DRDC-RDDC-2014-P127

Radio frequency identification (RFID) based corrosion monitoring sensors Part 1 – Component selection and testing Y. L. He*1, S. McLaughlin2, J. S. H. Lo1, C. Shi1, J. Lenos3 and A. Vincelli4

1CanmetMATERIALS, Natural Resources Canada, 183 Longwood Road South, Hamilton, ON, Canada, L8P 0A5

2DRDC Atlantic Research Centre - Dockyard Laboratory Pacific, CFB Esquimalt, Building 199, P.O. Box 17000 Stn. Forces, Victoria, BC, Canada, V9A 7N2

3Nanotechnology Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1

4Imperial Oil, Sarnia Manufacturing Site, 602 South Christina Street, P. O. Box 3004, Sarnia, Ontario, Canada, N7T 7M5

*Corresponding author, email: yohe@nrcan.gc.ca

Abstract

Cost-effective radio frequency identification (RFID) transponders (tags) were investigated for use as wireless corrosion monitoring sensors. Performance metrics of read rate, received signal strength indicator, and minimum activation power were established for the as received tags. The effects of orientation, distance between transceiver (reader) and tag, and a metallic backing were evaluated using these performance metrics. Laboratory tests to determine the effect of a corrodible electromagnetic interference (EMI) shielding layer were conducted with a simple steel foil applied onto the surface of the plastic encased RFID tags. This layer acted as a barrier against the penetration of radio waves, preventing communication between the reader and tag. When the shielded RFID transponders were exposed to accelerated corrosion tests, the degradation of the shielding layer decreased the EMI shielding effectiveness and resulted in a strengthened communication between the reader and tag. The use of different thicknesses of this layer demonstrated the effect of cumulative corrosion damage. With appropriate calibration, the amount of corrosion could be estimated from the change in strength of communication between the reader and tag.

Keywords: Corrosion monitoring, Wireless sensor, RFID, Electromagnetic interference, Coating

Introduction

Corrosion is one of the major factors that cause premature failures and high maintenance costs of infrastructures. Sensors that can detect early signs of corrosion or corrosion induced damage will significantly help reduce the service costs and increase the reliability, safety, and service life of the infrastructure. A great number of corrosion monitoring technologies have been developed based on various theories and principles,1–3 and many have already been implemented in real-world monitoring practices. Most of these sensors, however, require continuous physical connections to readers or data loggers which limit their use. Wireless sensors are emerging as an attractive alternative to the conventional wired sensors.4–9

Radio frequency identification (RFID) based sensing has appeared as a promising technology in a number of applications in recent years. Several investigations were conducted to explore the possibility of using RFID sensing technology to monitor the health of civilian infrastructures6,8 and the corrosion condition of military ground vehicles.9 One approach was to incorporate a corrosion sensitive element in the RFID transponder antenna.7 When the sensor element was exposed to the environment, corrosion of the element resulted in a discontinuity of the antenna, and cut off the communication between the transponder and transceiver.

Another approach employed was the fabrication of thin-film RFID sensors using nanocomposites.6 Electrochemical (e.g. corrosion/pH) sensing mechanisms were encoded into thin films, which were then patterned into a coil antenna through which inductive coupling between the transponder and transceiver can be established. When the pH on the surface of the thin film changed, both the film resistance and the system bandwidth would change accordingly. By mapping the relationship between pH and bandwidth, it was possible to monitor corrosion since pH was one factor integral to identifying the corrosion processes.

Dante and Friedersdorf9 used thin aluminium and copper coatings only a few micrometres thick, applied to RFID tags as an electromagnetic shield to attenuate or even completely block the signal. When the thickness of the coating was altered by corrosion, the shielding effectiveness and signal attenuation was reduced, re-establishing communication between the tag and reader. Problems included high corrosion resistance of the coating material, which was not ideal for corrosion monitoring, and difficulties associated with applying the thin coating to the tags.

The present exploratory research employs and builds upon the shielding concept developed by Dante and Friedersdorf,9 by using carbon steel foils and steel filled composite paints to replace the thin aluminium and copper coatings as the electromagnetic interference (EMI) shielding material. The advantage of using steel as a shielding material over aluminium or copper is that steel is a common structural material for infrastructure that requires corrosion monitoring, and thus, the coatings should undergo corrosion at a similar rate as exposed parts of the structure being monitored. The composite paints can be applied with a coating thickness on the order of hundreds of micrometres, which makes the sacrificial shielding material more comparable to real structures. The simple coating method can significantly reduce the sensor manufacturing costs as compared to the vacuum vapour deposition method employed by Dante and Friedersdorf.9 The basic sensing principle involves relating any changes in communication performance of an EMI shielded tag to the corrosion induced degradation of the EMI coating.

The objectives of Part 1 of this paper are to select a suitable RFID system, to establish performance metrics for unshielded RFID tags, and to prove the sensing concept using a carbon steel foil covering. Evaluation of different composite paints as a shielding material and application methods will be presented in Part 2 of this paper.

Background

Principle of RFID tag and reader operation

An RFID system10 is typically composed of tags, a reader, and a host computer to manage the information. There are many types of RFID tags and reader systems commercially available; most are designed for inventory management and product tracking. While there are many designs, there are two primary types of tags: active, which have a local power source (e.g. battery, solar, etc.); and passive, which do not have an integral power source. Passive tags are much less expensive and more reliable than active tags, due to the absence of the integrated power source, and are energised by the reader via electromagnetic coupling during interrogation.

Radio frequency identification systems generally employ one of two main coupling techniques, near-field or far-field, to communicate between the reader and tag, each with a different range of operating frequency or wavelength.10 Low and high frequency tags utilise the near-field coupling technique and have a relatively short reading distance up to about 1 m, whereas ultrahigh frequency (UHF) tags utilise the far-field coupling technique and can have longer reading distances from a few metres to tens of metres. The tags used in the present study are UHF (902–928 MHz), and thus employ the far-field coupling technique.

Shielding theory

The signal between the reader and tag can be partially or completely blocked by the application of an EMI shielding layer such as a metallic foil or a conductive polymer composite coating. The shielding effectiveness (Es) of electromagnetic radiation is the sum of the losses11–14

𝐸𝑠 = 𝑅 + 𝐴 + 𝑀 (1)

where R is the loss due to reflection, A is the loss due to absorption, and M is the loss due to multiple reflections of various surfaces and interfaces within the shield. The UHF radio signal can be considered a far-field plane wave if the distance between the radio source (the reader/antenna) and the shielding material is greater than λ/2π, where λ is the wave length.14 With the frequency range of 902–928 MHz, λ/2π is between 0.053 and 0.051 m, which is an order of magnitude smaller than the reader distance of 0.5 to 1 m used in this study. Thus, the shielding effectiveness for a far-field plane wave can be employed here, which is expressed as a function of material properties, frequency, and thickness of the shielding layer11,14

𝐸𝑠 = 168.2 + 10 log10 �𝜎𝑟𝜇𝑟𝑓

� + 131.4 × 10−6𝑡(𝜎𝑟𝜇𝑟𝑓)1/2 (2)

where the shielding effectiveness Es is measured in dB, σr is the relative conductivity of the shielding material (as compared to copper), f is the frequency in Hz, µr is the material’s relative permeability, and t is the thickness of the shielding layer in mm. In equation (2), the first two terms on the right-hand side represent the loss by reflection, while the third term is the loss due to absorption. The loss due to multiple reflections was ignored as it is usually only significant at low frequencies below about 20 kHz, and when the shielding layer is thinner than the skin depth.11–14 The skin depth (δ) is the distance measured in mm from the surface of the shield to where the electric field drops to 1/e of the incident strength, and is defined as11,14

𝛿 =65.23 × 103

(𝜎𝑟𝜇𝑟𝑓)1/2 (3)

It is clear from equation (2) that the shielding by both reflection and absorption increases with the conductivity (σr) of the shield. Shielding by reflection is inversely proportional to the product µrf, while the shielding by absorption is proportional to µrf.

From equation (2), it was calculated that for a material with µr=1 and thickness t=5 μm at the UHF range of 902–928 MHz, the material’s conductivity should be greater than about 0.8 S m-1 in order to have positive shielding values. With a high conductivity material such as a metallic foil, where depending on the material, the conductivity ranges from about 1×106 to 6.3×107 S m-1, the reflection term dominates, and with Es calculated at >80 dB, the signal would be 99.999 999% blocked [Es=10log(Powerout/Powerin)] even at extremely low thicknesses on the order of a few micrometres. However, using such small thicknesses of a foil for long-term corrosion monitoring would not be practical since even small discontinuities in the shielding layer due to physical or corrosion damage would likely sufficiently restore the signal such that any further damage could not be detected. Corrosion of a thicker shield would still create discontinuities by converting the conductive metal into low conductivity oxides in the shielding material; however, the decrease in the shielding effectiveness and associated increase in signal strength or communication between the tag and reader would be more gradual. We capitalised on this principle of a gradual restoration of the signal in our investigation of the RFID as a wireless corrosion sensor. With information on the correlation between corrosion damage and signal strength, the degree of corrosion damage can be estimated by measuring signal strength or communication metrics between the tag and reader.

Materials and methods

Selection of RFID tags and reader

For corrosion monitoring, passive tags are preferable over active tags because the corrosion sensors are expected to be in use for a relatively long time without intervention. Since one application of the corrosion sensor tags is on vehicles, and they may be expected to be read over a large distance, the UHF systems are preferred.

A rectangular ‘Fastenable Mount-on Metal’ type of UHF (902–928 MHz) tag (GAO RFID Inc., Toronto, ON, Canada) was selected as the sensor tag (Fig. 1a). These tags are made of laminated

polycarbonate and acrylonitrile butadiene styrene plastics with a built-in metal backplane; they are designed to be attached to a metallic surface with little or no interference of the signals. The tags can tolerate temperatures up to 200°C, high humidity, and pressures up to 1.5 MPa. They can also withstand vibration and mechanical shock. The rectangular flat surfaces made them ideal for coating applications. An Impinj Speedway Revolution R220 reader (Impinj Inc., Seattle, WA, USA) and associated control software were selected as the RFID transceiver to communicate with the tags. In the present work, the transmitted power range was 10 to 30 dBm with a maximum receive sensitivity of -82 dBm. The maximum read range was about 5 m.

Performance of RFID system

The response of any RFID tag/reader system is a function not only of the components themselves, but also the conditions under which the system is evaluated. An RFID system should therefore be tested for a given application. The tag-reader system selected in this study was first calibrated with as received UHF tags (uncoated) to provide a benchmark for comparison of coated and corroded tags. There are four metrics that could be used to establish the performance of the RFID system in a corrosion sensing application:

(i) tag response ratio

(ii) tag response speed

(iii) received signal strength indicator (RSSI)

(iv) minimum transmitted power to operate the tag.

The tag response ratio, sometimes called ‘response rate’,15 is the ratio of the number of successful reads to the number of read attempts. The tag response speed, also called ‘read rate’, is the number of responses per unit time and it represents the speed at which the reader can read a tag. This is an important parameter in conventional RFID application such as object identification or supply chain tracking. The read rate for the current system was taken as the number of successful reads in a period of 10 s, with a maximum number of 247, or 24.7 reads s-1.

The RSSI is an indication of the radio frequency (RF) signal level received by the antenna, and is given in arbitrary units.16 Although there is no standardised relationship between the RSSI value and the power level, the former is often represented by an integer value. The reader used in the present work employed an offset metric for reporting RSSI: a value of 270 was returned when there is no signal, and a larger (less negative) integer was returned for stronger signal strength. For the purpose of clarity, the RSSI reported throughout the present work has been transformed by adding 70 to it, such that the theoretical range is from 0 (tag not readable) to 70. However, the maximum transformed RSSI measured in the present work was 26, and if the tag was readable, the transformed RSSI generally fell in the range of 10 to 26. It was also noted that when the read rate was at its maximum value, the RSSI had not necessarily reached its peak value.

The minimum transmitted power to operate the tag is a direct measure of the threshold RF energy to activate a tag. Below this minimum power, there is no signal detected from the tag by the reader, and both the read rate and the transformed RSSI are zero. The reader system used in the present work provides three of these metrics (ii, iii and iv) for tag performance measures. All three of these metrics were used to assess the sensor tags before and after an EMI shielding layer was applied.

Tag reading platform

It is well known that the performance of an RFID system is affected by a number of factors such as the tag sensitivity, power effectiveness, orientation sensitivity, tag-reader distance, metal proximity, etc. A tag reading platform (Fig. 1b) was designed for reading the tags at angular orientations ranging from 0 to 360°, and distances from 0 to 100 cm. The platform was made of wood and plastic to minimise the effect of metal proximity on the performance readings. The tag was mounted on a rotary platform driven by a computer controlled step motor via a small O-ring belt and pulleys. The step motor has an angular resolution of 1.8° per step, which can be further reduced to 8 micro-steps by using a microcontroller, resulting in a resolution of 0.225° per step. The reader software was synchronised with the rotation of the tag by the step motor to automatically collect readings at each step.

Performance measures of uncoated tags

Three different sizes of the mount-on-metal type of UHF tags were tested: 38×10×3 mm, 75×15×2 mm and 150×18×3 mm. The larger tags (150×18×3 mm, Fig. 1a) were found to have longer reading distances and less orientation sensitivity, and thus they were selected for further study. The variation of performance between tags of the same size and model under similar reading conditions was found to be sufficiently small such that the performance measures could be carried out on one randomly selected tag, and the results apply to the batch. Each performance test was repeated four times on a single uncoated tag using the tag reading platform described above, and the average reading was recorded. For the read rate measurements, the software was set to read the tag as many times as possible in 10 s, and the results were reported as reads s-1.

Orientation sensitivity

The tag was read at various angular orientations relative to the reader antenna at distances from 50 to 100 cm in 10 cm steps, and at a power level of 10 dBm. This relatively low power level was chosen to maximise any effect that the orientation might have. The performance metrics of read rate and RSSI were logged at each of the orientation angles.

Effect of read power

Tag performance was evaluated at read powers ranging from 10 to 30 dBm with a step size of 0.25 dBm, for a total of 81 levels, at tag-reader distances of 50, 75 and 100 cm, and at four orientation angles of 0, 90, 180 and 270°. These angles were selected out of convenience, since initial results indicated that the angle of peak orientation sensitivity appears to shift almost arbitrarily with changes to the power and tag-reader distance. The performance metrics of read rate and RSSI were logged at each of the read powers.

Effect of backing metal

As discussed above, the mount-on metal UHF tags incorporate a built-in metal backplane, which enables the tag to be read even when mounted onto metallic objects. Although these tags are readable in close proximity to metal, any metal nearby usually has an effect on the reading performance. To evaluate this effect, uncoated tags were attached to two differently sized coupons of 0.2 mm-thick low carbon steel: 150×18 mm (the same size as the tag) and

225×152 mm. The performance tests were conducted at a power level of 10 dBm for distances of 50 to 100 cm with a step size of 10 cm, and orientation angles of 0 to 360° with a step size of 15° to evaluate the effect of tag-reader distance. Tests were also performed at a distance of 75 cm at orientation angles of 0 and 90°, and at power levels of 10 to 30 dBm with a step size of 0.25 dBm to examine the effect of read power.

Results and discussion

Orientation sensitivity

The long thin rectangular tags selected in this study have intrinsic orientation sensitivity due to their dipole antenna configuration. Figure 2 illustrates the influence of tag orientation and tag-reader distance on the performance metrics of read rate and RSSI of an uncoated tag at 10 dBm, which is the lowest power that the reader is capable of outputting. As shown in Fig. 2a, at a tag-reader distance of 50 cm the read rate was nearly constant at 24.7±0.1 reads s-1, and was generally unaffected by tag-reader orientation angle. However, after increasing the distance by 20% to 60 cm, a strong directional influence was evident. The read rate was near the maximum of 24.7 reads s-1 at the orientation angles of 45 and 225°, but was less than half of this value at orientation angles of 135 and 315°. At tag-reader distances of 70 to 90 cm, not only was the directional influence strongly evident, but the overall read rate also declined sharply to fewer than 8.0 reads s-1for all orientation angles. With a distance of 100 cm, the read rate was almost zero at all orientation angles.

The transformed RSSI showed a moderate orientation sensitivity at the tag-reader distance of 50 and 60 cm and power level of 10 dBm (Fig. 2b), where with the exception of three measurements, the RSSI fell between 19 and 24. The orientation sensitivity was much more pronounced for distances of 70 cm and greater at the same power level, where it ranged from 0 to 19, and declined sharply with increasing distance and ranged from 0 to 10 at 100 cm.

Effect of read power

From Fig. 2, it is clear that to minimise orientation sensitivity at lower powers, the tag-reader distance should be minimised. However, by increasing the read power, the orientation sensitivity can be reduced while still maintaining a larger tag-reader distance.

In Fig. 3a, c and e, the decline in the read rate with increasing tag-reader distances is again evident at the lowest read power of 10 dBm. Strong orientation sensitivity is also clear with the larger distances. By increasing the read power, the read rate increased rapidly, and reached a maximum of 24.7 reads s-1 at power levels between 10 and 14 dBm, depending on tag-reader distance and orientation. The directional influence on read rate is most clearly demonstrated at the larger tag-reader distances (Fig. 3e), where the maximum read rate was reached at a power of approximately 12 dBm for the 90 and 270° orientations, and approximately 14 dBm for the 0 and 180° orientations.

At power levels above 26.25, 27.00 and 28.75 dBm for tag-reader distances of 50, 75 and 100 cm respectively, there was a noticeable drop in read rate (Fig. 3a, c and e). This drop was attributed to ‘multiple-tag-to-reader’ interference, also known as ‘tag collision’, which occurred when the

high transmitted power energised nearby tags which were not meant to be read. This was found to occur even with the extra tags that were stored in a metal cabinet about 2 m away from the base of the tag reading platform. At lower powers, tags not directly under the antenna were not activated; but at high powers, tags in close proximity to the antenna, including those stored in the metal cabinet became activated. The activated tags all reflected their respective signals back to the reader with a mixture of scattered waves, and the reader was not able to differentiate among the energised tags. This effect was eliminated by moving the extra tags more than 10 m away from the reader (data not shown). However, this tag collision demonstrates the need to limit the power levels when sensors are in close proximity on a multiple sensor installation.

The RSSI showed an initial drop for increasing power levels up to about 11 and 14 dBm at tag-reader distances of 75 and 100 cm, respectively, but the initial drop was not clearly evident at a tag-reader distance of 50 cm (Fig. 3b, d and f). For all measurements, the minimum RSSI and maximum read rates appeared to occur at similar powers (Fig. 3a, c and e).

At reading powers beyond about 14 dBm, the RSSI showed an approximately linear relationship with read power for all tag-reader distances measured. This linear relationship is similar to the RSSI vs input power relationship observed in typical RF systems.17,18 Further, the slope of the linear relationship was approximately the same for all three tag-reader distances, although it is clear that the RSSI decreased with distance (Fig. 3b, d and f). The orientation effect on RSSI noted at the power of 10 dBm in Fig. 2b, is most clearly evident over the entire range of reading power from 10 to 30 dBm, particularly for the tag-reader distance of 100 cm (Fig. 3f), where the RSSI at the orientations of 0 and 180° are consistently higher than those at orientations of 90° and 270° over the entire range of reading power.

Effect of backing metal

Figure 4a illustrates the orientation and distance effects at a power of 10 dBm on the read rate of an uncoated tag attached to a 150×18×0.2 mm coupon of low carbon steel, which is the same size as the tag. Compared to Fig. 2a, Fig. 4a shows that at this low power, the steel backing actually enhanced the read rate and reduced the orientation sensitivity at all of the reading distances, with the exception of 50 cm where the orientation sensitivity increased slightly. Figure 4b shows that at a tag-reader distance of 75 cm, the full read rate was achieved at a power of about 12 dBm for the 0 and 90° orientations. In fact, when the read power was increased to above 12.5 dBm, the orientation sensitivity was eliminated for all orientations (data not shown).

When the size of the steel backing plate was increased to 225×152×0.2 mm, the minimum power needed to operate the tag also increased. While the minimum power to operate the tag with the small backing plate was 10 dBm for all orientations, the minimum power to operate the tag with larger backing plate was 16 and 14 dBm for the 0 and 90° orientations, respectively (Fig. 4b). The orientation sensitivity also increased as indicated by the large difference in read rates between 0 and 90° at a given read power.

Although the RSSI data for these tests are not shown, the trends were similar to the read rate: the use of backing metal enhanced the RSSI, especially at low read power and high read distances. The RSSI orientation sensitivities were also not as pronounced with the addition of the backing plate. However, the size of the backing metal noticeably influenced the RSSI, where the RSSI at

a given read power for the larger plate was considerably lower than for the plate the same size as the tag.

The performance metrics of read rate and RSSI on uncoated tags are clearly influenced by orientation, tag-reader distance, reader power and presence and size of a metallic backing plate. Since the orientation sensitivity of the read rate metric can usually be eliminated at high powers and/or short tag-reader distances, it is a preferred metric for the current study as this may reduce the number of affecting parameters to be considered. In addition, the environment in which the tags are being read needs to be taken into consideration, i.e. proximity to metallic backing plates and to other tags. This is not a trivial matter, and highlights the importance of thorough characterisation of the tag reader system and development of an installation and reading protocol, and a comprehensive algorithm which can accommodate all of the important factors influencing the tag-reader communication into a single metric which could be related to corrosion.

Proof of corrosion sensing concept

A series of plain carbon steel (AISI 1008) foils with thicknesses ranging from 25 to 300 μm and the same size as the tags were attached to the top surface of the RFID tags with double-sided tape. The shielded tags were then tested using the RFID reading system at a distance of 75 cm. The signals were completely blocked at all power levels up to and including 30 dBm. Even when the tag-reader distance was decreased to 35 cm, none of the shielded tags could be read at the highest power of 30 dBm. This indicates that a very thin layer of steel foil on the order of 25 μm is sufficient to block all of the RF signals and serves as an effective EMI shield.

The covered tags were then subjected to accelerated corrosion tests in the laboratory in an automated cyclic corrosion test chamber (CCT-10P, Singleton Corp., Cleveland, OH, USA). A modified SAE J233419 cyclic corrosion test was used with a salt solution of 0.5 wt-%NaCl, 0.1 wt-%CaCl2 and 0.075 wt-%NaHCO3 as the corrosive medium. Each 24-h corrosion cycle consisted of three stages:

(i) humid stage: 50°C and 100% humidity for 6 h

(ii) salt fog stage with fog at ambient conditions for 15 min

(iii) dry stage: 50°C and 50% humidity for 17 h 45 min.

The tags were exposed to the accelerated corrosion test for 24 cycles, and read after 12 and 24 cycles.

After the first 12 corrosion cycles, all of the steel foils had a similar appearance with a uniform layer of corrosion products on their surface. Only the tag covered with a 25 μm steel foil could be read; the minimum reading power was 15 dBm at the set distance of 75 cm. At relatively low reading power levels on the order of 15–20 dBm, there was an obvious orientation sensitivity in both the read rate and the RSSI (Fig. 5). In order to suppress the orientation sensitivity in the read rate metric at this tag-reader distance, a higher power of 25 dBm was required. This power was nearly double the power required to suppress the orientation sensitivity in the uncoated tags at the same tag-reader distance. The RSSI still showed some orientation sensitivity even at the highest power of 30 dBm.

After 24 corrosion cycles, substantial corrosion was observed on all of the foils. Figure 6 shows the cross-sections of the steel foils, which had all become stratified, and nearly all of the iron had been converted into corrosion products. The white regions in Fig. 6 are iron oxides as confirmed by energy dispersive X-ray spectrometry under a scanning electron microscope (data not shown). With the exception of the 25 μm thick foil which was nearly completely converted to a flaky and friable surface oxide, the oxide layers were mostly continuous, although there were numerous voids, which appear as either black (voids) or grey (voids filled with epoxy mount) regions within these layers in Fig. 6. The thickness of the foil layers increased considerably due to the formation of the iron oxides. As shown in Fig. 6a and b, the thicknesses of both the 25 and 50 μm foils increased to about 200 μm following accelerated corrosion testing.

Although the corrosion attack did not entirely perforate the steel layer in the 50, 150, 200 and 300 μm foils, the read tests indicated that the conversion of iron metal to low conductivity iron oxides was sufficient to reduce the shielding capability of the layer; all the tags could be read at a distance of 75 cm and orientation angle of 0°. For tags covered with 25, 50 and 150 μm thick steel foils, the powers at which the read rate reached the maximum value of 24.7 reads s-1 were approximately 14, 16 and 21 dBm (±0.25 dBm), respectively, while for tags covered with the 200 and 300 μm thick foils, the reading rates did not reach this value even when the maximum reading power of 30 dBm was applied (Fig. 7). This change in reading performance clearly demonstrates that the coated tags can be used to monitor cumulative corrosion, since it can be correlated to the absorption losses caused by the different oxide layer thicknesses formed (equation (2)). The correlation between the sensor response and the degree of corrosion attack will be further explored in Part 2 of this paper.

Summary and conclusions

The concept of using a corrodible EMI shielding on RFID transponders was employed to build wireless corrosion monitoring systems. Using the common RFID performance metrics of read rate, read power, and RSSI, the properties of a ‘mount-on-metal’, UHF passive type RFID tag were characterised.

It was found that a number of factors including orientation, reading power, reading distance, and presence of metal backing all influence the tag response. With a sufficiently high read power, the orientation effects could be minimised or eliminated with the metric of read rate. However, excessively high read powers would activate nearby tags, which would interfere with the signal. The data presented provided benchmarks for the construction of wireless corrosion sensing systems using RFID techniques.

Steel foils of different thicknesses were used as the EMI shielding layer, which effectively blocked the tag and reader from communicating. The corrosion of the steel foils featured the stratification of the foil layers and the conversion of the iron metal into iron oxides. Although the iron oxides formed had very low conductivity, they still had some EMI shielding capability, and thus the corrosion of the EMI shielding layer resulted in the partial restoration of the communication between the tag and reader. The strength of this communication was dependent on the thickness of the shielding layer, thus illustrating how RFID tags coated with steel could be used to monitor cumulative corrosion damage.

Acknowledgements

The authors gratefully acknowledge Mr Raul Santos, Mrs Jennifer Collier and Mr Denis Jacob for their assistance in laboratory activities and for constructing the tag reading platform.

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Figures

Figure 1: RFID system: a RFID tag, and b tag reading platform constructed from wood and plastic.

Figure 2: Orientation sensitivity of uncoated tag at various tag-reader antenna distances: reading power was 10 dBm and data represent average of four tests. a) read rate; b) received signal strength indicator (RSSI).

Back

Mounting Holes

Side

(a)

Front

Reader Antenna

Reader

Control Board

Tag

Step Motor

Height-Adjustable Antenna Holder

(b)

(a)

0°

180° 90°

270° 0°

180° 90°

270°

(b)

Figure 3: Performance metrics of averaged read rate and RSSI of as received tag under various reading power levels at different orientation angles and distances. a, b 50 cm; c, d 75 cm; e, f 100 cm.

50 cm, read rate (a)

50 cm, RSSI (b)

75 cm, read rate (c) 75 cm, RSSI (d)

100 cm, read rate (e) 100 cm, RSSI (f)

Figure 4: Effect of backing metal, orientation, transmitted power, and reading distance on tag read rate. a) uncoated tag attached to 150×18×0.2 mm low carbon steel backing plate at reading power of 10 dBm at various distances; b) read rate of uncoated tag attached to two different sized steel backing plates: 150×18×0.2 mm (small) and 225×152×0.2 mm (large), oriented at 0 and 90° at fixed reading distance of 75 cm and at various transmitted powers.

Figure 5: a) read rate and b) RSSI at different transmitted powers and orientations for RFID tag covered by 25 μm thick steel foil after 12 cycles of salt spray. Tag-reader distance was 75 cm, and there was no steel backing plate.

(a)

(a) (b)

(b)

Figure 6: Optical micrographs of cross-sections of steel foil covered RFID tags following 24 cycles of salt spray. Initial foil thicknesses are: a) 25 μm, b) 50 μm, c) 150 μm, d) 200 μm, e) 300 μm, and f) tag with 100 μm foil before corrosion showing cross-section. E: epoxy mount, S: corroded steel foil, A: adhesive, T: tag. Adhesive layers in a–e eroded away during corrosion process, whereas in f it is swollen due to moisture absorption from metallographic sample preparation process.

400 µm

(a)

S

E

T

A

400 µm

(b) E

S

T

A

400 µm

S

E

A

(c)

T

(d) E

S

A

T 400 µm

A

S

E (e)

400 µm Circuit/antenna

(f)

400 µm

Polycarbonate

Adhesive Steel foil

Epoxy mount

Figure 7: Read results at increasing powers of steel foil covered RFID tags after 24 cycles of salt spray. Tag-reader distance was 75 cm, and orientation angle was 0°.