Volume 9 Preprint 10
Pit realkalisation and its role in the electrochemical repair of reinforced concrete
G.K. Glass, N. Davison and A. C. Roberts
Keywords: Corrosion, Chloride, Concrete, Cathodic Protection,<br>Carbonation, Steel, Anode
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Volume 9 Paper 10
Pit Realkalisation and its Role in the
Electrochemical Repair of Reinforced Concrete
FaberMaunsell Beaufort House, Newhall Street, Birmingham B3 1PB,
Department of Civil and Building Engineering, Loughborough
University, Leicestershire, UK, LE11 3TU
A. C. Roberts
Department of Technology, Open University, Walton Hall, Milton
Keynes, Bucks, MK7 6AA, UK.
Open circuit steel passivity is induced by most electrochemical
treatments used to arrest ongoing corrosion of reinforcement in
concrete. This is achieved by the re-alkalisation of the acidic
corrosion sites on the steel and may be induced by a charge density as
low as 60 kC/m2 in moderately aggressive environments. The rapid
restoration of steel passivity is facilitated by using a sacrificial metal
anode in an impressed current role. This allows the delivery of current
densities at low driving voltages with no gas generation and limited
Keywords: Corrosion, Chloride, Concrete, Cathodic Protection,
Carbonation, Steel, Anode.
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science
and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at
http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it
should not normally be referenced in published work. © UMIST 2004.
Corrosion is the most important deterioration process affecting
reinforced concrete. Prior to corrosion initiation, the reinforcing steel
is protected by a passive film. The rate of steel corrosion is negligible
with typically 0.1µm of section loss occurring each year . Corrosion
initiation involves the disruption of a normal passive condition. Its
main causes are chloride contamination and carbonation of the
Many electrochemical treatments have been developed to arrest the
corrosion process . In most cases protection is achieved by the reestablishing steel passivity. This work is concerned with the
mechanism of steel passivity, the magnitude of the treatment required
to restore passivity and the delivery of such treatment.
Corrosion Cell Activity
An analysis of thermodynamic data illustrated in the interpreted
Pourbaix diagram for iron in Figure 1  shows that the oxides that
make up the passive film on iron are thermodynamically stable in the
alkaline environment in concrete. Carbonation may result in the loss of
concrete alkalinity hence rendering the passive film unstable.
However, chloride ions do not directly destabilise the passive film and
insoluble oxides remain the most stable products at alkaline pH values
when chloride ions are present .
Figure 1 Interpretation of the stability of iron, its oxides and its soluble species. The
presence of chloride has no effect on the stability of the iron oxides .
Chloride induced corrosion tends to be localised and the localised
breakdown of the passive film in concrete follows the model of pitting
corrosion. It is a two stage process in which the localised passive film
breakdown, termed pit nucleation, is followed by the establishment of
a sustained corrosion process, termed pit growth . However, most
nucleating pits do not grow. A pit nucleation event is likely to be
followed by the restoration of the passive film.
To establish a sustained corrosion process, pit nucleation must be
accompanied by a local fall in pH at the pit nucleation site. A local fall
in pH arises from the hydrolysis of dissolving iron ions. Dissolving
iron reacts with water to form iron hydroxides and hydrogen ions. The
positively charged hydrogen ions are balanced by the presence of
negatively charge chloride ions producing hydrochloric acid (Figure
2(a)). This stabilizes the local pH reduction and promotes further
dissolution of iron.
Figure 2 An illustration of the processes occurring in a corrosion cell on steel in
concrete (a) together with the potential field in the concrete cover (b) and the
potentials on the steel and concrete surface (c).
Figure 2(a) illustrates the processes occurring in a corrosion cell in
concrete. The dissolution of iron at a corroding site, also termed an
anode, is at least in part supported by the consumption of oxygen
(oxygen reduction) at a location (the cathode) away from the anode.
This process was modelled using a 2 dimensional model that was
governed by a description of the reactions occurring at the anode and
the cathode as well as by ensuring conservation of current within the
concrete . It was assumed that 10 mm long anodes were located at
210 mm centres.
The corrosion potential of the isolated anode was -770 mV (SCE),
while that of the isolated cathode was -140 mV. It should be noted
that, when the anode is connected to the cathode as is normally the
case, these potentials will move towards each other - the reactions are
said to be polarised. The potential contours within the concrete cover
are given in Figure 2(b) and the potential as a function of distance
along the steel surface and along the concrete surface are given in
Figure 2(c). The concrete resistivity was taken as 200 m.
The behaviour of the anodic and cathodic reactions on active steel
used in the model is shown in Figure 3. The combined behaviour,
labelled micro-cell, represents the isolated behaviour of an active steel
surface. These curves were calculated using an anodic Tafel slope (A )
of 60 mV, a migration resistance (Rm ) of 0.3 m2 a cathodic Tafel
slope (c) of 120 mV and a cathodic limiting current (iL ) of 100mA/m2.
The migration resistance is an empirical parameter similar to the
resistance in “pencil anode” models of pit growth or to the approach
resistance described by Evans . On passive steel the anodic reaction
kinetics were limited to 0.1 mA/m2 . For comparison, the behaviour of
active steel measured at a sweep rate of 1mV/min in a 350 kg/m2 OPC
concrete with a water cement ratio of 0.5 contaminated with 3 % cast
in chloride added as NaCl to the mix water is shown in Figure 4.
Figure 3 Theoretical potential-current behaviour of the anodic and cathodic
reactions on active steel used in the model .
Figure 4 Measured potential-current behaviour of steel in concrete contaminated
with 3 % cast in chloride.
The modelled corrosion cell in Figure 2 shows that, while a large
potential gradient exists at the steel, the gradient at the concrete
surface is much smaller. The modelling process also indicated that
the corrosion rate of an isolated active area of steel is likely to be close
to the limiting current for oxygen reduction and it may be increase by
a factor of 3 if it is connected to an adjacent steel cathode. It may be
noted that the potential of the passive steel adjacent to the anode area
is shifted by more than 300mV in the negative direction.
One interesting view of the model in Figure 2 is that it shows an
example of electrochemical protection. The actively corroding steel
area is providing a form of sacrificial cathodic protection to the
adjacent passive steel. However in this view an anomaly exists. The
passive steel closest to the site of active corrosion receives the most
protection as indicated by its large potential shift but this steel
remains at a high corrosion risk because of the lateral growth of the
corroding area. This risk is presented by the acidic nature of the
environment local to the actively corroding site. Hence the damaging
effect of the acidic nature of the environment overrides the substantial
effect of protection current.
In more typical electrochemical treatments applied to steel in concrete
such as cathodic protection and prevention, intermittent cathodic
protection, chloride extraction and re-alkalisation, the anode is
installed at some distance from the steel. The protective effects
include a negative potential shift that inhibits the dissolution of steel
to form positive iron ions (corrosion), the removal of chloride ions
from the steel surface to render the environment less aggressive to
passive steel, and the generation of hydroxyl ions at the steel surface
stabilising the formation of passive films on steel .
These effects are illustrated in Figure 5 which shows a section through
a temporarily installed inert carbon - gel anode and a carbonated
reinforced concrete beam after it had been treated with a high current
short term electrochemical treatment . A universal indicator was
applied to the concrete and anode section. The red colour at the
anode in indicates a pH less than 2, while the purple colour at the steel
indicates a pH greater than 12.
Figure 5 A section through an anode and carbonated reinforced concrete beam after
it had been treated with a high current short term electrochemical treatment .
Traditional understanding of reinforced concrete electrochemical
treatments suggests that different treatments rely on different
protective effects. In this understanding, the basis for cathodic
protection is the achievement of a negative potential shift. Realkalisation of carbonated concrete requires the generation of a
reservoir of hydroxide around the steel. Chloride extraction requires
the removal of chloride ions from the concrete. Intermittent cathodic
protection relies on changing the environment at the steel either by
removing chloride or by generating hydroxyl ions to inhibit steel
corrosion for a short period while the protection current is interrupted.
However this traditional understanding is challenged below.
Analysis of published data
It is postulated in this work that one mechanism is likely to have a
dominant effect on the success of all electrochemical treatments
applied to steel, namely the generation of hydroxide at the steel
interface. The applied charge (current for a given time) raises the pH
at corroding sites such that the local environment naturally supports
steel passivation. The generation of hydroxide at the steel is widely
accepted as the protective effect that is relied on in the application of
re-alkalisation to carbonated concrete. This is a less intensive
treatment than chloride extraction and its application to arrest
chloride induced corrosion would offer some practical advantages.
The importance of the generation of hydroxide at the steel is
supported by the following existing evidence.
The increase in the tolerance to chloride contamination of
reinforced concrete following a limited period of electrochemical
treatment prior to chloride contamination.
The successful arrest of high corrosion rates using relatively low
integrated protection current densities in intermittent cathodic
The widespread successful application of cathodic protection to
chloride contaminated reinforced concrete at relatively low
The hypothesis that short term electrochemical treatments like
chloride extraction result in the generation of a reservoir of hydroxide
at the steel to increase the tolerance to chloride contamination has
previously been presented. An example of the data supporting this is
given in Figure 6 . In the absence of electrochemical treatment, the
chloride content at the depth of the steel that is required to induce
corrosion initiation (the chloride threshold level) is strongly dependent
on the presence of defects at the steel concrete interface  with
high chloride threshold levels being measured at low void contents.
After electrochemical treatment, the effect of voids is largely lost and
the chloride threshold levels are generally high. A similar series of
tests showed the chloride threshold level increasing from between 0.2
to 0.6% by weight of cement for the controls to between 0.9 to 3.8%
following a period of electrochemical treatment . Because the
electrochemical treatment was completed before exposure of the
concrete specimens to a source of chloride ions in these tests, the
hydroxide hypothesis is the only protective effect of those that are
widely accepted that can induce these beneficial changes.
Figure 6 The effect of a limited period of electrochemical treatment prior to chloride
contamination and interfacial voids on tolerance to chloride contamination .
The precipitation of hydroxides at the steel surface to inhibit corrosion
initiation introduces the novel concept of solid phase inhibitors.
Visible evidence of the change in the environment at the steel is shown
in Figure 7 . The white deposit on the treated concrete that is
more intense of the surfaces receiving the most electrochemical
treatment is likely to be the result of a high concentration of sodium
and potassium in these locations. This coupled with the high pH at
the steel after treatment (cf. Figure 5) is likely to produce an alkaline
gel that would provide a reservoir of hydroxyl ions in a sparingly
soluble solid phase at the steel surface. This stabilises the formation
of passive films on steel by preventing the local pH reduction that
accompanies localised corrosion and accelerates the deterioration
Figure 7 Visible evidence of the change in the environment at the steel is shown by a
white deposit that is more intense of the surfaces receiving the most treatment .
A typical chloride extraction treatment requires the delivery of a
current of 1 A/m 2 for 6 weeks. This is equivalent to an applied charge
of 3630 kC/m2. Intermittent cathodic protection, like chloride
extraction, also relies on inducing steel passivity. It however uses a
much lower charge. Figure 8 shows specimens that were subjected to
12 months of intermitted cathodic protection in which the current was
applied for only 4% of the time . The integrated current densities
and initial corrosion rates are included. An analysis of the published
data shows that passivity in one of the intact specimens was induced
with a charge of less than 100 kC/m2 . This is very small compared to
the charge applied in a chloride extraction treatment.
Figure 8 Five specimens that were subjected to 12 months of intermitted cathodic
protection in simulated marine exposure conditions with the current being applied
for only 4% of the time .
Strong evidence of induced steel passivity and the arrest of chloride
induced corrosion arising from the use of a relatively small charge
density comes from an analysis of extensive data obtained in both
field and laboratory cathodic protection studies. Figure 9 shows the
change in the steel corrosion potentials determined on the Tay Road
Bridge before and after the first year of cathodic protection . Similar
data has been reported for many other structures. The negative
potentials initially observed, indicating the presence of areas of
significant corrosion activity, were shifted to substantially more
positive values indicating induced steel passivity. The protection
current was reduced from approximately 10 mA/m2 to 3 mA/m2 over
the period analysed and a charge of less than 200 kC/m2 (more than
an order of magnitude smaller than that required by chloride
extraction) was delivered. This represents the upper limit of the
charge required to induce open circuit passivity in this case.
Figure 9 The change in the steel corrosion potentials on the Tay Road Bridge before
and after a period of cathodic protection .
Steel passivity is likely to be induced after a relatively short period of
cathodic protection. Indeed it has been shown that, in order to
achieve the standard 100 mV potential decay criterion with a practical
protection current, the steel must already be in a near-passive
condition . Figure 10 shows the steel potential decay over 2 hours
determined after various periods of cathodic protection applied in a
heavily chloride contaminated specimen in a laboratory environment
. The substantial potential shift to more positive values of the
current-off potentials again indicates that steel passivity is being
induced. It should be noted that this positive shift in corrosion
potential occurs despite the observation that cathodic protection
drives the steel potential to more negative values to reverse the
process of iron dissolution (corrosion). This implies that the protective
effect of the driven negative potential shift is negligible compared to
the protective effects of changing the environment at the steel to
induce passivity which is accompanied by a positive shift in corrosion
Figure 10 Steel potential decay determined after various periods of cathodic
protection applied to a heavily chloride contaminated specimen .
The change in the corrosion potential observed in Figure 10 indicates
that steel passivity has been restored after less than 50 days. A
current of 20 mA/m2 (the upper limit of the recommended range of
reinforced concrete cathodic protection design current densities)
delivered for 50 days is equivalent to a charge of only 80 kC/m2 on the
steel. Again this is a conservative value and 60 kC/m2 (15mA/m2) may
be a more typical of the practical charge delivered in this period to a
corroding reinforced concrete structure in a moderately aggressive
There is overwhelming evidence that steel passivity is induced using
cathodic protection current densities that are substantially lower than
the localised steel corrosion rates in chloride contaminated concrete
. Average corrosion rates of 0.02 mm steel section loss per year
and localised corrosion rates greater than 0.1 mm per year are not
uncommon in chloride contaminated concrete. These equate to
corrosion current densities of approximately 20 and 100 mA/m2.
However cathodic protection design current densities are nearly always
less than or equal to 20 mA/m2 and applied current densities are
invariably lower than the design current densities (cf. Figure 8).
Two other factors further compound this surprising observation.
Firstly, the applied protection current is very inefficient in reducing the
corrosion rate in concrete exposed to the air. The technical reason for
this is that the cathodic reaction kinetics are weakly polarised in this
environment. To reduce the corrosion rate to a negligible value, the
protection current would need to be many times greater than the
corrosion rate. Secondly a study of current distribution in reinforced
concrete , showed that the current preferentially flows to the more
positive cathodes rather than the corroding anodes of the natural
corrosion cells that are formed in concrete.
In these conditions it is very unlikely that the applied current will result
in the extraction of any chloride from the corroding anodic sites.
Indeed a net anodic current will always be leaving sites of high
corrosion activity at typical cathodic protection current densities (cf.
Figure 2). However re-alkalisation of such sites is possible because a
pH gradient between the surrounding concrete and the corroding sites
will provide an additional force to move hydroxyl ions to these sites.
Furthermore the strong electric fields that maintained these
concentration gradients are weakened by the electric field imposed by
the cathodic protection system. Thus the process of establishing
actively corroding sites on the steel is reversed - the corroding sites
begin to shrink until a point is reached where insoluble iron oxides are
the most stable corrosion product over the whole steel surface and the
passive film is re-established.
Delivery of Rapid Treatments
The above analysis has shown that a charge as low as 60 kC/m2 may
be sufficient to arrest the corrosion process on steel in concrete. It is
preferable to deliver this treatment as rapidly as possible to prevent
further corrosion induced damage. To achieve this, an anode system
capable of delivering a high current density is required. Most
impressed current systems utilise an inert anode with a long life.
However a brief high current treatment may be delivered using a
sacrificial anode metal in an impressed current role.
Figure 11 compares the current delivered of an aluminium anode
embedded in plaster in a hole in concrete, with the current delivered of
an MMO coated titanium anode in the same environment . It may
be noted that the instant off potentials in Figure 11 represent the
potential of the anode reaction that is generating a current density
proportional to the anode reaction rate while the current-on potentials
include a component of the voltage drop through the concrete.
Figure 11 A comparison of the current delivered of an aluminium anode with that
delivered of an MMO coated titanium anode embedded in plaster in concrete .
The aluminium anode delivers more than 10 000 mA/m2 off its surface
at anode potentials that are not sufficiently positive to result in any
significant anode current being driven of the MMO coated titanium
surface. This high current density may be compared with the 110
mA/m2 that MMO coated titanium is normally limited to when
embedded in concrete.
The principal anodic reaction on the sacrificial anode is the dissolution
of the sacrificial metal. This is preferable to the oxidation of water
that occurs on inert impressed current anodes as this latter reaction
results in problems associated with the production of acid and oxygen
gas which attack the concrete and hinder anode performance (cf.
Figure 5). The dissolution of the sacrificial metal produces a metal
salt. The production of gas may be avoided and the only acid that is
produced is the result of the secondary hydrolysis reaction of the
metal salt that is controlled by the equilibrium between the metal salt,
the environment pH and the metal hydroxide.
The above analysis indicates that some substantial advantages may be
obtained by using a sacrificial in this role. These advantages include
very high current densities, low driving voltages, no gas generation
and limited acid generation.
Open circuit steel passivity may be induced by most electrochemical
treatments used to arrest ongoing corrosion of reinforcement in
concrete. This is achieved by the re-alkalisation of the acidic
corrosion sites on the steel.
Open circuit steel passivity may be induced by the application of a
charge density that is less than 100 kC/m2. A charge density as low as
60 kC/m2 may be sufficient to restore passivity in moderately
The rapid restoration of steel passivity may be achieved using a
sacrificial metal anode in an impressed current role. This allows the
delivery of current densities at low driving voltages with no gas
generation and limited acid generation.
‘Corrosion of steel in concrete, understanding, investigation and
repair.’ J.P.Broomfield London, UK: E & FN Spon. 1997.
‘Guide to the maintenance, repair and monitoring of reinforced
concrete structures.’ BRE Centre for Concrete Construction, D53
(DME5.1). Watford, UK: Building Research Establishment. January
‘Thermodynamics and Corrosion’, M.Pourbaix, Corrosion Science ,
30 pp.963-988, 1990.
Encyclopedia of Comprehensive Structural Integrity, Volume 6,
Environmentally Assisted Failure, Eds. I Milne, R Ritchie, B
Karihaloo, (Elsevier Science, Oxford) Chapter 7, pp. 321-350,
‘Protection of reinforcing steel’, G.K.Glass, Application No. GB
0415132.0, 11 January 2005.
‘The Corrosion and Oxidation of Metals’, U.R.Evans, pg.87,
London, Arnold, 1971.
‘An analysis of monitoring data on a reinforced concrete cathodic
Materials Performance 35(2),
‘Anode for impressed current re-alkalization and dechlorination
of reinforced concrete subjected to carbonation attack or
aggressive ion penetration’ J.F.Drewett, Publication No. GB 2 279
664, 11 January 1999.
‘The influence of the steel concrete interface on the risk of
chloride induced corrosion’, G.K.Glass, and B.Reddy, in COST 521:
Corrosion of steel in reinforced concrete structures - Final
Reports of Single Projects, edited by R. Weydert, (Luxembourg
University of Applied Sciences, Luxembourg) pp.227-232, 2002.
10. ‘Mechanism of corrosion protection in reinforced concrete marine
structures’, C.L.Page Nature, 258(5535): pp.514-515, 1975.
11. ‘Criteria for novel electrochemical treatments of steel in concrete’,
, Proceedings of the 7th International Conference on Concrete in
Hot and Aggressive Environments, G.K.Glass, A.C.Roberts and
N.Davison, Volume 2, p.477-492, 13-15 October 2003.
12. ‘Achieving high chloride threshold levels on steel in concrete’,
G.K.Glass, A.C.Roberts and N.Davison, Corrosion 2004, NACE,
Paper No. 04332, March, 2004.
A.M.Hassanein, Journal of Corrosion Science and Engineering,
Volume 4, Paper 7, 2003.
14. “The 100 mV decay cathodic protection criterion”, G.K.Glass,
Corrosion, 55(3), pp.286-290, 1999.
15. ‘Improvements related to the protection of reinforcement’,
G.K.Glass, Application No. GB 0423251.8, 4 May 2006.