Effect Of Surface Modification On Corrosion Resistance Of SiC_p/Al Composites

The Silicon carbide reinforced aluminum matrix composite?SiC_p/Al?has been widely used in aerospace and electronic packaging industry due to its advantages of low cost,simple preparation,high strength,high hardness and high thermal conductivity.However the internal structure of the composite is inhomogeneity because of the addition of SiC particles which cause it easy to form corrosion micro batteries in the environment of chloride ion when suffer pitting damage.This paper adopts SiC_p/Al composite prepared by pressure infiltration process,using electroless nickel?EN?plating and micro arc oxidation?MAO?for surface modification.Different surface layer of Si C_p/Al composites were prepared by changing the process parameters,through the methods of scanning electron microscopy?SEM?,energy dispersivespectroscopy?EDS?,Xraydiffraction?XRD?andscanning electrochemicalmicroscopy?SECM?and PARSTAT 2273 electrochemical workstation technology to study the Si C_p/Al composite morphology,structure,and its corrosion resistance in 3.5 wt%NaCl solution.The results show that the Ni-P layer of 5,8,15,20?m are obtained on the surface of Si C_p/Al composites separately when EN plating for 10,30,60 and 90 min.Ni particles are easier to deposit on Al substrate.The nickel layer is most compact and continuous when plated for 60 min,and Ni particles is slightly shedding when plated for 90 min.The longer the nickel plating time is,the lower bonding between the nickel layer and the matrix.The electrochemical test results show that the corrosion resistance of SiC_p/Al composites increased significantly after EN plating.The corrosion potential and pitting potential of the composites were the highest after EN plating for 60 min,and the corrosion resistance of which was the best.When the 60min Ni-plated SiC_p/Al heat treatment for 1 h at 250?,300?and 350?respectively,the nickel layer structure change from amorphous to crystalline transition with the increase of heat treatment temperature,the layer has been completely converted to crystalline with Ni₃P phase formationed when the heat treatment temperature reaches to 300?.Electrochemical results indicate that a highest pitting potential,a widest passivation range and a greatest value of AC impedance for 250?heat-treatment SiC_p/Al composite were obtained when soaked in 3.5 wt%NaCl solution,therefore the composite shows a better corrosion resistance.The SiC_p/Al composites were put into 10 g/L NaAlO₂+1.1 g/L NaOH electrolyte system to MAO for 5,10,20,30 min respectively,and 6,10,15 and 21?m oxidation layers were obtained.With the prolongation of oxidation time,the more the accumulation of molten oxide on the surface and the characteristics of the diffraction peak of the?-Al₂O₃ phase in the oxidation layer are most obvious when oxidation for30 min.The electrochemical test results show that the self corrosion potential and pitting potential of Si C_p/Al composites were-1.0687 V and-0.6275 V after oxidation for 30 min.At this time,the SiC_p/Al composites exhibited the best corrosion resistance.Similarly,the SiC_p/Al composites were also put into 10 g/L Na₂Si O₃+1.1g/L NaOH electrolyte system to MAO for 5,10,20,30 min respectively,and 7,10,22and 55?m oxidation layers were obtained.However,the surface of oxide film becomes smooth and dense,and the pores tend to merging with the prolongation of MAO time.The oxide film on block SiC surface is uniform and dense with diffraction peak of Al₄C₃ decreases.The diffraction peak of SiO₂ becomes more obvious when oxidized at 30 min.The electrochemical test results indicate that the pitting potential of SiC_p/Al composites was-0.5758 V after MAO for 30 min,the surface charge transfer resistance R₂ is the largest after soaking in solution.After 9 days immersed,the weightlessness value is the minimum of 0.0336 g which proved the corrosion resistance of SiC_p/Al composite that MAO for 30 min is the best.The corrosion resistance of SiC_p/Al composites MAO in sodium silicate electrolyte systems is higher than that MAO in sodium aluminate electrolyte systems.From the experiments of the SiC_p/Al composites modificated by the three methods,we can conclude that under the same thickness of films:the SiC_p/Al composite EN plated for 60 min showed the best corrosion resistance,the corrosion resistance of SiC_p/Al composite that MAO in sodium silicate system for 10 min relatively poor,SiC_p/Al composite MAO in sodium aluminate system for 20 min exhibited the worst corrosion resistance.