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Unanticipated drastic decline in pitting corrosion resistance of additively manufactured 316L stainless steel after high-temperature post-processing
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Short Communication
Unanticipated drastic decline in pitting corrosion resistance of additively manufactured 316L stainless steel after high-temperature post-processing Majid Laleha,b, Anthony E. Hughesb,c, Wei Xua, Pavel Cizekb, Mike Yongjun Tana,b,* a
Deakin University, School of Engineering, Faculty of Science and Technology, Geelong Waurn Ponds Campus, Waurn Ponds, Victoria, 3216, Australia Deakin University, Institute for Frontier Materials, Geelong Waurn Ponds Campus, Waurn Ponds, Victoria, 3216, Australia c Commonwealth Scientific and Industrial Research Organisation (CSIRO), Mineral Resources, Private Bag 10, Clayton South, Victoria, 3169, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Selective laser melting Post-processing Inclusion Pitting corrosion Stainless steel 316L
The pitting corrosion resistance of 316L stainless steel (316L SS) additively manufactured by selective laser melting (SLM) has been reported to be substantially higher than its commercial counterpart due to the elimination of MnS inclusions. Here we report that the pitting corrosion resistance of the SLM-produced 316L SS declines drastically after thermal post-processing above 1000 °C. This unanticipated drastic decline in pitting resistance is explained based on the formation of deleterious MnS inclusions in the SLM-produced 316L SS after high-temperature thermal post-processing. This finding may have wide implications for determining suitable post-processing and industry application conditions for SLM-produced 316L SS.
1. Introduction Stainless steels made via additive manufacturing (AM) are commonly exposed to high temperatures either during their thermal postprocessing or during industry applications. Thermal post-processing by solution annealing or hot isostatic pressing (HIP) is a common practice to homogenise the microstructure, remove porosity, and relieve residual stresses that usually exist in AM-processed parts with the aim of improving mechanical properties [1–4]. For instance, high-temperature solution annealing (at 1066 °C) has been shown to be a necessary step in post-processing for attaining a reasonable resistance to stress corrosion cracking (SCC) in high-temperature applications of 316L stainless steel (hereafter 316L SS) produced by powder-bed AM [5]. HIP processing at 1150 °C has been shown to improve the threshold value for crack growth (fatigue limit) in the 316L SS produced by selective laser melting (SLM, the metal AM process), mainly due to the coarser grains achieved upon HIP [6,7]. On the other hand, SLM-produced stainless steels are used in industries such as chemical and petrochemical, where elevated temperatures and highly corrosive environments exist. At present, the influence of exposure of SLM-produced 316L SS parts at such elevated temperatures on their pitting corrosion resistance is not clear, even though pitting corrosion resistance of the as-built SLMproduced 316L SS has been investigated in recent years [8–13]. It has generally been shown that the pitting corrosion resistance of as-built SLM-produced 316L SS is substantially higher than that of its
conventionally-produced counterpart [10,11]. The current view is that SLM processing is capable of improving the pitting corrosion resistance of 316L SS through the elimination of deleterious manganese sulphide (MnS) inclusion [9,11,14]. It is well-known that inclusions, especially MnS, play a prominent role in the pitting corrosion resistance of 316L SS [15–17]. For the SLM-produced 316L SS, inclusions have been reported to be spherical in shape, and mostly enriched with Mn, Si, and O [9]. This is obviously different from deleterious MnS inclusions in the conventionally-produced 316L SS, which are often the preferential sites for pit initiation upon exposure to corrosive environments [18,19]. In the present study, the effect of thermal post-processing on pitting corrosion behaviour of SLM-produced 316L SS was investigated by isothermal heat-treatment at temperatures ranging from 900 °C to 1200 °C. Microstructural characteristics, inclusions type and their chemical composition were examined using electron back-scattered diffraction (EBSD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) associated with energy dispersive spectroscopy (EDS). Potentiodynamic polarisation measurements and ferric chloride immersion tests were used to evaluate the pitting corrosion resistance. 2. Material and methods 2.1. Sample preparation Commercially available gas-atomized 316L SS powder (SLM
⁎ Corresponding author at: Deakin University, School of Engineering, Faculty of Science and Technology, Geelong Waurn Ponds Campus, Waurn Ponds, Victoria, 3216, Australia. E-mail address: [email protected] (M.Y. Tan).
https://doi.org/10.1016/j.corsci.2019.108412 Received 4 October 2019; Received in revised form 19 December 2019; Accepted 23 December 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Majid Laleh, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108412
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mechanically grounded with abrasive SiC papers (down to 4000, each step for almost 5 min) on a disc rotating at 150 rpm, followed by rinsing with distilled water and drying with hot air. For SLM-produced specimens, the electrochemical tests were done on the transverse plane (perpendicular to the building direction), since our preliminary experiments showed no obvious difference in the pitting corrosion at different planes, which is in agreement with the literature for pitting corrosion of SLM-produced 316L SS [20]. Polarisation tests were repeated at least five times for each specimen, and the average Eb values with standard deviation were reported. To clarify the correlation between inclusions and pitting corrosion, immersion tests were carried out in 6 wt% ferric chloride (FeCl3) solution for 48 h based on the ASTM G48 - 11(2015) standard test method. According the ASTM standard, the relative pitting corrosion performance of alloys in the ferric chloride solution can be correlated to performance in certain real environments, such as natural seawater at ambient temperature. The immersion tests were carried out at room temperature (24 ± 2 °C). The specimens were later removed from the solution, rinsed with distilled water and dried with hot air, and then analysed using SEM to examine the possible changes in inclusion morphology after immersion tests.
Table 1 Chemical composition (wt%) of wrought 316L SS and the powder used in SLM processing.
Powder As-built SLM wrought
Cr
Ni
C
Si
Mn
P
S
Mo
Fe
17.13 16.6 16.9
10.92 11.60 10.30
0.02 0.02 0.02
0.67 0.56 0.50
1.16 1.11 1.77
0.02 0.01 0.04
0.01 0.01 0.01
2.33 2.70 2.11
Bal. Bal. Bal.
Solutions, Germany) with particle size ranging between 5 and 40 μm was used as the feedstock material for SLM processing. The chemical composition of the powder is presented in Table 1. Chemical compositions of the commercial (wrought) and as-built SLM-produced 316L SS are also presented in Table 1 for comparison purposes. Samples of 10 × 10 × 10 mm3 were produced using SLM (SLM Solutions 125 H L) with an optimised set of processing parameters (laser power of 175 W, scanning speed of 400 mm/s, hatch spacing of 100 μm, and layer thickness of 30 μm). A multi-directional meander scanning pattern rotating by 67° between successive layers was used. SLM processing was conducted under an argon atmosphere to prevent the parts from being oxidised. The building plate was pre-heated to 200 °C to minimise buildup of residual stress. These SLM processing parameters were selected based on the preliminary experiments to produce highly dense specimens (over 99.5 %). The relative density of the SLM-produced specimen was measured according to the Archimedes method by immersing the specimens in distilled water using an Electronic Densimeter (Model SD-200 L) of 0.1 mg/cm3 resolution. The SLM-produced specimens were then solution annealed in the temperature range of 900−1200 °C for different holding times (15 and 60 min) followed by water quenching. The “as-built” term was hereafter referred to those untreated SLM-produced specimens.
3. Results and discussion Solution annealing heat-treatment which dissolves carbides into the solid solution of the γ-Fe matrix, is typically applied to austenitic stainless steels to increase corrosion resistance [21]. Subsequent rapid quenching tends to suppress the recurrence of carbides. For type 316L SS, the solution annealing temperature was reported to range between 1040 °C and 1120 °C [22]. In the present study, a broader range of temperature, i.e. between 900 °C and 1200 °C, was used to monitor the influence of thermal post-processing (i.e. heat-treatment) on the pitting corrosion resistance. Fig. 1a shows the representative potentiodynamic polarisation curves for the as-built and heat-treated SLM-produced 316L SS. A potentiodynamic polarisation curve of the commercial counterpart is also provided for comparison purposes. The average values of breakdown potential, Eb, are presented in Fig. 1b. The as-built SLM-produced 316L SS showed a substantially higher breakdown potential compared to its commercial counterpart. Heat-treated SLMproduced 316L SS, however, showed a different pitting corrosion performance. Specimens heat-treated at 900 °C and 1000 °C exhibited a similar pitting potential to that of the as-built SLM-produced 316L SS. This is in contrast to a drastic deterioration (almost 300 mV) of pitting corrosion resistance observed for those specimens heat-treated at higher temperatures of 1100 °C and 1200 °C. It is well-known that an important microstructural feature that plays a key role in the pitting corrosion of 316L SS is inclusions [15,19,23–25]. Typical inclusions present in the as-built SLM-produced 316L SS with their corresponding elemental maps are shown in Fig. 2; the inclusions are spherical and enriched in Mn, Si, and O (manganese silicate). Numerous small spherical inclusions are also visible, having a similar chemical composition to that of the larger inclusion in Fig. 2a, as evidenced by STEM-EDS analysis shown in Fig. 2b. The size of inclusions varies from a few microns to tens of nanometers, however, their chemical composition seems to be similar based on SEM-EDS and STEM-EDS analysis. Indeed, the substantially high pitting corrosion resistance of the as-built SLM-produced 316L SS is due to the absence of deleterious MnS inclusion, which is in agreement with the literature [9,11]. SEM-EDS analysis of the inclusions in the heat-treated SLM-produced 316L SS revealed the formation of new types of inclusions, as will be shown in the following. The major concern is the MnS inclusion, regarding its detrimental influence on the pitting corrosion resistance. No MnS inclusion was detected for specimens heat-treated at temperatures of 900 and 1000 °C. Interestingly, after heat-treatment at higher temperatures of 1100 and 1200 °C, numerous MnS inclusions
2.2. Characterisation The shape, chemical composition, and distribution of the inclusions were analysed using SEM-EDS (ZEISS SUPRA 55 V FEG, and JEOL JSM 7800 F FEG). Electron backscatter diffraction (EBSD) analysis was performed using a ZEISS LEO 1530 FEG SEM, with a scan step size of 0.3 μm at an accelerating voltage of 20 kV. Samples for SEM-EDS and EBSD analysis were mechanically polished to a mirror finish with the final polishing using a 0.04 μm oxide polishing suspension (OPS). Transmission electron microscopy (TEM) operated at 200 kV was employed for a detailed analysis of inclusions in the as-built SLM-produced specimen, using a JEOL JEM 2100 F microscope equipped with an EDS detector. EDS mapping was performed in a scanning-transmission TEM (STEM) mode using a spot size of 1 nm. For TEM analysis, thin slices with a thickness of approximately 500 μm were cut from the specimens and then mechanically ground to a thickness of around 100 μm. Discs with a diameter of 3 mm were subsequently punched from the slices and further ground using 4000 abrasive SiC paper until their thickness was reduced to 60−70 μm. Final thinning was carried out in a twin-jet electro-polishing unit using a Struers Tenupol 5 device. 2.3. Electrochemical tests Potentiodynamic polarisation tests were conducted in 0.6 M NaCl solution at room temperature (24 ± 2 °C) by sweeping the potential in the anodic direction from -0.2 V to +1.4 V vs. open circuit potential (OCP) with a scan rate of 10 mV/min in an electrochemical system comprising 316L SS specimens (with the surface area of almost 1 cm2) as the working electrode, a titanium mesh as the counter electrode, and a silver/silver chloride (Ag/AgCl) electrode as the reference electrode, according to ASTM G5. The specimens were immersed into the solution for one hour to get a stable OCP before starting the potentiodynamic polarisation measurements. Prior to corrosion tests, specimens were 2
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Fig. 1. (a) Potentiodynamic polarisation curves for the heat-treated SLM-produced 316L SS specimens recorded in 0.6 M NaCl solution at room temperature. (b) Values of breakdown potential obtained from polarisation curves, indicating lower pitting corrosion resistances for the specimens heat-treated at 1100 °C and 1200 °C compared to those heat-treated at 900 °C and 1000 °C. Error bars represent the standard deviation obtained from at least five measurements under identical conditions.
produced 316L SS is quite high (up to 106-108 °C/s [26,27]) so the material is not thermodynamically in the equilibrium condition, meaning that Mn and S did not have sufficient time to segregate and form MnS inclusions, and the matrix is left saturated with Mn and S [28]. Subsequent heat-treatment at temperatures above 1000 °C will move the whole system towards the thermodynamically equilibrium conditions and lead to MnS formation. Heat-treatment also resulted in other types of inclusions’ transformation in the SLM-produced 316L SS. SEM-EDS analysis of some representative inclusions (over 100 inclusions were characterised) in the heat-treated SLM-produced specimens are displayed in Fig. 4. No obvious difference was detected for the inclusions upon heat-treatment at 900 °C and 1000 °C; inclusions still remained spherical in shape and
were formed. The number density of the MnS inclusions versus heattreatment condition is presented in Fig. 3, indicating that the temperature and holding time both affect the population of MnS inclusions; the number density increased by increasing the holding time from 15 to 60 min and temperature from 1100 to 1200 °C (refer to supplementary material 1 to see the SEM-EDS images and distribution of MnS inclusions). Furthermore, from the SEM-EDS analysis in supplementary material 1, it can be seen that heat-treatment temperature and holding time do not affect the size of the inclusions to any obvious extent. The formation of MnS inclusion in the SLM-produced 316L SS after hightemperature heat-treatment has not been reported before and the underlying mechanism for such transformation is still unclear. One possible explanation is because the solidification rate for the as-built SLM3
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Fig. 2. (a) SEM-EDS analysis of a typical inclusion in the as-built SLM-produced 316L SS, showing the micron-sized inclusions are spherical in shape and mostly enriched in Mn, Si, and O. (b) High-magnification STEM-EDS analysis of the small inclusions in the as-built SLM-produced 316L SS, indicating a similar chemical compositions for nano-sized inclusions compared to those of the micron-sized ones.
recent study, Yan et al. [30] characterised nano-scale oxide inclusions in SLM-produced 316L SS, where they found that MnSiO3 oxide inclusions transformed to spinel MnCr2O4 upon heat-treatment at 1200 °C, which is in good agreement with the results reported here. In another study on an Fe-Cr alloy, Shibata et al. [31] reported a similar phenomenon of inclusion transformation, showing the change from manganese silicate to manganese chromite inclusions when subjected to heat-treatment at 1200 °C. It has been suggested that the decrease in the solubility of chromium oxide in manganese silicate at such temperature and also the reaction at the interface between the oxide inclusions and the metal matrix are possible factors leading to the transformation of inclusions. The results show that heat-treatment had a significant influence on inclusions in the SLM-produced 316L SS. The inclusions in the heattreated SLM-produced 316L SS can be classified into three categories based on their chemical composition. Type I is the manganese silicate inclusion, which is present in the as-built SLM-produced specimen and those heat-treated at 900 °C and 1000 °C for both holding times of 15 and 60 min. Type II is the manganese chromite inclusion, which is irregular-shaped and appears in the SLM-produced specimens heattreated at 1100 °C and 1200 °C. Type III is the MnS inclusion, which coexists with the type II manganese chromite inclusions in those specimens heat-treated at 1100 °C and 1200 °C. The formation of the MnS inclusion is a particular concern, because it has been known as the predominant site for the initiation of pitting in 316L SS [16,23,32–34]. The influence of inclusion compositional and structural change on the pitting corrosion resistance is shown in potentiodynamic polarisation curves in Fig. 1, indicating the drastic decline in the pitting resistance of the SLM-produced 316L SS heat-treated at temperatures above 1000 °C. Such a reduction in pitting corrosion resistance could be attributed to the change of inclusions, where deleterious MnS inclusions appear after subjected to heat-treatment at temperatures above 1000 °C. This is supported by the fact that after high-temperature heat-treatment, the
Fig. 3. Effect of heat-treatment conditions on the number density of the MnS inclusions in the SLM-produced 316 L SS.
enriched in Mn, Si, and O (manganese silicate), at both 15 and 60 min holding times (Fig. 4a and b). After 15 min of heat-treatment at 1100 °C and 1200 °C, SEM-EDS analysis (Fig. 4c and d) clearly showed the formation of Mn-Cr-O enriched inclusions (manganese chromite). The newly formed manganese chromite inclusions tended to be irregularshaped (angular), reflecting their spinel structure. Note that manganese chromite inclusions were detected in all specimens heat-treated at 1100 °C and 1200 °C. At shorter holding time (15 min), the manganese silicates partially transformed to manganese chromite (Fig. 4c and d). It has been reported that this transformation requires chromium diffusion towards the inclusions and manganese and silicon transfer away from the inclusions [29]. For those specimens heat-treated for 60 min at 1100 °C and 1200 °C, pure spinel manganese chromite inclusions (without residual manganese silicate) were found (Fig. 4e-h). In a 4
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Fig. 4. SEM-EDS analysis of the inclusions in the heat-treated SLM-produced 316L SS: (a) 900 °C-15 min, (b) 1000 °C-60 min, (c) 1100 °C-15 min, (d) 1200 °C-15 min, (e) 1100 °C-60 min, (f, g, h, i) 1200 °C-60 min, (j) 1100 °C-15 min. Red dashed lines in SEM images shows where the EDS line scans were made. The coloured lines in EDS analysis represent the following elements: light blue for Fe, red for Cr, blue for Mn, yellow for O, orange for Si, green for Al, and pink for S (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
built and heat-treated SLM-produced 316L SS specimens. Readers are referred to the authors’ previous publication to see the microstructure of the commercial 316L SS [36]. No obvious differences to the as-built condition were found in terms of average grain size and grain boundary character, as summarised in Table 2. This indicates that solution annealing at temperature range between 900 °C and 1200 °C for 15 min and 60 min followed by water quenching has no noticeable influence on the microstructure of the SLM-produced 316L SS. It should be mentioned here that the SLM-produced 316L SS specimens used in this study are highly dense (over 99.5 %), and heat-treatment did not show any influence on the porosity. So, porosity is not an influencing factor in the pitting corrosion measurements reported here. Moreover, based on EBSD phase-contrast analysis, no phase changes have been observed for the SLM-produced 316L SS specimens under the heat-treatment conditions applied in this study.
pitting corrosion behaviour of the SLM-produced 316L SS resembles the commercial 316L SS (see Fig. 1). Despite the more prominent role of MnS inclusions in the pitting corrosion of 316L SS, oxide inclusions might also act as pit initiation sites. Limited information is available in the literature regarding the influence of oxide inclusions on pit initiation, however, the lower surface potential of the oxide inclusions compared to the surrounding matrix has been generally suggested as the driving force for pitting corrosion in the presence of sulphur and chloride ions [35]. To get more insights on the susceptibility of different types of inclusions in the heattreated SLM-produced 316L SS, immersion tests in ferric chloride solution were conducted. Fig. 5 displays SEM micrographs of some representative inclusions before and after 48 h immersion in a ferric chloride solution. No noticeable change was detected for the manganese silicates and manganese chromite inclusions, indicating their superior resistance to dissolution (Fig. 5a-c). However, Fig. 5d shows that the MnS-containing portion of the inclusion has been dissolved from the interface with the metal matrix and has thus acted as a pit initiation site. Microscopy analysis has also been conducted to see the influence of the heat-treatment on the microstructural characteristics of the SLMproduced 316L SS. Fig. 6 shows the inverse pole figure (IPF), grain boundary, and Kernel average misorientation (KAM) maps for the as-
4. Summary This study has revealed a major decrease in pitting corrosion resistance for SLM-produced 316L SS specimens after being heat-treated at temperatures of 1100 °C and 1200 °C compared to the as-built SLMproduced specimen and those heat-treated at 900 °C and 1000 °C. SEMEDS analysis showed that the shape and chemical composition of the 5
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Fig. 5. SEM images of a number of inclusions before (left column) and after (right column) 48 h immersion in 6 wt % ferric chloride solution. No obvious change was observed in manganese silicate and manganese chromite inclusions (a–c) after the immersion test. However, pits were found to form at the site of MnS inclusions (d). Schematic representation of the chemical composition of the inclusions are also presented in the middle column, with green applied to manganese chromite, blue applied to manganese silicate, yellow applied to silicon oxide, and red applied to MnS (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
consideration for publication elsewhere.
inclusions in the as-built SLM-produced 316L SS changed after heattreatment at temperatures higher than 1000 °C. Typical manganese silicate inclusions in the as-built SLM-produced 316L SS were transformed to manganese chromite inclusions upon heat-treatment at temperatures above 1000 °C. In particular, deleterious MnS inclusion appeared after high-temperature heat-treatment, which was deemed as the main reason for drastic decline in pitting corrosion resistance of the SLM-produced 316L SS heat-treated at temperatures above 1000 °C. Indeed, it has been shown that the as-built SLM-produced 316 L SS have a substantially higher pitting resistance compared to its commercial counterpart, and subsequent high-temperature thermal post-processing decreases its pitting resistance. This finding suggests that high-temperature thermal post-processing might not be suitable for SLM-produced 316L SS and that SLM-produced 316L SS may have limitations for high-temperature applications.
Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement Author agreement Financial support from Deakin University Postgraduate Research Scholarship (DUPRS) is greatly appreciated. Deakin University’s Advanced Characterisation Facility is acknowledged for use of the microscopy instruments.
I certify that all authors have seen and approved the final version of the manuscript being submitted. They warrant that the article is the authors' original work, hasn't received prior publication and isn't under 6
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Fig. 6. Inverse pole figure (IPF) coloured orientation maps (top raw); grain boundary maps (middle raw), in which black lines represent high angle grain boundaries (θ > 15°), purple lines represent low angle grain boundaries (15° > θ > 2°), red and blue lines represent ∑3 and ∑9 CSL boundaries, respectively; Kernel average misorientation (KAM) maps (bottom raw) for the as-built and heat-treated SLM-produced 316L SS. EBSD acquisition was performed on the transverse (perpendicular to the building direction) plane (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.). Table 2 Microstructural characteristics for as-built and heat-treated SLM-produced 316L SS measured from the EBSD results presented in Fig. 6.
As-built SLM 900 °C-15 min 900 °C-60 min 1000 °C-15 min 1000 °C-60 min 1100 °C-15 min 1100 °C-60 min 1200 °C-15 min 1200 °C-60 min
Average grain size (μm)
Length of low-angle grain boundaries (2° < θ < 15°) (mm/mm2)
Length of high-angle grain boundaries (15° < θ) (cm/mm2)
Length of twin boundaries (∑3) (mm/mm2)
8.77 ± 0.86 9.65 ± 0.84 10.23 ± 0.85 9.16 ± 0.83 10.31 ± 0.83 10.43 ± 0.80 11.07 ± 0.77 10.39 ± 0.82 11.50 ± 0.76
14.92 12.12 13.84 18.16 12.28 15.16 17.64 16.88 16.20
35.60 33.76 28.76 33.64 30.32 29.24 26.52 27.88 26.96
38.16 33.20 25.20 32.32 36.20 31.20 31.12 22.48 30.76
Appendix A. Supplementary data
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Short Communication
Unanticipated drastic decline in pitting corrosion resistance of additively manufactured 316L stainless steel after high-temperature post-processing Majid Laleha,b, Anthony E. Hughesb,c, Wei Xua, Pavel Cizekb, Mike Yongjun Tana,b,* a
Deakin University, School of Engineering, Faculty of Science and Technology, Geelong Waurn Ponds Campus, Waurn Ponds, Victoria, 3216, Australia Deakin University, Institute for Frontier Materials, Geelong Waurn Ponds Campus, Waurn Ponds, Victoria, 3216, Australia c Commonwealth Scientific and Industrial Research Organisation (CSIRO), Mineral Resources, Private Bag 10, Clayton South, Victoria, 3169, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Selective laser melting Post-processing Inclusion Pitting corrosion Stainless steel 316L
The pitting corrosion resistance of 316L stainless steel (316L SS) additively manufactured by selective laser melting (SLM) has been reported to be substantially higher than its commercial counterpart due to the elimination of MnS inclusions. Here we report that the pitting corrosion resistance of the SLM-produced 316L SS declines drastically after thermal post-processing above 1000 °C. This unanticipated drastic decline in pitting resistance is explained based on the formation of deleterious MnS inclusions in the SLM-produced 316L SS after high-temperature thermal post-processing. This finding may have wide implications for determining suitable post-processing and industry application conditions for SLM-produced 316L SS.
1. Introduction Stainless steels made via additive manufacturing (AM) are commonly exposed to high temperatures either during their thermal postprocessing or during industry applications. Thermal post-processing by solution annealing or hot isostatic pressing (HIP) is a common practice to homogenise the microstructure, remove porosity, and relieve residual stresses that usually exist in AM-processed parts with the aim of improving mechanical properties [1–4]. For instance, high-temperature solution annealing (at 1066 °C) has been shown to be a necessary step in post-processing for attaining a reasonable resistance to stress corrosion cracking (SCC) in high-temperature applications of 316L stainless steel (hereafter 316L SS) produced by powder-bed AM [5]. HIP processing at 1150 °C has been shown to improve the threshold value for crack growth (fatigue limit) in the 316L SS produced by selective laser melting (SLM, the metal AM process), mainly due to the coarser grains achieved upon HIP [6,7]. On the other hand, SLM-produced stainless steels are used in industries such as chemical and petrochemical, where elevated temperatures and highly corrosive environments exist. At present, the influence of exposure of SLM-produced 316L SS parts at such elevated temperatures on their pitting corrosion resistance is not clear, even though pitting corrosion resistance of the as-built SLMproduced 316L SS has been investigated in recent years [8–13]. It has generally been shown that the pitting corrosion resistance of as-built SLM-produced 316L SS is substantially higher than that of its
conventionally-produced counterpart [10,11]. The current view is that SLM processing is capable of improving the pitting corrosion resistance of 316L SS through the elimination of deleterious manganese sulphide (MnS) inclusion [9,11,14]. It is well-known that inclusions, especially MnS, play a prominent role in the pitting corrosion resistance of 316L SS [15–17]. For the SLM-produced 316L SS, inclusions have been reported to be spherical in shape, and mostly enriched with Mn, Si, and O [9]. This is obviously different from deleterious MnS inclusions in the conventionally-produced 316L SS, which are often the preferential sites for pit initiation upon exposure to corrosive environments [18,19]. In the present study, the effect of thermal post-processing on pitting corrosion behaviour of SLM-produced 316L SS was investigated by isothermal heat-treatment at temperatures ranging from 900 °C to 1200 °C. Microstructural characteristics, inclusions type and their chemical composition were examined using electron back-scattered diffraction (EBSD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) associated with energy dispersive spectroscopy (EDS). Potentiodynamic polarisation measurements and ferric chloride immersion tests were used to evaluate the pitting corrosion resistance. 2. Material and methods 2.1. Sample preparation Commercially available gas-atomized 316L SS powder (SLM
⁎ Corresponding author at: Deakin University, School of Engineering, Faculty of Science and Technology, Geelong Waurn Ponds Campus, Waurn Ponds, Victoria, 3216, Australia. E-mail address: [email protected] (M.Y. Tan).
https://doi.org/10.1016/j.corsci.2019.108412 Received 4 October 2019; Received in revised form 19 December 2019; Accepted 23 December 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Majid Laleh, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108412
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mechanically grounded with abrasive SiC papers (down to 4000, each step for almost 5 min) on a disc rotating at 150 rpm, followed by rinsing with distilled water and drying with hot air. For SLM-produced specimens, the electrochemical tests were done on the transverse plane (perpendicular to the building direction), since our preliminary experiments showed no obvious difference in the pitting corrosion at different planes, which is in agreement with the literature for pitting corrosion of SLM-produced 316L SS [20]. Polarisation tests were repeated at least five times for each specimen, and the average Eb values with standard deviation were reported. To clarify the correlation between inclusions and pitting corrosion, immersion tests were carried out in 6 wt% ferric chloride (FeCl3) solution for 48 h based on the ASTM G48 - 11(2015) standard test method. According the ASTM standard, the relative pitting corrosion performance of alloys in the ferric chloride solution can be correlated to performance in certain real environments, such as natural seawater at ambient temperature. The immersion tests were carried out at room temperature (24 ± 2 °C). The specimens were later removed from the solution, rinsed with distilled water and dried with hot air, and then analysed using SEM to examine the possible changes in inclusion morphology after immersion tests.
Table 1 Chemical composition (wt%) of wrought 316L SS and the powder used in SLM processing.
Powder As-built SLM wrought
Cr
Ni
C
Si
Mn
P
S
Mo
Fe
17.13 16.6 16.9
10.92 11.60 10.30
0.02 0.02 0.02
0.67 0.56 0.50
1.16 1.11 1.77
0.02 0.01 0.04
0.01 0.01 0.01
2.33 2.70 2.11
Bal. Bal. Bal.
Solutions, Germany) with particle size ranging between 5 and 40 μm was used as the feedstock material for SLM processing. The chemical composition of the powder is presented in Table 1. Chemical compositions of the commercial (wrought) and as-built SLM-produced 316L SS are also presented in Table 1 for comparison purposes. Samples of 10 × 10 × 10 mm3 were produced using SLM (SLM Solutions 125 H L) with an optimised set of processing parameters (laser power of 175 W, scanning speed of 400 mm/s, hatch spacing of 100 μm, and layer thickness of 30 μm). A multi-directional meander scanning pattern rotating by 67° between successive layers was used. SLM processing was conducted under an argon atmosphere to prevent the parts from being oxidised. The building plate was pre-heated to 200 °C to minimise buildup of residual stress. These SLM processing parameters were selected based on the preliminary experiments to produce highly dense specimens (over 99.5 %). The relative density of the SLM-produced specimen was measured according to the Archimedes method by immersing the specimens in distilled water using an Electronic Densimeter (Model SD-200 L) of 0.1 mg/cm3 resolution. The SLM-produced specimens were then solution annealed in the temperature range of 900−1200 °C for different holding times (15 and 60 min) followed by water quenching. The “as-built” term was hereafter referred to those untreated SLM-produced specimens.
3. Results and discussion Solution annealing heat-treatment which dissolves carbides into the solid solution of the γ-Fe matrix, is typically applied to austenitic stainless steels to increase corrosion resistance [21]. Subsequent rapid quenching tends to suppress the recurrence of carbides. For type 316L SS, the solution annealing temperature was reported to range between 1040 °C and 1120 °C [22]. In the present study, a broader range of temperature, i.e. between 900 °C and 1200 °C, was used to monitor the influence of thermal post-processing (i.e. heat-treatment) on the pitting corrosion resistance. Fig. 1a shows the representative potentiodynamic polarisation curves for the as-built and heat-treated SLM-produced 316L SS. A potentiodynamic polarisation curve of the commercial counterpart is also provided for comparison purposes. The average values of breakdown potential, Eb, are presented in Fig. 1b. The as-built SLM-produced 316L SS showed a substantially higher breakdown potential compared to its commercial counterpart. Heat-treated SLMproduced 316L SS, however, showed a different pitting corrosion performance. Specimens heat-treated at 900 °C and 1000 °C exhibited a similar pitting potential to that of the as-built SLM-produced 316L SS. This is in contrast to a drastic deterioration (almost 300 mV) of pitting corrosion resistance observed for those specimens heat-treated at higher temperatures of 1100 °C and 1200 °C. It is well-known that an important microstructural feature that plays a key role in the pitting corrosion of 316L SS is inclusions [15,19,23–25]. Typical inclusions present in the as-built SLM-produced 316L SS with their corresponding elemental maps are shown in Fig. 2; the inclusions are spherical and enriched in Mn, Si, and O (manganese silicate). Numerous small spherical inclusions are also visible, having a similar chemical composition to that of the larger inclusion in Fig. 2a, as evidenced by STEM-EDS analysis shown in Fig. 2b. The size of inclusions varies from a few microns to tens of nanometers, however, their chemical composition seems to be similar based on SEM-EDS and STEM-EDS analysis. Indeed, the substantially high pitting corrosion resistance of the as-built SLM-produced 316L SS is due to the absence of deleterious MnS inclusion, which is in agreement with the literature [9,11]. SEM-EDS analysis of the inclusions in the heat-treated SLM-produced 316L SS revealed the formation of new types of inclusions, as will be shown in the following. The major concern is the MnS inclusion, regarding its detrimental influence on the pitting corrosion resistance. No MnS inclusion was detected for specimens heat-treated at temperatures of 900 and 1000 °C. Interestingly, after heat-treatment at higher temperatures of 1100 and 1200 °C, numerous MnS inclusions
2.2. Characterisation The shape, chemical composition, and distribution of the inclusions were analysed using SEM-EDS (ZEISS SUPRA 55 V FEG, and JEOL JSM 7800 F FEG). Electron backscatter diffraction (EBSD) analysis was performed using a ZEISS LEO 1530 FEG SEM, with a scan step size of 0.3 μm at an accelerating voltage of 20 kV. Samples for SEM-EDS and EBSD analysis were mechanically polished to a mirror finish with the final polishing using a 0.04 μm oxide polishing suspension (OPS). Transmission electron microscopy (TEM) operated at 200 kV was employed for a detailed analysis of inclusions in the as-built SLM-produced specimen, using a JEOL JEM 2100 F microscope equipped with an EDS detector. EDS mapping was performed in a scanning-transmission TEM (STEM) mode using a spot size of 1 nm. For TEM analysis, thin slices with a thickness of approximately 500 μm were cut from the specimens and then mechanically ground to a thickness of around 100 μm. Discs with a diameter of 3 mm were subsequently punched from the slices and further ground using 4000 abrasive SiC paper until their thickness was reduced to 60−70 μm. Final thinning was carried out in a twin-jet electro-polishing unit using a Struers Tenupol 5 device. 2.3. Electrochemical tests Potentiodynamic polarisation tests were conducted in 0.6 M NaCl solution at room temperature (24 ± 2 °C) by sweeping the potential in the anodic direction from -0.2 V to +1.4 V vs. open circuit potential (OCP) with a scan rate of 10 mV/min in an electrochemical system comprising 316L SS specimens (with the surface area of almost 1 cm2) as the working electrode, a titanium mesh as the counter electrode, and a silver/silver chloride (Ag/AgCl) electrode as the reference electrode, according to ASTM G5. The specimens were immersed into the solution for one hour to get a stable OCP before starting the potentiodynamic polarisation measurements. Prior to corrosion tests, specimens were 2
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Fig. 1. (a) Potentiodynamic polarisation curves for the heat-treated SLM-produced 316L SS specimens recorded in 0.6 M NaCl solution at room temperature. (b) Values of breakdown potential obtained from polarisation curves, indicating lower pitting corrosion resistances for the specimens heat-treated at 1100 °C and 1200 °C compared to those heat-treated at 900 °C and 1000 °C. Error bars represent the standard deviation obtained from at least five measurements under identical conditions.
produced 316L SS is quite high (up to 106-108 °C/s [26,27]) so the material is not thermodynamically in the equilibrium condition, meaning that Mn and S did not have sufficient time to segregate and form MnS inclusions, and the matrix is left saturated with Mn and S [28]. Subsequent heat-treatment at temperatures above 1000 °C will move the whole system towards the thermodynamically equilibrium conditions and lead to MnS formation. Heat-treatment also resulted in other types of inclusions’ transformation in the SLM-produced 316L SS. SEM-EDS analysis of some representative inclusions (over 100 inclusions were characterised) in the heat-treated SLM-produced specimens are displayed in Fig. 4. No obvious difference was detected for the inclusions upon heat-treatment at 900 °C and 1000 °C; inclusions still remained spherical in shape and
were formed. The number density of the MnS inclusions versus heattreatment condition is presented in Fig. 3, indicating that the temperature and holding time both affect the population of MnS inclusions; the number density increased by increasing the holding time from 15 to 60 min and temperature from 1100 to 1200 °C (refer to supplementary material 1 to see the SEM-EDS images and distribution of MnS inclusions). Furthermore, from the SEM-EDS analysis in supplementary material 1, it can be seen that heat-treatment temperature and holding time do not affect the size of the inclusions to any obvious extent. The formation of MnS inclusion in the SLM-produced 316L SS after hightemperature heat-treatment has not been reported before and the underlying mechanism for such transformation is still unclear. One possible explanation is because the solidification rate for the as-built SLM3
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Fig. 2. (a) SEM-EDS analysis of a typical inclusion in the as-built SLM-produced 316L SS, showing the micron-sized inclusions are spherical in shape and mostly enriched in Mn, Si, and O. (b) High-magnification STEM-EDS analysis of the small inclusions in the as-built SLM-produced 316L SS, indicating a similar chemical compositions for nano-sized inclusions compared to those of the micron-sized ones.
recent study, Yan et al. [30] characterised nano-scale oxide inclusions in SLM-produced 316L SS, where they found that MnSiO3 oxide inclusions transformed to spinel MnCr2O4 upon heat-treatment at 1200 °C, which is in good agreement with the results reported here. In another study on an Fe-Cr alloy, Shibata et al. [31] reported a similar phenomenon of inclusion transformation, showing the change from manganese silicate to manganese chromite inclusions when subjected to heat-treatment at 1200 °C. It has been suggested that the decrease in the solubility of chromium oxide in manganese silicate at such temperature and also the reaction at the interface between the oxide inclusions and the metal matrix are possible factors leading to the transformation of inclusions. The results show that heat-treatment had a significant influence on inclusions in the SLM-produced 316L SS. The inclusions in the heattreated SLM-produced 316L SS can be classified into three categories based on their chemical composition. Type I is the manganese silicate inclusion, which is present in the as-built SLM-produced specimen and those heat-treated at 900 °C and 1000 °C for both holding times of 15 and 60 min. Type II is the manganese chromite inclusion, which is irregular-shaped and appears in the SLM-produced specimens heattreated at 1100 °C and 1200 °C. Type III is the MnS inclusion, which coexists with the type II manganese chromite inclusions in those specimens heat-treated at 1100 °C and 1200 °C. The formation of the MnS inclusion is a particular concern, because it has been known as the predominant site for the initiation of pitting in 316L SS [16,23,32–34]. The influence of inclusion compositional and structural change on the pitting corrosion resistance is shown in potentiodynamic polarisation curves in Fig. 1, indicating the drastic decline in the pitting resistance of the SLM-produced 316L SS heat-treated at temperatures above 1000 °C. Such a reduction in pitting corrosion resistance could be attributed to the change of inclusions, where deleterious MnS inclusions appear after subjected to heat-treatment at temperatures above 1000 °C. This is supported by the fact that after high-temperature heat-treatment, the
Fig. 3. Effect of heat-treatment conditions on the number density of the MnS inclusions in the SLM-produced 316 L SS.
enriched in Mn, Si, and O (manganese silicate), at both 15 and 60 min holding times (Fig. 4a and b). After 15 min of heat-treatment at 1100 °C and 1200 °C, SEM-EDS analysis (Fig. 4c and d) clearly showed the formation of Mn-Cr-O enriched inclusions (manganese chromite). The newly formed manganese chromite inclusions tended to be irregularshaped (angular), reflecting their spinel structure. Note that manganese chromite inclusions were detected in all specimens heat-treated at 1100 °C and 1200 °C. At shorter holding time (15 min), the manganese silicates partially transformed to manganese chromite (Fig. 4c and d). It has been reported that this transformation requires chromium diffusion towards the inclusions and manganese and silicon transfer away from the inclusions [29]. For those specimens heat-treated for 60 min at 1100 °C and 1200 °C, pure spinel manganese chromite inclusions (without residual manganese silicate) were found (Fig. 4e-h). In a 4
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Fig. 4. SEM-EDS analysis of the inclusions in the heat-treated SLM-produced 316L SS: (a) 900 °C-15 min, (b) 1000 °C-60 min, (c) 1100 °C-15 min, (d) 1200 °C-15 min, (e) 1100 °C-60 min, (f, g, h, i) 1200 °C-60 min, (j) 1100 °C-15 min. Red dashed lines in SEM images shows where the EDS line scans were made. The coloured lines in EDS analysis represent the following elements: light blue for Fe, red for Cr, blue for Mn, yellow for O, orange for Si, green for Al, and pink for S (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
built and heat-treated SLM-produced 316L SS specimens. Readers are referred to the authors’ previous publication to see the microstructure of the commercial 316L SS [36]. No obvious differences to the as-built condition were found in terms of average grain size and grain boundary character, as summarised in Table 2. This indicates that solution annealing at temperature range between 900 °C and 1200 °C for 15 min and 60 min followed by water quenching has no noticeable influence on the microstructure of the SLM-produced 316L SS. It should be mentioned here that the SLM-produced 316L SS specimens used in this study are highly dense (over 99.5 %), and heat-treatment did not show any influence on the porosity. So, porosity is not an influencing factor in the pitting corrosion measurements reported here. Moreover, based on EBSD phase-contrast analysis, no phase changes have been observed for the SLM-produced 316L SS specimens under the heat-treatment conditions applied in this study.
pitting corrosion behaviour of the SLM-produced 316L SS resembles the commercial 316L SS (see Fig. 1). Despite the more prominent role of MnS inclusions in the pitting corrosion of 316L SS, oxide inclusions might also act as pit initiation sites. Limited information is available in the literature regarding the influence of oxide inclusions on pit initiation, however, the lower surface potential of the oxide inclusions compared to the surrounding matrix has been generally suggested as the driving force for pitting corrosion in the presence of sulphur and chloride ions [35]. To get more insights on the susceptibility of different types of inclusions in the heattreated SLM-produced 316L SS, immersion tests in ferric chloride solution were conducted. Fig. 5 displays SEM micrographs of some representative inclusions before and after 48 h immersion in a ferric chloride solution. No noticeable change was detected for the manganese silicates and manganese chromite inclusions, indicating their superior resistance to dissolution (Fig. 5a-c). However, Fig. 5d shows that the MnS-containing portion of the inclusion has been dissolved from the interface with the metal matrix and has thus acted as a pit initiation site. Microscopy analysis has also been conducted to see the influence of the heat-treatment on the microstructural characteristics of the SLMproduced 316L SS. Fig. 6 shows the inverse pole figure (IPF), grain boundary, and Kernel average misorientation (KAM) maps for the as-
4. Summary This study has revealed a major decrease in pitting corrosion resistance for SLM-produced 316L SS specimens after being heat-treated at temperatures of 1100 °C and 1200 °C compared to the as-built SLMproduced specimen and those heat-treated at 900 °C and 1000 °C. SEMEDS analysis showed that the shape and chemical composition of the 5
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Fig. 5. SEM images of a number of inclusions before (left column) and after (right column) 48 h immersion in 6 wt % ferric chloride solution. No obvious change was observed in manganese silicate and manganese chromite inclusions (a–c) after the immersion test. However, pits were found to form at the site of MnS inclusions (d). Schematic representation of the chemical composition of the inclusions are also presented in the middle column, with green applied to manganese chromite, blue applied to manganese silicate, yellow applied to silicon oxide, and red applied to MnS (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
consideration for publication elsewhere.
inclusions in the as-built SLM-produced 316L SS changed after heattreatment at temperatures higher than 1000 °C. Typical manganese silicate inclusions in the as-built SLM-produced 316L SS were transformed to manganese chromite inclusions upon heat-treatment at temperatures above 1000 °C. In particular, deleterious MnS inclusion appeared after high-temperature heat-treatment, which was deemed as the main reason for drastic decline in pitting corrosion resistance of the SLM-produced 316L SS heat-treated at temperatures above 1000 °C. Indeed, it has been shown that the as-built SLM-produced 316 L SS have a substantially higher pitting resistance compared to its commercial counterpart, and subsequent high-temperature thermal post-processing decreases its pitting resistance. This finding suggests that high-temperature thermal post-processing might not be suitable for SLM-produced 316L SS and that SLM-produced 316L SS may have limitations for high-temperature applications.
Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement Author agreement Financial support from Deakin University Postgraduate Research Scholarship (DUPRS) is greatly appreciated. Deakin University’s Advanced Characterisation Facility is acknowledged for use of the microscopy instruments.
I certify that all authors have seen and approved the final version of the manuscript being submitted. They warrant that the article is the authors' original work, hasn't received prior publication and isn't under 6
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Fig. 6. Inverse pole figure (IPF) coloured orientation maps (top raw); grain boundary maps (middle raw), in which black lines represent high angle grain boundaries (θ > 15°), purple lines represent low angle grain boundaries (15° > θ > 2°), red and blue lines represent ∑3 and ∑9 CSL boundaries, respectively; Kernel average misorientation (KAM) maps (bottom raw) for the as-built and heat-treated SLM-produced 316L SS. EBSD acquisition was performed on the transverse (perpendicular to the building direction) plane (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.). Table 2 Microstructural characteristics for as-built and heat-treated SLM-produced 316L SS measured from the EBSD results presented in Fig. 6.
As-built SLM 900 °C-15 min 900 °C-60 min 1000 °C-15 min 1000 °C-60 min 1100 °C-15 min 1100 °C-60 min 1200 °C-15 min 1200 °C-60 min
Average grain size (μm)
Length of low-angle grain boundaries (2° < θ < 15°) (mm/mm2)
Length of high-angle grain boundaries (15° < θ) (cm/mm2)
Length of twin boundaries (∑3) (mm/mm2)
8.77 ± 0.86 9.65 ± 0.84 10.23 ± 0.85 9.16 ± 0.83 10.31 ± 0.83 10.43 ± 0.80 11.07 ± 0.77 10.39 ± 0.82 11.50 ± 0.76
14.92 12.12 13.84 18.16 12.28 15.16 17.64 16.88 16.20
35.60 33.76 28.76 33.64 30.32 29.24 26.52 27.88 26.96
38.16 33.20 25.20 32.32 36.20 31.20 31.12 22.48 30.76
Appendix A. Supplementary data
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