X-Ray tomographic characterisation of pitting corrosion in lean duplex stainless steel

Journal Pre-proof X-Ray Tomographic Characterisation of Pitting Corrosion in Lean Duplex Stainless Steel Kenichiro Eguchi (Conceptualization) (Methodology) (Validation)Formal analysis, Investigation) (Data curation) (Writing - original draft) (Visualization) (Funding acquisition), Timothy L Burnett (Conceptualization) (Methodology) (Resources) (Writing - review and editing) (Supervision), Dirk L Engelberg (Conceptualization) (Methodology)Resources, Writing - review and editing) (Supervision) (Project administration)

PII:

S0010-938X(19)31354-X

DOI:

https://doi.org/10.1016/j.corsci.2019.108406

Reference:

CS 108406

To appear in:

Corrosion Science

Received Date:

2 July 2019

Revised Date:

10 December 2019

Accepted Date:

17 December 2019

Please cite this article as: Eguchi K, Burnett TL, Engelberg DL, X-Ray Tomographic Characterisation of Pitting Corrosion in Lean Duplex Stainless Steel, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108406

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X-Ray Tomographic Characterisation of Pitting Corrosion in Lean Duplex Stainless Steel

Kenichiro Eguchi1,2,3, Timothy L Burnett1, Dirk L Engelberg3

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Manchester X-ray Imaging Facility, Department of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom 2

Tubular Products & Casting Research Department, Steel Research Laboratory, JFE Steel Corporation, 2-2-3 Uchisaiwaicho, Chiyoda-ku, Tokyo 100-0011, Japan 3

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Materials and Performance Centre & Corrosion and Protection Centre, Department of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom

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Graphical_abstract

Highlights

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Grain size dependent lacy pit covers were observed. Pitting corrosion occurred via selective dissolution. Pit morphology was grain size dependent. Pit stability products <0.3 Am-1 (as received) and <0.6 Am-1 (heat treated) were obtained,

Abstract The nucleation and growth of pitting corrosion in grade 2202 lean duplex stainless steel has been observed via X-ray Computed Tomography. The charge measured using an electro-chemical in-situ cell was correlated with pit growth characteristics and associated pit shape morphologies. Lacy metal covers were observed with holes formed by selective dissolution of the ferrite. All pits grew into semiellipsoid shapes, with large fractions of un-dissolved austenite grains remaining inside pits in heattreated microstructures. For the as-received microstructure the stability product showed typical values below 0.3 Am-1, with larger stability values observed for the heat-treated sample.

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Background

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Duplex stainless steels have higher strength, better low temperature toughness and greater resistance to hydrogen embrittlement and stress corrosion cracking compared to single phase ferritic and austenite stainless steels. Duplex stainless steels combine many of the beneficial properties of ferritic and austenitic stainless steels with reasonable material cost. These materials are therefore widely used for marine and petrochemical applications [1], with lean duplex grades (2202, 2101 etc.) considered a viable replacement for type 316 austenitic stainless steel. However, not much information exists about the occurrence and associated kinetics of localised corrosion, such as selective dissolution, intergranular corrosion, or pitting corrosion in these lean duplex stainless steel grades.

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Pitting corrosion has been extensively studied in austenitic stainless steel. When the material is immersed in relatively benign environments, the initial pit development depends on the properties of the passive oxide film [2]. However, after this early stage, the concentration and diffusion of chemical species inside the pit becomes important for achieving pit stability. Metastable pit growth is initially stabilised by the presence of a lacy pit cover, which helps to maintain the aggressive local chemistry within the pit. This pit cover eventually collapses, providing access of the external environment to the local pit chemistry. A pit must survive this event in order to continue growing and transform to a stable pit [3]. Isaacs et al. indicated that anodic dissolution during the quasi steady state period is controlled by diffusion [4]. Sato examined the stability criteria for pitting dissolution of metals [5]. He suggested that for metal dissolution rates below a current density of ~0.1 A. cm-2 there is insufficient ion buildup for the initiation of stable pits on a flat metal surface. Galvele developed a one-dimensional pit growth model and calculated concentrations of Me2+, Me(OH)+, and H+ ions, as a function of pit depth and current density by assuming that metal ions hydrolyze inside the pits and the corrosion products are transported by diffusion [6]. By assuming a critical pH for pit initiation, he found that a pit stabilizes and grows when the product of pit depth and current density reaches a certain value, later termed as “pit stability product” [7]. According to their estimation[7], the pit stability product should be between 0.3 Am-1 and 0.6 Am-1 for stable pit growth in type 304 austenitic stainless steel. This idea of the stability product has been supported by some experiments for austenite stainless steel [8-11]. However, pit nucleation and growth kinetics for the various grades of duplex stainless steel have not yet been reported. To our

knowledge, there are very limited previous published studies observing and examining the stability of pitting corrosion of duplex stainless steel [12]. Garfias & Sykes observed metastable pitting in 25 wt% Cr duplex stainless steel and reported that there is a difference in the growth of metastable pits compared to single phase austenitic stainless steel [12]. The ferrite phase dissolved preferentially and the presence of the corrosion resistant austenite phase leads to current transients. These transients then decay, as the dissolution front impinges on the austenite regions and interferes with pit growth causing local re-passivation events. They suggested that a large grain size increases the stability of metastable pits due to less interaction with the austenite phase [12]. However, the specific effect of microstructure on pit growth for dual-phase steels has not been studied yet.

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X-ray CT (computed tomography) enables non-destructive three-dimensional (3D) observation and analysis of features of different density and atomic number with resolution at the micrometre scale. X-ray CT was applied to observe pit development in 3D for austenite stainless steel over time [9, 10, 13] and duplex stainless steel [14]. The stability product of austenitic stainless steel was found to lie between 0.3 Am-1 and 0.6 Am-1 with the pit depth squared proportional to time elapsed, and these results suggest that pit growth is under diffusion control of species inside pits even taking into account the 3D pit shape. The study on duplex stainless steel focused on atmospheric corrosion kinetics over time, by comparing localized corrosion of a lean grade 2202 duplex stainless steel with standard grade 2205 duplex stainless steel wires.

Experimental method

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In this study, we have applied X-ray CT to quantify pit growth characteristics in a lean grade 2202 duplex stainless steel immersed in HCl solution, and compared these results to austenitic stainless steel. Experiments were carried out under electro-chemical potential control. A quasi in-situ X-ray CT approach was applied to obtain 3D information of pit growth, pit shape, and internal morphology for a lean duplex stainless steel. The effect of microstructure on pit growth is also investigated.

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An as-received Grade 2202 lean duplex stainless steel wire of 500 μm diameter was used, with a composition of 22-23.8 wt% Cr, ≤0.45 wt% Mo, 2-2.8 wt% Ni and ≤2.0 wt% Mn. To optimise the X-ray CT observation and achieve the best balance of spatial resolution with a short acquisition time a small diameter (500 μm) wire was used in this study. A solution heat treatment of the wire was carried out at 1100 °C for 24 hours in a tube furnace to increase the grain size without precipitation formation. The aim of the heat treatment is to investigate the effect of grain size for pitting corrosion. Argon shielding gas was used to prevent oxidization of the wire, and all samples then cooled in air. Wire samples were mounted in epoxy resin for cross-sectional microstructure analysis, with the surface ground to 4000-grit SiC, followed by diamond paste polishing to 1/4 μm surface finish. After polishing, the mounted samples were electro etched in 10 wt.% oxalic acid at 5 V for about 10 seconds to observe the microstructure. The average grain size was calculated using the line intercept method. Another sample was further polished with colloidal silica for electron backscatter diffraction (EBSD) mapping. The EBSD mapping was carried out with an Oxford Instruments Nordlys detector interfaced to a FEI Sirion scanning electron microscope (SEM) at 15 kV. For electrochemical measurement wire samples 80 mm in length were prepared, with the surface ground using 1200-grit SiC paper. The surface after heat-treatment was additionally ground using 240-

grit to 1200-grit SiC paper to remove any oxide layer that formed. The wire was then cleaned in an ultrasonic bath in acetone for 5 min. All samples were immersed into 35% HNO3 at room temperature for 2 hours to passivate the sample surface. Both ends of the wires were then coated by immersion in a mixture of beeswax and colophony (3 :1), leaving around 3 mm of the wire uncoated ready to be observed with X-ray CT. Light scratches were then introduced with 120-grit SiC paper along the circumference of the exposed area, to locally weaken the passive film and provide potential initiation sites for pits. This set-up resulted in all pits nucleating along these scratches due to the locally weakened passive film.

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An Ivium CompactStat was used to measure the open circuit potential (OCP) and to carry out potentiodynamic and potentio-static polarization experiments. All experiments were carried out in 0.1M HCl at room temperature. A platinum electrode was used as a counter electrode and an Ag/AgCl electrode as the reference. A miniature electrochemical cell, 25 mm in diameter was used for the observation of pitting corrosion kinetics [9]. This cell allows the sample to be kept inside the electrochemical cell and the HCl aqueous environment during the X-ray CT experiment. The experimental sequences for both the as received and heat treated wires are shown in Table 1 and Table 2, respectively. Two X-ray CT scans were carried out for each sample, with electrochemical polarization employed before each scan in order to initiate and grow pits. The current output during polarization was recorded with a sampling rate of 1 Hz. The electrochemical conditions saw the application of 10 mC (equivalent to 3.31 x 105 μm3 of dissolved volume) of electric charge for pits to initiate. The cumulative charge from each polarization curve was converted to the dissolved volume by using Faraday’s law. The metal dissolution assumed an average metal cation charge of n = 2.19, atomic weight M = 55.79 g.mol-1 and density of ρ = 7.97 g.cm-3, and Faraday’s constant F = 96485 coulomb/mol [7].

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For the as-received sample, the open circuit potential (OCP) was measured for 15 min and the specimen then polarized (1st PD) from OCP at a rate of 1 mV/s to +950 mV vs. Ag/AgCl. The potential was then held at +950 mV vs. Ag/AgCl for 1684 s until the electric charge reached 10 mC (i.e. potentiostatic polarization (1st PS)). The sample was then scanned by X-ray CT (1st X-ray CT scan) followed by another OCP measurement. The 2nd potentio-dynamic polarization (2nd PD) was then carried out to +950 mV vs. Ag/AgCl. Three separate potentio-static polarization experiments at +950 mV vs. Ag/AgCl were carried out (2nd, 3rd and 4th PS) until the electric charge reached a total of 10 mC with each experiment set to a max time of 2000 s. After reaching 2000 s, the polarization was briefly stopped, the charge recorded and the next potentio-static scan started. A 2nd X-ray CT scan was then carried out. SEM observation was conducted after the test was terminated and the sample removed from the solution. Ultrasonic cleaning was carried out in acetone for 1 hour to break and remove the pit cover for a more detailed observation of pit morphologies, followed by a second observation in the SEM. EDX mapping was carried out with an INCA system interfaced to a scanning electron microscope (Zeiss Ultra-55) at 20 kV to observe areas containing selective dissolution around the pit. For the heattreated sample, a similar experimental procedure was adopted, summarized in Table 2. A Zeiss Xradia 520 Versa was used for all X-ray CT observations, by applying an accelerating voltage of 120 kV. 721 projections were recorded at 10 × optical magnifications, and an exposure time of 12 s for each projection. This resulted in a reconstructed voxel size of 1.64 μm3 and a field of view of 3270 μm x 3270 μm. Each scan took approximately 3 hours using the above setup. The data were reconstructed using the Feldkamp-Davis-Kress (FDK) approach using the Zeiss Reconstructor software including a beam hardening correction. Images were segmented and visualised using Avizo 9 software.

The maximum depth, width and height of pits were quantified from 2D virtual slices from the reconstructed X-ray CT data of the pit by choosing the virtual slice position, which maximised each dimension. The depth (c) was defined as a distance from top of the pit mouth (i.e. the position of the pit cover) to the pit base. The pit width (2a) was defined as the longest straight-line distance of the subsurface pit diameter, perpendicular to the longitudinal axis. The height (2b) was defined as the longest distance of the pit along the longitudinal axis of the wire. The top surface of a pit was defined by extending the surface around the pit, which allowed the pit volume, the pit mouth area, and the surface inside the pit to be estimated.

Results and discussion

Electrochemical polarization

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SEM images of the as-received and heat-treated samples are shown in Figure 1. For the as-received sample, the grain structure appears equiaxed in the transverse section while the grains are elongated along the longitudinal axis along the wire length. This grain morphology is related to the drawing process for producing wire material. The grains of the heat-treated sample are equiaxed in both transverse and longitudinal direction as shown in Figure 1(b) and (d). The average grain size along the transverse section of the as-received sample was 1.1 ± 0.1 μm, while that of the heat-treated sample was 9.6 ± 2.2 μm. The grain size of the as-received sample is quite small compared to a typical bulk duplex stainless steel [12], which is estimated to be related to the heavy deformation induced during the manufacturing of the wire. No precipitation was found in either the as-received and heat-treated sample by SEM observation. The phase ratio of the as-received sample was 58 : 42 (ferrite: austenite) and that of the heat-treated sample was 51 : 49.

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The potentio-dynamic and potentio-static current response for the as-received sample is shown in Figure 2. During the 1st potentio-dynamic polarization, a few small current spikes can be seen from +200 mV to +500 mV vs. Ag/AgCl, followed by a large current increment around +600 mV as shown in Figure 2(a). At around +800 mV vs. Ag/AgCl, another current increment occurred, showing a continuously rising current, indicating stable pit growth. In contrast, there was no discrete current increment relating to pit growth at the 2nd potentio-dynamic polarization scan as shown in Figure 2(c).

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The potentio-static current response curves at +950 mV vs. Ag/AgCl for the as-received samples are shown in Figure 2(b) and 2(d). For the 1st polarization curve in Figure 2(b), after 1300 s, a sudden current increment occurred, which most likely corresponds to pit growth. For the 2nd potentio-static polarization, no current increment was seen. For the 3rd potentio-static polarization a current increment started around 1900 s. For the 4th potentio-static polarization, the current started to rise around 1100 s, with the entire sequence summarized in Figure 2(d). Potentio-dynamic and potentio-static current response curves for the heat-treated sample are shown in Figure 3. For the 1st potentio-dynamic polarization, four events can be seen up to +800 mV vs. Ag/AgCl, and then stable pit growth started and continued until the end of the polarization as shown in Figure 3(a). No stable pit growth was observed at the 2nd polarization scan, with only one small current spike at +700 V vs. Ag/AgCl as shown in Figure 3(c).

The potentio-static current response curves at +950 mV vs. Ag/AgCl for the heat-treated sample are shown in Figure 3(b) and 3(d). For the 1st potentio-static current response, a sudden current increment, which corresponds to pit growth, occurred around 1200 s and lasted until the end of the polarization. For the 2nd potentio-static polarization, no current increment was seen. For the 3rd potentio-static polarization, current increments started after 400 s and continued until the charge reached 10 mC. These observations show that pits can nucleate at susceptible sites but once they have been activated and stifled, the surface is more resistant to pit nucleation. Longer exposure or higher potentials are needed to initiate further pits.

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3D observation of pits

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The X-ray CT reconstructed volumes for the as-received sample after the 1st and the 2nd CT scan are shown in Figure 4. Three pits (Pit 1-3) can be seen in the 1st reconstructed X-ray CT 3-D volume (Figure 4(a)), with an additional pit (Pit 4) then observed in the 2nd X-ray CT volume (Figure 4(b)). Figure 5 shows higher magnification images of each pit after the 1st and 2nd X-ray CT scan. None of the pit shapes conformed precisely to a hemispherical shape, which is the typical pit morphology observed in austenitic stainless steel [9, 10]. All pits appeared more elongated, ellipsoid shaped, with slightly topographic interior pit surfaces. In addition, Pit 3 and Pit 4 looked as if they consisted of an agglomeration of several smaller pits, suggesting that corrosion did not proceed uniformly at the bottom of pits as reported previously for austenitic stainless steel [11]. Pit 1 and Pit 2 did not grow between the 1st and 2nd X-ray CT scan, whereas, Pit 3 re-activated and grew into a slightly different shape. The newly formed part of Pit 3 is defined as Pit 3-2 for later analysis.

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Figure 6 shows rendered 3D images from the X-ray CT scan of the pits. These figures clearly show that the pit shapes are not uniform (especially Pit 4), suggesting that smaller corrosion sites may have grown at the bottom of pits. Initiation of Pit 3-2 can be seen clearly in Figure 6(d). The 2D slice in Figure 7 (b) shows a cross-section of the pit along the wire diameter, with the ellipsoid shape parallel to the longitudinal wire axis (Figure 7 (a)).

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The X-ray CT images for the heat-treated sample after the 1st and the 2nd X-ray CT scan are shown in Figure 8. One image shows one side of the sample, and the other images show the opposite side. Two pits can be seen in the 1st X-ray CT image, and another pit formed between the 1st and the 2nd X-ray CT scan, labelled Pit 1 to Pit 3. Figure 9 shows magnified image of each pit after the 1st and the 2nd Xray CT scan. The pit morphologies are very different compared to those observed on the as-received sample. Pit 1 and Pit 2 did not grow between the 1st and the 2nd X-ray CT scan. Figure 10 shows 3D images of all pits. The surfaces of all pits have very complex shapes. These seemed to be related to the presence of remaining bulk material and grains within the pit, shown in the virtual 2D slice Figure 11.

SEM/EDX analysis

Figure 12 shows SEM images of Pit 1 of the as-received sample before and after ultrasonic cleaning. Before ultrasonic cleaning, the pit has a lacy cover consisting of a lot of tiny holes with irregular shapes, with the width of these holes of ≈ 1 μm. However the morphology of the cover is quite different from the lacy pit covers that occur on austenitic stainless steel [10, 15-17], which have a large hole at the centre, surrounded by circular regions with smaller holes. After the pit cover was removed by ultrasonic cleaning, images of the bottom of the pit shows microstructure features. By comparing these two figures, the tiny holes of the pit cover seem to correspond to the duplex stainless steel grain size of the as received material. Figure 13 shows the SEM image of Pit 2 of the heat-treated sample before and after ultrasonic cleaning. The pit also has a lacy cover. However, the size of the holes is approximately 10 μm, with irregular shapes. After ultrasonic cleaning, facets of grains can be seen at the base of the pit, with the size of the holes of the pit cover also corresponding to the size of the grains.

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A SEM image with EDX maps of Cr and Ni are shown in Figure 14. In the as-received sample, there are bright and dark layered areas elongated along the longitudinal direction. Though some overlaps exist, the bright area in the Cr map (black line) corresponds to the dark area in the Ni map, and vice versa (yellow line). The bright area in the Cr map represents ferrite phase, while the dark area in the Cr map represents the austenite phase. The layered structure represents the microstructure of the manufacturing process shown in Figure 1(c). The black area corresponds to the corrosion of the ferrite; selective dissolution of the ferrite has occurred.

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In the heat-treated sample, local areas appearing bright and dark in the corresponding EDX maps can be seen. The black area in the maps (Figure 14(e),(f)) corresponds to the corroded area, which is surrounded by the austenite phase. In addition, the remaining phase inside the pit (marked by the arrow in Figure 14(d)) corresponds to the austenite phase as well. Therefore, the holes in the lacy cover also seem here to form by selective dissolution of the ferrite phase, which supports observations reported in literature [18]. Both phases have different corrosion properties because of their chemical compositions [19, 20]. Therefore, which phase corrodes preferentially depends on the exposure environment. Selective dissolution of ferrite phase is reported to occur in HCl solution [21] and in a FeCl3:MgCl2-containing salt deposit environment [22, 23]. The formation of the “lacy cover pit structure” in lean duplex stainless steel is related to selective dissolution of one phase.

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The heat-treated sample had large austenite grains remaining inside pits, surrounded by the dissolved region of ferrite. In contrast, for the as-received sample a typical pit cavity was observed without indication of a remaining phase. This may be explained by the more rapid dissolution and disintegration of smaller austenite grains, within the pit providing a very aggressive environment due to metal-ion hydrolysis. However, small, isolated austenite grains are also difficult to detect using Xray CT, and the SEM images in Figure 12 (c,d) are inconclusive. On the other hand, the grain size of the heat-treated sample is far larger, so the austenite phase of the heat-treated sample would need much longer to dissolve. Corrosion of exposed austenite grains inside these pits is visible in Figure 13, indicating that the galvanic protection exerted by the presence of both phases is significantly reduced under these circumstances. First the ferrite dissolved selectively, followed by dissolution of the austenite phase. The formation process of perforated pit cover for austenitic stainless steel is different from that of duplex stainless steel. In austenitic stainless steel, lacy pit cover is believed to form by repeating active

corrosion and re-passivation processes [15]. The passive film is undercut with lateral pit growth and new holes are formed, resulting in re-passivation around the holes by dilution of the environment inside pit. The passive film is then undercut with lateral pit growth. By repeating this process, the lacy pit cover is formed on austenite stainless steels. The dissolution process in duplex stainless steel, in contrast, seems to be more related to preferential dissolution of the ferrite phase.

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Figure 15 shows a correlation of images obtained by X-ray CT, SEM and EDX elemental maps outlining ferrite and austenite regions. Figure 15 shows a facet (red line) in the 3D image and the virtual 2D image, with the same regions outlined in the SEM image (Figure 15 (a), (b) and (c)). The EDX maps (Figure 15(d) and (e)) clearly shows that the remaining phase corresponds to austenite phase. The convoluted nature of both phases is also outlined, supporting the interdependency of both microstructure constituents.

Dissolved Pit Volumes

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The total dissolved volume for each electrochemical step was 3.31 x 105 μm3, based on the 10 mC charge limit. Figure 16 gives a direct comparison of the segmented pit volumes from X-ray CT data, with the estimated volume from the measured current (total electric charge) of each polarization experiment. A high electrochemical potential was applied to both samples to grow pits. The current from the oxygen evolution at +950 mV vs. Ag/AgCl was around 1 µA (Figure 2(c)); the latter should not be neglected for evaluating the dissolved pit volumes. For the potentio-dynamic polarization test, the start of pit growth was defined at the potential where a significant current increment was observed. The current from the oxygen evolution was removed from each polarization curve and the remaining current then assumed to come from pit growth. For potentio-static polarization, the start point of pit growth was defined when a significant current increment started and continued until the end of the polarization. Each individual pit volume has been determined, and the relationship to the measured charge is estimated.

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For the as-received sample, pits 1, 2 and 3 nucleated during the 1st X-ray CT scan (1st PD and 1st PS), with Pit 4 and Pit 3-2 then present after the 2nd X-ray CT scan. By correlating the X-ray CT measured pit volume with the current response, it seems that pit 2 initiated during the 1st potentio-dynamic polarization cycle (1st PD), followed by pit 1 and 3 during the 1st potentio-static polarization exposure (1st PS). During the 3rd potentio-static cycle (3rd PS), Pit 3-2 grew, with Pit 4 initiating during the at 4th potentio-static polarization scan (4th PS). The 2nd potentio-dynamic polarization (2nd PD) and 2nd potentio-static polarization Scan (2nd PS) were not taken in consideration for this estimation, since there was no indication of metal dissolution during these polarization scans. To deconvolute the current response associated with pit 1 and 3 during the 1st PS scan of the as-received sample, the following assumptions were applied to simplify subsequent analysis. Pit 1 and Pit 3 were assumed to form at the same time, when a sudden current increase occurred, with both pits then simultaneously growing until the potentio-static polarization was stopped. For the heat-treated sample, by comparing their X-ray CT measured volumes, pit 1 is assumed to have initiated during the 1st potentio-dynamic polarization cycle (1st PD), then Pit 2 during the 1st potentiostatic (1st PS) exposure, followed by Pit 3 initiated during the 3rd potentio-static polarization scan (3rd

PS). Also here, the 2nd potentio-dynamic polarization scan and 2nd potentio-static polarization scan were not taken into consideration. The current vs. time plots in Figures 2 and 3 are then linked to the measured pit volumes via X-ray CT in Figure 16, allowing the size evolution of pits to be estimated. This provides an effective way of deconvoluting the contributions of each pit to the overall recorded current, which is summarised in Figure 17. The current for each pit fluctuated significantly, suggesting that pits did not grow in a coherent, continuous manner. All pits seemed to grow in a cyclic fashion, with current peaks and troughs indicating a dynamic pit growth process. This observation supports previous reports of the dissolution of the ferrite phase, followed by local impingement of the dissolution front onto the resistant austenite phase, resulting in current fluctuations [12].

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Analysis of the dissolution process by assuming a mean, stable current consumption allows the current density for each pit internal surface to be estimated. The dimensions of an ellipsoidal pit shape are shown in Figure 18. The pit depth (c), width (2b), and length (2a) can then be calculated via the measured pit volume (V) from X-ray CT data. The factor 𝑓 corresponds to the volume fraction of the dissolved phase (see Equation 1). This factor is necessary because only part of the observed pit volume contributes to the overall measured current. For the as received sample a factor (f=1) was assumed, whereas a factor of (f=0.5) was applied for the heat-treated sample, allowing to compensate for the non-dissolved austenite phase.

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To estimate pit growth, the ratio of pit depth, width and height was assumed to remain constant, during growth. Table 3 and 4 show the difference between the measured pit dimensions by X-ray CT and the calculated pit dimension via Faraday’s law at the end of each polarization for the as-received and the heat-treated sample, respectively. The differences for the as received pit volumes are reasonably small, with maximum uncertainties of up to 15-20%. This overestimation of the measured vs. dissolved volume is possibly related to the uncertain contribution of the dissolved austenite phase here, and where some of the measured current may even be linked to this dissolution process. In contrast, the heat-treated sample indicates differences of only 1-2%, suggesting that the assumptions made for the pit dissolution process are reasonable.

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Pit stability product

Pit growth kinetics of austenitic stainless steel has typically been studied using estimation of the pit stability product. The stability product is obtained via the current density, which is generated along the inner surface of the growing pit, multiplied by the depth (c) of the pit. The surface area development over time and associated depth of each pit can be estimated, by assuming a semi ellipsoid pit shape with the anodic dissolution assumed to occur homogeneously distributed along the inner pit surface. The surface area of each pit was then calculated by the following equation [24], where, S is surface area, a, b and c are half of the length of the principal axes (Fig.16). The selective dissolution along the pit surface is taken into consideration by using the volume fraction of dissolved

phase, in our case z = 0.5. As shown before in Table 3 & 4, a small fraction of the austenite in the as received microstructure may also be faradaically active, but this is not considered here in our estimation. Additionally, for estimating the active pit surface of the heat-treated sample, the convoluted shape of the dissolving ferrite is also neglected, with an overarching focus on the dissolution front rather than the morphology.

𝑆=𝑧 ×

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The pit depth evolution over time can be back-extrapolated via the measured volume by X-ray CT, summarised in Tables 3 & 4. The measured current over the estimated surface area by the above methods are used to estimate the stability products of both samples, with Figure 19 providing a summary of these results. The stability products of as-received sample increased with time and reached typical values of 0.05 Am-1 to 0.15 Am-1, shown in Figure 19(a). These values are smaller compared to literature data for pits in austenitic stainless steel [9, 10], with theoretical values of 0.3 to 0.6 Am-1[7]. The stability products of the heat-treated sample increased over time with variation shown in Figure 19(b). For Pit 1, the stability product exceeds 0.3 Am-1, the critical value of stable pit growth, which is completely different from the result of the as-received sample. For Pit 2 and Pit 3, the stability product initially exceeds 0.3 Am-1, and then fell below 0.3 Am-1.

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For the as-received sample, the stable pit growth below 0.3 Am-1 suggests that diffusion of species in the pit is restricted by the micrometre-sized holes in the pit cover. However, the stability product of the heat-treated sample increased beyond 0.3 Am-1, suggesting that larger holes in the pit cover and the remaining phase inside the pit provide conditions for stable pit growth. Because of the existence of the remaining austenite phase gives the pit a complex shape where the diffusion of species may be restricted, resulting in a longer diffusion route from pit bottom to pit mouth. The pathway is more convoluted, schematically shown in Figure 20. However, this effect is possible relatively small, judging from the pit stability product differences between as received and heat-treated microstructure.

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For Pit 2 and Pit 3 of the heat-treated sample, the stability product decreased below 0.3 Am-1. The stability product is calculated by assuming that corrosion happens on the ferrite phase at the bottom of the pit. However, if corrosion only happens along part of the bottom of the pit, the local stability product is going to be higher, perhaps exceeding 0.3 Am-1. Under diffusion control, pH inside a pit should gradually change with the distance from the bottom of the pit [6]. However, if diffusion in some part of pit is prevented by the remaining phase, pH inside the pit is going to vary and re-passivation may occur in some parts. The fact that this sudden drop of stability product does not occur on the asreceived sample supports this idea.

Estimation of Diffusivity Parameter To provide further insight of the pitting corrosion behaviour of lean duplex stainless steel, the pit diffusivity parameter (D∆C) can be calculated [8-10], and compared to values available for austenitic stainless steels. The density of material (ρ), exposure time (t), and molecular mass (M) are combined with the effective diffusion coefficient (D) and the concentration difference between pit bottom and

mouth (∆C), with (c) defining the pit depth. A saturation concentration (∆C) of 4.2M has been assumed here in this study [25]. For type 304 austenitic stainless steel, diffusivity parameter values of 1.68 to 3.04 x 10-8 mol.cm-1.s-1 [9] have typically been observed. 𝑐2 =

3𝑀𝐷∆𝐶 𝜋𝜌

𝑡

(3)

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Figure 21 and 22 show the relationship between the pit depth and the time for both samples. The pit depth squared (c2) over time is proportional to the diffusivity parameter slope. For the as-received samples, the slope changed when a sudden current decrease occurred (arrows in Figure 17), suggesting that the solution inside the pit is diluted in some part. For Pit 2 and the first slope change in Pit 3 of the heat-treated sample, the same phenomenon seems to occur. On the other hand, the slope increased when the current suddenly increased for Pit 1 and for Pit 3, suggesting some part of the pit was activated. Changes of slope on pit depth with time squared have also been reported for austenitic stainless steel [9,10].

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The mean diffusivity parameter is summarized in Table 5, with individual diffusivity parameters of each slope segment given in Table 6. The diffusivity parameters for as-received sample are far smaller than the parameters for austenitic stainless steel [9-11, 15, 26, 27, 28]. This result suggests that either the effective diffusion coefficient of as-received duplex stainless steel wire is smaller than that of austenitic stainless steel or smaller metal ion concentrations can still allow for pit growth. If the effective diffusion coefficient of metal ion is same as that of type 304 austenitic stainless steel, the critical metal ion concentration at the pit bottom for the duplex stainless steels used in this study should be 0.65 M to 2.2 M to sustain stable pit growth. In our knowledge, there is no literature data about the metal ion concentration inside stable pit for duplex stainless steel. Therefore, it is difficult to conclude that the critical metal ion concentration of duplex stainless steel is different with that of type 304 austenitic stainless steel. The diffusivity parameters of all heat-treated samples were similar to the ones reported for austenitic stainless steel. If the chemical composition of two materials are same, thus the critical metal ion concentration for the two materials can be expected to be equal since the corrosion resistance of the two materials are the same unless harmful phase forms. The chemical compositions of the ferrite phase is expected to be equal in as-received sample and heat-treated sample, because the heat-treatment in this study is not expected to affect the chemical composition [29]. In this assumption, the critical metal ion concentration of both as-received and heat-treated sample are the same, thus the effective diffusion coefficient of as-received sample should be smaller than that of heat-treated sample. The difference of the effective diffusion coefficient between asreceived and heat-treated sample may be related to the difference of pit cover morphology, based on the microstructure. Further work is required to ultimately fully understand the pit growth behaviour of different duplex stainless steels.

Conclusion

X-ray CT was used to obtain 3D image of pits in lean duplex stainless steel Grade 2202 over time. The pit growth and morphology was investigated for an as-received and a heat-treated sample giving two different microstructures. Pits initiated in 0.1 M HCl by potentio-dynamic and potentio-static polarization.

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Declaration of interests

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Pits in lean duplex stainless steels contain pit covers, with holes corresponding to their grain size; holes in the pit cover seem to form by selective dissolution of the ferrite phase.  The existence of the holes on pit cover of as-received sample seem to decrease stability product and associated diffusivity parameter by preventing diffusion of metallic ions in the pit.  For the heat-treated sample, the range of stability product and diffusivity parameter were the same as for austenitic stainless steel, suggesting that the effect of remaining phase had only a small effect on the diffusion of species inside the pit.  Stability products of the as-received material was estimated giving a range of 0.05 Am-1 to 0.15 Am-1. This stability range is smaller than the corresponding values for austenite stainless steel.  Stability product of the heat-treated sample was larger than that of the as-received sample, giving a range of 0.1 Am-1 to 0.6 Am-1.  Diffusivity parameter of the as-received sample is smaller than that of austenitic stainless steel, while that of the heat-treated sample is compatible with that of austenitic stainless steel.  The microstructure of duplex stainless steel seems to affect pitting corrosion growth via the formation of their perforated pit cover morphology. Acknowledgements The experimental facilities in the Henry Royce Institute for Advanced Materials established through EPSRC funding for Henry Moseley X-ray Imaging Facility under grant Nos. EP/M010619, EP/K004530, EP/F007906, EP/F001452, EP/I02249X, EP/F028431 is acknowledged in addition to HEFCE funding through the UK Research Partnership Investment Funding (UKRPIF) Manchester RPIF Round 2 for the Multiscale Characterisation Facility.

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.

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Author statement

Kenichiro Eguchi: Conceptualization, Methodology, Validation, Formal analysis,

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Investigation, Data Curation, Writing-Original Draft, Visualization, Funding acquisition

Timothy L Burnett: Conceptualization, Methodology, Resources, Writing-Review & Editing, Supervision

Dirk L Engelberg: Conceptualization, Methodology, Resources, Writing-Review & Editing, Supervision, Project administration

References

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[19] Wen-TaTsai, Jhen-RongChen, Galvanic corrosion between the constituent phases in duplex stainless steel, Corrosion Science 49(2007) 3659-3668. [20] P. Reccagni, L. Guilherme, Q. Lu, M. Gittos, D. Engelberg, Reduction of AusteniteFerrite Galvanic Activity in the Heat-Affected Zone of a Gleeble-simulated Grade 2205 Duplex Stainless Steel Weld, Corrosion Science, in press. [21] K. Eguchi, Y. Ishiguro, H. Ota, Corrosion Behaviour of Multi-Phase Stainless Steel in 15% Hydrochloric Acid at a Temperature of 80°C, CORROSION 71(2015) 1398-1405. [22] C. Örnek, X. Zhong, D.L. Engelberg, Low-Temperature Environmentally Assisted [23] Cracking of Grade 2205 Duplex Stainless Steel Beneath a MgCl2:FeCl3 Salt Droplet, Corrosion 72(2016) 384-399. [24] D.L. Engelberg, C. Örnek, Probing propensity of grade 2205 duplex stainless steel towards atmospheric chloride-induced stress corrosion cracking, Corrosion Engineering, Science and Technology 49(2014) 535-539. [25] O. Ersoy, Surface area and volume measurements of volcanic ash particles by SEM stereoscopic imaging, Journal of Volcanology and Geothermal Research 190(2010) 290296. [26] H.C. Kuo, D. Landolt, Rotating-disk electrode study of anodic dissolution or iron in concentrated chloride media, Electrochim. Acta 20 (1975) 393–399. [27] G.T. Gaudet, W.T. Mo, T.A. Hatton, J.W. Tester, J. Tilly, H.S. Isaacs, R.C. Newman, Mass-transfer and electrochemical kinetic interactions in localized pitting corrosion, AlChE J., 32 (1986) 949-958. [28] N.J. Laycock, R.C. Newman, Localised dissolution kinetics, salt films and pitting potentials Corrosion Science 39 (1997) 1771-1790. [29] A.G. Carcea, E.Y.W. Yip, D.D. He, R.C. Newman, Anodic kinetics of NiCr Mo alloys during localized corrosion: I. diffusion-controlled dissolution, Journal of Electrochemical Society 158 (2011) C215-C220. [30] S.L. Manchet, T. Mesquita, E. Chauveau, B. Chareyre, Corrosion Resistance of the Lean Duplex Material UNS S32202 for Oil and Gas Applications, NACE-international Corrosion Conference Series. 2015

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FIGURES AND CAPTIONS (colours required for all figures)

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Figure 1: SEM images along the transverse sections of Grade 2202 duplex stainless steel for (a) asreceived and (b) heat-treated (1100 °C for 24 hours) wires, with images of longitudinal sections of the (c) as-received and (d) heat-treated sample.

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Figure 2: Electrochemical data for the as-received sample exposed and tested following the protocol in Table 1, showing (a) 1st potentio-dynamic polarization and (b) 1st potentio-static polarization response, with (c) 2nd potentio-dynamic polarization and (d) 2nd, 3rd and 4th potentio-static polarization response in 0.1M HCl.

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Figure 3: Electrochemical data for the heat treated sample exposed and tested following the protocol in Table 2, showing (a)1st potentio-dynamic polarization and (b)1st potentio-static polarization response, with (c) 2nd potentio-dynamic polarization and (d) 2nd and 3rd potentiostatic polarization response in 0.1M HCl.

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Figure 4. 3D volume rendering from the X-ray CT showing as-received sample for (a) 1st CT scan and (b) 2nd CT scan.

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Figure 5. High magnification 3D volume rendering of the X-ray CT data showing all pits in the asreceived sample.

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Figure 6. 3D volume rendering of X-ray CT data showing as-received sample for (a) Pit 1, (b) Pit 2(right) and Pit 4(left) after the 2nd CT scan, (c)Pit 3 after the 1st CT scan and (d) the grown part of Pit 3-2 (arrow) after 2nd CT scan.

Figure 7. (a) 3D volume rendering showing pit from the surface and (b) virtual cross section CT image for Pit 1 of the as-received sample. The line on side view corresponds to the cross sectioned position.

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Figure 8. 3D volume rendering from the X-ray CT showing the heat-treated sample after the 1st CT scan (a & b) showing both sides, and (c & d) after the 2nd scan.

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Figure 9. High magnification 3D volume rendering from the X-ray CT showing the pits in the heattreated sample.

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Figure 10. 3D volume rendering from the X-ray CT showing the heat-treated sample for (a) Pit 1, (b) Pit 2 and (c) Pit 3 after the 2nd CT scan.

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Figure 11. (a) Virtual cross sectional CT image and (b) 3D volume rendering from the X-ray CT showing surface view for Pit 2 of the heat-treated sample. The line on side view corresponds to the cross sectioned position.

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Figure 12. Secondary electron SEM image of Pit 1 for as-received sample for (a) overview before ultrasonic cleaning, (b) high magnification image before ultrasonic cleaning and (c) overview after ultrasonic cleaning and (d) high magnification image after ultrasonic cleaning from inside the pit.

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Figure 13. Secondary electron SEM image of Pit 2 for heat-treated sample for (a) overview before ultrasonic cleaning, (b) high magnification image before ultrasonic cleaning, (c) overview after ultrasonic cleaning and (d) high magnification image after ultrasonic cleaning from inside the pit.

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Figure 14. (a) Secondary electron SEM image , (b)Cr EDX spectrum map and (c) Ni EDX spectrum map for the as-received sample, and (d) Secondary electron SEM image, (e) Cr EDX spectrum map and (f) Ni EDX spectrum map for Pit 1 of the heat-treated sample (Yellow line: austenite phase; Black line: ferrite phase; Green line: Edge of the pit; Arrow: remaining phase).

Figure 15. Observation on the heat-treated sample with (a) 3D volume rendering of X-ray CT data showing the pit edge, (b) virtual cross-section CT of the pit, (c) Secondary electron SEM image, (d) Cr

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EDX spectrum map and (e) Ni EDX spectrum map of Pit 2 after the 2nd CT scan. (The red line shows a facet of a grain; Yellow line: Edge of the pit; Green line: Ferrite phase on the un-corroded area)

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Figure 16. Individual pit volumes measured with X-ray CT and dissolved volumes converted from charges on each polarization step for (a) as-received sample and (b) heat-treated sample.

Figure 17. Individual current trace for (a) Pit 1, Pit 3 and Pit 4 and (b) Pit 2 and Pit 3-2 for the asreceived sample and (c) Pit 1 to 3 of the heat-treated sample (the arrows show sudden current incrment or decrement).

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Figure 18. Schematic diagram of dimention (a, b and c), surface area (S) and volume (V) of a pit.

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Figure 19. Stability product of (a) as-received samples and (b) heat-treated samples.

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Figure 20. Schematic diagram of cross-sectional image of (a) as-received sample and (b) heat-treated sample based on Pit 2. Broken line: pit mouth, Grey area: remaining phase, Grey line: pit bottom and Arrow: diffusion path.

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Figure 21. Pit depth squared over time of (a) Pit 1, Pit 3 and Pit 4 and (b) Pit 2 and Pit 3-2 of asreceived sample.

Figure 22. Pit depth squared over time for each pit with time of heat-treated sample.

Table 1. Experimental sequence for the as-received sample. (PD: Potentiodynamic polarization, PS: Potentiostatic polarization) Step 1

OCP measurement for 15 min PD from OCP to 950 mV vs. Ag/AgCl (1st PD)

1st X-ray CT scan

3

OCP measurement for 15 min PD from OCP to 950 mV vs. Ag/AgCl (2nd PD) PS at 950 mV vs. Ag/AgCl for 2000 sec (2nd PS)

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PS at 950 mV vs. Ag/AgCl for 1684 s (1st PS)

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PS at 950 mV vs. Ag/AgCl for 1929 sec (4th PS) 2nd X-ray CT scan + SEM observation (1)

5

Ultrasonic cleaning in acetone for 1 hour

6

SEM observation (2)

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Table 2. Experimental sequence for the heat-treated sample. (PD: Potentiodynamic polarization, PS: Potentiostatic polarization) Step 1

OCP measurement for 15 min PD from OCP to 950 mV vs. Ag/AgCl (1st PD)

2

1st X-ray CT scan

3

OCP measurement for 15 min

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PS at 950 mV vs. Ag/AgCl for 1365 sec (1st PS)

PD from OCP to 950 mV vs. Ag/AgCl (2nd PD)

PS at 950 mV vs. Ag/AgCl for 909 sec (3rd PS) 2nd X-ray CT scan + SEM observation (1)

5

Ultrasonic cleaning in acetone for 30 min

6

SEM observation (2)

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PS at 950 mV vs. Ag/AgCl for 2000 sec (2nd PS)

Table 3. Difference between the measured pit dimensions by X-ray CT and the calculated pit dimension via Faradays’s law at the end of polarization for as-received sample.

Pit 1

Pit 2

Pit 3

Pit 3-2

Pit 4

(Only grown part) Measured Calculated Difference Measured Calculated Difference Measured Calculated Difference Measured Calculated Difference Measured Calculated Difference

μm

μm

μm

μm

μm

37.3 8.7%

29.8

26.1 14.3% 54.1

50.1 8.1%

21.1

18.1 16.7% 69.0

68.1 1.3%

Max width

65.4

60.4 8.2%

36.1

31.3 15.2% 87.0

80.1 8.6%

25.3

21.7 16.9% 80.5

79.1 1.8%

Max height 87.2

80.6 8.2%

54.8

48.0 14.1% 123.1 114.1 7.9%

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Max depth 40.5

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30.7

26.4 16.5% 116.3 114.5 1.6%

Table 4. Difference between the measured pit dimensions by X-ray CT and the calculated pit dimension at the end of polarization for heat-treated sample. Pit 1

Pit 2

Pit 3

Measured Calculated Difference Measured Calculated Difference Measured Calculated Difference

μm

μm

μm

43.7

43.0

1.6%

55.7

55.7

-0.1%

89.1

89.1

0.0%

Max width

94.3

92.9

1.6%

188.2

188.4

-0.1%

123.6

123.0

0.5%

Max height 116.7

114.4

2.0%

143.4

143.8

-0.3%

151.6

151.5

0.1%

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Max depth

Table 5. Average diffusivity parameter of as-received and heat-treated sample. Pit ID

D∆C (mol.cm-1.s-1)

As-received sample

Pit 1

4.64x10-9

Pit 2

8.31x10-9

Pit 3

8.34x10-9

Pit 3-2

5.96x10-9

Pit 4

8.91x10-9

Pit 1

2.12x10-8

Pit 2

2.77x10-8

Pit 3

2.20x10-8

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Table 6. Individual diffusivity parameter of as-received and heat-treated sample. Pit ID

Segment

D∆C (mol.cm-1.s-1)

As-received sample

Pit 1

1

3.24x10-9

2

8.01x10-9

1

5.83x10-9

2

1.44x10-8

1

1.90x10-8

2

4.80x10-8

1

2.51x10-8

2

1.60x10-8

1

3.60x10-8

2

1.56x10-8

Pit 2

Pit 3

2.44x10-8

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Material