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

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 ℃ 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.