A Novel Approach for the Evaluation of Under Deposit Corrosion in Marine Environments Using Combined Analysis by Electrochemical Impedance Spectroscopy and Electrochemical Noise

- The first stage of UDC is initiation of a pit or crevice.
- The second stage of UDC is either general corrosion propagation or passivation.
- The transition between these stages could be assessed by EIS analysis.
- Mechanistic information of each stage could be specified by EPN transients.
- SEM analysis confirmed the correctness of the combined EIS and EN research approach.

Different techniques have been employed for the investigation of under deposit corrosion (UDC). However, most of these methods suffer from complications regarding the mechanistic analysis of UDC in terms of localized electrochemical factors that influence the corrosion process. The present work investigates a novel methodology based on complementary evaluation of UDC in marine environments by electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) analysis using Hilbert spectra. Two different electrodes (AISI 316L and mild steel) covered by calcareous deposit and exposed in 3 wt.% NaCl solution were studied, employing the proposed measurement approach coupled with microscopic observations. It was found that the combined investigation of the instantaneous frequency decomposition of transients in the electrochemical potential noise (EPN) signals and damage indices calculated from EIS measurements led to a more detailed mechanistic understanding of UDC evolution. Changes in the corrosion process over time were observed for both electrodes, including two different stages: the onset of pit formation followed by a large timescale process in terms of active-passive behavior of the substrates. Low-frequency variations of the impedance magnitude, as well as the corresponding damage indices, indicated a transition stage of UDC. Finally, these stages were identified by the analysis of the EN signals, together with investigation of the micrographs of the damaged surface.

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