TEM Examination of the Film Formed on Corroding Mg Prior to Breakdown

Taheri, Mehdi; Danaie, Mohsen; Kish, Joseph R.

doi:10.1149/2.017403jes

Cited by 51 publications

(40 citation statements)

References 42 publications

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“…There is a greater emphasis on corrosion in aqueous conditions, rather than in the relatively dry atmospheres in this study. Nevertheless, it is found that surface films identified in aqueous conditions and the present results in moist air are similar in that both comprise a thin MgO inner layer and a thicker Mg(OH)2 outer layer, and that the outer layer thickens by hydration of the inner [27,28].…”

Section: Discussionsupporting

confidence: 64%

Corrosion of the alloys Magnox AL80, Magnox ZR55 and pure magnesium in air containing water vapour

et al. 2016

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General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms CORROSION OF THE ALLOYS MAGNOX AL80, MAGNOX ZR55 AND PURE MAGNESIUM IN AIR ABSTRACTUnderstanding the corrosion susceptibility of Magnox alloys during short-and long-term storage in moist air is important. Corrosion of Magnox AL80, Magnox ZR55 and magnesium has been studied in air containing water partial pressures between 200 vppm and ~80000 vppm, at temperatures of 46 °C to 96 °C.Specimens were exposed to air, with and without CO2, and argon gas. Metal consumption rates were measured and corrosion products characterised, using scanning electron and optical microscopy, energy dispersive x-ray microanalysis, x-ray diffraction and secondary ion mass spectrometry. Results show the importance of CO2 gas for suppressing breakaway corrosion in moist air. INTRODUCTIONOf the fleet of UK gas-cooled Magnox-type nuclear power stations that were designed, constructed and commissioned during the 1950s and 1960s, the last ceased generating electricity at the end of 2015 [1]. All the other reactors, most of which achieved a 40-year operating life, are now in various stages of decommissioning. The fuel elements in these first generation gas-cooled, graphite-moderated nuclear reactors consist of metallic uranium rods within sealed cans of Magnox AL80 alloy. This alloy is magnesium-based, nominally containing 0.80 wt% aluminium together with 50 ppm by weight beryllium.This alloy normally undergoes negligible corrosion in the dry carbon dioxide (CO2) gas used to cool the reactors at power. During reactor operation, the fuel cans achieve temperatures within the range 250 °C to 450 °C [2]. There are, however, occasions where Magnox alloy fuel elements are exposed to air. These include outages for maintenance, sometimes quite prolonged, and, most significantly, during defueling following the final shutdown of the reactor. In these circumstances, the temperature of the Magnox alloy cans is, at most, about 100 °C and the water vapour partial pressure is that of the ambient air outside the reactor, about 2000 Pa to 3000 Pa.To underwrite the safe storage of fuel in the shutdown condition, it is necessary to have an understanding of the extent and rate of corrosion of the Magnox alloy cans in moist air. In particular, it is essential to know whether the resulting corrosion product is protective. If it is, then the extent of corrosion can be neglected; if it is not, and the corrosion obeys a linear rate law; a significant quantity of corrosion product can be formed.During decommissioning, if the corrosion leads to penetration of a Magnox alloy can then the irradiated fuel will be exposed, with consequential safety implications. There are no data available that consider the corrosion of Magnox AL80 under these shutdown conditions. Friskney [3] has published data on the reaction in dry steam at a pressure of ~10 5 ...

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Section: Discussionsupporting

confidence: 64%

Corrosion of the alloys Magnox AL80, Magnox ZR55 and pure magnesium in air containing water vapour

et al. 2016

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“…The film was non-uniform in thickness, typical of the aqueous corrosion of Mg alloys (especially in the presence of NaCl). 16,[20][21][22]28,[31][32][33]35,36,45,47,66 The film structure was complex, but generally morphologically duplex, with an overall "convex appearing" outer film region that consisted of a fine layered/striated morphology varying in total thickness on the order of ∼10-100 nm thick, overlying an inner, nodular-like film extending inward ∼0.5 to 1 μm for the location imaged. The outer convex film likely corresponded with the smooth, globular features evident in the secondary electron surface image shown in Fig.…”

Section: Resultsmentioning

confidence: 97%

Tracer Film Growth Study of the Corrosion of Magnesium Alloys AZ31B and ZE10A in 0.01% NaCl Solution

Brady¹,

Fayek²,

Leonard³

et al. 2017

J. Electrochem. Soc.

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A sequential isotopic tracer study of corrosion film growth for Mg-3Al-1Zn-0.25Mn (AZ31B) and Mg-1.2Zn-0.25Zr-<0.5Nd (ZE10A) was conducted by 4 h immersion in H 2 18 O or D 2 16 O, followed by a 20 h immersion in a 0.01 wt% NaCl H 2 18 O or D 2 16 O solution. Sputter depth profiles were obtained for 16 O, 18 O, H, and D using secondary ion mass spectrometry (SIMS). Compared to the previous tracer study for these alloys in salt-free water, the addition of 0.01 wt% NaCl resulted in a transition from oxygen inward-dominated film growth to a component of mixed inward/outward film growth for both alloys. The hydrogen tracer behavior remained inward growing for AZ31B, and short-circuit, inward growing for ZE10A, in both pure water and in 0.01 wt% NaCl solution, with extensive penetration of D beyond the film and into the underlying alloy also observed for ZE10A. Analysis of the films by X-ray photoelectron spectroscopy (XPS) and cross-section scanning transmission electron microscopy (STEM) indicated intermixed Mg(OH) 2 and MgO, with the relative fraction of Mg(OH) 2 peaking near the center of the film. These findings suggest a decoupled film growth mechanism, with initial formation of oxide followed by NaCl-accelerated conversion to hydroxide, likely by both solid-state and dissolution-precipitation processes. Low density Mg alloys are of great interest for the production of lightweight vehicle parts to achieve increased fuel efficiencies and better performance.1-6 Key challenges to achieve widespread adoption in automotive vehicle applications include the cost of primary Mg metal, the formability of wrought Mg alloys (to expand application beyond current Mg cast parts), and the rapid corrosion of Mg in many aqueous and humid environments.6 From a corrosion perspective, the high reactivity of Mg, which leads to micro-and/or macro-galvanic corrosion coupling, and the inability of Mg alloys to sufficiently establish and/or maintain a protective surface film are major challenges in many environments. [7][8][9][10][11][12][13][14] Of particular detriment is the acceleration of Mg corrosion in salt-containing environments, which has been linked to increased local micro-galvanic coupling at second phases/metallic impurity sites and enhanced Mg dissolution, as well as disruption of the Mg(OH) 2 /MgO based surface film. The successful application of isotopic tracer studies employing secondary ion mass spectrometry (SIMS) to gain insights into the film growth mechanism of Mg alloys in room-temperature water and 85• C humid air was recently demonstrated. 45,46 Isotopic tracer studies have been widely used in oxidation studies of Al-, Fe-, Ni-, and Zr-base alloys, as well as SiC, with significant new insights gained regarding the growth mechanism of the oxide films. 45,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] However, application of this experimental approach to Mg is particularly challenging due to the formation of both MgO and Mg(OH) 2 products in the film, necessitating the use of both ...

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“…Since Mg corrosion is under cathodic control, the anodic dissolution rate and net anodic current would be expected to decrease when the corrosion film covers the surface. Mg(OH) 2 is not a suitable passive layer due to its porous nature which leads to crack formation and spalling of the hydroxide revealing fresh areas of Mg metal to be oxidized, 16,53 whilst the Mg(OH) 2 may even allow electron transfer itself. The noise at the end of the −1.4 V SCE and −1.45 V SCE scans observed in Figure 1a can be interpreted as a result of crack formation and spalling of the oxide/hydroxide corrosion film.…”

Section: Discussionmentioning

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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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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TEM Examination of the Film Formed on Corroding Mg Prior to Breakdown

Cited by 51 publications

References 42 publications

Corrosion of the alloys Magnox AL80, Magnox ZR55 and pure magnesium in air containing water vapour

Corrosion of the alloys Magnox AL80, Magnox ZR55 and pure magnesium in air containing water vapour

Tracer Film Growth Study of the Corrosion of Magnesium Alloys AZ31B and ZE10A in 0.01% NaCl Solution

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

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