An Investigation on the role of crystallographic texture on anisotropic electrochemical behavior of a commercially pure nickel manufactured by laser powder bed fusion (L-PBF) additive manufacturing

Abstract

The layer-wise deposition of the laser powder bed fusion (L-PBF) process offers a tailorable microstructure with anisotropic properties. This research aimed to investigate the contribution of crystallographic texture on the anisotropic electrochemical/corrosion behavior of a L-BPF processed commercially pure nickel (Ni) cube along surfaces oriented parallel and perpendicular to the building direction. In order to exclusively assess the role of crystallographic texture on the corrosion behavior, the electrochemical measurements were performed on different surfaces of the L-PBF processed cube in alkaline and acidic chloride-free solutions. Orientation microscopy analysis showed that the morphology of grains along the parallel surface to the building direction was columnar, and their crystallographic orientation distribution was uniform. In contrast, the morphology of grains on the surface aligned normal to the building direction was equiaxed and highly oriented toward [110]. This bias in the orientation distribution led to the highest residual affinity for oxidation in comparison to the other planes in the FCC (Face Centered Cubic) crystal structure. In the case of 1 M NaOH solution, the hydroxide layer formed at the exposed surface did not effectively control the cation ejection from the surface. Also, surfaces oriented normal to the building direction, with lower surface atomic density, showed an elevated dissolution rate. Conversely, in the case of 1 M H₂SO₄ solution, a more integrated and thicker oxide film formed in comparison to the surface aligned parallel to the building direction. This fact led to reduced surface dissolution. This study shows that engineering the topology and microstructure based on the desired properties can remarkably promote the performance of additively manufactured materials in extreme environments.

Introduction

Metallurgical compatibility and high oxidation resistance of nickel (Ni) provide superior performance for a wide range of Ni-based superalloys. This fact makes Ni a technologically viable material in energy, transportation, aerospace, semiconductor, oil, and gas industries [1,2]. Also, Ni and Ni-based oxides are used extensively as catalysts and electrocatalysts [3,4] in reagentless sensors [5,6], hydrogen production and oxygen evolution reactions [7,8], wastewater treatment and microbial fuel cells [9], [10], [11], proton exchange membrane in alkaline fuel cells [7,12,13] and energy storage devices [3,14,15]. However, the application of these alloys is limited due to the high production cost and difficulties in conventional manufacturing processes such as investment casting, hot forging, and complicated machining [16,17].

Additive manufacturing (AM) has been providing an opportunity for the development of a new generation of products with higher accuracy and flexibility in design. In AM processes, parts are fabricated by the deposition of layers over layers only in areas guided by design, which considerably reduces the waste of materials [18,19]. Recent developments in AM processes, e.g., laser powder bed fusion (L-BPF), of metallic parts, enable fabrication of near-net-shape components with complicated geometries and miniature designs [20], [21], [22], [23]. Furthermore, with the advent of the L-BPF technique, prototype components can be fabricated readily in the early stages of mass production at a minimal cost. This fact enables incremental testing and iterative designs [17,24]. The cyclic melting and solidification during the layer-wise deposition process form complex and tailorable microstructures [25], [26], [27], [28] with a remarkable anisotropy in properties (e.g., yield strength) [29], [30], [31], [32], [33], [34]. Interestingly, this anisotropy can be implemented positively by optimizing the part topology and the desired properties with respect to the building direction [32,[35], [36], [37]].

In general, the corrosion and the electrochemical behavior of an AM component are governed by the existing phases and microstructural features such as the grain size distribution, morphology, and crystallographic orientation distribution. Although many studies have investigated the role of manufacturing and post-processing parameters on the corrosion behavior of L-PBF processed alloys [38], [39], [40], [41], [42], [43], only a limited investigations reported the anisotropic corrosion behavior of a multi-phase part with respect to the building direction [44], [45], [46], [47], [48], [49]. The anisotropic localized corrosion (e.g., pitting) is primarily assigned to the formation of porosities and different distributions of phases along the surfaces, which result in localized corrosion with a wide range of corrosion rate values [50]. For instance, for a corrosion investigation of an additively manufactured Ti-6Al-4V in 3.5 wt.% NaCl solution, Wu et.al. [45] showed that surfaces parallel to the building direction (side) have a superior corrosion performance compared to the surfaces aligned perpendicular to the building direction (top). This behavior was attributed to the formation of a coarser α-phase on the side surface due to a relatively slower cooling rate compared to the top surface. Similarly, Li et al. [51] attributed the anisotropic anodic dissolution of an additively manufactured Ti-6Al-4V in 15 wt.% NaCl solution to the distribution and size of α-phase on the top and side surfaces. Shahriari et al. [44] showed that the localized corrosion of a L-PBF processed maraging stainless steel in 3.5 wt.% NaCl solution was more severe on the top surface compared to the side surface. This anisotropic performance in corrosion was attributed to a higher level of grain boundary density, a finer morphology of laths, and pre-existing pores on the top surface compared to the side surface. Also, for an additively manufactured TiC reinforced Inconel 718, it is reported that insufficient development of dendrites and agglomeration of TiC particles along surfaces aligned vertically to the building direction lead to the formation of large pores and inhomogeneous microstructures. This phenomenon undermines the corrosion performance of these surfaces in a 3.5 wt.% NaCl solution [47]. However, to the best of the authors' knowledge, the role of crystallographic texture was not highlighted in any of the mentioned studies and, there is no comprehensive investigation on the role of texture on the electrochemical behavior of additively manufactured metal parts yet.

As discussed, previous studies on the corrosion behavior of AM have merely focused on chloride contaminated media as the aqueous corrosive environment. Noteworthy, the charge and the interfacial behavior between the metal (electrode) surface and electrolyte were found to be significantly modulated by the nature of ions. The variation in the corrosion behavior is assigned to the presence of cosmotrope (water-structure-makers) or chaotrope (water-structure-breakers) ions in the examining electrolyte [52], [53], [54]. As such, the latter ions which have a weak hydration shell can initiate localized corrosion, while the former ions are surrounded by a tight hydration shell, which leads to a uniform corrosion behavior [53,55]. Since chloride ions are chaotrope ions, chloride contaminated environments are well-known in inducing pitting corrosion. Accordingly, in this investigation, in order to undermine the effect of localized corrosion and highlight the uniform corrosion behavior, the electrochemical measurements were conducted in alkaline and acidic environments using OH⁻ and SO₄⁻ which are known as cosmotrope ions.

This research aimed to assess the role of unusual crystallographic texture, which is particularly induced by L-PBF processes, on the electrochemical behavior and corrosion performance of an L-PBF manufactured Ni part. Considering the fact that alloying elements and formation of different phases may change the trend of electrochemical responses, to study the role of texture on corrosion behavior, a single-phase part was fabricated by a relatively pure feedstock part. By employing electrochemical measurements on a single-phase electrode in cosmotrope electrolytes, this work provides an insight into the relationship between the anisotropic electrochemical response and the non-uniform crystallographic orientation distribution forms in additively manufactured parts. The collected result showed that AM processes can potentially provide an additional degree of freedom to tune the corrosion and electrochemical behavior by optimizing the printing direction and engineering the texture evolutions.