Imaging of Concentration Distributions and Hydrogen Evolution on Corroding Magnesium Exposed to Aqueous Environments Using Scanning Electrochemical Microscopy

Abstract

1IntroductionMagnesium and its alloys have ag reat potential for several industriala nd biomedical applications due to their non-toxicity and high strength-weight ratio, though their practical use is severely limited by their poor corrosion resistance in most aqueouse nvironments [1][2][3][4].I no rder to slow down this degradation, it is mandatory to gather precise knowledge about the mechanism of the process. Unfortunately,t he mechanism of magnesium corrosion remains controversial, and it presents certain uniquef eatures when compared to the corrosion of other active metals,i np articular with regard to the involvedh ydrogen evolution. While the main cathodic half-reaction leads to proton reduction into hydrogen gas,t his reaction has been recordedt oo ccur on magnesium not only under cathodic polarization, as expected, but also when increasing anodic overpotentials are applied to the material. This phenomenon, usually termed as the "Negative Dif-ferenceE ffect" [1,3],h as been traditionally invokedt o support the formationo fm agnesium (I) cationsa si ntermediate speciesd uring the degradation of this metal [ 1]. That is,o xidation of magnesium at the anodic sites would produceM g(I) species which subsequently react with water producing the hydrogeng as [1,5].Indeed, there are published reports suggesting that the apparent valence of the anodic half-reaction during the corrosiono fm agnesium was smaller than 2.0, which has been regarded the main evidence of the mechanism involvingu nivalent Mg cations [ 6].H owever, the advent of an ew rangeo fs ensitive microelectrochemical and spectroelectrochemical methods is delivering new experimental evidences that anodic dissolution produces solely Mg(II) instead, through a2 -electron exchange process [7][8][9][10][11].C oupling of electrochemicalm easurements with atomic emission spectroelectrochemistry supported that the apparentu nbalanceb etween Mg dissolution and hydrogenr elease rather arises from the formation of both soluble and insoluble Mg 2 + species, the latter increasing with the elapse of time,t hus effectively correlating to an n = 2m echanism [12].T his would relatet ot he formation of dark bi-layered film precipitates on anodicallyp olarised magnesium in chloride solutions [13][14][15][16],b ecause this film has been reported to exhibit enhancedc atalytic activity towards hydrogen evolution [15,17].T hese lattero bservations can be regardedm ore consistent with the expectation that no direct determination of the Mg(I) should be feasible due Abstract:S canning ElectrochemicalM icroscopy (SECM) is presented as an essential tool for the local characterization of the still uncertain mechanismf or magnesium corrosion. Ther eaction leading to magnesium release and hydrogen evolution from separated magnesium cathodes and anodes has been imaged using an adequate combination of the operation modes available in SECM. Magnesium ion selective microelectrodes (Mg-ISMEs) were used for the visualization of the heterogeneously distributed release of magnesium (II) spe...

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The local electrochemical behavior of the AA2098‐T351 and surface preparation effects investigated by scanning electrochemical microscopy

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Scanning Electrochemical Microscopy (SECM) Study of the Electrochemical Behavior of Anodized AZ31B Magnesium Alloy in Simulated Body Fluid

“…Thus, H 2 generation can be sensed in an oxidation reaction at the UME probe potential of 0.0 V versus Ag/AgCl. Several researchers studied the flux of H 2 gas evolving from the surface of magnesium based materials 10,14,[16][17][18][19] .…”