JOM, Vol. 70, No. 8, 2018 https://doi.org/10.1007/s11837-018-2940-y 2018 The Author(s) NUCLEAR MATERIALS, OXIDATION, SUPERCRITICAL CO2, AND CORROSION BEHAVIOR Identiﬁcation and 3D Reconstruction of Cr S Precipitates Along 5 6 Grain Boundaries in Fe13Cr 1,2,3 1 ¨ ¨ KATHRIN NUTZMANN , NICOLE WOLLSCHLAGER , 1 1 CHRISTIAN ROCKENHAUSER , AXEL KRANZMANN , 1,2 and CHRISTIANE STEPHAN-SCHERB 1.—Bundesanstalt fu ¨ r Materialforschung und -pru ¨ fung (BAM), Unter den Eichen 87, 12205 Berlin, Germany. 2.—Freie Universita ¨ t Berlin, Fachbereich Geowissenschaften, Malteserstraße 74-100, 12249 Berlin, Germany. 3.—e-mail: firstname.lastname@example.org Metal sulﬁde grain boundary precipitates of a ferrous model alloy with 13 wt.% chromium formed at 650C under a gas atmosphere containing 0.5% SO and 99.5% Ar were investigated after ageing for 3 h, 6 h, and 12 h. The precipitates formed along grain boundaries were identiﬁed as Cr S using 5 6 energy-dispersive x-ray spectroscopy in transmission electron microscopy and electron backscatter diffraction analysis. Serial focused ion beam slicing was conducted followed by three-dimensional reconstruction to determine the number, size, and penetration depth of the precipitates evolved at the differ- ent time steps. There was a linear increase in the number of precipitates with time, while their average size increased only for the initial aging time but became constant after 6 h. Based on these results, a model for grain boundary sulﬁdation of ferritic alloys is discussed. formed precipitates in grain boundaries due to INTRODUCTION corrosion might also have a signiﬁcant inﬂuence Ferritic high-temperature alloys with Cr content on the mechanical properties of the component. up to 16 wt.% are used as components of different Sulﬁde precipitates mentioned in literature mostly 7–9,11 energy systems, such as coal- and biomass-ﬁred refer to CrS; no deeper consideration regarding power plants. These materials, especially VM12- phase composition and penetration depth is given SHC (12 wt.% to 13 wt.% Cr), are used as super- but is essential to understand formation processes heater tubes and are exposed to different tempera- and conditions. Sulfur phases in contact with the tures, process pressures, and reactive atmospheres, base material of VM12 SHC have already been such as H O/steam inside the tube and ﬂue gas observed. Sulﬁde precipitates within the base containing CO ,SO ,H O, NO , and O on the material were noticed in 9Cr-1Mo steels under 2 2 2 x 2 ﬁreside. High temperatures in combination with sulfurous environments in the reﬁnery industry. 7–9,11 reactive atmospheres cause massive corrosion, In all the mentioned previous studies, the which leads to a change of security-related proper- inﬂuence of different alloying elements and gas ties. In particular, sulfurous gases such as SO and components on formation of grain boundary sulﬁdes SO are known to increase corrosion rates and cannot be clearly distinguished. This work uses a material loss of high-temperature iron-based model alloy of Fe with 13 wt.% Cr and SO as 1–5 alloys. The role of sulfur in combustion gases in reactive gas to study formation of sulﬁde precipi- corrosion of device materials and sulﬁdation of grain tates along grain boundaries. The objectives are to boundaries is still under discussion. Usually, sul- identify grain boundary sulﬁdes precisely and furous corrosion proceeds by formation of sulﬁde obtain information about the formation process by crystals within corrosion scale, resulting in faster analyzing the size, density, and location of diffusion of Fe cations and faster material loss. precipitates. Sulﬁde precipitates in grain boundaries of the base Focused ion beam (FIB) serial slicing was con- metal are described in literature only occasionally ducted site-speciﬁcally at grain boundaries to cap- 8,9 and mostly for oxygen-free atmospheres. Newly ture the shape and size of precipitates. Several 1478 (Published online May 29, 2018) Identiﬁcation and 3D Reconstruction of Cr S Precipitates Along Grain Boundaries in Fe13Cr 1479 5 6 approaches in literature underline the potential of images, labeling of precipitates, and generation of FIB slicing to study local reaction mechanisms in 3D ﬁgures using triangular approximation (Supple- 13,14 engineering materials. Transmission electron mentary Fig. S1). microscopy (TEM) analysis at one of the formed A lamella with large sulﬁde precipitates within a precipitates provided chemical information. The grain boundary of the 12-h sample was prepared for results support a formation model of grain-bound- TEM (see sample preparation in ESM). TEM inves- ary sulﬁdes in thermodynamic and microstructural tigations were performed using a JEOL JEM- aspects. 2200FS (JEOL, Japan) microscope with ﬁeld-emis- sion gun operating at 200 kV. Phase identiﬁcation was performed by combining scanning transmission EXPERIMENTAL PROCEDURES electron microscopy (STEM), EDX (energy resolu- An iron-based model alloy with 13 wt.% Cr tion of 138 eV, JEOL, Japan), and selected-area (Fe13Cr) was cut into 15 mm 9 20 mm rectangular electron diffraction (SAED) analysis. Diffraction coupons of 3 mm thickness. The sides of each patterns were simulated using the software package specimen were ground to 1200 grade using SiC JAVA Electron Microscopy simulations version paper and polished to 1 lm using diamond suspen- 4.3431U2015 by Stadelmann. sion. Ageing experiments for 3 h, 6 h, and 12 h time were performed at 650C in an infrared light RESULTS AND DISCUSSION furnace with a gas mixture of 0.5% SO and FIB Tomography Results 99.5% Ar [see Electronic Supplementary Material (ESM) for further details]. The Ar/SO gas mixture FIB 3D slicing conﬁrmed sulﬁde precipitates was not equilibrated to minimize the SO content or within grain boundaries solely. The precipitates reach the gas composition of oxyfuel combustion had disc-like shape with widest dimension parallel 16,17 processes. to the grain boundary. Figure 1a shows the recon- For each sample, semiautomatic serial slicing was structed 3D ﬁgure of the 6-h sample with precipi- performed using a FIB/scanning electron micro- tates colored in red, oxide top layer in green, and scopy (SEM) system (Quanta 3D FEG, FEI Com- porosity in blue. The volume fraction of precipitates pany) equipped with gallium source and energy- was determined from the 3D ﬁgure. The correlation dispersive x-ray (EDX) detection system. After of microstructure and precipitates formed in the 3-h localizing the region of interest along the grain- sample is shown in Fig. 1c. boundary path of the bulk material, a protective and The analyzed volume of each sample contained smoothing platinum layer was deposited on the different grain boundary areas. To show the evolu- surface using an in situ chemical vapor deposition tion of precipitates quantitatively, their number and system. A surrounding volume material of 20 lm/ total volume were normalized to the grain boundary 20 lm/10 lm(x/y/z) was excavated, and a ﬁducial length on the surface. Figure 2a shows a surface structure was generated for beam alignment. Sub- after 6 h of SO treatment, exemplarily. The grains sequently, the volume was cut slice per slice using are still visible with grown oxide islands in the grain acceleration voltage of 30 kV, aperture of 0.5 nA, center consisting of FeCr O ,Fe O , and Fe O . 2 4 3 4 2 3 and slicing distance of 50 nm. After each cutting The white framed area was chosen for slicing. step, an SEM image was recorded. The resulting Figure 2b shows the remodeled sample surface image stack was postprocessed using Amira 5.3 obtained from the sliced x–y images after alignment software (Thermo Fisher Scientiﬁc) for alignment of to display the surface grain boundary shape. The Fig. 1. Exemplarily reconstructed volume of Fe13Cr after 6 h of SO treatment. (a) x–y view showing precipitates (red) on grain boundaries (transparent) with pores (blue) in the oxide scale at the surface (green). (b) x–z view showing larger pores (yellow arrows) along the grain boundary. (c) Precipitates formed after 3 h of SO treatment correlate with the microstructure of the Fe13Cr base metal. 2 1480 Nu ¨ tzmann, Wollschla ¨ ger, Rockenha ¨ user, Kranzmann, and Stephan-Scherb Fig. 2. Evolution of precipitates as a function of time. (a) Fe13Cr surface after 6 h ageing; marked area was sliced in (b) top view (x–z) directly under the surface with visible grain boundaries (picture remodeled from x–y images). (c) Length of the grain boundary was measured as 31 lmin this example. (d) Correlation between number of precipitates normalized to 1 lm of surface grain boundary and time. (e) Correlation between total volume of all precipitates normalized to 1 lm of surface grain boundary. grain boundary directly under the thin oxide layer clearly smaller, while the volume distribution for is clearly visible. For the 6-h sample shown in 6 h and 12 h ageing time seemed to be similar. No Fig. 2c, this grain boundary length is determined to statement is made about the absolute precipitate be 31 lm. The number of precipitates along 1 lmof depth in our case. Only in the case of 3 h ageing surface grain boundary increased linearly with time time was the last particle found at depth of around (Fig. 2d). After 12 h ageing time, 2.3 precipitates 6 lm. The maximum penetration depth of the 6-h were formed along 1 lm surface grain boundary, and 12-h samples exceeded the depth possible using statistically, compared with 1 precipitate after 3 h the 3D slicing method. ageing time. Longer ageing time leads to a higher For each ageing time, the largest precipitates number of precipitates, and larger precipitates. were concentrated in a region between 2 lm and After 12 h ageing time, a total volume of 3.7 lm 6 lm depth. With increasing penetration depth, chromium sulﬁdes was formed in the sliced volume their size decreased again. Very small grains could along 1 lm surface grain boundary (Fig. 2e). be found at each depth. Directly under the surface, According to Fig. 2d, this volume is divided by 2.3 only small grains were found. Figure 3b shows the precipitates, resulting in an average volume of correlation of the distance, volume, and number of 1.6 lm per particle. After 6 h, the average particle precipitates for the 12-h sample, exemplarily. Sulfur volume was about 1.4 lm . Compared with the diffusion along grain boundaries is not limited to average volume per particle of 0.08 lm after 3 h, the near surface, consequently. the deviation between 6 h and 12 h is not signiﬁ- cant. The precipitates grew in number and volume TEM Results until 6 h ageing time, but thereafter only in num- TEM–EDX results veriﬁed an alloy composition of ber. Since the number of precipitates increased Fe and Cr, whereas the precipitate contained Cr linearly, a constant nucleation rate is assumed. and S solely. Cr depletion in the alloy was observed Therefore, the diffusion of S atoms along the grain around the particle and along the grain boundary boundary must be constant, too. Under the assump- (Supplementary Fig. S2 and details). An EDX line tion that S atoms diffuse constantly per time unit scan across the precipitate showed average concen- through the grain boundary and more and more trations of 53.1 ± 0.6 at.% S and 46.9 ± 0.6 at.% precipitates consume S atoms, precipitates grow Cr, indexing a composition of murchisite (Cr S ), slower because less S is left for each particle. 5 6 which contains 54.44 at.% S and 45.45 at.% Cr at Figure 3a shows the distance from each precipi- nominal composition. To conﬁrm the crystal struc- tate to the surface along the grain boundary for each sample. The volumes of the 3-h precipitates were ture of the Cr S precipitate, SAED along the [3 1 5 6 Identiﬁcation and 3D Reconstruction of Cr S Precipitates Along Grain Boundaries in Fe13Cr 1481 5 6 Fig. 3. Penetration depth of sulfur correlated with time: (a) dependence of volume of chromium sulﬁde precipitates along grain boundaries with time. (b) Volume and number of chromium sulﬁde precipitates versus their distance along the grain boundary after 12 h of SO treatment. zone, while S diffuses continuously through the 2 0] zone axis was performed. The reﬂection 15,21 oxide. One part of S is consumed by (Fe,Cr)S positions of the experimental diffraction pattern sulﬁde growth in the inner corrosion zone below the matched the reﬂection positions of the simulated external oxide layer at the scale–alloy interface, diffraction pattern according to the crystal structure while another part accumulates in pores above the given by Vanlaar very well, conﬁrming the hexag- grain boundary. The increased sulfur partial pres- onal structure expected for murchisite (Supplemen- sure triggers S inward diffusion along the grain tary Fig. S3 and details). boundary. The reconstructed 3D ﬁgure shows a high Metal sulﬁde grain boundary precipitates formed number of pores elongated along the grain boundary in Fe13Cr aged for 24 h at 700C under the same below the oxide layer (Fig. 2b). S together with Cr gas atmosphere were also identiﬁed as murchisite from the alloy, diffusing faster outwards at the using electron backscatter diffraction (EBSD) (Sup- grain boundaries than within the grains, can plementary Fig. S4 and details). nucleate Cr S at the grain boundaries. Due to the 5 6 higher amount of Cr below 2 lm distance from the Formation Model original alloy surface, the precipitates in this region The presented results show distinct sulﬁdation of become larger. Cr is consumed to form oxides on top grain boundaries in Fe13Cr even at an early stage of and precipitates within the grain boundary, which corrosion. The number and size of precipitates creates a depletion zone (Fig. 4a). increased with time, leading to the assumption of Formation of Cr sulﬁdes is thermodynamically continuous sulfur diffusion along grain boundaries preferred compared with Fe sulﬁde formation. during the ageing experiment. At the surface, Formation of Cr S precipitates requires sulfur 5 6 formation of a Cr–rich oxide in the beginning is partial pressure of p =10 Pa and oxygen partial S2 thermodynamically preferred. According to a phase pressure of p =10 Pa (Supplementary Fig. S5). O2 diagram calculated using FactSage 7.0 for the The main driving force for formation of Cr S 5 6 same system, this Cr-rich oxide is most likely precipitates is therefore supersaturation through chromite FeCr O . Chromite consumes a high constant S diffusion. TEM–EDX/SAED as well as 2 4 amount of Cr from the alloy, in this case 2 lm EBSD analysis identiﬁed metal sulﬁde grain bound- ary precipitates as Cr S (murchisite). The precip- under the original surface, which agrees with the 5 6 common oxidation model of Fe–Cr alloys. The itates at the grain boundary with the greatest formation of the largest Cr sulﬁde particles is possible distance to the surface were used for observed between penetration depth of 2 lm and analysis by TEM-SAED and EBSD. However, the 6 lm. It is assumed that SO decomposes at the transition to CrS, as suggested in the phase dia- oxide surface layer, and oxygen is consumed to form gram (Supplementary Fig. S5), in deeper regions or further oxides such as Fe O and Fe O in the outer after longer ageing time, cannot be excluded. In the 2 3 3 4 corrosion zone and FeCr O in the inner corrosion 6-h and 12-h samples, more sulfur could diffuse into 2 4 1482 Nu ¨ tzmann, Wollschla ¨ ger, Rockenha ¨ user, Kranzmann, and Stephan-Scherb Fig. 4. (a) Elemental distribution at a cross-section after 12 h ageing time measured using EDX spectroscopy. (b) Schematic formation model of sulﬁde precipitates based on experimentally found relations here and in former studies. Arrows mark the direction of diffusion. the metal to form larger precipitates compared with source, provide a link to the Creative Commons li- the 3-h sample. The question then arises of why the cense, and indicate if changes were made. particle size is not signiﬁcantly larger in case of 12 h compared with 6 h ageing time. In fact, the 12-h sample has higher total volume and higher number of precipitates. The loss of growth is more likely due to more particles that consume S and Cr. If sufﬁ- ELECTRONIC SUPPLEMENTARY cient Cr is no longer present at one location, S must MATERIAL diffuse further into the material to form new precipitates. The online version of this article (https://doi.org/ 10.1007/s11837-018-2940-y) contains supplemen- CONCLUSION tary material, which is available to authorized Grain boundary sulﬁdation is a prominent phe- users. nomenon in alloys used for superheater tubes in power plants, impacting the service lifetime of con- REFERENCES struction devices. This work identiﬁed and recon- 1. P. Kofstad, High Temperature Corrosion, 1st ed. (London/ structed grain boundary sulﬁdes as Cr S 5 6 New York: Elsevier Applied Science, 1988), pp. 437–464. (murchisite) for a Fe13Cr model alloy, already being 2. M. Kutz, Handbook of Environmental Degradation of Materials, 1st ed. (Norwich: William Andrew Pub, 2005), present at an early stage of corrosion. The number pp. 132–142. and volume of precipitates increased with time. The 3. S. Mrowec, Oxid. Met. 44, 177 (1995). particle size was nearly constant after 6 h ageing 4. H.J. Grabke, E. Reese, and M. Spiegel, Corros. 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