Electrochemical impedance models for molten salt corrosion

Abstract

Taking into account the chemical stability and scaling features of metals, four electrochemical impedance models were proposed to represent their electrochemical impedance responses in molten-salt systems at the open-circuit potential. Electrochemical charge transfer for the non-active metals is the rate-limiting process. For the active metals, the transfer of ions in the scale and the diffusion of oxidants in melts become increasingly important as compared with the electrochemical process. When a non-protective scale forms on the metal surface, the impedance diagram may present the characteristics typical of a diffusion-controlled reaction, i.e., a semi-circle at high frequency and a line at low frequency. When a protective scale forms on the metal surface, the Nyquist plot is composed of double capacitance loops, and the transfer of ions in the scale is rate limiting. An equivalent circuit of double layer capacitance in series with oxide capacitance can be used to represent this kind of impedance response. In the case of localized corrosion, the Nyquist plot also consists of double capacitance loops, which can be described by an equivalent circuit of double layer capacitance parallel to oxide capacitance. Impedance measurements of Pt, Ni 3Al and FeAl intermetallics in molten-salt systems were conducted to verify the proposed impedance models.