Activation of an Al-Zn-Mg-Li alloy by the Presence of Precipitates to be used as Sacrificial Anode
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Activation of an Al-Zn-Mg-Li alloy by the presence of precipitates to be used as sacrificial anode S. Valdez1, M. A. Talavera2, J. Genesca2 and J. A. Juarez-Islas1 1 Instituto de Investigaciones en Materiales, 2Fac. de Quimica, Universidad Nacional Autonoma de Mexico, Circuito Escolar S/N, Cd. Universitaria, 04510, Mexico, D.F. Mexico. ABSTRACT This work reports the influence of microstructure on the electrochemical efficiency of Alalloys to be used as sacrificial anodes. The microstructure of as-cast Al-ingots was characterized by X-ray diffractometry and scanning electron microscope (equipped with WDS microanalysis facilities). The electrochemical behavior was investigated in 3% NaCl solution simulating sea water at room temperature. Regarding microstructure, X-ray diffractometry detected the presence of the α-Al phase, and the τ-Al2Mg3Zn3, Mg7Zn3 and δ-AlLi precipitates. SEM observations confirmed the presence of α-Al dendrites with τ-Al2Mg3Zn3 and Mg7Zn3 particles in matrix. In interdendritic regions, it was observed the presence of the eutectic α-Al + τ-Al2Mg3Zn3 plus some primary Mg7Zn3 particles. δ-AlLi particles were identified at this step only by X-ray diffractometry. Finally, it was detected that an uniform distribution of precipitates in the α-Al dendrites together with an uniform distribution of a fine eutectic gave as a result in as-cast ingots an electrochemical efficiency up to 61.4 %. INTRODUCTION Aluminium alloys have been developed for several applications [1]. Regarding its use as sacrificial anodes for cathodic protection, its interesting properties, such as low specific weight, low electrode potential and high current capacity are sometimes hindered by the presence of an oxide film [2]. This oxide film is the cause of a rapid polarization, when aluminium is placed under corrosion load in a cathodic protection circuit, reducing its efficiency. The performance of aluminium alloys as sacrificial anodes has been directly related to its electrochemical efficiency [3]. In order to have a good performance of the aluminium sacrificial anodes, research has been focused towards the development of aluminium alloys capable to breakdown this oxide film. For that purpose, several approaches have been carried out, including the effect of alloying additions [2, 3, 7], the effect of casting parameters [8] and microstructure control [9], in order to promote an uniform structure together with an uniform distribution of precipitates in the α-Al matrix, capable to prevent the formation of the continuous, adherent and protective oxide film on the alloy, thus permitting a continued galvanic activity of the aluminium sacrificial anode [2-6]. The purpose of this work is to report advances on the development of Al-alloys to be used as industrial sacrificial anodes. EXPERIMENTAL DETAILS Aluminium alloys were prepared by using commercial aluminium, zinc and magnesium elements, with 99.8 % of purity. Lithium was used as a wire of 3.2 mm in diameter and with a AA3.5.1
purity of 99.99%. The previously weighted elements wer
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