日本金属学会誌

J. Japan Inst. Metals, Vol. 57, No. 11 (1993),
pp. 1293-1300

Degradation Mechanism of Corrosion Resistance for Al--0.9 mass%Mg2Si Alloy with Ga

Motoshige Hibino1, Takehiko Mae2, Hisahiro Sakai3, Satoshi Sunada1 and Koichi Arai1

1Department of Metallurgical Engineering, Faculty of Eng., Toyama University, Toyama
2Toyama National College of Technology, Toyama

3Kyoshin Heat-treatment Co., Ltd., Toyama

Abstract:

The effect of Ga content on the characteristics of precipitation for Al--0.9 mass%Mg2Si alloys was investigated by the measurements of DTA curves of aging, transmission electron microscopy, and electron diffraction of precipitates. Furthermore, the relationship between the precipitation behavior and corrosion resistance for the alloys was estimated by means of an electrochemical measurement of the polarization curves, immersed corrosion tests, and scanning electron microscopic observations of the corroded surfaces for the alloys with various phases in a dilute HClO4 solution.
The main results obtained are summarized as follows:

(1) The effect of the added Ga on the characteristics of precipitation occurs mainly in G.P. II zone, and its initiation temperature of Mg2Si precipitation is lowered with increasing Ga content in the alloys. However, the distribution and composition of precipitates with transition and equilibrium phases are independent of Ga content.

(2) The activation potentials (Eact.) of Al--0.9 mass%Mg2Si alloys in a dilute HClO4 solution decrease in the order of super saturated solid solution>equilibrium phase>transition phase. On the contrary, the anodic current densities (log (Ia)) of the alloys under a potentiostatic condition increase in the order of super saturated solid solution
(3) The corrosion of the alloys is uniform at the super-saturated solid solution phase. However, the corrosion of the alloys containing Ga is enhanced by a selective dissolution of Mg or Al from the boundary layers of the alloys. On the other hand, galvanic corrosion occurred between the Mg2Si precipitates and a matrix phase for the alloys with the transition and equilibrium phases regardless of the Ga content, and the Mg2Si precipitates act as an anode. On the contrary, the matrix phase acts as a cathode. The selective dissolution of Mg from the precipitates is observed on the corroded surfaces.


(Received July 2, 1993)

Keywords:

aluminium--magnesium--silicon--gallium alloy, scanning electron microscopic observation, electrochemical measurements, uniform corrosion, galvanic corrosion, immersed corrosion test, aging precipitation behavior, perchloric acid


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