Comparison of Modeling Approaches for a Porous Salt Film

Abstract

In most models of anodicatly formed salt films, the concentrations of the ionic components of the salt are assumed to be at their saturation values at the film/electrolyte interface. The consequences of also imposing the saturation condition inside a porous film are compared to results of the more commonly employed models that allow supersaturation. It is shown that film thickness and porosity derived from fitting simulations to experiment can depend significantly on this assumption. In contrast, for iron dissolution into an HC1 electrolyte, estimates of spatial pH variations inside the FeC12 film are not affected by the modeling approach.Porous salt films are ubiquitous in studies of high-rate corrosion, secondary batteries, and oscillatory, anodic reactions. These films are often unstable and nearly impossible to study by ex situ methods. Consequently, transport phenomena inside porous salt films are often investigated by transient electrochemical methods such as ac impedance spectroscopy. Interpretation of these experiments requires extensive modeling efforts in order to infer the environment inside the film. Although they can have a dramatic effect on derived film properties, the modeling assumptions have not always been evaluated critically.To explain anodic limiting currents, it is generally accepted that, at the film/electrolyte interface, the concentration of the ionic components of the salt are equilibrated with the solid film, i.e., the ionic concentrations are related to one another through a solubility product. Unless an additional phenomenon is introduced, it is logically consistent to impose the saturation condition inside the film. Nevertheless, supersaturation is normally permitted. The validity of allowing for supersaturation inside a porous film, where the electrolyte is in intimate contact with the salt, can reasonably be questioned.The purpose of this paper is to demonstrate that models that prescribe a saturation condition inside the film can lead to very different estimates of film thickness and porosity from the values obtained from supersaturation models. It is also shown that estimates of surface pH are independent of the mechanism for transport through the film. For the remainder of this paper, the model that imposes a saturation condition is known as Model SAT, and the model permitting supersaturation is called Model SUPER. Although this paper shows that care should be taken in developing models of porous films, only experiment can determine the more appropriate approach.To make the discussion concrete, the mass-transfer-limited dissolution of iron into a concentrated HCI or NaCI electrolyte is used as an example. Various authors have used these systems as experimental models of electropolishing and localized corrosion. I-3 The dissolution valence on the limiting-current plateau is two 4'~The limiting current is due to the formation of an FeCI~ film. 2 The dynamic behavior of the film indicates a complicated st/'ucture, ~,6 which is best modeled by a duplex film consisting of an inner, ...

Carbon Paste Composite with Co3O4 as a New Electrochemical Sensor for the Detection of Allura Red by Reduction

“…where n is the number of electrons exchanged in the redox process, F is the Faraday constant, A is the electroactive area of the electrode, D and C are the diffusivity and concentration of the electroactive species (FeCN 6 3−/4− ) and I lim . is the limit current of the diffusion process, which at the open circuit potential can be replaced by the current exchange ( I 0 ) , where I 0 =7.14×10 −6 mA . As seen in Table , the diffusion resistance of the CoO X /CPE is almost one order of magnitude lower than that exhibited by the CPE.…”

Sensitive and Profitable Electrochemical Detection of Uric Acid in the Presence of Dopamine with a Novel Carbon Paste Electrode Decorated with a Copper(II) Complex

“…Values of the parameters used to fit the impedance results using the electrical circuit shown in the inset of Figure 1B. replaced by the current exchange (I 0 ) [35] where I 0 = 7.14 × 10 À 6 mA [36]. As seen in Table 1, the diffusion resistance of the Cu C /CPE is almost half of that exhibited by the CPE.…”

Mathematical Models for Under-Deposit Corrosion

“…In the bulk region, the eddy diffusivity term D (t) i was used to account for the influence of turbulence on mass transfer. 18,19 [16] The corresponding expressions for the eddy diffusivity are [17] where A 1 and A 2 are constants, ν is the kinematic viscosity and z + = √ τ 0 ρ/ν is the dimensionless distance from the metal surface,…”