Kuppan T. Heat Exchanger Design Handbook
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574
Chapter
1
I
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oj‘
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Hayashi,
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An analysis procedure for fixed tubesheet exchangers,
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71.
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72.
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74.
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12
Corrosion
1
BASICS
OF
CORROSION
Most common metals and their alloys are attacked by environments such as the atmosphere,
soil, water, or aqueous solutions. This destruction of metals and alloys is known as corrosion.
It is generally agreed that metals are corroded by
an
electrochemical mechanism. With practi-
cally all commercial processes engineered on a continuous basis of operation, premature failure
from corrosion
of
various types of equipment, including heat exchangers, piping, and others,
may mean costly shut downs and expensive maintenance operations. It is especially trouble-
some
in
oil refining, chemical industries, and electric power plants on land and sea, as well as
in food and liquor processing, paper manufacture, refrigeration, air conditioning, etc. There-
fore,
an
understanding of corrosion principles and corrosion control should be of great interest
to industry and the general public.
1.1
Reasons
for Corrosion
Studies
There are two main reasons for concern about and study of corrosion:
(1)
economics and
(2)
conservation
of
materials. Of these, the economic factors mostly favor study and research into
the mechanisms of corrosion and the means
of
controlling corrosion. Economic reasons for
corrosion study include:
1.
Loss
of efficiency: Corrosion can result in the build up of corrosion products and scale,
which can cause a reduction in heat transfer as well as an increase in the power required
to pump the fluid through the system.
2.
Loss
of product due to leakage: High fuel and energy costs as a result of leakage of
steam, fuel, water, compressed air,
or
process fluid that absorbed energy.
3.
Possible impact on the environment: If the leaking fluid is corrosive in nature,
it
will
attack its surroundings, and if lethal or poisonous, it will create hazards and environmen-
tal problems. Discharge of copper and chromate-treated water is severely regulated to
conserve aquatics and biosphere.
579
580
Chapter
12
4.
Lost production as a result of a failure.
5.
High maintenance costs.
6.
Warranty claims on corroded equipment and the consequent loss of customer confidence,
sales, and reputation.
7.
Contamination and loss of product quality, which can be detrimental to the product,
such
as foodstuffs, soap products, discoloration with dyes, etc.
8.
Extra working capital to carry out maintenance operations and to stock spares to replace
corroded components.
9.
Overdesign: In many instances, when the corrosive effect
of
a system is known, addi-
tional thickness to components is provided for in the design. This is known as corrosion
allowance and involves additional material cost and extra weight of new units.
10.
Highly corrosive fluids may require the use of expensive materials such as titanium,
nickel-base alloys, zirconium, tantalum, copper-nickels, etc. The use of these materials
contributes to increased capital cost.
11.
Damage to adjacent equipment and the system components.
1.2
Corrosion Mechanism
According to electrochemical theory, the combination of anode, cathode, and aqueous solutions
constitutes a small galvanic cell (Fig.
I),
and the corrosion reaction proceeds with a flow of
current in a manner analogous to the way current is generated by chemical action in a primary
cell or in a storage battery on discharge. Due to the electrochemical action, the anode is
dissolved. For a current to flow, a complete electrical circuit is required. In a basic corroding
system as shown in Fig. la, the circuit is made up four components:
Anode
Electrolyte
Cathode
An external circuit
Anode: The anode is the electrode at which oxidation (corrosion) takes place and current
in the form of positively charged metal ions enters the electrolyte. At the anode, the metal
atom loses an electron, oxidizing to an ion.
Electrolyte: The electrolyte is the solution that surrounds, or covers, both the anode and
the cathode. The conductivity of the solution is the key to the speed of the corrosion
process. A solution with low conductivity produces a slow corrosion reaction, while a
solution with high conductivity produces rapid corrosion [l]. In the total absence of an
electrolyte little or no corrosion takes place. For example, iron exposed to dry desert air
remains bright and shiny since water necessary to the rusting process is not available. and
in arctic regions no rusting is observed because ice is a nonconductor
[2].
The electrolyte
need not be liquid. It can be a solid layer also. For example, at elevated temperatures,
corrosion
of
a metal can occur in the absence of water because a thick metal oxide scale
acts as the electrolyte (Fig.
2).
The surface metal oxide is the cathode, and the metal oxide
and metal interface are the anode
[2].
Cathode: The cathode is the electrode at which reduction takes place and current enters
from the electrolyte.
External circuit: If there are two pieces of metal, they must either be in contact or have
an external connection in order for the corrosion to take place. The external circuit
is
a
metallic path between the anode and cathode that completes the circuit. Where the anode
and cathode are on the metal surface, as shown in Fig.
3,
the metal itself acts
as
the
external circuit.
Corrosion
581
Current
t
low
r
-.--.----T
,/
I
a
"'
2H+
+
OH-,
Fe
(OH12
b
Figure
1
Corroding system. (a) Basic corroding system (From Ref.
6);
and
(b)
corrosion of iron in
water (From Ref.
3.)
Basic Corrosion Mechanism
of
Iron in Aerated Aqueous System
The corrosion principle is explained by the basic corrosion mechanism
of
iron in an aerated
aqueous system (Fig lb). In its simplest
form,
this
reaction consists
of
two parts: (1) the
dissolution
of
iron at the anode, and
(2)
cathodic reaction in the absence
of
oxygen is or the
reduction
of
oxygen to
form
hydroxyl ions at the cathode: Hence, the overall theoretical corro-
sion reaction becomes
Fe
=
Fe'2
+
2e-
(1)
tl€
I
AC
Figure
2
Thick metal oxide scale as the electrolyte. [From Roger Pludek,
Design
and
Corrosion
Control,
The Macmillan Press Ltd., London
(1977).]
582
Chapter
12
I:
lectro
lyte
Current
ter
s
Qnode
Cathode
net01
Figure
3
Anode and cathode
on
the same metal surface.
(From
Ref.
6.)
2H’+ 2e-
-+
H2
(2)
1
-
O2
+
H,O
+
2e-= 20H-
(3)
2
Fe
+
-
1
O2
+
H20
=
Fe”
+
20H-
(4)
2
followed by
Fe‘
+
20H-
+
Fe(0H):
(5)
In practice, however, these reactions are much more complex. The rate of corrosion is governed
by the rate of either the cathodic reaction or anodic reaction or less frequently by the electrical
resistance of the electrolyte
[2].
When the anodic reaction is severely limited by films that
form on the metal surface, as in the case of stainless steels, the metal is called passive.
1.3
Forms
of
Electrochemical Corrosion
Various forms of electrochemical corrosion are the (1) bimetallic cell or dissimilar electrode
cell, (2) concentration cell, and
(3)
differential temperature cells.
Bimetallic Cell
In a bimetallic cell, two dissimilar metas are in contact and are immersed in an electrolyte.
The farther the two metals are apart in the electromotive series, the more severe
is
the corro-
sion. Bimetallic corrosion also takes place when a metal is nonhomogeneous. One part of a
metallic structure may be anodic to another part if it is not exactly the same alloy.
A
bimetallic
cell is also known as a dissimilar electrode cell.
Concentration Cell
A
concentration cell is produced in an identical electrode, in contact with an electrolyte of
differing concentration. The area of metal in contact with the dilute solution will be anodic
and it will corrode. It is often called crevice corrosion because a crevice acts as a diffusion
barrier and corrosion occurs most often within a crevice. There are mainly two kinds of concen-
tration cell, as shown in Fig. 4: (1) the salt concentration cell, formed due to differences in
concentration of the salt in the electrolyte (Fig. 4a), and
(2)
the differential aeration cell, where
the oxygen concentration varies on the electrode surface, with the anodic area being the one
Corrosion
583
c
\
nE
TAL
ARfQ
OF
LOW
ION
CONCENTRQTION
0
.ow
0
Figure
4
(a) Salt concentration cell; and
(b)
oxygen concentration cell. (From Ref.
3.)
having lower oxygen concentration (Fig. 4b). Such cells account for localized corrosion of
metals at crevices formed by overlapping joints, threaded connections, or by microorganisms
growing on metal surfaces
[2].
The shielded area tends to be low in dissolved oxygen concen-
tration and hence become anodic to the outside area with higher oxygen concentration.
Differential Temperature Cells
Components
of
these cells are electrodes
of
the same metal, each of which is at a different
temperature, immersed in an electrolyte
of
the same initial composition. The surfaces at higher
temperatures are generally anodic with respect to cooler ones.
1.4
Corrosion Potential and Corrosion Current
The driving force for current and corrosion is the potential developed between the metals.
When no current flows between the anode and cathode, the potential difference between them
is at a maximum known as the open circuit potential. The current that flows between the anode
and cathode when a metal corrodes in an electrolyte is called the corrosion current, and the
net potential
of
the corroding surface is the corrosion potential.