Electrochemical impedance spectroscopy (EIS) of composite polymer coatings on metal substrates

• V.A. Golovin, S.A. Dobriyan and V.E. Kasatkin

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation

Abstract: The electrochemical properties of multilayer composite polymer coatings consisting of a Zn-filled inhibited primer layer and an external insulating polymer coating are investigated by the EIS method. During long-term exposure in aggressive media, the kinetics of the properties of coatings obtained on corrosion-inert (Pt) and corrosion-active (mild steel St3) substrates were compared. It is shown that the impedance spectrum in the studied systems is determined by the imposition of two relaxation processes: high-frequency (HF), due to the processes of electrolyte transfer, and low-frequency (LF) associated with the Faraday process of under-coating corrosion of Zn powder in the primer layer or substrate corrosion. The frequencies of the HF process for all investigated composite coatings, including those applied to different substrates, are close, while the frequency of the LF process depends on the coating structure and the substrate type. It is shown that frequencies of LF processes at high exposure times in composite coatings with Zn-protective primer on different substrates (mild steel St3 or Pt) are identical (9.3 Hz). This fact is the base for the conclusion that the Faraday process in both cases is the same and deals with corrosion of Zn powder. The coating structure is a factor determining the frequency of the LF process for coatings on a steel substrate. In composite systems with Zn-rich primer, the Faraday low-frequency process is observed at significantly lower frequencies (9.3 Hz) than in the case of applying only the insulating coating (250 Hz), not only at the initial stage, but also at high exposure times in 3% NaCl. This can be seen as evidence of prolonged protective action of the Zn powder in composite coatings. So the nature of the under-coating corrosion process can be determined by its EIS characteristic frequency.

Keywords: electrochemical impedance spectroscopy, corrosion inhibitors, protective polymer coatings

Int. J. Corros. Scale Inhib., 2018, 7, no. 4, 697-709 PDF (1 521 K)
doi: 10.17675/2305-6894-2018-7-4-15

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