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INTRODUCTION

METALLIC Mg is mainly used as an additive in Al alloys, a desulfurizing agent in steel production, and a reducing agent in Ti metal production.[1,2] Further, Mg-based alloys (such as Mg-Al-Zn alloy) are excellent lightweight structural materials and are used to fabricate products such as automotive parts and laptop casings. Global Mg production has been increasing, mainly owing to the increase in Mg production in China. The global production of primary Mg grew from approximately 500,000 ton in 2002 to as high as 900,000 ton in 2015.[2] The global market for Al alloy and die-cast Mg alloy is predicted to grow steadily. The amount of metallic Mg used in Ti smelting would also increase because of the increasing demand for Ti alloys in the aircraft industry. Thus, the demand for Mg and its alloys is expected to increase steadily in the future. As shown in Figure 1, metallic Fe does not form intermetallic compounds with Mg.[3] The solubility of Fe in Mg is small (< 1 mass pct) below the boiling temperature of Mg [1364 K (1091 °C)] under atmospheric

YU-KI TANINOUCHI, KATSUHIRO NOSE, and TORU H. OKABE are with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Contact e-mail: [email protected] Manuscript submitted March 12, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS B

pressure. Therefore, steel vessels are often used for melting and casting Mg and its alloys. In Mg production by molten salt electrolysis, steel materials are sometimes used as a cathode onto which liquid Mg is precipitated.[4] Furthermore, in Mg and Ti smelters, steel containers and pipes are often used for storage and transportation of liquid Mg.[4,5] Although the solubility of Fe in liquid Mg is relatively low, the dissolution of Fe from steel materials during melting and casting contaminates Mg, which can degrade some of its material properties and be problematic for some applications. For example, Fe contamination often deteriorates the corrosion resistance of Mg and its alloys.[1,6] The corrosion resistance of as-cast pure Mg decreases drastically when Fe contamination exceeds 170 mass ppm as a result of galvanic corrosion between the precipitated Fe particles and Mg matrix. In Mg alloys employed for structural applications, the Fe concentration is usually regulated to less than 50 mass ppm to ensure sufficient corrosion resistance.[1,7] In order to reduce the dissolved Fe content, Mn is often added to liquid Mg-Al alloy; the Mn combines with Fe to form Fe-Mn-Al intermetallic compounds, which are removed from the melt as precipitates. In Ti smelting based on the Kroll process, metallic Mg is used as a reducing agent and Fe impurities in the Mg deteriorate the quality of the produced Ti. In Ti smelters, metallic Ti (Ti sponge) is produced by reacting TiCl4 with liquid Mg in a steel vessel at around 1173 K (900 °C).[4,5] During this reduction step, Fe and other

Fe content, CFe (mass%) 0

1

2

3

4

1700 0.69

1600

Liquid

1629 K

L + δ-Fe

Temperature, T / K

1500

L