Transactions of Nonferrous Metals Society of China The Chinese Journal of Nonferrous Metals

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Transactions of Nonferrous Metals Society of China

Vol. 20    No. 7    July 2010

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Applications of carbonic acid solution for developingconversion coatings on Mg alloy
YU Bing-lung(余秉隆), LIN Jun-kai(林俊凱), UAN Jun-yen(汪俊延)

Department of Materials Science and Engineering, National Chung Hsing University,
250 kuo-kuang Rd., Taichung 402, Taiwan, China

Abstract:Works on exploring an environmentally clean method for producing an Mg,Al-hydrotalcite (Mg6Al2(OH)16CO3·4H2O) layer and/or calcium carbonate (CaCO3) layer on Mg alloy in a carbonic acid solution system (aqueous HCO3/CO32− or Ca2+/HCO3) at 50 ˚C were reviewed. Conversion treatment for the Mg,Al-hydrotalcite conversion coating was as follows. Mg alloy was treated  first in acidic HCO3/CO32− aqueous for precursor layer formation on Mg alloy surface and then in alkaline HCO3/CO32− aqueous to form a crystallized Mg,Al-hydrotalcite coating. Duration of an Mg,Al-hydrotalcite coating on Mg alloy surface was reduced from 12 h to 4 h by the conversion treatment. On the other hand, for reducing the formation time of CaCO3 coating on Mg alloy, the aqueous Ca2+/HCO3 with a saturated Ca2+ content was employed for developing a CaCO3 coating on Mg alloy. A dense CaCO3 coating could yield on Mg alloy surface in 2 h. Corrosion rate (corrosion current density, Jcorr) of the Mg,Al-hydrotalcite-coated sample and CaCO3-coated AZ91D sample was 7−10 μA/cm2, roughly two orders less than the Jcorr of the as-diecast sample (about 200 μA/cm2). No corrosion spot on the Mg,Al-hydrotalcite-coated sample and CaCO3-coated sample was observed after 72 h and 192 h salt spray test, respectively.


Key words: Mg alloy; AZ91D alloy; corrosion; conversion coating; carbonic acid

ISSN 1004-0609
CN 43-1238/TG

ISSN 1003-6326
CN 43-1239/TG

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