Impedance corrosion rate sensors
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 take place, making impedance behavior difficult to interpret. Consequently, the protective performance of the aluminum oxide film is characterized in terms of impedance. It is possible to assess corrosion rates by capturing this dynamic behavior—specifically, by connecting a thermistor or a concentric corrosion sensor and applying high- and low-frequency AC voltages. The sensor—designed to monitor atmospheric conditions or coating degradation—utilizes two concentrically arranged electrodes to eliminate anisotropy caused by electrode geometry; it consists of a central disk electrode surrounded by a second electrode. Measurements are conducted using electrochemical impedance spectroscopy, and the observed spectra can be interpreted via a transmission-line model. Fundamentally, the data obtained are identical to those from the aforementioned two-electrode corrosion sensor.
In atmospheric corrosion systems—whether the metal electrode is immersed in an electrolyte or covered by a thin water film—the corrosion reaction of a sample electrode (of a given material and surface area) can be represented by the electrical equivalent circuit shown in the figure above. Here, Z represents impedance, Rp is polarization resistance (proportional to corrosion resistance), Cdl is electric double-layer capacitance, and Rs is solution resistance. Since Z ≈ Rs as ω → ∞ and Z ≈ Rs + Rp as ω → 0, the solution resistance (Rs) can be determined from the high-frequency range, and the polarization resistance (Rp) can be calculated by subtracting the high-frequency value from the low-frequency value. Because the reciprocal of polarization resistance correlates with the corrosion rate, this method allows for the monitoring of changes in the corrosion rate over time.
Noise reduction and elimination of cable stray capacitance; achieves noise reduction and shielding/guarding functions by integrating 1 to 8 cycles.
