Principle of EIS Type Sensors

Wet corrosion can be further classified into general corrosion and localized corrosion.Dissolved substances influence corrosion rates, particularly in environments subject to repeated wet-dry cycles where water film thickness and salt
concentrations fluctuate rapidly. Because corrosion occurs amidst these dynamic changes, complex state transitions arise, making the corrosion process difficult to fully comprehend.
Electrochemical Impedance Spectroscopy (EIS) is a powerful technique for investigating a system's complex impedance; it is highly sensitive to changes in both surface phenomena and bulk material properties.
The protective nature of surface films and coatings can be characterized in terms of impedance. By monitoring these dynamic changes—akin to observing a "dance"—it becomes possible to assess corrosion rates. This is achieved by
connecting sensors (such as thermistors or concentric corrosion sensors) and applying high- and low-frequency alternating voltages.
The corrosion rate sensor consists of two concentrically arranged electrodes—a central disk electrode surrounded by an outer electrode—to eliminate anisotropy caused by electrode geometry. Measurements are conducted using the
electrochemical impedance method, and the resulting spectra can be interpreted via a transmission-line model. Fundamentally, the data obtained is equivalent to that of the aforementioned dual-electrode corrosion sensor.

Principle: The corrosion reaction can be represented by the equivalent circuit shown in the figure above. Here, Z represents impedance, Rp represents polarization resistance
(proportional to corrosion resistance), Cdl represents double-layer capacitance, and Rs represents solution resistance. When ω → ∞, Z ≈ Rs; when ω → 0, Z ≈ Rs + Rp.
In this scenario, since the relationship Z ≈ Rs + Rp holds, the solution resistance (Rs) can be determined from the high-frequency range, and the polarization resistance (Rp) can be
calculated by subtracting the Rs value. The reciprocal of the polarization resistance corresponds to the corrosion rate.
Noise reduction: Noise reduction and shielding/protection functions are achieved by integrating 1 to 8 cycles, thereby reducing noise and eliminating cable stray capacitance.