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Research On Preparation And Corrosion Resistance Of Titanate Conversion Coatings On AZ91Magnesium Alloy

Posted on:2014-03-26Degree:MasterType:Thesis
Country:ChinaCandidate:J F GouFull Text:PDF
GTID:2251330422450957Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Chemical conversion coating is a useful method to improve the corrosion resistanceof magnesium alloy, especially the conventional chromate conversion coating which hasbetter results. But the chromate conversion coating contains toxic hexavalent chromium(Cr6+), it is being inhibited by many countries. So, research on the non-chromateconversion coating is an important task of researchers, which can replace the chromateconversion coating. In this article, titanate conversion coating was prepared on the AZ91magnesium alloy by using Ti(SO42and H2TiF6, both of which contains titanium. Theeffect of parameters, which contained concentration of Ti(SO42and H2TiF6,concentration of oxidizers, concentration of accelerator, immersion time, temperatureand pH value of solution, on the morphology, composition and corrosion resistance oftitanate conversion coating were studied. The morphology was observed by SEM, thecomposition was analyzed by EDS and XPS, the corrosion resistance was estimated bypotentiodynamic polarization and electrochemical impedance spectroscopy (EIS).The corrosion resistance results illustrated that corrosion resistance of magensiumalloy was improved by using Ti(SO42and KMnO4to prepare conversion coating, butthis method didn’t prepare titanate conversion coating which had compact morphologyon magnesium alloy. The tianate conversion coating was prepared by using H2O2andTi(SO42on the same magnesium alloy, which was homogeneous and compact.Extending immersion time resulted in large openings on the conversion coating.Increasing quantity of H2O2was bad for the homogeneity of conversion coating, whichresulted large openining. Pieces of films on the conversion coating fell off if thetemperature of solution was increased. If the pH value of solution was increased, theconversion coating was compact, but raction rate was limited and thinkness of coatingwas reduced. Increasing the concentration of Ti(SO42has the similar effect on theconversion coating with increasing immersion time. According to the analysis of EDSand XPS, the conversion coating was composed mostly of titanium (Ti), magnesium(Mg), aluminium (Al), oxygen (O), carbon (C). Ti existed in the form of TiO2and Ti2O3.According to the anticorrosion performance, tiatanate conversion coating improved thecorrosion resistance. The corrosion potential was raised250mV and corrosion currentdensity was decreased by two orders of magnitude.Titanate conversion coating was prepared by using H2TiF6and Na2SiO3on AZ91magnesium alloy, the former provided titanium and the latter was accelerator. Accordingto the analysis of EDS and XPS, the coating was composed mostly of Ti, silicon (Si),fluorine (F), Mg, Al, O. Ti existed in the forms of TiO2and TiF3,Si existed in the formsof silicate and silicon. As the increase of concentration of H2TiF6, the conversioncoating became compact and quantity if Ti was decreased. As the increase of concentration of Na2SiO3, composition of coating changed until the concentration ofNa2SiO3was larger than3g/L, the quantities of elements of conversion coating didn’tchange. As the increase of solution temperature, conversion coating appeared peeling,especially when the temperature was higher than50℃, contents of F in the samplesprocessed changed obviously. This is attributed to the formation of much morecompound containing F when the temperature was increased. Exstending immersiontime, the thickness of coating was increased. When the immersion time extended10min,the composition of coating didn’t change. Thick coating didn’t form when the pH valueof solution was increased. The titanate conversion coating could improve the corrosionresistance of AZ91magnesium alloy according to measurements of potentiodynamicpolarization. The corrosion current density was reduced by two orders of magnitude.
Keywords/Search Tags:titanate conversion coating, AZ91magnesium alloy, morphology, composition, corrosion resistance
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