Journal of the Korean institute of surface engineering (한국표면공학회지)

The Korean Institute of Surface Engineering (한국표면공학회)
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TEM Analysis on Oxide Films of Al1050 and Al7075 Exposed to 24-month Atmospheric Conditions

24개월 대기 노출된 Al1050 및 Al7075 알루미늄 합금 산화막에 대한 투과전자현미경 분석

  • Kim, Dae-Geon (School of Mechanical Engineering, Pusan National University) ;
  • Kim, Ga-Rim (School of Mechanical Engineering, Pusan National University) ;
  • Choi, Wonjun (School of Mechanical Engineering, Pusan National University) ;
  • Bahn, Chi Bum (School of Mechanical Engineering, Pusan National University)
  • Received : 2019.01.17
  • Accepted : 2019.03.16
  • Published : 2019.04.30
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Abstract

Al1050 and Al7075 alloy specimens were exposed to atmospheric conditions for 24 months and analyzed by Transmission Electron Microscopy to characterize their corrosion behavior and oxide film characteristics, especially focusing on intergranular corrosion or oxidation. In general, the intergranular oxygen penetration depth of Al1050 was deeper than Al7075. Since O and Si signals were overlapped at the oxidized grain boundaries of Al1050 and Mg is not included in Al1050, it is concluded that Si segregated along the grain boundaries directly impacts on the intergranular corrosion of Al1050. Cr-Si or Mg-Si intermetallic particles were not observed along the grain boundaries of Al7050, but Mg-Si particle was barely observed in the matrix. 10-nm size Mg-Zn particles were also found all over the matrix. Mg was mainly observed along the oxidized grain boundary of Al7075, but Si was not detected due to the Mg-Si particle formation in the matrix and relatively low concentration of Si in Al7075. Therefore, it is thought that Mg plays an important role in the intergranular corrosion of Al7075 under atmospheric corrosion conditions.

Keywords

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Fig. 1. SEM surface images of (a) Al1050 and (b) Al7075 after 24-month exposure to atmospheric conditions

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Fig. 2. SEM cross-section images of Al1050 surface exposed to atmospheric conditions for 24 months

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Fig. 3. TEM cross-section image and EDS elemental mapping of pitting area in Al1050 exposed to 24-month atmospheric conditions; (a) dark field image (b) Al (c) O

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Fig. 4. TEM cross-section panorama images and EDS elemental mapping of surface oxide layer formed on Al1050(non-pitting); (a) dark field image (b) O

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Fig. 5 TEM cross-section image and EDS elemental mapping for oxygen penetration region of Al1050; (a) dark field image (b) O (c) Al

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Fig. 6. EDS elemental mapping of oxygen penetration area in Al1050 (higher magnification of area shown in Fig. 5); (a) O (b) Al (c) Si

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Fig. 7. Line EDS analysis results for oxygen penetration regions of Al1050 (Note each area number is designated in Fig. 5.)

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Fig. 9. TEM cross-section image and EDS elemental mapping of oxygen penetration area in Al7075 exposed to 24-month atmospheric conditions (Right side of each picture is external surface.); (a) Dark field image (b) O (c) Al

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Fig. 10. TEM cross-section panorama image and EDS elemental mapping of oxygen penetration area in Al7075 (Right side of each picture is external surface); (a) O (b) Mg (c) Si (d) Fe (e) Cr

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Fig. 11 SEM image and EDS elemental mapping near the Mg-Si particle in Al7075

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Fig. 12 TEM image and EDS elemental mapping near the oxygen penetration tip in Al7075; (a) dark field image, (b) Mg, (c) O, Cr, Mg, (d) Al, (e) Cr, (f) Zn

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Fig. 8. (a) SEM cross-section image of surface oxidation region and (b) TEM specimen image after FIB sectioning for Al7075

Table 1. Chemical composition(wt%) of aluminum alloy Al7075 and Al1050 specimens.

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