Monitoring of corrosion processes in chloride contaminated mortar by electrochemical measurements and X-ray tomography

Materials and Corrosion 2010, 61, No. 6 DOI: 10.1002/maco.200905364 475

Monitoring of corrosion processes in chloride contaminatedmortar by electrochemical measurements and

X-ray tomography

M. Beck*, J. Goebbels, A. Burkert, B. Isecke and R. Baßler

Corrosion of steel reinforcement in concrete exposed to chloride containing

environments is a serious problem in civil engineering practice. Electrochemical

methods, e.g., potential mapping, provide information whether the steel

reinforcement is still passive or depassivation has been initiated. By applying

such techniques no information on the type of corrosion, its extent and

distribution of corrosion products is available. Particular the corrosion progress

is a significant problem. Especially in the case of macrocell corrosion in

reinforced concrete structures, the development at the anode cannot be

separated into corrosion damage resulting from macrocell corrosion or self-

corrosion. Until now also in laboratory tests it is impossible to collect such

information without destroying specimens after electrochemical testing was

performed. To overcome this problem it was tried to study the steel surface

within the mortar specimens by X-ray tomography (CT). Within the scope of

these investigations it could be shown, that X-ray tomography is suitable to

make corrosion pits and their development visible which are embedded in a

mortar with a cover thickness of about 35mm. In this publication the

time-dependent corrosion damage of reinforced steel is documented by

X-ray tomography.

1 Introduction

Corrosion of steel reinforcement in concrete exposed to chloride

containing environments is a serious problem in civil engineer-

ing practice. Electrochemical methods, e.g., potential mapping

[1, 2], provide information whether the reinforced steel is still

passive or depassivation has been initiated.

By applying such techniques no information on the type of

corrosion, its extent and distribution of corrosion products is

available. Until now it is impossible to collect such information

without destroying specimens after electrochemical testing has

taken place.

Particular the progress of corrosion is a significant problem.

Especially in the case of macrocell corrosion in reinforced

concrete structures, the development at the anode cannot be

separated into corrosion damage causing by macrocell corrosion

or self-corrosion. Within the scope of a German research project

[3], investigations were performed, to determine the part of self-

corrosion within the total amount of corrosion (macrocell or

M. Beck, J. Goebbels, A. Burkert, B. Isecke, R. Baßler

BAM Federal Institute for Materials Research und Testing, Unter den

Eichen 87, 12205 Berlin (Germany)

E-mail: [email protected]

www.matcorr.com

element corrosion). The rate of self-corrosion was determined in

investigations [4] to be 25–50% of total corrosion. The continously

carried out measurements showed, that in the case of self-

corrosion measurements a number of fluctuations of the

electrochemical values of the same series is inevitable [5]. It is

important to notice that the aim of this examination is to correlate

the electrochemical values to the real steel surface. For this

purpose the dismantled steel surface of one specimen (after a

certain period of time), is correlated to the electrochemical values

of the whole series.

From the observations obtained so far it can be concluded

that due to the different conditions of each specimen this

procedure has to be used with caution. The electrochemical

values give information about trends but no absolute data about

the development of self-corrosion. The real appearance of the

steel surface can only be observed by the dismantled steel

cylinder, after destroying the concrete cylinder. Therefore a

continous monitoring of the same steel surface is not possible.

To overcome this problem it was tried to study the steel

surface inside the mortar specimens by X-ray tomography (CT).

Previous investigations on mineral building materials [6–10]

showed promising possibilities. Also in pre-tests the applicability

of X-ray tomography for observation of corrosion processes in

chloride contaminated mortar was confirmed. Therefore in the

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

476 Beck, Goebbels, Burkert, Isecke, and Baßler Materials and Corrosion 2010, 61, No. 6

Figure 2. Damaging cycles and dates of X-ray tomography

present investigations the steel-surface of the embedded rebar

was documented by X-ray tomography. Moreover, the dimension

of the pits in a scale of a few micrometers were measured by

sections from CT analysis and verified metallographically after

destroying the specimen. Aim of this work is to show the time-

dependent corrosion damage development, by X-ray tomography.

Furthermore for comparison the mass loss of the specimen was

calculated by the data from X-ray tomography and electrochemi-

cal measurements.

2 Experimental

2.1 Electrochemical pre-damaging

For the electrochemical investigations, mortar cylinders with

embedded electrodes were prepared. The mortar cylinders had a

height of 120mm and a diameter of 80mm. The concrete

cylinder simulates a constant concrete cover (35mm). In the

center of the specimen a small steel cylinder (working electrode)

welded to a stainless steel filler wire is fixed. The steel cylinder

had a diameter of 9mm and a length of 10mm which resulted

in a surface of 4.1 cm2. In addition to the working electrode a

counter electrode (platinized titanium net) was embedded (Fig.

1). The concrete mixtures were made according to DIN-EN 196-1

(1995) [11] with 4.0% chloride by weight of cement. After

producing the mortar cylinders, they were stored for 28 days

according to DIN EN 196-1 (1995).

Following this procedure, the anode was pre-damaged

gradually by static galvanic polarization, starting at 5mA/cm2,

up to 25mA/cm2, during 40 days (Fig. 2). The current is related to

the whole steel surface (4.1 cm2). The experimental setup is

presented in Fig. 1. Before starting the damaging cycles and after

several cycles (7, 14, 24, 34, and 40 days) the specimens were

investigated by X-ray tomography (circles). Subsequently, the

Figure 1. Specimen and experimental arrangement for

electrochemical pre-damaging


mortar cylinders were destroyed and the bar was dismantled,

cleaned and the mass loss was determined.

2.2 CT-examination

The measuring system for the 3D-computer tomography consists

of an X-ray tube, a manipulator and a detector. The experimental

arrangement is illustrated in Fig. 3. This is realized by rotation of

the test object r in the X-ray cone beam. A radiography of the test

object for many different viewing angles is taken. The radio-

graphy is an image of attenuation of the primary X-ray beam by

differences in material density and material thickness. The

choice of appropriate X-ray energy is a parameter influenced by

the sample geometry and the material compound. The sample

volume is reconstructed from all radiographic images.

The usage of an X-ray tube with small focal spot (microfocus

X-ray-tube) and magnification technique enables a high resolu-

tion. As detector a flat panel detector with 2048� 2048 pixel was

used, whereas four elements were combined (binning 2). The

parameters were: X-ray energy 200 kV, 100mA. A prefilter of

0.3mm Cuþ 1.0mm Ag was selected to reduce the beam

hardening artifact.

The number of frames for one resolution was 900/3608. Themagnification was 9.2�. The reconstructed image matrix had a

volume of 1023� 1023� 915 pixel. This corresponds to 915 slices

with 1023� 1023 pixel. The spatial resolution (voxel size) was

45mm3. The X-ray absorption in each voxel, represented by the

material specific X-ray absorption coefficient, was normalized to 8

bit gray values.

Figure 3. Principal configuration of computer tomography

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Materials and Corrosion 2010, 61, No. 6 Monitoring of corrosion processes by X-ray tomography 477

Figure 4. Specimen with markers

On the surface of the concrete cylinder, small markers (steel

balls) were placed thus providing a coordinate system (Fig. 4).

The markers were necessary, to justify the specimen in the

same position during the recurrent measurements by X-ray

tomography.

2.3 Determination of mass loss

Aim of this examination is to monitor the steel surface of the

embedded rebar; in addition the current mass loss of the bar

should be determined. For this purpose three ways of evaluation

have been employed.

Up to now the mass loss was determined by comparing the

mass of the steel cylinder by weighing before embedding and

after destroying the concrete cylinder. Therefore, the embedded

steel cylinder was dismantled, pickled and the mass after testing

was determined by weighing (Equation 1).

Dmgrav ¼ m0;grav �mi;grav (1)

Dmgrav, gravimetric mass loss (g); m0,grav, mass of specimen

before pre-damaging (g); mi,grav, mass of specimen after pre-

damaging (g).

Moreover the mass loss was determined, time dependent,

employing electrochemical parameters and calculating the actual

mass loss, using Faradays law.

DmEC ¼ M

zFIcorrt (2)

DmEC, mass loss determined by electrochemical data (g); M,

molar mass (g/mol); I, current (A), t, time of pre-damaging (s); z,number of valence electrons (–); F, Faraday constant

(1F¼ 96 487As/mol).

Another completely new way to determine the actual weight

of the embedded steel cylinder is using volume data of the steel

cylinder from X-ray tomography. Therefore, the current mass of

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the steel cylinder is calculated from the measured steel volume

and the known density of the steel (Equation 3).

DmCT ¼ m0;grav � Vsteel;CTrsteel� �

(3)

DmCT, mass loss determined by data from X-ray tomography (g),

m0,grav, mass of specimen before pre-damaging (g); Vsteel,CT,

resulting steel volume calculated by data from X-ray tomography

(cm3), rsteel, density of steel (g/cm3).

3 Results

The following results show the determined mass loss of the

embedded steel cylinder using the three different types of

methods. Moreover a realistic impression of the steel cylinder

from inside the concrete cylinder is given by using the data from

CT examination [Fig. 5].

In the first image (0 days) the volume of the original

unaffected steel cylinder is reconstructed from the CTdata. In the

following images (7–40 days) the advancing corrosion damage is

visible. Therefore, the actual CT data were fitted into the volume

of the undamaged steel cylinder. The difference is the volume

of the corroded steel cylinder, calculated by data of X-ray

tomography. In all cases the extensions of corrosion damage, and

additionally the ongoing corrosion damage on the steel surface

can be observed. It is obvious that the corrosion damage spread

out preferentially at the bottom of the steel cylinder. A number of

small damaged areas of uniform corrosion slowly grew up and

grew together to significant corrosion damaged areas. The image

at the age of 34 days of pre-damaging, show first corrosion attack

at the circumferential surface. In addition, in the later stage areas

of pitting corrosion are visible.

The results of mass loss, determined by weighing,

electrochemical measurement and X-ray tomography are com-

pared in Fig. 6.

After destroying the concrete cylinder only one value for the

gravimetric mass loss is available (Dmgrav¼ 139mg). In addition

the mass loss can be characterized time dependently and non-

destructively by electrochemical measurements and X-ray

tomography.

It is obvious that the mass loss, determined by electro-

chemical measurements is a little bit higher than the mass loss

determined by the data from CT. This trend continues

approximately to the 24th day of pre-damaging. Afterward

the relationship inverts. In the following (34, 40 days), the mass

loss determined by data fromX-ray tomography is on a significant

higher level than the mass loss determined by electrochemical

values.

The total mass loss, determined by weight is about 40–60%

higher than the mass loss determined by X-ray tomography or

electrochemical measurements. But it must be mentioned, that

the mass loss, determined by CT, corresponds much more to the

total mass loss, than the mass loss determined by electrochemical

measurements.


478 Beck, Goebbels, Burkert, Isecke, and Baßler Materials and Corrosion 2010, 61, No. 6

Figure 5. Specimen surface reconstructed by data of X-ray tomography

Figure 6. Mass loss determined by different methods


4 Discussion and conclusions

Within the scope of these investigations it could be shown that

X-ray tomography is a suitable tool to visualize propagating

localized corrosion attack on reinforced steel in mortar with a

cover thickness of about 35mm. Moreover it was shown, that it is

possible to determine the mass loss by X-ray tomography

continuously at various stages of corrosion.

It is obvious, that there is a good correlation between

the mass loss calculated from electrochemical parameters and

those from the data of X-ray tomography. But there must be some

reasons for the differences between the calculated mass loss from

EC and the mass loss from CTcompared with the total mass loss,

determined by weighing after 40 days. In addition the difference

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Materials and Corrosion 2010, 61, No. 6 Monitoring of corrosion processes by X-ray tomography 479

Figure 7. Development of pitting corrosion (hydrolysis) at the bottom

of the specimen

between calculated mass loss by electrochemistry and X-ray

tomography must be discussed.

If deep pits are present in X-ray tomography the separation of

the density of steel and corrosion products becomes more

difficult. The software application must be specific for this case

and is an ongoing process of adjustment.

If the the mass loss is determined by electrochemical data

another reason applies. It should be noted that the calculation of

the mass loss using electrochemical parameters preferentially

gives satisfying results if the corrosion attack is uniform over the

surface of the sample. The evaluation of pitting corrosion involves

several difficulties. The reconstructed surface of the steel cylinder

in Fig. 7 shows only a few pits and shallow pits after 14 days of

testing.

At this stage of testing the main attack was caused by

uniform corrosion. During the galvanostatic pre-damaging up to

40 days the appearance of the surface changes significantly.

The number of pits as well as their depth increased. It may

be assumed that during the galvanostatic pre-damaging the

corrosion system changes from primarily uniform corrosion to

pitting corrosion. Due to hydrolysis reactions in the pits which

leads to acidification of the pit electrolyte a prognosis of mass loss

based on electrochemical parameters is strongly limited.

A comparison of the experimental arrangement with real

situations in practice show that the experimental conditions are

similar to the self-corrosion in the case of macrocell corrosion in

reinforced concrete structures. Under practical conditions usually

large passive (cathodic) areas polarize small anodic areas which is

equivelant to the galvanostatic polarization in laboratory tests.

For a real structure this means that the portion of self-

corrosion may increase with exposure time and calculations

based exclusively on the evaluation of macrocell currents become

not reliable. So it can be concluded from these results that the rate

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of self-corrosion in element corrosion is in the range of 40–60%

corresponding to the values described in [5].

To detect this part of self-corrosion, the electrochemical

measurements were supported by the X-ray tomography and

become part of the ongoing research project, to observe and

describe the development and the extension of corrosion

phenomenon.

From the present data it can be concluded, that CT is suitable

to detect corrosion pits on embedded steel bars. Furthermore, this

method enables the observation of corrosion pits and their growth

in situ without destroying the specimens. Moreover the

development at the surface can be observed and described by

X-ray tomography.

Acknowledgements: This project as a part of the research group

no. 537 is founded by the German Research Foundation

[Deutsche Forschungsgemeinschaft (DFG)], which is gratefully

acknowledged. The authors would like to thank the laboratory

staff of the divisions VIII.3 ‘‘Radiological Methods’’ and VI.1

‘‘Corrosion and Corrosion Protection’’. Special thanks to DietmarMeinel and Daniel Hanke who performed the X-ray tomography

measurements. Gino Ebell and Rico Ehrlich, who prepared and

performed the electrochemical measurements, are also gratefully

acknowledged.

5 References

[1] Interessengemeinschaft Potentialmessung Stahlbeton (IGPot), Durchfuhrung und Interpretation der Potentialmessungan Stahlbetonbauten, Schweizerischer Ingenieur- und Archi-tekten-Verein, SIA 2006.

[2] DGZfP-Merkblatt uber elektrochemische Potentialmessungenzur Ermittlung von Bewehrungsstahlkorrosion in Stahlbetonbau-werken, DGZfP, Berlin 1990.

[3] P. Schießl, K. Osterminski, Mater. Corros. 2006, 57, 911.[4] B. Isecke, Kathodischer, in: Handbuch des kathodischen Kor-

rosionsschutzes W. v. Baeckmann, W. Schwenk, (Eds.), 4Auflage, Wiley-VCH, Weinheim New York 1999, pp. 396–407.

[5] M. Beck, B. Isecke, J. Lehmann,Mater. Corros. 2006, 57, 914.[6] F. Weise, Y. Onel, F. Dehn, International Conference on

Concrete Repair, Rehabilitation and Retrofitting, Cape Town,South Africa, 2005, pp. 170–171.

[7] A. Badde, B. Illerhaus, SPIE Opt. Methods Arts Archaeol.2005, 58570U-1–58570U-8.

[8] F. Weise, U. Muller, J. Goebbels, Weathering and Conserva-tion (HWC-2006) Conference, Madrid 2006.

[9] W. Berger, U. Kalbe, J. Goebbels, Appl. Clay Sci. 2002, 21, 89.[10] M. Siitari-Kauppi, N. Marcos, P. Klobes, J. Goebbels, J.

Timonen, K.-H. Hellmuth, The Palmottu Natural AnalogueProject – Physical rock matrix characterisation, Report YST-118 Geological Survey of Finland GTK, 2003.

[11] DIN-EN 196-1, Prufverfahren fur Zement, DIN DeutschesInstitut fur Normung, Berlin 2005.

(Received: April 27, 2009)

(Accepted: June 10, 2009)

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Monitoring of corrosion processes in chloride contaminated mortar by electrochemical measurements and X-ray tomography