Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

Abstract

The filiform-like corrosion of extruded Mg alloy AM30 immersed in a dilute near-neutral NaCl solution was investigated using electrochemical techniques coupled with transmission electron microscopy (TEM). Scanning vibrating electrode technique (SVET) measurements showed that the filament-like corrosion consisted of an intensely anodic propagation front supported by a cathodicallyactivated filament behind. The TEM examination of the corroded and intact surface films in cross-section using thin foils prepared by focused-ion-beam (FIB) milling indicated that the cathodic activity was likely a combined result of the formation of a thick, highly-defective MgO film and the ability of Al-Mn intermetallic particles to catalyze the cathodic H 2 gas evolution reaction. The formation of an Al-rich layer at the film/alloy interface with time, due to the incongruent dissolution of the alloy, was not able to stop the initiation and propagation of the filaments. Mg and Mg alloys have been shown to be susceptible to the formation of laterally-spreading products, which tend to manifest as thread-like "filaments" or radially-expanding "discs" during corrosion in aqueous NaCl solutions. [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. 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. 1The mechanism, however, by which the cathodic reaction (H 2 gas evolution) is enhanced on corroded Mg or Mg alloys continues to be unclear. Proposed mechanisms include (i) the enrichment of noble secondary phase constituents such as iron (Fe) impurity particles (for pure Mg) or Al-Mn intermetallic particles (for AZ31B) within the corroded regions 5,9,13,14 (ii) an increased catalytic tendency of the corroded regions caused either by undissolved metallic Mg "chunks," 15 a significantly roughened surface film 5 or the formation of Mg(OH) 2 as a corrosion product. 16 The SVET measurements reported by Williams et al. suggested that a significantly reduced cathodic activation was exhibited by the as-cast AZ31B corrosion filaments 9 relative to the corrosion discs on pure Mg.5 This finding agrees well with the noble secondary phase theory,...

Corrosion Performance of Friction Stir Linear Lap Welded AM60B Joints

“…The intact film formed on both materials is relatively thin (~400 nm) and reasonably compact, relative to the much thicker and more defective corrosion product that is the dark corrosion product filament on Mg-Al alloys. 26,36 Although not shown here, selected area diffraction (SAD) patterns collected from the both intact surface films are consistent in part with crystalline MgO. Typical EDS maps associated with the intact film of the base material and stir zone is shown in Fig.…”

“…[16][17][18]22 However, the ability of the sub-surface Al distribution and any associated Al enrichment layer to alter the surface film to avoid cathodic activation has not been investigated in a unified context. The scanning vibrating electrode (SVET) technique has eloquently revealed that breakdown of Mg 23,24 and single phase Mg-Al alloys [25][26][27] in NaCl (aq) electrolytes involves laterally-spreading focal anodes that leave behind cathodically-activated corrosion products. Thus, identifying microstructural features that can alter surface films to avoid cathodic activation is desirable from a corrosion performance perspective.…”

Exploring the Effects of Intermetallic Particle Size and Spacing on the Corrosion of Mg-Al Alloys Using Model Electrodes

“…It is clear that E(r) is elevated for a radial distance of ∼2000/4000 μm. The potential distribution around the electrode, E(r), may trigger (anodic dissolution induced) cathodic activation of the nearby Mg-matrix in this zone, as is seen in many Mg-alloys 26,32,33 -with corrosion initiation and propagation proximate to cathodic electrodes. For electrodes with different compositions but the same area fraction and electrode size, as presented for both Al and Fe electrodes in Mg (Figure 3), it was observed that the affected radial zone around the Fe electrode was larger than that around the Al electrode because the Fe electrode is a 'stronger cathode', whereby it has more rapid HER kinetics for a given cathodic potential.…”

“…21 The dissolution of Mg at these sites has been observed to develop as dark regions which propagate through the a mechanism with filiform-like morphology typically cited on many Mg alloys, including AZ31 and AM30. 32,33 The Mg-alloy corrosion rate was also found to correlate with the percent area of these dark, corroded regions in chloride environments. 9,34 Rapid HER on Mg-alloys was also correlated with increased corrosion or cathodic activity at these dark sites.…”

The Effect of Deep Cryogenic Treatment on the Corrosion Behavior of Mg-7Y-1.5Nd Magnesium Alloy

“…The change in the Bode diagrams represented the dynamic characteristics of the dissolving surfaces. The larger high frequency capacitive loop was related to the electric double layer at the electrode/electrolyte interface [46,47], and its diameter was equal to the charge transfer resistance of the working electrode. The smaller low frequency capacitive loop was due to the relaxation of mass transport of the partially protective oxide layer [48].…”