| Co-Investigator(Kenkyū-buntansha) |
YAMAMOTO Atsushi University of Hyogo, Graduate School of Engineering, Associate Professor (70220449)
FUKUMOTO Shinji University of Hyogo, Graduate School of Engineering, Associate Professor (60275310)
INOUE Hiroyuki Osaka Prefectural University, Graduate School of Engineering, Assistant Professor, 大学院, 講師 (40203252)
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| Budget Amount *help |
¥15,300,000 (Direct Cost: ¥15,300,000)
Fiscal Year 2005: ¥2,500,000 (Direct Cost: ¥2,500,000)
Fiscal Year 2004: ¥3,000,000 (Direct Cost: ¥3,000,000)
Fiscal Year 2003: ¥9,800,000 (Direct Cost: ¥9,800,000)
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| Research Abstract |
Three kinds of new technique for surface modification on magnesium and its alloys have been developed. In the first technique, magnesium hydroxide formed in corrosion reaction on magnesium alloys is noticed. The hydroxide is uniformly formed to coat the corroded surface in the early stage of corrosion reactions, and shows a relatively weak corrosion resistance. However, the hydroxide film can not suppress the subsequent corrosion reaction, filiform corrosion. Glancing the chemical formula of the magnesium hydroxide, Mg(OH)_2, leads to the dehydration reaction to form magnesium oxide, MgO. By immersing the specimens of magnesium alloys into high pH solutions, stable films of magnesium hydroxide were formed on the specimens, artificial corrosion treatment, and then the specimens were heated in air at 673K, oxidation treatment. When the magnesium alloys are directly heated in air without artificial corrosion, brittle and porous oxide films are formed, in contrast to the reaction, when the
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specimens are heated after the artificial corrosion, the specimens are coated with stable oxide surface films. The oxide film can suppress the filiform corrosion in salt immersion tests, for example, in the case of AZ31 alloy, filiform corrosion occurred with duration time of about 30ks, while it occurred after only 1.7ks on the non-treated specimen. Evolution of hydrogen bubbles which show the corrosion reaction of magnesium is completely suppressed in the early stage of the salt immersion tests. In the second technique, electrochemical method is adopted to deposit the hydroxide films, instead of immersion. Oxidation treatment is the same, heating in air. Immersion tests using a 3% NaCl solution showed that the corrosion rate of the specimen of AZ31 alloy surface-treated by this technique was 4.2mg/cm^2/d, while that on the specimen treated by the first technique was 8.6mg/cm^2/d. The third technique is the fluoride treatment. Molten salt of NaBF_4 was used. Specimens were immersed into the molten salt at 673K for few tens ks. The resultant surface film was composed of MgF_2 and NaMgF_3 with about 10μm in thickness. The duration time for suppressing the filiform corrosion in 1% NaCl solution was prolonged to 1296ks (15 day) in the case of the specimen of AZ31 alloy by applying the third technique. Moreover, the treated specimens showed corrosion resistance even in the acid solutions such as 1% HCl and 1% HNO_3 solutions for 20 to 30ks. All these three techniques would not deteriorate the recycle ability in magnesium alloys. Less
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