H₂SO₄-NaCl水溶液中の局部腐食特性に及ぼすAl-9 mass%Mg合金中のGaの影響

抄録

The effects of Ga content and aging treatment on local corrosion characteristics of Al-9 mass%Mg alloys containing a small amount of Ga content were investigated by the measurements of the polarization curves and potentiostatic corrosion and immersed corrosion tests. The scanning electron microscopic observation of precipitates of the secondary phase with aging treatment and surfaces of the specimens corroded in NaCl containing 1×10⁻² kmol/m³ H₂SO₄ solution at 303 K were also performed. The main results obtained are summed up as follows:
(1) The Al₃Mg₂ compounds of secondary phase of Al-9 mass%Mg alloys aged at 453 K for 6.0 ks precipitate along the grain boundary irrespective of the specimens used. However, the rodlet precipitates are observed even within the grain, and the precipitate free zones are formed along the grain boundary of the specimens aged at 453 K for 307.2 ks.
(2) The pitting potential E_pit of Al-9 mass%Mg alloys is shifted to a more basic potential with increasing NaCl concentration, Ga content in the alloys and aging time intervals, and thus the degradation of local corrosion resistance is observed.
(3) The apparent anodic dissolution rates of the specimens under the potentiostatic condition are enhanced with increasing Ga content in the alloys and aging time irrespective of the specimens used. The rates are increased in the order of super-saturated solid solution<the specimens aged at 453 K for 6.0 ks<the specimens aged at 453 K for 307.2 ks.
(4) The corrosion of super-saturated Al-9 mass%Mg alloys in a H₂SO₄-NaCl solution is mainly pitting attack. However, the corrosion form of the aged specimens is observed both intergranular and pitting attacks.