Localized Corrosion of Magnesium in Chloride-Containing Electrolyte Studied by a Scanning Vibrating Electrode Technique

Abstract

An in situ scanning vibrating electrode technique ͑SVET͒ is used to investigate localized corrosion occurring on unpolarized magnesium ͑Mg͒ samples immersed in 5% w/v aqueous sodium chloride electrolyte. Corrosion is characterized by the appearance of circular, blackened areas which expand radially at a constant rate and evolve hydrogen vigorously. These are shown to consist of a cathodically active center surrounded by a 0.5 mm wide anodic ring. Any localized corrosion currents emerging from the intact ͑uncorroded͒ Mg surface are negligible by comparison. Local anodic current density is shown to be directly proportional to the radius of the local cathode, while corresponding local cathodic current density remains relatively constant with time. Estimates of time-dependent rates of total equivalent Mg loss and H 2 evolution, obtained by numerical integration of SVET-derived normal current density distributions, indicate that corrosion rate is controlled by the area of local cathodic activity. The empirical findings are consistent with a mechanism involving cathodic H 2 evolution on the dark, film-free region which is galvanically coupled with anodic attack of the intact Mg surface. It is proposed that cathodic activation of the film-free, corroded "disk" is caused by a combination of elevated pH and enrichment in noble iron-containing impurity phases.

Role of Segregated Iron at Grain Boundaries on Mg Corrosion

“…36,37 It has to be noted that there is no direct evidence of the formation of Mg + species as indicated by several authors. 16,[38][39][40][41] The other mechanisms such as breakdown of corrosion films can be influenced by the surface reactivity of Mg substrate, which depends on the presence of alloying elements or noble metal impurities, leading to increased catalytic activity of Mg. The noble metal impurities, such as Fe, Cu or Ni, 34 which have been considered as the active sites of the cathodic reaction, 38,[42][43][44] lead to formation of dark filiform patterns 13,16 and increase the hydrogen production in aqueous chloride solutions.…”

“…Data acquisition and post processing analyses were performed using Ion-Spec software (version 4.1). 24 Mg + ions characteristic of metallic and/or oxidized magnesium, 40 MgO + and 41 MgOH + characteristic of magnesium oxide and hydroxides, respectively, and 13 Al + , 25 Mn + and 26 Fe + characteristic of metallic (and/or oxidized) impurities (Al, Mn and Fe, respectively), were chosen for ion depth profiling and imaging.…”

“…The black spots appear at specific points and, then, their size increases in radius with time as already reported. 39,40 As a result of these measurements and from optical microscopic observations performed on the Mg electrodes after different times of immersion at OCP, a 2-minute immersion time was finally selected and systematically applied before all chronoamperometric measurements and analysis by surface techniques (XPS and ToF-SIMS). At this immersion time, dark corrosion spots and hydrogen evolution are not yet visible on the surface of Mg electrode while open circuit potential has reached a steady value of −1.58 V vs SCE.…”

The use of a multiscale approach in electrochemistry to study the corrosion behaviour of as-cast AZ91 magnesium alloy

“…Filiform corrosion has been observed in high purity magnesium of unpolarised and galvanostatically polarised specimen in aqueous sodium chloride electrolyte [52,56]. A relatively widespread surface coverage of what is the typically observed filiform-like corrosion morphology [63] was observed for pure Mg. The presence of alloyed As (0.37 %) decreases cathodic reaction and modifies the resultant corrosion morphology upon Mg, mitigating the typically observed filiform-like corrosion [64].…”

Local Hydrogen Fluxes Correlated to Microstructural Features of a Corroding Sand Cast AM50 Magnesium Alloy

“…Local scanning probe techniques such as several modes of scanning electrochemical microscopy (SECM), [13][14][15][16][17][18][19] scanning Kelvin probe force microscopy (SKPFM), [20][21][22] the scanning vibrating electrode technique (SVET) [23][24][25] and local electrochemical impedance spectroscopy (LEIS) 26 have been used to probe the heterogeneous surfaces of Mg alloys and to assess the electrochemical behavior of the different microstructural constituents responsible for microgalvanic corrosion. Among these techniques, SECM has the ability to quantify electrochemical fluxes in situ with high lateral resolution using a microelectrode (ME).…”