Quantifying chloride-induced corrosion from half-cell potential

CEMENTand CONCRETERESEARCH. Vol.23, pp. 1443-1454, 1993. Printedin the USA. 0008-8846/93. $6.00+00. Copyright© 1993PergamonPress Ltd.

QUANTIFYING CHLORIDE-INDUCED CORROSION FROM HALF-CELL POTENTIAL

R K Dhir, M R Jones and M J McCarthy Concrete Technology Unit Department of Civil Engineering, University of Dundee Dundee DDI 4HN, Scotland UK (Communicatedby C.D.Pomeroy) (ReceivedFebruary22, 1993)

ABSTRACT The paper describes a study undertaken to examine a methodology to determine the rate and severity of chloride-induced corrosion of steel embedded in concrete using the half-cell potential test. Preliminary studies undertaken in the laboratory, using a wide range of concrete variables, indicated that an effective relationship between half-cell potential (Ecorr) and corrosion current (Icorr), from polarisation resistance measurements, did indeed exist. The study was subsequently extended to an external marine environment where sections of full-scale beam, slab and column elements were exposed to seawater attack for a period of 5 years. By instrumenting these elements prior to exposure, it has been possible to demonstrate that by establishing a local relationship between Ecorr and Icorr, the extent of corrosion damage can be reasonably estimated over the full section. The practical implications of this and a proposed test procedure are discussed.

Introduction Methods for assessing the state of corrosion of reinforcing steel have been under development for over 25 years <1-4~,however, a number of difficulties remain and the need still exists for the introduction of a simple, quick and practical tool which can be used to survey complete structures. Although methods such as polarisation resistance (Rp) achieve some of the required characteristics, it like others, is limited by equipment costs and the need to know the area of steel under test. The latter drawback is particularly problematic in-situ and the development of guard-rings has done little to help in a practical, meaningful way. The need for a method to locate and quantify non-destructively corrosion activity has grown ever more pressing for large structures such as highway bridges <5). With this background, it was decided to look 1443

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again at the half-cell test and investigate the feasibility of quantifying chloride-induced corrosion. This was made possible by the considerable amount of data generated at Dundee University t6t°~, where corrosion current (Icorr) and half-cell potential (Ecorr) have been measured for a wide range of environmental and concrete characteristics.

Half-Cell Potential a n d C o r r o s i o n C u r r e n t Figure 1 illustrates schematically the current paths and potentials set up due to electrochemical activity of corroding reinforcement. The basis of corrosion assessment has, therefore, to be mainly directed at the measurement of these parameters.

Half.Cell Potential (Ecorr).

This is in effect a measure of the relationship between a standard reference electrode on the concrete surface (ie a half-cell) and the overall potential difference set-up between the anodic corroding site and cathodic non-corroding area. As corrosion intensifies, there is generally an associated shift in potential to more negative values. The test methodology for the technique is most commonly that of ASTM C876 Cm

Corrosion Current (lcorr). This is a measure of the current flowing in the freely corroding condition. Since overall there is no net transfer of charge, the steel/concrete system has to be perturbed by the application of an external signal and the response as the system equilibrates measured. In the polarisation resistance test method a potential of _+10mV is normally applied with respect to Ecorr and the current flowing to a counter electrode measured.

Figure 1. Schematic representation of the occurrence of corrosion currents and potentials.

By plotting the V/I graph, the polarisation resistance (Rp) can be determined from the slope of this relationship and used in the Stern-Geary equation (12>to obtain Icorr, as :

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CHLORIDES,CORROSION,HALF-CELLPOTENTIAL

Icorr

-

B

B

B. AI

Rp

Aria/

Av

1445

where B is a constant normally taken as 26mV for steel in a corroding condition °3).

Test

Programme

Two series of tests were carried out as detailed in Table 1. Series 1. A preliminary laboratory study was carried out with a range of different binder types, concrete workabilities and grades and periods of curing to establish whether a general relationship of sufficient practical facility between Ecorr and Icorr exists. In these tests small scale reinforced concrete specimens (l(X)mm cubes) were subjected to 5M salt spray conditions. Measurements of Ecorr and Icorr were taken at monthly intervals over an exposure period of 1 year. Series 2. It was considered vital for a programme of this nature to test the practicality of laboratory data generated in an accelerated chloride environment, against a real exposure with full-scale sections. Thus, for the second series, it was decided to manufacture elements of typical full-scale beam, colmrm and slab sections and locate them at a marine exposure site. The results of the tests reported here were obtained after an exposure period of 5 years.

Materials and Mix Proportions An ordinary Portland cement to B S I 2 (1989) and a pulverized-fuel ash to BS3892 : Part 1 (1982) were used as binders. The properties of these materials are summarised in Table 2. The aggregates were a gravel (10ram and 20ram) and a zone M natural sand to BS882 (1983). Concrete grades from

Table 1. Details of experimental programme.

BINDER

CONCRETE

GRADE

CURING

SPECIMEN TYPE

EXPOSURE

100 iron cubes

5M Salt

Series 1

OPC

C20, C30

28 days standard

OPC/PFA

C40, C60

water and air

Spray

(20°C/65% RH)

(laboratory)

Series 2

OPC OPC/PFA

* See Figure 2.

C20, C40 C60

Air

Beam* (500x425x325mm)

Marine

(20°C/95% RH)

Column (500x375x340mm) Slab (675x425x250mm)

exposure (external)

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Table 2. Main properties of OPC and PFA.

MAJOR OXIDE, % CaO SiO2 A1203 Fe203 MgO SO3 K20 Na~O TiO 2 MnO P205 LOI Fineness, m2/kg

OPC 63.3 19.1 4.8 3.2 2.8 3.0 0.5 O.1 0.2 0.1 0.1 1.9 306

PFA 3.7 46.8 33.3 7.5 1.6 0.9 1.0 0.0 1.5 0.1 0.6 2.3 5.7*

Retained on 45 ~nn sieve, % weight 2(i) to 6(i) N/ram 2 (C20 to C60) were considered. The effect of different concrete workabilities was also tested at grade 40 by varying the paste content for the same water/binder ratio. Details of the mix proportions t'or the OPC and PFA concretes tested are given in Table 3.

Test Specimens The test specimens used in Series 1 were 100mm cubes, where 5 sides, except for the as-cast face, were sealed using a surface coating system of silane primer and paraffin wax. These specimens contained 10ram diameter high yield steel bars arranged in a manner to provide a corroding bar (working electrode at a cover of 25ram) and a non-corroding bar at a deeper cover of 75mm (counter electrode). The end 20% of corroding steel bar was sealed against chloride attack by the application of a pitch epoxy to give an exposed area of approxhnately 30cm 2. Further details of the specimens may be obtained from reference 10. In Series 2, tests were carded out on eighteen different elements of full-scale sections, see Table 1, with reinforcement configurations typical of those used in practice, viz, beam, column and slab, see Figure 2. The main reinforcement in these spechnens was 201ran diameter high yield steel with 10ram mild steel stirrups. In each of these specimens one of the main reinforcing bars was electrically isolated by housing on insulating pads between stirrups and coating the ends of the bar with a pitch epoxy resin. These had a known area, as indicated in Table 4, and could be used to measure corrosion rates. In addition, four small isolated probes of 10ram diameter and 100mm length were also included in each specimen at the depth of the main steel (35mm) to allow corrosion rate measurements to be carried out.

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Initial Curing and Exposure Conditions For the Series 1 tests, all spechnens were cured under hessian for 24 hours and then in water at 20°C to 28 days. As indicated in Table 1, selected concretes for this Series (grade 40) were also cured in air at 20°C and 65% RH to 28 days. The large specimens, used in Series 2, were cured in air at 20°C and 95% RH to 28 days. Following initial curing, all specimens were soaked in water for 3 days such that they all had a similar moisture condition. The cube specimens were then exposed to a 5M salt-laden fog, for a one hour period twice a day, in a custom-built salt spray tank. Series 2 specimens were located in the splash zone at an external exposure site on the Tay Estuary. Details of the site location and the composition of the seawater are given in Figure 3 and Table 5 respectively. The temperature range of the seawater at the site was between 4 and 14°C.

Table 3 Mix proportions. CONSTITUENT MATERIALS, k g / m 3

MIX

Aggregate Water

OPC

PFA

SLUMP (mm)

Sand

10mm

20mm

820 785 785 785 730 630

390 375 390 390 365 375

765 745 775 775 725 750

75 75 25 75 125 75

775 650 590 570 555 410

390 405 435 420 410 440

780 810 880 840 820 880

75 75 25 75 125 75

OPt? Concrete C20 C30 C40

185 185 165 185 195 185

225 280 285 325 340 430

PFA Concrete C20 165 C30 165 C40 145 165 175 C60 165

170 210 210 235 250 345

C60

80 120 120 130 140 120

Chloride Measurements Both water-soluble and total chloride contents of all test spechnens were measured at the full depth of cover to assess the degree of contmnination that was occurring in the laboratory and at the outdoor exposure site. The test methods used have previously been reported in Reference 7.

Corrosion Measurements

Half-cell potentials were measured in accordance with ASTM C876 ~1) using a Ag/AgC1 standard reference electrode (Ag/AgCI).

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Polarisation resistance was measured using the stone reference electrode, a potentiostat (Sycopel Scientific DD 10) with positive feedback IR compensation and a rmnp unit. Potentials were measured and then the test specimen polarized at a rate of 0.hnV/sec to +10mV and Icorr calculated. BEAM

SLAB A l l Faces Sealed E x c e p t As Cast Face

COLUMN Top & Bottom F a c e s Sealed

Top & End F a c e s Sealed

~

6'

J

t

/ R l O ~ i O tentre~75 425 250

425

6R10-100 c e n t r e s

325

340

(All dimensions mm, cover depth to all links = 25mm) Figure 2. Details of elements of full-scale structural sections.

Table 4. Exposed areas of isolated main reinforcement.

ELEMENT

EXPOSED STEEL AREA cm 2

Beam

190

Column

190

Slab

250

Results

Series I : Laboratory Tests The chloride contents at the mean depth of cover (27.5mm) were up to 6.0% by weight of binder, Figure 4. These quantities are typical of accelerated chloride tests in the laboratory ~9)and are in the range encountered in practical situations - ~5~. The concentrations reflect the major effect that the concrete characteristics have on chloride ingress and binding capacity. In line with the chloride contents, the Ecorr results fell within a range from -50 to -550 mV (Ag/AgC1), which is typical of the values expected ~14). The Icorr values again mirrored the chloride contents with a wide range of corrosion currents from negligible at 0.01 laA/cm2 to quite severe at 6.0 laA/cm 2. At the completion of tests, after 12 months accelerated chloride exposure, a number of test specimens were broken open to assess visually the state of the surface of the steel. In all cases, inunediately

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CHLORIDES,CORROSION,HALF-CELLPOTENTIAL

1449

Figure 3. Location of marine exposure site.

Table 5. Typical seawater composition at marine exposure site. MAIN COMPOSITION

CONCENTRATION, ppm

CI

14,095

(SO4)-

2,020

Na ÷

8,800

Mg 2÷

945

K÷

375

Ca 2÷

238

Total Dissolved Solids

28,975

after breaking open, brown corrosion products were noted. This would suggest that oxygen availability was not restricted. This is further supported by the slope of the linear regression line of 156 mV/decade. Other investigators ~5) have suggested that an Ecorr/Icorr slope of this order is indicative of aerobic corrosion.

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It is clear from Figure 4 that there is indeed a close linear relationship between Ecorr and Icorr for the range of different concretes considered and that there is no significant effect on the results in respect of variations in concrete grade, curing or binder type and content. The 95% confidence limits for this relationship suggest that it is possible to estimate Icorr to a factor of 2. The results offered sufficient evidence that, under the conditions used in the laboratory, the general relationship had potential to be used in practical situations. Therefore, the study was extended to test the relationship in a more practical way. 0 "~m= .1001

~,~?t"-~ "~l~._~ ~l~,

'_= .,=I ~ , m

C o e f f i c i e n t of

-~ ~ • ~ 1 ~11[ "~ mm~m~,' .

-200 _

~

~ff- 95% Confidence Limit

'~

•

<

,,im~i~ilim

Correlation, r = 0.96 "1

.~Ir m il ='~-~ll~Imm m ~. • imlmlL~m

mere

~,

-300 E

m "~m~.. -400

-

_

.

.

.

.

m m m m \ ~.

" " "

..

Range of Chloride Contents

~_ • ql]=']]lll~ • "~ •

-500

W -

-600

a

~

l

Total n

i

i

i

lllh

10"2

"l Cm_l _li[~ m • re'Iti~.

cem
t

I

I

I

I [llll

10"

I

I

I

I IIII

10 °

10'

lcorr, laA/cm 2 Figure 4. Laboratory developed relationship between Ecorr and lcorr. S e h e s 2 : External Exposure Tests

To assess the relationship developed in the laboratory on a practical basis, the following tests were carried out on the large outdoor exposed specimens, i. A local relationship between Ecorr and lcorr was obtained from the small embedded probes. ii. Ecorr was measured on the main reinforcing bars. iii. The mean Ecorr ineasurements were used in the local relationship obtained in i. to estimate Icorr. iv. Icorr was directly measured on the main (isolated) reinforcing bars using the polarisation resistance method. v. Estimated and actual Icorr values were compared. The local relationship obtained between Ecorr and Icorr and the range of chloride contents present after the five year exposure period are given in Figure 5. As noted in Series I the chloride contents varied with concrete grade and binder type. The level of

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CHORIDES,CORROSION,HALF-CELLPOTENTIAL

1451

contamination in the spechnens from the marine environment were, not surprisingly, lower than those previously noted in the laboratory (Series 1). This was reflected in the less negative Ecorr and lower Icorr results noted in Figure 5. However in both cases, these results are typical of what has been • real structures.(5). It can be seen from Figure 5 that PFA did not change the relationship noted m between Ecorr and Icorr, although due to the lower chloride contents, corrosion activity was generally lower in the PFA specimens.

0

-100

• .. -.. / 95% "- ~ ...

Confidence Limits

Coefficient of n

U ,<

,

,

:

096

-200

-300 t_

R__ange°f _Chi°rlde C°ntea t s -400

-500

Water.Soluble

0.01 - 2.0qb

Total

0.03 - 2 . 7 ~

I

I

10-:

l0 -1

""

~-

~

"~

, ~" "-

wt cem

wt cem

I 100

101

Icorr, laA/cm 2 Figure 5. Local relationship established between Ecorr and Icorr for sections in seawater exposure. Linear regression carded out on this data gave a gradient of 147 mV/decade. The slope of this locally obtained relationship is close to that obtained in the laboratory. This gives confidence that the type and rate of corrosion activity was indeed shnilar in both the accelerated and natural exposures. This further supports the premise in this case that oxygen supply to the sites of corrosion was in excess of its rate of consumption. The mean Ecorr values obtained on the main reinforcing bars, including the isolated section, are given in Table 6. As indicated, differences in potential over a section were generally less than 100mV. These results were used to obtain estimated values for Icorr. Polarisation resistance tests were used to obtain the actual Icorr value for the isolated portion of the main steel and compared with the estimated value in Figure 6. This indicates that there is good agreement between the actual measured corrosion and that esthnated from the mean potentials measured.

Practical I m p l i c a t i o n s It has been shown from the results that it is possible to extend the application of the half-ceU test method to obtain an esthnation of the severity of corrosion. With the present state of knowledge, where the structural and environmental conditions are unknown,

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Table 6. Mean and (range) of potentials from modelled structural elements CONCRETE TYPE

Ecorr, mV vs Ag/AgC! BEAM

COLUMN

SLAB

OPC

-400 (-351 to -417)

-390 (-370 to -426)

-370 (-331 to -395)

PFA

-353 (-320 to -383)

-391 (-341 to -420)

-361 (-317 to -408)

OPC

-343 (-306 to -389)

-372 (-308 to -401)

-346 (-316 to -395)

PFA

-296 (-271 to -321)

-346 (-316 to -395)

-256 (-248 to -273)

OPC

-270 (-211 to -315)

-253 (-223 to -291)

-239 (-187 to -261)

PFA

-176 (-150 to -213)

-206 (-171 to -254)

-210 (-163 to -247)

20N/mm 2

40N/mm z

60N/mm 2

.//

10 Line of Equality

== a

~

<

1.0 ,< [-.

0.1 , . 4 0.1

, 1.0 ESTIMATED

Icorr, ItA/cm 2

Figure 6. Comparison of the estimated and actual Icorr measured for the seawater exposed sections

10

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CHLORIDES,CORROSION,HALF-CELLPOTENTIAL

1453

it will be necessary to establish a local relationship between Ecorr and Icorr to ensure adequate confidence in the estimation of corrosion. This is, in fact, the usual practice with non-destructive tests and is analogous with the general and local relationships that exist between concrete compressive strength and, for example, rebound hamlner or pulse velocity. The consequence of this is that it will be necessary to carry out 'calibration' tests for half-cell and corrosion current for the structure and location under test. This can be carded out by the use of polarisation resistance equipment employing a guard l'ing ¢16) and relating measurements to the measured potentials in the region. This would have to be done at a sufficient number of points on the structure for the relationship to be developed. Once the local relationship has been established then potential mapping of the whole structure can be carded out to establish the corrosion intensity. It is likely that certain forms of corrosion will not obey the general relationship particularly, for example, that of macro-cell corrosion or where oxygen starvation induces anaerobic conditions, although it may be possible to produce similar relationships for such cases. From the present study, however, it is important to note that the general relationship between Econ" and Icorr developed in the laboratory was in fact duplicated for the large sections exposed in the marine environment, although in the case of the former relationship the 95% confidence limits were much greater. This suggests that it may indeed be possible to formulate a universal relationship between Ecorr and Icon" for certain classes of concrete and environmental characteristics. This means that it would be unnecessary to carry out a local determination of the Ecorr/Icorr relationship. Clearly more work is necessary to confirm this hypothesis, but the initial work reported in this study together with the important advantages that the proposed method offers support further studies.

Conclusions 1. The results of an extensive laboratory and outdoor exposure study have shown that it is possible to obtain more pertinent information from the widely used haft-cell test to enable the additional estimation of Icon. from the measurement of Econ.. 2.

A general relationship between Ecorr and Icon. has been developed from a wide range of laboratory tests on OPC and PFA concretes, which have been exposed to accelerated chloride ingress. The relationships between Econ. and Icorr was linear, indicating that their binder type, grade, curing etc did not alter the basic relationship.

. An additional series of outdoor exposure tests were carded out with sections of full-scale elements, which were exposed over a period of 5 years to a marine splash environment. The local relationship was then used to esthnate Icorr and this was found to be accurate when compared against actual Icon. values measured on the main reinforcement using polarisation resistance.

4. As with all non-destructive tests a local calibration obtained for the particular structure and environment under test improves the accuracy of the estimation of Icorr. In the present study the duplication of the Ecorr/Icorr relationship in the laboratory and marine exposure suggest that it is possible to develop a universal relationship to estimate Icorr without the need for an in-situ calibration. Further work is needed to examine this.

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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

P R VASSIE. Transport and Road Research Laboratory, Report 953, 1978 J L DAWSON. Proc Soc Chem Indust Conf, 1983, pp 59 S G MCKENZIE. Corr Prev Contr, 34, 1987, pp 5 R K DHIR et al. Concrete, 25, 1, 1991, pp 15 E J WALLBANK. Report prepared for the Department of Transport by G.Maunsell and Partners, April 1989. HMSO London. R K DHIR et al. Mag Concr Res, 42, 132, 1990, pp 177. R K DHIR et al. Cem Concr Res, 20, 1990, pp 579. R K DHIR et al. Cem Concr Res, 26, 6, 1991 pp 1092 R K DHIR et al. Mag Concr Res, 43, 154, 1991, pp37 R K DHIR et al. Proc Inst Civ Eng, 94, 3, 1992, pp 335. AMERICAN SOCIETY FOR TESTING AND MATERIALS C876, Annual Book of Standards, 04.02, 1988, pp 420. Philadelphia. M STERN and A L GEARY. J Electrochem Soc, 104, 1, 1957, pp 56. C ANDRADE and J A GONZALEZ. Werk und Corr, 29, 1978, pp 515. B SORENSEN. Steel in Concr Newsletter, 2,1979, pp 6.

15. C L PAGE and J HAVDAHL. Mat et Constr, 18, 103, 1985, pp41. 16. J L DAWSON et al. Proc Soc Chem Indust Conf, 1990, pp358