Research on Anodic Oxidation and Electrolytic Coloring Processingof Al-12.7Si-0.7Mg Alloy and Its Corrosion Resistance
Supervisor:liu chang sheng wang lin shan
Al-12.7Si-0.7Mg is a novel kind of magnesium containing high silicon aluminum alloy extrusion profile,which was developed domestically in China recently,and possesses excellent mechanical properties as well as broad market prospects.Nevertheless,this kind of material is urgently in need of surface treatment technologies.The thesisaims in developing anodic oxidation and electrolytic coloring processing for Al-12.7Si-0.7Mg alloy to improve the corrosion resistance and prolong the service life.The research had the vital significance to expand application field of Al-12.7Si-0.7Mg alloy.In the current work,the orthogonal design and single factor experiments were applied to optimize for composition of electrolytic solutions and conditions of anodic oxidation and electrolytic coloring processing.Microstructures of anodic oxide films and colored films were studied bySEM,EDS,XRD,XRF and XPS.Examinations in performances of anodic oxide films and colored films were evaluated by alkaline etching test,potassium dichromate spot test,acidic erosion test,neutral salt spray test(NSS)and potentiodynamic polarization experiment.The major achievements in the presentdissertationare includedin the following:(1)The optimal conditions of anodic oxidation were 170 g·L-1 of H2SO4,1.5 A·dm-2 of current density,30 min of anodized time and 5 mL of additive.Voltage-time curvesfor Al-12.7Si-0.7Mg alloy in four kinds of electrolytes conformed to the typical characteristic curve of aluminum anodic oxidation.Formed in the mixed electrolyte of sulfuric acid and citric acid,the surface of anode oxide filmwhose essential component of amorphous Al2O3exhibited advantages in smoothness,well-distribution and excellent densification.The results of NSS test,potassium dichromate spot test and potentiodynamic polarization indicated that citric acid was the most effective additive for enhancing corrosion resistance of anodic oxidation film for Al-12.7Si-0.7Mg alloy.(2)The optimal processing parameters of stearic acid sealing treatment were as follows:pure stearic acid,sealing time of 45 min and sealing temperature of 95 ℃.The porous layers of anodic films were sealed entirely and exhibited excellent corrosion resistance.Different sealing techniques exhibited different influences on the corrosion resistance.Stearic acid sealing method was more effective than untreated samples for improving corrosion resistance of anodic film of Al-12.7Si-0.7Mg alloy.(3)Stabilizer land stabilizer Ⅱ were used to inhibit hydrolysis of Sn2+.Electrolytic coloring conditions,determined by single factor tests,were 10-15 min of electrolytic time,20-25 ℃ of temperature and 12-25 V of AC voltage.By controlling operation conditions,electrolytic coloring films with colors from light to dark were obtained,respectively.(4)For Al-12.7Si-0.7Mg alloy,the optimal experimental conditions of tin-nickel mixed salt electrolytic coloring processing were obtained by orthogonal design and single factor experiments.By controlling the operation conditions,the uniform tin-nickel mixed salt electrolytic coloring films with different color were obtained.XRF was applied to analyze anodization film and colored film,which verified the existence of tin in the colored film,which is in agreement with the EDS analysis results.The colored filmswere uniform,dense and firmly attached to the substrate.Tin-nickel colored film possessed better corrosion resistance,comparing with untreated sample and anodized film.(5)The orthogonal design and single factor experiments were applied to optimize for composition of nickel salt electrolyte and conditions of electrolytic coloring processing.The optimal constituents of nickel salt electrolyte were NiSO450-52 g·L-1,MgSO420-23 g·L-1,(NH4)2SO440-42 g L-1,stabilizer Ⅲ 8-8.5 g L-1,H3O330-31 g L-1.Nickel salt electrolytic coloring conditions were 5-7 min of electrolytic time,25 ℃ of coloring temperature and 15-17 V of AC voltage.Nickel salt electrolytic coloring films with black color were obtained by controlling the operation conditions.Analysis of microstructure(SEM/EDS,XPS,XRD)showed that nickel particles(NiO)had been deposited in the colored film.The porous was filled by Ni deposition,and the surface roughness of colored film was reduced,the corrosion resistance of colored film was improved.(6)The concentration of Al3+was increased with thecontinuoususe of nickel salt electrolyte.Hydrolysis of A13+ was observed which made the electrolyte denature,and the performance of the films became poor.The quality of nickel salt electrolytic coloring solution could be effectively improved by increasingpH and removing the Al(OH)3.