Study On The Corrosion Behaviors Of SiC_p/2009Al Composite

Particle reinforced Al matrix composites(PRAMCs) have superior comprehensive properties, such as high specific stiffness and strength, high thermal conductivity, controllable coefficient of thermal expansion, good resistance to fatigue, and superior performance at elevated temperature. So that PRAMCs are generally thought to be one of the most competitive metal matrix composites. 2009 Al alloy based composites reinforced by silicon carbide particle(Si C_p/2009) is a kind of typical PRAMCs developed during the past 20 years. And these composites have been extensively applied in field of aviation, aerospace, and microelectronics, etc. As the using environment become more and more complicated, the environmental corrosion and protection techniques are attaching much more attentions. The behaviors and underlying mechanisms of corrosion of these composites have been an important research subject of metal matrix composites.Corrosion of Si C_p/2009 Al composites is relatively complicated, under the interactive influence of alloying elements, reinforcing particles, interface and crystalline defects. In the present dissertation, Si C_p/2009 Al composites produced by powder metallurgy was investigated, which has significant engineering applications. Simulated environment tests, together with electrochemical measurement were carried out, to explore the corrosion and failure behavior, and their governing factors. Corrosion of 15%Si C_p/2009 Al composite in 3.5% Na Cl solution was investigated by using methods of macro- and micro-electrochemical, scanning electronic microscopy(SEM) and X-Ray diffraction, etc. Besides, effect of cryogenic treatment on corrosion was also explored. Anodic oxidation film was fabricated on the composite by using sulfuric acid anodic oxidation processing, of which the polarization curve and electrochemical impedance spectroscopy was observed to examine the corrosion resistant, and corrosion process. Results of the present dissertation were concluded as follows.Neutral saltwater fog test, damp heat test and xenon lamp weathering resistant test were carried out on the composite and the corrosion extent varied in the order from heavy to gentle. Within the environment of saltwater fog, Si C_pCorrosion of both the Si C/2009 Al is observed to have inferior corrosion resistant in compared to 2009 Al.p/2009 Al and 2009 Al includes two stage, corrosion inoculation stage and corrosion extending stage, of which the equivalent circuit model is R(Q(R(CR))) and R(Q(R(C(RW)))), respectively. In particular, the corrosion extending stage of the composite is earlier than 2009 Al alloy by about 6 hours, with higher corrosion rate and lower general impedance in the initial of corrosion. The corrosion current curve as a function of immersing time of both the composite and 2009 Al shows periodical fluctuates with alternate decrease and increase. Average corrosion current(ACC) of 2009 Al is lower than that of the composite in the beginning, and gradually increases with time. In contrast, ACC of the composite is always high throughout the test, with minor increase with time. ACC of 2009 Al is eventually close to that of the composite after 40 h immersing test.Corrosion of both the composite and 2009 Al alloy occurred mainly by pitting. For 2009 Al, the dominant mechanism is galvanic corrosion between the Cu-rich intermetallics of Al2 Cu and Al2 Cu Mg and the Al matrix around. In particular, with the addition of Si C particles phase composition of the Al alloy matrix of the composite changes to Al2 Cu and Mg2 Si, accompany with disappear of Al2 Cu Mg. The composite corrosion mainly occurs in the following three areas, the Al matrix around the rich copper phase、the Al matrix around the Si C particles and the multiphase mixture zone including Mg2Si、Si C and Al2 Cu phase. Also, addition of Si C particles reduces the size and increases the amount of Al2 Cu particles, as one of the main precipitate phase of 2009 Al. As a result, galvanic corrosion around the precipitate particles is accelerated because of the increased active Al area.Interestingly, corrosion resistance of the composite was appropriately improved after cryogenic treatment, which suggests a probable approach to protection prevention of this material. Corrosion resistance of the composite was remarkably improved by the coating protection by sulfuric acid anodic oxidation. Anodic oxidation is expected to be an effective method of corrosion protection.