The Relationship Between Corrosion Behavior And Microstructure In 7055 Aluminum Alloy

Al-Zn-Mg-(Cu)alloy is a kind of typical age-hardenable aluminum alloy,which is widely used in aerospace and transportation areas because of its high strength,good formability and good welding performance.But this type alloy is susceptible to localized corrosion in corrosive environments like pitting,intergranular corrosion and exfoliation corrosion.The localised corrosion behavior of this alloy has a great relationship with its microstructure.In this work,the relationship between microstructure and corrosion behavior of 7055 aluminum alloy are investigated by means of intergranular corrosion test,exfoliation corrosion test,hardness test,scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The first part(Chapter 3)is to understand the effects of artificial ageing temperature and time on the microstructure and corrosion behavior of 7055 aluminum alloy.The second part(Chapter 4)is to investigate the effect of quenching transfer time on the microstructure and corrosion behavior of 7055-T6 Aluminum alloy.The third part(Chapter 5)is to study the effect of cold rolling deformation ratio before solution treatment on the microstructure and corrosion behavior of 7055 aluminum alloy.The main results are as follows:(1)TEM quasi-in situ immersion corrosion results show that when the alloy is aged at 120 ?,the grain boundary precipitates(GBPs)are the preferential corrosion sites;however,when the alloy is aged at 400?,the PFZ near the GBPs is the preferential corrosion site rather than the GBPs.(2)With the ageing temperature and ageing time changing,the corrosion morphology of 7055 aluminum alloy will evolve in the following order:typical intergranular corrosion morphology(IGC)(120 ? early ageing),pitting-like corrosion morphology(120 ? longer time ageing and 200 ? ageing),quasi IGC morphology(400 ? ageing).TEM and elemental mapping of X-ray energy dispersive spectra(EDS)results show that:when the alloy is aged at 120 ? for early stage,because of the continuous distribution and relatively low Cu content of GBPs makes the corrosion speed of grain boundary much higher than the corrosion speed of grain internal,so the corrosion morphology shows typical intergranular corrosion morphology.When the alloy is aged at 120 ? for longer time or aged at 200 ?,the Cu content in the GBPs will increase to some degree,which leads to a decrease in the corrosion potential difference difference between grain internal and the GBPs.And it makes the corrosion speed of grain boundary a little higher than the corrosion speed of grain internal,so the corrosion morphology shows pitting-like corrosion morphology.When the alloy is aged at 400 ?,Cu elements atoms will concentrate in the precipitates,especially in the GBPs,and the precipitate free zone(PFZ)deplete Cu element.As such,the corrosion potential of GBPs becomes higher than the PFZ,causing the PFZ dissolution and the corrosion morphology shows quasi IGC.(3)With the quenching transfer time extending,the intergranular corrosion and exfoliation corrosion resistance of 7055-T6 aluminum alloy decrease.Under rapid quenching condition,due to GBPs enrichment Cu element atoms compared with grain internal of 7055-T6 aluminum alloy makes the corrosion potential difference decrease between GBPs and grain internal,so the corrosion morphology shows pitting-like morphology.When the quenching transfer time extends to 20s,due to Cu element uniformly distributing in the grain boundary and grain internal makes the corrosion speed of grain boundary much higher than the corrosion speed of grain internal,so the corrosion morphology shows typical intergranular corrosion morphology.When the quenching transfer time extends to 240s,the PFZ deplete Cu leading corrosion expanding along the PFZ,so the corrosion morphology shows typical intergranular corrosion morphology.(4)With the increase of cold rolling deformation ratio before solution treatment,the grain size of 7055 aluminum alloy decrease after solution treatment and ageing,and the GBPs become more discontinuous,the intergranular corrosion resistance of 7055 aluminum alloy is gradually increased.