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

• Published in: Transactions of Nonferrous Metals Society of China
• Main Authors: Yu, B.L., 汪俊延, Lin, J.K., Uan, J.Y.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2010
• Subjects: Mg alloy; AZ91D alloy; corrosion; conversion coating; carbonic acid; az91d magnesium alloy; corrosion-resistance; formation mechanism; zn; alloy; film; protection; performance; stannate; cerium; layer; Carbonic acid
• Online Access: http://hdl.handle.net/11455/43489; https://doi.org/10.1016/s1003-6326(09)60300-9

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.