The Effect of Halides on the Capacity and Resistance of the Magnesium Electrode in Aqueous Solutions

Abstract

A study of potential-time decay curves at anodica]ly and cathodically polarized magnesium by the d-c current interrupter method is presented. Potential-time oscillograms and polarization curves were determined for magnesium electrodes in buffered and unbuffered aqueous solutions containing chloride, bromide, iodide, and fluoride. Film capacity, solution double layer capacity, and resistance data determined from the oscillograms using the electrical analog R s Itdis presented where Cd and Rd are the capacity and resistance of the HelmholtzGouy double layer. The effect of specific adsorption of anions and the physical nature of the film on the magnesium electrode are discussed.Local corrosion (1-3) and electrochemical formation of Mg § (4-10), Mg~ § (11), and Mg.Mg § (3) have been suggested to explain the negative difference effect observed for magnesium. The existence of the ions Mg § Mg~ §247 and Mg-Mg § has not been definitely established, and the negative difference effect of Mg may, in fact, be explained solely on the basis of local corrosion. It can be stated with certainty, however, that even if Mg* is formed, local corrosion must also occur. This conclusion follows from the experimental facts: (a) values of the apparent Mg valency (determined coulometrically) less than unity as well as greater than unity have been observed; (b) a positive difference effect is observed for Mg at low anodic current densities in solutions containing tIC1 or NH,C1 (9, 12). Observed Mg valencies greater than unity may be explained by electrochemical formation of Mg + § either directly or via the reaction, Mg § Mg § + e (5).From the above considerations it may be concluded: (a) the potential of Mg is a mixed anodiccathodic potential, and (b) although the.mechanism of the local anodic reaction is uncertain it consists, in part, of the electrochemical formation of Mg § either directly or via monovalent Mg. The local cathodic reaction is generally the hydrogen evolution reaction but may under some experimental conditions consist both of the h.e.r, and the oxygen adsorption reaction (13, 14).The potential of the Mg electrode may be under anodic, cathodic, or mixed control depending on the experimental conditions. In acid solutions (pH's ~ 3 in unbuffered solutions) the corrosion reaction is probably under cathodic control; the potential becomes more noble with increasing hydrogen ion concentration (15) and the corrosion proceeds under H~O § diffusion control (16,17). At high pH's (pH 11 in unbuffered solutions), the potential increases sharply in the more noble direction with increasing pH and anodization causes passivation (15) which indicates that at high pH's the reaction is under anodic control. In the intermediate pH range the potential of Mg is practically independent of pH and a transition from cathodic to anodic control probably occurs. Film formation at high pH's undoubtedly plays a major role in the transition from cathodic to anodic control and there are indications that transport phenomena through surface film may be part...

Surface Modification of Biodegradable Magnesium Alloys

“…44,45 However, the native oxide film on the alloy surface is prone to pitting corrosion, which is facilitated mostly by halide ions. 46 In their presence the alloy undergoes localized corrosion even at the corrosion potential, and a severe dissolution of the alloy occurs at potential values slightly positive than E corr (Fig. 5).…”

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

“…To explain the magnesium corrosion and more particularly this NDE phenomenon, several mechanisms have been proposed including the formation of magnesium hydrides, 6 metastable monovalent ions, [7][8][9][10][11][12] and magnesium hydroxides and oxides. [13][14][15][16][17][18][19][20][21][22][23] Song et al 11,12 recently proposed a new mechanism for the NDE. They explained that the formation of a partially protective layer of Mg͑OH͒ 2 plays an important role in the corrosion behavior of magnesium.…”

Surface Modifications of the Mg Alloy by Self-Assembled Monolayers of Fatty Acids

“…The observed behavior needs further investigation. (29). At the corrosion potential the electrode surface is partially covered by the surface film, while at the film-free areas magnesium dissolution occurs with the formation of Mg + ions, which are oxidized in a reaction with water (30):…”

“…Mg(OH) 2 as an insoluble substance precipitates on the electrode surface as a corrosion protective layer. However, in the presence of chloride ions in an amount greater than 30 mmol L -1 , pitting corrosion starts to occur due to the conversion of Mg(OH) 2 surface film to soluble MgCl 2 ECS Transactions, 41 (24) 81-91 (2012) (29). At the corrosion potential the electrode surface is partially covered by the surface film, while at the film-free areas magnesium dissolution occurs with the formation of Mg + ions, which are oxidized in a reaction with water (30):…”