Corrosion behaviour of additively manufactured 316L and CoCrNi

Alloys such as 316L and the medium‐entropy alloy CoCrNi are known for their excellent strength and corrosion properties. In the present study, bulk samples of 316L and CoCrNi (without and with 0.11 wt.% N) alloys fabricated using powder bed fusion laser beam (PBF‐LB) were tested in the as‐printed state for their corrosion behaviour in 0.5 M H2SO4 without and with added 3 wt.% NaCl. The tests were done using potentiodynamic measurements and the results were compared with those of the conventionally manufactured 316L. By means of angle‐resolved X‐ray photoelectron spectroscopy (ARXPS), the passive film characteristics were studied in terms of composition and film thickness. The 316L fabricated using PBF‐LB showed favourable passivation and corrosion behaviour as compared with its conventionally manufactured counterpart. It was observed that all the alloys fabricated using the PBF‐LB showed similar corrosion behaviour, but with CoCrNi and CoCrNi‐N showing better passivation behaviour than 316L alloys in the presence of NaCl. The ARXPS showed the presence of both hydroxide and oxides in all the alloys, with outer hydroxide layer and inner oxide layer. The ARXPS of both 316L alloys showed the expected presence of Cr–Fe oxide on the surface of as‐passivated samples, whereas the presence of sulphide was also depicted for the conventionally manufactured 316L, supposed to be detrimental to its corrosion behaviour. The CoCrNi‐based alloys showed the presence of only Cr2O3 layer in their passivated state, with Co and Ni acting as noble elements in the formation of the passive film. Upon micro‐alloying with the strong solid solution strengthener N, CoCrNi did not show any negative effect on either the corrosion behaviour or the passivation behaviour of the alloy.


| INTRODUCTION
Additive manufacturing (AM) processes refer to the processes that manufacture components layer by layer to near net shape in a bottom-up approach, as opposed to the traditional top-down subtractive manufacturing processes. 1,2 Among the AM techniques, powder bed fusion-laser beam (PBF-LB) uses high-power laser to rapidly melt and solidify thin layers of powder selectively and rapidly. Once a layer of material is selectively fused, a new layer of material is applied, and the process is repeated until a three-dimensional component with near neat shape and nominally full densification is produced. [1][2][3] Due to the inherent high thermal gradient and solidification rate, the resulting microstructure of the PBF-LB components typically consists of the epitaxially grown columnar grains and is unlike the microstructure of the components produced by traditional manufacturing techniques. [4][5][6][7][8][9][10] The ability to produce the parts on-demand, with near-fulldensification and near-net-shape components, makes PBF-LB a desirable technique for industrial applications. With the growing material portfolios of PBF-LB, there is also growing interest in the wide range of properties for the materials, and one such properties is the corrosion behaviour of the alloys. [11][12][13] The focus on the corrosion properties of the alloys manufactured through PBF-LB has been mainly on Al-based, Fe-based and Ti-based alloy systems. 11,12 Among the Fe-based alloys, 316L is the most studied alloy for its corrosion properties with studies mainly focusing on understanding the influence of the microstructure, porosity and inclusions on the corrosion behaviour. When compared with the wrought variant of 316L, the PBF-LB variant typically showed better corrosion resistance mainly owing to its unique microstructure and rapid solidification rate, which supresses the formation of undesirable inclusions that are detrimental to the corrosion performance. [14][15][16] The porosity of the parts is also an important aspect when it comes to the corrosion properties of the alloys, specifically considering the re-passivation behaviour. Studies on the influence of the porosity of the PBF-LB parts show that pore characteristics such as pore shape, size and distribution seem to heavily influence the pitting behaviour of the alloys. 11,12,17 With the present state of the art, the PBF-LB 316L provides nominally porefree materials; see, for example, ref. [18][19][20] High-entropy alloys (HEAs) and medium-entropy alloys (MEAs) are members of the novel class of alloys that comprise several principal elements in nearly equiatomic proportions. These alloys have attained increased interest over the last two decades due to their excellent mechanical properties and damage tolerance. [21][22][23][24][25] With increasing interest in this novel class of alloys, there is also increasing interest in understanding their corrosion behaviour. Due to the compositional complexity of these alloys, the corrosion behaviour will also vary based on the alloying elements in these alloys. In case of the single-phase face-centred cubic HEAs, the presence of high amount of Cr is known to significantly enhance the corrosion behaviour of the alloys. 26 Similarly, addition of elements such as Al, Ti, Mo, B and N is also known to further enhance the corrosion resistance of HEAs. 27 The corrosion behaviour of HEAs were reportedly either comparable or in most cases better than the stainless steels. Although there are increased efforts on the development of HEAs and MEAs using AM, very limited studies are available on the corrosion behaviour of these alloys, especially in comparison with their stainless steel counterparts.
Equiatomic CoCrNi is one of the most widely studied MEA owing to its excellent strength, ductility, oxidation and corrosion resistance, enhanced hydrogen embrittlement resistance and excellent cryogenic mechanical properties. [28][29][30] Addition of nitrogen as an interstitial is known to not only improve the strength but also to improve the corrosion behaviour of the wrought CoCrNi MEA. 28 The improved corrosion performance of the nitrogen-containing MEA was attributed to the presence of relatively higher fraction of oxides/hydroxides on the surface. With relatively higher Cr content as compared with the stainless steels, combined with the better mechanical properties, these alloys could be engineered to potentially perform better than traditional 316L in the corrosive environment.
The aim of this study is to understand, compare and benchmark the corrosion and passivation behaviour of CoCrNi and CoCrNi-N (0.11 wt.% N) manufactured by PBF-LB, with 316L (AM and conventionally manufactured). Conventional 316L is one of the most commonly used alloys in the field of marine engineering. [31][32][33] However, the conventional 316L is also susceptible to pitting corrosion in the saline environments owing to the presence of MnS inclusions though the absence of such precipitates in additively manufactured 316L is known to result in the improved pitting resistance. 14,15,34,35 The testing approach includes potentiodynamic polarisation studies in sulphuric acid without and with sodium chloride addition to simulate and understand their behaviour in the acidic and saline environments.
Detailed investigations of synthetically passivated samples using Xray photoelectron spectroscopy (XPS) were performed to depict the correlation between the passivation behaviour and the passive film chemistry of the MEAs and stainless steels.

| Materials
The materials used in the present study are shown in Table 1. The PBF-LB fabrication of 316L and CoCrNi and CoCrNi-N was performed using the EOS M100 machine equipped with 200 W Yb-fibre laser, with a focus diameter of 40 μm. In all cases, pre-alloyed powder in the size range of 20-50 μm supplied by Höganäs AB, were used for the studies. Cylindrical samples of 12 mm in diameter and 30 mm in height were printed in the vertical orientation with optimised parameters, yielding relative densities greater than 99.9% in all cases. As a benchmark, conventional 316L of same dimensions was prepared from extruded and machined rod specimens. Samples manufactured by PBF-LB and conventional routes were machined to the shape of cylindrical discs with a diameter of 10 mm and thickness of 2 mm. The machined samples were ground down to 4000 grits SiC paper followed by fine polishing using 3 and 1 μm suspended diamond solutions. All materials are fully austenitic and expected to contain oxide inclusions, 9 and the conventional 316L is expected to also contain manganese sulphide inclusions. 34 T A B L E 1 List of studied materials and their manufacturing route.

Material
Manufacturing route

| XPS studies
XPS analysis on as-passivated samples was conducted by using the PHI 5000 Versaprobe III instrument equipped with monochromatic AlK α X-ray source (E = 1486.6 eV), with a beam diameter of 100 μm and under ultra-high vacuum of 10 À9 mbar. Before the measurements, calibration of the instrument was carried out using Au4f 7/2 (83.96 eV),  Table 2.
As compared with the conventionally manufactured 316L, the PBF-LB 316L shows higher free corrosion potential (E corr ) and smaller corrosion current density (I corr ) in both environments. Also, the passivation current density is observed to be significantly smaller for the 316L manufactured using PBF-LB (Figure 1). These results suggest that the PBF-LB 316L has better corrosion resistance as compared with its wrought counterpart, as has been observed by Chao et al. 36 This improved corrosion resistance in the 316L manufactured using PBF-LB was attributed to the fast solidification rate, which suppresses the formation of phases, which could be detrimental to the corrosion behaviour of these alloys.  In the case of 316L alloys, it is clear from Figure 2 Figure 2). • The AM-fabricated 316L shows better corrosion properties than the conventional 316L used as benchmark in the current study. On all measures like free corrosion potential, corrosion current density, passivation current density and in particular passivation current density, the AM-fabricated 316L shows superior performance.

| CONCLUSIONS
• The AM-fabricated MEAs CoCrNi and CoCrNi-N show attractive corrosion properties, with free corrosion potential and corrosion current density better than the AM-fabricated 316L.
• Compared with the conventionally manufactured 316L, the AMprocessed material shows better corrosion properties in the pitting corrosion environment such as 0.5 M H 2 SO 4 + 3 wt.% NaCl. The actual mechanism will be subject to further studies. However, it could be anticipated that even if AM generally means incorporation of nm-size oxide inclusions (formed from powder surface oxide), these inclusions are suggested to not be harmful to the corrosion properties, whereas the conventionally manufactured 316L contains MnS inclusions, which are supposedly detrimental to its pitting corrosion resistance.
• The XPS analyses confirms that the passive film on the AMfabricated 316L consists of Cr-Fe oxide, rich in Cr. Also, in line with the behaviour of stainless steels, the passive film is found to be a mixture of hydroxide/oxide with higher hydroxide content on the top surface.
• For both CoCrNi and CoCrNi-N alloys, a mixture of Cr oxides and Cr hydroxides are observed. This is supposed to be a result of the fact that both Co and Ni act as the noble alloying elements and they do not take part in the oxide film formation.