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Data Sheet

AD5933

Rev. E | Page 33 of 40

ELECTRO-IMPEDANCE SPECTROSCOPY

The AD5933 has found use in the area of corrosion monitoring.

Corrosion of metals, such as aluminum and steel, can damage

industrial infrastructures and vehicles such as aircraft, ships,

and cars. This damage, if left unattended, may lead to premature

failure requiring expensive repairs and/or replacement. In

many cases, if the onset of corrosion can be detected, it can

be arrested or slowed, negating the requirement for repairs or

replacement. At present, visual inspection is employed to detect

corrosion; however, this is time consuming, expensive, and

cannot be employed in hard-to-access areas.

An alternative to visual inspection is automated monitoring

using corrosion sensors. Monitoring is cheaper, less time

consuming, and can be deployed where visual inspections are

impossible. Electrochemical impedance spectroscopy (EIS) has

been used to interrogate corrosion sensors, but at present large

laboratory test instruments are required. The AD5933 offers an

accurate and compact solution for this type of measurement,

enabling the development of field deployable sensor systems

that can measure corrosion rates autonomously.

Mathematically, the corrosion of aluminum is modeled using an

RC network that typically consists of a resistance, RS, in series

with a parallel resistor and capacitor, RP and CP. A system metal

would typically have values as follows: RS is 10 Ω to 10 kΩ,

RP 1 is kΩ to 1 MΩ, and CP is 5 µF to 70 µF. Figure 38 shows

a typical Bode plot, impedance modulus, and phase angle vs.

frequency, for an aluminum corrosion sensor.

100k

0.1

FREQUENCY (Hz)

100k

–75

–50

–25

100

10k

100

10k

Figure 38. Bode Plot for Aluminum Corrosion Sensor

To make accurate measurements of these values, the impedance

needs to be measured over a frequency range of 0.1 Hz to 100 kHz.

To ensure that the measurement itself does not introduce a

corrosive effect, the metal needs to be excited with minimal

voltage, typically in the ±20 mV range. A nearby processor

or control unit such as the ADuC702x would log a single

impedance sweep from 0.1 kHz to 100 kHz every 10 minutes

and download the results back to a control unit. To achieve

system accuracy from the 0.1 kHz to 1 kHz range, the system

clock needs to be scaled down from the 16.776 MHz nominal

clock frequency to 500 kHz, typically. The clock scaling can be

achieved digitally using an external direct digital synthesizer

like the AD9834 as a programmable divider, which supplies a

clock signal to MCLK and which can be controlled digitally by

the nearby microprocessor.

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