Gases Evolved at Magnesium Anodes And Cathodes in Neutral Salt Solutions

347

226

The corrosion behavior of pure magnesium in sodium sulfate solutions was investigated using voltammetry and electrochemical impedance spectroscopy with a rotating disk electrode. The analysis of impedance data obtained at the corrosion potential was consistent with the hypothesis that Mg corrosion is controlled by the presence of a very thin oxide film, probably MgO, and that the dissolution occurs at film-free spots only. This hypothesis was substantiated both by the superposition of the EIS diagrams, obtained for different immersion times and for two Na 2 SO 4 concentrations once normalized, and by use of scanning electrochemical microscopy in the ac mode to sense the local conductivity of the material. On the basis of the electrochemical results, a model was proposed to describe magnesium corrosion at the open-circuit potential. Simulation of the impedance diagrams was in good agreement with the experimental results.

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confidence: 99%

An Impedance Investigation of the Mechanism of Pure Magnesium Corrosion in Sodium Sulfate Solutions

Baril

Galicia

Deslouis

et al. 2007

347

226

“…To account for this, Kleinberg and co-workers (1)(2)(3) have proposed that the metal enters solution as a univalent ion subsequently reacting with water to produce hydrogen. Others (4)(5)(6)(7) propose that the anodic dissolution is film controlled, and as such the behavior of magnesium can be explained without the need of uncommon valency ions. More recently the "chunk effect" (8) has been advanced as a third possibility.…”

mentioning

confidence: 99%

The Difference Effect on Magnesium Dissolving in Acids

James¹,

Straumanis²,

Bhatia

et al. 1963

The difference (A) effect on Mg metal was measured in HC1, HC104, and tteSO~ of concentrations between 0.05 and 0.50N at 25~ Generally the effect was positive, but the usual linear relationship between A and I existed only at low current densities and high acid concentrations. In YI2SO4 and HC1, the positive effect changed to negative at increased current densities and low acid concentration. Attempts to attribute the A effect to the anodic expulsion of uncommon valency Mg ions result in unrealistic values of cationic charges for positive values of A. The difference effect is explained on the basis of anodic polarization, film disruption, and disintegration of the magnesium anode, i.e. "chunk effect."In the past sixty years considerable work has been done to arrive at a mechanism for the anodic dissolution of magnesium metal in aqueous solutions. It has been found that magnesium anode consumption measured in terms of hydrogen evolution is very much greater than predicted by Faraday's law for divalent magnesium ion formation. To account for this, Kleinberg and co-workers (1-3) have proposed that the metal enters solution as a univalent ion subsequently reacting with water to produce hydrogen. Others (4-7) propose that the anodic dissolution is film controlled, and as such the behavior of magnesium can be explained without the need of uncommon valency ions. More recently the "chunk effect" (8) has been advanced as a third possibility.Accordingly a study of the difference effect exhibited by magnesium while dissolving in various acids was undertaken in order to arrive at a mechanism which best explains the experimental results. ExperimentalThe difference effect on magnesium metal of 99.995% purity (as given by Dr. R. Gadeau of L'Aluminium Francais) was measured in hydrochloric, perchloric, and sulfuric acids at 25 ~ • 0.10~ The concentration of the acids employed varied from 0.05 to 0.50N.The apparatus and procedure used have been thoroughly described previously (9).Three hundred milliliters of acid of known concentration were poured into the reaction flask and the system allowed to come to the desired temperature. The electrodes were then immersed (with external circuit switch open) in the acid, the stirring mechanism turned on, and the volume of gas recorded for definite time intervals until the rate became constant. At this time, the external circuit was closed and two consecutive volume readings were taken at equal time intervals together with the average milliammeter reading. Then, two consecutive volume readings were made with the cir-I Present address: Monsanto Chemical Company, St. Louis, Missouri. s Present address:cult open. The above procedure of taking two volume readings without an external current and two with an external current flowing was repeated at several current densities. ResultsThe difference effect is defined as the difference between hydrogen evolution rate V1, from an electrode without current flowing and hydrogen evolution rate V2 from the same electrode with current flowing, i.e. .~= v~--...

“…This paper was presented at the Philadelphia Meeting, Oct. [9][10][11][12][13][14] 1966. This is Paper No.…”

Section: Acknowledgmentmentioning

confidence: 99%

“…They are: (a) complex ion (Cd 9 Cd +'~) formation, (b) uncommon valence ion (Cd +) formation, (c) film control, (d) local corrosion, and (e) disintegration. The uncommon valence ion theory has been widely accepted, but recent papers have somewhat discounted its validity, especially as pertains to Be (7), Mg (4,14,15), and Zn (9,16). Experiments carried out in this laboratory with amalgamated anodes also make it doubtful that the theory applies to Cd (17).…”

mentioning

confidence: 99%

The Anodic Dissolution of Cadmium

Johnson¹,

Deng²,

Lai³

et al. 1967

The anodic dissolution of cadmium has been studied in aqueous solutions containing CI-, Br-, I-, Ac-, SO4 =, and NO3-ions. The normal valence (+2) was found in all solutions with the exception of NO3-. The apparent valence (calculated) of cadmium ions in nitrate solutions varied from 1.2 to 2.0 and was found to be a function of NO3-concentration, current density, and temperature. An anodic dissolution mechanism has been proposed involving local corrosion and disintegration of the anode which is consistent with the experimental results.