Iron Dissolution Mechanism Research Articles

Mechanism of Iron dissolution and passivation in an aqueous environment: Active and transition ranges

Mechanism of Iron dissolution and passivation in an aqueous environment: Active and transition ranges

Mecanisme de dissolution du fer en milieu sulfurique

Potential step methods are used to obtain accurate informations about iron dissolution mechanism in sulfuric acid. The results do not fit with catalyzed (Heusler) or non catalyzed (Bockris) mechanisms. It is shown that the following sequence is needed: 1) a one electron-transfer high-rate step with H 3O + and adsorbate species formation; 2) a rate-determining step without changing of the species; 3) a high rate electrochemical step. Such a mechanism was proposed by Bignold and Fleischmann with, as a second step, a movement of adsorbed FeOH onto the iron surface. In addition, it is observed that dissolutions kinetics are not modified by sulfate concentration if potential and pH remain constant and no diffusion occurs. Impedance measurements support those results and give the following values for rate constants: 2.10 4 < k 1 < 2.10 6 s −; k 1 ≌ 10 4 mol −1 dm 3 s − (potential vs. NHE); k 2 ≌ 9.10 2 s −1 . Values of the Tafel slope obtained in steady-state conditions ( ≌ 40 mV/decade instead of 60 mV when only transfer process is involved) and the inductive loop observed at low frequencies on complex impedance diagrams are consistent with pH variations at the metal-solution interface.

The Effect of Solvent on the Electrochemistry of Iron

A study was made of the electrochemical behavior of the active iron electrode in acidic ethanol‐water media. The pA (pH in pure water solvent) and potential dependence of the iron dissolution and hydrogen evolution reactions were determined. The results of this investigation served to resolve the role of water in the iron dissolution mechanism. Basic to the proposed mechanism was the validity of an “absolute acidity scale” for solutions.

Electrodissolution Kinetics of Iron in Chloride Solutions: IV . Alkaline Solutions Containing Benzotriazole

The anodic dissolution of iron in alkaline chloride solutions in the presence and absence of benzotriazole has been investigated by potentiostatic, galvano‐static, and potential sweep polarization. The steady‐state and transient polarization experiments indicate that iron dissolution occurs in the absence (active region) and presence (prepassive region) of a porous prepassive surface film. Anodic Tafel slopes of 60 mV/decade were obtained below (active region) and above (prepassive region) the formation potential of the prepassive film on both uninhibited and inhibited iron indicating the same mechanism of iron dissolution. The results suggest that the prepassive film merely decreases the effective surface area for iron dissolution. The effect of benzotriazole is to decrease further the effective surface area.

Zur anodischen auflösung von reineisen im bereich zwischen aktivem und passivem verhalten

The kinetics of anodic iron dissolution in sulphate ions containing solutions, which are free of oxygen, are investigated in a over potential range between active and passive states of the metal. The measurements were carried out on a rotating disk electrode in the pH-range between 3·5 and 5·5 using the potentiostatic triangular voltage sweep method under quasi-steady-state conditions. The measured current density-potential curves show two current maxima. At relatively low anodic overvoltages, i.e. before the maxima I is reached, the used recrystallized iron dissolves in the active state corresponding to the consecutive (Bockris-) mechanism. At high anodic overvoltages the passive state arises after passing the maxima II. In the transition range, i.e. in the potential range of the maximum I, a second mechanism of iron dissolution is discussed. This mechanism takes place parallel to the consecutive one. An intermediate [Fe (OH) 2] ads will be formed, which dissolves in a following rate-determining chemical step or is changed in an oxide phase respectively.

The Active Iron Electrode: I . Iron Dissolution and Hydrogen Evolution Reactions in Acidic Sulfate Solutions

The pH and potential dependencies of the steady‐state iron dissolution and hydrogen evolution reactions were determined for zone‐refined iron in H2‐saturated acidic sulfate solutions. The mechanistic significance of the data is examined in detail. The results of this investigation serve to resolve the conflict concerning the mechanism of iron dissolution in noninhibiting media and to provide an experimental and mechanistic base for the interpretation of the electrochemical behavior of the active iron electrode in inhibiting media.