Preparation And Characterization Of Ce-Mn Conversion Coating On Aluminum Alloy With Cl~-as Promoting Agents

Rare-earth conversion coating is one of the most promising green alternatives ofsurface-treatment techniques of aluminum alloy.Currently most rare-earth metals conversioncoating technologies are complicated in the process, time consuming and elevatedtemperature is needed. Although some technologies can effectively increase the coatingformation rate at room temperature by adding the accelerant, but the stability of passivationsolution is poor. These defects which passivation solution can easily become ineffective andthe coating formation speed was negatively correlated with the stability of the passivationsolution limit its large-scale applications. The research in this thesis was based on preliminarystudies of the Ce-Mn conversion coating system, Cl^-was selected as an additive into theRare-earth passivation solution, and a high efficient conversion coating technology based onrare-earth metals which can effectively keep the passivation solution of the stability wasobtained. The organization, structure, the detailed mechanism of coating formation and thestability of passivation solution are discussed.The conversion coating technology was developed by adding Cl^-in the Ce-Mnpassivation as promoting agents. The treatment time was9min.The optimal pre-treatment forthe Ce-Mn passivation after adding Cl^-was developed:3min soaking in home made mixedacid solution at room temperature, followed by1min soaking in another home made mixedacid solution.The results of the study show that The conversion coating was more compact afteradding Cl^-in the Ce-Mn conversion bath solution and Cl^-can greatly shorten the formationtime and obviously improve corrosion resistance of aluminum alloy, Cl^-is a good accelerantfor Ce-Mn conversion coating technologies. Scanning electron microscopy （SEM）, X-raydiffraction （XRD）, energy dispersive spectrometry （EDS） and X-ray photo-electronspectroscopy （XPS） were used for the conversion coating characterization. The results showthat the Conversion coating was amorphous,and mainly composed of Ce、Mn、Al and Oelements, the Cl^-in the passivation solution can not change the composition of the conversioncoating. Ce was found in the coating with the valence state of+3and+4,while the valencestate of Mn was+4.The comprehensive experimental results show that the conversion coatingmainly composed of CeO₂、MnO₂、Ce（OH）₃、Ce（OH）₄and Al₂O₃.The conversion coating forming process is a dynamic balance process between corrosionand growth of the conversion coating. Cl^-has strong corrosive to the aluminum alloy matrix.It can accelerate the dissolution of mic-anode aluminum, and accelerate the exposure of the cathode point on the surface of aluminum alloy to accelerate the conversion coating formationprocess. The presence of Cl^-also can hinder the secondary reactions in the passivationsolution, which are not conductive to the growth of conversion coating. Observed the stabilityof the passivation solution, there was no sediment within12days. The stability of thepassivation solution is much better than which selected F-or HF₂^-as accelerant. However, Cl^-is less effective than F-and HF₂^-on promoting the growth of Ce-Mn conversion coating, thereason is that F-can have chemical reaction with Ce³⁺directly but Cl^-can not.