Abstract:
A fiber optic magnetic field gradiometer uses the first derivative of a magnetic field associated with a corrosion process to determine the location on the surface of a metal of the corrosion at its onset or very early stages, whereas eddy current type detectors detect the corrosion only after a significant amount of the metal has already been removed. Two adjacent flat magnetic transducers made of magneto-strictive glass, onto which are secured two optical fibers, are immersed in an applied magnetic field to null out material differences in the earth&#39;s magnetic field. The optical fibers are secured to a surface of each transducer to provide a relatively flat sandwiched pair of transducers. The flat magnetic transducers are sandwiched together and scanned over the metal surface. A magnetic field associated with the corrosion process in the direction of the axis of the flat magnetic transducers causes an optical path length change in the fibers. The overall path length&#39;s change is proportional to the first derivative of the magnetic field.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to surface and subsurface corrosion detectors and in particular to a fiber optic magnetic field gradiometer for detecting magnetic gradients generated by electrochemical corrosion reactions. 
     The flow of metal ions between two metal surfaces occurs during the corrosion process. Since magnetic fields are associated with current flow, a corrosion process has an associated magnetic field. Therefore, corrosion can be located and identified by detecting these magnetic fields. 
     A variety of techniques have been developed for studying electrochemical corrosion, including actually weighing the material lost from a sample, monitoring the voltage which develops between two corroding electrodes, or even determining the equivalent circuit of a multielectrode cell by measuring its frequency response. However, the above-mentioned techniques are necessarily invasive and/or destructive. It is of interest to detect corrosion currents in a non-invasive non-destructive manner. 
     Magnetic fields associated with corrosion have been detected and reported by J. G. Bellingham and M. L. A. MacVicar in &#34;SQUID Technology Applied to the Study of Electrochemical Corrosion,&#34; (IEEE Transactions on Magnetics, Vol. MAG-23, No. 2, March 1987). There a SQUID gradiometer was used to detect the magnetic fields associated with electrochemical reactions or corrosion. Since a SQUID device must be immersed in liquid helium, its associated dewars and insulation restrict the sensor to be at least 2 centimeters away from the surface. In addition, such bulky dewars make it impractical to actually scan a SQUID sensor over a surface. 
     Accordingly, it is an object of the present invention to provide a method and apparatus capable of scanning and detecting corrosion at the surface or subsurface of a metal. It is a further object of the invention to provide an apparatus that is small and lightweight. Other objects and advantages of the invention will be more apparent hereinafter in reference to the detailed description of the preferred embodiments and the drawings. 
     SUMMARY OF THE INVENTION 
     Two flat magnetic transducers are each wrapped with a single mode optical fiber and sandwiched on top of each other. The fibers are so attached to the transducers that good coupling results when the transducers are subjected to a magnetic field of magnitude which is desired to be detected. The transducers along with their respective fibers are immersed in an applied magnetic field to balance out material differences and the earth&#39;s magnetic field, and to establish an appropriate bias field level. Light from a laser is then launched into the two single mode optical fibers and recoupled after passing through the fibers. A magnetic field along the sensitive axis, of the transducer causes magneto-strictive glass in the transducer to expand or contract, thereby changing the optical path length. The output of the coupler is then used to determine the overall path length change which is proportional to the first derivative of the magnetic field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents the relationship between a line current in the Z direction confined to the X-Z plane and its associated magnetic field. 
     FIG. 2 is a perspective view of a first embodiment of the invention. 
     FIG. 3 is a top view of a transducer made in accordance with the invention and showing an optical fiber tacked onto the surface of a magnetostrictive foil. 
     FIG. 4 is cross-sectional view of the corrosion detector head taken along plane 44 of FIG. 2. 
     FIG. 5 is a cross-sectional view of the corrosion detector head as taken along a plane perpendicular to plane 44 of FIG. 2. 
     FIG. 6 is a block diagram of the first embodiment of the invention. 
     FIG. 7 is a diagrammatic view of the corrosion detector head on a surface 1. 
     FIG. 8 is a graph of the x component of the magnetic field. 
     FIG. 9 is a diagrammatic representation of a second embodiment of the invention. 
     FIG. 10 is a schematic representation illustrating the driving of several corrosion detector heads using one laser. 
     FIG. 11 is a schematic diagram showing multiple corrosion detector outputs with no multiplexing. 
     FIG. 12a is a diagrammatic representation of a multiple corrosion detector head system with two corrosion detector heads on one optical interferometer. 
     FIG. 12b is a corresponding schematic diagram of the multiple corrosion detector head system including a detector system and lock-in amplifier(s). 
     FIG. 13a is a schematic diagrams showing a multiple corrosion detector head system using two optical interferometers each with two corrosion detector heads. 
     FIG. 13b shows a corresponding system using a star coupler in place of the 2×2 couplers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The corrosion detector in accordance with the invention depends on the detection of small magnetic field gradients arising from the corrosion currents at or near the specimen surface. In order to &#34;simulate&#34; a corrosion field with a known magnitude, consider a single thin wire carrying current, i(t) in the Z direction running through the point P with coordinates (0, 0, Z p ) as shown in FIG. 1. The component of the magnetic field in the X direction, B x  is given by, ##EQU1## where .sup.μ o is the permeability in a vacuum 
     The derivative of B x  in the Y direction is, ##EQU2## 
     If the median plane is a distance Y=d above the surface 1, then ##EQU3## 
     At X=0, δB x  /δY has its maximum value of -.sup.μ oi/2πd 2 . Thus, obtaining the first derivative of the magnetic field provides the ability to locate corrosion activity on the surface of a metal. 
     A corrosion detector head, generally designated by reference numeral 3 is shown in FIG. 2. The corrosion detector head 3 is constructed using two vertically offset magnetic transducers each of which has a magnetically sensitive axis oriented along the field direction whose first derivative is to be measured. In the example of FIG. 2, transducers 5 and 35 are offset in the y direction relative to one another, and each transducer is oriented with its magnetically sensitive axis along the x direction. Transducers 5, 35 are in the form of thin rectangular boxes, each containing a thin layer of magneto-strictive glass 7, 27. FIG. 3 shows a top view of a transducer core 11, 31 which is located inside each of the transducers 5, 35 respectively. Optical fiber 9, 29 is secured to the surface of the magneto-strictive glass 7, 27 using an epoxy tack. The magnetostrictive glass is typically on the order of 0.001&#34; thick. Spacer strips 10, 30 are placed on either side of the magneto-strictive glass 7, 27 alongside the optical fiber 9, 29, respectively. As seen in FIG. 2, the transducer core 11, 31 is placed inside a mandrel 13, 33 around which a wire 14, 34 is wrapped, respectively. 
     Each transducer is then wrapped in a metal foil 16, 36. The transducers 5, 35 are then sandwiched on top of each other with a piece of tissue paper 26 between them as shown in FIGS. 2, 4 and 5. It is noted that FIG. 4 is not to scale in relation to FIG. 3, and only a few of the many passages of optical fiber 9 are shown in FIG. 4. 
     In operation, the sandwiched transducers are placed on another piece of tissue paper 37 and placed on top of a base plate 38 and clamped down with a hold down piece 39, which is secured to the base plate 38 as seen in FIG. 2. The base plate 38 has a coating 40 on the side opposite the transducers in order to prevent the base plate 38 from scratching the surface 1 to be measured. Rollers or wheels can be placed at the base plate 38 to facilitate scanning over the surface 1. 
     The corrosion detector head 3 must remain as thin as possible in order to detect weak magnetic fields near the surface 1. Thus, the corrosion detector detects magnetic fields and their gradients arising from currents at the corrosion sites in the near field. FIG. 4 shows a cross-sectional view of the sandwiched transducers as taken along the plane 44 of FIG. 2. FIG. 5 shows the cross-sectional view of the sandwiched transducers as taken in a plane perpendicular to plane 44. Typical thicknesses are as follows: the spacer strips 10, 30 are 0.030&#34;, the walls of the mandrel 13,33, are 0.030&#34;, the wires 14, 34, have a diameter of 0.002&#34;, the walls of the aluminum foil 16,36, are 0.003&#34;, the tissue paper 26, is 0.001&#34;, the base plate 38, is 0.030&#34;, and the base plate coating 40, is 0.001&#34;. Hence, the corrosion detector has a total stack thickness of about 0.230&#34;. 
     Operation of the corrosion detector is similar to the operation of the fiber optic field gradiometer described in U.S. Pat. No. 4,814,706 and incorporated herein by reference. Balance is continuously nullable as described in U.S. Pat. No. 4,904,940 and incorporated herein by reference. In this case, however, only two magnetic transducers 5, 35 are used. Referring to FIG. 6, corrosion detector 62 is illustrated as comprising magnetic transducers 5,35 and associated optics. Light from a laser 52 is launched through a coupler 54 into the single mode fibers 9, 29 and recoupled after passing through the transducers 5, 35 at a coupler 56. The output of the coupler 56 is fed through two optical fibers 57 into detector system 58, wherein the detector system 58 contains for instance two PIN diode detectors to detect the output of each of the fibers 57. 
     In a manner similar to that described in U.S. Pat. No. 4,814,706, the two transducers 5, 35 having fibers 9, 29 recoupled at coupler 56 comprise a Mach-Zender interferometer with both fibers 9, 29 operating as measurement arms. Detector system 58 provides active stabilization in order to maintain the corrosion detector in quadrature. Hence, the corrosion detector 62 is not affected by random phase drift due to small environmental changes and the phase of the interference signal emanating from the coupler 56 is locked. Circuits 64 and 68, connected to the magnetic wires 14 and 34 respectively, serve the same roles as the applied magnetic field circuit 30 in U.S. Pat. No. 4,814,706. The wires 14 and 34 are wrapped around the mandrels 13 and 33 respectively with sufficient turns to provide easy control of the applied magnetic field of appropriate magnitude. Each circuit 64, 68 is equipped with a phase shifter 71,74 and a variable attenuator 72,75 so that AC and DC magnetic fields created in transducers 5 and 35, respectively may be independently adjusted for amplitude and phase. In addition, a static DC magnetic field bias may be applied by adjusting the magnitude of the voltage at a DC supply 73,76 contained in each circuit 64, 68, respectively. The DC magnetic biases are applied in order to compensate for any material response differences, to compensate for earth&#39;s magnetic field, and to provide the appropriate bias field to ensure that the magneto-striction is quadratic in field strength. Also, the AC field is used to up convert a DC signal of interest to a convenient frequency f well away from the effects of noise at extremely low frequencies. 
     An object of the invention is to provide a compact device which can be scanned across a surface in order to locate corrosion activity. Referring to FIGS. 1 and 7 a corrosion current i(t) can be detected by scanning the corrosion detector head 3 back and forth in the X direction over the corrosion current i(t), where plane Y2 corresponds to the magneto-strictive glass 7 in transducer 5 and plane Y1 corresponds to the magneto-strictive glass 27 in transducer 35. As the corrosion detector head 3 passes over the point P (see FIGS. 1 and 8) the value of the magnetic field component B x  varies at Y=Y1, Y=d, and Y=Y2 as shown in FIG. 8. Typically, a null condition is initially achieved during set up or calibration so as to compensate for small differences in any defective coupling of the fibers to the magneto-strictive glass at each transducer. After nulling, the corrosion detector head is passed over the corrosion current i(t), and the magnetostrictive glass 7 and 27 expands or contracts. However, the X component B x  of the magnetic field will not change at the same rate in the planes Y1 and Y2. Since the magnitude of B x  is larger at plane Y1 than at plane Y2, the magneto-strictive glass 27 will expand or contract more than the magneto-strictive glass 7. Hence, the effective optical path length difference between the two fibers 9 and 29 is changed. Also, as mentioned above, with the appropriate field biasing, this path length change is quadratic in the magnetic field. Hence, at balance, the phase of the remaining signal at frequency, f is proportional to the difference between the DC external field at planes Y2 and Y1. 
     At balance, the difference in the value of the X component of the magnetic fields, B x  between the plane Y1 and Y2 is proportional the intensity at the output of coupler 56. Provided that the value of B x  varies monotonically over the distance s between the planes Y1 and Y2, the corrosion detector provides an output proportional to the gradient of B x , δB x  /δy| y0  evaluated at some point Y0, where Y1&lt;Y0&lt;Y2. Y does not necessarily equal the midpoint d. However, this discrepancy remains fixed as long as the distance d corresponding to the median plane and the distance s (see FIG. 1) remains fixed. Therefore, the discrepancy may be compensated by rescaling, or increasing the number of passes of fiber on the transducer 5. 
     Another object of the invention is to provide a corrosion detector with high enough sensitivity to detect corrosion currents. The present sensitivity of gradient detection, using magneto-strictive foil that has not been annealed, is 8.3×10 -9  Gauss/ centimeter for approximately 52 meters of fiber on each transducer. Here, in order to detect corrosion currents distributed over small areas, it is desirable to keep the spatial resolution of the detector high, e.g. ±1/4 centimeters on a side. Hence, the magneto-strictive glass should be about 1&#34; by 1&#34;, and consequently about 192 one centimeter passes of fiber 9, 29 fit on each magneto-strictive glass 7, 27. This decreases the sensitivity of the corrosion detector to about 3×10 -7  Gauss/ centimeter. If the magneto-strictive glass 7, 27 is annealed, this value will be one order of magnitude more sensitive, or 3×10 -8  Gauss/centimeter. This is sufficient to measure the field magnitudes. By increasing the area of the magneto-strictive glass 7, 27, which in turn allows both an increased number of passes of fiber 9, 29 and each pass of fiber to exceed one centimeter, the sensitivity of the corrosion detector can be increased to as high as 10 -9 , 10 -10  or even 10 -11  Gauss/centimeter. 
     A second embodiment of the invention is shown in FIG. 9, where multiple corrosion detector heads 79 can be placed in parallel and/or in series. Here, as in the previous cases, each corrosion detector head 3 can be scanned across the surface 1 and/or the surface 1 can be moved across the corrosion detector head. FIG. 9 may be implemented with a plurality of lasers, one laser corresponding to each detector 3 as shown in FIG. 6. Alternatively, a single laser 80 together with 2×2 couplers 82 can supply radiation to multiple corrosion detector heads as shown in FIG. 10. In FIG. 10, the light output from the laser 80 is launched into the fiber 81 and is split into 8 optical fiber outputs 84, which are inputted into the multiple corrosion detector heads 79 and then recombined at the fiber optic couplers 86, whose optical fiber outputs 100 provide the same information as the optical fibers 57 in FIG. 6. 
     The signal at the fiber optic outputs 100 can be detected using detector systems 110 as shown in FIG. 11. In this case, the signals appearing as outputs on lines 112 and fed to lock-in amplifier (LIA) 115 may have the same frequency f or different frequencies. The LIA 115 is used to demodulate the signal. Other phase detection/demodulation devices may be used in place of LIA 115. 
     FIG. 12a shows a multiple corrosion detector head system using a single optical interferometer with two corrosion detector heads 3 placed in series. Each detector head can be driven at different frequencies, e.g. F and F&#39;. The fiber optic output is then detected by the detector system 110 shown in FIG. 12b using the two signals, one at each frequency F and F&#39;. The LIA 115 can then lock in on either the signal at frequency F or the signal at frequency F&#39;. A second LIA 115 could be used to simultaneously detect signals at frequencies F and F&#39;. Using multiple lock-in amplifiers 115 as many as 10 such signals could fit into a 30 Hz bandwidth. When the multiple corrosion detector head system is at balance the particular detector head 3 which is proximate corrosion activity causes a change in phase of the signal with a frequency corresponding to that particular detector head 3 and hence a change in output voltage of the lock-in amplifier 115 locked in to that particular frequency, thereby permitting location of the corrosive area. 
     FIGS. 13a and 13b show a schematic diagram of a multiple corrosion detector head system with multiple optical interferometers each having multiple detector heads. FIG. 13a shows how five 2×2 couplers 82 are used to create two optical interferometers. FIG. 13b shows that a single star coupler 90 can replace three 2×2 couplers 82. As was discussed with reference to FIG. 12b, each corrosion detector head 3 is at a different frequency. Again, when the system is at balance, the particular detector head 3 which is proximate corrosion activity causes a change in phase of the signal with a frequency corresponding to that particular detector head 3, and hence a change in output voltage of the lock-in amplifier 115 locked in to that particular frequency. 
     The advantages of the present invention are numerous. It will find great utility as a small, light-weight corrosion detector capable of scanning the surface of a specimen such as an aircraft wing. The corrosion detector head itself is significantly less expensive to manufacture than SQUID devices. It is also rugged, and easy to use in the field, since it does not suffer from drastic directional sensitivity as does a vector magnetometer. It requires no shielded environment, and is immune from environmental field gradients unless they vary drastically over a 1 centimeter distance at the location of the sensor. 
     Although the invention has been described relative to specific embodiments, thereof, it is not so limited, and numerous variations and modifications thereof will be readily apparent to those skilled in the art in light of the above teaching. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise then as specifically described.