| Co-Investigator(Kenkyū-buntansha) |
FUKUMOTO Shinji University of Hyogo, Graduate School of Engineering, Research Associate, 助手 (60275310)
YAMAMOTO Atsushi University of Hyogo, Graduate School of Engineering, Professor, 教授 (70220449)
IWASAK Hajime Previously, Himeji Institute of Technology, Graduate School of Engineering, Professor, 教授 (80047589)
INOUE Hiroyuki Osaka Prefecture University, Division of Engineering, Graduate School, 大学院・工学研究科, 講師 (40203252)
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| Research Abstract |
Surface modification of magnesium alloys has been attempted in order to improve corrosion resistance in magnesium alloys by coating the surface with high purity magnesium applying vapor deposition technique. Principle of the technique is very simple ; pure magnesium with 3N grade or magnesium alloys are heated at about 973 K in vacuum with about 10^<-3> Pa, magnesium with high vapor pressure can be vaporized under the conditions, while heavy metal impurities such as Fe, Ni and Cu, which have deteriorative effects on corrosion resistance in magnesium alloys, can not be vaporized because of their low vapor pressure, purified magnesium vapor depositis on magnesium alloy substrate set at lower temperature region. Salt immersion tests have revealed that corrosion on the deposition coated specimen is decreased about one tenth of that on the un-coated specimen.
In order to clarify the mechanism for improvement in corrosion resistance, in-situ laser microscopic observations were carried out. It
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has been shown that "filiform corrosion" occurred on AZ31 magnesium alloy and pure magnesium with 3N grade, while the filiform corrosion was never observed on high purity magnesium, 6N-Mg, and on the deposition coated specimen. Purification effect intrinsically included in vaporization-deposition process can improve the corrosion resistance in magnesium alloys.
Specimens with small sizes, 10×10×6mm, were used in the preliminary experiments. large size specimens, 60×100×6mm, were attempted to be used for practical usage, and conditions for fabricating defect-less coating were optimized. Hot pressing and HIP treatment were also tried in order to improve adhesion strength. between the coated film and substrates. Corrosion resistance was also improved by hot pressing and HIP treatment.
On the other hand, analyses of the corrosion products lead to another study for improving corrosion resistance. It has been shown that magnesium hydroxide formed by the corrosion reactions is easily changed into magnesium oxide by heating in air. That is, when the specimen is heated in air at about 673 K after immersion in salt solution with pH value of about 12, surface of the specimen was covered by magnesium oxide which is expected to show superior corrosion resistance. Indeed, time for occurring filiform corrosion on the specimen treated by the "artificial corrosion-oxidation method" is prolonged by a factor of twenty in comparison with that on the un-treated specimen.
The deposition coated specimens were provided for the "Solid recycling group" in the same Scientific Research on Priority Areas, and superior property for recycle ability on the deposition coated specimen has been shown. Less
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