  Corrosion of metal structures is often prevented using cathodic protection
systems, that employ sacrificial anodes that corrode more preferentially
relative to the metal to be protected. In-situ monitoring of these sacrificial
anodes during early stages of their useful life could offer several insights
into deterioration of the material surrounding the infrastructure as well as
serve as early warning indicator for preventive maintenance of critical
infrastructure. In this paper, we present an Electro-Mechanical Impedance (EMI)
measurement-based technique to quantify extent of corrosion of a zinc
sacrificial anode without manual intervention. The detection apparatus consists
of a lead zirconate titanate (PZT) transducer affixed onto a circular zinc
disc, with waterproofing epoxy protecting the transducer element when the
assembly is submerged in liquid electrolyte (salt solution) for accelerated
corrosion by means of impressed current. We develop an analytical model for
discerning the extent of corrosion by monitoring shift in resonance frequency
for in-plane radial expansion mode of the disc, that also accurately models the
nonlinearity introduced by partial delamination of the corrosion product (zinc
oxide) from the disc. The analytical model thus developed shows excellent
agreement with Finite Element Analysis (FEA) and experimental results. Our work
establishes the efficacy of the proposed technique for monitoring the state of
health of sacrificial anodes in their early stage of deterioration and could
thus be widely adopted for structural health monitoring applications within the
internet of things.
