Passivity and Localized Corrosion of AZ31 Magnesium Alloy in High pH Electrolytes

Alsagabi, Sultan; Ninlachart, Jakraphan; Raja, Krishnan S.; Charit, Indrajit

doi:10.1007/s11665-016-2101-9

Cited by 8 publications

(4 citation statements)

References 48 publications

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“…10d), this may be attributed to passivity of Mg in alkaline solution and the close separation of the anodes and cathodes. 29,33 The slight cathodic and anodic currents densities measured in the alkaline regime in the uncoupled sample, however, do not reflect the true value due to limitations associated with the vibrating microelectrode.…”

Section: Resultsmentioning

confidence: 99%

“…These results are concordant with reports in the literature. 6,7,13,33 SVET maps reveal that severity galvanic coupling effect is well pronounced in very acidic or alkaline environment. MgZn 2 is anodic relative to Al at pH 2 but cathodic relative to Al at pH 12, while self-dissolution prevails under near neutral conditions with Al being almost in a passive state (Figs.…”

Section: Discussionmentioning

confidence: 99%

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SVET and SIET Study of Galvanic Corrosion of Al/MgZn₂in Aqueous Solutions at Different pH

Ikeuba

Zhang

Wang

et al. 2018

J. Electrochem. Soc.

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The galvanic corrosion behavior of synthesized MgZn 2 intermetallic particle coupled with Al in 0.1 M NaCl solutions across pH 2 to12 has been studied using the scanning vibrating electrode technique (SVET), the scanning ion-selective electrode technique (SIET) and X-ray photoelectron spectroscopy (XPS). Results indicate the galvanic dissolution of MgZn 2 depends on pH as well as exposure time. SVET current density maps reveal that MgZn 2 is anodic relative to Al at pH 2 but cathodic at pH 12, while self-dissolution of MgZn 2 prevails at pH 4 and 6. However, the galvanic coupling effect is more pronounced at pH 2 and 12. SIET maps of H + and Cl − ion distribution reveal evidence of local alkalization on MgZn 2 and migration of Cl − ions away from MgZn 2 surface at pH 4 and 6 due to significant accumulation of OH − ions. High strength 7xxx series Al alloys, which are parts of structural components in the aerospace and automotive industry, are vulnerable to localized corrosion in the presence of chloride ions. The localized corrosion manifests in the form of pitting, crevice, inter-granular and exfoliation corrosion, which increase stress corrosion cracking susceptibility.1,2 Localized corrosion is initiated at microstructural inhomogeneities such as grain boundaries or coarse intermetallic phases. Anodic dissolution of grain boundary precipitates (intermetallic particles) such as MgZn 2 is always considered a probable reason for stress corrosion cracking in Al 7xxx series alloys.3 Galvanic corrosion can occur between the intermetallic particles and the surrounding alloy matrix due to the difference between the electrochemical activity of the particles and the matrix, this leads to the deterioration of Al alloys in engineering materials. 4 The electrochemical behavior of MgZn 2 and its role in corrosion of 7xxx series alloys has been investigated by various researchers using various traditional methods.5-8 For instance, electrochemical testing carried out on MgZn 2 by Birbilis et al. 6,7 and Wloka et al. 9 using the microcell method indicates that MgZn 2 is anodic relative to the matrix and corrodes freely above their corrosion potential. Potentiodynamic polarization carried out by Ramgopal et al. 4 on a thin film analog of MgZn 2 indicates that it is anodically very active. The scanning Kelvin probe force microscopy (SKPFM) investigations on Al 7075 by Andreatta and coworkers 5,10,11 revealed that the initiation of the attack at the intermetallic sites is caused by strong potential differences between the matrix and intermetallics, in particular, MgZn 2 particles along grain boundaries. However, the kinetic parameters relating to the galvanic couples could not be provided. It then becomes obvious that some issues remain regarding the galvanic corrosion behavior of MgZn 2 due to lack of information on the kinetics of local processes occurring at the matrix/ solution interface. 6,7,12,13 These issues include the galvanic current density distribution as well as the pH and ion distributions especially in near neutral...

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

SVET and SIET Study of Galvanic Corrosion of Al/MgZn₂in Aqueous Solutions at Different pH

Ikeuba

Zhang

Wang

et al. 2018

J. Electrochem. Soc.

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“…The solubility of MgSO4 is 33.7 g/100 g is much higher the solubility of Mg(OH)2 0.0009628 g/100 g that forms a semi-protective 'passive' layer. The higher solubility of MgSO4 would lead to general corrosion at potentials closer to the OCP and localized corrosion at more anodic potentials (18). The higher cathodic Tafel constant indicates the cathodic controlled corrosion process.…”

Section: Resultsmentioning

confidence: 99%

The Effect of Grain Size on the Corrosion Behavior of Mg-RE Alloy ZE10A

2018

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Mg-RE alloy Electron717 (ZE10A, UNS M11600) has a nominal composition of 1.2wt % Zn, 0.25wt% Zr, <0.5%Zr, 0.007wt%Mn, 0.004wt %Fe, 0.001wt%Ni and balance Mg. The alloy was investigated in three different heat-treated conditions such as (1) as received (AR) (average grain size dav = 7.90 µm), ( 2) solution treated at 525 o C for 8 hours (ST8 with dav: 26.24 µm) and ( 3) solution treated at 525 o C for 24 hours (ST24 with dav: 41.37 µm). Anodic and cathodic polarizations were carried out in three different electrolytes (pH unadjusted) such as: a) 0.6M NaCl, b) 0.5M NaNO3, and c) 0.1 M Na2SO4. The results are compared with commercial purity magnesium with an average grain size of 16 μm. The effect of grain size on the corrosion rate was observed for ZE10A alloy in the 0.1 M NaNO3 solution. Increase in the grain size increased the corrosion current density. The grain size effect diminished when polarized to high anodic potentials. Commercial purity magnesium showed lower corrosion current density than the ZE10A specimens in all the solutions tested in this study.

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“…Magnesium and its alloys show passivation in aqueous alkaline solutions (pH > 10.5) by formation of surface layers consisting of Mg(OH) 2 , MgO, and RE 2 O 3 phases where RE refers to the rare earth elements such as Nd, Gd, and Y (18,19,20). The bulk energy gap of the MgO is reported as 7.77 eV, and the surface energy gap is 6.3 eV (21,22).…”

Section: Introductionmentioning

confidence: 99%

Effect of Light Illumination on the Corrosion Behavior of Mg-RE Alloy EV31A in Chloride-Alkaline Solutions

Ninlachart

Raja

2018

ECS Trans.

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The effect of white light illumination on the corrosion behavior of a Mg -Rare Earth (RE) alloy EV31A specimens in peak aged condition was investigated in 0.1 M NaOH solution containing 0, 80, 100 and 200 ppm chloride ions at room temperature. The passivation current density decreased upon illumination in the 0.1 NaOH solution without chloride addition. On the other hand, increased passive current densities were observed in the chloride containing solutions upon illumination. The white light illumination did not affect the open circuit potential of the EV31A. This result indicated that alloying additions did not significantly alter the bandgap of the passive layer or induce bandgap states in the passive layer in order to harvest the longer wave lengths of the white light. The incubation time for passivity perturbation decreased with illumination. The charge carrier density increased significantly upon illumination. The exchange current density for hydrogen evolution increased under the illuminated conditions. The Mott-Schottky results pointed out that light illumination induced surface defects of n-type character. These defects could have been potential sites for chloride adsorption that decreased the stability of the passive film. The white light illumination did not result in induced photo potential due to wide bandgap of the passive layer. Since the band bending was not altered, the electric field remained the same in both illuminated and non-illuminated condition. Overall, illumination of white light did not offer significant benefit to the localized corrosion resistance of the Mg-RE alloy in alkaline-chloride solutions.

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Passivity and Localized Corrosion of AZ31 Magnesium Alloy in High pH Electrolytes

Cited by 8 publications

References 48 publications

SVET and SIET Study of Galvanic Corrosion of Al/MgZn₂in Aqueous Solutions at Different pH

SVET and SIET Study of Galvanic Corrosion of Al/MgZn₂in Aqueous Solutions at Different pH

The Effect of Grain Size on the Corrosion Behavior of Mg-RE Alloy ZE10A

Effect of Light Illumination on the Corrosion Behavior of Mg-RE Alloy EV31A in Chloride-Alkaline Solutions

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Passivity and Localized Corrosion of AZ31 Magnesium Alloy in High pH Electrolytes

Cited by 8 publications

References 48 publications

SVET and SIET Study of Galvanic Corrosion of Al/MgZn2in Aqueous Solutions at Different pH

SVET and SIET Study of Galvanic Corrosion of Al/MgZn2in Aqueous Solutions at Different pH

The Effect of Grain Size on the Corrosion Behavior of Mg-RE Alloy ZE10A

Effect of Light Illumination on the Corrosion Behavior of Mg-RE Alloy EV31A in Chloride-Alkaline Solutions

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SVET and SIET Study of Galvanic Corrosion of Al/MgZn₂in Aqueous Solutions at Different pH

SVET and SIET Study of Galvanic Corrosion of Al/MgZn₂in Aqueous Solutions at Different pH