GB2489048A

GB2489048A - Passive wireless corrosion sensing device

Info

Publication number: GB2489048A
Application number: GB201104621A
Authority: GB
Inventors: Victor Alexandrovich Kalinin
Original assignee: Transense Technologies PLC
Current assignee: Transense Technologies PLC
Priority date: 2011-03-18
Filing date: 2011-03-18
Publication date: 2012-09-19
Also published as: GB201104621D0

Abstract

A passive, batteryless, wireless corrosion sensing device 120 includes an electrical circuit 26, comprising a plurality of electrically conducting corrosion sensors 22 and an antenna 10. Each sensor 22 comprises a corrodible sacrificial switch member 6A, 6B, 6C, 6D coupled with a passive RF transponder 24A, 24B, 24C, 24D based on acoustic waves, the device 120 being arranged so that, in use, each switch member in an intact condition permits its respective coupled transponder 24 to generate a response signal 62 to a radio frequency interrogation signal 60, but in a corroded, broken condition does not permit its coupled transponder 24 to generate the response signal 62 to the interrogation signal 60. The transponders may be surface acoustic wave (SAW) resonators or reflective delay lines and the circuit may include a reference transponder. The device may be used to monitor the corrosion of structures such as reinforcement members in concrete.

Description

I

Passive, Batterviess, Wireless Corrosion Sensing Device The present invention relates to passive, batteryless, wireless S corrosion sensing devices.

Gradual deterioration of concrete structures as a result of corrosion of steel reinforcing rebar causes a lot of problems all over the world. In the normal alkaline environment inside concrete, a protective passivation oxide film created on the rebar surface prevents its corrosion. However, if the atmospheric carbon dioxide mixed with water creates an acidic medium, and H-'-ions can penetrate into the bulk of the concrete, reach the rebar surface and destroy the passivation film, then corrosion can begin. In the case of a salty environment (sea water, de-icing agents), chloride ions can also 1 S penetrate the concrete and initiate corrosion. Corrosion of rebar in its turn eventually leads to destruction of the concrete. If corrosion is detected at early stages when it is still invisible, repairs of the concrete structures are considerably less expensive than replacement of damaged concrete parts.

Thus, a constant monitoring of corrosion of steel reinforcement prevents disastrous destruction of concrete structures and reduces cost of their maintenance.

Conventionally, it is known to provide corrosion sensing devices which are based on any one of a number of different techniques. However these known devices suffer a number of disadvantages. The devices may not directly monitor corrosion, but may sense or monitor a corrosion effect, such as pH or chloride concentration. The devices may require a power supply in the form of a battery or a wired connection, and may require a wired connection to an interrogation device or data logger, which may be located on the surface of the concrete.

In this specification, the following terms will be used. SAW is an abbreviation of "surface acoustic wave", and the term SAW device is used to denote a surface acoustic wave device. An active device is a device which incorporates a dc power supply such as a battery or a rectifier converting ac signal into direct current; a passive device does not incorporate a dc power supply.

According to a first aspect of the present invention there is provided a passive, batteryless, wireless corrosion sensing device, the device including an electrical circuit, the circuit comprising one or more electrically conducting corrosion sensors and an antenna, the or each sensor comprising a corrodable sacrificial switch member coupled with a passive RF transponder based on acoustic waves, the device being arranged so that, in use, the or each switch member in an intact condition permits its respective coupled transponder to generate a response signal to a radio frequency interrogation signal, but in a corroded, broken condition does not permit its coupled transponder to generate the response signal to the interrogation signal.

Possibly, the circuit includes a reference transponder, which may generate a response signal to the radio frequency interrogation signal irrespective of the condition of the or each of the switch members.

Possibly, the or each transponder comprises an acoustic wave transponder, and may comprise a surface acoustic wave resonator, a bulk acoustic wave resonator or a surface acoustic wave reflective delay line.

Possibly, the or each resonant transponder has a Q factor of greater than 300. In some embodiments, the Q factor may be at least 1000, and may be greater than 5000.

Possibly, the delay line transponder has a delay within the range from 0.01 microsecond to 10 microseconds.

Possibly, the or each transponder includes an interdigital transducer, which may be mounted on a mounting substrate. Possibly, the device includes a plurality of interdigital transducers, each of which are mounted on the same mounting substrate. Possibly, the mounting substrate and the interdigital transducers together comprise an acoustic wave device, and may comprise an SAW device.

Possibly, the or each transponder operates at between 300 MHz and GHz. Possibly, the or each transponder operates at UHF, and may operate at between 300 MHz and 3 GHz.

Possibly, the or each switch member is formed of an electrically conducting corrodable material, and may be formed of a metal.

Possibly, the device includes a plurality of sensors, and the switch members of the sensors may be arranged so that the time period to reach the corroded broken condition is different for each switch member. Possibly, the switch members are of different cross sectional areas, and may have different thicknesses and/or different diameters. Possibly, the switch members are formed of different materials. Possibly, the switch members are spaced apart, and may be spaced apart at regular intervals. Possibly, in use, the device is embedded in a substrate. The switch members may be located at different depths in the substrate. The substrate may be concrete.

Possibly, the resonant transponders generate response signals of different frequencies and the delay line transponders generate response signals at different times.

Possibly, the circuit includes a ground.

The device may be for monitoring the corrosion of a structure, which may be embedded in the substrate. The structure may comprise one or more reinforcement members. The or each of the switch members may be formed of the same material as the structure. The switch members may be electrically connected to part of the structure. The structure may form a ground for the circuit, and may form a ground for the antenna.

S According to a second aspect of the present invention, there is provided a method of monitoring corrosion, the method including providing a passive, batteryless, wireless corrosion sensing device, f7g5the device including an electrical circuit, the circuit comprising one or more electrically conducting corrosion sensors and an antenna, the or each sensor comprising a corrodable sacrificial switch member coupled with a passive RF transponder based on acoustic waves, the device being arranged so that, in use, the or each switch member in an intact condition permits its respective coupled transponder to generate a response signal to a radio frequency interrogation signal, but in a corroded, broken condition does not permit its coupled transponder to generate the response signal to the interrogation signal.

Possibly, the device includes any of the features described in the previous paragraphs.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:-Fig. I is a schematic diagram of an electrical circuit of a corrosion sensing device according to the invention; Fig. 2 is a schematic diagram of an SAW device for a corrosion sensing device; Figs. 3A to 30 are plots of the frequency response of the corrosion sensing device of Fig I in use; Fig. 4 is a view of the corrosion sensing device of Fig 1 in use in an installed condition; Fig. 5 is a schematic diagram of an electrical circuit of another corrosion sensing device according to the invention; Figs. 6A and 6B are plots of impulse response against time of the corrosion sensing device of Fig 5 in use; Fig. 7 is a schematic diagram of an electrical circuit of a third corrosion sensing device according to the invention; Fig. 8 is a schematic diagram of an electrical circuit of a fourth corrosion sensing device according to the invention; Fig. 9 is a schematic diagram of an electrical circuit of a fifth corrosion sensing device according to the invention; Fig 10 is a perspective schematic view of a sixth corrosion sensing 1 0 device in an installed condition in use; and Fig 11 is a perspective schematic view of a seventh corrosion sensing device in an installed condition in use.

Fig. 1 shows a passive, batteryless, wireless corrosion sensing device 120, the device 120 including an electrical circuit 26, the circuit 26 comprising a plurality of electrically conducting corrosion sensors 22 and an antenna 10.

Each sensor 22 comprises a corrodable sacrificial switch member 6 coupled with a passive RF transponder 24 based on acoustic waves, the device 120 being arranged so that, in use, each switch member 6 in an intact condition permits its respective coupled transponder 24 to generate a response signal 62 to a radio frequency interrogation signal 60, but in a corroded, broken condition does not permit its coupled transponder 24 to generate the response signal 62 to the interrogation signal 60.

The circuit 26 includes a reference transponder 5, which generates a response signal to the radio frequency interrogation signal irrespective of the condition of the or each of the switch members 6.

In one example, each transponder 24 and the reference transponder 5 could comprise an SAW resonator 2, each of which could be arranged to resonate at a different frequency.

The circuit 26 includes a ground 28.

Each of the switch members 6 could be in the form of wire, rod or bar.

The switch members 6 are spaced apart at regular intervals, in the form of a ladder. The switch members 6 could each have a different cross sectional S area, with the area increasing from left to right as shown in Fig 1, from 6A to 6D. In one example, the switch members 6 could be circular in cross section, and the diameter of each of the switch members 6 could be different. The different cross sectional areas of the switch members means that the time period to reach the corroded broken condition is different for each switch member.

Referring to Fig 4, the corrosion sensing device 120 includes a protective, relatively rigid housing 44 in which the circuit 26 is located. The housing 44 defines an aperture 46 which permits exposure of the switch members 6 to a corrosion inducing environment. The housing 44 could be formed of a relatively rigid plastics material.

In use, the device 120 is located in a corrosion inducing environment, to monitor the corrosion of a structure 50 located in the same environment.

For example, the device 120 could be located close to or against a metal structure. As shown in Fig. 4, the device 120 could be located in a substrate 52 surrounding the structure 50.

In one example of a method of use of the device 120 of the invention, the structure 50 is a concrete reinforcement structure which comprises a plurality of reinforcement members 54 (which may be referred to as "rebar") and the substrate 52 is concrete. The device 120 is located against, close to, or attached to the reinforcement members 54, and the structure 50 and the device 120 then embedded in the concrete. To provide a close correlation between the rate of corrosion of the switch members 6 and the rate of corrosion of the reinforcement members 54, the device 120 could be located at approximately the same depth as the shallowest reinforcement member 54, as, in general, corrosion in the concrete substrate 52 increases towards the surface of the concrete. Also, for the same reason, the switch members 6 could be formed of the same material as the reinforcement members 54 of the structure 50.

S The example of the packaged sensor attached to the steel rebar is shown in Fig. 4 in the case if a simple dipole antenna 10 is used. The antenna is oriented normal to the reinforcement members 54 in order not to short-circuit its electric field. Both shoulders of the antenna 10 are isolated from the reinforcement members 54 in this case.

The frequency response of the device 120 when all of the switch members 6 are in an initial intact condition before the switch members 6 are affected by corrosion is shown in Fig. 3A. In the initial intact condition, all of the respective coupled transponders 24 generate a response signal 62 to the radio frequency interrogation signal 60. After a period of time, corrosion affects the ability of switch member 6A to pass current, weakening the frequency response, and Fig. 3B shows the response in the course of corrosion when the switch 6A is corroded but not totally broken. Fig. 3C shows the response when the switch 6A is in a corroded, broken condition and the switch 6B is affected by corrosion. In the corroded broken condition, switch member 6A does not permit its coupled transponder 24A to generate a response signal 62 to the interrogation signal 60. Thus, by analysing the change of the frequency response of the device 120 over a period of time and knowing the cross-section of the switch members 6, the rate of corrosion can be determined.

The applicant has found that one of the most important considerations in selecting a suitable resonant transponder is the Q factor of the transponder which determines the frequency bandwidth of operation of the transponder.

Another important consideration is the frequency of operation of the transponder. The frequency of operation has to lie within the range of frequencies which are defined by regulatory authorities as being freely available for use without licence for short range (eg less than 1 metre) transmission. Ideally, the frequency of operation will lie within the range of frequencies which are defined as the industrial-scientific-medical (ISM) band, Transponders utilising inductors and capacitors have been considered by the applicant, but have been found unsuitable as the Q-factor of the LC circuits of around 30 -100 is such that the bandwidth occupied by the transponder is rather broad in comparison with the frequency of operation.

The frequency bands available for licence-free short-range devices at these frequencies are severely restricted by national and international regulations.

For instance, only the 14 kHz band is available in the EU at the frequency around 13.5 MHz and only the 40 kHz band is available at 40.7 MHz.

Transponders based on lumped inductors and capacitors have thus been found by the applicant to be too broadband to be used within the ISM bands.

Such transponders also suffer the disadvantage that their operating frequency is usually relatively low (well below 100 MHz) so that the antenna dimensions needed for wireless operation are too large for practical use. Thus, instead of electro-magnetic coupling, either inductive (magnetic) or capacitive (electrostatic) coupling is used in practice, but this limits the interrogation range for such transponders to around 10-20 cm.

In contrast, in some embodiments the invention utilises acoustic wave resonant transponders, which have been found to have higher Q factors of greater than 300 and ideally at least 1000. In some embodiments, the Q factor may be greater than 5000. The higher Q factors of the transponders of the invention are achievable at higher frequencies, which permit the use of antennas of moderate size, giving a greater read range of up to 1 metre and a having a narrower frequency bandwidth. The narrower frequency bandwidth means that several transponders at different frequencies can be located in the same device.

In some examples of the invention, the acoustic wave transponders are resonators such as surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators (such as FBAR and similar devices). The resonators operate at UHF, i.e. from 300 MHz to 3 GHz. The wavelength at these frequencies is sufficiently small to allow using a compact and yet efficient antenna with a size less than 20 cm. Advantageously, there are a number of sufficiently wide ISM bands within the UHF range available for operation of the device.

In the case of SAW resonators, the Q-factor can be as high as 10000.

As a result, the bandwidth occupied by a single SAW resonator working within the European 433 MHz ISM band is only around 50 kHz. Providing so-called "guard" bands of 250 kHz between each of the resonators and at the boundaries of the band, with a width of 1.74 MHz, one can provide five SAW resonators operating at five different frequencies with a 300 kHz interval between the frequencies.

Taking the example shown in Fig 1, the transponders 24 could be in the form of one-port SAW resonators 2 which are connected to the antenna through the switches 6 made in the form of a wire or a bar of the same metal as the reinforcement members 54 (or the same metal as any other structure where corrosion is to be monitored). The switch members 6 have different cross-sectional areas in order to allow determination of the corrosion rate. The minimum diameter or thickness of the switch members 6 depends on the minimum corrosion depth (ie the depth of penetration of corrosion into the surface of the reinforcement members 54) that needs to be detected, and could be around 20 microns. The reference transponder 5 could also be an SAW resonator, which is directly connected to the antenna 10 in order to allow detection of the device 120 after all the switch members 6 are broken as a result of corrosion, or to indicate a fault in the device 120. The positions of the antenna 10 and the ground 28 relative to the switch members 6 can be swapped if required.

The antenna 10 in Fig. I is electro-magnetically (wirelessly) coupled to the antenna of an interrogation unit (not shown), that can be positioned either in the near field zone of the sensor antenna 10, or in its far field zone. The interrogation unit has the task of measuring the frequency response of the device 120. It can be performed either in the frequency domain or in the time domain. One of the methods is to radiate a short (C 1 microsecond) RF pulse at one particular carrier frequency and then, after the end of the pulse, receive the natural oscillations excited in all the resonators of the device. They will last for about 20 microseconds for the Q factor of around 10000. Then, performing a Fourier transform of the response signal 62, one can find the frequency response of the device 120.

Another method is to use a longer RF pulse (between 1 and 10 micro seconds) that can excite natural oscillations more efficiently in one of the resonators and then sweep the carrier frequency of the pulse over the entire range occupied by the resonators in order to obtain their responses sequentially. In this case, the frequency response of the device can be found 1 5 just by measuring the magnitude of the natural oscillations received after the end of the interrogation pulse.

Another way, less sensitive to overloading of the front-end circuit of the RF receiver, is to calculate the Fourier transform of the response signal 62 and find approximately the position of the maximum of its energy spectrum. It should be noted here that there is no need for accurate measurement of the resonant frequencies. The only aim of the interrogation algorithm is to detect the presence or the absence of the resonant response of a particular resonator.

The power of the interrogation signal 60 should not exceed 10 mW in the EU in the 433 MHz ISM band, and it should be somewhat less for this frequency in the USA. Nevertheless, if the device antenna 10 is buried not deeper than 10-15 cm in the concrete, this power should be enough to ensure at least 1 m read range.

The SAW corrosion sensor described above can also work in other ISM bands, not necessarily in the 433 MHz band. In Europe, the 868 MHz ISM band can be used, in the USA the 915 MHz ISM can be used. If the Q-factor is sufficiently high, each of the resonators of the sensor can occupy one of the channels of the band and can be interrogated by the RF interrogation signal 60 employing random frequency hopping between the channels. In this case, the regulations allow a considerable increase of the interrogation power from 10 mW up to 1 W which gives a respective increase of the device read range.

Other types of resonators can be used in the corrosion sensing device instead of the SAW resonators. For instance, bulk acoustic wave resonators such as FBAR devices usually have Q factors of between 300 and 1000 and can work at frequencies between 1 and 10 GHz. The international 2.45 GHz ISM band offers a license-free bandwidth of up to 80 MHz that can fit around six different resonators with five different corrosion-sensitive switches.

Referring to Fig 2, each of the transponders 24 comprises a sensor interdigital transducer 34S, and the reference transponder 5 comprises a reference interdigital transducer 34R. In one example, as shown in Fig 2, all of the interdigital transducers 34 could be mounted on the same mounting substrate 32, and the mounting substrate 32 and the interdigital transducers 34 together comprise an SAW device 30. The mounting substrate 32 could be a piezoelectric substrate such as a quartz substrate. Its cut and the SAW propagation direction can be selected in such a way that the variation of the resonant frequencies with temperature is minimized in order to fit as many resonators with different resonant frequencies within the ISM band as possible. One of the possibilities is to use ST-X cut quartz as a substrate. The layout of the SAW device 30 may look like it is shown in Fig. 2. The SAW resonators 2 are made of thin Al film deposited on the surface of the mounting substrate 32, and comprise the interdigital transducers 34 as described above, with an array 36 of reflecting metal strips extending on each of two opposed sides of each interdigital transducer 34. The interdigital transducers 34 are connected to a ground contact 38. The sensor interdigital transducers 34S are connected to signal contacts 40 that are in their turn connected to the corrosion-sensing switch members 6. The reference interdigital transducer 34R of the reference resonator 5 is connected to a signal contact 42 which is directly connected to the sensor antenna 10.

S Figs 5 to 11 show other embodiments of the invention, many features of which are similar to those already described in relation to the embodiment of Fig 1. Therefore, for the sake of brevity, the following embodiments will only be described in so far as they differ from the embodiment already described. Where features are the same or similar, the same reference numerals have been used and the features will not be described again.

Types of acoustic wave transponders other than resonators can also be used as passive transponders in the corrosion sensors. Fig. 5 shows a second corrosion sensing device 220 in which the transponders 24, 5 are in the form of SAW reflective delay lines 58, each reflective delay line 58 comprising an interdigital transducer 34 with an associated reflector 56. The reflectors 56 are spaced from their respective transducers 34 at different distances to provide differing time responses as will be described below. The range of delays achieved is usually within the range from 0.01 microseconds to 10 microseconds.

In the example shown, the interdigital transducers 34 could operate, for instance, in the 2.45 GHz ISM band. The mounting substrate 32 could again be a piezoelectric substrate, e.g. YZ-LiNbO3. The terminals of the interdigital transducers 34 are connected to the antenna 8. Other terminals are connected to the corrosion-sensitive switch members 6 that are grounded on the opposite ends. One of the delay lines is directly connected to ground forming a reference transponder 5.

In use, a wireless interrogator (not shown) measures the impulse response of the corrosion sensing device 220 using known methods of interrogation, either in the time domain or in the frequency domain. With all of the switch members 6 in an initial intact condition, before corrosion affects the first switch member 6A, the impulse response looks as shown in Fig. 6A. Fig 6B shows the impulse response after a period of time, when switch member 6A is in the corroded broken condition and does not permit its coupled transponder to generate a response signal 62 to the interrogation signal 60, S and switch member 6B is partially corroded. By observing variation of the impulse response over sufficiently long period of time, one can measure the corrosion rate.

Fig 7 shows a third corrosion sensing device 320, which is an alternative embodiment of the SAW reflective delay line corrosion sensing device shown in Fig. 5. In the example shown, the device 320 comprises an an input interdigital transducer 64 and the reflectors are formed by the interdigital transducers 34, which are all positioned in a single surface acoustic wave channel. In use, when one of the switch members 6 is broken by corrosion, the amplitude of the reflected surface acoustic wave changes, which is detectable by the interrogation unit (not shown). The advantage of this approach is that the condition of the switch members 6 does not affect matching of the antenna 10 to the input interdigital transducer 64. One of the interdigital transducers 34R is always short-circuited and forms a reference transponder 5.

Fig 8 shows a fourth corrosion sensing device 420. In the arrangement of the third corrosion sensing device 320, internal reflections of surface acoustic wave can occur between the interdigital transducers 34 forming the reflectors, that distort the impulse response. To reduce the amount of internal reflections, in the fourth corrosion sensing device 420, the interdigital transducers 34 forming the reflectors are positioned in different acoustic channels, in a similar arrangement to that of the second corrosion sensing device 220 shown in Fig 5.

Fig 9 shows a fifth corrosion sensing device 520. Quite often, corrosion of rebar in concrete is facilitated by so-called macro-cells with corroding anodic sites well separated in space from the cathodic sites. If the switch members of a corrosion sensing device are electrically isolated from the steel reinforcement they can be under a different potential and can experience a different rate of corrosion to the rebar. In order to keep the switch members under the same electro-chemical conditions, the switch S members can be electrically connected to the rebar and the antenna can be of a monopole rather than dipole type.

The arrangement of the reinforcement members 54 also affects the choice of type and orientation of the antenna. If a dipole antenna is used for 1 0 the sensor as shown in Fig. 4, its arms should be normal to the reinforcement members 54 in order not to short the electric field generated by the antenna 10. If the structure 50 comprises reinforcement members orientated in a number of different directions (eg both vertical and horizontal), and the distance between the reinforcement members is comparable or smaller than the antenna size (or half a wavelength) then those wires not normal to the arms of the antenna will short-circuit the dipole antenna and drastically reduce its efficiency. Effectively, a dense mesh of wires will work as a ground plane.

In this case, it is better to use an antenna design that does require a ground plane for its operation, for instance a monopole (whip antenna) normal to the rebar plane or a patch antenna. Similarly, the monopole and the patch antenna can be used if the corrosion sensing device is installed on a metal sheet.

In the fifth corrosion sensing device 520, the switch members 6 are electrically connected to ground 28 which could comprise the structure 50 and/or the reinforcement members 54 (rebar or any other metal structure to be monitored) to put the switch members 6 under the same electro-chemical conditions as the rest of the structure 50. The antenna 10 can be of any type requiring the ground plane for its operation, e.g. a monopole or a patch.

Fig. 10 shows a sixth corrosion sensing device 620. In this embodiment, the antenna 10 is in the form of a monopole whip. As described for the fifth corrosion sensing device 520 above, the switch members 6 are electrically connected to one of the reinforcement members 54 of the structure which forms the ground 28. The structure 50 is in the form of a wire mesh, but could comprise any continuous metal surface.

S Fig. 11 shows a seventh corrosion sensing device 720. In this embodiment, the antenna 10 is in the form of a patch antenna. The patch antenna 10 comprises a conducting patch 66 positioned on a dielectric substrate 68 electrically connected via a contact 29 to the reinforcement members 54 of the structure 50 which form the antenna ground plane 28.

The switch members 6 of the sensors 22 are connected between the ground plane 28 and the signal terminals 40 (see for reference Fig 2) of the SAW device 30. The common terminal 38 of the SAW resonators 24 of the sensors 22 is connected to the patch antenna.

Various other modifications could be made without departing from the scope of the invention. The corrosion sensing device could comprise any suitable number of sensors. In one example, the corrosion sensing device could comprise just one sensor with an antenna.

The corrosion sensing device could be arranged and used differently.

In one example, the switch members could be of the same or similar cross sectional area, but could be located at different depths in the substrate, in order to monitor the depth of corrosion penetration into the substrate. in another example, the switch members could be formed of a number of different materials, for example, to assess different forms of corrosion.

Although the corrosion sensing device proposed here was mainly discussed in the context of monitoring the state of metal rebar in concrete structures it can equally be used to monitor corrosion in other metallic structures such as bridges, towers, pipelines, tanks, etc. It is not necessary that the material of the switch members is the same as that of the structure. It may be, for example, that the corrosion of a structure can be correlated to the corrosion of a switch member of a different material.

Any of the features or steps of any of the embodiments shown or described could be combined in any suitable way, within the scope of the

overall disclosure of this document.

There is thus provided a passive, batteryless, wireless corrosion sensing device. The device does not require batteries and is relatively simple 1 0 and robust in construction, and can therefore be left in situ for long periods without requiring attention or maintenance. The device is simple and inexpensive, and can locally detect corrosion and monitor its rate. The device has a read range of at least 1 m and that can operate within an industrial-scientific-medical (ISM) frequency band that does not require licensing. The device can be used for non-contact monitoring of steel reinforced concrete buildings, bridges, tunnels etc, metal bridges, towers, pipes and other structures where a visual inspection and installation of wired corrosion sensors is impossible or difficult.

Claims

Claims 1. A passive, batteryless, wireless corrosion sensing device, the device including an electrical circuit, the circuit comprising one or more electrically conducting corrosion sensors and an antenna, the or each sensor comprising a corrodable sacrificial switch member coupled with a passive RF transponder based on acoustic waves, the device being arranged so that, in use, the or each switch member in an intact condition permits its respective coupled transponder to generate a response signal to a radio frequency interrogation signal, but in a corroded, broken condition does not permit its coupled transponder to generate the response signal to the interrogation signal. r
2. A device according to claim 1, in which the circuit includes a reference transponder, which generates a response signal to the radio frequency interrogation signal irrespective of the condition of the or each of the switch members. (4
3. A device according to claims 1 or 2, in which the or each transponder comprises an acoustic wave transponder.
4. A device according to claim 3, in which the acoustic wave transponder comprises a resonant transponder, and may comprise a surface acoustic wave resonator or a bulk acoustic wave resonator.
5. A device according to claim 4, in which the or each resonant transponder has a Q factor of greater than 300.
6. A device according to claims 4 or 5, in which the Q factor is at least 1000, and may be greater than 5000.
7. A device according to any of claims 4 to 6, in which the resonant transponders generate response signals of different frequencies.
8. A device according to claim 3, in which the acoustic wave transponder comprises a surface acoustic wave reflective delay line.
9. A device according to claim 8, in which the delay line transponder has a delay within the range from 0.01 microsecond to 10 microseconds.
10.A device according to claims 8 or 9, in which the delay line transponders generate response signals at different times.
11.A device according to any of the preceding claims, in which the or each transponder includes an interdigital transducer. i-15

0
12.A device according to claim 11, in which the interdigital transducer is mounted on a mounting substrate. (0
13.A device according to claim 12, in which the device includes a plurality of interdigital transducers, which are mounted on the same mounting substrate.
14.A device according to claims 12 or 13, in which the mounting substrate and the interdigital transducer(s) together comprise(s) an acoustic wave device, and may comprise an SAW device.
15.A device according to any of the preceding claims, in which the or each transponder operates at between 300 MHz and 10 GHz.
1 6.A device according to claim 15, in which the or each transponder operates at UHF, and may operate at between 300 MHz and 3 GHz.
17.A device according to any of the preceding claims, in which the or each switch member is formed of an electrically conducting corrodable material, and may be formed of a metal.
18.A device according to any of the preceding claims, in which the device includes a plurality of sensors.
19.A device according to claim 18, in which the switch members of the sensors are arranged so that the time period to reach the corroded broken condition is different for each switch member.
20.A device according to claim 19, in which the switch members are of different cross sectional areas, and may have different thicknesses and/or different diameters. i-15

0
21.A device according to claims 19 or 20, in which the switch members are formed of different materials. (0
22.A device according to any of claims 19 to 21, in which the switch members are spaced apart, and may be spaced apart at regular intervals.
23.A device according to any of the preceding claims, in which, in use, the device is embedded in a substrate.
24.A device according to claim 23, in which the switch members are located at different depths in the substrate.
25.A device according to claims 23 or 24, in which the substrate is concrete.
26.A device according to any of the preceding claims, in which the device is for monitoring the corrosion of a structure.
27.A device according to claim 26 when dependent on any of claims 23 to 25, in which the structure is embedded in the substrate.
28.A device according to claims 26 or 27, in which the structure comprises one or more reinforcement members.
29.A device according to any of claims 26 to 28, in which the or each of the switch members is formed of the same material as the structure.
30.A device according to any of claims 26 to 29, in which the switch members are electrically connected to part of the structure.
31.A device according to any of claims 26 to 30, in which the structure forms a ground for the circuit, and may form a ground for the antenna. i-15

0
32.A method of monitoring corrosion, the method including providing a passive, batteryless, wireless corrosion sensing device, the device (Q including an electrical circuit, the circuit comprising one or more electrically conducting corrosion sensors and an antenna, the or each sensor comprising a corrodable sacrificial switch member coupled with a passive RF transponder based on acoustic waves, the device being arranged so that, in use, the or each switch member in an intact condition permits its respective coupled transponder to generate a response signal to a radio frequency interrogation signal, but in a corroded, broken condition does not permit its coupled transponder to generate the response signal to the interrogation signal.
33.A method according to claim 26, in which the device includes any of the features defined in any of claims I to 31.
34.A passive, batteryless, wireless corrosion sensing device substantially as hereinbefore described and/or with reference to any of the accompanying drawings.
35.A method of monitoring corrosion substantially as hereinbefore described and/or with reference to any of the accompanying drawings. r r (0 (4