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Project D1: Surface properties of Mg alloys |
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| D1.1: Surface properties of Mg alloys |
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Key researchers: A. Kanta, T. Abbott (AMT), B.C. Muddle |
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| Summary |
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| Pure magnesium (Mg) ignites in the molten state when exposed to air. However, research in this project has demonstrated that additions of yttrium (Y), beryllium (Be), and calcium (Ca) suppress the oxidation of molten Mg, and efforts are continuing to elucidate how these alloying elements alter the surface behaviour of magnesium. |
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| Yttrium accumulates on the free surface of as-cast magnesium in the form of Y2O3, and this research shows that low Y concentrations are sufficient to completely replace MgO films. Minor alterations in the Mg-Y alloy composition cause major modifications of the free surface of Mg-Y alloys. In the case of BeO and CaO large enrichments are not detected. Beryllium, and in particular calcium, appear to lend more protection to the molten magnesium than what is expected from thermodynamic considerations. It seems that Ca modifies the properties of MgO films instead of replacing them – Ca additions do not have any effect on the surface composition despite clear effects on melt protection. |
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| This observation has been confirmed by the addition of Y2O3 and CaO powders into the melt – Y2O3 does not protect molten Mg but CaO lends remarkable protective properties to Mg alloys even at very low concentrations. Initial work demonstrates that it is possible to mitigate the ignition of Mg alloys. Small concentrations of alloying elements have a significant influence on the nature of surface films on the free surface of as-cast magnesium alloys. |
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| In order to understand the mechanism behind the melt protection of molten Mg by alloying, the free surfaces of as-cast Mg alloys (Mg-Y, Mg-Be, and Mg-Ca) have been probed using a combination of X-ray photoelectron spectroscopy depth profiling, secondary ion mass spectroscopy, and transmission electron microscopy. Comparable concentrations of the alloying elements have been investigated, and the work is focussed on the near-surface regions of the as-cast surfaces (approx. 10 nm). |
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| The exposed surfaces of as-cast Mg-Y alloys, with varying Y additions, are strongly affected by Y alloying. Y concentrations are enhanced in the immediate vicinity of the free surfaces. Lower Y concentrations result in the coexistence of MgO and Y2O3 in the near-surface region; higher Y concentrations lead to the formation of a robust layer of Y2O3 and a Mg-depleted sub-surface region. Minor alterations in the alloy composition thus cause major modifications of the free surface of the Mg-Y alloys. Additional experiments showed good correlation between observed behaviour and thermodynamic predictions for other rare earths, namely La, Ce, and Nd. |
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| The surface composition and thus properties of as-cast Mg-1Al-Be alloys are also affected by small concentrations of Be (Al was used as part of a master alloy). Beyond a threshold composition, such surfaces are Be-enriched relative to the nominal bulk composition. Alloying of magnesium with Be decreases the thickness of MgO surface films formed on the as-cast surfaces, and increasing Be content promotes the formation of BeO at the expense of MgO within these films. There is a perfect agreement between our XPS depth profiling data and TEM cross-sectional profiles. Surface films formed in the presence of Be are less permeable to dissociated water. |
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| The exposed surfaces of as-cast Mg-Ca alloys, with varying Ca additions, differ substantially from the surfaces of Mg-Y and Mg-Be alloys. The addition of Ca imparts protective properties to the surface of Mg by suppressing ignition of the metal, but no Ca enrichment is detected in the near-surface region of the as-cast Mg-Ca alloys for comparable concentrations. The near-surface region of the free surfaces of as-cast Mg-Ca alloys predominantly consists of MgO films, and the thickness of these MgO films does not depend on Ca alloying. This is in stark contrast with comparable concentrations of Y and Be whose preferential oxidation leads to substantial surface enrichments. As CaO appears thermodynamically stabilised with respect to MgO, a different factor seems to diminish the potency of Ca. A current hypothesis is that the effect of Ca is reduced due to the low solubility of CaO in MgO. However, additional work is required to confirm this hypothesis. |
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| All three elements tested limit the susceptibility of Mg melts to ignition but the mechanism through which this is achieved appears different for each case. Our investigation shows that surface films formed during casting and solidification play a vital role in controlling the susceptibility to ignition, and that it is possible to reproducibly control the near-surface region of the free surfaces of as-cast Mg alloys. This is vital for optimisation of both the casting process and the concentrations of alloying elements required for constructive modifications. |
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| As part of the effort to improve understanding of surface behaviour modification of Mg alloys, the group is developing a furnace capable of melting Mg alloys under carefully controlled environments. This equipment is currently being built and is expected to be fully operational by the end of March 2008. The furnace will prove vital for preparation of as-cast Mg alloys under well-defined conditions. |
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| D1.2: High throughput experimentation to define variable space in Zinc metal degradation for multi-scale modelling |
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Key researchers: N. Shahzad, I. Cole, B.C. Muddle |
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| Summary |
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| The PhD project of Mrs Nasia Shahzad is collaboration between the ARC Centre of Excellence for Design in Light Metals and CSIRO Melbourne, and is part of a broader international investigation into corrosion of light metals and alloys. The main objective of the project is to provide quantitative correlation between the properties of atmospheric aerosols and corrosion of zinc. The focus is on elucidation of the effect of droplet size, pH, and aerosol composition on atmospheric corrosion. |
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| Technical Report |
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| The major goal is to quantify the effect of droplet size on the atmospheric corrosion of zinc surface. The specific objectives include elucidation of the secondary spreading phenomenon of sea water droplets as a function of droplet size, droplet composition, and surface conditions. In addition, we will: |
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Develop a consistent method to place coarse and fine droplets on the zinc surface. |
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Analyse the depth and chemistry of damage caused by droplets of various sizes and compositions. |
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Establish a relationship between chemical mass loss and spectroscopic data after removal of corrosion products caused by coarse and fine droplets. |
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Design an experiment matrix to find correlation between relative humidity, temperature, pH, and concentration of major sea salts (NaCl, Na2SO4, MgCl2, MgSO4). |
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| Progress to date |
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| A comprehensive literature review has been compiled to understand the current state of research. A basic standardization of the zinc surface was done in order to obtain a consistent interface for subsequent corrosion experiments. Preliminary results have been obtained for coarse-sized droplets to demonstrate feasibility of this project and obtain a reference point for micro-droplets. It has been found that secondary spreading is a function of droplet size and surface conditions. Corrosion products built under the coarse-sized droplets have been characterized by Raman and Infrared spectroscopy as well as EDS-SEM microscopy. Samples have been prepared by a Focused Ion Beam miller to obtain cross-sectional profiles, and secondary electron images have been acquired to analyse corrosion products are a function of depth. In brief, the feasibility of the project has been successfully demonstrated for the coarse-sized droplets. |
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