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Ashworth
This article is a revision of the Third Edition article 10
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Ashworth, volume 2, pp 10:3–10:28, ß 2010 Elsevier B
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References

Historical Background
Electrochemical Principles
Aqueous Corrosion
Cathodic Protection
Oxygen Reduction
Hydrogen Evolution
Methods of Applying Cathodic Protection
Impressed Current Method
Sacrificial Anodes
Proof of Protection
Steel
Other Metals
Steel in Concrete
Potential Measurements
Current Requirements
Coatings and Cathodic Protection
Calcareous Deposit
Potential Attenuation in Impressed-Current Systems
Summary

Abbreviations
AC Alternating current
BS British Standard
DC Direct current
emf Electromotive force
EN European Norm
NACE National Association of Corrosion Engineers
SRB Sulfate-reducing bacteria

Symbols
E Potential
E Equilibrium potential
Ea Anodic potential
Ec Cathodic potential
Ecorr Corrosion potential
I Current
Ia Anodic current
Ic Cathodic current
Icorr Corrosion current
Ilim Limiting current

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2762

IR Ohmic drop (equivalent to a voltage)
R Resistance
h Overpotential

4
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1 Historical Background
In recent years, it has been regarded as somewhat
´
passe to refer to Sir Humphry Davy in a text on
cathodic protection
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In
1824, Davy presented a series of papers to the Royal
Society in London,1 in which he described how zinc
and iron anodes could be used to prevent the corrosion of copper sheathing on the wooden hulls of
British naval vessels
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Several practical
tests were made on vessels in harbor and on seagoing ships, including the effect of various current
densities on the level of protection of the copper
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The first ‘full-hull’ installation on a vessel in service was applied to the frigate HMS Samarang in
1824
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So effective was the system that the prevention of corrosion of the copper resulted in the loss
of the copper ions required to act as a toxicide for
marine growth, leading to increased marine fouling of
the hull
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The beneficial action of the copper ions in
preventing fouling was judged to be more important
than preventing deterioration of the sheathing
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2
It is interesting that the first large-scale application
of cathodic protection by Davy was directed at protecting copper rather than steel
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4
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2 Electrochemical Principles
4
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2
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It is convenient to consider separately the metallic
and nonmetallic reactions in eqn [1]:
2Fe ! 2Fe

2þ

þ 4e

À

O2 þ 2H2 O þ 4eÀ ! 4OHÀ

schematic representation of aqueous corrosion
occurring at a metal surface
...
Equation [3], which represents consumption of electrons and dissolved species in the environment, is termed a cathodic reaction
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Moreover, the metal normally takes up a
more or less uniform electrode potential, often called
the corrosion or mixed potential (Ecorr)
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18
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2

Cathodic Protection

It is possible to envisage what might happen if an
electrical intervention was made in the corrosion
reaction by considering the impact on the anodic
and cathodic reactions
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It follows that the rate of metal consumption
would increase
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This is the basis of cathodic
protection
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The rate of dissolution
O2 + 2H2O

Fe2+

½2Š
½3Š

To balance eqns [2] and [3] in terms of electrical charge, it has been necessary to add four electrons to the right-hand side of eqn [2] and to the
left-hand side of eqn [3]
...

We conclude that corrosion is a chemical reaction [1] occurring by an electrochemical mechanism
(eqns [2] and [3]), that is, by a process involving
electrical and chemical species
...
Note that the electrons released in
the anodic reaction are consumed quantitatively in the
cathodic reaction, and that the anodic and cathodic
products may react to produce Fe(OH)2
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Figure 3
shows the effect of a greater electron supply; corrosion ceases since the external source provides all the
requisite electrons
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Clearly,
this would be a wasteful exercise
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The diagram
O2 + 2H2O

2OH–

Fe2+

2OH–

Environment

Fe

Fe
2e

Metal
From external source

Figure 2 Schematic illustration of partial cathodic
protection of steel in an aerated environment
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O2 + 2H2O

Environment
4OH–
Fe

Fe
Metal

Fe
4e
From external source

Figure 3 Schematic illustration of full cathodic
protection of steel in an aerated environment
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shows how the rates of the anodic and cathodic reactions (both expressed in terms of current flow I) vary
with electrode potential E
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At Ec, the net rate
of the cathodic reaction is zero and it increases as the
potential becomes more negative
...
) The two reaction rates are electrically equivalent at Ecorr, the corrosion potential, and the
corresponding current Icorr is an electrical representation of the rate of the anodic and cathodic reactions
at that potential, that is, the spontaneous corrosion
rate of the metal
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To hold the metal at any potential other than Ecorr
requires that electrons be supplied to, or be withdrawn from, the metal surface
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If the metal is to
be maintained at E 0 , the shortfall of electrons given
by (|I 0 c| – I 0 a) must be supplied from an external
source
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At Ea, the net anodic reaction rate is zero (there
is no metal dissolution) and a cathodic current equal
to I 00 c must be available from the external source to
maintain the metal at this potential
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There
will, however, be a higher interfacial hydroxyl ion
concentration
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3

Oxygen Reduction

In illustrating the corrosion reaction in eqn [1],
the oxygen reduction reaction [3] has been taken
as the cathodic process
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While there is
a range of cathodic reactions that can provoke the
corrosion of a metal (since to be a cathodic reactant,
any particular species must simply act as an oxidizing

2750

Electrochemical Protection

Ec
Fe2+ + 2e

Fe

Icorr

Ecorr

E
EЈ

|IЈc|

IЈa
Ea

O2 + 2H2O + 4e

4OH

|IЈЈc |
log [ I ]
Figure 4 Polarization diagram representing corrosion and cathodic protection
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If the potential is to be lowered to E0 , a current equal to (|I 0 c| – I 0 a) must
be supplied from an external source; the metal will then dissolve at a rate equal to I 0 a
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It is
for this reason that the oxygen reduction reaction has
been emphasized here
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The rate of the
cathodic reaction is governed by the rate at which
electrical charge can be transferred at the metal surface
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Because oxygen is not very soluble in aqueous
solutions ($10 ppm in cool seawater, for example),
it is not freely available at the metal surface
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The
cathodic reaction rate is then controlled by the rate
of arrival of oxygen at the surface
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Since oxygen is
an uncharged species, its rate of arrival is unaffected
by the prevailing electrical field and responds only to
the local oxygen concentration gradient
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e
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Only an increase in oxygen concentration or an
increase in flow velocity will permit an increase in the
limiting value
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Ec
Fe

Fe2+ + 2e

E
Ecorr
EЈ
Ea

IЈa

|IЈc |
ЈЈ
|Ic | = Ilim = Icorr
O2 + 2H2O + 4e

4OH–

log [ i ]
Figure 5 Polarization diagram representing corrosion and
cathodic protection when the cathodic process is under
mass transfer control
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Figure 5 demonstrates that, even when semilogarithmic cathodic kinetics is not observed, partial or total
cathodic protection is possible
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2
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In
practice, this cathodic reaction is barely significant
because the reduction of any oxygen present is both
thermodynamically favored and kinetically easier
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Nevertheless, hydrogen evolution is important
in considering the cathodic protection of steel
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This too is demonstrated in Figure 6
where, it must be remembered, the current supplied
from the external source at any potential must be
sufficient to sustain the total cathodic reaction, that
is, both oxygen reduction and hydrogen evolution
reactions at that potential
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4
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3 Methods of Applying Cathodic
Protection
There are two principal methods of applying
cathodic protection, namely the impressed current technique and the use of sacrificial anodes
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3
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The circuit comprises the power source, an
auxiliary or impressed current electrode, the corrosive
solution, and the structure to be protected
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The structure is thereby cathodically polarized (its potential is lowered), and the positive current returns through the circuit to the power
supply
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The power supply is usually a transformer/rectifier
that converts AC power to DC
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Thus,
fairly substantial driving voltages and currents are
available
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It will be seen that the impressed current electrode discharges positive current, that is, it acts as an
anode in the cell
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The consumable electrodes undergo an anodic reaction that
involves their consumption
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The dotted line shows the total cathodic current
due to oxygen reduction and hydrogen evolution
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2752

Electrochemical Protection

The ferrous ions then enter the environment as a
positive current carrier (although in practice the current will be carried in the environment by aqueous
ions such as Naþ and ClÀ
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In practice, the loss for an iron anode is
approximately 9 kg AÀ1 yearÀ1
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This poses some problems in design because, as the anode dissolves, the
resistance it presents to the circuit increases
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Nonconsumable anodes sustain an anodic reaction
that decomposes the aqueous environment rather
than dissolves the anode metal
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Because these anodes are not consumed faradaically, they should not, in principle, require replacement during the life of the structure
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This is particularly true of the
platinized electrodes because they can operate at
high current density (>100 A mÀ2) without detriment, but will then produce high levels of acidity
(pH < 2) and large volumes of gas
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Although the anodes are described as nonconsumable, they do suffer some loss of the thin
($2
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This loss, which
unfortunately has become known as the wear rate
although there is no question of the loss being due to
mechanical wear, is usually small, related to the total
current passed, and increased if the applied current has
an AC component
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Negative-going current spikes, such as may be
induced by a poorly designed thyristor switching

device, even given otherwise clean DC, can produce a
100-fold increase in the rate of loss
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This is because the
anodic reaction is shared between oxidizing the
anode material (causing consumption) and oxidizing
the environment (with no concomitant loss of metal)
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Table 1 gives a brief summary of the behavior of
some impressed current anodes, and protection by
impressed current is discussed in more detail
elsewhere
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2

Sacrificial Anodes

Using the impressed-current technique, the driving
voltage for the protective current comes from a DC
power source
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No power
source is employed
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Thus, while the impressed-current
anode may be more noble or more base than the
protected structure because the power source forces
it to act as an anode, the sacrificial anode must be
spontaneously anodic to the structure, that is, be
more negative in the galvanic series for the given
environment
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Figure 8 illustrates the use of a sacrificial anode for cathodic protection
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There are a variety of reasons for this,
which includes the need for
1
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a high and reproducible capacity (A h kgÀ1) for the
anode;
3
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freedom from any loss of activity by the anode due
to passivation
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5–10 A mÀ2

Buried or immersed use; consumption (<1 kg AÀ1 yearÀ1); Mo reduces
consumption in seawater
Consumption rate very much less than steel or cast iron (< 1 kg AÀ1
yearÀ1); chloride ions reduce consumption

<50–200 A mÀ2
(in seawater)
<50–500 A mÀ2
(in seawater)
< 1000 A mÀ2
(consumption)

PbO2 film restrains consumption

Consumable:
Scrap iron
Cast iron
Semiconsumable:
Silicon cast iron
(Fe–14Si–(3 Mo)
Graphite
Nonconsumable:
Lead alloys:
1
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Pt-activated
Platinized Ti, Ta, or
Nb

PbO2 film protective
Discontinuities in Pt coat protected by oxide film on subtrate; sensitive
(< 100 Hz) AC ripple in DC or negative current spikes causing
electrode consumption; maximum operating potential with Ti
substrate: 9 V

A more detailed treatment of cathodic protection
by sacrificial anodes is provided elsewhere in this
book
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1
Mn+
Sacrificial
anode
(M)

Protected
structure
Mn+
Positive current
flow

Figure 8 Schematic diagram of cathodic protection using
sacrificial anodes
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is required to keep disadvantageous tramp elements
(notably iron and copper) below defined threshold
levels
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A guide to minimum
quality standards has been produced
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It will be apparent that the
driving voltages that are available from sacrificial
anodes are substantially less than those available
from power sources
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Steel

Figure 4 demonstrates that the rate of dissolution of
iron (or any other metal) decreases as the potential is
made more negative
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Thus Figure 6 shows
that, in the absence of oxygen, the current requirement
would be low, but would increase to a value approximating to the limiting current in its presence
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4 It follows that the current required to prevent
corrosion completely (i
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, in principle to achieve Ea)
will vary according to the environment and the environmental dynamics
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Current supplied is not, therefore, an unequivocal
indication of the effectiveness of protection
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Ag/AgCI/seawater
(V)

Driving voltagea
(V)

Capacity
(Ah/kg)

Al–Zn–Hg
Al–Zn–ln
Al–Zn–ln
Al–Zn–Sn
Znb
Znb
Mg–Al–Zn
Mg–Mn

Seawater
Seawater
Marine sediments
Seawater
Seawater
Marine sediments
Seawater
Seawater

À1
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05
À1
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10
À0
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05
À1
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05
À0
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03
À0
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03
À1
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7

0
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25
0
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30
0
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25
0
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25
0
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23
0
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23
0
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9

2600–2850
2300–2650
1300–2300
925–2600
760–780
750–780
1230
1230

It is often important to control impurities and especially Fe, Cu, Ni, and Si, although a controlled Si concent is essential to some aluminum
alloys
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b
US Military Specification
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Almost without exception, all the accepted criteria for full cathodic protection of iron are based
on a potential measurement
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S
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5–11 The current British
Standard Code of Practice12 gives one
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Only the first three are
useful; the remaining ones are of dubious value or
expressions of pious hope
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85 V with respect to a
Cu/CuSO4 reference electrode
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95 V (versus Cu/CuSO4) is the preferred
protection potential because of the possible presence
of active sulfate-reducing bacteria (SRB)
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0 V
versus Cu/CuSO4 for steels in the 700–800 MPa tensile
strength range)
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The consequence
is that the useful window of potential in which the steel
can operate is severely restricted, especially under
anaerobic conditions
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The standard
equilibrium potential (E  ) for the iron/ferrous-ion is
–0
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Table 3
Cathodic protection criteria: after British
Standard Code of Practice12 and NACE Recommended
Practices5–11
À0
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r
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Negative shift !300 mV when current applied
3
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More negative than beginning of Tafel segment of
cathodic polarisation (E – log I) curve
5
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Polarize all cathodic areas to open circuit potential of
most active anode areas
1
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e
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Thus, Ea = –0
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93 V versus Cu/CuSO4
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85 V)
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85 V (versus Cu/CuSO4), the
ferrous ion concentration then present is sufficient to
permit deposition of the hydroxide ion

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