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, J_corr) of the Mg,Al-hydrotalcite-coated sample and CaCO₃-coated AZ91D sample was 7−10 μA/cm², roughly two orders less than the J_corr 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