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Investigation On Preparation And Characterization Of Colored Ti-Zr Conversion Coating On AZ91D Magnesium Alloy

Posted on:2014-08-27Degree:MasterType:Thesis
Country:ChinaCandidate:J WangFull Text:PDF
GTID:2181330422982745Subject:Materials engineering
Abstract/Summary:PDF Full Text Request
Magnesium alloys have been widely used in industrial areas such as the aerospace,automobile, electronic products due to their light weight and high specific strength. However,magnesium is very easy to be corroded because of its low potential, which restricts widerapplication of magnesium alloys. Surface protective treatment of magnesium alloys caneffectively improve their corrosion resistance. As one important method of surface protectivetreatments, chemical conversion/spraying process has been widely used in industries due toits easy operation and low cost. Traditional chromate conversion treatment method has beenlimited since it is harmful to human body and the environment. Ti-Zr conversion treatment isconsidered to be one of the most hopeful methods to replace chromate technics. Based on ourgroup’s previous researches on colored Ti-Zr conversion coating on aluminum alloy, thefeasibility of the Ti-Zr technology on magnesium alloy surface was firstly carried out in thisstudy. Golden conversion coating on magnesium alloy was obtained at room temperature.The processing parameters were optimized by orthorgonal and single factor experiments. Themicrostructures and corrosion resistance were deeply characterized for the colored Ti-Zrconversion coating on AZ91magnesium alloy. The formation mechanism of the coloredcoating was also discussed.The feasibility of the Ti-Zr conversion coating system on magnesium alloy was provedby a series of trying experiments. The optimal compositions and their concentrations of thebasic system of the conversion technic were: H2TiF60.5g/L, H2ZrF61.5g/L, organic acid1.5g/L, pH=2.2~2.5. The conversion time was3min at room temperature. The key factorswhich decided the color of the film were the organic acid and H2TiF6, conversion solutionshould contain these two compotents in order to color the coating. In the optimalfilm-formation time (about3min), the conversion coating was golden and uniform. In theearly film-formation time (<1min), the color of the conversion coating was not significant.With prolonging the conversion time, the color deepened and appeared the hang ashphenomenon.The compositions and microstructures of the conversion coating were characterized by SEM, EDS, XPS and FT-IR methods. The Ti/Zr conversion coating was compact anduniform with some tiny micro-cracks. The conversion coating constitution elements includedMg, C, O, Ti, Zr, etc, and was mainly composed of Mg(OH)2、MgO、TiO2、ZrO2、MgF2andthe organic complex. The color of the conversion coating should be originated from theorganic complex, which was the product of the react between the organic acid hydrolysis andTi4+ion.The corrosion potential of magnesium alloy matrix was-1.53V while the conversioncoating was-0.37V. The corrosion current density for matrix was93.72μA/cm2while theconversion coating was1.074μA/cm2. By EIS test, the total resistance for the conversioncoating was1597Ω·cm2, which was more than7times than that of the matrix (212Ω·cm2).These results indicated that the conversion coating can reduce the corrosion rate ofmagnesium alloy. The corrosion resistance of magnesium alloy could be improved since thecolored Ti/Zr conversion coating can effectively provide anti-corrosion protection.
Keywords/Search Tags:magnesium alloy, chemical conversion treatment, Ti-Zr conversion coating, coloring, corrosion resistance
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