DEVELOPMENT OF AN INTELLIGENT ELECTROMAGNETIC
SENSOR TO DETECT FERROUS CORROSION PRODUCTS UNDER
STRUCTURAL COATINGS
J. M. Liu and R. L. Ruedisueli
Metals Department, Naval Surface Warfare Center Carderock Division, West Bethesda,
MD 20817-5700
ABSTRACT. A sensor based on the change in the magnetic field-induced microwave absorption
due to the presence of ferrous corrosion products on steel is described. This type of sensor for
detection of corrosion under structural coatings is unique in that its sensitivity is to chemical, rather
than physical alterations, to steel surfaces by corrosion processes. Preliminary results suggest that this
sensor may be particularly sensitive to ferrous corrosion products found in the initial phases of the
corrosion process.
INTRODUCTION
For protection against the environment or operational purposes, many metallic
structures are painted or otherwise covered by protective, often thick, coatings, jackets or
tiles. Unexpected loss of such coatings has occurred in the past with severe ramifications
in repair costs and in loss of service. In many of these cases rust undercutting was the
cause for coating loss and failure.
Because of the need for "reduced manning," and "reduction in maintenance cost,"
a change in the philosophy for maintenance from "time-based" to "condition-based" has
been promoted by various government organizations. To translate this philosophy into
practice, sensors need to be developed that enable rapid surveillance of the conditions of
materials under paint, camouflage, insulation, or other protective layers without costly
material removal. Paint or other coating removal and reapplication on Navy ships, for
example, is very a labor-intensive endeavor. A sensor that can reliably ascertain the
initiation of corrosion by identifying the presence of corrosion products underneath paint
or ceramic coatings will provide not only a more intelligent maintenance scheduling and
cost savings, but also a reduction in the environmental hazards associated with
unnecessary coating removal.
Recent advances are evident in imaging NDE techniques using thermography,
ultrasound, eddy current and other technologies to detect hidden corrosion. Imbedded
sensors using special signal excitation and reception techniques, including those based on
telemetry, also show promise for such an application. However, in most current systems
the detection mechanism is based on the difference in the physical properties such as
elastic moduli, thermal and electrical conductivity, or the presence of geometrical features
such as cracks, voids, or surface roughness between the corroded and base material. There
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
2003 American Institute of Physics 0-7354-0117-9
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has not been much attention to exploiting the difference in specific chemical properties
between the corroded and the base material.
In the following paragraphs, we briefly review some examples of corrosion
detection based on the chemistry of corrosion products under structural coatings that can
be exploited for "smart" sensor development. This approach has the advantage that the
effects of property variations unrelated to corrosion are reduced. One particular example
[1] is a concept based on the magnetic field induced microwave absorption in some of the
corrosion products in steel. This absorption changes the strength of microwave reflected
from the coated steel when an external magnetic field is applied. A sensor based on this
mechanism should be particularly sensitive to the initiation of corrosion for which some of
the current NDE techniques may be insensitive
Development in corrosion sensors can take different approaches. Some are based
on sensing the changes in the local electrochemical environment (e.g. a technique called
Electrochemical Impedance Spectroscopy [2]), electrical conductivity, or other physical
properties. Most chemical sensors tend to respond to the combined effects of all the
chemical constituents present, and require the presence of an electrolyte to function.
Various practical, nondestructive evaluation (NDE) techniques [3, 4] based on
ultrasound, radiography, thermography, and electromagnetic technology are currently in
use to detect the occurrence of hidden corrosion (marine and high temperature). At
present, none of these commercialized techniques can pinpoint the occurrence of chemical
changes associated with corrosion or oxidation. Frequently, the interpretation for the
results of measurements is complicated by the effects of material geometry, dielectric
properties of the coating, or the static permeability of the metal unrelated to corrosion or
oxidation.
In order to improve the technology for corrosion or oxidation detection unbiased
by uncontrollable but acceptable variations in material thickness and physical properties,
we need to know how a property associated with the corrosion products can produce a
unique signature "upon request," so that their presence can be unambiguously identified
and the stages of corrosion or oxidation can be reliably assessed. It will be shown in the
following paragraphs that the microwave absorption in the magnetic species in the
corrosion products, which can be turned on or off by controlling an external magnetic
field, is such a property providing such a unique signature. This technique is not affected
in any way by the nonmagnetic properties or geometry of the coating or the base metal.
A sensor based on this mechanism should be particularly sensitive to the initiation
of corrosion for which some of the current NDE techniques may be insensitive. In the
following paragraphs, we present the physical basis and preliminary experimental results
that illustrate this concept.
THEORY
Ferromagnetic resonance absorption of iron oxidation has been reported [5]. It
turns out that the frequency of this ferromagnetic resonance absorption in metals and
magnetic oxides can be described in general [6, 7] by an expression of the following type,
in Equation (1), when the magnetic field is parallel to the surface of the specimen:
H(H + 47iM)
(1)
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Where f is the frequency in Hertz, y is a constant (2.8 MHz / Oersted), H is the
externally applied magnetic field, and 4 n M is the saturation magnetization for a magnetic
species. Magnetite, FesO^ has a saturation magnetization at room temperature of
approximately 6000 Oersted; for iron it is approximately 21,000 Oersted [8, 9]. Not only
does the frequency but the polarization of the microwave with respect to the direction of
the DC magnetic field vector has to satisfy this relationship.
EXPERIMENTAL APPROACH
Experiments were performed in a set up with the DC magnetic field directed
parallel to the surface of the material. Microwaves polarized perpendicularly to the
direction of this DC magnetic field were beamed towards the material via a coupling hole
in the bottom of a standard resonator constructed from a segment of X-band wave guide.
Either a laboratory electromagnet or a portable electromagnet (used for magnetic particle
NDE applications) can be used to establish the requisite magnetic field. A schematic of
the measurement system is shown in Figure 1 .
The material was placed either in the middle or at the fringe of this DC magnetic
field. The magnetic flux was measured with a gauss meter. The change in the microwave
signal reflected from the resonator coupled to the material as the DC magnetic filed was
applied was used to indicate the occurrence of absorption by the magnetic species present
in the corrosion products. The resonator was operated in the "under coupled' mode so that
the signal was always increased when absorption occurred.
FIGURE 1. Schematic of measurement set up for the detection of magnetic field-induced microwave
absorption in coated steel.
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1.8
1.4
1,2
Steel
0,8
0.6
0.4
Fe3O4 on Steel
0.2
0,0
1,0
2,0
3,0
4.0
5,0
6.0
7.0
8.0
Magnetic Induction (KG)
FIGURE 2. Reflected microwave signal from pristine steel and steel covered with Magnetite as a function
of applied DC magnetic field.
1.6
PAINTED MED STEEL
1.2
0.8
0,4
0.2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
DrtvtogVott-g.fVolt)
FIGURE 3. Reflected microwave signal from painted steel as function of applied DC magnetic field.
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1.6
1,4
PAINTED RUSTED STEEL
1.2
0.8
0,6
0.2
0,0
tO
2,0
3,0
4,0
5,0
6.0
7,0
FIGURE 4. Reflected microwave signal for painted and rusted steel as a function of applied DC magnetic
field.
CONCLUSIONS
We have demonstrated the feasibility of a new technique to detect the presence of
corrosion products on the surface of steel, using the magnetic field induced microwave
absorption. This approach has the benefit that the physical properties and geometry of the
coating as well as the static magnetic properties of the steel (e.g. magnetic permeability)
should not affect the measurements. With further development, this could form the basis of
a sensor for corrosion detection with improved intelligence and reliability.
ACKNOWLEDGEMENTS
Authors appreciate very useful discussions on various aspects of ferromagnetic
resonance with Prof. Sam Lofland, Jr., Rowan University, Glassboro, NJ, and Prof. Sam
Bhagat, University of Maryland, College Park, MD.
REFERENCES
1. J. M. Liu, "Nondestructive Detection of Steel Surface Corrosion," U.S. Patent No.
6411105 Bl, June 25,2002.
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2. Electrochemical Impedance and Noise, R. Cottis, S. Turgoose, and B. Syrett, Series
Editor, NACE International, Houston, TX (1999).
3. G. A. Matzkanin and J. K. Easter, "NDE of Hidden Corrosion- A State-of-the-Art
Report," NTIAC-SR-98-03, Nondestructive Testing Information Analysis Center,
Texas Research Institute Austin, Inc., Texas, 1998.
4. K. G. Lipetzky, M. R. Novack, I. Perez, and W. R. Davis, "Development of
Innovative NDE Technologies for the Inspection of Cracking and Corrosion under
Coatings," NSWCCD-61-TR-2001/21, Naval Surface Warfare Center Carderock
Division, Bethesda, Md. 2001.
5. Z. Frait, et al, "Ferromagnetic Resonance in Surface Oxidized Iron Single Crystals,"
Czech. J. Phys., B25, 906-915 (1975).
6. S. M. Bhagat, "Resonance Methods-Magnetic Materials," Chapter 8, Techniques in
Metals Research. Vol. VI, Part 2, pp.79-164, John Wiley (1973).
7. S. M. Bhagat, ASM Handbook. Vol.10,267 (1986).
8. F. N. Bradley, Materials for Magnetic Functions. Hayden Book Co., New York (1971).
9. J. Smit and R P. J. Wijn, Ferrites. John Wiley (1959).
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