Volume 6 Preprint 32
Red Mud as a Corrosion Inhibitor for Reinforced Concrete
M. Cabeza, A. Collazo; X. R. NÃƒÂ³voa and M. C. PÃƒÂ©rez
Keywords: Red Mud, Corrosion inhibitors, Concrete, Electrochemical Techniques
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Volume 6 Paper C077
RED MUD AS A CORROSION INHIBITOR FOR
M. Cabeza, A. Collazo; X. R. Nóvoa, M. C. Pérez
University of Vigo, E.T.S.E.I, Lagoas – Marcosende, 9. 36200 VIGO.
The effectiveness of red mud as corrosion inhibitor of carbon steel
embedded in concrete was evaluated using cyclic voltammetry and
electrochemical impedance spectroscopy. The testing media were red
mud solutions (pH=12), alkaline solutions having the same pH (NaOH
0.08M and Ca(OH)2 0.04M), and reinforced mortar. In all cases chloride
additions were used for evaluating the inhibition effectiveness. The
results show that red muds are able to maintain steel passivity in
presence of high chloride concentration, i.e., in mortar 3% CaCl2 by
weight of cement, and in solution a ratio Cl-/OH- equal to 5 (maximum
Keywords: Red Mud, Corrosion inhibitors, Concrete, Electrochemical
The alumina plants based on the Bayer Process generate great
amounts of caustic slurries. For each ton of Al2O3 produced almost two
tons of residues have to be stocked . These residues are constituted
mainly by iron oxides, titanium oxide and “bayer sodalite”, an
alumino-silicate, along with caustic soda. The name “Red Mud”, RM,
comes from to their reddish colour due to iron oxides. The huge
volume of RM produced and their alkaline character (pH ≈ 13)
represent an important environmental problem in the areas were these
industries are installed, as it is the case of the ALCOA factory in San
Cibrao (Lugo, N.W of Spain) which generates daily 1200 tons of RM.
So, the problem is of major importance in the area and different
solutions are currently in study to reuse and valorise those wastes .
Sodalites are zeolite-type compounds with an extremely high ion
exchange capacity, which makes RM a good adsorbent of for heavy
metals (as oxyanions)  and influences the surface properties of RM
slurries . Concrete, due to its high pH, protects steel by formation
of a passive film that hinders corrosion. However, in presence of
chlorides, the breakdown of the passive layer occurs and rebars
corrode actively, although the chloride level (Cl- to OH- ratio) depends
on electrode potential . Numerous studies have been carried out to
hinder chlorides depassivation by using inhibitors either inorganic, of
redox/buffering power as nitrites , or organics that block the steel’s
surface . In this context RM can be considered as good candidate
for inhibitor of chloride action because of the proved redox activity
and buffering power [4,8], and complexing properties  that reduce
Cl- to OH- ratio at the metal-concrete interface.
In the present is aimed to analyse the possible use of red mud as
corrosion inhibitor of chloride attack reinforcing steel. The study is
developed in two parts: a first one where the inhibiting properties are
studied in solutions simulating concrete pore solution, and a second
one focused on the behaviour of steel embedded in mortar prepared
with RM additions.
Red Mud slurry was supplied by ALCOA-Europe factory in San Cibrao
(Lugo, Spain). The chemical characterization was made by chemical
analysis of cations and X-Ray diffraction of RM powder. Figure 1
identifying the different iron oxo-hydroxides present (goethite and
hematite) as well as titanium oxide (rutile) and aluminium hidroxides
sodalite. The chemical analysis allows obtaining quantitative results
that, given as upper oxides, are (in %w/w): Fe2O3 (37%), TiO2 (20%),
Al2O3 (12%), CaO (6%), Na2O (5%), H2O (1000 ºC).
Figure 1: X-ray diffraction analysis of red mud powder showing the
identified species present. X-axis corresponds to Bragg’s angle 2θ.
The RM suspensions were prepared by adding 20g of dry powder to
one litre of distilled water. Once vigorously shacked, the overnightdecanted
suspensions have about 30 mg.L-1 of fine particles and their pH is
close to 12. The behaviour of carbon steel in these suspensions was
compared with that in an alkaline solution of similar pH (NaOH 0.08M).
This pH is also close to that found in concrete pore solution after 28
days curing . In order to induce passive film breakdown, chloride (as
NaCl) was added to those solutions up to a upper limit Cl-/OH- =5.
performed in solution using a conventional three-electrode cell, where
the working electrode was carbon steel of standard construction
quality (AISI1023). The same material was employed for preparing
mortar specimens. The exposed surface was 0.53 cm2. A graphite
sheet mesh was used as large area counter electrode, and the
reference electrode was Hg/HgO 0.1M KOH. In the experiments, the
potential was scanned from –0.6V to 0.6V vs. Hg/HgO, 0.1M KOH, at
1mVs-1 scan rate. Just before the voltammetric tests, electrochemical
impedance spectra were registered from 10 kHz to 1 mHz at the
corresponding open circuit potential.
Mortar was prepared using ordinary Portland cement with water to
cement ratio of 0.5, and normalized sand (UNE-EN 196-1:1996), using
three parts of sand by one of cement. The mixed were made with
additions of CaCl2 (1 and 3% by weight of cement) and red mud’s
powder (3% by weight of cement). Mixes were cast in cylindrical
moulds with the rebar (13 cm2 exposed surface) placed along the
rotation axis in order to guarantee proper electric field distribution
specimens were demoulded after one day and stored in humid
chamber until performing measurements (after one month ageing).
An AUTOLAB 30 Potentiostat (from EcoChemie) was used for cyclic
voltammetry and electrochemical impedance measurements.
Results and discussion
a) Carbon steel in NaOH 0.08M and red mud solutions
The voltammograms obtained for the carbon steel in NaOH 0.08M, and
red mud solutions at one day and 25 days of immersion time are
depicted in figure 2a and 2b.
As it can be observed in Figure 2a, the corrosion potential is about
−0.4V in both cases (although slightly lower for RM), and both show
high current values in the theoretical passive domain (0 to 0.5V);
however, the measured current density is higher for the RM system.
The high current observed here can be attributed to the oxidation of
Fe(II) species remaining at the metallic surface after mechanical
cleaning (no electrochemical reduction was performed). However, RM
correspond to oxidation of RM particles adhered to the metallic
substrate. RM particles can be oxidised at those potentials, as
demonstrated elsewhere .
The effect of ageing in both solutions appears in Figure 2. The
measured current decreases by about one order of magnitude
although now the passivity current is lower for the RM system
indicating the formation of a better (more resistive) passive layer, so
hindering RM particles oxidation and oxygen evolution (at about 0.6V).
Current Density / µA cm
Potential / V vs. Hg/HgO
Current Density / µA cm
Potential / V vs. Hg/HgO
Figure 2: Cyclic Voltammograms obtained for carbon steel in unstirred NaOH
0.08M and red mud solutions at one day (a) and 25 days (b) of immersion
time, from –0.6V to 0.6V vs. Hg/HgO at 1mVs-1.
The effect of chlorides is summarised in figure 3. Sodium chloride was
added to both solutions so that Cl-/OH-=5 was reached. The cathodic
domain is similar for both systems, but the corrosion potential
continues being lower for the RM solution. At +75 mV the carbon steel
pits in NaOH 0.08M + 0.4M NaCl while in RM solution (same pH, same
chloride concentration) the anodic current remains steady.
log |i| / µA. cm
Red Mud solution + 0.4 M NaCl
NaOH 0.08M + 0.4 M NaCl
Potential / V vs. Hg/HgO, 0.1M KOH
Figure 3: Cyclic Voltammograms obtained for carbon steel in NaOH 0.08M
and red mud solution with 0.4 mol/l of NaCl added.
Figure 4: Optical images (100X) of carbon steel obtained after
potentiodynamic cycling (figure 3) in NaOH 0.08M + 0.4M NaCl (left)
and Red Mud + 0.4M NaCl solution (right).
Figure 4 shows the aspect of both carbon steel surfaces after
performing the electrochemical test depicted in figure 3. It can be seen
that the sample immersed in NaOH+NaCl solution suffers severe
corrosion while no signs of attack are observed in RM+NaCl solution,
in accordance with electrochemical results.
Red muds maintain the corrosion potential more cathodic than NaOH
solution which, according to reported results  increases the critical
Cl-/OH- due to hydration of the iron oxides layer .
Electrochemical Impedance Spectroscopy
The impedance spectra were recorded at the corresponding rest
potentials as a function of immersion time. Figure 5 and 6 show the
spectra obtained for carbon steel in NaOH 0.08M and red mud
solutions at one day and twenty-five days of immersion. The measured
corrosion potential at one day of immersion was –0.31 V and –0.37 V,
respectively for NaOH and red mud solutions. With immersion time the
rest potentials shift anodically to -0.15 V and -0.20 V.
- Imaginary part
Real Part / k Ω. cm
Figure 5: Nyquist diagrams obtained for carbon steel in NaOH 0.08M
and red mud solution at one day of immersion time.
The observed shift in open circuit potential is associated to an
important variation in the low frequency impedance limit. As figures 5
and 6 show, this limit increases for the RM solution, while it decreases
for the NaOH solution, although both remain in resistance values
typical of passive state . This result agrees with cyclic voltammetry
experiments that show lower current level in RM solution after 25 days
ageing (see figure 2b). The only apparent discrepancy concerns data
presented in figures 2a and 5 (one day ageing). Carbon steel in RM
solution shows higher low frequency limit than in NaOH solution
(figure 5), while the corresponding recorded current is higher (figure
2a). This effect can be understood in terms of capacitive contribution
to the measured current (I=C dE/dt): for the same sweep rate (dE/dt),
the measured current will be higher for higher capacitance, C. From
figure 5, the low frequency capacitance is higher for the RM system, so
the measured current will be higher when the sweep rate approaches
the time constant of the system. Details on this effect have been
reported elsewhere .
- Imaginary part
Real Part/ k Ω. cm
Figure 6: Nyquist diagrams obtained for carbon steel in NaOH 0.08M
and red mud solution at 25 days of immersion time.
The Nyquist impedance spectra obtained for carbon steel in RM and
NaOH solutions with NaCl added are presented in Figures 7 and 8,
respectively for one and twenty five days of immersion. The recorded
OCP were for one day of immersion about –0.45V in NaOH solution
and –0.48V for red mud solution. The obtained impedances are similar
(figure 7) although the low frequency limit is slightly higher for the
RM+NaCl system, the corresponding Rp value gives corrosion currents
(icorr = 26 mV/Rp) one order of magnitude higher than the 0.1 µA.cm-2
Nevertheless, this is the initial situation that can be considered as
normal for the passive layer build up in the initial stages of immersion.
More important is the evolution on immersion time.
0.08M NaOH + 0.4M NaCl
Red Mud + 0.4M NaCl
- Imaginary Part
Real Part/ k Ω . cm
Figure 7: Nyquist diagrams obtained for carbon steel in NaOH 0.08M
and red mud solution with NaCl added, at one day of immersion.
At 25 days of immersion, the potential shifted anodically when the
electrode is immersed in red mud solution (it reaches -0.5 V); however
the potential shift in NaOH solution follows the opposite direction and
reached –0.78V. The corresponding impedances (figure 8) are now
very different: while for the RM+NaCl system the low frequency limit is
very high indicating passive state, in NaOH+NaCl the measured
impedance is very small which can be related to active corrosion. In
fact, plenty of pits were visually observed on the surface, result
consistent with polarisation data given in figure 3.
The problem of whether a shift in the electrode potential below the
magnetite formation peak (toward the cathodic protection domain)
leads or not to active corrosion is still an unresolved problem. If there
is no change in the cathodic reaction and the corrosion potential
decreases only as a consequence of a lack of oxygen, the measured
resistance will be smaller and not corresponding to the corrosion
kinetics but to redox processes ; nevertheless, if the potential shift
occurs because of changes in the cathodic reaction (from oxygen to
proton reduction, formed in the present case as a consequence of
hydrolysis processes inside pits), the measured small resistance will
correspond to corrosion kinetics. Some aspects of this problem have
been discussed elsewhere .
0.08M NaOH + 0.4M NaCl 1 mHz
Red Mud + 0.4M NaCl
- Imaginary Part
7 10 kHz
Real Part / k Ω . cm
Figure 8: Nyquist diagrams obtained for carbon steel in NaOH 0.08M
and red mud solution with NaCl added, at twenty five days of
b) Carbon steel embedded in concrete
In order to compare results in solution with the possible behaviour in
real structures, specimens of reinforced mortar were prepared with
different additions of CaCl2 (1% and 3%) and red mud powder (3%).
Figure 6 and 7 correspond to the voltammograms obtained for the
specimens without additives (reference) and with CaCl2 (1% and 3%)
red mud (3%), after one month aging. No pitting potentials were found,
however, some interesting features can be noticed. In the anodic
sweeps, the voltammograms show a cathodic displacement of
corrosion potential in the specimens with chlorides respect to the
specimen without additives however the specimens with red mud keep
the corrosion potential close to that of the reference specimen. This
fact suggests (according to the result in solution) an activation of the
metal surface in absence of RM. The inhibiting effect of RM is also
noticed in the recorded passivity currents, much higher in presence of
chlorides, but close to the reference values when RM is present.
log |i| / µA cm
With 1% CaCl2
With 1% CaCl2 + 3% Red Mud
Potential / mV
Figure 6: Cyclic voltammograms obtained on rebars embedded in
different mortars: without additives, with 1% CaCl2 and with
1%CaCl2+3% Red Mud, at 30 days ageing. The scan rate was 1mV s-1
With 3% CaCl2
With 3% CaCl2 + 3% Red Mud
log |i| / µA cm
Potential / mV
Figure 7: Cyclic voltammograms obtained on rebars embedded in
different mortars: without additives, with 3% CaCl2 and with
3%CaCl2+3% Red Mud, at 30 days ageing. The scan rate was 1mV s-1.
Non-linear fitting of the polarisation curves to the Butler-Volmer
equation around the corrosion potential allows obtaining corrosion
rate values that, from data in figure 7 result: 0.027, 0.028 and 0.051
µAcm-2, respectively for the reference sample, that with RM + Cl-, and
that with Cl-. All values are, at this age of the samples, below the
referred limit for passive state, but it is clear also from these data that
red muds are able to inhibit the action of chlorides in mortar
The effectiveness of red mud as inhibitor corrosion was examined in
model solutions and in reinforced mortar specimens, with and without
added chlorides, using cyclic voltammetry and EIS.
In the model solution, the results evidence that red muds are good
inhibitors of the chlorides attack. It has been verified that, in presence
of chlorides, the surface of the electrodes immersed in red mud’s
containing solutions do not evidences signs of corrosion, while
generalised pitting was observed in alkaline solution of the same pH
voltammetry and EIS results.
Red muds are also effective corrosion inhibitors of reinforcing steel
embedded in chloride contaminated mortar.
According to the obtained results, it can be said that red muds
constitute a promising way to improve the rebar protection in
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