Abstract:
A method and an apparatus for remotely and automatically monitoring for corrosion of a tank, pipe, or container with a gas or liquid environment using corrosion coupons that do not have to be removed from the environment for inspection and evaluation. The coupons are designed to fail when a specified level of corrosion occurs. A permanent magnet located on a coupon sensing system inside the corrosive environment is used to transmit the failure of the coupons outside the corrosion environment and across a wall or other boundary surface without requiring a power supply.

Description:
This application claims the benefit of provisional application No. 60/392,984, filed Jun. 28, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   A method for detecting the presence of corrosion of a structure (e.g., wall, container, vessel, tank, or pipe) using a magnetically coupled sensing system that remotely monitors the health of one or more corrosion coupons. It is best used when physical access to the coupon is difficult, costly, or impossible. 
   2. Description of the Prior Art 
   Corrosion will reduce the useful life of a structure. Corrosion may result in the thinning of the structure, pitting of the structure, or cracking of the structure. The type of corrosion that may occur and the type of corrosion monitoring systems needed to assess the degree of corrosion will depend on the service environment of the structure and the condition and operational use of the structure. There are three basic approaches to corrosion monitoring. The first is to make a “direct” measurement of the physical properties of the structure itself. The second is to use a “surrogate” material positioned in the service area, which is identical to the material in the structure, and infer the corrosion of the structure from the surrogate material. The third is to monitor the “chemistry” of the solution or gas upstream, downstream or within the service environment and then infer the effects of corrosion on the structure from an empirical or theoretical relationship that relates the measured quantity to the corrosion-induced damage. 
   The objective of all three corrosion-monitoring methods is to predict the remaining useful life of the structure of interest from an estimate of the corrosion measured or inferred with the monitoring method. In the case of monitoring the structure directly, a simple extrapolation can be made once several time-sequenced measurements have been made. In the case of either monitoring corrosive chemistry or monitoring surrogates, an inference must be made that correlates the corrosion measurement taken to the actual impacts on the structure. 
   Direct monitoring is a preferred method, but due to access, safety, or cost implications, this approach is not always viable. Direct monitoring may involve visual or photographic inspection of the structure, or physical measurements of the dimensions of the structure (e.g., (1) wall thickness; (2) pit depth, diameter or pit density; or (3) crack depth, width, length or density). For many structures in which one side of the structure is accessible, the use of non-destructive examination equipment such as ultrasonic or eddy current techniques can be used to provide general wall thickness data or cross-sectional imaging. The main problem with direct monitoring is the access to the structure is needed and in many instances, access is not possible. Such measurements cannot be practically be made, for example, in radioactive storage containers, or on the walls of underground or the floor of aboveground storage tanks and piping containing petroleum or other hazardous substances and hazardous waste. For these types of applications, surrogate monitoring and chemistry monitoring systems are normally employed. 
   There are commercially available corrosion monitoring techniques that involve direct monitoring of a surrogate. The surrogate material is typically made of the same material as the structure of interest. The most common surrogate monitoring approach is the direct placement of corrosion coupons in the environment of interest. Corrosion coupons are the lowest-tech method of corrosion monitoring via surrogates. A corrosion coupon is a piece of material similar (identical) to the material of interest. The corrosion coupon(s) are placed in similar service conditions and then removed from the service area and evaluated at a later date. These coupon inspections are done periodically and are not linked to a specific level of corrosion. The coupons may be analyzed using destructive metallography. They may be inspected for appearance and/or weighed and compared to the pre-service weight do determine material loss. The use of corrosion coupons, while viewed as a very good method of assessing corrosion, is typically expensive and inconvenient to use. In some instances, the structure needs to be taken out of service to remove the coupons from the service area, which is expensive and may have health and safety implications. As presently used, corrosion coupons do not give any early warning of impending failure until they are retrieved and examined. 
   Electrical Resistance (ER) and Linear Polarization Resistance (LPR) probes both rely on electrical current being passed through a surrogate material and measuring changes in the resistance of the electrical circuit as the surrogate material degrades. Essentially, current is passed through a known cross-section; as metal disappears, resistance increases. Both ER and LPR probes are effective means for measuring uniform corrosion; however, correlating the change in resistance of an ER or LPR probe to the physical changes to the structure caused by corrosion can be imprecise and not yield good answers for many applications. 
   Developed for, and applied at, the U.S. Department of Energy&#39;s (DOE&#39;s) Hanford tank farms, electrochemical noise corrosion probes measure corrosion current and potential (voltage) between three surrogate electrodes. The relationship between corrosion current and corrosion potential on each electrode is indicative of the type and magnitude of corrosion on the electrodes, which can then infer the type and magnitude of corrosion on the structure. While electrochemical noise is a viable technology for early warning of stress corrosion cracking and pitting, its ability to quantify corrosion in a new application requires confirmatory laboratory corrosion studies in order to reliably correlate corrosion probe data with degradation of the structure. 
   A Thin-Wall Membrane Corrosion Probe is a one-shot vacuum chamber with a thin-wall membrane and a sensor. When the thin wall is breached by a through-wall pit, a signal is generated. This device operates somewhat like a balloon; when the balloon is “popped”, the pressure change is used to indicate the breach. This device is an excellent pitting corrosion detector. 
   The present invention describes a method and apparatus for remotely and automatically determining the amount and rate of corrosion of a structure or the material in the structure in a difficult to access environment without the need to handle or remove the corrosion coupon from the environment. The patent literature does not describe any such invention using corrosion coupons. U.S. Pat. No. 4,120,313 describe holding and/or handling systems for corrosion coupons. There are however, numerous inventions in the patent literature that describe electrical noise, electrical resistance and linear polarization methods and apparatuses. For example, U.S. Pat. Nos. 3,609,549; 3,936,737; 4,181,882; 4,238,298; 5,139,627; 5,446,369 describe such inventions. 
   In U.S. Pat. No. 6,499,353, Douglas, et. al., describes a magnetically coupled pressure gauge that measures the pressure or temperature inside a seal container and generates magnetic signal outside the container that yields a continuous measurement of pressure or temperature. In U.S. Pat. No. 5,284,061, Seeley, et. al., describes an apparatus for measuring pressure change of a specified amount in a sealed container that is mainly intended to detect a gas leak due to a loss of pressure. In U.S. Pat. No. 6,182,514, Hodges describes a pressure monitoring system for seal containers using bellows and magnet to monitor pressure. In U.S. Pat. No. 6,067,855, Brown, et. al., describes an apparatus for measuring liquid level in a container, which communicates the level changes of a float riding on the liquid surface to the outside of the container using magnetic sensing strip. None of these systems monitor corrosion and none of these systems use corrosion coupons. 
   The present invention was initially conceived to address a potential corrosion problem in a sealed stainless steel container holding radioactive material in a specialized container system known as a 3013 canister. However, the invention has extensive application to corrosion monitoring in general. It can be used to monitor corrosion in storage tanks and pipelines containing liquids and gases that may be corrosive to the walls of the tank or pipe. It has the potential for use in many less obvious application like furnaces and other structures where access is difficult. 
   API 653 requires the floor of an aboveground storage tank containing petroleum products be periodically inspected. The time between inspections can be increased and the inspections improved if the rate of corrosion of the floor or inside walls of the tank can be measured. The same is true for pipelines. 
   The present invention automates the use of corrosion coupons and mitigates the common and important disadvantages this approach. The coupon does not need to be removed from the service area for assessment, and periodic assessments are not required. Also, the present invention does not disturb the service environment, which occurs when coupons are removed. More importantly, the present invention indicates when a certain specified level of corrosion occurs. A time sequence of measurements can be made using multiple coupons. Coupons with different physical characteristics and/or loadings can be used to determine different types and different levels of corrosion. For example, a thin coupon can be used to indicate that corrosion is occurring, but at a level of negligible impact to the structure. A thicker coupon can be used to give an early warning of an important level of corrosion and may indicate that a more thorough inspection of the structure is required. Finally, an even thicker coupon may indicate that the structure needs replacement or upgrading. 
   In view of the prior art described above, it is apparent that there is a need and a wide range of applications for a method and apparatus that can remotely and automatically measure corrosion using coupons without requiring the removal of the coupons from the service environment. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a method and an apparatus for measuring corrosion of a structure using one or more corrosion coupons without removing, handling, observing or assessing, qualitatively or quantitatively, the condition of the coupon or coupons. 
   It is another object of the present invention to provide a method and an apparatus for measuring corrosion using an automatic, remote monitoring, corrosion coupon system. 
   It is another object of the present invention to provide a method and an apparatus for measuring corrosion inside a tank, a pipe, or another type of container system storing, transferring, or processing liquids, gases, mixed phase solutions, or slurries (e.g. water, petroleum, radioactive substances) that do not require penetrating the walls of the containment system. 
   It is another object of the present invention to provide a method and an apparatus for measuring corrosion inside a sealed container that does not require penetrating the sealed container. 
   It is a still further object of the present invention to provide a method and an apparatus for measuring the failure of a corrosion coupon inside a containment system, especially those that are completely sealed, that does not require internal electrical power. 
   It is another object of the present invention to provide a method and an apparatus for measuring the failure of a corrosion coupon inside a containment system using permanent magnets for transmitting the corrosion data and not requiring internal electrical power. 
   Briefly, the present invention includes a method and an apparatus for remotely and automatically measuring corrosion in a difficult to access area or containment system, especially sealed containers storing or transporting radioactive materials, or storage tanks or piping storing or transferring water, petroleum products, or other hazardous substances/chemicals. It consists of a corrosion coupon system that is positioned inside and used to monitor for corrosion in a containment system in which access to or penetration of the walls is not desirable. The corrosion coupon system may contain a multiplicity of coupons and transmits a detectable magnetic signal as each coupon fails that is sensed outside the containment system. Each corrosion coupon inside the containment system will fail when a specified level of corrosion occurs. The magnetic signal is produced from a magnet that moves in response to the coupon failure. The coupon failure will allow the rotation or translation of a spring-loaded element. The magnetic signal produced by a change in position of the magnet, is measured/detected by a magnetic sensing system located on the outside of the containment system. The measured magnetic signal is then displayed using either a mechanical or electrical readout system. 
   The method and apparatus are comprised of (1) corrosion coupon transmitter, which is placed in the service environment to be monitored and (2) a receiver, which is usually located outside the service area, although this is not necessary and for some applications, it cannot be. The transmitter is comprised of (a) one or more corrosion coupons that are identical to the material in the structure that might corrode; (b) a rigid mounting system that holds and positions the corrosion coupons in the potentially corrosive environment; (c) a spring or coil in compression or tension that will change its position if the tension or compression is removed because of a coupon failure; and (d) a magnet that can be attached to the spring or coil so that the magnet or the magnetic field of the magnet changes if the spring or coil changes position because of a coupon failure. Alternatively, the magnet can be attached to another element that will change its position and the position of the magnet positioned on the element when a coupon fails. The movement of the magnetic may involve a rotation, translation or combination. 
   The receiver apparatus is comprised of a magnetic sensing unit that measures the change in the magnet field if the corrosion coupon fails. The magnetic sensing unit can be a mechanical system such as a compass whose needle will give a different reading when the magnet in the coupon transmitter apparatus changes position. The magnetic sensing unit can also be an electronic sensor, comprised of commercially available sensors such as coil or magnetoresistive sensors that will sense the change in magnet field when the magnet in the coupon transmitter changes position. The rate of corrosion can be easily computed if the time between the installation of the coupon and the failure of the coupon is known. If two or more coupons of different thickness are used, more information about the rate of corrosion can be determined. The magnetic field can change, because the magnet is physically displaced with any movement of the spring when the coupon fails, or the magnet rotates or moves in a known pattern (e.g., rotation or translation) as the coupon fails. The output of the receiver unit can be, transmitted via a wireless communication system such as a radio frequency (RF) tag to a computer located at another location for additional analysis or for archiving. 
   An advantage of the present invention is that it provides a safe method of measuring corrosion in a nuclear waste container, or a storage tank or piping containing corrosive fluids. 
   Another advantage of the present invention is that it provides a safe method of measuring corrosion inside a containment system without penetrating the walls of the containment system. 
   A further advantage of the present invention is that it provides a method of measuring corrosion a containment system without requiring electrical power. 
   A further advantage of the present invention is that it provides a method of measuring corrosion a containment system without removing, handling, inspecting, or assessing the condition of the corrosion coupons. 

   
     IN THE DRAWINGS 
       FIG. 1  is a block diagram of the preferred embodiment of the present invention. 
       FIG. 2   a  is an embodiment of the present invention for a one-coupon system mounted on a C-spring that produces a change in the position of a magnet when the coupon fails; 
       FIG. 2   b  is an embodiment of the present invention for a one-coupon system mounted on a C-spring after the coupon fails; 
       FIG. 3   a  is a mechanical embodiment of the receiver used to sense the magnetic field generated by the corrosion coupon transmitter positioned inside a containment system. 
       FIG. 3   b  shows an enlarged view of the mechanical receiver. 
       FIG. 4   a  is an embodiment of the present invention for a one-coupon system that produces a rotation change in the position of a magnet when the coupon fails; 
       FIG. 4   b  is an illustration of an embodiment of the present invention foe a one-coupon system after the coupon fails; 
       FIG. 5   a  is an illustration of an embodiment of the present invention for a two-coupon system that produces a rotational movement of the magnet when a coupon fails; 
       FIG. 5   b  is an illustration of an embodiment of the present invention for a two-coupon system after one of the coupons fail; 
       FIG. 6  is an illustration of an embodiment of the present invention for a multiple-coupon system mounted on a coil spring that produces a rotational movement of the magnet when the coupons fail; 
       FIG. 7  is an illustration of the corrosion coupon subassembly; 
       FIG. 8   a  is an illustration of an embodiment of the present invention for, a multiple-coupon system that produces a rotational movement of the magnet when the coupons fail; 
       FIG. 8   b  shows the rotational change of the magnet after the first corrosion coupon fails; 
       FIG. 9   a  is an illustration of an embodiment of the present invention for a one-coupon system that produces a translational movement of the magnet when the coupon fails; 
       FIG. 9   b  is an illustration of an embodiment of the present invention for a one-coupon system after the coupon fails; 
       FIG. 10   a  is an illustration of an embodiment of the present invention for a two-coupon system that produces a translation movement of the magnet when a coupon fails; 
       FIG. 10   b  is an illustration of an embodiment of the present invention after one of the coupons fails; 
       FIG. 11  is an illustration of an embodiment of the present invention for a one-coupon system in a rigid structure entirely open to the service environment that produces a translation movement of the magnet when the coupon fails; 
       FIG. 12  is an illustration of the use of the present invention for monitoring corrosion in an aboveground storage tank. 
       FIG. 13  is an illustration of the present invention implemented in a magnetic mounting system for monitoring the corrosion in an aboveground storage tank; 
       FIG. 14  is an illustration of the present invention implemented in a horizontal frame for monitoring the corrosion of the floor of an aboveground storage with a translation coupon-failure signal. 
       FIG. 15  is an illustration of the present invention implemented in a horizontal frame for monitoring the corrosion of the floor of an aboveground storage tank with a rotational coupon-failure signal; and 
       FIG. 16  is a simplified illustration of an embodiment of the present invention for measuring corrosion inside a pipe. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The method and several embodiments of the apparatus are illustrated for three applications of the present invention: (1) sealed containers storing nuclear materials; (2) an aboveground storage tank containing petroleum or other substances, both hazardous and non-hazardous (e.g., water); and (3) pipelines containing petroleum or other substances, both hazardous and non-hazardous. However, the present invention is applicable for many other applications. The present invention is applicable for any service environment in which a corrosion coupon can be used, including those environments comprised of liquids, gases, mixed-phase solutions, slurries, and radioactive materials. It is best implemented for containers, areas, or structures that are difficult, inexpensive, or inconvenient to access. For those skilled in the art, the method and the apparatuses described can be applied to a much wider range of corrosion problems. 
   The preferred embodiment of the method and apparatus of the present invention is illustrated in FIG.  1 . It illustrates how the invention can be applied to a sealed container or a container in which penetrations are not desired, unsafe, or expensive to implement. According to the present invention, an apparatus  10  is provided for determining the corrosion of an environment inside a sealed container  18 . The apparatus  10  includes a corrosion coupon transmitter apparatus  12  (or transmitter apparatus  12 ) that is separated from a receiver apparatus  14 , by a wall  16  of the container  18 . The container  18  wall  16  is preferably constructed of a non-magnetic or weakly magnetic material. For storage of nuclear waste material, the container is preferably constructed of stainless steel, a weakly magnetic metal. Other non-magnetic or weakly magnetic metals, plastics, and composite materials are also included in the spirit of the present invention. In addition, magnetic metals are also included in the spirit of the present invention. 
   The transmitter  12  is self contained and does not require any physical connections or holes through the wall  16  for communication to the receiver  14 . The transmitter  12  is constructed to respond to the failure of the corrosion coupon  24  inside  20  of the container  18 . The transmitter  12  includes a corrosion coupon  24  responsive to the environment and a magnetic field generator  26  to provide the magnetic field  22  with a characteristic indicative of the binary status of the corrosion coupon (either in an intact or failed condition). The characteristic may be a magnetic field orientation, the presence or absence of a detectable magnetic signal, or the strength of the magnetic field. The term “radiate” may be used in the following text and claims as a general term referring to the existence or creation of a magnetic field, even though in the case of a permanent magnet the field is not usually moving outward, but is static and therefore does not require an energy supply to sustain energy radiated from the magnet. The receiver  14  includes a receiver transducer  28  responsive to the field  22  to cause an indicator  30  to provide a communicative indication of the state of the corrosion coupon  24 . The communicative indication can be any of various types, including display apparatus such as a needle and scale, or a digital read-out using LEDs, etc. The term “magnetic field generator” applies to any of the devices known by those skilled in the art for providing a magnetic field, and is preferably achieved using permanent magnets. The term “characteristic” applies to any property of a magnetic field that can be altered by movement (i.e., a change in position) of the magnet due to failure of a corrosion coupon. A particular and important embodiment of the present invention is the application of the disclosed apparatus to monitor for corrosion inside a container used for storage of radioactive material, including nuclear waste. 
     FIG. 2  is a simplified illustration of a simple corrosion coupon transmitter  32  of an embodiment of the present invention. A transmitter  32  inside a container  36  is shown and includes a holder  34  for a corrosion coupon  40  and magnet  42  as the field generator. In this embodiment, a magnet  42  is secured to one end of a “C”-shaped spring  44  preferably made out of stainless steel or another non-magnetic material. The other end of the “C” spring would be attached to the inside of a frame  34  that allows direct communication between the coupon and the corrosive environment  38  in the container  36 . For storage of nuclear materials, the steel container  36  would be made of stainless steel; for other types of stored materials, liquids, or gases, it can be made of other materials. A thin, narrow “ribbon” coupon  40  made out of the same material as the container would be welded across the ends of the spring in such a way that the “C” spring was held in tension ( FIG. 2   a ). Being made of the same material as the container, the corrosion coupon  40  would be subject to the same corrosion effects as everything else within the sealed container  36 ; if the canister were corroding, the coupon would also be attacked and corrode. But being made of much thinner material than the sealed container itself, the corrosion would cause the coupon to fail first. When the coupon failed, the spring, being relieved of the tension, would expand  46  out to its equilibrium shape ( FIG. 2   b ). This would translate the magnet  42  away from its in-tension position. 
   An external sensor  52 , which is capable of detecting the change in the magnetic field produced when the coupon  40  fails (i.e., breaks), such as a Hall Effect device  52  or a magnetoresistive device  52 , would sense the change in position of the magnet  46  and signal a corrosion-caused failure of the coupon. The external sensor is typically battery operated  54  and installed in a mounting device that can be positioned outside the container whose inside environment  38  is being monitored for corrosion. This change in the magnetic field would be an indication that corrosion was occurring in the container, In this case, the change in the magnetic field would be very close to binary in nature (absence or presence of a detectable magnetic field); that is, the output of the sensor would determine that the coupon  40  had failed or not. 
   Depending on the thickness and/or shape of the coupon  40 , the coupon could give an earlier indication of corrosion than other techniques, including destructive testing. Furthermore, this invention would not require the containers to be destructively tested, radiographed, or even removed from their storage environment. 
   As shown in  FIG. 3   a , the receiver  68  could also be a mechanical system such as a compass-like measurement system where the position or alignment (pointing direction) of the needle  82  indicates the location of the magnet in the transmitter apparatus  50 .  FIG. 3   a  shows the receiver  68 , container  56 , and corrosion coupon transmitter  50 .  FIG. 3   b  shows an enlarged view of the receiver  68 . The receiver apparatus  68  could be positioned close to the wall  58  of the container  56  within sensing range of the magnetic field  60  produced by the magnet  52  in the transmitter apparatus  50 . The receiver has a magnet  70  attached to a pin  72  that is pivotally mounted to the base  74  of the receiver housing  76 . The pin  72  extends upward through the top  78  and attaches to a pointer  80  for pointing to a calibrated scale  82  indicative of the failed or intact (OK) state of the coupon  54  in the container  56 . The field  64  transmitted (radiated/extended) from the magnet  52  of transmitter  50  extends to the wall  58  and passes through the wall  58  without being altered if the wall  58  is not magnetic. If the wall is magnetic, the field aligns magnetic domains in the magnetic wall, and the magnetized wall portion  58  then radiates-extends a corresponding magnetic field exterior through the container  56 . The magnet  70  of the receiver, being free to rotate, then aligns itself with the field extended by the magnet  62 , which is in a position dependent on the state (failed or intact) of the corrosion coupon  40 . The resulting orientation of the magnet  70  is transferred via pin  72  to pointer  80  to point at the scale  82  indicating whether or not the coupon has failed. 
   The apparatus of  FIG. 2  is illustrative of a very simple embodiment of the present invention. Other transmitter and receiver constructions for responding to the state of one or more coupons in the corrosive environment and transferring a magnetic indication of a value of the property through a boundary, which may also have magnetic properties, and detecting the magnetic field and displaying a parameter value indication will be apparent to those skilled in the art, and these are included in the spirit of the present invention. 
   There are many variants of the present invention. The “right” design of apparatuses based on the present invention would depend on what potentially corrosive environment needs to be measured and how accurately or precisely it needs to be measured. As an example, the “C” spring could be replaced by a coiled spring; in this case, the failure of the coupon would be indicated as a change in the rotation angle of the magnet.  FIG. 4  is a simplified illustration of a transmitter  102  in a sealed container  100 . A coiled spring  110  is mounted to the holder  104 , which bottom is open to the environment  114  of the container  100 . A magnet  112  is attached to the coiled spring  110  in such a way that it will rotate as the coiled spring  110  rotates. Here, the coupon  108  failure would cause a pre-determined rotation of the magnet  112  that would be measured by the receiver such as the ones shown in  FIGS. 1-3 . Coupons (ribbons, wires, or rods) of incrementally increasing thickness could be mounted to the “C” or “coiled spring” in such a way that as each failed in turn with continued corrosion, the corrosion rate could be estimated.  FIG. 4   b  illustrates the rotation  116  of the magnet after the coupon fails. 
     FIG. 5  illustrates a two-coupon transmitter similar to the transmitter described in FIG.  4 .  FIG. 5   a  illustrates the position of the coiled spring  132  and the magnet  130  before any of the coupons  124 ,  128  fails. When coupon  128  fails, as shown in  FIG. 5   b , the coiled spring  132  rotates, which in turn rotates the magnet  130 , which changes the magnetic field radiated to the receiver. Each failed coupon would cause the magnet  130  to rotate (or translate) a pre-determined distance; a line of magnetic sensors in the receiver would measure the incremental translation and indicate which of the coupons had been broken. The receiver could also consist of a electronic coil or magnetoresistive receiver shown in  FIG. 2  or the mechanical compass-like receiver illustrated in FIG.  3 . The rotation angle of the magnet  130 —measured by the receiver—would be a measure of the number of the coupons that had been corroded. 
   For nuclear corrosion measurements, the magnet  130  would preferably be a samarium-cobalt (SmCo) magnet, because it has several advantages. First, it has a very high magnetic strength; this allows a small magnet to send out a large and easily detectable magnetic field. Second, SmCo is the magnet material most commonly used in radiation environments where high field strength is needed—it resists demagnetization due to radiation-induced depolarization of the magnetic dipoles. 
   A simplified illustration of another embodiment of the present invention with a coiled spring is shown in FIG.  6 . In this embodiment, the corrosion coupon can be placed under a pre-determined stress loading, which is required for estimates of stress-corrosion cracking.  FIG. 6  illustrates a six-coupon transmitter  148 . However, the transmitter can be implemented with as few as one coupon or as many as is physically possible within the transmitter. 
     FIG. 6  illustrates the embodiment in a three-dimensional view of the transmitter  148 . The magnet  162  is mounted onto a ball bearing-supported spindle  156  that includes a finger  158 . The spindle finger  158  touches a finger stop  160  that is part of a corrosion coupon  180  subassembly, discussed below. The base of the transmitter  148  secures one end of a torsion spring  152  while the other end is attached to the rotary assembly causing the spindle assembly to  156  (attempt to) rotate. For a corrosion coupon  170  that is intact (i.e., that has not “failed”), the rotation of the finger  158  and magnet  162  is inhibited by the presence of the finger stop  160 . The base of the transmitter  140  also supports the bottom end of the corrosion coupon subassemblies  180 . 
   When sufficient corrosion occurs, a coupon  170  fails and the coupon separates at the specimen region—the location of the subassembly where the coupon is designed to fail when it corrodes and weakens. When the coupon fails, a set of Belleville springs  166  within the case causes the upper portion of the coupon assembly to “pop up”. This moves the finger stop  160  out of the way of the finger  158  on the spindle allowing the spindle assembly  156  with the magnet  162  to rotate to the next finger stop  182 . The receiver, as illustrated in  FIGS. 1-3  measures the rotation of the magnet signaling failure of that particular coupon. As illustrated in the figure, multiple coupons can be employed, each with a decreasing stress applied in order to clearly establish a corrosion rate of the process occurring inside the container. 
   The details of the corrosion coupon subassembly  180  are shown in FIG.  7 . This subassembly (made from stainless steel or any other high-tensile material designed to measure the corrosion process) has a one-piece, partially threaded, corrosion coupon shaft  174  that includes a region for the corrosion coupon  170 . A stack of Belleville washers  174  are compressed against the base  140  of the transmitter with a tensioning nut  164 ; this provides the stress to the specimen region and allows the finger stop  158  to pop up when the corrosion coupon  170  fails. The top end of the shaft  174  incorporates the finger stop  158  and the bottom end of the shaft  174  has a shoulder  168  that fits into a foot  142  at the bottom of the body. Only the lower portion of the corrosion coupon—that portion of the subassembly containing the specimen region—is exposed to the corroding environment. 
   The transmitter illustrated in  FIG. 6  incorporates six corrosion coupons, each arranged at 60-degrees intervals.  FIG. 8   a  shows the corrosion mechanism in the “armed” or “ready” position where the corrosion coupon  170  on the corrosion coupon subassembly  180  has not yet failed. As noted above, the specimen region of the mechanism is external to the case and is exposed to the corrosive material; this allows the coupon specimen to experience the same corrosion processes being experienced by the container being measured. 
   In the “armed” or “ready” state, as illustrated in  FIG. 8   a , the finger  200  on the torsion-spring-loaded spindle assembly (i.e., coiled spring) is prevented from rotating by the interference of the finger stop  202  portion of the corrosion coupon shaft. As shown in  FIG. 8   b , when the corrosion coupon  170  fails, the finger stop  202  “pops up” allowing the spindle finger  200  to pass by the finger stop  202  clearance section to the next corrosion coupon subassembly  182  where it is stopped by the finger stop  206 . 
   Each of the six coupons illustrated in  FIGS. 6 and 8 , which are arranged around the periphery of the case interior, may be under different stress and will be arranged in the order that the coupons are expected to fail. These coupons could be spaced at uniform 60-degree intervals. However, a non-uniform spacing interval can also be used such that there is a unique relationship between the coupon and the angular change observed between coupon failures. This type of spacing arrangement ensures that when a coupon fails, the measured angular change will be an indicator of which coupon failed. This arrangement assures that the corrosion process can continue to be monitored and quantified, even if the zero-coupon index position is lost. 
   The stress applied to the coupon section is strictly a function of the applied force imposed by the Belleville discs and the cross-section of the corrosion coupon. Either one may be varied to achieve the desired stress. For example, a stack of seven fully compressed, off-the-shelf Belleville washers will exert a force of about 430 pounds. For this force, the applied stress can be simply determined from the cross-sectional area of the specimen region. Table 1 shows the stress (in psi) that can applied, for various diameters of a circular-shaped specimen region. 
   
     
       
             
           
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Specimen diameter required for specified stress at 430# force 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Stress (psi) 
               30,000 
               25,000 
               20,000 
               15,000 
               10,000 
             
             
               Diameter (in.) 
               0.135 
               0.148 
               0.165 
               0.191 
               0.234 
             
             
                 
             
           
        
       
     
   
     FIGS. 9-11  are simplified illustrations of alternative embodiments of the present invention where a coupon failure results in a translation (rather then rotation) of the magnet. The transmitter  300  in  FIG. 9  results in the same information and has the same functionality as the rotational transmitters illustrated in  FIGS. 4-8 . The transmitter  300  is comprised of a spring  320  attached to a rigid structure  350  open to the service environment  310 , and a corrosion coupon  340  attached by a wire or rod  360  to the mounting structure  340  and the magnet  330 . A receiver with a magnetic sensing device  370  is located above the transmitter  300  in the “armed” or “ready” position of the magnet. When the corrosion coupon fails, as illustrated in  FIG. 9   b , the spring  320  pulls or translates the magnet away from its “armed” or “ready” position. The magnetic sensing system in the receiver  370  can easily detect the decrease in the magnetic field that occurs when the magnet is pulled away from its “armed” or “ready” position. The receiver could also be positioned where the magnet will come to rest after the coupon fails. It that position, it will sense an increase in the magnetic field. 
     FIG. 10  illustrates an embodiment of the present invention shown in  FIG. 9 , but with two corrosion coupons  382 ,  384 , in which one of the coupons  382  is thinner than the other coupon  384 .  FIG. 10   a  illustrates the transmitter in the “armed” or “ready” position, and  FIG. 10   b  illustrates the transmitter after the thinner coupon  382  has failed. Clearly, a multiplicity of coupons of different designs and thicknesses can be used. For the clearest interpretation, the coupons should be installed in the transmitter  380  in the order they are expected to fail.  FIG. 11  illustrates an embodiment of the transmitter  400  of the present invention shown in  FIG. 9 , but with a rigid frame  460  open to the environment  410  on all sides holding the spring  420 , magnet  430 , and corrosion coupon  440 . The receiver  470  will detect the presence or absence of a magnetic field that corresponds to the failure status of the coupon  440 . 
     FIG. 12  illustrates the application of the transmitter shown in  FIG. 6  in an aboveground storage tank. The main difference between this application and the nuclear container application is that a storage tank is typically constructed of steel, a magnetic material, and the nuclear containers are typically constructed of stainless steel, a weakly magnetic material. The receiver can detect the magnetic field changes even for magnetic materials, because the strength of the magnetic field is strong and localized due to the rotation (or translation) of the magnet in the transmitter. In  FIG. 12 , the transmitter and receiver pair  360  is located on or near the wall of the tank. The transmitter  362  is located inside the tank and is submerged in the fuel. Only the corrosion coupon  364  is immersed in the fuel environment. The receiver  370  is located on the outside part of the tank wall  352 . The receiver shown in  FIG. 12  includes a battery, a magnetoresistive chip, and a wireless communication system. 
     FIG. 13  is a simplified illustration of an embodiment of the present invention for monitoring corrosion of the inside walls or floor of an aboveground storage tank. In this case, a magnetic mounting system is used to mount the transmitter on the inside wall of the tank and a similar mounting system is used to mount the receiver on the outside of the tank. The receiver  386  and the transmitter  380  are aligned so that any movement of the magnet inside the transmitter  380  can be properly interpreted. The receiver is comprised of a battery, an electronic magnetic sensing chip, a battery and a wireless communication system to transmit the measured data to a computer. The receiver may also include a digital display. The receiver could also be simplified by replacing the electronic receiver by a mechanical one similar to the one shown in FIG.  3 . 
     FIG. 14  is an illustration of an apparatus  510  for monitoring the corrosion of the floor of an aboveground storage tank  500 . The transmitter  520  and the receiver  580  are positioned on opposite sides of the tank wall  502 . The transmitter  520  is comprised of a corrosion coupon subassembly  540  that is very similar to the one illustrated in  FIGS. 4-8 . The main difference is that the magnet  564  is attached to a long rod  550  that separates the corrosion coupon subassembly  540  from the magnet  564 . The corrosion coupon subassembly  540  is comprised of the a base foot  562  that hold the assembly  540  at the base, Belleville washers  560  putting the assembly under compression, a nut  558  located on a threaded rod  556  to anchor the subassembly at the top, and a corrosion coupon  570 . The couplings  554  and  552  are only used if the rod  550  is long or needs an adjustment in length. The assembly  542  comprised of the corrosion coupon subassembly  540 , rod  550 , and magnet  564  is positioned in a mounting frame  530  that is open to the fuel environment and rests on the bottom floor of the tank. The distance between the magnet  564  and the tank wall  502  controls the strength of the magnetic field that needs to be sensed by the receiver  580 . In the “armed” or “ready” position before the corrosion coupon  570  has failed, it is desirable that the magnetic field being sensed by the receiver be small (negligible). The number of Bellville washers can be varied so that the magnet will move close enough to the tank wall  502  and receiver  580  when the corrosion coupon  570  fails to produce a strong and very detectable magnetic signal at the receiver  580 . While not explicitly illustrated in  FIG. 14 , the internal parts of the corrosion coupon subassembly can be enclosed in a sealed container, leaving only the corrosion coupon exposed to the fuel environment. 
   An array of transmitters and receivers, like the transmitter  520  and receiver  580  shown in  FIG. 14 , can be inserted into the tank with different coupon stresses and thickness to give the same information as the cylindrical transmitter illustrated in FIG.  6 . The corrosion coupons for an aboveground storage tank application will be developed with different diameters to detect when certain levels of corrosion have occurred; these diameters should be selected to identify when certain levels of maintenance, repair or replacement of the aboveground storage tank need to be performed. The first corrosion coupon would have a small diameter and be designed to break in a short period of time if active corrosion in the tank is occurring. The fifth or sixth corrosion coupon would indicate that the thickness of the tank wall or floor is sufficiently thin that replacement of the aboveground storage tank or major sections of the aboveground storage tank may be necessary. 
     FIG. 15  illustrates a rotational implementation of the transmitter  590 . The transmitter is similar to the one illustrated in  FIG. 6 , except the transmitter  590  magnet  596  is located outside the enclosed portion of the transmitter  598  and on the end of the extension rod  582  near the tank wall  594  and the receiver  584  mounted on the tank wall  594 . The corrosion coupon  592  is located away from the wall  594  and closer to the center of the tank. Multiple corrosion coupons can be implemented in the transmitter  590 , as illustrated in the transmitter shown in FIG.  6 . 
   For all embodiments of the present invention, if the magnet movement is sufficient, the receiver only needs to be able to detect the presence of absence of the signal. The location of the received signal, whether it is due to a translation, or rotation, is indicative of which corrosion coupon failed. By knowing the time between failures of the coupons, the thickness and loading of each coupon, both the amount and rate of corrosion can be determined and used to assess the life cycle of the structure or containment system. 
     FIG. 16  is an illustration of a simplified drawing of an apparatus  600  for a pipe. The corrosion coupon subassembly shown in  FIG. 7  is attached to the pipe in such a way that the corrosion coupon is positioned in the pipe fluid. The transmitter  630 , with the corrosion coupon subassembly  640  and receiver  650 , is threaded into the pipe  610  through a special coupling  612  that is welded onto the pipe. The transmitter  640  and receiver  650  pair  600  is very similar to the embodiment shown in  FIG. 14 , except the rod  550  and couplings  554 ,  552  extending the rod in  FIG. 14  have been removed, and the received is positioned on the top of the transmitter  640  than on the wall separating the tank environment from the ambient air environment. The transmitter  640  works identically to the transmitter illustrated in FIG.  14 . For pipe applications, the part of the transmitter  640  subject to the ambient air environment outside the pipe should be enclosed in a small housing. 
   Although the present invention has been described above in terms of a few specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.