Understanding Corrosion Behavior of Magnesium Surface by x-Ray Irradiation for Improved Surface Design and Applications
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https://doi.org/10.1007/s11837-020-04403-1 2020 The Minerals, Metals & Materials Society
SURFACE ENGINEERING: APPLICATIONS FOR ADVANCED MANUFACTURING
Understanding Corrosion Behavior of Magnesium Surface by x-Ray Irradiation for Improved Surface Design and Applications KATSUYOSHI KONDOH ,1,4 KEISUKE FUNATSU,2 MAKOTO TAKAHASHI,1 SHUFENG LI,3 FUMITERU AKAMATSU,2 and JUNKO UMEDA1 1.—Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 5670047, Japan. 2.—Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. 3.—School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China. 4.—e-mail: [email protected]
An advanced surface modification processing design has been developed to improve the corrosion resistance of magnesium (Mg) samples via x-ray irradiation in humid air. OHÆ radicals generated by water ionization during x-ray irradiation reacted with natural oxide films mainly consisting of Mg(OH)2, which resulted in the formation of an approximately 50-nm-thick magnesium oxide (MgO) dense surface film as a protection layer against corrosion. After xray irradiation for 24 h, pure Mg and its alloy (AZ91D) specimens underwent the saltwater immersion test and exhibited a remarkable reduction in corrosion rate (80-85%). They were also compared to as-received samples after polishing treatment. This surface modification process is environmentally friendly because it does not release any hazardous materials because of the high recycling ability.
INTRODUCTION Magnesium (Mg) and its alloys are the representative eco-materials that remarkably improve fuel efficiency and reduce CO2 gas emission because of their high specific strength and Young’s modulus when applied to the structural materials used in the automotive and aircraft industries.1,2 It is, however, well known that Mg alloys have poor corrosion resistance; in particular, the galvanic corrosion phenomenon at the interface of dissimilar bonded materials easily occurs by local cell formation.3 This is because pure Mg has a standard electrode potential (SEP) of 2.37 V,4 which is the lowest of the structural metal materials. On the other hand, aluminum (Al) and titanium (Ti), which also have a low SEP of 1.66 V and 1.63 V, respectively,4 show excellent corrosion resistance due to the formation of protective surface layers made of each dense oxide. In Mg alloys, however, the porous (Received June 25, 2020; accepted September 22, 2020)
surface films, consisting of magnesium oxide (MgO), hydrous oxide (Mg(OH)2), and hydride (MgH2), are not protective against water and moisture and result in severe corrosion phenomena.3,5,6 The conventional surface modification using chemical conversion treatment7 and anodic oxide coatings8 is often employed to suppress the surface corrosion and galvanic corrosion phenomena of Mg alloys.3 From the viewpoint of the environmental burden reduction, they are not the best techniques because the hazardous hexavalent chromium element
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