Stub resonators for online monitoring early stages of corrosion

Here we demonstrate the proof-of-principle of a new type of sensor to assess effects due to corrosion of metal surfaces. The method can be applied to all situations where metals are exposed to a corrosive (fluidic) environment, including, for instance, the interior of pipes and tubes. The sensing device is based on the operating principle of a quarter wave length open-ended stub resonator. In the absence of corrosion, inner and outer conductors of the resonator are separated by a single dielectric, i.e., a fluid. Oxidation of the metal surface of inner and/or outer conductor changes the properties of the dielectric between inner and outer conductor because it introduces a dielectric permittivity that differs from that of the fluid. Additionally, corrosion affects the skin effect and the effective resistance of the corroding inner and outer conductors of the stub resonator. As a result the recorded amplitude-frequency (AF) plot shows a shift of the resonance frequency and/or a change of shape of the resonance peak(s). The two types of transmission line designs explored are coaxial and coplanar stripline (CPS). Irrespective the design, the method outlined here offers an equipment-undemanding, low maintenance and cost-efficient in-line early warning system to detect (the onset of) corrosion. The additional advantage of the system is the freedom of design as for its geometry, from coax to one embedded in printed circuit boards, both designs with hardly any constraints on the particular dimensions. Depending on the precise geometry, the sensor may be able to detect corrosion in tap water after just 1 h. Experimental data (recorded after four days) are in close agreement with simulation data representing a 17 mu m homogeneous oxide layer.