The effect of small addition of tin and indium on the corrosion behavior of aluminium in chloride solution

Abstract

The corrosion behavior of Al, Al–In, Al–Sn and Al–Sn–In alloys in 2 M NaCl solution has been studied by using potentiodynamic polarization, open circuit potential and impedance measurements as well as volumetric determination of evolved hydrogen. Polarization measurements indicated that the alloying elements considerably change the electrochemical behavior of aluminium. Changes are especially pronounced in the anodic branch of the polarization curve, and manifest in significant shift of the corrosion and the pitting potentials in the negative direction, reducing the passive potential region, and increasing the current output already at low anodic overpotentials. The open circuit corrosion process of tested samples was monitored through the period of 17 days. It was established that the dissolution of Al and its alloys is mainly accompanied by the cathodic reaction of hydrogen evolution, with In inhibits the cathodic reaction of hydrogen evolution, while Sn catalyzes the same one. The impedance measurement performed also during the 17 days of corrosion test, provided a continuous monitoring of the parameters, which characterize the properties of the surface and their changes as corrosion occurs. Different equivalent circuits were employed to account for the electrochemical processes taking place at different stages of corrosion.

Introduction

The low atomic mass of aluminium, and its high energetic capacity (2980 Ah kg⁻¹), along with the negative value of standard electrode potential (−1.66 V vs. NHE [1]) make the metal potentially attractive as an anode material in Al-batteries and in cathodic protection systems. However, the hindrance for the realization of these theoretical possibilities is the protective oxide film, which is spontaneously formed on Al surface in air and in aqueous solutions. Due to presence of oxide film, the corrosion potential of Al electrode is shifted in the positive direction (almost for 1.0 V), and the active dissolution of Al is slowed down considerably. This causes a significant loss of available energy and makes Al unattractive as energetic anode material.

However, if Al is alloyed with small quantities of elements such as Hg, In, Ga, Zn, Sn and others [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], its anodic activity improves as the structure of the aluminium/aluminium oxide film/chloride electrolyte system changes. The activation of Al is manifested by a displacement of the solution potential in the electronegative direction, the system's ability to deliver high anodic current density, and the maintenance of an active surface that corrodes in a nearly uniform manner. In order to make the Al/air batteries or cathodic protection systems more economical, the possibilities of replacement of the special alloys made from super-pure Al with binary or ternary alloys of technical purity have been investigated lately [5], [19], [20]. Activation of Al can also be achieved by adding small quantities of suitable metal cations, like In³⁺, Ga³⁺, Hg²⁺, Sn⁴⁺, Sn²⁺ to the electrolyte [9], [15], [22], [23], [24], [25], [26], [27].

The present paper describes the corrosion behavior of technical grade Al (99.8%) and its alloys Al–0.1%In, Al–0.2%Sn, Al–0.1%In–0.2%Sn in stagnate 2 M NaCl solution. The selection of technical grade Al, its binary alloys and ternary alloy for this study was based on insufficient examination of application of these materials as anode materials in Al/air batteries in which 2 M NaCl is a common used electrolyte. According to our knowledge, no literatures have reported usage of Al–0.2%Sn–0.1%In alloy in this application. This combination of elements taken into consideration is entirely novel. In the available literature electrochemical studies were usually performed on samples of binary alloys of high-purity Al (99.999%) with Sn and In.

The main objective of this work was to detect, measure and monitor corrosion of tested samples. For this purpose, just after immersing the tested electrodes in the solution, the variation of the open circuit potential, volume of the evolved hydrogen, as well as pH value of solution, were measured trough the period of 17 days. Impedance measurements were also performed at various exposure times during the 17 days corrosion test, while the surface damage after the test was observed by optical microscopy. In addition, prior to long term corrosion test, the polarization curves for each sample were recorded in wide potential range.