Corrosion rate of steel in
concrete—Measurements
beyond the Tafel law
B. Elsener
*Institute for Building Materials, Swiss Federal Institute of Technology (ETH), ETH H€onggerberg, CH-8093 Zurich, Switzerland
Department of Inorganic and Analytical Chemistry, University of Cagliari, Italy Available online 12 September 2005
Abstract
The rapid galvanostatic pulse technique was applied on site on a large number of measuring points with reinforcement varying from severely corroding to passive state. The measurements provide reliable results on corrosion potential, ohmic resistance and polarization resistance in very short time. The overall scatter of the data is not bigger than with any other instrumen-tation used for corrosion rate determination in the field. For actively corroding zones the cur-rent from the counter electrode is self-confined, no guard-ring is needed and the corrosion rate can be calculated on the basis of the reinforcement area under the counter-electrode. For pas-sive zones the calculated ‘‘corrosion rate’’ is overestimated (on a very low level). Corrosion rate calculated from polarization resistance data are always instantaneous values. For engi-neering application (residual service life) the daily and seasonal changes in corrosion rate have to be considered. In the frequent case of chloride induced localized corrosion the local pene-tration rates calculated from Rpdata can vary upto a factor of 5–10 and local penetration
rates of 1 mm/year may occur. This uncertainty on a very high level of corrosion rate is much more important than variations induced by using devices with or without guard ring. 2005 Elsevier Ltd. All rights reserved.
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.06.021
*
Address: Institute for Building Materials, Swiss Federal Institute of Technology (ETH), ETH Ho¨nggerberg, CH-8093 Zurich, Switzerland. Tel.: +41 44 6337291; fax: +41 144 6331147.
E-mail address:[email protected]
Keywords: A. Steel reinforced concrete; B. Galvanostatic; Polarization resistance; C. Pitting corrosion; Non-uniform current distribution
1. Introduction
Corrosion of the reinforcement is the main cause of damage and early failure of reinforced and pre-stressed concrete structures with very high costs for maintenance, restoration and replacement worldwide[1]. Maintenance, planning of the restoration and more recently residual service life calculations of these structures need rapid, non-destructive inspection techniques able to detect corrosion of the rebars at an early stage, to define adequately which areas of structures require repair and provide a quantitative measure of the corrosion rate. The use of half-cell potential measure-ments to determine areas of corrosion risk of reinforcing steel in concrete was pio-neered in the United States and resulted in the development of an ASTM standard (ASTM C 876)[2]. Great progress has been made in the measuring tech-nique, data representation and interpretation [3–5]. Today potential mapping is ‘‘state of the art’’ to locate corroding zones precisely and an international RILEM recommendation has been worked out and published[6]. The extent of any corrosion problem of the structure being investigated can be mapped prior to more detailed and costly examination and repair. Potential readings however do not provide infor-mation on the corrosion rate of the reinforcement.
Quantitative information on the corrosion rate of steel in concrete is of great importance for the evaluation of repair methods in the laboratory, for service life prediction and structural assessment of corroding structures as well as for control of repair work on-site. Such measurements are a challenging research topic since the early 1970. A recent RILEM recommendation ([7]and literature cited therein) gives the basics of the measurements, discusses in detail the specific problems encountered when measuring corrosion rate on large reinforced structures and de-scribes the procedure, documentation, interpretation and use of corrosion rate data. As stated [7], the corrosion rate is determined electrochemically by means of the polarization resistance Rp, the Stern–Geary equation with the proportionality
con-stant B containing the Tafel concon-stants of the anodic and cathodic reactions (see be-low). Several stationary or non-stationary (transient) techniques can be used to determine the polarization resistance Rp. For practical purpose a single measurement
should be rapid (duration less than 10 s), allowing a ‘‘corrosion rate mapping’’ sim-ilar to half-cell potential maps—a requirement that can be fulfilled only by transient techniques such as the galvanostatic pulse technique introduced for field application already in 1988 [8].
This paper presents new results of the application of galvanostatic pulse technique to determine polarization resistance on site and discusses the influence of non-uniform current distribution and localized corrosion attacks on the corrosion rates calculated from Rpdata.
2. From polarization resistance to corrosion rate
Corrosion rate of steel in concrete can be determined by weight loss measurements or (more related to practice) by the reduction in cross-section of the reinforcement. These two techniques present as result an average corrosion rate, meaning that mea-surement times of several years are involved. None of the two techniques is non-destructive. Thus the recommended way[7]to determine the corrosion rate of steel in concrete is by measuring the polarization resistance Rp. It has to be taken into
ac-count that Rpmeasurements give as result an instantaneous corrosion rate, influenced
greatly by climatic changes (temperature, humidity)[9,10]. A comparison with aver-age corrosion rate can be performed only after integrating Rpdata over time.
Polarization resistance Rp is determined as the slope of the polarization curve in
the vicinity of the corrosion potential (Fig. 1). The polarization curve is the sum isof
the anodic (ia) and cathodic (ic) partial reactions, both of them depend exponentially
on the overpotential, the Tafel constant baand bcdefining the slope of the curves.
is¼ iaþ ic¼ icorrf½expðe ecorrÞ=ba þ ½expðecorr eÞ=bcg ð1Þ
Fig. 1. Partial and sum polarization curves of a corroding metal electrode with linear polarization resistance.
Rearranging Eq. (1)and substituting the exponential function expx(x! 0) with x, the Stern–Geary formula[11]results:
is¼ icorr½e ecorr ½1=baþ 1=bc ð2aÞ
Dis=De¼ icorr=B¼ R1p ð2bÞ
icorr¼ B=Rp ð2cÞ
The constant B is related to TafelÕs constants baand bcby the equation
B¼ ðba bcÞ=2:303ðbaþ bcÞ. ð3Þ
Despite a widespread use of the polarization resistance technique in the system steel/ concrete[7], only few data on Tafel slopes for steel in concrete are reported in liter-ature. Solution experiments in neutral to acidic pH show bavalues between 73 and
98 mV/dec[12], in good agreement with 75 mV/dec at pH 1[13]. It should be noted that Tafel slopes decrease with increasing temperature[13].
Cathodic Tafel slopes for the oxygen reduction reaction in neutral and alkaline solutions are found at 230 mV/dec[12,14]. As reported, the oxygen reduction de-creases with ageing of the passive film and the Tafel slope inde-creases[14]. The catho-dic polarization curves for steel in mortar exposed for up to 12 month to 93% RH are shown in Fig. 2b. The cathodic Tafel slope is 200–230 mV/dec[15], it decreases with increasing temperature and increases with time as found for steel in alkaline solutions [13]. The anodic polarization curves of steel in mortar (Fig. 2a) do not show a Tafel behaviour[15].
Calculation of the B value according to Eq. (3)shows (Fig. 3) that the function ‘‘product/sum’’ reacts smoothly on quite large variations in anodic and cathodic Tafel slopes. A value of 26 mV for corroding steel in concrete as reported in early work of Andrade [16] and recommended [7] is a reasonable choice as confirmed by the quite good agreement between Rpand weight loss data[7,16].
3. Polarization resistance from galvanostatic pulse measurements
Galvanostatic pulse method is a transient polarization technique working in the time domain. The technique is based on the usual three-electrode setup(counter, ref-erence and working electrode) and operates with a potentiostat in the galvanostatic mode or a constant current source.
A short time anodic current pulse is imposed galvanostatically on the reinforce-ment from a counter electrode (here with a diameter of 14 cm) placed on the concrete surface. The applied current is usually in the range of 10–100 lA and the typical pulse duration is up to 10 s with a sampling rate of 10 points/s (more details are given in[17]). The reinforcement is polarized in anodic direction compared to its free corrosion potential. The resulting change of the electrochemical potential of the rein-forcement is recorded by a reference electrode (usually in the centre of the counter electrode) as a function of polarization time. Typical potential transient response is shown in Fig. 4for a corroding area (Fig. 4a) and a fully passive area (Fig. 4b).
When the constant current Iappis applied to the system (at t = 0 s), an immediate
ohmic potential jump and subsequently a slight polarization of the rebars occur (Fig. 4). Under the assumption that a simple Randles circuit describes the transient behaviour of the rebars, the potential of the reinforcement, Vt(t), at a given time t
can be expressed as[18]:
VtðtÞ ¼ Iapp½Rp½1 expðt=RpCdlÞ þ RX ð4Þ
where Rpis the polarization resistance, Cdlis the double layer capacitance and RXis
the ohmic resistance.
In order to obtain the values of Rp and Cdl and the ohmic resistance RXEq. (1)
has to be evaluated further based on the experimental values. In our work an
Fig. 2. Polarization curves of reinforcing steel in mortar after exposure for 12 months to 93% relative humidity at different temperatures. (a) Corroding (in chloride contaminated mortar), (b) passive[13].
exponential curve fitting procedure[19]is used. Eq.(4)can be transformed in a form suitable for curve fitting to determine all the relevant parameters:
VtðtÞ ¼ K0 K1expðt=K2Þ ð5Þ
Fig. 3. Plot of the constant B calculated according Eq.(3)versus the cathodic Tafel slope for different anodic Tafel slopes (60–90 mV/dec).
Fig. 4. Raw data of the galvanostatic pulse data (potential versus time) for different applied currents. (a) On actively corroding zone (Ecorr=0.355 VSCE), (b) on a passive zone (Ecorr= +0.06 VSCE).
K0 (IappRp+ IappRX) [mV]
K1 IappRp [mV]
K2 (RpCdl) = time constant s [s]
An example of curve fitting is shown inFig. 5. Extrapolation of the fitted poten-tial response Vt(t) to time zero allows calculating the ohmic resistance RX(Eohm/Iapp),
from extrapolation to infinity (t! 1 according to Eq.(5)) the steady state polari-zation resistance Rp (already corrected for RX) can be determined. Note that this
procedure is equal to a long-term steady state measurement but avoids changing of the pore solution chemistry near the rebars because the charge flowing for a single measurement is very small.
4. Results 4.1. Test-site
The tests were performed on the outside of a reinforced girder of a post-tensioned bridge built in 1965, where at several points corrosion of the reinforcement had started due to leaking salt water from the traffic lane. Several measuring points with different corrosion potential, from corroding zones (0.4 VCSE) to passive zones
(+0.05 VCSE) were used for the measurements. At every measuring point several
gal-vanostatic pulses with currents between 20 and 100 lA were applied. The counter electrode used had a diameter of 14 cm. Concrete cover depth was 30 mm. Temper-ature during the measurements was 6C.
Fig. 5. Results of curve fitting of a potential/time curve obtained by galvanostatic pulse measurements. Dots: original potential versus time curve, line: fitted curve according to Eq.(5), delta: error (difference between original and fit). Eohm: ohmic potential, Ep: polarization.
4.2. Raw data
Typical potential transients measured with the computer-assisted equipment developed at IBWK ETH [17] are shown in Fig. 4. It can be noted that most of the potential change measured by the reference electrode is due to the ohmic poten-tial ‘‘drop’’; the effective polarization of the reinforcement is always below 20 mV. Results obtained from the evaluation of the transients at some of the measuring points are summarized inTable 1. The ohmic resistance, the polarization resistance and the time constant are independent of the applied current. The scatter (standard deviation) of all three values is below 10% of the average, thus the data are very reproducible.
An overview of all data is given inFigs. 6–9and presented briefly:
• The measured polarization resistance Rp(Fig. 6) increases exponentially with the
potential, at potentials Ecorr>50 mV CSE no further increase is found. The
measured Rpvalues remain in a range of 800 ± 200 X (±25%).
• The measured ohmic resistance RX (Fig. 7) increases in a similar way with the
potential, indicating that a more positive potential is associated to a more dry concrete. No upper limit of RXis detected.
• The time constant s (Fig. 8) shows a clear distinction between fully active corrod-ing zones (s 2 s) and the passive zones. The scatter in the results obtained at dif-ferent points and with different currents is quite high (s = 4 ± 1 s) in zones with only slight corrosion or passive rebars. Note that a single point is more reproduc-ible (Table 1).
• The measured ohmic resistance and the measured polarization resistance are pro-portional in a wide range; values are within a factor of ±2 (Fig. 9).
From these results it can be concluded that the galvanostatic pulse technique developed for field application at ETH Zurich provides reliable results on corrosion
Table 1
Results from galvanostatic pulse measurements evaluated according to Eq.(5)
Ecorr(VCSE) Current, Iapp(lA) RX(kX) Rp(kX) t const (s)
Point 1 0.351 20 1.55 0.156 1.78 0.349 50 1.74 0.144 1.94 0.348 70 1.66 0.168 1.72 0.345 100 1.66 0.137 1.85 Point 2 0.192 20 3.45 0.275 3.44 0.183 50 3.56 0.278 3.55 0.176 70 3.51 0.280 3.35 Point 3 0.003 10 9.86 0.75 5.1 0.004 20 9.95 0.67 4.1 0.002 50 10.0 0.73 4.8
Diameter of the counter electrode 14 cm, area polarized 150 cm2
potential, ohmic resistance and polarization resistance in very short time. The overall scatter of the data is not bigger than in any other instrumentation used for corrosion rate determination in the field[7,20,21]. A first distinction between actively corroding steel and passive or slightly corroding steel can be based on the time constant s as proposed recently also by Feliu et al. [22]. These authors report values for s of 1.5 ± 1 s for actively corroding rebars.
Fig. 6. Experimentally measured polarization resistance Rpversus corrosion potential for all measured
points on the bridge girder.
Fig. 7. Experimentally measured ohmic resistance RXversus corrosion potential for all measured points
5. Discussion—From polarization resistance to corrosion rate
In contrast to the laboratory, where homogeneous field distribution between working and counter electrode can be achieved, on real structures the area of the
Fig. 8. Calculated time constant s versus corrosion potential for all measured points on the bridge girder. Note the clear difference of the actively corroding points.
Fig. 9. Measured polarization resistance Rpversus the ohmic resistance RXfor all measured points. Good
counter electrode put on the concrete surface is much smaller than that of the work-ing electrode (reinforcement). Measurements of the polarization resistance Rpand of
the ohmic resistance RXon site are thus influenced by geometrical parameters (cover
depth of the concrete and diameter of the counter electrode of the measuring device) in addition to the concrete resistivity and the corrosion state of the rebars, all gov-erning the current distribution between the CE and the rebars [22–24]. It is thus not surprising (and has been reported also in the SHRP tests[19,20]) that different devices measure different values of RXand Rp.
5.1. Ohmic resistance RX
The ohmic resistance for a given specific concrete resistivity q and concrete cover d depend on the ‘‘cell constant’’ k given by the ratio volume/area, thus the CE size influences directly the value of RX: the larger the area of the CE the smaller will result
the measured ohmic resistance. This has been confirmed in a comparative study of two devices[25]. The ohmic resistance is governed by the primary current distribu-tion[26], thus does not depend on the corrosion state (active or passive) of the rein-forcement. This behaviour can be explained by the fact that measurements of the ohmic resistance are performed with AC impedance at high frequencies[24]or with pulse techniques at short times. In such condition the impedance Z of the rebars below the CE is determined by the double layer capacitance Cdl, the impedance Zc=
1/2pfCdlbeing very small at high frequencies f, thus no current spread-out is
occur-ring. For the measurement of the ohmic resistance RXthe current is self-confined and
a guard ring is not necessary[24].
5.2. Polarization resistance Rpand corrosion rate
For a correct determination of Rp, the compensation of the IR drop is considered
essential[7,24]. Compared to linear polarization resistance (LPR), the galvanostatic pulse technique has the advantage that RXis determined as integral part of the
mea-surement (Fig. 5), thus the Rp values obtained are IR-free.
Measurements of Rp are performed at or near DC conditions (low frequency or
long times). In these conditions a non-uniform current distribution between the small counter electrode (CE) on the concrete surface and the large rebar network results, the electrical signal tends to vanish with increasing distance from the counter elec-trode. As a result, the measured polarization resistance Rp (from Eq. (2)) cannot
be related a priori to the reinforcement under the CE area[7,22–24]. One way in try-ing to overcome the problem of current spread-out is the use of an additional con-centric counter electrode, a guard-ring, to confine the current to the area under the central CE [7,27,28]. The most advanced confinement uses a modulated guard-ring
[29].
Due to the changing conditions on site, both the concrete resistivity and the specific polarization resistance Rp can vary in a wide range: the specific polariza-tion resistance varies from 2 kX cm2 for heavily corroding reinforcement to more then 500 kX cm2 for passive steel in concrete. As practical experience [19],
outdoor-exposure studies [22,30] and simulation calculations [22,31] show, no measuring device can work reliably in all these conditions:
• In the active state (R
p low), a self-confinement of the current occurs[24,31]and
instruments without guard ring measure correct Rp values, thus provide correct
corrosion rates. Instruments with a guard ring show an over-confinement and the Rpvalues measured are upto a factor 3 too high; as a consequence the
cor-rosion rates are strongly underestimated [30,31]. Compared to weight loss data an underestimation of a factor 4–6 was reported for a device with guard ring[30]. • In the passive state (R
pvery high) current spread-out occurs. More accurate values
of the polarization resistance are achieved with the guard-ring confinement[31]. Instruments without guard ring measure Rpvalues that are upto a factor 5 too low.
The experimentally measured Rp data (Table 1, Fig. 6) can now be analysed in
more detail:
• The measured values obtained on points with reinforcement in the active state (Point 1 in Table 1) are correct. To obtain the corrosion rate icorr according
Eq. (2)the measured Rpvalues have to be multiplied by the area of the
reinforce-ment under the CE. For a CE diameter of 12 cm[17]and bar diameters of 16 mm a rebar area under the CE of 60 cm2
can be calculated, the specific polarization resistance at point 1 (Table 1) results to be 9 kX cm2and a instantaneous corro-sion rate icorr of 3 lA cm2 or 30 lm per year can be calculated. The corrosion
rate will be twice as high at 20C and three times higher at 30 C[13]and reaches upto 0.1 mm/year.
• The measured polarization resistance values in the passive state (point 3 inTable 1) are around 0.75 kX (see also other Rp values at potentials >50 mV CSE in
Fig. 6). Assuming the same geometrical area (60 cm2) for the reinforcement polar-ized a specific polarization resistance of 45 kX cm2would result—much too low for passive reinforcement with Rp values determined in the laboratory of 500 kX cm2
. This is due to the spread-out of the current signal, calculations show that the current reaches an area with diameter of 45 cm, thus ±15 cm around the counter electrode. This agrees with calculations of the critical length for current spread-out for passive reinforcement [30]. The non-uniform current distribution results in a unknown area of polarized passive reinforcement-higher specific Rp values of the passive reinforcement lead to higher current spread-out
and thus the measured Rpvalue on passive reinforcement (Rp/polarized area)
can-not exceed a certain value (plateau in Fig. 6). The spread-out tends to diminish with higher concrete resistivity (Fig. 7).
5.3. Engineering application of corrosion rate
Regarding the engineering application of the corrosion rates calculated from Rp
corrosion rates that strictly apply only to the measuring conditions. Exposure con-ditions, especially temperature and concrete humidity can alter icorrin chloride
con-taminated concrete by a factor of upto 10[9,10]. A simple comparison between two similar measurements performed in different days (with different humidity and tem-perature condition) may be highly misleading[22]. Thus for residual live time calcu-lations several Rpmeasurements over a year should be integrated [7]. The effect of
lower temperatures during night (usually no measurements!) has to be considered, too. Second, so far only homogeneous (fully active or fully passive) reinforcements have been considered in the discussion. In the frequent case of chloride induced localized corrosion, the average corrosion rate determined from Rp measurements
underestimate the real, local penetration rates by a factor of 5–10[7,19,24]. Assum-ing localized corrosion at the point with active corrosion (see above), the local pen-etration rate could vary between 0.15 and 0.3 mm/year at temperature of 6C and increase upto 1 mm/year at 30C. From an engineering point of view such high local reduction in cross-section of the reinforcement is very dangerous for the safety of structures when rebars are located in zone of high tensile or shear forces and is much more important than small variations induced by using devices with or without guard ring[22].
6. Conclusions
A rapid galvanostatic pulse technique has been applied on site to determine the polarization resistance at points with varying corrosion state of the reinforcement. The results have shown
1. The galvanostatic pulse technique developed for field application at ETH Zurich provides reproducible results on corrosion potential, ohmic resistance and polar-ization resistance in very short time (10 s). The overall scatter of the data is not bigger than in any other instrumentation used for corrosion rate determination in the field.
2. It has been confirmed that for actively corroding zones the current from the coun-ter electrode is self-confined and no guard-ring is needed. The calculation of the corrosion rate can be performed with the reinforcement area under the counter-electrode. For passive zones the calculated ‘‘corrosion rate’’ is overestimated (on a very low level).
3. The calculated corrosion rates based on polarization resistance measurements are instantaneous values. To obtain a representative average corrosion rate needs information on the changes of exposure conditions (especially temperature and humidity) during daily and seasonal cycles.
4. In the case of chloride induced local corrosion attacks the local penetration rate can be upto 5 or 10 times higher than the calculated corrosion rate. This uncer-tainty is much more relevant for residual lifetime calculations than the compara-tively small variations induced by operating with devices with or without guard ring.
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