Corrosion Science and Technology

The Corrosion Science Society of Korea (한국부식방식학회)
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Effect of Annealing on the Pitting Corrosion Resistance of Anodized Aluminum-Magnesium Alloy Processed by Equal Channel Angular Pressing

  • Son, In-Joon (Department of Materials Process Engineering, Graduate School of Engineering, Kyushu University) ;
  • Nakano, Hiroaki (Department of Materials Science & Engineering, Faculty of Engineering, Kyushu University) ;
  • Oue, Satoshi (Department of Materials Science & Engineering, Faculty of Engineering, Kyushu University) ;
  • Kobayashi, Shigeo (Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kyushu Sangyo University) ;
  • Fukushima, Hisaaki (Department of Materials Science & Engineering, Faculty of Engineering, Kyushu University) ;
  • Horita, Zenji (Department of Materials Science & Engineering, Faculty of Engineering, Kyushu University)
  • Published : 2007.12.01
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Abstract

The effect of annealing on the pitting corrosion resistance of anodized Al-Mg alloy (AA5052) processed by equal-channel angular pressing (ECAP) was investigated by electrochemical techniques in a solution containing 0.2 mol/L of $AlCl_3$ and also by surface analysis. The Al-Mg alloy was annealed at a fixed temperature between 473 and 573 K for 120 min in air after ECAP. Anodizing was conducted for 40 min at $100-400A/m^2$ at 293 K in a solution containing 1.53 mol/L of $H_2SO_4$ and 0.0185 mol/L of $Al_2(SO_4)_3$. The internal stress generated in anodic oxide films during anodization was measured with a strain gauge to clarify the effect of ECAP on the pitting corrosion resistance of anodized Al-Mg alloy. The time required to initiate the pitting corrosion of anodized Al-Mg alloy was shorter in samples subjected to ECAP, indicating that ECAP decreased the pitting corrosion resistance. However, the pitting corrosion resistance was greatly improved by annealing after ECAP. The time required to initiate pitting corrosion increased with increasing annealing temperature. The strain gauge attached to Al-Mg alloy revealed that the internal stress present in the anodic oxide films was compressive stress, and that the stress was larger with ECAP than without. The compressive internal stress gradually decreased with increasing annealing temperature. Scanning electron microscopy showed that cracks occurred in the anodic oxide film on Al-Mg alloy during initial corrosion and that the cracks were larger with ECAP than without. The ECAP process of severe plastic deformation produces large internal stresses in the Al-Mg alloy; the stresses remain in the anodic oxide films, increasingthe likelihood of cracks. It is assumed that the pitting corrosion is promoted by these cracks as a result of the higher internal stress resulting from ECAP. The improvement in the pitting corrosion resistance of anodized AlMg alloy as a result of annealing appears to be attributable to a decrease in the internal stresses in anodic oxide films

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References

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