Influence of rare earth metals on the characteristics of anodic oxide films on aluminium and their dissolution behaviour in NaOH solution

Abstract

The characteristics of oxide films on Al and Al1R alloys (R = rare earth metal = Ce, Y) galvanostatically formed (at a current density of 100 μA cm⁻²) in borate buffer solution (0.5 M H₃BO₃ + 0.05 M Na₂B₄O₇·10H₂O; pH = 7.8) were investigated by means of electrochemical impedance spectroscopy. EIS spectra were interpreted in terms of an “equivalent circuit” that completely illustrate the Al(Al1R alloy)/oxide film/electrolyte systems examined. The resistance of the oxide films was found to increase on passing from Al to Al1R alloys while the capacitance showed an opposite trend. The stability of the anodic oxide films grown in the borate buffer solution on Al and Al1R alloys was investigated by simultaneously measuring the electrode capacitance and resistance at a working frequency of 1 kHz as a function of exposure over a period of time to naturally aerated 0.01 M NaOH solution. Analyses of the electrode capacitance and resistance values indicated a decrease in chemical dissolution rate of the oxide films on passing from Al to Al1R alloys.

Introduction

The electrochemical behaviour of aluminium and aluminium alloys is a subject of a large number of studies due to increased industrial application of these materials. These applications are often possible because aluminium, like other valve metals (Ta, Ti, Zr, Nb, W), spontaneously forms stable oxide films in air and in aqueous solutions. Oxide films can be also formed artificially by anodising processes. The growth of anodic oxide films on aluminium in aqueous electrolytes takes place by counter migration of Al³⁺ and O²⁻ ions through the film thickness under the high electric field (10⁶–10⁷ V cm⁻¹), with formation of amorphous alumina at the metal/film and film/electrolyte interfaces.

The behaviour of aluminium during the anodising process has been extensively studied [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and it has been found that the anodic film properties depend on the anodising electrolyte, current density, passivation potential, time of anodising, as well as on the composition of aluminium substrates. If foreign ions are incorporated into the oxide film on aluminium, its properties can also change. Anions present in the solution may substitute some O²⁻ ions in the oxide films. Similarly, cations, formed by oxidation of the alloying element in an alloy, may replace some of the aluminium ions in the Al₂O₃ lattice.

The physical and chemical characteristics of the rare earths and their alloying behaviour have recently been analysed by Lundin and Wilson [19]: the rare earth metals are reported to improve mechanical properties like tensile strength, toughness and fatigue resistance in alloys such as AlLi, AlFeVSi and AlSi. Moreover, a beneficial effect of rare earth metals on the corrosion resistance of Mg- and Al-based alloys has recently been evidenced [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30].

Very few papers concern the effect of rare earth elements on the characteristics of anodic oxide films on aluminium and no data were found on their stability in alkaline solutions.

Thompson et al. investigated the anodic oxidation of AlCe, containing up to 27 at.% Ce, and Al–10 at.% Nd alloys in 0.1 M ammonium pentaborate solution [31], [32]. Cerium and neodymium ions, as well as Al³⁺ ions, are incorporated into the mainly amorphous anodic oxide films at the alloy/film interface in the same proportion as the concentration of the respective elements in the bulk alloys. The cerium and neodymium ions migrate outward in the films more rapidly than Al³⁺ ions leading to the formation of a layer of relatively pure cerium or neodymium oxide. For dilute AlCe alloys, a cerium-enriched layer of alumina adjacent to the alloy/film interface is observed. However, studies focusing on the influence of rare earth elements on the dissolution behaviour of anodic oxide films on aluminium have not yet been reported.

The aim of the present work is to examine the effect of cerium and yttrium additions on the characteristics of anodically formed oxide films on aluminium and their stability in sodium hydroxide solution. Cerium and yttrium were chosen as representatives of the light and heavy rare earths, respectively.