Applications of carbonic acid solution for developing conversion coatings on Mg alloy

作者：Yu Bing lung; Lin Jun kai; Uan Jun yen^*

来源：Transactions of Nonferrous Metals Society of China, 2010, 20(7): 1331-1339.

DOI：10.1016/S1003-6326(09)60300-9

• Mg alloy
• AZ91D alloy
• corrosion
• conversion coating
• carbonic acid

Works on exploring an environmentally clean method for producing an Mg,Al-hydrotalcite (Mg(6)Al(2)(OH)(16)CO(3)center dot 4H(2)O) layer and/or calcium carbonate (CaCO(3)) layer on Mg alloy in a carbonic acid solution system (aqueous HCO(3)(-)/CO(3)(2-) or Ca(2+)/HCO(3)(-)) at 50 degrees C were reviewed. Conversion treatment for the Mg,Al-hydrotalcite conversion coating was as follows. Mg alloy was treated first in acidic HCO(3)(-)/CO(3)(2-) aqueous for precursor layer formation on Mg alloy surface and then in alkaline HCO(3)(-)/CO(3)(2-) aqueous to form a crystallized Mg,Al-hydrotalcitc 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 CaCO(3) coating on Mg alloy, the aqueous Ca(2+)/HCO(3)(-) with a saturated Ca(2+) content was employed for developing a CaCO(3) coating on Mg alloy. A dense CaCO(3) coating could yield on Mg alloy surface in 2 h. Corrosion rate (corrosion current density, J(corr)) of the Mg,Al-hydrotalcite-coated sample and CaCO(3)-coated AZ91D sample was 7-10 mu A/cm(2), roughly two orders less than the J(corr) of the as-diecast sample (about 200 mu A/cm(2)). No corrosion spot on the Mg,Al-hydrotalcite-coated sample and CaCO(3)-coated sample was observed after 72 h and 192 h salt spray test, respectively.

• 出版日期2010-7