Study On The Chrome-free Anticorrosion Conversion Coating Of Mg-Li Alloys

As the lightest metal materials, Mg-Li alloys have a series of advantages. However, thepoor corrosion resistance restricts their further development and extensive applications.Chemical conversion is a practical and effective way in surface treatment, and the traditionaltoxic chromate conversion will be replaced by other environmentally friendly methods. In thisthesis, molybdate and tungstate were used as bases to form the conversion coatings on thesuiface of LAZ532Mg-Li alloy, respectively. The microstructure, thickness and compositionof films were measured with SEM, EDS and XRD. The binding force was judged by thermalshock test. The corrosion resistance was analyzed with potentiodynamic polarization andelectrochemical impedance spectroscopy.The optimum processing technical parameters of Mo-based conversion coating on theMg-Li alloy were optimized by orthogonal experiments, as follows: solution concentration is35g/L, treating temperature is25℃, treating time is10min and solution pH is4. The orderof the degree of various factors impacting on corrosion current density is: treating time>solution concentration> solution pH> treating temperature. The taupe molybdate conversioncoating was crackle-like morphology, which was made of oxide and undissolving salts. Theresults of thermal shock test and corrosion experiments indicated that molybdate conversioncoating had a general binding force with matrix and it could improve the corrosion resistanceof LAZ532Mg-Li alloy.The optimum processing technical parameters of W-based conversion coating on theLAZ532alloy were optimized by orthogonal experiments, as follows: solution concentrationis35g/L, treating temperature is25℃, treating time is15min and solution pH is3. Theorder of the degree of various factors impacting on corrosion current density is: solutionconcentration> treating time> treating temperature> solution pH. The rufous tungstateconversion coating which was made of oxide and undissolving salts as the same as molybdateconversion coating was nest-pit-like morphology. The results of coating properties andelectrochemistry tests revealed that tungstate conversion coating not only had a excellentbinding force with matrix, but also could provide LAZ532Mg-Li alloy with remarkableprotection from corrosion.