V/W And V/Zr Conversion Coatings On AZ31B Mg Alloy

Magnesium alloys, which have unique characteristics of low density, highstrength-to-weight ratio and fatigue limit, excellent castability，good heat and electricconductivity, and electromagnetic shielding properties，are widely used in aerospace,automobile,3C and other fields. However, the poor corrosion resistance of magnesiumalloys plays a major factor to limit its practical applications. Mg alloys are often treatedby chromate treatment technology which is economic and effective in order to improvetheir anti-corrosion performance. Because of carcinogen, Cr(VI) was limited to use inthe metal surface treatment by the European Union RoHS Directive (Restriction ofcertain hazardous substances in electrical and electronics equipment) as early as2006,and“Heavy metal pollution comprehensive prevention and control12thfive year plan”,the first12thfive year plan, was put forward in2011, giving strict control requirementsto the five heavy metals including hxavalent Cr. Therefore，it is necessary for thedevelopment of an environmentally friendly methods.V/W and V/Zr composite conversion coatings were prepared on AZ31Bmagnesium alloy. The optimum processing conditions of two conversion coatings wereinvestigated by neutral salt spray test. The performance of the coatings wascharacterized and analyzed by scanning electron microscope (SEM) and energy dispersespectroscopy (EDS) and polarization curve. The main results obtained were as follows:(1) The V/W composite conversion coating was obtained under the condition ofNH4VO32.4g/L, Na2WO4of0.6g/L,20mins,50℃and pH2.2. The salt spray time ofthe coating was up to60h. The conversion coating, which was an amorphous structureand consisted of53.68%Mg,4.6%Al,0.77%Zn,24.22%O,2.09%F,3.36%W and 10.68%V, made the corrosion potential of Mg alloy increase273mV and the corrosioncurrent density decrease about6times, the corrosion resistance of Mg alloy was greatlyimproved.(2) The V/Zr composite conversion coating was obtained under the condition ofNH4VO32g/L, K2ZrF62g/L,15mins,60℃and pH2.5. The salt spray time of thecoating was up to80h. The conversion coating,, which was an amorphous structure andconsisted of70.54%Mg,1.88%Al,0.77%Zn,10.63%O,10.36%F,4.75%Zr and1.05%V, made the corrosion potential of Mg alloy increase269mV and the corrosioncurrent density decrease about8.5times, the corrosion resistance of Mg alloy wasgreatly improved.(3) The salt spray time of V/W and V/Zr composite conversion coatings waslonger than present8h industrial application standard. The corrosion resistance of V/Zrcomposite conversion coatings was better than that of V/W composite conversioncoatings.