Study On Surface Ceramization Technology Of Aluminum Alloy And Microstructure And Properties Of Ceramic Film

Aluminum is widely distributed in the earth’s crust,mostly in the form of compounds(oxides,hydroxides,bauxite,etc.),rarely in the form of elemental aluminum,and abundant in reserves.Its elemental content in the earth’s crust is second only to Oxygen and silicon account for about 7.45%,and the content ranks first among all metal elements.Aluminum and aluminum alloys have become widely used industrial metal materials because of their low density,high specific strength,good corrosion resistance,easy alloying,good electrical conductivity,easy heat treatment,high thermal conductivity,plasticity and ductility.It is widely used in aerospace,transportation,construction and machinery industries.However,in the natural environment,aluminum alloys are highly susceptible to corrosion.In order to effectively protect the aluminum alloy,the chromate passivation treatment is currently widely used in the industry.However,hexavalent chromium is toxic and carcinogenic,and the discharge of waste liquid seriously pollutes the environment,so it is necessary to develop a surface treatment process that can replace chromate passivation.pottery treatment process includes zirconium,titanium,organosilane,etc.,and is considered to be the most promising green treatment process to replace chromate passivation.In this paper,6061 aluminum alloy was used as the main research object,and hexafluorozirconic acid was selected as the main film-forming agent.The complexing agent and oxidant were used as additives.The effects of various factors on the corrosion resistance of ceramic film were studied by orthogonal experiment and single factor experiment.Finally,the best potting process at room temperature was determined: hexafluorozirconic acid 2.0 g/L,complexing agent 1.0 g/ L,oxidant 3.0 g / L,pH 5,treatment time 3 min.The corrosion resistance,surface morphology and composition of the ceramic film were studied by means of electrochemical workstation,scanning electron microscope(SEM),copper sulfate drip corrosion test and energy dispersive spectroscopy(EDS).The results show that the obtained ceramic film is light yellow with a thickness of 2.74 μm,which is about10.5 times of the thickness of the additive-free film.There are many particles with a diameterof more than 1 μm distributed on the upper surface.These larger particles contain mostly M-based compounds,followed by zirconium compounds.The composition distribution of the film was characterized.The results show that the distribution of oxygen and M elements is very uneven,and the distribution is the most dense in large particles.The distribution of zirconium and carbon in the film is relatively uniform.The obtained ceramic film was subjected to a 24 h salt spray test,and the corrosion surface was less than 5%.The self-corrosion current density(icorr)was reduced by 3 orders of magnitude compared to the untreated specimen,and the polarization resistance(Rp)was 744 times that of the untreated specimen.In this paper,the effects of various factors on the corrosion resistance,surface morphology and composition of ceramic films were investigated.The results show:1)The color of the membrane is pale yellow,which is mainly due to the presence of oxidants in the conversion liquid,and the addition of a complex agent can promote the growth of the membrane and make the membrane more dense.2)The thickness of the ceramic film increases with the concentration of hexafluorozirconium acid,but when the concentration of hexafluorozirconium acid is greater than 2 g/L,cracks begin to appear around the large particles on the surface of the ceramic film,and with the concentration of hexafluorozirconate increases,the cracks extend and widen.When the concentration of hexafluorozirconic acid ranges from 1 to 2 g/L,there are very few cracks around the larger particles in the membrane layer and the membrane layer is dense.3)When the concentration of the complexing agent is greater than 1.5 g/L or the treatment time is 4 min,pits left by the falling off of larger particles appear on the film layer.This may be because as the film layer grows,the stress of the larger particles and the surrounding film layer is greater,causing cracks to occur around it.As the film continues to grow,the cracks expand and eventually cause the particles to fall off.4)The pH value of the conversion solution has a great influence on the film formation.When the pH value is less than 4,the trace of corrosion of the film layer can be clearly observed;when the pH value is greater than 6,the precipitation starts to be formed in the conversion liquid,and the obtained The film layer is very uneven,and the content of zirconium element and M element is very low,so the optimum film forming pH range is 4-5.