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The films formed on ultrahigh purity Mg, Elektron 717 (ZE10A), and AZ31B in water at room temperature were characterized by TEM, XPS, and SIMS. The films consisted primarily of MgO, with surface regions also containing Mg(OH) 2 and MgCO 3 . SIMS suggested H throughout the films and into the underlying metal. Segregation of Zn to the metal/film interface and Al in the film was observed for AZ31B. Similar Zn film segregation was also detected for Elektron 717, along with Nd at the alloy/film interface and nano-size Zn 2 Zr 3 precipitates throughout the film. Implications of these findings on film growth are discussed. Magnesium and its alloys are lightweight structural materials of great interest for reduced vehicle weight and improved fuel efficiency in the automotive industry due to their good strength, low density, amenability to casting, and ease of recycling.1-3 A major drawback, however, is the poor corrosion resistance of Mg under many conditions. 4 Magnesium is considered the most reactive structural material and is susceptible to multiple forms of corrosion; including general dissolution, galvanic coupling, localized corrosion, and stress corrosion cracking. 2,3,5,6 Surface treatments/coatings are needed for many applications which result in increased cost and can be a source of component durability issues. 1The inability of Mg alloys to establish a continuous and fully protective surface film under many exposure conditions is a key factor underlying their susceptibility to corrosive attack. Films formed on Mg alloys under humid air or aqueous corrosion conditions are typically based on mixtures of MgO and Mg(OH) 2 .4,7-12 Although MgO can offer a degree of protection under ambient dry atmospheric corrosion conditions, it can be readily hydrated in the presence of water or water vapor to form Mg(OH) 2 , which offers limited stability and reduced protectiveness. 4 Further, considerations based on Pilling-Bedworth (P-B) ratio have also been invoked for explaining the inability of Mg to establish a fully protective oxide film. 13 Magnesium has a P-B ratio of ∼0.8, which places surface MgO in tension, and can result in a porous, cracked, nonprotective oxide layer. (Typically a P-B ratio of 1-2 is considered a minimum, but not sufficient prerequisite for protective film formation). Corrosion can be further significantly accelerated when aggressive electrolyte species such as chloride are present. 5,14 Alloying has been shown to modify surface film performance; 15 however, a detailed mechanistic understanding of how and why is frequently lacking.Extensive studies based on electrochemical assessment and surface film chemistry depth profiling of Mg alloys have been pursued by techniques such as Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), etc. [16][17][18][19][20][21][22][23] However, what is lacking is a detailed nano/micro structural level picture of the film nature and alloy/film interface regions, particularly with regards to segregation of...

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Small angle neutron scattering (SANS) and scanning transmission electron microscopy (STEM) were used to study film formation by magnesium alloys AZ31B (Mg-3Al-1Zn base) and ZE10A (Elektron 717, E717: Mg-1Zn + Nd, Zr) in H 2 O and D 2 O with and without 1 or 5 wt% NaCl. No SANS scattering changes were observed after 24 h D 2 O or H 2 O exposures compared with as-received (unreacted) alloy, consistent with relatively dense MgO-base film formation. However, exposure to 5 wt% NaCl resulted in accelerated corrosion, with resultant SANS scattering changes detected. The SANS data indicated both particle and rough surface (internal and external) scattering, but with no preferential size features. The films formed in 5 wt% NaCl consisted of a thin, inner MgO-base layer, and a nano-porous and filamentous Mg(OH) 2 outer region tens of microns thick. Chlorine was detected extending to the inner MgO-base film region, with segregation of select alloying elements also observed in the inner MgO, but not the outer Mg(OH) 2 . Modeling of the SANS data suggested that the outer Mg(OH) 2 films had very high surface areas, consistent with loss of film protectiveness. Implications for the NaCl corrosion mechanism, and the potential utility of SANS for Mg corrosion, are discussed. Magnesium and its alloys are of great interest for a wide range of structural and functional applications, including lightweight automotive and aircraft structural materials, biomedical implants, fuel cells and hydrogen storage, batteries, etc.1-7 However, the high reactivity and rapid corrosion of Mg in many aqueous and humid environments is a key issue. [8][9][10][11] Of particular challenge and interest is the acceleration of Mg corrosion in the presence of salt species. Mechanisms of accelerated attack in salt solutions include enhanced local micro-galvanic coupling at alloy second phases and/or impurities (e.g. Fe, Ni, Cu etc.) and disruption of the MgO and/or Mg(OH) 2 base surface films by Cl-. 3,9,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Film formation by Mg alloys under aqueous conditions can be quite complex, involving MgO, Mg(OH) 2 , and MgCO 3 base phases, as well as complexes of those phases when alloy additions (e.g. Al) or environmental species (e.g. Cl-) become incorporated into parts or all of the film structure. [15][16][17]21,[27][28][29][30][31][32][33][34] In some cases, porous and even filiform-like corrosion films have been reported on bare or surface-modified Mg alloys. 11,14,16,18,24,[35][36][37][38] Small angle neutron scattering (SANS) has emerged as a powerful technique to assess structural and morphological features in the ∼1 to up to ∼200-300 nm feature size range (specific range accessed depends on the instrument capabilities and settings, as well as the nature of neutron scattering from the test samples). 39,40 Information can be obtained regarding size, morphology, and number density of scatterers (e.g. 2 nd phase precipitates, phase separation, grain structure, voids, pores, etc.), as well as surface area a...

An isotopic tracer study of the film growth mechanism for pure magnesium, AZ31B, and ZE10A (Elektron 717, E717) magnesium alloys in water at room temperature was performed. A series of individual and sequential exposures were conducted in both H 2 18 O and D 2 16 O, with isotopic tracer profiles obtained using secondary ion mass spectrometry (SIMS). The water-formed films consisted primarily of partially hydrated MgO. The SIMS sputter depth profiles indicate that H and D penetrated throughout the film and into the underlying metal, particularly for the Zr-and Nd-containing E717 alloy. Film growth for the UHP Mg involved aspects of both metal outward diffusion and oxygen/hydrogen inward diffusion. In contrast, the film on the Al-containing AZ31B alloy grew primarily by inward oxygen and inward hydrogen diffusion. The 18 O and D profiles for the film formed on E717 were the most complex, with the 18 O data most consistent with inward lattice oxygen diffusion, but the D data suggests inward, short-circuit diffusion through the film. It is speculated that preferential inward short circuit hydrogen transport may have been aided by the presence of nano Zn 2 Zr 3 particles throughout the E717 film. Such hydrogen penetration may have implications from both a corrosion resistance and hydrogen storage perspective. Magnesium alloys are of great interest because they can be used to manufacture lightweight automotive and aircraft structural materials that reduce vehicle weight and improve fuel efficiency.1-3 However, the poor corrosion resistance of Mg is a major challenge.4-6 A key contributor to the poor corrosion resistance of Mg is the inability to establish and/or maintain protective surfaces films. Surface films formed on Mg under ambient air and water conditions typically consist of mixtures of Mg(OH) 2 and MgO, with small amounts of MgCO 3 often also reported.7-12 They provide adequate protection under some circumstances, but are particularly vulnerable to disruption by salt species.The ambient corrosion of Mg differs from many corrosion-resistant structural alloy classes in that the protective surface films can become quite thick, on the order of tens to hundreds of nanometers, rather than the few nanometers typically encountered for protective films on stainless steels, for example. As such, corrosion resistance is influenced not only by classical thin film electrochemical passivity considerations, but also thermodynamic and kinetic considerations typically encountered in thick-film (> 0.5 micron range), high-temperature alloy oxidation phenomena.Isotopic tracer studies have been widely applied to studies of the high-temperature oxidation of alloys (Al, Fe, Ni, and Zr base; SiC), with significant new insights gained regarding the growth mechanism of the oxide films. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Such insights have proven particularly useful for understanding the influence of various alloying additions on film growth, and provide a basis for improved alloy design. For examp...