Surface Acoustic Wave Delay Line and Its Potential Applications for Corrosion Monitoring

Surface acoustic wave (SAW) devices has been used as sensors for temperature, pressure, traces of vapor and gases as well as filters and resonators in communication
systems. It has also been demonstrated as a corrosion sensor designed to monitor atmospheric corrosion of copper. In this work, ruggedness of SAW sensor will be
further explored and applied to sense and monitor corrosion. As a deliverable, a novel technique to extract corrosion rate of iron using SAW sensor is demonstrated.
SAW is typically generated and detected with inter-digital transducers (IDT) on a piezoelectric substrate. For sensing application, sensing elements are placed in between the IDTs. Any physical changes in the sensing elements, alter the frequency or phase of the detected SAW. The SAW is designed to be generated at pre-designed central
frequency using impulse response (IR) model. The modulated SAW due to the changes to sensing element is then investigated using coupling of mode (COM) model. As
expected, the thinning of sensing element showed a significant phase shift in SAW. Based on the design, SAW device is fabricated using typical lithography process.
Its transfer characteristics i.e. transmission S21 and reflection S11 coefficients are then
measured using vector network analyzer (VNA). In dry condition, the transfer characteristics are similar to the simulated ones. In partially wet condition i.e. a small
corrosive droplet on the sensing element, the transfer characteristics are modulated and later saturated after a certain time. The modulation is believed to be due to the mass unloading effect i.e. thinning of metallic sensing element and evaporation of droplet. Analyzing the phase of S21 reveals that the unwrapped phase of S21 can be correlated with the thinning of metallic sensing element. From the correlation, a corrosion rate is
determined. The corrosion rate obtained from this technique agrees reasonably well with the corrosion rate obtained from linear polarization resistance (LPR) technique.