EIS and potentiodynamic polarization studies on immiscible monotectic Al–In alloys

Highlights

• The Al–In alloy microstructure is characterized by In droplets spread in an Al matrix.

• The scale of phases forming the microstructure affect the electrochemical behavior.

• Larger interphase spacing/droplet diameter is related to higher corrosion resistance.

• The deleterious effect in corrosion resistance is due to strains and galvanic cells.

Abstract

The electrochemical behavior of monotectic Al–In alloys is experimentally investigated. Electrochemical impedance spectroscopy (EIS), potentiodynamic anodic polarization techniques and an equivalent circuit analysis were used to evaluate the corrosion response in a stagnant and naturally aerated 0.5 M NaCl solution at 25 °C. It was found that a better galvanic protection can be provided for microstructures having indium droplets of larger diameters and larger interphase spacings. From five samples extracted along the length of a directionally solidified Al–In casting, that having smallest interphase spacing (λ = 18 μm) and droplet diameter (d = 0.7 μm) had its corrosion resistance significantly decreased (about 2 and 3 times in terms of the current density and polarization resistance) when compared with that of the sample having the coarsest microstructure (λ = 60 μm and d = 2.5 μm). Such behavior is attributed to both localized strains between aluminum and indium boundaries and the corrosion potential of the indium particles.

Introduction

A number of Al-based monotectic binary alloys having limited solubility in the liquid state have significant potential for many industrial applications, such as electronic components, high temperature superconductors, self-lubricated bearings [1], [2], [3], [4], [5] and optical devices [6]. Serious problems can be associated with conventional melting and casting techniques of these alloys, caused by severe gravity segregation in casting due the large densities differences between the liquid phases [2], [5]. It is important to remark that Al-based monotectic alloys have an Al-rich liquid phase L₁, at the monotectic temperature, which is decomposed into an Al-rich solid phase S₁ and a liquid phase L₂. During cooling, a continuous Al-rich matrix is formed with the liquid minority phase being retained in a discontinuous way within the solid matrix in the form of isolated pockets [5], [7]. The competition between the growth of the minority phase and the rate of displacement of the solidification front will determine if the prevalent morphology of the microstructure will be characterized by fibers or droplets [7].

Kamio et al. [8] reported the development of the microstructure of a monotectic Al–17.5 wt.% In alloy during the steady-state growth in an unidirectional solidification set-up at various growth rates and temperature gradients. The resulting microstructures were shown to be formed either by small indium droplets or by fine and regularly aligned fibers along the growth direction disseminated in the Al matrix, which depended on the range of thermal gradients and growth rates imposed during solidification. They reported that the droplet morphology prevailed when a growth rate of about 1.1 × 10⁻⁶ m/s was attained. Kamio et al. [8] have also compared their experimental results of interphase spacing and growth rate with those reported in studies by Grugel and Hellawell [9], [10]. It was shown that the interphase spacing decreases with the increase in the growth rate [8], [9], [10]. Yasuda et al. [4] used a static magnetic field in order to control the monotectic solidification of Al–In alloys. They observed the presence of both indium rods and droplets segregated close to the bottom of the casting [4]. Liu et al. [2] have shown that the size of indium particles increased with the increase in distance from the chilled surface.

Despite the potential for the use of Al–In alloys in a number of industrial applications, the literature is scarce on studies interrelating microstructural effects such as the morphology of the indium particles and the interphase spacing to the corresponding electrochemical corrosion behavior. It is expected that the control of the resulting microstructural array of a monotectic Al–In alloy would permit to prescribe guidelines with a view to preprogramming a required corrosion resistance.

The present study focus on the effect of the scale of the interphase spacing of a microstructure, formed by indium droplet-like particles embedded in an Al-matrix, on the resulting EIS and potentiodynamic polarization plots of a monotectic Al–In alloy, with tests performed into a naturally stagnant 0.5 M NaCl solution at 23 (±3) °C.