3D printed N-doped CoCrFeNi high entropy alloy with more than doubled corrosion resistance in dilute sulphuric acid

The development of materials with both high strength and excellent corrosion resistance is a significant challenge in materials science. Traditional approaches often involve trade-offs between these properties. High strength typically requires microstructures that are detrimental to corrosion resistance, and vice-versa. For instance, single-phase alloys often possess low strength, while engineering secondary phases to enhance strength can lead to galvanic corrosion due to electrode potential differences. Similarly, thermomechanical treatments, while improving strength, can introduce intergranular corrosion along grain boundaries. This challenge is particularly acute in marine applications requiring materials with high strength and resistance to corrosive environments. High-entropy alloys (HEAs) offer a promising avenue to address this dilemma. Their multi-principal-element composition can promote single-phase formation, potentially enhancing corrosion resistance through the enrichment of anti-corrosion elements such as Cr. Selective laser melting (SLM) is a net-shape additive manufacturing technique that has shown promise in producing alloys with hierarchically heterogeneous microstructures, potentially improving strength, work hardening, and ductility. Previous research has explored the corrosion behavior of FeCoCrNi-based HEAs produced by various methods, including SLM, but the potential for significantly enhanced corrosion resistance through microstructure control using SLM has not been fully explored. This study investigates the potential of SLM to produce N-doped CoCrFeNi HEA samples that exhibit exceptional strength-ductility combinations and substantially improved corrosion resistance in acidic environments.

Existing literature highlights the challenges in achieving high strength and corrosion resistance simultaneously in alloys. Studies have shown that single-phase alloys typically lack sufficient strength, while introducing secondary phases or precipitates to increase strength often results in galvanic corrosion. Secondary thermomechanical treatments can enhance strength but can also lead to intergranular corrosion. The use of high-entropy alloys (HEAs) has emerged as a potential solution, leveraging their unique multi-principal-element composition and high entropy to achieve a single-phase structure that may improve corrosion resistance. However, processing methods significantly impact the resulting microstructure and thus the final properties. Prior studies on the corrosion behavior of various HEAs, such as FeCoCrNi-based compositions, fabricated via different techniques like melting, rolling, laser cladding, and additive manufacturing (SLM and DED) have shown varying results, suggesting that careful control of the microstructure is crucial for enhancing corrosion resistance. Specific attention has been given to the effect of SLM-induced microstructural features such as submicron dislocation walls on corrosion resistance. This work builds on the findings of these previous studies, focusing on the use of SLM to manipulate the microstructure of an N-doped CoCrFeNi HEA to achieve superior corrosion resistance.

The researchers fabricated N-doped equiatomic CoCrFeNiN0.07 HEAs using three different processing methods: selective laser melting (SLM), hot isostatic pressing (HIP), and HIP followed by cold rolling (CR). Pre-alloyed CoCrFeNiN0.07 powders (2–100 μm) were used as starting material, produced by gas atomization under a nitrogen atmosphere. For SLM, an FS271M machine (Farsoon, China) was used with a 400 W laser, a scanning speed of 1200 mm/s, a beam size of approximately 120 μm, a scanning distance of about 100 μm, and a layer thickness of around 30 μm. Nitrogen atmosphere was maintained throughout the SLM process, followed by annealing at 673 K for 3 h and water quenching. HIP processing involved degassing at 673 K for 10 h followed by consolidation at 1523 K and 140 MPa for 2 h, with furnace cooling. For CR, samples were cut from the HIP billet and cold rolled to reduce thickness from 10 mm to 2 mm. The densities of the SLM, HIP, and CR samples were measured using the Archimedes Drainage Method. Microstructure characterization employed various techniques, including X-ray diffraction (XRD) to identify phase structures, field-emission scanning electron microscopy (SEM) and EBSD to analyze surface corrosion morphologies and microstructures, focused ion beam (FIB) for cross-sectional analysis of passive films, and atom probe tomography (APT) to characterize elemental distributions. Transmission electron microscopy (TEM) provided detailed microstructural information, including bright-field and dark-field images and selected area electron diffraction (SAED) patterns. X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical composition of passive films. Electrochemical testing in 0.5 M H2SO4 involved open circuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS) to evaluate corrosion resistance, and potentiodynamic polarization (PDP) to assess passive behavior. Mott-Schottky (M-S) tests were conducted to analyze the semiconductive properties of passive films. All electrochemical tests were repeated at least three times for data reliability.

The SLM-processed CoCrFeNiN0.07 HEA exhibited a significantly lower passive current density (ip) compared to the HIP and CR samples (6.1 A/cm² vs. 13.8 A/cm² and 19.6 A/cm², respectively). Using 1/ip as a measure of corrosion resistance, the SLM sample demonstrated more than twice the corrosion resistance of the HIP sample and more than three times the resistance of the CR sample. EIS measurements showed the SLM sample had the largest diameter of the capacitive semicircle in Nyquist plots and the highest polarization resistance at 0.1 Hz in Bode plots, indicating superior corrosion resistance. The passive film on the SLM sample was approximately twice as thick (9.37 nm) as those on the HIP and CR samples (5.65 nm and 5.24 nm, respectively). Mott-Schottky plots revealed p-n heterojunction behavior in all samples, but the donor and acceptor densities for the SLM sample's passive film were about half those of the CR sample, indicating a more compact and less defective oxide film. XPS analysis revealed that the passive films were Cr-rich, with the SLM sample's passive film having a higher Cr2O3/Cr(OH)3 ratio compared to the HIP and CR samples (approximately 40% vs. 30% and 25%, respectively). This difference in Cr2O3 fraction is related to the high Cr concentration in the SLM's microstructure. The SLM sample showed a significantly faster drop in current density during the active to passive transition, highlighting rapid passive film formation. The SLM microstructure featured a dislocation cell structure with Cr segregation along cell boundaries, providing numerous nucleation sites for oxide formation and promoting rapid passive film growth. The cell structure also acted as a 3D diffusion network, enhancing cation transport and contributing to the rapid formation of the protective Cr oxide layer.

The superior corrosion resistance of the SLM-processed HEA is attributed to the unique 3D dislocation cell microstructure created by the SLM process. This microstructure facilitates Cr segregation along the cell boundaries, which act as numerous nucleation sites for the formation of a protective Cr oxide passive film. Furthermore, the cell structure enhances the diffusion of Cr ions from the bulk material to the surface, leading to the rapid growth of a thick, protective passive film. The enhanced diffusion is likely caused by the high density of dislocations that act as fast diffusion paths. The combination of these two factors (enhanced nucleation and accelerated growth) results in superior corrosion resistance compared to the HIP and CR samples. This finding directly addresses the research question of how to enhance both strength and corrosion resistance, demonstrating that SLM can create a microstructure which surpasses the limitations of traditional methods. The results have significant implications for the design of corrosion-resistant alloys, suggesting that manipulating microstructures through advanced processing techniques can overcome the traditional trade-off between strength and corrosion resistance.

This study successfully demonstrated that SLM processing can produce an N-doped CoCrFeNi HEA with substantially improved corrosion resistance in dilute sulfuric acid, significantly exceeding that of conventionally processed counterparts. This enhanced corrosion resistance is directly linked to the unique 3D dislocation cell microstructure produced by SLM, which promotes Cr segregation and rapid passive film formation. This work highlights the potential of additive manufacturing for creating alloys with superior combinations of mechanical and corrosion properties. Future research could explore the effects of different SLM parameters on microstructure and corrosion behavior, investigate other HEA compositions, and extend these findings to different corrosive environments.

The study focused on a specific corrosive environment (0.5 M H2SO4) and HEA composition. The generalizability of the findings to other environments and compositions requires further investigation. While the study indicates excellent corrosion resistance in the specific environment tested, long-term corrosion behavior and performance under more complex conditions should be explored. The relatively small sample size used in the electrochemical experiments might influence the statistical significance of the findings.