Effect of pH on the Grain Size Dependence of Magnesium Corrosion

Abstract

Prior works show that grain size can play a role in the corrosion of a metal; however, such works are nominally executed in a single electrolyte/environment at a single pH. In this work, the anodic and cathodic reaction kinetics of pure Mg specimens with grain sizes ranging from approximately 8 μm to 590 μm were compared as a function of pH in 0.1 mol dm -3 sodium chloride (NaCl) electrolytes using anodic polarization experiments and an in situ scanning vibrating electrode technique (SVET). Anodic polarization experiments showed that grain size is important in determining overall electrochemical response, but the environment dictates the form of the grain size vs. corrosion rate relationship (i.e., pH is the overall controlling factor). Consequently, the role of grain size upon corrosion cannot be fully assessed unless a variation in environment is simultaneously studied. For example, the anodic reaction, which dictates active corrosion, also dictates passivation, so the corrosion rate vs. grain size relationship has been shown to "flip" depending on pH. Further, SVET analysis of unpolarized Mg immersed in 0.1 mol dm -3 NaCl electrolyte at neutral pH showed that breakdown of passivity of cast Mg occurred after ~1 h immersion, giving filiform-like corrosion tracks. The front edges of these corrosion features were revealed as intense local anodes, while the remainder of the dark-corroded Mg surface, left behind as the anodes traversed the surface, became cathodically activated. In contrast, grain-refined Mg samples were significantly less susceptible to localized corrosion, and breakdown was not observed for immersion periods of up to 24 h.

Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

“…The formation of discs versus filaments was suggested to depend on whether the catalytic efficiency of the locally corroded region allowed for galvanic coupling with its perimeter, or merely along the most anodic pathways of the microstructure. 7 This "differential electrocatalytic" mechanism was also shown to drive filiform corrosion on coated Mg as determined by scanning Kelvin probe (SKP) measurements. 1 The mechanism, however, by which the cathodic reaction (H 2 gas evolution) is enhanced on corroded Mg or Mg alloys continues to be unclear.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12] Williams et al have offered critical insights into the electrochemical mechanism of this unique localized corrosion mode on pure Mg 1,5,7 and the Mg-Al-Zn-Mn alloy AZ31B 9 by employing the scanning vibrating electrode technique (SVET), where their SVET measurements showed that the corrosion filaments/discs became cathodically-activated following their formation. It was proposed that the net cathode on the locally corroded regions galvanically coupled with the adjacent intensely anodic regions to drive the lateral propagation of the corrosion products across the exposed surface.…”

On the Evolution of Cathodic Activity during Corrosion of Magnesium Alloy AZ31B in a Dilute NaCl Solution

“…Mg and Mg alloys have been shown to be susceptible to the formation of laterally-spreading corrosion products, which tend to form thread-like "filaments" or radially-expanding "discs" in neutral-pH sodium chloride (NaCl) solutions with concentrations ranging from 0.01 M to 0.86 M. [1][2][3][4][5][6][7][8][9][10][11][12] Williams et al have employed the scanning Kelvin probe (SKP) 1 and the scanning vibrating electrode technique (SVET) 1,5,9 to show that this behavior is driven by a "differential electrocatalytic" mechanism, whereby areas consumed by the corrosion products catalyze the cathodic hydrogen evolution (HE) reaction. The HE reaction has been proposed to be catalyzed by non-alloyspecific mechanisms including undissolved metallic Mg "chunks" and a significantly roughened surface film, 5,13 in addition to alloyspecific mechanisms including enrichment of noble (relative to Mg) alloy constituents.…”

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

“…It has been found that the propagation of filaments occurs with voluminous gas evolution at the head while the body immediately behind it passivates [62]. 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 this resistant surface film was revealed by EDS measurements. Filiform corrosion has previously been observed in pure magnesium [13,[49][50][51][52][53][54][55][56] as well as Mg-Al [57,58], Mg-Y [50], Mg8Li [55], AM30 [59], AZ31 [60,61], AZ80 [60], AZ91D [13] and AZ91 [51] alloys after immersion in chloride-containing solutions. It has been found that the propagation of filaments occurs with voluminous gas evolution at the head while the body immediately behind it passivates [62].…”