Effects of temperature and corrosion thickness and composition on magnetic measurements of structural steel wires

Effects of temperature and corrosion thickness and composition

on magnetic measurements of structural steel wires

Varsha Singh*, George M. Lloyd, Ming L. Wang

Department of Civil and Materials Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607, USA

Received 8 July 2003; revised 1 February 2004; accepted 14 February 2004

Available online 8 June 2004

Abstract

A tubular circulating electrolytic cell was developed in order to prepare specimens of structural steel rods to evaluate non-destructive

testing of corrosion layer thickness at different temperatures using magnetic property measurements. Hysteresis curves were obtained for

ferromagnetic steel reinforcement subjected to a low frequency magnetic field. The response is measured without contact using Faraday’s

law to infer stress and corrosion. Experiments were performed to estimate the effect of temperature and composition of the steel on its

magnetic properties. Magnetic measurements were obtained using a previously calibrated solenoidal measurement system controlled by a

computer at different temperatures and for different reductions in cross-section due to corrosion for 1140 mm long specimen. Magnetic

measurements on nickel rod were performed to validate the measurement procedure. The results obtained from the present work will be

helpful in quantitative evaluation of attainable sensitivity, signal to noise ratio, and to minimize the uncertainities due to temperature

changes.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Hysteresis; Corrosion; Reinforcement steel; Temperature

1. Introduction

Structural steel is a ferromagnetic material which exhibits

a spontaneous magnetization even at comparatively low

fields due to the elementary magnetic moments coupled with

an internal field proportional to magnetization itself [3].

Response of the magnetization obtained is sensitive to

variation of stress in the structural steels (,1 N/A2/kN/mm2)

in the case of incremental permeability. Thus, a component

of the structure itself can be utilized to act as its own load

sensor, provided the magnetic properties can be measured

non-destructively [2]. Application of this principle for

measurement of stress in prestressed and post-tensioned

cables and rebars has been successfully performed by several

investigators (Wang et al., 2000) [5]. In structural steels,

stress is uniaxial and homogeneous along the length of the

cable or rebar, hence the sensor axis can be fixed beforehand.

However, the magnetic properties of the steel are sensitive to

the temperature variations (,0.01 N/A/C) and corrections

for temperature changes are necessary while correlating the

measured magnetic properties to variation in stress and

corrosion of the structural steels [4,7].

Corrosion testing and evaluation play a vital role in

addressing one of the most important problems in the

existing structures. Measurement of magnetic properties to

measure the corrosion has been attempted by several

investigators (Ghorbanpoor et al., 1996), which involved

recording the perturbance of the magnetic field as an

electrical charge in the medium surrounding the flaw, but

the accuracy of the obtained results has been insufficient to

draw meaningful conclusions. Several other techniques

including the electrochemical methods have less accuracy

and applicability for in-field measurements for the existing

structures.

In this paper we follow a more basic approach by

investigating the sensitivity and accuracy of direct magnetic

property measurements to the effects of corrosion. The

voltage induced in a sense coil is proportional to the rate of

change of flux, which in turn is equal to the cross-sectional

area of the material and the average magnetic induction, B:

Corrosion can be inferred from the change in cross-

sectional area of the material and thus be associated with

0963-8695/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ndteint.2004.02.006

NDT&E International 37 (2004) 525–538

www.elsevier.com/locate/ndteint

* Corresponding author. Tel.: þ1-312-413-3376/2426; fax: þ1-312-413-

2426.

E-mail address: [email protected] (V. Singh).

http://www.elsevier.com/locate/ndteint

the change in magnetic flux. The objective of this paper is to

estimate the thermal dependence of magnetic properties of

the structural steel and to correlate quantitatively, corrosion

to the measured magnetic properties.

2. Description of corrosion reactor

The three electrode cell (Figs. 1 and 2) was used for the

quantitative investigation of the corrosion properties of steel

specimen. The working electrode is the steel specimen

being investigated. Platinum foil (Surface area ¼ 320 mm2)

served as counter electrode and an Ag/AgCl reference

electrode provided the stable datum point against which the

potential of the working electrode was measured. An EG&G

potentiostat and galvanostat was used for applying and

measuring current and potentials. The data acquisition and

plotting was performed by the software, SoftCorr III.

The schematic of the experimental set-up for corrosion is

shown in Fig. 2. For the calibration curve Fig. 3, 20 mm

long, 6.2 mm diameter stress proof rebar specimen was

corroded by galvanostatic corrosion. This calibration curve

has been utilized to calculate mass loss for long specimens

(1140 mm). Since the total mass of rod is directly

proportional to the cross-sectional area of a rod, the

mass% loss for specimen due to corrosion is equal to

%loss in cross-sectional area of the steel specimen.

Corrosion is the degradation of a metal by an electro-

chemical reaction with its environment. The free energy

difference between a metal and its corrosion product, DG;

represents the tendency of the metal to corrode [1].

Nomenclature

AG area of the current loop

A0 cross-sectional area of the air gap between the

sense coil and the steel rod

AF cross-sectional area of the steel specimen

B magnetic induction (N m/A m2)

Bsat saturation magnetic induction (N m/A m2)

E potential (V)

F Faraday’s constant, 96,494 (C/mole)

G Gibbs free energy (V C/mole)

H magnetic field (kA/m)

Hc coercive force (kA/m)

I current (A)

Mj molecular weight of the substance

n number of electrons in the half-cell reaction

OCP open circuit potential (V)

Q amount of charge (C)

t time (s)

Vint 2Ð

Vsc dt; analog integration of Vsc

Vsc voltage from sense coil of N turns

n the number of moles of the substances involved

in the electrochemical reaction

m0 permeability of free space 4p £ 1027 (N/A2)

m permeability

ma permeability at technical saturation

f magnetic flux density (N/A/m)

t time constant of analog integrator

Fig. 1. Block diagram of the three electrode corrosion cell. The steel

specimen was corroded in standard solution by galvanostatic corrosion

method. Fig. 2. Schematic of set-up for corrosion measurement.

V. Singh et al. / NDT&E International 37 (2004) 525–538526

The free energy change can be expressed as following

according to Faraday’s law:

DG ¼ Eð2nFÞ ð1Þ

The non-equilibrium potential generated by performing

the reaction, with [reactants] being the reaction concen-

tration and [products] being the product concentration is as

follows

E ¼ E0 20:059

nlg

½products�

½reactants�ð2Þ

An open circuit implies there is no current flowing and

when there is no current, there is no flow of electrons or

ions; therefore, corrosion or degradation of the anodic

material does not occur. The amount of anodic material

corroded or consumed when the current passes through

the circuit is governed by two Faraday’s laws of

Electrolysis [11].

The number of equivalents of corroded or dissolved

material is given as

Q

F¼

It

Fð3Þ

If m is the mass of the substance dissolved (oxidized) or

set free (reduced), then

Number of equivalent weight:

m

Mj

£n

nj

¼Q

Fð4Þ

The mass of substance:

m ¼njQ

nFMj ¼

njIt

nFMj ð5Þ

Hence, the number of moles

m

Mj

¼njQ

nF¼

njIt

nFð6Þ

Here,

n; the number of moles of the substances involved in the

electrochemical reaction.

F; Faraday’s constant, 96,494 C/mole

n; the number of electrons in the half-cell reaction.

Mj; the atomic mass or the molecular weight of the

substance.

t; time in seconds.

Samples sufficiently long (,1 m) were used in order

to permit future mechanical stressing, as well as to

reduce to negligible levels the demagnetizing field so that

solenoidal magnetic measurements were possible.

Measurements were performed for 6.2 mm diameter,

1140 mm long Stress proof steel specimen. The specimen

was surface cleaned and coated with lacquer except for

the middle 50 mm for corrosion. For each sample, the

open circuit potential (OCP) was measured directly by

the potentiostat. Tafel plot was obtained for the 6.2 mm

diameter steel sample as shown in Fig. 4 along with the

Cyclic Polarization curve (not shown).

Accelerated corrosion was performed by galvanostatic

corrosion method using 0.1 A current for 4300 s for the

desired 1 mass% loss (Fig. 5).

The electrolyte for the corrosion process was alkaline

solution simulating pore water solution of concrete

(0.075 M KOH þ 0.025 M NaOH þ 0.0025 M Ca(OH)2)

having pH of 12.5. To simulate the concrete contaminated

Fig. 3. Calibration curve for determination of mass loss. This curve was utilized to estimate the mass% loss for the long steel specimen.

V. Singh et al. / NDT&E International 37 (2004) 525–538 527

with deicing salts, 0.1 M NaCl þ 0.05 M CaCl2 salts were

added.

Photograph of the corroded 6.2 mm diameter stress proof

steel specimen, at various intervals during the experiment, is

shown in Fig. 6. EDS and SEM (scanning electron

microscope) analysis and Raman spectroscopy were per-

formed to clarify the composition and phases of iron oxides

of the corrosion laver formed on the steel specimen.

These results are compiled and discussed at the end of

paper.

3. Magnetic measurements

Measurement of the magnetic properties, variation of

magnetic induction B with magnetic intensity, H (hysteresis

curves) were obtained using the magnetic test set-up (Fig. 7).

The sample was corroded in the corrosion set-up. The

results obtained for the magnetic measurement for different

mass% loss (i.e. %loss in cross-sectional area) are shown

and discussed in the next section.

The temperature inside the tube encasing the steel (Fig. 8)

was increased by joule heating using a high current source

and magnetic measurements were made at approximately

every 5 8C increase in temperature. To ensure uniform

temperature distribution, the steel wire was maintained in

vacuum (Bi < 0). Magnetic field, H was measured using

hall sensors and the integrated voltage is related to the

change of magnetic induction (Eq. (9)). The system is

described in further detail in Refs. [4,7].

In the corrosion experiments, the cross- sectional area of

the bar, AF was reduced due to corrosion, which can be

described as the change in magnetic flux and hence related

to magnetic induction Bz (Eq. (9)). Variation of magnetic

properties with the change in temperature was measured

for a 7 mm diameter high strength specimen (yield

strength ¼ 1390 N/mm2, tensile strength ¼ 1660 N/mm2)

and 6.2 mm diameter stress proof steel specimen (yield

strength ¼ 762.4 N/mm2, tensile strength ¼ 915 N/mm2).

The initial magnetization curve and loops were obtained

as described in Ref. [7].

The magnetic flux f; though a filamentary current loop G

is defined as the surface integral of the magnetic induction.

Consequently, the time rate of change of the flux through G

is given by Ref. [9]

_f ¼d

dt

ððG

B·A ¼ðð

G

›B

›tdA ¼

dBz

dtAG ð7Þ

Here it has been assumed that B ¼ BzðrÞez; the radial

dependence is small enough to be ignored, and the loop is

stationary. Faraday’s law states _f ¼ 2emf; where emf is

the potential induced in the loop G. For a coil of N loops, the

induced voltage will thus be VscðTÞ ¼ N _f; and Eq. (7),

integrated between two arbitrary times, becomes

1

N

ðt0þDt

t0

VscðtÞdt ¼ 2AG

ðBzðt0þDtÞ

Bzðt0ÞdBz ð8Þ

The integral on the left hand side is performed with an

analog integrator with time constant t ¼ RC; resulting in an

integrated voltage, Vint;where the minus sign accounts for the

inversion due to analog integration. Solving for DBz gives

DBzðTÞ ¼ tVintðtÞ=ðAGNÞ ð9Þ

The integrator reset occurs at the end of the demagneti-

zation interval. Hence t0 is identified with this point and DBz

can be identified with the magnetic induction of the

specimen, provided that demagnetization was effective. A

real sense coil has finite length and pitch and consequently

the surface AG is difficult to define precisely, here it is taken as

the normal cross-section at the midpoint of the sense coil.

Eq. (9) is applicable to a solid ferromagnetic sample

with a tightly wound sense coil directly contacting

the sample. When this situation does not hold (reduced

cross-section due to corrosion) the flux distribution is

inhomogeneous and modifications to Eq. (9) are necessary.

Fig. 5. Potential and time plot (E (V) vs t (s)) for galvanostatic corrosion

test for 6.2 mm diameter stress proof steel specimen. The specimen was

subjected to 0.1 A current for 4300 s to get 1 mass% loss.

Fig. 4. Tafel plot for 6.2 mm diameter stress proof rod. Open

circuit potential (OCP) for the sample was EðI ¼ 0Þ ¼ 2569:0 mV.

Tafel slopes were measured as banodic ¼ 334:9 £ 1023 V/decade and

bCathodic ¼ 160:6 £ 1023 V/decade.


Hence the cross-section contains a ferromagnetic material

and the remaining is air (permeability m0) due to reduced

cross-section. Eq. (9) with Faraday’s law becomes

_f ¼ðð

Af

_B dA þðð

A02AF

_B dA

¼dBz

dtðA0 2 AFÞ þ m0

dHz

dfðAFÞ ð10Þ

_f ¼ 21

NVscðtÞ ð11Þ

The resulting hysteresis loops are shown in the next

section. The loops were extrapolated to the surface of the

steel specimen by a three-point quadratic fit using the

tangential field measurements obtained from three hall

sensors attached to the sense coil. The hall sensors were

Fig. 7. Schematic of set-up for magnetic measurements. Three hall sensors were attached to the sense coil as shown in the inset above. Quadratic extrapolation

of the results from hall sensors was utilized to obtain flux at the surface of the steel specimen.

Fig. 6. Photograph of the corroded sample (corroded length ¼ 50 mm, length of sample ¼ 1140 mm) at various intervals of the galvanostatic corrosion

measurement (4300 s) (A) at the beginning of the experiment (B) 1800 s after experiment is started (C) 3600 s after the start of experiment (D) specimen at the

end of the experiment, taken out of the corrosion reactor.


calibrated after installation onto the sense coil bobbin by

situating the sense coil at the middle of the excitation coil

and applying currents (0–20 A). The set-up is described in

detail in Ref. [7,8].

4. Results and discussion

In order to validate the experimental procedure for

measurement of magnetic properties of structural steel wire,

Fig. 8. Hysteresis curves at different temperatures for Nickel rod. Obtained data coincides with the data in Ref. [2], which validates the experimental

measurement procedure.

Fig. 9. Hysteresis curves for 7 mm diameter steel specimen at increasing temperatures. Ipeak was 2.5 A, quadratic extrapolation was employed to extrapolate the

curves to the surface of the specimen using the results from three hall sensors.


experiments were performed on Nickel rod. The variation of

magnetic induction, B with the magnetic field, H is shown in

Fig. 8, for different temperatures. The hysteresis curves

for Nickel are in close agreement with the curves given

in Ref. [2].

Magnetic property measurements were made for 7 mm

diameter steel specimen at different temperatures (Fig. 9). It

can be seen from the hysteresis curve (Fig. 9) for 7 mm

diameter cable that the coercive force, Hc varies with the

change in temperature. In general, the variation in coercive

Fig. 10. Hysteresis curve for 6.2 mm stress proof rod at increasing temperatures. The steel specimen was maintained in a vacuum to ensure isothermal

condition.

Fig. 11. Comparison of coercive force, Hc and saturation magnetic induction, Bsat at different temperatures for 7 mm steel specimen and 6.2 mm steel

specimen. Hc and Bsat differ for 7 and 6.2 mm mainly due to different rolling method during formation and different composition of steel.


force, Hc is dependent on the composition of the steel, the

type of heat treatment it is subjected to during formation of

rod and on temperature variation [2].

Hysteresis curve measurements for 6.2 mm. diameter

stress proof steel specimen were performed at approximately

the same temperature intervals as 7 mm diameter steel

specimen. The variation of magnetic induction, B with the

applied field, H is shown in Fig. 10.

Comparison of the hysteresis curves for 7 and 6.2 mm

diameter steel specimens shows (Fig. 11) that the variation

Fig. 12. Major hysteresis curves for corroded 6.2 mm diameter stress proof steel (1 mass% loss) specimen at different temperatures. Uncorroded cross-sectional

area was used to calculate magnetic flux, B:

Fig. 13. Comparison of major hysteresis curves for 6.2 mm. diameter Stress proof steel specimen at room temperature (28.9 8C) with different cross-sectional

area due to corrosion.


of Hc for 7 mm specimen is different from that for 6.2 mm

stress proof specimen. Composition of Stress proof 6.2 mm

steel specimen, % by weight is Fe (96.95%), Mn (1.4%),

C (0.41%), Si (0.22%), S (0.3%) and for 7 mm steel is Fe

(93.78%), Mn (1.8%), Si (0.87%) and C (0.7%). Hence the

higher carbon and silicon content gives 7 mm steel higher

strength. Also, 7 mm steel rod has been cold stretched

extensively to achieve high strength whereas the 6.2 mm

diameter stress proof steel rod is medium strength steel that

has been heated to austenite structure (above 800 8C), hot

rolled and then cooled in air. From the Fig. 11, it can be

observed that Bsat increases with temperature for 7 mm wire

whereas it decreases for 6.2 mm. The decrease in Hc with

temperature for 7 mm. wire is higher than that for 6.2 mm

steel specimen.

The 6.2 mm diameter steel specimen was then corroded

in the corrosion reactor (Fig. 2) for 4300 s, a time

precalculated, to achieve 1 mass% loss using the calibration

Fig. 14. Saturation magnetic induction, Bsat and Coercive force, Hc for different mass% loss as a function of temperature for 6.2 mm diameter steel specimen.

The measurement precision is from Ref. [4]. Reduced cross-sectional area was used to calculate magnetic induction, B. The saturation induction plotted here is

defined as Bsat ¼ m0ðH þ MsatÞ:


curve in Fig. 3. Hysteresis curves were obtained for the

corroded specimen as shown in Fig. 12. Curves were

obtained at two different locations, the sense coil centerlined

with the corroded length (50 mm), and the sense coil at an

offset of 10 mm from the corroded end. The curves obtained

for different locations along the corroded length were

comparable, implying uniform corrosion.

To verify the consistency of the variation in magnetic

properties with change in cross-section due to corrosion, the

steel specimen was again corroded in the corrosion reactor

for 4300 s to obtain a total of 2 mass% loss. The specimen

was then placed in the magnetic test set-up to perform

magnetic measurements (Fig. 13).

From the hysteresis curves obtained for 6.2 mm steel

specimen, it was difficult to observe the visible changes

in magnetic properties with variation in temperature. For

that reason the variation of coercive force, Hc with

temperature and variation of saturation magnetic induc-

tion, Bsat with temperature were plotted as shown in

Fig. 14. The hysteresis curves shown are the final

Fig. 15. Coercive force, Hc and saturation induction, Bpsat for varying mass% loss for 6.2 mm diameter steel specimen as a function of temperature. Uncorroded

cross-sectional area was used to calculate magnetic induction, B:


measurements referred to the tangential field extrapolated

to surface.

To obtain magnetic induction, respective reduced cross-

sectional area was used in Eq. (10) based on the calibrated

mass% loss from the calibration curve.

Coercive force, Hc is the property of the material hence

should not change with the change in cross-sectional area.

Hence, it is necessary to state here that the precision of

the measurement of magnetic field, H was 0.02 kA/m

which suffices the fact that the coercive force Hc

measured for different cross-sectional area is within the

%error of the measurement system. Similarly for the

magnetic induction, B the estimated experimental error is

0.013 T which justifies the consistency of material

magnetic properties [10].

The plot provides an important information about the

variation of coercive force, Hc with temperature that as

the temperature is increasing, Hc is decreasing. Although

the points are randomly distributed, it is possible to find

a linear fit for the plot (as shown in Fig. 14) and

Fig. 16. EDS spectra for 6.2 mm diameter steel specimen showing the composition of the metal surface (top plot) and composition of the corrosion layer

(bottom plot). Optical micrograph of the steel specimen from SEM scans for the uncorroded metal and corroded metal surface (insets) are shown with their

respective EDS plots.


the subsequent corrections can be made to the measure-

ment of magnetic field.

From Eq. (11), it can be observed that as the cross-

section of the steel specimen is reduced due to corrosion,

the flux through the air gap between the steel and the

sense coil is increased. Thus for the calculation of

magnetic induction ðBpsatÞ the cross-section of the steel is

assumed to be the same then for the same applied

magnetic field, the magnetic induction obtained ðBpsatÞ

with the decrease in steel cross-section.

Fig. 15 shows the variation of saturation induction with

temperature for different mass% loss considering the

uncorroded cross-section. Consistency of the variation of

magnetic induction with the variation in mass loss was

observed. The coercive force is unaffected by the change in

cross-sectional area for the reasons stated earlier. Hence it is

possible to estimate corrosion in a steel rod by measuring a

portion of steel which is not corroded and then obtaining

magnetic measurements for the corroded section. A

comparison of the magnetic induction for the two cases,

considering the uncorroded cross-sectional area for obtain-

ing B can lead to an estimate of corrosion.

5. Spectroscopic study of corrosion layer

SEM and EDS (energy dispersive spectrometer) were

used to analyze the composition of the corroded layer

formed during galvanostatic corrosion in standard solution

with deicing salts (Fig. 16). Chemical analysis (micro-

analysis) in the SEM was performed by measuring the

energy or wavelength and intensity distribution of X-ray

signal generated by a focused electron beam on the

specimen. With the attachment of the EDS, the precise

elemental composition of materials was obtained with high

spatial resolution. The microstructure of the 6.2 mm Stress

proof bar is shown in Fig. 16.

The spectrum for metal surface gives the composition %

by weight as Fe (96.95%), Mn (1.4%), Si (l.23%), S (0.42%).

The % composition of the corroded layer is as shown in

Table 1. The corrosion layer formed showed uniformity in

composition when three different locations were compared.

To clarify the chemistry of corrosion films formed on the

steel rod, Raman spectroscopy was performed in addition to

EDS and SEM scans. Raman spectroscopic measurements

were conducted by Reinshaw 2000 spectrometer using

514.5 mm line of air cooled Argon ion laser equipped with

Olympus microscope having a spatial resolution of less than

1 mm. Raman spectra were recorded in the range of

200–1500 cm21 (Raman shift) which corresponds to

6.25–50 mm wavelengths.

Raman Spectra for the corroded steel rod shown in

Fig. 17, confirms the presence of magnetite (Fe2O3),

maghemite (g-Fe2O3), hematite (a-Fe2O3) and lepodocro-

cite (g-FeOOH). Hematite obtained from Sigma Aldrich

was used for conditioning the equipment and was also used

as the reference for comparison of the peaks.

Fig. 17. Raman spectroscopy for 6.2 mm diameter steel specimen corroded in standard solution. The corrosion layer was uniform over the surface, hence three

locations were chosen randomly on the corroded steel specimen surface. The plot shows spectra for these three locations.

Table 1

Composition of the corrosion layer for 6.2 mm diameter steel rod

Ion composition %Weight

Fe (iron) 58.36

O (oxygen) 32.31

Mn (manganese) 3.27

Cl (chloride) 2.91

S (sulphur) 1.6

Ca (calcium) 1.19

K (potassium) 0.37


At location A, the peak at 225 coincides with a-Fe2O3, at

576 coincides with magnetite (Fe2O3). The short peak at

1464 is due to influence of maghemite (g-Fe2O3) Similarly

at location B, the peaks at 213, 282 and 1278 resemble the

spectra for hematite (a-Fe2O3) and the peak at 549

coincides with magnetite (Fe2O3), and the broader peak at

866–921 can be assigned to maghemite (g-Fe2O3) The peak

at 269 is related to red-brown lepodocrocite (g-FeOOH)

which has a major peak around 255 cm21. The stability of

lepodocrocite (g-FeOOH) is usually attributed to the

presence of chlorides which in our case is due to the

deicing salts added to the standard solution. At location C,

the peak at 275 and 1340 corresponds to hematite (a-Fe2O3)

and peak at 1474 can be attributed to the presence of

maghemite (g-Fe2O3) [12].

The two most prevalent products resulting from cor-

rosion in standard pore water solution with deicing salts are

Fe2O3 and Fe3O4. The magnetic measurements assumed

Fig. 19. Hystereis curve measurement for Fe3O4 powder at 25 8C.

Fig. 18. Hystereis Curve Measurement for Fe2O3 powder at 25 8C.


that the influence of the properties had they not been

mechanically removed, was small. It is of interest to

determine the extent to which this is true. To measure the

magnetic properties of Fe2O3 and Fe3O4 powder was

compacted into a glass tube and porosity computed based

on the particle density and the volume of the powder filled.

Two samples each for Fe2O3 and Fe3O4 were prepared

and the results are shown in Figs. 18 and 19. The magnetic

measurements obtained were reduced to the response due to

Fe2O3 or Fe3O4 by following equation

BðFe2O3Þ ¼fðFe2O3Þ

ð1 2 PÞ £ AIDoftube

Here, P is porosity of oxide in the glass tube and f is

measured flux.

There is little published data for comparison, although

the inferred coercive force for magnetite appears reasonable

[6]. The effect of magnetite at high fields was estimated to

account for one third of the flux deficient due to the corroded

volume. Further work is required to determine the effect of

this corrosion product on sensitivity.

6. Conclusions

Measurement of hysteresis curves (B–H curves) for

structural steel bars were performed at elevated tempera-

tures for different mass% loss due to corrosion in order to

estimate the effects of temperature changes on magnetic

properties of the structural steels. The change in magnetic

induction, B with %change in cross-sectional area of the

steel due to corrosion was correlated to predict the corrosion

in terms of mass% loss of structural steel. Hence, the

variation of magnetic properties can be compared to the

calibrated uncorroded sample with the in-field steel rods

under investigation and thus can give an estimate of the

mass% loss or change in cross-sectional area. The results

obtained were consistent with the theoretical data incorpor-

ating the experimental uncertainities. The variation of

coercive force, Hc with change in temperature is a

significant estimate of the temperature corrections for the

magnetic properties of the steel. A linear fit over the

temperature range investigated was found to be adequate.

The variation of magnetic induction with decrease in cross-

section of steel due to corrosion was consistent with the

theoretical and measured mass% loss. It was also verified

that the composition of the steel and the heat treatment

during rolling and temperature also the coercive force, Hc

and magnetic induction, Bsat. Hence it can be concluded that

the temperature corrections are imperative when magnetic

measurements are made for structural steels subjected to

environmental temperature variation.

Acknowledgements

The authors wish to acknowledge support provided by

the National Science Foundation under Grant NSF-

0085204. The bipolar current supply was designed by

Peter Fabo and Andrej Jarosevic of the Department of

Radiophysics, Commenius University (Bratislava, Slova-

kia); their assistance is greatly appreciated.

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Documents

Effects of temperature and corrosion thickness and composition on magnetic measurements of structural steel wires