Wireless Corrosion Sensors: Review and Status

Abstract

WIRELESS CORROSION SENSORS: Review and StatusThe traditional methods to study corrosion of metallic materials in occluded environments involve external electrical connections to electrochemical instrumentation, and they are intrusive. Advances in microelectronics have made possible new sensors based on RFID and wireless interrogation [1] “Wired” connections could be optical fibers used for spectroscopic detection of species, or for corrosion of metal films for example [4,5]. Publications that discuss probes and sensors with “wired” connections for corrosion are in references [6-16]. Most traditional monitoring techniques characterize the behavior of macroscopic samples either exposed to simulated conditions or exposed in service conditions. Broomfield, et al.[17] recently reviewed techniques for studying corrosion degradation: (1) linear polarization resistance (LPR); (2) critical threshold concentrations of damaging species (e.g. chloride ions, or pH); (3) resistivity of the damaging medium, and; (4) half-cell potential measurements. Methods (1) to (4) average over sample area (e.g, corrosion rate by LPR), or determine the concentrations or permeation rates averaged over large dimensions. These traditional methods have provided a valuable resource for predicting behavior and corrosion failure, but the “wired” approach limits the extension to buried interfaces and isolated damaging fluid exposure. Other approaches designed for industrial conditions are cited by Dean [18], and a new textbook is an excellent resource for a fundamental background on corrosion science [19]. To take advantage of wireless techniques to characterize the corrosion of “buried” metals, New Wireless Sensors should be:(1) Small so that they are not intrusive and do not alter local conditions in occluded regions;(2) Inexpensive so that multiple units can be embedded at distributed locations;(3) Tailored to enable unequivocal predictions of the metal corrosion process intechnological conditions, and; (4) Robust to ensure durability before the onset of corrosionPrevious wireless chemical (corrosion) sensor research involve passive sensors or threshold sensors. For example, a major type of passive RFID sensor (Class A) does not have a corrosion monitoring function. An example was reported by Andringa, et al [20](2005) with a design “window” for exposure of a portion of the sensor antennae to a damaging fluid. If the sensor fails to respond to interrogation, it is assumed to have “failed”. Similar sensors by Dickerson, et al [21](2006) and by Materer, et al [22](2009) have similar functions. However, these techniques can give a false positive, because the failure mechanism may be unrelated to corrosion. Other sensors measure threshold concentrations of damaging species such as H+, or Cl-, or other properties [23-24]. With integrated electronics, one can design the voltage readout function to exclude signals below the desired threshold. A passive RFID chip has been used to exclude measurements of the Cl- ion that were below a threshold concentration [25]. A third sensor type is activated when an embedded conductivity array becomes immersed in a condensed fluid. An “always-on” battery component is embedded with the conductivity array to provide power for the impedance measurement. These and other sensor types will be reviewed.An alternative active sensor design is to use a battery-assisted threshold chip for an “off/on” signal for wireless transmission. A battery is inactive until a damaging fluid arrives at the sensor location, and the battery-sensor turns “on” and powers a transmitter that signals the onset of damaging condition. The relative merit and advantages of the passive, threshold, and active sensors will be described.References available upon request – space in not sufficient