Research On Preparation And Performance Of 6063 Al Alloy Chemical Conversion Coatings Based On Bath Containing Phosphoric Acid And Titanium-tungstate

As a light-weight structural material,aluminum alloy is the most widely applied metal material in non-ferrous metals.It has has established an unshakable position in the fields of electronics,machinery manufacturing,aerospace and so on.However,the Al alloy is easily oxidized to form an oxide film,The film has poor corrosion resistance under severe conditions,which easily cause corrosion invalidation of the equipment and breed safety accidents.Therefore,a layer of chemical conversion film is often prepared on the surface of the aluminum alloy to enhance its corrosion resistance.The application of hexavalent chromium chemical conversion has been very mature,but due to the extremely toxic use of Cr⁶⁺,therefore,the development of a new environmentally friendly chromium-free chemical conversion technology has become a hot issue in this field.In this paper,the 6063 Al alloy as the research object,the combination of phosphating and titanium to prepare a new composite system conversion film.On the basis of this,in order to further improve its corrosion resistance,systematically studied the molybdenum sodium sealing post-processing technology.Main works and conlusions are as follows:1.Chemical conversion solution effective components selection,formula and process conditions optimization.Choose the macroscopic morphology of the conversion film,the instillation time of copper sulfate,and the film surface density as the main technical indicators.By selecting the effective components of the chemical conversion solution and the influence of the effective components on conversion effect was studied to get the optimal formula and process conditions for the chemical conversion solution are:H₃PO₄ 10mL/L,accelerator 3.5g/L,K₂TiF₆ 1.1g/L,Na₂WO₄ 1g/L,complexing agent 1.5g/L,pH=2.40,solution temperature 40℃and treatment time 10 min.2.Sealing Treatment of Chemical Conversion Containing Phosphoric Acid and Titanium-tungstate.Based on the best conversion technology,in order to further improve the corrosion resistance of the coating,the effects of silicate and molybdate systems on the sealing treatment of conversion coatings were studied.The results indicate that the optimal conditions of sealing process was:sodium molybdate 14g/L,solution temperature 50°C and treatment time 30 min.After the sealing treatment,the conversion coating obtained was uniform,close binding force,excellent corrosion resistance and golden brown.3.Typical sample performance analysis The typical sample obtained typical conversion sample,typical closed sample,Alodine sample and blank sample were analyzed by SEM,EDS,immersion corrosion test,copper sulfate drop test,potentiometric polarization curve（PDP）test.SEM analysis results show that the surface of the typical conversion sample is completely covered by the Ti-W conversion coating,and a large number of shallow pits are uniformly distributed on the surface.The microstructure is"honeycomb",and pores exist between the coating layers.After the sealing treatment with sodium molybdate,the SEM showed that a new coating is formed at the crevice of the existing coating layer.There are a lot of micro-cracks on the surface of the new coating.The results of immersion corrosion test and drop test indicate that the corrosion resistance of the conversion coating is typically closed sample>Alodine sample≥typical conversion sample>blank sample.The PDP test results show that the E_corr of typical conversion samples,Alodine samples and typical closed samples are-0.668V,-0.612V and-0.425V,respectively,which are shifted by 303mV,359mV,and 546mV compared to the blank samples;i_corr of typical conversion samples,Alodine samples and typical closed samples are 4.02×10^-3μA/cm²,1.26×10^-3μA/cm² and 0.691×10^-3μA/cm²,three orders of magnitude lower than that of Al contest sample.Based on the above results,it can be seen that the typical conversion sample can provide Al alloy products with corrosion protection during the transition period,the typical closed sample can be used as a final protective layer,which can provide medium and long-term corrosion protection.