water.
2. inter-granular
corrosion ‘weld
decay’
In un-stabilised
stainless steel.
Almost any
corrosion mechanism
is made worse if the
material is subject to
simultaneous
corrosion and
abrasion.
Abrasion removes
the protective
passive film that
forms on the surface
of many metals,
exposing the
underlying metal.
An example is the
so-called ‘island
attack’ n the walls of
pipes containing fast-
flowing fluids and
suspended solids.
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