An Algorithm of Galvanostatic Pulse Method to Determine the Corrosion Status of Reinforcement in...

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UNIVERSITY OF GAZIANTEP FACULTY OF ENGINEERING CIVIL DEPARTMENT

CE-547

Corrosion of Plain &Reinforced concrete

Report #5 About :

(An article research about galvanostatic pulse method to determine the

corrosion status of reinforcement in concrete )

Submitted to: Do.Dr. Mehmet GESOLU

Prepared by:

Chalak Ahmed Mohammed [email protected]

2014 45056 Date : 13.04. 2015

Corrosion Science 42 (2000) 675686

An algorithm of galvanostatic pulse method to determine the corrosion status of

reinforcement in concrete

Cui Lu, Yan Peiyu*

Depavtment of Ci1il Fngineeving, Zzinghua Uni1evzit5, Beijing 100084, Peoplez Republic of China

Received 22 December 1998; accepted 12 August 1999

Abstrast

An algorithm was developed to Fourier-transform the measuring data of galvanostatic

pulse method (GPM) from the time domain into the frequency domain. The polarization

resistance and ohmic resistance are determined in the time domain; the impedance spectrum

and disperse parameter are obtained in the frequency domain to characterize the corrosion

status of reinforcement in concrete. Based on the investigation upon the diRerent corrosion

status of rebars in solution and in concrete, the criterions of depassivation are

discussed. g 2000 Elsevier Science Ltd. All rights reserved.

Ke5wovdz Rebar; Concrete; Galvanostatic; Time domain; Frequency domain

1. Introdustion

The corrosion status of steel rebar in concrete is commonly examined by several

electrochemical techniques [1], such as linear polarization response (LPR) and

electrochemical impedance spectrometry (EIS). Linear polarization response is a

relatively mature method which can be achieved galvanostatically [2]. LPR has

been used extensively on site and its data are processed normally in time domain.

The LPR results can reveal only limited information about steel corrosion in

* Corresponding author. Tel. +86-1062785836; fax +86-1062771132.

F-mail addvezz [email protected] (Y. Peiyu).

0010-938X]00]$ - see front matter g 2000 Elsevier Science Ltd. All rights reserved.

PII S 0 01 0 - 9 38 X ( 9 9 ) 00 1 02 - X

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676 C. Su, Y. Pei5u ] Covvozion Science 42 (2000) 675686

concrete and can not distinguish the contributions of various processes in the

whole sample system, e.g. charge-transfer resistance, concentration polarization

and ohmic resistance. EIS is a traditional measuring method in frequency domain

and a powerful tool for identification of corrosion mechanism [3]. Repeated

perturbations upon the system and long measuring time in low frequency range

may result in change of the corrosion status and error to the outcome. EIS

measurement needs more sophisticated instruments. Therefore, EIS is mainly used

for the researching work in laboratory. Owing to the distinct advantages of the

two methods, both of them have been developed, respectively. However, the

diRerence between the time domain and frequency domain makes great diAculties

to compare and inoculate the experience results of EIS and LPR.

Besides the steady-state measuring methods, some non-steady-state measuring

methods are also developed to determine the corrosion status of rebar in concrete,

such as galvanostatic pulse method (GPM) [48] and coulostatic transient method [9]. Since the extensive review of the electrochemical technique by Gonzalez in

1985 [10,11], transient methods have made a big progress. Among them, GPM

gains special attention. Bohni used GPM successfully on site [5]. He simplified the

reinforced concrete system to a modified Randles equivalent circuit consisting of

an ohmic resistance in series with a parallel combination of the double-layer

capacitance Cdl and the linear polarization resistance Rp. Newton [6] suggests a

complex model of multi-series RC elements to depict the system. He fitted the

potential response curve by a multi-exponential function

AV = Iapp

. n

.

R0 ..

Ri Riet/Ri Ci .

i=1

(l)

However, in some cases, especially when the steel is badly corroded, too many RC

elements can be found to give them appropriate physical meanings. A complex

analysis tends to be subjective and sometimes does not result in a unique

interpretation of measured data. Millard [7] used the similar program of Newtons to characterize the corrosion rate of rebar in concrete.

The data of transient measure are analyzed mainly in time domain at present,

which limit the practical information obtained in the experiments. As EIS shows

that the resistances and capacities of the steelconcrete interface equivalent circuit are frequency dependent, precise analysis about the corrosion mechanism in time

domain will become diAcult. It is well known that the impedance spectrum may

be obtained by Laplace or Fourier transformation of transient data recorded in

the time domain. Glass [9] made a good example based on the coulostatically

induced transient by Laplace transformation. But the great impedance of concrete

cover may induce large error to the measuring result of coulostatic transient

method. In this paper, a new algorithm is presented. It can Fourier-transform the

data of GPM measurement from time domain into frequency domain and

determine the corrosion status of the rebar in concrete.

=

C. Su, Y. Pei5u ] Covvozion Science 42 (2000) 675686 677

2. Wheory

A modified Randles equivalent circuit (Fig. 1) is adopted to describe the

electrochemical system in reinforced concrete. Rp(v) and Cdl(v) are frequency- independent [12]. The input signal of GPM, a galvanostatic pulse (Fig. 2), is the

same as that of galvanostatic linear polarization method. The main diRerence between the two methods is the technique used to record the potential transient.

GPM records the entire potential charging procedure, while LPR records mainly the steady potential. The sampling rate of GPM is much higher than that of LPR.

On the basis of experience of LPR and EIS, the galvanostatically-induced potential transient (Fig. 3) can be divided into three parts [12,13]

1. The first part of the transient is related to the concrete electrolyte or the

corrosion product. It can be considered as an ohmic resistance with a time

constant 0 in the range of s, or much smaller. It is far less than the sampling period used in GPM measurement. Thus, in the galvanostatically induced

response curve, a sharp potential step, AV, is gained at the beginning of transient, from which the concrete resistance can be got

AV R =

Iapp (2)

where AV is the potential step in the potential, Iapp the applied polarization

current.

2. The second part of the transient corresponds to the charging process of double

layer. The curve will eventually attain a limitation. This part contains the

information of the corrosion reaction.

3. The third part of the transient, which continuously increases without

attainment of steady state, corresponds to diRusion eRect. Ualter [14] found

that the transient corresponding to the diRusion eRect developed in a

characteristic way

. 8 .1/2

AVdiff

n Iapp o t

1/2 (3)

Fig. 1. Simplified Randles circuit [12].

5


Fig. 2. The galvanostatic pulse input into the sample system.

where AVdiff is the diRusion over-potential, Iapp, is the applied polarization

current, o is Uarburg coeAcient, and t is the time.

The diRusion transient AVdiff becomes linear when it is plotted against t1/2. Thus, this part can be easily removed by subtracting the t1/2 element from the charging

curve.

Then the rest of potential transient can be attributed to the linear polarization

resistance and double layer capacity. From the steady state value of the curve, we

can get the polarization resistance

Rp = AVp

Iapp (4)

where AVp is the polarization potential of the interface of corrosion product and concrete.

The GPM measuring data in time domain can be Fourier-transformed into an

impedance spectrum in frequency domain

Fig. 3. The potential response of the sample system.

( ( )

V(v)


Rp

X(v) = U v

= R 1 jv 1a (3)

where U(v) and V(v) are the frequency function of input and output signal, respectively, R is the concrete resistance, Rp is the polarization resistance, v is

the angular frequency, = RC is the time constant of circuit, and a is the dispersion parameter ranged from 0 to 1.

Similar to EIS, the calculated impedance, X(v), can be expressed in terms of its real, X', and imaginary, X", components. Thus, the data collected by GPM can be translated into the same form as EIS. Uith the aid of the algorithm, a few

minutes detection by GPM can give similar results as those from several hours

EIS experiments. It has been proved that the error produced by data

transformation from the time domain into the frequency domain is less than 4%

[15].

Hachani [12] found by EIS that the dispersion parameter a is related with the

homogeneity of the product formed directly on the steel surface in concrete. Therefore, the dispersion parameter a was recommended to characterize the

corrosion of the specimens. The depassivation of rebar begins when a > 0.1. The larger the a value, the heavier the corrosion, especially the more dangerous the pitting corrosion. McCaRertys method [16] is used to extract the dispersion

parameter a to characterize the corrosion status of rebar in concrete. Defining the arc chord vectors U and V (Fig. 4)

V = X R (6)

U = .R Rp

. X (7)

Combining Eqs. (5), (6) and (7), it can be obtained

Fig. 4. Definition of the arc chord vectors U and V and the colecole figure with diRerent a values.

)

7

. .

. .

680

. V

. .

C. Su, Y. Pei5u ] Covvozion Science 42 (2000) 675686

.(a1) . U . = vCdlRp (8) . .

.. V

.. ln = (a 1)ln v (a 1)ln.CdlRp

. (9)

. .

. U .

Thus, a plot of log(v) vs. log(U/V ) gives a straight line (Fig. 5). a can be obtained from the slope, while Cdl from the intercept as Rp has been determined in time domain.

3. Experimental method

The test specimens are $8 mm ~ 80 mm mild steel bars. The bars were subsequently polished to 1000 grit with SiC paper and then degreased with

acetone. Both ends of the bar were masked by adhesive tape to expose 1000 mm2

area. The testing systems were bars immersed in solution and reinforced concrete.

$25 mm ~ 100 mm concrete cylinders with bars embedded in center were prepared. The composition of used concrete was shown in Table 1. The testing samples and

conditions were shown in Table 2. The used cement was PS-425 blended Portland

cement. The sand was natural river sand with a fineness modulus of 2.43. The size

range of coarse aggregate was 510 mm. After curing the concrete cylinders with bar at 20 2C and 90% humidity for 28 days, the specimens were immersed in solution. A 100 A]cm2 anode current was applied to sample No. 4 to accelerate the corrosion. After 72 h, cracks produced by the expansion of corrosion product

could be seen on the surface of the concrete cylinder.

The testing bars in solution or concrete acted as working electrode (UE) during

the electrochemical measurement. An external saturated calomel electrode was used as reference electrode (RE). The counter electrode (CE) was an aluminum

cylindrical shell ($80 mm ~ 60 mm) to obtain a better current distribution. A galvanostatic pulse supplied by a 18 V DC battery was controlled by a series-

Fig. 5. Determination of disperse parameter a.


Table 1

The composition of concrete

E]C Uater

(kg]m3)

Cement

(kg]m3)

Aggregate

(kg]m3)

Sand

(kg]m3)

Superplasticizer

(wt% of cement) CaCl2 (wt% of cement)

0.44 268 609 456 1065 0.2 0

0.44 268 609 456 1065 0.2 3

wound resistor and a relay. The amplitude of the input signal was adjusted to

make the output signal not higher than 25 mV. The amplified potential response

was converted to digital signal by a 12-bit A]D converter and then recorded into a

personal computer. The polarization resistance and the ohmic resistance were

determined in time domain; other parameters were obtained in frequency domain.

The accuracy of the measurement was tested by analyzing a dummy Randles type

combination of electrical resistors and capacitors.

4. Results and dissussion

The potential response curves of sample Nos. 1 and 2 are shown in Figs. 6 and

7. Their calculation results are shown in Table 3. Sample No. 2 that is immersed

in Cl contaminated solution has smaller time constant of electrochemical reaction and larger diRusion eRect than No. 1, that is immersed in saturated Ca(OH)2 solution. The impedance spectrum of No. 1 (Fig. 8) is close to an ideal semicircle. Therefore, there is basically one RC element corresponding to the

electrochemical reaction on the interface of steel and solution. The rebar in No. 1

must be passive now. The impedance spectrum of No. 2 (Fig. 9) contains two arcs

with diRerent curvatures. There are two RC elements in the system. One

represents the electrochemical reaction and another, the eRect of corrosion

product layer that induces the diRusion eRect. The rebar in No. 2 must be active

now.

The corrosion current of No. 1 is 0.36 A]cm2, calculated from the Stern

Fig. 6. The potential response of sample No. 1.

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Table 2

Samples and the experiment conditions

No. Sample Electrolyte solution Measuring time

1 Rebar in Ca(OH)2 saturated

solution Ca(OH)2 saturated solution At 4 h after rebar was immersed in

solution

2 Rebar in Ca(OH)2 saturated

solution containing 1%CaCl2

Ca(OH)2 saturated solution

containing 1% CaCl2

At 4 h after rebar was immersed in

solution

3 Rebar in concrete without Cl Ca(OH)2 saturated solution At 4 h after rebar was immersed in solution

4 Rebar in concrete with Cl Ca(OH)2 saturated solution

containing 1% CaCl2

After the current is input for 72 h

Geary equation

B Icorr =

p

(lO)

where Tafel constant B is defined as 52 mV for the passive state.

Since corrosion current of No. 1 is larger than 0.1 A]cm2, the rebar comes into activation. It is contrary to the fact and the criterion of the disperse parameter

that is 0.077 and remains in the range of passivation. The corrosion current of

No. 2 is 5.75 A]cm2 (Tafel constant B is defined as 26 mV for the active state). Its disperse parameter is 0.28. Both of them show that the rebar begins to corrode

and the surface non-uniformity increases because Cl ion accelerates the corrosion

rate and increases the possibility of pitting corrosion.

The potential response curve of the rebar in plain concrete (No. 3) (Fig. 10) has

a large time constant and an obvious diRusion eRect because oxygen diRuses with

diAculty in uncracked and water-saturated concrete. Its impedance spectrum

contains two arcs corresponding to the electrochemical reaction and the interface

between rebar and concrete, respectively (Fig. 11). The disperse parameter is very

small (Table 3). It means that the rebar in plain concrete is passive and its surface


R


Fig. 8. The impedance spectrum of sample No. 1.

Fig. 9. The impedence spectrum of sample No. 2.


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Table 3

The GPM results of corrosion status of rebar in solution or concrete

No. Ohmic

resistance

Polarization

resistance

Double-layer

capacitance

Corrosion

current

Disperse

parameter

(kM cm2) (kM cm2) (F]cm2) (A]cm2)

1 = 0 144.69 48.6 0.36 0.077

2 = 0 4.52 247.2 5.75 0.28

3 0.15 267.97 28.8 0.19 0.0075

4 8.48 2.71 83.8 9.60 0.422

is smooth and uniform. But there is once again a contradiction between the

disperse parameter and the corrosion current that is 0.19 A]cm2 larger than the limit of passivation.

After acceleration of corrosion by 72 h input of anodic current, the rebar in the

concrete cylinder containing Cl (No. 4) corrodes heavily and the concrete cover has cracks. At this time, both the diRusion resistance of oxygen in concrete and the polarization resistance of the electrochemical reaction decrease obviously. The

potential response curve has small time constant and low diRusion eRect (Fig. 12).

The ohmic resistance increases because there is a thick isolate layer of corrosion

product on the surface of rebar. Therefore, there is a significant jump in the

beginning of response. The impedance spectrum contains three arcs with diRerent

curvatures (Fig. 13). They represent three RC elements of electrochemical

reaction, the interface of rebar and corrosion product and the interface of

corrosion product and concrete, respectively. Both the disperse parameter of 0.422

and the corrosion current about 10 A]cm2 show that there is badly non-uniform corrosion on the surface of rebar.




5. Conslusion

An algorithm is developed to Fourier-transform the measuring data of

galvanostatic pulse method from the time domain into the frequency domain. The

polarization resistance and ohmic resistance are determined in the time domain;

the impedance spectrum and disperse parameter are obtained in the frequency

domain to assess the corrosion status of the rebar in concrete.

The criterion of corrosion current may over-estimate the beginning time of

depassivation when the status of rebar is just below the critical depassive level. In

this case, the criterion of disperse parameter should be prior used. Both criterions

of corrosion current and disperse parameter can judge accurately the corrosion

status of rebar in the case of heavy corrosion.


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Asknowledgements

This work was funded by Chinese National Hey Projects on Basic Research and

Applied Research Applied Research on Safety and Durability of Major Construction Projects.

Referenses

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London, 1997, p. 68.

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termination of corrosion rate of reinforcement in concrete structure, British Corrosion Journal 29

(1994) 2532.

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