An XPS investigation of the oxidation/corrosion of melt-spun Mg
Introduction
The low density and high specific stiffness of magnesium-based alloys make them extremely attractive for aerospace and automotive industries where light weight is important. However, despite their competitive mechanical properties, the use of magnesium alloys is not fully realized, largely due to their poor corrosion resistance.
In general, magnesium alloys produced by conventional ingot metallurgy processing exhibit poor corrosion resistance. The application of rapid solidification (RS) processing can result in the refinement of both matrix grains and intermetallic particles, extension of solid solubility, formation of non-equilibrium phases (including crystalline, quasi-crystalline and non-crystalline) and improved chemical homogeneity. RS processing of appropriate magnesium alloys can improve their corrosion resistance by contributing towards the formation of protective films and the elimination of microgalvanic effects. As a result, a number of workers [1], [2], [3] have studied the effects of RS processing and alloy additions on the corrosion resistance of Mg-based alloys.
The resistance of a metal and alloy to attack by aggressive solutions or gases is primarily related to the properties of the surface oxide film. In order to control these properties to optimize corrosion/oxidation resistance in a given material, it is necessary to understand the surface properties of the film. A fundamental study of corrosion-resistant oxides formed on the surface of magnesium alloys has to be based on knowledge of the structure and properties of the oxide on pure magnesium and the manner in which its properties are modified as a result of alloying. However, fewer studies have been done on the structure and morphology of oxides formed naturally on magnesium under various conditions. In this work, the surface chemical species of rapidly solidified pure Mg exposed to air, immersed in distilled water and 3% NaCl solution saturated with Mg(OH)2 were studied by X-ray photoelectron spectroscopy (XPS). The growth mechanisms of oxide surface films formed under these corrosion conditions are discussed.
Section snippets
Experimental
Pure Mg (99.99%) ribbons were obtained by melt spinning using a single melt spinner under argon atmosphere at a wheel speed of 25 m/s. A 3% NaCl solution saturated with magnesium hydroxide was used for corrosion testing. These tests were carried out for various duration in a glass container containing the solution at 25±0.1°C. After corrosion testing, the specimens were briefly washed with acetone and drained of excess solution on tissue paper, and finally air-dried. XPS measurements were
Results and discussion
XPS depth profiling spectra of Mg2p and O1s of the surface film formed in air for 7 years are shown in Fig. 1. There are two main Mg2p component peaks in Fig. 1(a). The lower BE peak at 49.5±0.1 eV with a narrow full width half maximum (FWHM) of ∼0.70 eV is assigned to metallic Mg [5], [6], [7], [8], [9]. The broader higher BE peak is centered between 50.7 and 51.6 eV, indicating the co-existence of Mg oxide, Mg hydroxide or Mg carbonate (demonstrated by C1s peak at BE about 4.5–5.0 eV higher
Conclusions
Our XPS results revealed two distinct oxygen species on the surface films formed in air, distilled water and 3% NaCl solution: one assigned to O2− in MgO, the other to OH− in Mg(OH)2. Depth profiling revealed that the two species had different depth distributions in the films. The films on pure Mg ribbon exposed under different conditions were mixtures of MgO and Mg(OH)2, with Mg(OH)2 formed predominantly at the outermost surface. For the sample exposed to air, excluding a thin monolayer air
Acknowledgements
The assistance of Mr. J.W. Chai from the Institute of Materials Research and Engineering on the VG ESCALAB 220iXL is gratefully acknowledged.
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