Investigation On Densification Technology And Electrochemical Properties Of Cerium-based Conversion Coating On Magnesium Alloy

Due to the unique properties, magnesium is becoming the third-largest metalengineering materials after steel and aluminum, knowm as the “21st Century GreenEngineering”. However, the application of magnesium alloys has been limited due totheir poor corrosion resistance. Traditional chemical conversion treatments, which areusurally carried on in the acidity or Cr（Ⅵ）-based solutions, are harmful to human andenvironment. Recently, the employment of rare earth elements, particularly cerium,has been the focus in anticorrosion coating design because of low dosage, nontoxicity,friendliness, and high efficiency. The rare earth conversion coatings have excellentpotential for use in protective coatings. Nevertheless, almost all the rare earthconversion coatings have a common problem, that is, severe cracking tends to occurand thus leading to poor coating adhesion to the metallic substrate. Research hasshown that the cracks in films are produced due to the removal of crystal water in thefilms after deposition. In several reports,the corrosion protection of Ce-basedconversion coating was improved by the use of a phosphate densification process,which leads to a reduction in surface cracking.In this paper, cerium-based conversion coating has been obtained on AZ31magnesium alloy. Scanning electron microscopy （SEM） indicates that there are lots ofcrackings, which are wide and deep. SEM also reveals a two-layered structurecomprising of compact and fibrous layers sequentially formed on top of themagnesium plate. The results of the EDS and XPS indicate that the surface of thecoating was made up of Ce, O and Mg, and Ce exists with two states comprising ofCe（Ⅲ） and Ce（Ⅳ）, in which Ce（Ⅳ）accounts for85%and mainly made up of CeO₂and CeO·2H₂O（i.e.Ce（OH）₄）. The protective performance of conversion coating wasevaluated by potentiodynamic polarization and EIS measurements, and the results indicate that conversion coating improves the corrosion properties with limited extent.After the formation of the film, some cracks induced by dehydration of the coatingand the different adhesion between the two layers will appear.A phosphate densification process for Ce-based conversion coating with apurpose of reduction in surface cracks was studied. SEM results indicate that thenumber and wide of cracks obviously diminish after densification process, and thethickness decreases. The results of the EDS and XPS indicate that the surface of thecoating was made up of Ce, O, Mg and P, and the proportion of Ce（Ⅳ）decreases to60%. Stable cerium phosphate CePO₄is formed on the surface. The potentiodynamicpolarization and EIS results reveals that the anti-corrosion property of conversioncoating with densification process is far better than that without that. Process activeduring densification process are strongly influenced by time and temperature.The phosphate densification process for cerium-based conversion coating onmagnesium alloy experienced three stages:（1） the magnesium matrix as anodedissolved and Mg₃（PO₄）2produced;（2） the cerium oxidation state altered from Ce4+toCe3+, and the hydrated CeO₂and Ce（OH）₄species present in the coating changed tostable CePO₄;（3） a new, compact coating formed with the growing of CePO₄. Theformation of Mg₃（PO₄）2and CePO₄in which the magnesium substate exposed, andthe content of crystal water decreases greatly, leading to improvement of the filmperformance with less cracks. The improvement of the coating reatrains thepermeation of corrosion product and aggressive medium, such as Cl^-, H₂O and O₂.Equivalent circuits and associated physical representations reveal that theimprovement of the electrochemical properties of the conversion coating are mainlycontribute to a combination of increasing resistance and capacitance of the coatingafter densification process.