Volume 4 Paper 7
Surprisingly Effective Cathodic Protection
G. K. Glass and A. M. Hassanein
Keywords: Concrete, Cathodic Protection, Current Distribution, Corrosion Cell, Chloride, Passivity, Reinforcing Steel
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JCSE Volume 4 Paper 7
Submitted 10th February 2003, revised version published 3rd November 2003
Surprisingly Effective Cathodic Protection
G. K. Glass*, A. M. Hassanein#
Department of Civil and Environmental Engineering, ImperialCollege, London, SW7 2BU
* current address: Fosroc International
Ltd., Coleshill Road, Tamworth, B78 3TL, mailto2('gareth_glass','fosroc.com')
# current address: CAPCIS Systems Ltd., Sycamore Court, Witney, Oxford, OX29 6SW, mailto2('alaa.hassanein','capcissystems.com')
§1 This work examines the effect of current
distribution on the basis for cathodic protection of steel in concrete. It is
noted that cathodic protection at integrated current densities that are small
compared to the corrosion rate will induce changes in the local environment at
the steel that promote steel passivity in chloride contaminated concrete. In
conditions characterised by weakly polarised cathodic reaction kinetics,
cathodic protection of steel in concrete ultimately induces anodic polarisation
on actively corroding steel. This renders low current densities surprisingly
effective in achieving protection of actively corroding steel. The
instantaneous protective effects of cathodic polarisation are negligible. The
protection current tends to flow to the more positive cathodes in the
active-passive macro-cells that exist on steel in chloride contaminated
concrete, even when geometry and resistivity might favour current flowing to
the anodes. Such currents reduce the macro-cell activity that sustains the
concentration gradients in the concrete environment necessary for pit growth.
§2 Keywords: Concrete, Cathodic Protection, Current Distribution, Corrosion
Cell, Chloride, Passivity, Reinforcing Steel
§3 Cathodic protection is a widely accepted
repair technique for concrete structures. Its development has largely been
empirical, with the success of early trials on experimental systems resulting
in its widespread acceptance . The design of these systems tends to be based
on previous experience. This work reviews the theoretical basis for reinforced
concrete cathodic protection and examines the effects of current distribution
in the presence of macro-cell corrosion activity on the achievement of
Basis for Protection
§4 The conventional basis for cathodic
protection relies on inducing a negative steel potential shift . This
reduces the tendency for iron to dissolve as positive ions. It is achieved by
polarising the cathodic reaction kinetics, hence the name cathodic protection
. However the conventional understanding of cathodic protection needs to be
extended to include the significant polarisation of the anodic reaction
kinetics on the protected steel surface [3,4]. A cathodic current also results
in a less aggressive environment at the cathode that promotes steel passivity
[5,6]. As a result, a significant reduction in the open circuit corrosion rate
of the steel occurs.
§5 The effect of inducing a reduction in the
open circuit corrosion rate is particularly important when the cathodic
reaction kinetics are weakly polarised as is often the case for steel in
atmospherically exposed concrete. It has been noted that conventional cathodic
protection in conditions characterised by weakly polarised cathodic reaction
kinetics may be rendered uneconomic by a very high protection current
§6 The effects of anodic polarisation on the
protected steel are illustrated in Figs.1 to 5. The data points in Fig. 1 give a typical relationship between the corrosion potential and corrosion rate in reinforced concrete . An increase in the steel corrosion rate is often accompanied by a more negative corrosion potential. This
potential-corrosion rate relationship is observed when the cathodic reaction
occurs relatively easily (the cathodic reaction kinetics are weakly polarised).
An increase in corrosion rate is caused by depolarisation of the anodic
reaction kinetics (the dissolution of iron occurs more easily) . The Evans
diagram illustration of this is included in Fig. 1 (red and blue lines).
§8 Fig. 1 Corrosion potential- corrosion rate
relationship and its Evans diagram explanation.
§9 The visual effect of a range of cathodic
protection current densities applied to reinforced concrete cylinders
containing 2% chloride by weight of cement exposed to simulated sea water for
half an hour twice a day, and to dry circulating laboratory air at other times
is given in Fig. 2 . The protection current was applied only while the specimens where immersed in seawater. The current densities reported in Fig. 2 are integrated current densities.
§11 Fig. 2 Effect of the protection current density
on the condition of 100mm diameter reinforced concrete cylinders containing 2%
§12 At integrated protection current densities
below 5 mA/m2 of steel, corrosion induced cracking occurred although
the time to cracking increased with the application of a protection current. At
higher current densities, corrosion induced cracking was prevented. It was also
observed that the applied protection current densities preventing cracking were
sometimes very low compared to the initial corrosion rates determined on these
specimens. This suggests that protection may be achieved with a cathodic
current that is small compared to the corrosion rate.
§13 The open circuit potentials of the steel in
the specimens in Fig.2, determined 8 hours after the current was last applied,
are given in Fig.3. In cases where corrosion induced cracking was prevented, a
positive shift of more than 200 mV occurred. When corrosion was not arrested,
the potentials remained unchanged or fell to even more negative values .
§15 Fig. 3 Effect of integrated protection current
density on the corrosion potential of the steel (cf. Fig.2) determined 8 hours
after interrupting the current.
§16 A positive shift in the open circuit steel
potential has been reported in many practical reinforced concrete cathodic
protection installations at the locations where the most negative potentials
were initially determined . The initial corrosion potentials as well as the
open circuit potentials determined after 12 months of cathodic protection by
interrupting the current for 24 hours on the piers of a bridge are given in
Fig.4. This shows that the most negative potentials associated with the highest
corrosion risk have moved by at least 150 mV in the positive direction . It
may be noted that relatively low current densities between 3 and 12 mA/m2
were applied in this case.
§18 Fig. 4 Analysis of open circuit potential data
determined 24 hours after interrupting the protection current applied to bridge
§19 The explanation for a 200 mV positive shift
(cf. Fig.3) in the open circuit corrosion potential for a steel bar that is
initially corroding at 30 mA/m2 is illustrated in Fig.5. Such a
positive shift in the open-circuit potential would result from a current
induced change in the environment at the cathode. In this case an open-circuit
corrosion rate of 30 mA/m2 would be reduced to less than 1 mA/m2
§21 Fig. 5 Evans diagram interpretation of the
positive shift in open circuit potentials observed on cathodic protection
§22 Included in Fig.5 is the effect of applying
a relatively high current density (40 mA/m2) on the rate of steel
dissolution when the steel is initially corroding at 30 mA/m2 and
the anodic reaction kinetics remain unchanged. The rate of steel dissolution
reduces from 30 to 15 mA/m2 as the result of the negative potential
shift induced. This is a relatively small reduction and implies that the
effects of a negative potential shift are not that significant.
§23 It may also be noted that it would be
practically difficult to sustain a protection current density of 40 mA/m2.
Reinforced concrete cathodic protection design current densities up to 20mA/m2
are used and the applied current densities will be lower .
§24 Another proposed explanation for the
effectiveness of practical cathodic protection of steel in concrete is that the
corrosion of reinforcing steel is a localised phenomenon and that the
protection current is concentrated at the actively corroding anodes. This
hypothesis might support the more conventional basis for cathodic protection
but it is challenged in this section.
§25 Theoretical and empirical studies have been
undertaken to investigate the distribution of current in reinforced concrete
cathodic protection systems [14,15]. Non-linear cathodic boundary conditions
exhibiting activation controlled anodic and cathodic kinetics at small current
densities and mass transfer controlled cathodic kinetics at large current
densities relate the corrosion rate of the cathode to the corrosion potential
which affects current distribution when the steel is not corroding at a uniform
rate. Changes in the open circuit corrosion rate with time resulting from the
electric field driving a variety of beneficial changes have also been modelled
. Some models are now freely available as public domain software .
Cathodic Kinetics and Geometry
§26 The boundary conditions at the cathode are
mainly determined by the cathodic micro-cell behaviour. The cathodic
polarisation curves measured on concrete specimens containing a range of
chloride contents to produce steel bars corroding at rates between 1 and 16 mA/m2,
are given in Fig.6. After a sufficient negative potential shift has been
achieved (the local cathodic reaction has been increased and anodic reaction
reduced such that the cathodic reaction kinetics dominate the behaviour
observed), the rate of cathodic reduction appeared to be independent of the
initial corrosion rate. This suggests that the cathodic reaction kinetics, described
mainly by the cathodic Tafel slope and exchange current density for oxygen
reduction, are independent of corrosion rate.
§28 Fig. 6 Cathodic polarisation curves determined
on steel in concrete exhibiting a range of initial corrosion rates resulting
from variations in chloride content.
§29 Any dependency of the cathodic reaction
kinetics on the presence of a passive film was masked by the factors that also
render corrosion potential an imperfect indicator of corrosion rate. This suggests
that changes in the cathodic reaction kinetics on passive and active steel
would not significantly favour current distribution to active areas over
§30 The effect of increasing the applied
cathodic current density on the current distribution to bars corroding at
ostensibly the same rate, but embedded at different depths from a surface
applied anode was measured for combinations of bars exposed to the same
chloride contaminated concrete. The concrete specimens consisted of prisms with
a 100 mm square section containing parallel 10mm diameter bars spaced at 110mm
intervals. The specimen length depended on the number of bars in the prism.
An anode was installed on the end of the prism with the first bar being centred
55mm from the anode.
§31 Fig.7 gives an example of the specimen
layout and the effect of the applied current density on the current
distribution between two passive bars. At low current densities (< 5 mA/m2)
very uniform current distribution was achieved. As the applied current increased,
significantly more current started to flow to the bar closest to the anode. It
received 55% and 59% of the total current at 10 and 25 mA/m2
§33 Fig. 7 Effect of the applied current on the
current distribution to passive bars at different depths.
§34 Fig.8 gives the same effect on specimens
containing two active bars. In this particular case the current distribution
was very poor initially, probably as the result of some macro-cell activity.
It improved as the applied current was increased with the bar closest to the
anode receiving 71% and 65% of the total current at 10 and 25 mA/m2
respectively. It was generally noted that at low current densities the
distribution of current was very dependent on the initial corrosion rate with
very good current distribution being achieved on passive steel. At high current
densities the distribution of current was dominated by environment resistivity
and geometry irrespective of whether the steel was passive or not.
§36 Fig. 8 Effect of the applied current on the
current distribution to active bars at different depths.
§37 The explanation for this comes from the
effect of the cathodic reaction kinetics on current distribution. In Fig.9 the
cathodic polarisation behaviour obtained on the “near passive” steel in Fig.6
has been plotted on a linear (as opposed to a log) scale. The slope of the
curve (change in potential over change in current) gives an indication of the
resistance to current flow across the interface presented by the cathodic
reduction reaction. It is evident that, at low rates of cathodic reduction
associated with passive steel and low current densities, the steel-concrete
interface exhibits a high resistance to current flow. This has a beneficial
effect on current distribution.
§39 Fig. 9 Illustration of the effects of the rate
of cathodic reduction (current) on the resistance to cathodic reduction (slope
of potential vs. current) (cf. Fig.6).
§40 As the rate of cathodic reduction increases
an apparent reduction in the resistance at the interface occurs. The
resistivity of the environment and the geometry of the system then dominate
current distribution. A high rate of cathodic reduction may be induced by
either a high corrosion rate or a large applied current. Thus, inferior current
distribution may occur even when all the steel is passive if a large current
density is applied, although a small current density will always give rise to a
large potential shift on passive steel.
Macro-Cell Corrosion Activity
§41 The distribution of current in reinforced
concrete cathodic protection systems is complicated by the existence of
active-passive macro-corrosion cells on the steel. Not only do different areas
of the steel exhibit different corrosion rates but they also have different
corrosion potentials (cf. Fig.1). The polarization curves measured on active
and passive steel (cf. Fig.6) suggest that the current required to shift the
potential of passive steel to that of active steel would be of the same order
as the active steel corrosion rate.
§42 The effect of the applied current density
on the distribution of current in an active-passive galvanic couple is given in
Fig.10. It is evident that, although geometry and resistivity favour current
flowing to the corroding steel, the only effect of the applied current is to
overcome the galvanic effects of the active-passive couple at low current
densities. It was generally observed that, when a cathodic protection current
density below the open circuit corrosion rate of the active steel in an active
- passive galvanic couple is applied, the current flows to the more positive
passive sites .
§43 This contradicts the idea that current will
flow to the active areas of the steel. The effect of the protection current is
to reduce the galvanic current in the active-passive couple. This supports a
pH gradient in the local environment necessary to sustain localised corrosion
activity. The oxides that form the passive film on steel in concrete are
thermodynamically stable in the high pH environment in concrete, even when
chloride ions are present. The removal of the galvanic current arising from an
active-passive couple allows the pH at the active anode to rise to that in the
§45 Fig. 10 Effect of the applied current on the
current distribution to bars at different depths in an active-passive galvanic
§46 As the applied current density increases
above the open circuit corrosion rate of the active sites and the protection
potential falls below the open circuit active corrosion potential, the
distribution of current is increasingly dominated by the resistivity and
geometry of the environment and the flow of current to actively corroding sites
may be favoured by a lower local environment resistivity. However the high
localised corrosion rates of active steel (typically up to 100 mA/m2
or 100 µm/yr) may render a favourable distribution of current difficult to
achieve. The absence of evidence suggesting that current will preferentially flow
to actively corroding sites at low applied current densities supports the
hypothesis that the negative potential shift achieved in cathodic protection of
steel in concrete has a negligible effect in arresting active corrosion.
§47 The success of the widespread use of low
current densities in the cathodic protection of steel in concrete results from
the current induced improvements in the environment at the cathode. This
includes the removal of chloride and the continuous production of hydroxyl ions
at the steel as a result of the cathodic reduction reaction. A source of
hydroxyl ions will sustain a high pH environment at the steel and prevent the
local reduction in pH that is necessary to sustain pit growth . The
experimental data given above (cf. Fig.2) suggests that low current densities
are surprisingly effective at arresting corrosion of steel in reinforced
§48 Cathodic protection at integrated current
densities that are small compared to the corrosion rate will induce changes in the
local environment at the steel that promote steel passivity in chloride
§49 A basis for the cathodic protection of
steel in concrete in conditions characterised by weakly polarised cathodic
reaction kinetics is that it polarises the anodic reaction kinetics on actively
§50 The instantaneous protective effects of a
negative steel potential shift induced by a relatively small practical current
density may be neglected in concrete where oxygen has easy access to the steel.
In other words, the effects of cathodic polarisation induced by cathodic
protection of steel in atmospherically exposed concrete are negligible.
§51 The protection current tends to flow to the
more positive cathodes in the active-passive macro-cells that exist on steel in
chloride contaminated concrete, even when geometry and resistivity might favour
current flowing to the anodes. Low protection current densities reduce the
corrosion cell activity that sustains the concentration gradient in the
concrete environment necessary for pit growth.
§52 This work was undertaken in the Concrete
Durability Group at Imperial College. The authors would like to thank the
Engineering and Physical Sciences Research Council for funding the research
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