A Compilation of Corrosion Potentials for Magnesium Alloys

Cain, Taylor; Bland, Leslie G.; Birbilis, Nick; Scully, John R.

doi:10.5006/1257

Cited by 75 publications

(44 citation statements)

References 45 publications

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“…These potentials are representative of typical E corr measurements for Mg and AZ31, as seen in current literature. [59][60][61][62][63][64] By increasing the [Cl -], there is an increase in the anodic dissolution rate. This is also consistent with a decrease in the measured E corr (via activation of the anodic reaction).…”

Section: Resultsmentioning

confidence: 99%

Assessing the Corrosion of Commercially Pure Magnesium and Commercial AZ31B by Electrochemical Impedance, Mass-Loss, Hydrogen Collection, and Inductively Coupled Plasma Optical Emission Spectrometry Solution Analysis

Bland

King

Birbilis³

et al. 2015

CORROSION

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The corrosion of commercially pure magnesium (Mg) and AZ31B-H24 with simultaneous measurements of electrochemical impedance (EIS) and hydrogen gas evolved over a 24 h immersion period was studied in solutions of three chloride concentrations. The corrosion rate was determined from the Stern-Geary approach. The integral electrochemical-based mass loss was compared to the gravimetric mass loss and inductively coupled plasma optical emission spectrometry (ICP-OES) solution analysis of the total Mg concentration released. The use of ICP-OES to support the other assessment methods has not been previously reported. Assuming Mg dissolves as Mg 2+ , there was agreement using these four unique measures of Mg corrosion. The integration of the polarization resistance (R P ) over time, as evaluated from EIS at the low frequency limit incorporating full consideration of the pseudoinductive impedance behavior of Mg, provided excellent correlation to the cumulative mass loss, ICP-OES solution analysis, and volume of hydrogen collected for commercially pure Mg and reported for the first time for AZ31. The choice of using the Tafel slope in the Stern-Geary approach, as well as the subsequent comparison of results to corrosion rate data in the literature, are discussed.

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Section: Resultsmentioning

confidence: 99%

Assessing the Corrosion of Commercially Pure Magnesium and Commercial AZ31B by Electrochemical Impedance, Mass-Loss, Hydrogen Collection, and Inductively Coupled Plasma Optical Emission Spectrometry Solution Analysis

Bland

King

Birbilis³

et al. 2015

CORROSION

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“…4 Commercial Mg alloys possess corrosion potentials less than −1.4 V SCE in a wide variety aqueous environments, where the hydrogen evolution reaction (HER) is the primary cathodic reaction and any transition metal (TM) contaminants can support HER without the thermodynamic tendency for dissolution. 5 Rapid rates of Mg corrosion occur in aqueous environments of near-neutral pH compared to other engineering metals on the basis that: (1) Mg is not thermodynamically stable below pH ∼11, (2) there is no mass transport limitation on the cathodic reaction (since water, which is available in abundance, is being reduced, not oxygen), (3) the lack of a protective (passive) film, and (4) the non-polarizable nature of Mg also contributes to very high rates of anodic dissolution, since low overpotentials correspond to high current densities. This latter point is the kinetic reason why galvanic coupling of Mg to other, more noble, engineering metals such as steels is particularly detrimental.…”

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confidence: 99%

Evidence of the Enrichment of Transition Metal Elements on Corroding Magnesium Surfaces Using Rutherford Backscattering Spectrometry

Cain

Madden

Birbilis

et al. 2015

J. Electrochem. Soc.

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103

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The increasing rate of hydrogen evolving from magnesium (Mg) surfaces under anodic polarization was characterized through simultaneous hydrogen volume collection, mass loss, potentiostatic and potentiodynamic polarization, and inductively coupled plasma optical emission spectroscopy (ICP-OES). This is distinct from the literature in that all four techniques are not often performed in the same test. Results indicate Mg dissolves as Mg 2+ with anodically induced increases in hydrogen evolution rates due to increases in the water reduction reaction rate. In order to contribute to a mechanistic understanding of the cause for anodically induced increases in hydrogen evolution, quantitative surface spectroscopy for accurate chemical analyses of the Mg surface subject to dissolution was carried out via post exposure Rutherford Backscattering Spectrometry. Surface enrichment of transition elements other than Mg were confirmed, which provides a foundation for understanding the origin of enhanced hydrogen evolution. Magnesium (Mg) alloys remain attractive for potential weight reduction in the automotive and aerospace industries.1-3 With the growing demand for Mg products, the corrosion of Mg in aqueous electrolytes remains an important technological issue. 4 Commercial Mg alloys possess corrosion potentials less than −1.4 V SCE in a wide variety aqueous environments, where the hydrogen evolution reaction (HER) is the primary cathodic reaction and any transition metal (TM) contaminants can support HER without the thermodynamic tendency for dissolution. 5Rapid rates of Mg corrosion occur in aqueous environments of near-neutral pH compared to other engineering metals on the basis that: (1) Mg is not thermodynamically stable below pH ∼11, (2) there is no mass transport limitation on the cathodic reaction (since water, which is available in abundance, is being reduced, not oxygen), (3) the lack of a protective (passive) film, and (4) the non-polarizable nature of Mg also contributes to very high rates of anodic dissolution, since low overpotentials correspond to high current densities. This latter point is the kinetic reason why galvanic coupling of Mg to other, more noble, engineering metals such as steels is particularly detrimental. Several attempts have been made to increase the intrinsic corrosion resistance of Mg alloys, 6-14 however design of improved Mg alloys requires a fundamental understanding of the corrosion mechanisms. One widely studied phenomenon of Mg corrosion is the so called negative difference effect or NDE (i.e., increased rates of the HER at increasing anodic polarization) which confounds the understanding of dissolution mechanisms, establishment of the Tafel law, and other issues. 8,[15][16][17][18][19][20][21][22][23][24][25][26][27] Several explanations of this anomalous behavior have been proposed among which include impurity element enrichment, formation (and disruption) of a partially protective film, metal spalling, univalent Mg based anodic dissolution, and magnesium hydride based models as revie...

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“…[14][15] Any Fe particles or Fe containing particles in the Mg matrix thus experience significant cathodic polarization (at the Mg alloy OCPs), and the HER takes place at extremely high rates at such sites. It is now widely known that the HER on pure Mg increases during anodic polarization, 11,[16][17][18][19][20][21][22][23][24][25] posing an anomaly on the conventional analysis of the mixed potential theory.…”

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On the Fe Enrichment during Anodic Polarization of Mg and Its Impact on Hydrogen Evolution

Lysne

Thomas

Hurley

et al. 2015

J. Electrochem. Soc.

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Iron (Fe) is an unintentional impurity present in pure magnesium (Mg) and Mg alloys, albeit nominally in low and innocuous concentrations (< 100 ppmw). Since Fe, like most metals, is more noble than Mg, the presence of Fe impurities can serve as cathodic sites within the Mg matrix. During anodic polarization of Mg, incongruent dissolution can lead to undissolved Fe impurities accumulating upon the Mg surface, permitting an increase in the overall rate of hydrogen evolution. The experimental manifestation of the incongruent dissolution of Mg, has not yet been clarified, wherein, the extent and efficiency of Fe enrichment during anodic polarization is not known, and also the increase in the hydrogen evolution rate due to Fe enrichment has not been quantified. In this work, Mg specimens with Fe concentration between 40 to 13,000 ppmw were examined in 0.1 M NaCl to obtain a quantitative relation between the Fe concentration and the rate of cathodic hydrogen evolution. These base-line alloys were then anodically polarized to facilitate surface Fe enrichment, and subsequently again cathodically polarized to determine the impact of prior dissolution and Fe enrichment on the subsequent hydrogen evolution. A simple model to predict Fe enrichment was used to analyze the electrochemical data and predict the extent and efficiency of Fe enrichment.

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A Compilation of Corrosion Potentials for Magnesium Alloys

Cited by 75 publications

References 45 publications

Assessing the Corrosion of Commercially Pure Magnesium and Commercial AZ31B by Electrochemical Impedance, Mass-Loss, Hydrogen Collection, and Inductively Coupled Plasma Optical Emission Spectrometry Solution Analysis

Assessing the Corrosion of Commercially Pure Magnesium and Commercial AZ31B by Electrochemical Impedance, Mass-Loss, Hydrogen Collection, and Inductively Coupled Plasma Optical Emission Spectrometry Solution Analysis

Evidence of the Enrichment of Transition Metal Elements on Corroding Magnesium Surfaces Using Rutherford Backscattering Spectrometry

On the Fe Enrichment during Anodic Polarization of Mg and Its Impact on Hydrogen Evolution

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