The Difference Effect on Magnesium Dissolving in Acids

Abstract

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~--...

The pH Dependence of Magnesium Dissolution and Hydrogen Evolution during Anodic Polarization

“…This phenomenon is termed the negative difference effect (NDE), and has been well documented for many decades. [6][7][8][9][10][11] The physical conditions responsible for the NDE remain an important research topic in the context of the complete understanding of Mg as an electrode.…”

Investigating the Real Time Dissolution of Mg Using Online Analysis by ICP-MS

“…This is important to note, because theories based on the existence of Mg + have been proposed in more recent years in the field of corrosion research, with no reasonable proof of its existence, [2][3][4][13][14][15] in spite of several historical papers providing alternative explanations to a Mg + theory. [16][17][18][19] Of relevance to Mg corrosion studies, the Mg + theory is purported to account for the so-called 'negative difference effect' (NDE), which is the phenomenon by which the amount of hydrogen evolved from a Mg electrode increases as the electrode potential is polarized anodically, to more noble potentials. According to the Mg + theory, superfluous hydrogen arises from a chemical reaction of Mg + with water at some unknown distance away from the metal surface.…”

The negative-difference effect during the localized corrosion of magnesium and of the AZ91HP alloy

“…This can cause misleading interpretations of mass loss measurements, since the mass loss is due to the mechanical desintegration of the Mg electrode rather than to the electrochemical dissolution. [8][9][10] Formation of magnesium hydride: [11][12][13] magnesium hydride can form at sufficiently negative potentials, according to Mg + 2H + + 2e -→ MgH 2 . Hydrogen, in this model, is released by the decomposition reaction of the hydride, which is unstable in contact with water: MgH 2 + 2H 2 O → Mg 2+ + 2OH -+ 2H 2 .…”