Conduction Mechanism of the Passive Film on Iron Based on Contact Electric Impedance and Resistance Measurements

Abstract

The application of a mixed conduction model and a new contact electric impedance ͑CEI͒ technique to predict quantitatively the electronic and ionic transport properties of oxide films on iron in a nearly neutral tetraborate solution is discussed. The mixedconduction model emphasizes the coupling between the ionic defect structure and the electronic conductivity in an anodic film. Conventional electrochemical techniques have not been sufficient to characterize properly the electronic and ionic properties of anodic films on metals. The CEI technique makes it possible to distinguish between processes taking place at different rates within oxide films. Using this technique together with the contact electric resistance technique, we have found that the diffusion coefficient for the electronic conduction in the anodic film on iron is several orders of magnitude higher than that for the ionic transport. This shows that the passive film on iron is predominantly an electronic conductor. The fitting of the experimental results to the mixed conduction model gives a good agreement and thus supports the validity of this model in the present case.The objective of this paper is to predict quantitatively the electronic and ionic transport properties of oxide films on iron from the ionic defect structure in the film. This prediction is based on a newly proposed mixed conduction model ͑MCM͒ for anodic passive films and experimental determination of the required parameters using a new contact electric impedance ͑CEI͒ technique.We believe that the transport properties of the film are closely related with the corrosion behavior of metals and alloys. Thus the results of this work serve to assess the resistance of metals and alloys to both general and localized corrosion.We have chosen iron in a nearly neutral environment to be the model system in our work. A considerable amount of work on iron has already been carried out to clarify the film growth and dissolution kinetics, 1-3 the electronic properties of the oxide film, 4-14 and the mechanism of ionic transport through it. [1][2][3]15,16 The close correlation between the dissolution kinetics and the electronic properties of the passive film on iron and synthetic iron oxides has been established in recent work. [17][18][19][20][21][22] The application of spectroscopic techniques based on synchrotron radiation and scanning probe microscopy 23,24 has revealed that anodic layers on iron have a spinel-like nanocrystalline structure. Consequently, the concepts based on point defects in crystalline materials 25-29 can with justice be applied to model the electronic and transport properties of thin passive films on iron. Quantitative correlations between the electronic and ionic transport properties of passive films, film composition, and film structure have not been established so far.Because of the need to combine the understanding of passive film structure and transport in the film, we have proposed a model for the coupling between ionic defect structure and electronic conduction in stea...

The passivity of AISI 316L stainless steel in 0.05 M H2SO4

“…Taking into account the error caused by the assumption of k in Eq. 6, the diffusion coefficient of defects in the passive film formed on AISI 316L in 0.05 M H 2 SO 4 is estimated to be in the range of 10 -17 -10 -16 cm 2 /s, which is in the order of the results reported for diffusion coefficient of defects in the passive film on carbon steel in borate solution at room temperature [25,26]. Figure 6 shows a linear relationship between the steadystate film thickness (L ss ) and the formation potential.…”

Corrosion modelling of iron based alloy in nuclear waste repository

“…the Point Defect Model [1][2][3][4][5][6][7][8][9][10][11] and the Mixed Conduction Model [12][13][14][15][16]. Both models assume that the metal is covered by a dense oxide layer.…”

Fundamental Understanding of Crack Growth in Structural Components of Generation IV Supercritical Light Water Reactors

“…It has been previously demonstrated [36,37,43] that the potential drop at the M/F interface is independent of the external applied potential in the steady state. In other words, only the rate constant of consumption of the ionic current carrier at the F/S interface depends on the applied potential: …”

“…Recently, a combination of impedance measurements in an asymmetrical and a symmetrical configuration has been employed to characterize the conduction mechanism of the passive film on pure Fe [36], Fe-Cr and Fe-Cr-Mo alloys [37] in a borate solution at room temperature. The CEI measurements in the symmetrical configuration are an extension of the CER technique [38,39] to the frequency domain [33,36,37]. TLEC is used to measure the asymmetric configuration.…”

“…Both the steady state current and impedance response of a metal covered with a passive film in contact with an electrolyte have been derived and successfully compared to experimental data [44][45][46][47]. In recent years, the model was further developed at VTT [36,43,48,49] and by others [50,51]. The main addition proposed by VTT in terms of the mixedconduction model (MCM) was the derivation of the Young impedance (Eq.…”