Shigenori Yamamoto, Masahiro Kawakami and Toshihide Takenaka
Faculty of Engineering, Toyohashi University of Technology, Toyohashi
Exfoliation corrosion proceeds laterally along planes parallel to the surface of high strength aluminum alloys. The ASTM G34(EXCO) test is widely used to predict exfoliation corrosion susceptibility. The susceptibility is determined by visual examination in which the surface profile is compared with the reference standard photograph. This method is ambiguous, so that it is difficult to distinguish general corrosion (overall corrosion, pitting) from exfoliation.
In this study, a quantitative estimation method of exfoliation corrosion susceptibility based on ASTM G34 was developed, and the effect of composition and heat treatment of the alloys on exfoliation corrosion susceptibility were examined by this method.
The specimens used were of 7XXX, Al-Li series alloys treated in T6, T7 and T4 modes. The normal procedure of ASTM G34 was carried out. Moreover, as quantitative measurements, the mass losses and quantity of exfoliation were evaluated by the following procedure. The specimens were soaked in nitric acid for 2 min and rinsed in water after being rated. After drying for about 30 min, the mass losses were measured. One week later, piles on the specimens were removed from the surface by a tooth brush. The piles and residues in solution were treated with hydrochloric acid, so that the weight of the metallic component was estimated by the amount of evolved hydrogen gases. The quantity of exfoliation was defined by the weight.
The results obtained in this study were as follows;
(1) The quantity of exfoliation was slightly correlated with ASTM G34 rating. The exfoliation corrosion could be distigushed from the overall corrosion and pitting.
(2) Exfoliation susceptibility was especially high in the alloys containing from 0.7 to 1.0%Cu. The alloy containing 0.37%Cu was the lowest.
(3) The heat treatment in T7 gave high susceptibility of exfoliation corrosion, while that in T4 gave high susceptibility of overall corrosion and pitting.
(Received March 27, 1996)
corrosion, exfoliation corrosion, high strength aluminum alloy, estimation method, ASTM G34(EXCO), nitric acid treatment, hydrochloric acid treatment
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