| Surface protection treatment is an important approach to improve the corrosion resistance of aluminum alloy,and rare earth conversion process is one of the most promising processes to replace chromate conversion one.In order to solve the problem of poor stability of acidic rare earth conversion bath,an alkaline cerium based conversion process for aluminum alloy with good bath stability was developed.The corrosion resistance,surface morphology and microstructure of the conversion coatings were characterized,and the film-forming mechanism of the conversion coatings under alkaline condition was discussed.The Ce coating obtained by alkaline cerium salt conversion treatment was colorless,and the self corrosion current density was reduced to 2/5 of the aluminum alloy substrate.Based on the alkaline cerium salt conversion process,the basic system of alkaline Ce-Mo conversion composed with Ce(NO3)3 and Na2MoO4 was determined.Through orthogonal test and single factor experiments,the optimum process parameters of alkaline Ce-Mo basic system were determined as follows:Ce(NO3)3 4 g/L,Na2MoO4 12 g/L,EDTA-2Na 8 g/L,conversion temperature 70?,p H 8.5,treatment time 20 min.The results showed that BTA is the best additive through comparative experiments and single factor experiments,and the optimal process parameters of optimized alkaline Ce-Mo conversion system were obtained as follows:Ce(NO3)3 4 g/L,Na2MoO4 12 g/L,EDTA-2Na 8 g/L,BTA 1 g/L,conversion temperature 40?,p H 8.7,treatment time 15 min.Polarization curve analysis showed that compared with the self corrosion current density of 10.98?A·cm-2 for aluminum alloy substrate,the self corrosion current density of Ce coating,Ce-Mo coating and Ce-Mo-BTA coating decreased to 4.38?A·cm-2,0.61?A·cm-2 and 0.15?A·cm-2,respectively.The results of electrochemical impedance analysis showed that the impedance arc diameter of the samples after treatment was larger than that of the aluminum substrate,and the impedance modulus value and phase angle of Ce-Mo-BTA coating were the largest,which could provide the most effective protection for aluminum alloy.SEM,EDS,XRD,XPS and FT-IR were used to characterize the composition and structure of the conversion coatings.The results showed that the Ce coating is porous,and the thickness of the coating is about 0.8?m,which is mainly composed of Al2O3,cerium oxides(Ce2O3,CeO2)and cerium hydroxides.Microcracks were distributed on the surface of Ce-Mo coating and Ce-Mo-BTA coating,but the latter was more uniform and dense,and the thickness of the two coatings were about 1.2?m and 1?m respectively,which were mainly composed of amorphous species.Besides the composition of the Ce coating,Ce-Mo coating also contained molybdenum oxides(MoO2,MoO3),and Ce-Mo-BTA coating also contained metal-BTA complex.By analyzing the composition of the coatings and the influence of different treatment time on the conversion coatings,the film-forming mechanism of alkaline cerium based conversion coatings was determined:the dissolution reaction of Al under alkaline condition led to the instability of MoO42-and AlO2-and the formation of corresponding oxides/hydroxides;the rapid decrease of OH-concentration in the liquid layer near the matrix resulted in the dissociation equilibrium of Ce complex being broken and Ce3+was released,which made Ce3+and Ce4+oxidized by MoO42-form cerium hydroxides.With the addition of BTA,the ionized BTA anion reacted with metal ions to form metal-BTA complex adsorbing on the alloy surface. |
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