Probing corrosion initiation at interfacial nanostructures of AA2024-T3

Highlights

• Early stages of corrosion of AA2024-T3 were investigated at near-atomic scales.

• Compositions of the corroded nanostructures were quantified by atom probe.

• High-density near-surface defect structures were found to trigger corrosion.

• Dislocation arrays were discovered as pathways for corrosive species.

Abstract

High-strength aluminium alloys such as AA2024-T3 are often developed by the introduction of plastic deformation to a precipitate containing microstructure. These alloys contain complex, near-surface nanostructures whose effects on localised corrosion processes have not been well understood due primarily to the difficulty of characterising these heterogeneous compositions and structures. In this work, we observed entangled oxide networks co-located with dislocation structures piled up at corroded intermetallic particles of an AA2024-T3 alloy. It was revealed that dislocation arrays act as pathways for corrosive species and promote structural degradation at interfacial regions, providing a new insight into corrosion initiation at the nano-scale.

Introduction

Advanced structural materials such as high-strength aluminium alloys are widely used in industrial applications. Their desired mechanical strength can be achieved through engineering of the embedded nanostructures such as precipitates [1]. During precipitation hardening, dislocation-precipitate interactions can improve mechanical strength considerably [1] and [2], however, such high-density nanostructures (the total dislocation length in a cubic metre of a strained alloy can be as long as a light year (∼10¹⁶ m) [3]) could also have a serious impact on the localised corrosion behaviour of these alloys. It is therefore necessary to understand the effect of these nanostructures on the durability of these alloys in order to prevent corrosion induced failure of these materials [4] and [5]. Historically, corrosion studies have focused on the micron-scale (˃0.5 μm) [6] heterogeneities, such as intermetallic particles (IMPs), that are known to play a critical role in localised corrosion mechanisms such as pitting [5] and [6] and galvanic corrosion [6] and [7]. There are various studies of localised corrosion initiation at the nano-scale including passive film breakdown [4], [5], [8] and [9] and galvanic interactions around nano-sized particles [7], [10] and [11]. However, the impact of complex nano-scale heterogeneities on the onset of corrosion in these advanced alloys has not been fully understood [4], due primarily to the difficulty of characterising compositional and structural heterogeneities at the atomic scale. For example, dislocation structures such as partial dislocations, stacks of dislocations, dislocation loops, and dislocation cells are proposed to serve as active entities during the corrosion processes, possibly due to high levels of stored strain energy, however there has been a lack of experimental evidence [12], [13], [14], [15], [16], [17] and [18]. Previous studies of corrosion at the nano-scale have only looked at simplified examples of precipitates [4], [6], [19], [20], [21], [22], [23] and [24]. For example, Ralston et al. [10] investigated the role of hardening precipitates (˂0.2 μm) [6] and suggested that, while the matrix oxide might be able to bridge a small Al₂CuMg (S-phase) precipitate (less than 3 nm in size), the oxide covering a larger precipitate would breakdown and destroy the passive layer. In other cases, researchers [8], [25] have investigated the role of individual crystallographic defect structures, e.g. (sub)grain boundaries, in nano-scale corrosion initiation of nearly pure metals (e.g. Cu) free of IMPs. So far, the possible collaborative role of high-density crystallographic defects, in particular dislocations, during the early stages of corrosion has not been investigated.

For the first time, we have examined the combined role of precipitates and defect (e.g. dislocation) structures on localised aqueous corrosion of nano-sized samples, taking the advantage of the unique capabilities of atom probe tomography (APT) supported by other high resolution techniques. To facilitate the examination of the role of dislocation structures in the earliest stages of corrosion, we have chosen a typical high-strength alloy, drawn AA2024-T3 wire, to have a high density of dislocations together with IMPs, as demonstrated previously [15] and [26]. Using APT on freshly prepared samples, we have been able to quantify compositions and reveal three-dimensional (3D) corroded structures with near-atomic spatial resolution and ultra-high chemical sensitivity [27]. To our knowledge, previous APT studies have been confined to the study of oxidation at elevated temperatures, using the focused ion beam (FIB) technique to fabricate samples from specific regions of interest (i.e. site-specific samples) of either pre-oxidised or pre-corroded alloys [28], [29], [30], [31], [32]. In conjunction with APT, other complementary techniques such as transmission electron microscopy (TEM) and scanning electron microscopy/electron backscattered diffraction (SEM/EBSD) have also been used to show how heterogeneous nanostructures of AA2024-T3 contribute to the initiation stages of corrosion over several orders of magnitude.