The Study Of Anode Oxidation Process And Corrosion Resistance Of Anodic Oxidation Ffilm Of Al-4%Si Alloy

Al-Si alloy is widely used in construction, aerospace and many other areas in ourdaily life because of the advantages such as light weight, high specific strength, lowdensity and favourable casting properties. In Al-Si alloy, α+Mg2Si+Si phase is formedwhich can increase the strength but decrease the corrosion resistance of Al alloy. Al-Sialloy can be surface treated properly to significantly improve the properties of service tomeet the different requirements. The corrosion resistance, wear resistance and hardness canbe improved by anodic oxidation which is already a common protection method withsimple operation process and low cost, besides, the anodic oxidation surface can be used asthe transition laver of the external coating and make Al-Si alloy decorated.The oxide film thickness of Al-4%Si alloy under different conditions are prepared bybeing oxidized by sulfuric acid through changing the electrolyte concentration, currentdensity, oxidation time and temperature, and the component, structure and thecomposition of the film are analyzed by SEM, TEM and EDS, finally the optimalparameters of the oxidation process is determined. Under the optimum conditions, theoxalic acid, citric acid, sulfosalicylic acid, cerium sulfate are added into sulfuric acidelectrolyte as a single additive. The influence of the additives on the film thickness, and thechanging of the polarization curves of oxide film in3.5%NaCl solution are analyzed. Theoptimal content of additives is determined by analyzing the influence of additives on thecorrosion resistance through drop tests and drops of alkali experiments.The results indicate that the porous and dense oxide film can be formed on the surfaceof Al-4%Si alloy with sulfuric acid oxidation. The thickness of the oxide film is increasedas the increase of electrolyte concentration, current density, oxidation time. But theparameters cannot be exaggerated because of the thermal effect and acid etching and theoptimum parameters are as flow: sulfuric acid concentration is180g/L, current density is1.5A/dm2, oxidation time is60min. The oxide film can be effectively prevented from being corroded by sulfuric acid and the quality of film is improved through adding citricacid, oxalic acid, sulfosalicylic acid, cerium sulfate and other additives into the sulfuricacid electrolyte. The optimum content of the additives above can be determined byanalyzing the corrosion resistance through polarization curves and drop tests: oxalic acid is30g/L, sulfosalicylic acid is10g/L, citric acid is25g/L, cerium sulfate0.7g/L. The filmunder the condition of optimum parameters is colored by Alizarin Red Solution and it isobserved by FESEM. The film is colored uniformly and continuously and nano-pores onthe surface are uniform and orderly which means the oxide film exhibits good quality.