Preparation And Characterization Of Ce-Mn/Mo Converison Coating On6063Aluminum Alloy

The application of aluminum alloys in the field of structural metallic materials is justafter steel because of its good mechanical properties and machining performance. However,wider application of the aluminum alloys is limited by their relatively poor corrosionresistance. Surface protection treatment is an important method to improve the corrosionresistance of aluminum alloy, among which the chemical conversion/spray processingtechnology is the most widely used in industries. Traditional chromate conversion process isrestricted due to its toxicity to both human body and the environment. Therefore, it isimperative to develop an environmentally-friendly alternative conversion process. Rare earthconversion process is considered to be one of the most promising alternate to chromeconversion. Currently, conversion coating can be formed by some rare earth conversionprocess at room temperature rapidly, but its performance is not as good as chromateconversion coating in terms of corrosion resistance, weather resistance and wear resistance.On the basis of Ce-Mn conversion coating system studied by our research group, a newtreatment process in which the passivation solution mainly include Na₂MoO₄was developedin this thesis, this treatment was used just after Ce-Mn conversion. A Ce-Mn/Mo conversioncoating with excellent corrosion resistance, weather resistance and abrasion resistance wasobtained after the treatment. The organization, structure, composition of the coating wasanalysed, and the coating-forming mechanism was discussed.An efficient Ce-Mn conversion process at room temperature was firstly obtained byscreening the previous research results of our group:10g/L Ce（NO₃）₃,2g/L KMnO₄,0.6g/LNaCl, conversion time of9min. Then, the concentration of Na₂MoO₄and the processparameters were optimized by using orthogonal experiment and single-factor experiment tooptimize, the optimal basic process was obtained:15g/L Na₂MoO₄, conversion temperature of45℃and time of20min. In order to make the coating more homogeneous and to furtherimprove its properties, C18H29NaO₃S was selected as an effective additive into the basicpassivation solution. Through optimization of solution composition and processparameters, the optimal conversion process is18g/L Na₂MoO₄,1.5g/L C18H29NaO₃S,45℃and20min. The surface morphology, cross section, composition and valence state of the Ce-Mn/Moconversion coating were analyzed by SEM, BES, EDS, XRD, XPS. The results showed thatthe coating surface was reticular with uniform distribution of micro-cracks, also had smallamount of pits locally. With the conversion time extending, the cracks widened and the pitsdeepened. The coating was amorphous phase and mainly composed of Mo, Ce, O, Mn, Al, inwhich Mo was found in the valence of+6and+4，Ce in the valence of+4,+3, Al in thevalence of+4, O in the form of O₂-and OH-, while Mn mainly in the valence of+4. Thecoating consisted of MoO₃、MoO（OH）2、MoO₂、CeO₂、Ce（OH）₄、Ce（OH）₃、MnO₂、Al₂O₃and Al（OH）₃.It was proved that the binding force between Ce-Mn/Mo composite conversion coatingand substrate achieves the requirements of GB9286-98, the coating has good corrosionresistance, weather resistance and wear resistance.Based on the good corrosion resistance,especially the excellent weather resistance and wear resistance of the coating, the Ce-Mn/Moconversion coating is expected to use as a final protective coaitng in the case of mildenvironment.