Corrosion behaviour of extruded AM30 magnesium alloy under salt-spray and immersion environments

Song, Weiwei; Martin, Holly J.; Hicks, A.; Seely, Denver; Walton, C.; Lawrimore, William B.; Wang, Paul T.; Horstemeyer, M.F.

doi:10.1016/j.corsci.2013.10.020

Cited by 67 publications

(42 citation statements)

References 38 publications

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“…19,20 Studies documenting the corrosion behavior of AM30 in aqueous NaCl solutions have revealed that filaments form during exposure. 10,20 Thus, AM30 was well suited for the intended purpose.…”

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confidence: 99%

“…[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.…”

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Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

Cano

McDermid

Kish

2015

J. Electrochem. Soc.

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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,...

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Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

Cano

McDermid

Kish

2015

J. Electrochem. Soc.

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“…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.…”

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

Cano

Kish

McDermid

2015

J. Electrochem. Soc.

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Sequential potentiodynamic polarization and scanning vibrating electrode technique measurements were made on Mg alloy AZ31B immersed in dilute 0.05 M NaCl solution to corroborate the suspected role of noble metal (relative to Mg) solute enrichment at the filament/alloy interface in sustaining the cathodic activation exhibited by corrosion filaments. The electrochemical measurements revealed that the extent of cathodic activation (i.e. current density) measured over a corrosion filament was sustained with immersion time. This resulted in a continual increase in the total anodic current measured over the AZ31B surface as new filaments initiated and propagated. The sustained cathodic activation coupled with observations of zinc (Zn) enrichment at the filament/alloy interface during the life of a corrosion filament is argued to be indicative of a solute enrichment mechanism being a significant factor in this process. 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. 1,5,9 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. 5,9,14,15 However, a consensus has not been reached in the literature to date regarding the dominant source(s) of cathodic activity.Williams et al. 9 proposed that the main source of cathodic activation on corrosion filaments formed on as-cast Mg alloy AZ31B were Al-Mn intermetallic particles which were originally present in the alloy microstructure and were left behind as the Mg matrix phase was selectively dissolved. Al-Mn phases are well-known to promote cathodic activity in Mg alloys due to their higher corrosion potentials (ranging from −1.0 to −1.5 V SCE ) relative to α-Mg (−1.6 V SCE ).16 A subsequent transmission electron microscopy (TEM) investigation by Cano et al.12 provided physical evidence of Al-Mn particles embedded in the corrosion filaments formed on wrought AZ31B-H24 sheet. However, these intermetallic particles were found to be embedded in the intact surface film as well, casting reasonable doubt concerning the role played by these intermetallic particles in sustaining cathodic activation. In light of this, two alternative sources of cathodic activation were identified by Cano et al.: 12 (1) the MgO-based corrosion filament itself and (2) the metallic zinc (Zn)-enriched layer formed at the filament/alloy interface during prolonged immersion in the NaCl solution. The first of these sources is regarded as less likely due to the poor cathod...

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“…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].…”

Section: Global and Local Electrochemical Behaviour Of Az91 In 01 M mentioning

confidence: 99%

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

Kot

Krawiec

2015

J Solid State Electrochem

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The microstructure of as-cast AZ91 alloy is complex, with the presence of Mg 17 (Al, Zn) 12 precipitates surrounded by an eutectic phase, AlMn intermetallic particles and an α-Mg solid solution. In addition, the distribution of aluminium in the alloy was found to be heterogeneous. Under these conditions, a multiscale approach coupling global and local electrochemical measurements seems to be a promising method to study its corrosion behaviour. Results show that the multiscale approach can be used in 0.1 M NaCl. In this case, AZ91 is susceptible to pitting corrosion. By contrast, the multiscale approach is not valid for AZ91 in 0.1 M Na 2 SO 4 . At the global scale, filiform corrosion and pitting corrosion are observed in the matrix, whereas only pits initiate when using the microcapillary techniques. The obtained results show that the local electrochemical techniques have to be used carefully in some environments.

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Corrosion behaviour of extruded AM30 magnesium alloy under salt-spray and immersion environments

Cited by 67 publications

References 38 publications

Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

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

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

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