Role of second phases in the corrosion resistance and cerium conversion coating treatment of as-extruded Mg-8Al-4Ca magnesium alloy

The presence of a second phase in the Mg-8Al-4Ca (at. %) alloy plays a significant role on both its corrosion behavior and the chemical conversion coating processes. Using scanning Kelvin probe force microscopy (SKPFM), a lower Volta-potential of the second phase pre-sent on the surface has been measured. The b-Al-Ca phase has a higher electrochemical activity than the a-Mg matrix and may act as the micro-galvanic anode in a local elec-trochemical corrosion process. Transmission electron microscopy (TEM) examinations reveal that the b-Al-Ca phase is more susceptible to corrosion than the a-Mg matrix in an aqueous solution, and its higher activity and higher corrosion rate accelerate the hydrogen evolution rate on the a-Mg matrix in the cerium (Ce) conversion coating process. It's also been discovered that by immersing the bare Mg-Al-Ca alloy in deionized (DI) water, the b-Al-Ca phase, exposed on the surface, can be dissolved and converted in situ into aluminum hydroxide (Al(OH)3), and the Ce conversion coating can be deposited via replacement re-actions in the subsequent conversion coating process. A thicker Ce coating with smaller blisters has then been produced on the DI-treated Ce-coated Mg specimen; it indeed im-proves the corrosion resistance. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).