고강도 7xxx 알루미늄 합금의 응력부식균열에 미치는 부식환경과 응력속도의 영향

Abstract

High strength 7xxx aluminum alloys have been applied to automotive bump back beam of the some limited model for light weight vehicle. The aluminum bump back beam is manufactured through extrusion, bending and welding. The residual stress given on these processes combines with the corrosive atmosphere on the road spreaded with corrosive chemicals to melt the snow to occur the stress corrosion cracking. The composition of commercial 7xxx aluminum has the Zn/Mg ratio about 3 and Cu over 2wt% for better strength and stress corrosion cracking resistivity. But this composition isn't adequate for appling to the automotive bump back beam with high resistance to extrusion and bad weldability. In this study the composition of 7xxx aluminum alloy was modified to high Zn/Mg ratio and low Cu content for better extrusion and weldability. To estimate the resistivity against stress corrosion cracking of this aluminum alloy by slow strain rate test, the corrosion atmosphere and strain rate separate the stress corrosion cracking from the conventional corrosion must be investigated. Using 0.6Mol NaCl solution on slow strain rate test the stress corrosion cracking induced fracture was not observed. By adding 0.3% H₂O₂ and 0.6M Na₂SO₄ to 1M NaCl solution, the corrosion potential and current density of polarization curve moved to active potential and larger current density, and on the slow strain rate test the fracture energy in solution was lower than that in pre-exposure. These mean the stress corrosion cracking induced fracture can be estimated in this 1M NaCl + 0.3% H₂O₂ + 0.6M Na₂SO₄ solution. When the strain rate was below 2공10-6, the stress corrosion cracking induced fracture start to be observed.