Abstract:
A static DC magnetic field is externally applied to a targeted surface portion of protectively coated steel to vary the amount of microwave energy absorbed therein. Measurements of varying amounts of absorbed microwave energy are compared by coordination with corresponding measurements of the strength of the applied magnetic field varied in response to coating hidden deposit of corrosion products on the targeted surface portion of the steel, to provide a basis for detection of the corrosion involved.

Description:
The present invention relates in general to the detection of corrosion on a steel surface underlying a non-magnetic coating thereon. 
     BACKGROUND OF THE INVENTION 
     The detection of corrosion underneath a protective coating on the surface of steel has been traditionally achieved by electrochemical techniques involving the use of electrolytes which render detection cumbersome and time consuming. Non-destructive evaluation techniques have also been utilized for detection of defects, involving ultrasound, eddy current, radiography or thermography reflecting changes in base metal caused by the defects. Such non-destructive evaluation techniques detect changes in mass density, elastic stiffness or conductivity of an electrical or thermal type in physically local environments of metal oxide mixtures or metal voids created by corrosion. Since typical steels have expectedly wide variations in properties associated with such local environments, it would be difficult to distinguish between such variations and those resulting from corrosion hidden underneath a coating on the steel. Also some of the foregoing existing techniques are sensitive to material thickness and geometrical effects unrelated to corrosion so as to render corrosion detection unreliable. 
     The detection of hidden corrosion in aluminum alloys involving use of nuclear magnetic resonance, is disclosed for example in U.S. Pat. No. 5,905,376 issued May 18, 1999. The corrosion detection technique, as disclosed in such patent, is not however applicable to steels. It is therefore an important object of the present invention to provide a reliable technique for detection of corrosion in the form of magnetic oxidation products hidden underneath a protective coating covering a targeted surface of steel. 
     SUMMARY OF THE INVENTION 
     In accordance with a corrosion detection method of the present invention, microwave energy of an appropriate frequency is absorbed in a body of steel by transmission through a protective coating on its targeted surface when a static DC magnetic field of less than 0.5 Telsa is externally applied thereto. Such absorbed microwave energy reflected from the steel body is measured through a portable sensor to provide microwave measurement data that is compared by coordination with data on variations in the strength of the magnetic field before and after corrosion of the targeted steel surface for reliable and readily available corrosion detection. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     A more complete appreciation of the invention and many of its attendant advantages will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein: 
     FIG. 1 is a block diagram of the corrosion monitoring system of the present invention; 
     FIG. 2 is a partial section view and diagram depicting electronic apparatus associated with one embodiment of the system depicted in FIG. 1; 
     FIG. 3 is a graphical representation of absorbed microwave energy reflected from painted steel before corrosion; 
     FIG. 4 is a graphical representation of absorbed microwave energy reflected from painted steel after corrosion; 
     FIG. 5 is a graphical representation of microwave energy absorbed in pristine steel; 
     FIG. 6 is a graphical representation of microwave energy absorbed by one type of oxide (magnetite) in corrosion products; and 
     FIG. 7 diagrammatically illustrates electronic corrosion monitoring associated with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawing, FIG. 1 outlines a corrosion monitoring system for detection  10  of corrosion products in the form of magnetic oxides underlying a protective, non-magnetic coating on a targeted steel surface  12 . The procedure associated with the corrosion monitoring system is initiated by generation  14  of variable field strength energy for a static DC magnetic field  16  externally applied to the targeted steel surface  12 . Microwave energy of an appropriate frequency from a source  13  is absorbed by the steel as a result of the action of the magnetic field  16  on its target surface  12 . The absorbed microwave energy then reflected from the target surface  12  undergoes sensing  18  to provide for reflected microwave intensity measurement  20 . The data so obtained by the microwave intensity measurement  20  together with the value data on variable magnetic field strength of the applied magnetic field  16 , produced by generation  14 , are both utilized for data coordination  22  in order to provide an output as the detection  10  of the magnetic oxide corrosion products. 
     FIG. 2 illustrates by way of example a body of pristine steel in form of a plate  24  on which is located the targeted surface  12  denoted in FIG. 1 to be monitored for corrosion pursuant to the present invention. Such targeted surface on the steel plate  24  underlies a non-magnetic type of protective coating  26 , such as paint, insulation or camouflage layers. The coating  26  hides the corrosion products which tend to form on the targeted steel surface during initial stages of corrosion or oxidation. Typical of the corrosion product magnetic oxides is magnetite (Fe 3 O 4 ) having an inverted spinnel crystal structure and a permanent magnetic moment of 4 Bohr magnetrons as compared to that of iron (Fe) having a body center cubic crystal structure and an experimentally observed moment of 2.2 Bohr magnetrons. Other magnetic oxides include gamma (Fe 2 O 3 ). 
     With continued reference to FIG. 2, an electromagnet  28  is positioned as shown on the plate  24  to externally apply the static magnetic field  16  (as denoted in FIG. 1) to the coated steel plate surface between magnet pole portions  30  and  32 . Such magnetic field is induced through the electromagnet  28  in response to electrical energy fed thereto from the power supply  34 . Such generation  14  of the magnetic field  16  imposes a strength thereon that is controllably varied through an on/off control  36 . The strength of the static magnetic field is typically less than 0.5 Tesla so that the apparatus involved is relatively small and lightweight. 
     The microwave source  13  as outlined in FIG. 1, includes an antenna  38  as diagrammed in FIG. 2 focused on the plate  24  between the pole portions  30  and  32  of the electromagnet  28 . The antenna  38  is connected through a signal separator  40  to a microwave energy source  42  from which the microwave energy is transmitted by the antenna  38  to the steel plate  24  at a suitable frequency and polarization entirely orthogonal to the direction of the magnetic field. Part of the microwave energy so absorbed by the material in the steel plate  24  under inducement of the magnetic field when applied, is reflected back and picked up by the same antenna  38  for direction through the signal separator  40  to a detector  44  so as to undergo the previously referred to intensity measurement  20  diagrammed in FIG. 1 through a power indicator  46  connected to the detector  44  as diagrammed in FIG.  2 . 
     In regard to data coordination  22  as diagrammed in FIG. 1, the different magnetic properties of the pristine steel of plate  24  and the magnetic corrosion oxides deposited on its targeted surface give rise to peak absorption of the microwave energy at different strength values of the applied magnetic field to provide a basis for distinguishing between corroded steel and uncorroded steel coated for example with paint which prevents visual observation of corrosion. By choosing a value of the magnetic field strength when it is applied, which is larger than the peak value in pristine steel but smaller than the peak value for oxides, the absorbed microwave increases under the influence of the corrosion product oxides to indicate its presence by detection  10  because of the respectively different residual magnetization of the pristine steel and the magnetic oxides. 
     FIG. 2 also diagrams use of a reflection method to sense the presence or absence of microwave absorption on the targeted surface of the steel plate  24 . Antenna in the form of a portable microwave sensing resonator  38  having an aperture  50  is positioned on the targeted surface between the magnet poles  30  and  32 . The measured coefficient of microwave energy reflection from such targeted surface being monitored is proportional to the differences between waveguide impedance and the impedance of the resonator  38  which varies with magnetic field applied through the magnet  28  and the presence of corrosive oxides so as to provide a more reliable basis through detection  10  for indicating corrosion as reflected by experiments conducted with a magnetic field of 0.1 Tesla applied. The results of such experiments conducted on uncorroded and corroded steel plates coated with paint, are graphically depicted in FIGS. 3 and 4 in terms of measured reflected power with a magnetic field of 0.1 Telsa applied to the pristine steel of plate  24 . A comparison of the graphs in FIGS. 3 and 4 respectively associated with uncorroded and corroded steel shows an increase in the reflected power of the absorbed microwave energy when corrosion products are present. 
     In regard to the metal oxide corrosion products involved herein, their electrical conductivity is less than that of the pristine steel of plate  24 . Also, attenuation of the microwave energy transmitted through such oxides is governed by the skin effect or thickness of the coating layer  26  so as to affect the amount of the microwave energy reaching the substrate of the steel plate  24 . Since the chemistry and crystallographic nature of the steel and the corrosion oxides are very different, the amounts of microwave absorptions therein are well separated as reflected by a comparison of the experimentally derived graphs respectively depicted in FIGS. 5 and 6. Based on the foregoing referred to differences between pristine steel and corroded steel, equipment is calibrated under coordination  22  as diagrammed in FIG. 1 by use of pristine metal and magnetite sheets to provide opposite responses in electronic equipment in order to facilitate detection  10  by recognition of the distinction between pristine steel and corroded steel. 
     FIG. 7 diagrams a modified embodiment of the electronic system for monitoring corrosion as hereinbefore described in connection with the steel plate  24  having the coating  26  thereon, underlying the opposite pole portions  30  and  32  of the electromagnet  28  and with the resonator sensor  38  positioned thereon. Superimposed in the system with the on/off control  36  and the magnetic power supply  34  through which the DC magnetic field is generated by the magnet  28 , is phase modulation involving an AC modulator  56  connected to energizing coils  58  and  60  on the electromagnet  28 , also having a DC bias  62  imposed thereon through a biasing coil  64 . The output of the microwave absorption sensing resonator  38  is applied to an oscilloscope display  68  and to the AC modular  56 . Depending on the presence or absence of corrosion products and the nature of the base steel in plate  24 , absorbed microwave radiation reflected and sensed by resonator  38  as hereinbefore described effects a change in phase of the absorbed microwave energy, which would lag the modulation imposed alone through the pristine steel of plate  24  by the signal output of AC modulator  56 . This phase would be an advance of such imposed modulation when corrosion products are present, providing more readily achieved detection of corrosion products on the targeted surface of the steel plate  24 . Other embodiments may similarly involve pulse modulation for enhancing detection of corrosion. 
     Obviously, other modifications and variations of the present invention may be possible in light of the foregoing teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.