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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
D.V. RIBEIRO | J.A. LABRINCHA
|
M.R. MORELLI
made of the onset of rebar corrosion, i.e., the moment when the
potential was below -274 mV (the probability of corrosion exceeds
90% for the saturated calomel electrode). Results obtained by
BAUER apud SANTOS [13] show that, in most cases, the evalu-
ation of the corrosion onset period based on corrosion potential
tests is consistent with assessments based on more accurate elec-
trochemical parameters such as corrosion intensity (i
corr
).
Figure 4 presents the results of the corrosion potential measure-
ments. In the first 63 days, the specimens were not subjected
to wetting and drying cycles in NaCl solution until the “safe po-
tential” (-124 mV) was reached, and the tests were interrupt-
ed when the “unsafe potential” (-274 mV) was reached in two
consecutive dry state measurements. The values represent the
average of six measurements taken for each composition. The
error bars were not placed in graphs because they would render
the graphs very confusing and difficult to visualize. However, it
can be stated that the results were highly reproducible, with a
variation of less than 6%.
One of the aspects of the test procedure adopted is that the rebar
corrosion potential varied throughout the test, showing more nega-
tive or more positive values depending on the semi-cycle to which
the specimen was subjected.
The most positive corrosion potential values were recorded after
the drying cycles because, due to the decreasing amount of elec-
trolyte, the concentration of dissolved substances increased. In
fact, according to the Nernst equation, the equilibrium potential in-
creases with the increase in activity, i.e., the increase in the effec-
tive concentrations of oxidized substances. Studies by SANTOS
[13] confirm this behavior and show an inverse correlation between
the corrosion potential and the concrete’s moisture content, indi-
cating that an increase in moisture content implies a decrease in
the measured rebar corrosion potential.
According to the results shown in Figure 4, the reference samples
showed a greater difference between the corrosion potential mea-
Table 1 – Probability of rebar corrosion activity as a function of ranges of corrosion potential
of various reference electrodes in accordance with the ASTM C-876/91 standard
Electrode
Probability of Rebar Corrosion Activity
< 10%
10% - 90%
> 90%
a
NHE
> 0.118 V
(0.118 V) –(-0.032 V)
< -0.032 V
Cu/CuSO ,Cu +
4
2
(ASTM C 876)
> -0.200 V
(-0.200 V) – (-0.350 V)
< -0.350 V
Hg,Hg Cl /KCl
2 2
b
(saturated solution)
> -0.124 V
(-0.124 V) – (-0.274 V)
< -0.274 V
Ag,AgCl/KCl (1M)
> -0.104 V
(-0.104 V) – (-0.254 V)
< -0.254 V
a
Normal Hydrogen Electrode (NHE) standard.
b
Saturated Calomel Electrode (SCE) used in this work.
Table 2 – Chemical composition of red mud estimated by XRF
a
LOI = loss of ignition
Component
Al O
2 3
Fe O
2 3
Na O
2
CaO
SiO
2
K O
2
MnO
TiO
2
Others
a
LOI
Content (wt.%)
19.87
19.85
7.35
4.61
14.34
1.87
0.21
2.66
1.01
27.20
Figure 3 – X-ray diffraction (XRD) pattern
of dry red mud