New insights into the corrosion of magnesium alloys — The role of aluminum

Esmaily, Mohsen; Blücher, Daniel Bengtsson; Svensson, Jan‐Erik; Halvarsson, Mats; Johansson, Lars‐Gunnar

doi:10.1016/j.scriptamat.2016.01.008

Cited by 90 publications

(56 citation statements)

References 26 publications

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“…However, the range of applications has been limited by poor wear and corrosion resistance [1,3,4]. For this reason, surface tratments are needed in order to improve the surface properties and thereby extend the use of magnesium and its alloys.…”

Section: Introductionmentioning

confidence: 99%

“…A C C E P T E D M A N U S C R I P T 4 In this work, a new alkaline electroless Ni-P coating was deposited on the magnesium and AZ31B magnesium alloy, using an optimized bath composition that provides enhanced corrosion resistance. The selected process avoids the use of Cr(VI) species or hydrofluoric acid, as demonstrated in our previous works [8,10].…”

Section: Introductionmentioning

confidence: 99%

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Study of the formation of alkaline electroless Ni-P coating on magnesium and AZ31B magnesium alloy

Zuleta

Correa

Castaño

et al. 2017

Surface and Coatings Technology

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In this work, alkaline electroless Ni-P coatings were directly formed on commercial purity magnesium and AZ31B magnesium alloy substrates using a process that avoided the use of Cr(VI) compounds. The study focused on two aspects of coating formation: (i) the effect of the substrate roughness on the kinetics of the electroless Ni-P deposition process on magnesium; (ii) the morphological and chemical evolution of the coating on both magnesium and the AZ31B alloy. For these purposes, gravimetric measurements, scanning electron microscopy (SEM), X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and open-circuit potential (OCP) measurements were employed. It is shown that a relatively rough substrate promotes the rapid formation of the Ni-P coating on the substrate surface in comparison with smoother substrates. Furthermore, the presence of fluoride ions derived from the NH 4 HF 2 reagent in the electroless Ni-P plating bath leads to formation of MgF 2 a few seconds after immersion in the bath; the MgF 2 . Subsequently, crystals of NaMgF 3 , with a cubic morphology, are developed, which later become embedded in the Ni-P matrix. The presence of fluorine species passivates the substrate during coating formation and hence restricts the decomposition of the electroless Ni-P plating bath, which can occur due to release of Mg 2+ ions. Finally, according to gravimetric measurements, SEM and XRD, the plating process is initially faster on magnesium than on the alloy.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Study of the formation of alkaline electroless Ni-P coating on magnesium and AZ31B magnesium alloy

Zuleta

Correa

Castaño

et al. 2017

Surface and Coatings Technology

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“…20 The oxide/hydroxide layer formed in chloride solutions was previously shown to contain increased levels of impurities caused by incongruent dissolution and/or replating of noble elements onto the surface of the alloy. [21][22][23] These impurities were suggested to be the cause for the increased cathodic behavior of the corrosion products that accelerated the dissolution of magnesium and its alloys. In AZ31, the oxide formed on the α-Mg matrix phase offers poor corrosion resistance, allowing the anodic reaction to proceed.…”

Section: Resultsmentioning

confidence: 99%

Corrosion Mechanisms in Dissimilar AZ31/AZ80 Friction Stir Welds

Savguira

North

Thorpe

2018

J. Electrochem. Soc.

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Corrosion of dissimilar friction stir welds (FSW) made in AZ31/AZ80 magnesium alloys was investigated using the scanning reference electrode technique (SRET), and microcapillary polarization technique, complemented by optical and SEM/EDX microscopy. The corrosion rate of the base metals along with the welded specimen was estimated by mass loss testing. The stir zone material in both alloys showed a higher corrosion potential than the base metal due to the partial dissolution of β-Mg 17 Al 12 and Al-Mn particles. The basic corrosion mechanism in dissimilar welds was determined to be different from that of a similar joint. The corrosion behavior of the dissimilar FSW joint was governed by the galvanic coupling of the two alloys, and not by the microstructural evolution occurring during the welding process. The corrosion behavior of the joint was governed by the galvanic coupling between the α-Mg matrix in AZ31 and the Al-rich intermetallics in AZ80. The welded specimens exhibited the highest corrosion rate, while AZ80 was the most corrosion resistant material. Magnesium alloys can be successfully joined by friction stir welding (FSW) to achieve excellent mechanical integrity in completed welds. 1,2 One challenge arising from the utilization of this joining method is an increased susceptibility to localized corrosion in the material immediately adjacent to the welded joint. 3-6 Previous microcapillary polarization and scanning reference electrode technique (SRET) studies have revealed that the stir zones in AZ31B spot and seam welds are more cathodic than the bulk material. [3][4][5][6] Due to the difference in electrochemical potentials, a macrogalvanic cell is formed causing pitting corrosion in the thermo-mechanically affected zone (TMAZ) and the heat affected zone (HAZ) regions, which are located between the noble stir zone and the active base metal. 5 The differences in the electrochemical behavior of different regions in completed joints have been related to their microstructural evolution that occurred during the welding operation. The stir zone microstructure comprised a dynamically recrystallized matrix which was nominally devoid of intermetallics. The increase in the corrosion potential of the stir zone has been attributed to the dissolution of β-Mg 17 Al 12 and Al-Mn particles via two mechanisms: 4,5 1. Dissolution of intermetallics, which increases the concentration of Al in the α-Mg matrix, resulting in a higher corrosion potential of this region, 7 and 2. The absence of the intermetallics, which eliminates the microgalvanic coupling between the intermetallic particles (IMPs) and the α-Mg matrix, and reduces the susceptibility of the welded joint to localized pitting corrosion. [8][9][10][11] Many industrial applications require joining of dissimilar alloys, and therefore it is essential to understand the influence of the welding process on the corrosion behavior of dissimilar joints. It is currently unknown whether the corrosion behavior of dissimilar welds is governed by the microstructural changes o...

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“…This is different to studying the role of Al-content, which has been summarized recently elsewhere. 40 …”

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confidence: 99%

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

Bland

Birbilis

Scully

2016

J. Electrochem. Soc.

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The effect of the area fraction, size and distribution of model cathodic, intermetallic particles (IMPs) in an anodic Mg matrix on corrosion was investigated. Model Mg-Al electrodes were developed to study IMP effects in isolation from other metallurgical effects, with particles simulated by Al electrodes embedded in a Mg matrix. Arrays of model Mg-Al electrodes were constructed using high purity Al as a surrogate for Al-rich IMPs and flush mounted in commercial purity Mg. The area fraction, size and spacing of these electrodes each altered the corrosion rate and cathodic reaction kinetics assessed after a 24 and 48 hour immersion period at the open circuit potential. Corrosion rate increased with increasing area fraction of Al electrodes but decreased with increasing electrode spacing given a fixed area fraction. The affected zone around electrodes and at the Al/Mg interface was explored to ascertain its impact on the resultant global corrosion rate and kinetics. The effect of local pH at the Al electrode on the prospects for Al corrosion and chemical redeposition were also explored. Magnesium (Mg) alloys continue to be of growing interest due to their good balance of specific properties (i.e. properties relative to weight).1-3 However, due to the inherently negative electrochemical potential of Mg and its alloys, 4,5 Mg-alloys are highly reactive compared to other engineering metals. Mg-alloys are susceptible to several forms of localized corrosion, whilst also highly prone to macro-as well as micro-galvanic corrosion. Due to the low solid solubility of most alloying elements in Mg 6 and particularly low solubility limits for most transition elements; secondary phases readily form during most types of material processing, including casting 7,8 and welding, 9-15 which can adversely alter the corrosion performance.There are many secondary phases, or intermetallic particles (IMPs), which are particularly common in Mg alloys. Each IMP 16 has its own unique dissolution or reduction kinetics, dependent on its composition, size and dispersion within the material. For example, in the Mg-Al alloys which contain Mn (such as AZ31, AZ91 and AM50), Al-Mn IMPs that are rich in Al such as Al 4 Mn, Al 6 Mn, display relatively low rates of cathodic kinetics in comparison to other IMPs that are rich in Mn such as Al 8 Mn 5 ; the latter displaying relatively rapid cathodic kinetics. 1,7,17 Similarly, the so called Al-Mn-Fe IMPs, such as Al 8 (Mn,Fe) 5 function as highly potent cathodic sites in Mg, although the Mn has been shown to prevent some of the detrimental galvanic effects of Fe impurities by incorporating the Fe into the Al-Mn IMPs. 1,6,16 Furthermore, Mg-Zn IMPs have approximately the same open circuit potential (OCP) as many Mg alloys. However, the cathodic kinetics of this IMP are more rapid than Mg, 16 attributed to the presence of Zn. Mg-Zn IMPs are cathodic to the α-Mg and, therefore, tend to cause localized corrosion through micro-galvanic coupling which is often manifest at the Mg matrix/IMP interface. Some ...

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New insights into the corrosion of magnesium alloys — The role of aluminum

Cited by 90 publications

References 26 publications

Study of the formation of alkaline electroless Ni-P coating on magnesium and AZ31B magnesium alloy

Study of the formation of alkaline electroless Ni-P coating on magnesium and AZ31B magnesium alloy

Corrosion Mechanisms in Dissimilar AZ31/AZ80 Friction Stir Welds

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

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