Anodic Dissolution of Iron in Acidic Sulfate Electrolytes: I . Formation and Growth of a Porous Salt Film

A first principle mathematical model of the formation of calcareous deposits on a cathodically protected steel rotating disk electrode in seawater is presented. The model includes equations which transport phenomena, electrochemical reactions, precipitation reactions, and a homogeneous reaction involved in the formation of calcareous deposits on an electrode surface. Predicted concentration profiles show that a high concentration of OH-ions on the electrode surface leads to the formation of calcareous deposits. The calcareous deposits contain mostly CaCO3, but the initial deposits are predicted to contain more Mg(OH)2 than CaCO3. The predicted calcareous deposits on the electrode surface reduce the active surface area available for the electrochemical reactions, which results in a decrease in the cathodic current density. The predicted current density as a function of time during the formation of deposits agrees qualitatively with experimental data.

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“…Consequently, combining Eq. 11 for i = 6 and 7, yields the governing equation for CO~- 21 OCco~-+ OCHco~ = _ (V "Nco~-+ V-NHco~) […”

Section: ] Otmentioning

confidence: 99%

Mathematical Modeling of the Formation of Calcareous Deposits on Cathodically Protected Steel in Seawater

Yan

Nguyen

White

et al. 1993

J. Electrochem. Soc.

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“…It is determined by requiring that the potential at y = 0 yields i~,m when substituted into a kinetics equation. Here, a Tafel relationship is assumed i11~, = io exp (~) [13] where the exchange current density io in the presence of a film should be expected to be different from that measured on a bare metal. Since the applied potential V is known, Eq.…”

Section: N1 = ~_m~ [9] Zlrmentioning

confidence: 99%

Comparison of Modeling Approaches for a Porous Salt Film

West

1993

J. Electrochem. Soc.

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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, ...

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“…The electrodissolution of iron in various media (mainly aqueous H 2 SO 4 ) was studied most intensively, both experimentally and theoretically (cf. e.g., [6][7][8][9][10][11][12][13][14][15][16]). Other thoroughly studied oscillatory electrodissolution systems include, among others, copper in phosphoric acid [17][18][19][20][21], copper in chloride media [22,23], and nickel in sulfuric acid [24,25].…”

Section: Introductionmentioning

confidence: 99%

Electrochemical oscillations and bistability during anodic dissolution of vanadium electrode in acidic media—part I. Experiment

Gorzkowski

Wesołowska

Jurczakowski

et al. 2011

J Solid State Electrochem

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Dynamic instabilities, current oscillations and bistability observed during anodic dissolution of both stationary and rotating disk vanadium electrode in acidic phosphoric media, were reported using both dc and ac techniques. The effect of various experimental conditions, concentration of H 3 PO 4 , temperature, disk rotation rate, and external resistance, was analyzed. Systematic studies allowed the construction of bifurcation diagrams, showing the regions of oscillations and bistability, including the complex behaviors like coexistence of both dynamic regimes. Analogous comparative measurements and analyses were performed for other media-sulfuric, nitric, perchloric, and trifluoroacetic acids-also indicating complex dynamic behaviors. These complexities arise not only due to the difficulties with the precise identification of the composition of the passive layer in every acid but are also due to relatively fast dissolution of V electrode, even in the presence of passive layer, which causes permanent drift of the system's characteristics. Due to these experimental difficulties, theoretical modeling seems to be an appropriate method to analyze the essential nonlinear dynamic properties of the studied system.

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Anodic Dissolution of Iron in Acidic Sulfate Electrolytes: I . Formation and Growth of a Porous Salt Film

Cited by 69 publications

References 8 publications

Mathematical Modeling of the Formation of Calcareous Deposits on Cathodically Protected Steel in Seawater

Mathematical Modeling of the Formation of Calcareous Deposits on Cathodically Protected Steel in Seawater

Comparison of Modeling Approaches for a Porous Salt Film

Electrochemical oscillations and bistability during anodic dissolution of vanadium electrode in acidic media—part I. Experiment

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