Pitting Corrosion Resistance and Repassivation Behavior of C-Bearing Duplex Stainless Steel

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Article Pitting Corrosion Resistance and Repassivation Behavior of C-Bearing Duplex Stainless Steel

Hanme Yoon 1,2, Heon-Young Ha 2,* , Tae-Ho Lee 2, Sung-Dae Kim 2, Jae Hoon Jang 2 , Joonoh Moon 2 and Namhyun Kang 1,*

1 Department of Materials Science and Engineering, Pusan National University, Busan 46241, Korea 2 Ferrous Alloy Department, Korea Institute of Materials Science, Changwon 51508, Korea * Correspondence: [email protected] (H.-Y.H.); [email protected] (N.K.); Tel.: +82-55-280-3422 (H.-Y.H.); +82-51-510-3274 (N.K.) Received: 24 July 2019; Accepted: 22 August 2019; Published: 26 August 2019

Abstract: The eﬀects of C-substitution for part of the N content, on the pitting corrosion resistance and repassivation tendencies of duplex stainless steels (DSSs) were investigated. For this investigation, normal UNS S32205 containing N only (DSS-N) and the C-substituted DSS (DSS-NC) were fabricated. Microstructural analyses conﬁrmed that the two DSSs had dual-phase microstructures without precipitates, and they possessed similar initial microstructure, including their grain sizes and phase fractions. Polarization and immersion tests performed in concentrated chloride solutions revealed that the DSS-NC was more resistant against stable pitting corrosion and possessed a higher repassivation tendency than the DSS-N. Furthermore, the corrosion pits initiated and propagated to a less corrosion resistant α phase. Polarization tests and corrosion depth measurements conducted in an HCl solution indicated that the DSS-NC exhibited lower galvanic corrosion rate between the α and γ phases than the DSS-N. Therefore, the growth rate of pit embryo was lowered in the DSS-NC, which shifted the potentials for the stable pit initiation and the pit extinction to the higher values.

Keywords: duplex stainless steel; pitting corrosion; repassivation behavior; galvanic corrosion

1. Introduction The beneficial effects of alloying nitrogen (N) [1–9] and carbon (C) [10–20] on mechanical properties and resistance to the localized corrosion of austenite stainless steels (γ-SSs) are widely investigated, and their mechanistic roles in the performance improvements are well established. N and C are economical and strong γ formers, thus they can be used in the γ-SS in replacement of Ni. In addition, they improve the mechanical strength of γ-SS by solid solution strengthening with a minimal loss of elongation. Moreover, it is reported that alloying N and C enhances the resistance to localized corrosion by strengthening the protectiveness of the passive film. Therefore, various γ-SSs have been modified and newly developed by alloying N and/or C [2,4,5,7,10–12,15]. It is worth mentioning that the advantages of the alloying N and C can be manifested as long as they remain in solid solution state in the matrix. Because the alloyed N and C exceeding solubility limits is prone to form Cr-consuming precipitates, such as Cr2N[21–23] and Cr23C6 [19,24], which degrade both mechanical and corrosion properties, thermomechanical processing and welding for N and/or C-bearing γ-SSs should be managed with caution. Duplex stainless steels (DSSs) are a family of SSs consisting of equal volume of ferrite (α) and γ phases [25–28]. A combination of the α and γ phases exerts beneficial influences on mechanical properties and resistance to corrosion. DSSs are particularly well known for their outstanding resistance to stress corrosion cracking in Cl−-containing environments [29]. Moreover, DSSs generally contains less Ni content (1–7 wt%) than the commercial 300-series γ-SS (i.e., FeCrNi-based SS contains, generally

Metals 2019, 9, 930; doi:10.3390/met9090930 www.mdpi.com/journal/metals Metals 2019, 9, 930 2 of 13

8–24 wt% Ni), thus DSSs are known to be more economical than FeCrNi-based γ-SSs. Because of the desirable performances and economic benefits, demand for the DSSs as structural materials shows a continuous increase in various industrial fields requiring high mechanical strength and corrosion resistance. Alloying N is frequently adopted for DSSs because of its advantages in performance improvement [25,30–34]. Thus, commercial DSSs including UNS S32101, UNS S32304, UNS S32205, and UNS S32750, contain 0.1–0.3 wt% N. However, alloying C to DSSs has rarely been attempted, although C is found to be beneficial to performances of γ-SSs, similarly to N. Generally, C content was carefully controlled to be less than 0.03 wt% for commercial DSSs, because DSSs can provide more sites to Cr23C6 precipitations such as phase boundaries than the γ-SS [19,35,36]. But as long as C remains in solid solution state, it is expected to take advantages of the alloying C in promoting the physico-chemical properties of DSSs, the possibility of which was supported by the previous studies on C-bearing γ-SSs [10–20]. This study investigated the effects of C on the corrosion properties of DSS. For this study, type UNS S32205 DSS in which some N content was substituted with C, and normal UNS S32205 DSS containing only N, were produced. The resistance to pitting corrosion of the DSSs was assessed, and the role of C-substitution in the corrosion properties was discussed in terms of the repassivation tendency and pit growth rate.

2. Materials and Methods In the present study, Fe-22Cr-5Ni-3Mo-1Mn-based (in wt%, type UNS S32205) DSSs containing diﬀerent N and C contents were investigated. The detailed chemical compositions of the DSSs denoted as DSS-N and DSS-NC are given in Table1.

Table 1. Chemical compositions (in wt%) of the investigated DSSs.

Alloy Fe Cr Ni Mo Mn N C S P DSS-N 22.48 5.17 3.10 1.46 0.24 0.002 0.005 0.005 Balanced DSS-NC 22.43 5.18 3.14 1.46 0.13 0.090 0.006 0.004

The DSS ingots of 30 kg were produced by vacuum induction melting in Ar. Then they were homogenized at 1250 ◦C for 2 h in Ar gas and hot-rolled, followed by water quenching. The hot-rolled plates with a thickness of 2 mm of the DSS-N and DSS-NC were solution treated at 1150 ◦C and 1130 ◦C for 1 h, respectively, in order to obtain α + γ matrix without precipitates and to control the phase fraction (α:γ) to be 1:1. The temperatures for the solution treatments were determined based on the equilibrium phase fraction versus temperature diagram (Figure1), which was calculated using Thermo-Calc software (version 2017b, database TCFE 7.0, Stockholm, Sweden). Microstructures of the DSSs were examined using light optical microscopy (LOM, Epiphot, Nikon, Minato, Japan), scanning electron microscopy (SEM, IT-300, JEOL, Akishima, Japan), and electron backscatter diffraction (EBSD, HKL Nordlys Channel 5, Oxford Instruments, Abingdon, UK) in a SEM (JSM-7001F, JEOL, Akishima, Japan). For the LOM and SEM analyses, the specimens (10 mm 10 mm 2 mm) were mechanically polished using suspension containing 1 µm-sized × × diamond particles. The polished specimens were chemically etched in a mixed acid solution (20 mL HNO3 + 30 mL HCl + 50 mL deionized water) for 1–3 min [33,35,37,38], which allowed us to measure the phase fractions (α:γ) of the specimens. Then we electrochemically etched in a 40% HNO3 solution by applying 4.5 V for 20–30 s [39,40] in order to identify the grain boundaries. Based on the SEM images taken at 5 different locations, the phase fractions (α:γ in vol%) of the DSSs were measured by image analysis. For the EBSD analysis, the specimens were polished using colloidal silica with a particle size of 0.02 µm. The EBSD analysis was performed on a scanning area 120 100 µm. with a step × size of 0.5 µm, and the grain boundaries were identified with the critical misorientation angle of 10◦. Alloying element partitioning in the DSSs was examined by an energy dispersive X-ray spectroscope Metals 2019, 9, 930 3 of 13

(EDS)Metals 2019 equipped, 9, x FOR on PEER the REVIEW SEM, and the chemical compositions were quantitatively examined through3 of 14 electron probe micro analysis (EPMA, SX100, CAMECA, Gennevilliers, France).

Figure 1. Equilibrium phase fractions of the (a) duplex stainless steel containing N only (DSS-N) and Figure 1. Equilibrium phase fractions of the (a) duplex stainless steel containing N only (DSS-N) and (b) duplex stainless steel containing N and C (DSS-NC) as functions of temperature, calculated using (b) duplex stainless steel containing N and C (DSS-NC) as functions of temperature, calculated using Thermo-Calc software (version 2017b, database TCFE 7.0). Thermo-Calc software (version 2017b, database TCFE 7.0). Pitting corrosion resistance and repassivation tendencies of the DSSs were assessed through linearMicrostructures and cyclic potentiodynamic of the DSSs were polarization examined tests using in NaCl light solutions. optical microscopy The polarization (LOM, tests Epiphot, were performedNikon, Minato, in a multi-neckJapan), scanning glass cell electron consisting micros of acopy saturated (SEM, calomelIT-300, referenceJEOL, Akishima, electrode Japan), (SCE), aand Pt plateelectron counter backscatter electrode, diffraction and a specimen (EBSD, servingHKL Nord as a workinglys Channel electrode. 5, Oxford The specimensInstruments, were Abingdon, mounted inUK) cold in a epoxy SEM resin(JSM-7001F, and ground JEOL, using Akishima, SiC emery Japan) paper. For ofthe up LOM to 2000 and grit.SEM The analyses, exposed the area specimens for the polarization(10 mm × 10 test mm was × 2 controlled mm) were to mechanically be 0.2 cm2 using polished electroplating using suspension tape. The containing tests were 1 performed µm-sized withdiamond a potentiostat particles. The (Reference600 polished specimens+, GAMRY, were Warminster, chemically PA, etched USA). in Linear a mixed polarization acid solution tests (20 were mL HNO3 + 30 mL HCl + 50 mL deionized water) for 1–3 min [33,35,37,38], which allowed1 us to measure conducted in a 5 M NaCl solution at 50 ◦C with a potential sweep rate of 2 mV s− . For the cyclic the phase fractions (α:γ) of the specimens. Then we electrochemically etched in a 40% HNO3 polarization tests, a 2 M NaCl solution at 50 ◦C was used, and the potential was elevated from 0.05 V solution by applying 4.5 V for 20–30 s [39,40] in order to identify the grain boundaries. Based− on the versus corrosion potential (Ecorr) to the potential value where the current density exceeded 0.15 mA SEM2 images taken at 5 different locations, the phase fractions (α:γ in vol%) of the DSSs were cm− , and was then lowered to the repassivation potential (Erp) with a potential sweep rate of 2 mV measured1 by image analysis. For the EBSD analysis, the specimens were polished using colloidal s− . Repetitive polarization tests were done at least 5 times, conﬁrming the reproducibility. In addition, immersionsilica with a tests particle were size employed of 0.02 inµm. order The to EBSD assess analysis the resistance was performed to pitting corrosionon a scanning of the area DSSs. 120 For × 100 the tests,µm. with the specimens a step size (30 of mm 0.5 30µm, mm and2 the mm) grain were boundaries mechanically were ground identified using 600with grit the SiC critical paper, misorientation angle of 10°. Alloying× element× partitioning in the DSSs was examined by an energy and they were immersed in a 6% FeCl3 + 1% HCl solution at 50 ◦C for 12 h. Before and after the immersiondispersive test,X-ray the spectroscope weight of specimen (EDS) equipped was measured on the in SEM, order and to calculate the chemical the weight compositions loss during were the tests.quantitatively Three specimens examined for eachthrough DSS wereelectron tested probe and the micro average analysis weight loss(EPMA, ratio (weightSX100, lossCAMECA,/original weightGennevilliers,100, %) France). was calculated. × ThePitting sites corrosion for the pitting resistance corrosion and repassivation were observed tendencies using the SEMof the after DSSs immersion were assessed in a 0.2 through M HCl linear and cyclic potentiodynamic polarization tests in NaCl solutions. The polarization tests were solution (pH 0.62) at 25 ◦C under the open circuit condition for 4 h. The strong acid containing Cl− was usedperformed for the in immersion a multi-neck test glass in order cell toconsisting simultaneously of a saturated identify calomel the phase reference boundary electrode and the (SCE), pit sites. a Pt plateUniform counter (orelectrode, general) and corrosion a specimen behavior servin of theg DSSsas a wasworking examined electrode. through The linear specimens polarization were mounted in cold epoxy resin and ground using SiC emery paper of up to 2000 grit. The1 exposed area tests in the 0.2 M HCl solution (pH 0.62) at 25 ◦C with a potential sweep rate of 2 mV s− . In addition, 2 galvanicfor the polarization corrosion rate test between was controlled the α and toγ bephases 0.2 cm in the using DSSs electroplating were quantiﬁed tape. by The measuring tests were the corrosionperformed depth with a of potentiostat the α phase (Reference600+, as compared byGA theMRY,γ phase.Warminster, The DSS PA, specimensUSA). Linear were polarization polished −1 usingtests were a suspension conducted with in a 15 µMm-sized NaCl solution diamond at particles,50 °C with and a potential then the sweep specimens rate of were 2 mV immersed s . For the in thecyclic 0.2 polarization M HCl solution tests, (pHa 2 M 0.62). NaCl After solution the 40at 50 min °Cof was immersion, used, and thethe three-dimensionalpotential was elevated corroded from −0.05 V versus corrosion potential (Ecorr) to the potential value where the current density exceeded 0.15 mA cm−2, and was then lowered to the repassivation potential (Erp) with a potential sweep rate of 2 mV s−1. Repetitive polarization tests were done at least 5 times, confirming the reproducibility. In addition, immersion tests were employed in order to assess the resistance to pitting corrosion of the DSSs. For the tests, the specimens (30 mm × 30 mm × 2 mm) were mechanically ground using

Metals 2019, 9, 930 4 of 13 morphology and the corrosion depth was investigated using a surface optical proﬁler (Wyko NT8000, Veeco, Plainview, NY, USA). The corrosion test conditions employed in this study are listed in Table2.

Table 2. Experimental conditions for the corrosion tests.

Purpose Experiment Test Condition

Linear potentiodynamic Solution: 5 M NaCl (50 ◦C) Measurement of Epit 1 Evaluation of resistance polarization test Potential sweep rate: 2 mV s− to pitting corrosion Measurements of Epit Cyclic potentiodynamic Solution: 2 M NaCl (50 ◦C) 1 and Erp polarization test Potential sweep rate: 2 mV s− Measurement of weight solution: 6% FeCl + 1% HCl Immersion test 3 loss (50 ◦C) Immersion time: 12 h Solution: 0.2 M HCl (25 C) Observation of pit initiation sites Immersion test ◦ Immersion time: 4 h Measurement of Ecorr, Linear potentiodynamic Solution: 0.2 M HCl (25) Evaluation of resistance 1 icorr, and i polarization test Potential sweep rate: 2 mV s to uniform corrosion crit − Evaluation of galvanic Solution: 0.2 M HCl (25 C) Immersion test ◦ corrosion rate Immersion time: 40 min

3. Results

3.1. Microstructure Figure1a,b shows the equilibrium phase fraction versus temperature plots of the DSS-N and DSS-NC, respectively. Figure1a informs that precipitates including sigma ( σ) phase and Cr2N can be formed in the DSS-N below 1000 ◦C, and Figure1b indicates that, in addition to the σ and Cr2N, Cr23C6 can be formed below 1020 ◦C in the DSS-NC. Based on Figure1, the temperatures for solution treatments were determined as 1150 ◦C for the DSS-N and 1130 ◦C for the DSS-NC. Figure2a,b shows the microstructures of the DSS-N and DSS-NC, respectively. LOM images (Figure2a-1,b-1) and SEM images (Figure2a-2,b-2) show that the two DSSs have typical dual-phase microstructures, and non-metallic inclusions are rarely observed in both DSSs. The high-resolution SEM images (Figure2a-3,b-3) show that the precipitates, including Cr 23C6, Cr2N, and σ are not formed. Elemental distribution maps in Figure3 indicate that Ni being a γ-stabilizer, it is enriched in the relatively bright phase, whereas Cr and Mo being α-stabilizers are concentrated in the dark phase. Therefore, the relatively bright phase in the SEM image was found to be the γ and the matrix phase was the α. The image analyses on the SEM micrographs revealed that the two DSSs had the mostly same α volume fractions; speciﬁcally, those of the DSS-N and DSS-NC were 51.5 3.2 and 52.6 2.2 ± ± vol%, respectively. The grain boundaries were identiﬁed in the electrochemically etched specimens in a HNO3 solution by applying 4.5 V (Figure2a-4,b-4). Image analysis on the Figure2a-4,b-4 showed that the two DSSs were similar in grain size with a diameter of 7.5–8.5 µm. The EBSD analysis gave more precise analysis results. Figure4 shows the phase maps measured through EBSD analysis, in which the green phase is the α phase and the red phase with annealing twin is the γ phase. For the DSS-N and DSS-NC, the average grain sizes of the α phases were 7.71 and 6.97 µm, respectively, and those of the γ phases were 8.05 and 8.20 µm, respectively. Metals 2019, 9, x FOR PEER REVIEW 6 of 14 Metals 2019, 9, x FOR PEER REVIEW 6 of 14 Metals 2019, 9, 930 5 of 13 Metals 2019, 9, x FOR PEER REVIEW 6 of 14

Figure 2. Microstructures of the (a) DSS-N and (b) DSS-NC: (a-1,b-1) Light optical micrographs and

SEMFigure micrographs 2. Microstructures taken at of(a -2,theb -2)(a) lowDSS-N and and (a-3, (bb)-3) DSS-NC: high magnifications (a-1,b-1) Light of opticalthe specimens micrographs etched and in Figure 2. Microstructures of the (a) DSS-N and (b) DSS-NC: (a-1,b-1) Light optical micrographs and aSEMFigure mixed micrographs 2. acid Microstructures solution taken (20 at of ml(a -2,the HNOb -2)(a) 3lowDSS-N + 30 and ml and ( aHCl-3, (bb )-3) +DSS-NC: 50 high ml magnificationsdeionized (a-1,b-1) water).Light of optical the (a-4, specimensb micrographs-4) Light etched optical and in SEM micrographs taken at (a-2,b-2) low and (a-3,b-3) high magniﬁcations of the specimens etched micrographsaSEM mixed micrographs acid of solution the takenspecimens (20 at ml(a-2, etchedHNOb-2) 3low in+ 30a and 40% ml ( a HClHNO-3,b -3)+3 50solution high ml magnificationsdeionized by applying water). of4.5 the V.(a -4,specimensb-4) Light etched optical in in a mixed acid solution (20 ml HNO3 + 30 ml HCl + 50 ml deionized water). (a-4,b-4) Light optical micrographsa mixed acid of solution the specimens (20 ml etchedHNO3 in+ 30a 40% ml HClHNO +3 50solution ml deionized by applying water). 4.5 V.(a-4, b-4) Light optical micrographs of the specimens etched in a 40% HNO3 solution by applying 4.5 V. micrographs of the specimens etched in a 40% HNO3 solution by applying 4.5 V.

Figure 3. SEM micrographs of the (a) DSS-N and (b) DSS-NC with EDS maps (Cr, Mo, and Ni). FigureFigure 3. SEMSEM micrographs micrographs of of the the ( (aa)) DSS-N DSS-N and and ( (b)) DSS-NC DSS-NC with with EDS EDS maps maps (Cr, Mo, and Ni). Figure 3. SEM micrographs of the (a) DSS-N and (b) DSS-NC with EDS maps (Cr, Mo, and Ni).

Figure 4. Electron backscatter diﬀraction (EBSD) phase maps of the (a) DSS-N and (b) DSS-NC. Figure 4. Electron backscatter diffraction (EBSD) phase maps of the (a) DSS-N and (b) DSS-NC. The The grain boundaries were identiﬁed with the critical misorientation angle of >10 . (red: Sace-centered grainFigure boundaries 4. Electron were backscatter identified diffraction with the (EBSD) critical phase misorientation maps of the angle (a) DSS-Nof >10°. and◦ (red: (b )Sace-centered DSS-NC. The cubic (γ) and green: Body centered cubic (α). cubicgrainFigure (boundariesγ 4.) and Electron green: were backscatter Body identified centered diffraction with cubic the ( α(EBSD) ).critical phase misorientation maps of the angle (a) DSS-Nof >10°. and (red: (b )Sace-centered DSS-NC. The cubicgrain (boundariesγ) and green: were Body identified centered with cubic the (α ).critical misorientation angle of >10°. (red: Sace-centered cubic (γ) and green: Body centered cubic (α).

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3.2.3.2. Partitioning Partitioning of of Alloying Alloying Elements Elements IndividualIndividual chemical chemical compositions compositions of of the the αα andand γγ phasesphases of of the the DSSs DSSs were were measured measured using using the the EPMAEPMA and and they they were were indicated indicated in in (Figure (Figure 55).). The αα phasesphases of of both both DSSs DSSs contained contained only only 0.05 0.05 wt% wt% N N (Figure(Figure 55a),a), whichwhich waswas thethe solubilitysolubility limitlimit ofof NN inin thethe α phase [38,41–43]. [38,41–43]. Most Most of of the the alloyed alloyed N N was was concentratedconcentrated in in the the γγ phase,phase, whose whose concentration concentration was was 0.40 0.40 wt% wt% for for the the DSS-N DSS-N and and 0.26 0.26 wt% wt% for for the the DSS-NCDSS-NC (Figure 5 5a).a). NiNi contentscontents of of the theγ γphases phases of of the the two two DSSs DSSs were were 6.44–6.53 6.44–6.53 wt%, wt%, and and those those of theof theα phases α phases were were 4.14–4.16 4.14–4.16 wt% (Figurewt% (Figure5b). The 5b).γ-stabilizing The γ-stabilizing elements, elements, i.e., N and i.e., Ni, N were and enriched Ni, were in enrichedthe γ phases. in the However, γ phases. the However,α-stabilizing the α-stabilizing elements, i.e., elements, Mo and i.e., Cr, Mo were and concentrated Cr, were concentrated in the α phase. in theThe α two phase. DSSs The contained two DSSs 3.71–3.77 contained wt% 3.71–3.77 Mo in the wt%α phases Mo in andthe α they phases had approximatelyand they had approximately 2.3 wt% Mo in 2.3the wt%γ phases Mo (Figurein the γ5c). phases The Cr (Figure contents 5c). of The the Crα phases contents were of 23.81the α wt% phases for thewere DSS-N 23.81 and wt% 24.07 for wt%the DSS-Nfor the and DSS-NC, 24.07 wt% and thosefor the of DSS-NC, the γ phases and those were of 21.00 the γ wt% phases for were the DSS-N 21.00 wt% and for 20.74 the wt%DSS-N for and the 20.74DSS-NC. wt% Figure for the5b,c DSS-NC. indicates thatFigure the 5b,c partitioning indicates of that Ni and the Mo partitioning was not significantly of Ni and a Moffected was by not the significantlyC-substitution affected for N inby the the DSSs. C-substitution In contrast, for the N C-substitution in the DSSs. In for contrast, N notably the aff C-substitutionected Cr partitioning for N notablyin the DSSs affected (Figure Cr5 d).partitioning Regarding in C contentsthe DSSs (Figure (Figure5e), 5d). C partitioningRegarding ofC thecontents DSSs was(Figure estimated 5e), C partitioningthrough thermodynamic of the DSSs calculationwas estimated using through Thermo-Calc. thermodynamic Since the DSS-N calculation contained using only Thermo-Calc. 0.002 wt% C, Sincethe C the partitioning DSS-N contained could be only negligible. 0.002 wt% However, C, the forC partitioning the DSS-NC, could the C be contents negligible. calculated However, in the forα theand DSS-NC,γ phases the were C contents significantly calculated different: in the 0.031 α and wt% γ and phases 0.153 were wt%, significantly respectively. different: 0.031 wt% and 0.153 wt%, respectively.

Figure 5. Chemical compositions of (a) N, (b) Ni, (c) Mo, and (d) Cr of the α and γ phases in the DSS-N Figureand DSS-NC 5. Chemical measured compositions using the EPMA.of (a) N, For (b each) Ni, phase(c) Mo, of and each (d DSS,) Cr 10of pointsthe α wereand γ analyzed phases in using the DSS-Nan EPMA, and andDSS-NC the averages measured and using the the standard EPMA. deviation For each valuesphase of (scatter each DSS, band) 10 werepoints calculated. were analyzed (e)C usingcontents an EPMA, of the α andand theγ phases averages of the and DSSs the standard calculated deviation by Thermo-Calc values (scatter software. band) were calculated. (e) C contents of the α and γ phases of the DSSs calculated by Thermo-Calc software. 3.3. Resistance to Pitting Corrosion 3.3. ResistanceThe resistance to Pitting to pitting Corrosion corrosion of the DSSs was assessed by linear potentiodynamic polarization testsThe in a 5resistance M NaCl solutionto pitting at 50corrosion◦C, and theof polarizationthe DSSs was curves assessed were presentedby linear in potentiodynamic Figure6a. In this environment, both DSSs exhibited passive behavior at the Ecorr of 0.33 V , and as the applied polarization tests in a 5 M NaCl solution at 50 °C, and the polarization− curvesSCE were presented in Figurepotential 6a. increased,In this environment, stable pitting both corrosion DSSs exhibited occurred passive in both behavior DSSs at at certain the Ecorr potentials. of −0.33 V TheSCE, and pitting as thepotential applied (E potentialpit) of the increased, DSS-N was stable 0.06 pitting VSCE, and corrosi thaton for occurred the DSS-NC in both was DSSs 0.15 at V SCEcertain(Figure potentials.6b and TheTable pitting3). From potential the repetitive (Epit) of the polarization DSS-N was tests 0.06 in V theSCE, 5and M NaClthat for solution, the DSS-NC the average was 0.15 E corrVSCE,E (Figurepit and 6bpassive and Table current 3). densityFrom the (ipassive repetitive) were polarizatio measuredn astests given in the in Table5 M NaCl3. solution, the average Ecorr, Epit and passiveThe Epit currentand E densityrp were (i measuredpassive) were through measured cyclic as given potentiodynamic in Table 3. polarization tests in a 2 M NaCl solution at 50 ◦C (Figure7a). A more dilute NaCl solution compared with that for the linear polarization test (Figure6) was used in order to extend the passive potential range for the measurement of Erp. Additionally, in this solution, both DSSs exhibited passivity from Ecorr to Epit. The Epit of the DSS-NC was 0.21 VSCE and that of the DSS-N was 0.16 VSCE, which also conﬁrmed that the DSS-NC exhibited a higher Epit than the DSS-N. It is noted that the Erp of the DSS-NC was also higher than that of the DSS-N: The Erp of the DSS-NC was 0.05 VSCE and that of the DSS-N was approximately 0 VSCE (Figure7b and Table3). In Table3, the average E corr,Epit,Erp, and ipassive obtained from the cyclic polarization curves in a 2 M NaCl solution at 50 ◦C were summarized.

Metals 2019, 9, x FOR PEER REVIEW 7 of 14

3.2. Partitioning of Alloying Elements Individual chemical compositions of the α and γ phases of the DSSs were measured using the EPMA and they were indicated in (Figure 5). The α phases of both DSSs contained only 0.05 wt% N (Figure 5a), which was the solubility limit of N in the α phase [38,41–43]. Most of the alloyed N was concentrated in the γ phase, whose concentration was 0.40 wt% for the DSS-N and 0.26 wt% for the DSS-NC (Figure 5a). Ni contents of the γ phases of the two DSSs were 6.44–6.53 wt%, and those of the α phases were 4.14–4.16 wt% (Figure 5b). The γ-stabilizing elements, i.e., N and Ni, were enriched in the γ phases. However, the α-stabilizing elements, i.e., Mo and Cr, were concentrated in the α phase. The two DSSs contained 3.71–3.77 wt% Mo in the α phases and they had approximately 2.3 wt% Mo in the γ phases (Figure 5c). The Cr contents of the α phases were 23.81 wt% for the DSS-N and 24.07 wt% for the DSS-NC, and those of the γ phases were 21.00 wt% for the DSS-N and 20.74 wt% for the DSS-NC. Figure 5b,c indicates that the partitioning of Ni and Mo was not significantly affected by the C-substitution for N in the DSSs. In contrast, the C-substitution for N notably affected Cr partitioning in the DSSs (Figure 5d). Regarding C contents (Figure 5e), C partitioning of the DSSs was estimated through thermodynamic calculation using Thermo-Calc. Since the DSS-N contained only 0.002 wt% C, the C partitioning could be negligible. However, for the DSS-NC, the C contents calculated in the α and γ phases were significantly different: 0.031 wt% and 0.153 wt%, respectively.

Figure 5. Chemical compositions of (a) N, (b) Ni, (c) Mo, and (d) Cr of the α and γ phases in the DSS-N and DSS-NC measured using the EPMA. For each phase of each DSS, 10 points were analyzed using an EPMA, and the averages and the standard deviation values (scatter band) were calculated. (e) C contents of the α and γ phases of the DSSs calculated by Thermo-Calc software.

3.3. Resistance to Pitting Corrosion The resistance to pitting corrosion of the DSSs was assessed by linear potentiodynamic polarization tests in a 5 M NaCl solution at 50 °C, and the polarization curves were presented in Figure 6a. In this environment, both DSSs exhibited passive behavior at the Ecorr of −0.33 VSCE, and as the applied potential increased, stable pitting corrosion occurred in both DSSs at certain potentials. The pitting potential (Epit) of the DSS-N was 0.06 VSCE, and that for the DSS-NC was 0.15 VSCE (Figure Metals 2019, 9, 930 7 of 13 6b and Table 3). From the repetitive polarization tests in the 5 M NaCl solution, the average Ecorr, Epit and passive current density (ipassive) were measured as given in Table 3.

Metals 2019, 9, x FOR PEER REVIEW 8 of 14 Figure 6. Resistance to pitting corrosion of the DSSs: (a) Linear potentiodynamic polarization curves of 1 Figurethe DSSs 6. Resistance measured into apitting 5 M NaCl corrosion solution of atthe 50 DSSs:◦C with (a) aLinear potential potentiodynamic sweep rate of 2polarization mV s− and curves (b) the ofaverage the DSSs and measured standard deviationin a 5 M NaCl values solution (scatter at band) 50 °C of with the pittinga potential potentials sweep obtained rate of 2 frommV s 5−1 repeatedand (b) thepolarization average and tests. standard deviation values (scatter band) of the pitting potentials obtained from 5 repeated polarization tests. Table 3. The corrosion characteristics of the DSS-N and DSS-NC measured through polarization tests in 5 M NaCl (Figure6) and 2 M NaCl solutions (Figure7) at 50 C. The Epit and Erp were measured through cyclic potentiodynamic◦ polarization tests in a 2 M NaCl solutionTest at 50 °C (Figure 7a). A more dilute NaCl solution compared with that for the linear 5 M NaCl at 50 C 2 M NaCl at 50 C polarizationCondition test (Figure 6) was used◦ in order to extend the passive potential◦ range for the

measurement of Erp. Additionally, in thisipassive solution,at both DSSs exhibited passivity from ipassiveEcorr toat Epit. The EpitAlloy of the DSS-NC Ecorr,VSCE was 0.21Epit,V VSCE and that0.1 V SCEof the, EDSS-Ncorr,VSCE was 0.16Epit,V VSCESCE, whichErp,V alsoSCE confirmed0.1 VSCE that, − − µA cm 2 µA cm 2 the DSS-NC exhibited a higher Epit than the DSS-N.− It is noted that the Erp of the DSS-NC was− also DSS-N 0.326 0.057 6.997 0.192 0.164 0.001 2.696 higher than that of− the DSS-N: The Erp of the DSS-NC− was 0.05 VSCE and that of the DSS-N was approximatelyDSS-NC 0 V0.324SCE (Figure 0.1477b and Table 6.875 3). In Table0.189 3, the 0.211average Ecorr 0.048, Epit, Erp, 2.471and ipassive − − obtained from the cyclic polarization curves in a 2 M NaCl solution at 50 °C were summarized.

Figure 7. Resistance to pitting corrosion and repassivation behavior of the DSSs: (a) Cyclic Figure 7. Resistance to pitting corrosion and repassivation behavior of the DSSs: (a) Cyclic potentiodynamic polarization curves of the DSSs measured in a 2 M NaCl solution at 50 ◦C with a potentiodynamic polarization curves1 of the DSSs measured in a 2 M NaCl solution at 50 °C with a potential sweep rate of 2 mV s− and (b) the average and standard deviation values (scatter band) potential sweep rate of 2 mV s−1 and (b) the average and standard deviation values (scatter band) of of the pitting and repassivation potentials (Epit and Erp, respectively) obtained from five repeated the pitting and repassivation potentials (Epit and Erp, respectively) obtained from five repeated polarization tests. polarization tests. The polarization tests in the concentrated NaCl solutions indicated that the DSS-NC exhibited betterTable resistance 3. The corrosion to localized characteristics corrosion of than the DSS-N the DSS-N. and DSS-NC Twelve measured hours of through immersion polarization tests in 6% tests in 5 M NaCl (Figure 6) and 2 M NaCl solutions (Figure 7) at 50 °C. FeCl3 + 1% HCl solution at 50 ◦C confirmed the finding. Figure8 shows the corrosion morphologies Testof Condition the DSSs after the immersion 5 M NaCl tests. at Multiple 50 °C pits are shown in the DSS-N 2 M (Figure NaCl 8ata), 50 while °C only one or two pits are observed in the DSS-NC (Figure8b). The average weight loss ratio ofi the DSS-N at −0.1 was V , −2 passive SCE 3.46Alloy0.50%, E andcorr, thatVSCE ofE pit the, VSCE DSS-NC ipassive at was −0.1 1.93 VSCE,0.42%. µA cm Ecorr, VSCE E pit, VSCE E rp, VSCE ± ± µA cm−2 DSS-N −0.326 0.057 6.997 −0.192 0.164 0.001 2.696 DSS-NC −0.324 0.147 6.875 −0.189 0.211 0.048 2.471

The polarization tests in the concentrated NaCl solutions indicated that the DSS-NC exhibited better resistance to localized corrosion than the DSS-N. Twelve hours of immersion tests in 6% FeCl3 + 1% HCl solution at 50 °C confirmed the finding. Figure 8 shows the corrosion morphologies of the DSSs after the immersion tests. Multiple pits are shown in the DSS-N (Figure 8a), while only one or two pits are observed in the DSS-NC (Figure 8b). The average weight loss ratio of the DSS-N was 3.46 ± 0.50%, and that of the DSS-NC was 1.93 ± 0.42%.

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Figure 8. Corrosion morphologies of (a) DSS-N and (b) DSS-NC after immersion in 6% FeCl3 + 1% FigureHClFigure solution 8. 8. CorrosionCorrosion at 50 °C morphologies morphologiesfor 12 h. of of (a (a) )DSS-N DSS-N and and ( (bb)) DSS-NC DSS-NC after after immersion immersion in in 6% 6% FeCl FeCl33 ++ 1%1% HClHCl solution solution at at 50 50 °C◦C for for 12 12 h. h. The sites for the pitting corrosion were examined. After immersion in the 0.2 M HCl solution, it was possibleTheThe sites sites to for forsimultaneously the the pitting pitting corrosion corrosion identify were the were pitexamined examined. initiation. After sites After immersion with immersion the phase in inthe boundaries. the 0.2 0.2 M MHCl HCl Figuresolution, solution, 9a,b it wasexhibitsit was possible possible the pit to tosimultaneouslymorphologies simultaneously of identify the identify DSS-N the the pit and pit initiation initiationDSS-NC, sites sites respectively. with with the the phase phaseAfter boundaries. boundaries.4 h of immersion, Figure Figure 9a,b 9thea,b exhibitsstableexhibits pits the the with pit morphologies morphologiesa size of several of of the theµm DSS-N DSS-N were and observedand DSS-NC, DSS-NC, at respectively.the respectively. α side of After the After 4phase h of4 immersion,h boundary of immersion, thefor stableboth the µ α stableDSSs.pits with pits a with size ofa size several of semveral were µm observed were observed at the sideat the of α the side phase of the boundary phase boundary for both DSSs. for both DSSs.

Figure 9. Pit morphologies of (a) DSS-N and (b) DSS-NC after immersion in a 0.2M HCl solution at Figure 9. Pit morphologies of (a) DSS-N and (b) DSS-NC after immersion in a 0.2M HCl solution at 25 C for 4 h. Figure25 °C◦ for 9. 4Pit h. morphologies of (a) DSS-N and (b) DSS-NC after immersion in a 0.2M HCl solution at 3.4.25 Resistance °C for 4 h. to Uniform Corrosion 3.4. Resistance to Uniform Corrosion 3.4. ResistanceThe resistances to Uniform to uniformCorrosion corrosion of the two DSSs were evaluated by measuring the Ecorr andThe the resistances dissolution to rates uniform from corrosion the linear of the polarization two DSSs testswere performedevaluated by in measuring a 0.2 M HCl the E solutioncorr and The resistances to uniform corrosion of the two DSSs were evaluated by measuring the Ecorr and the(Figure dissolution 10a). Once rates the from pit isthe initiated linear polarization by the passive tests film performed breakdown, in a the 0.2 electrolyte M HCl solution confined (Figure inside the dissolution rates from the linear polarization tests performed in a 0.2 M HCl solution (Figure 10a).the pitOnce cavity the pit becomes is initiated acidified by the because passive of film the hydrolysisbreakdown, reaction the electrolyte of metal confined ion [44 ,inside45]. Thus the pit the 10a). Once the pit is initiated by the passive film breakdown, the electrolyte confined inside the pit cavityevaluation becomes of the acidified uniform because corrosion of the behavior hydrolysis in the reaction acidified of chloride metal ion solution [44,45]. can Thus provide the evaluation important cavity becomes acidified because of the hydrolysis reaction of metal ion [44,45]. Thus the evaluation ofinformation the uniform about corrosion the pit growthbehavior reaction in the inside acidified the pit. chloride In this acidsolution solution, can bothprovide DSSs important exhibited ofinformation the uniform about corrosion the pit growth behavior reaction in the inside acidified the pit. chloride In this acidsolution solution, can bothprovide DSSs important exhibited typical active–passive transition behavior in the potential range from the Ecorr to approximately 0.15 informationtypical active–passive about the transitionpit growth behavior reaction in inside the potential the pit. rangeIn this from acid the solution, Ecorr to bothapproximately DSSs exhibited −−0.15 VSCE, and passive behavior was observed within the potential range from 0.15 VSCE to 0.80 VSCE. typical active–passive transition behavior in the potential range from the Ecorr− to approximately −0.15 VAlthoughSCE, and passive passive behavior ofwas both observed DSSs was within similar the to potential each other, range the di fromfferences −0.15 were VSCE distinguishable to 0.80 VSCE. VSCE, and passive behavior was observed within the potential range from −0.15 VSCE to 0.80 VSCE. Althoughin the active–passive passive behavior transition of behavior.both DSSs Figure was 10 similarb enlarged to theeach active–passive other, the differences transition curves.were Althoughdistinguishable passive in thebehavior active–passive of both transitionDSSs was behavior. similar Figureto each 10b other, enlarged the thedifferences active–passive were Ecorr of the DSS-N was 0.41 VSCE and that of the DSS-NC was 0.38 VSCE. The difference in the distinguishabletransition curves. in Ethecorr active–passiveof− the DSS-N wastransition −0.41 Vbehavior.SCE and that Figure of the−10b DSS-NC enlarged was the −active–passive0.38 VSCE. The Ecorr values was small but it was consistently observed in the repetitive polarization tests. Moreover, corr SCE SCE transitiondifference curves.in the E corrE values of the was DSS-N small was but it−0.41 was Vconsistently and that observed of the DSS-NC in the repetitive was −0.38 polarization V . The the corrosion rate (icorr) and critical anodic current density (icrit) of the DSSs were obviously reduced differencetests. Moreover, in the Ethecorr corrosionvalues was rate small (icorr but) and it was critical consistently anodic current observed density in the (irepetitivecrit) of the polarization DSSs were tests. Moreover, the corrosion rate (icorr) and critical anodic current density (icrit) of the DSSs were

Metals 2019, 9, 930 9 of 13

Metals 2019, 9, x FOR PEER REVIEW 10 of 14 by substitution of C for N. As shown in Figure 10c and Table4, the average i corr of the DSS-N was obviously reduced2 by substitution of C for N. As2 shown in Figure 10c and Table 4, the average icorr of 117.3 µA cm− and it decreased to 61.4 µA cm− for the DSS-NC. Moreover, the icrit of the DSS-N was the DSS-N was2 117.3 µA cm−2 and it decreased to 61.4 µA cm−2 for the DSS-NC. Moreover,2 the icrit of 158.3 µA cm− (at 0.29 VSCE), and that of the DSS-NC also decreased to 104.9 µA cm− (at 0.26 VSCE). the DSS-N was 158.3− µA cm−2 (at −0.29 VSCE), and that of the DSS-NC also decreased to 104.9− µA cm−2 The corrosion characteristics, including Ecorr, icorr, polarization resistance, corrosion rate (milli-inches (at −0.26 VSCE). The corrosion characteristics, including Ecorr, icorr, polarization resistance, corrosion per year, MPY), and icrit which were obtained from Figure 10 are given in Table4. The MPY was rate (milli-inches per year, MPY), and icrit which were obtained from Figure 10 are given2 in Table 4. calculated in accordance with the following equation; MPY = 0.13 icorr (µA cm− ) equivalent The MPY was calculated in3 accordance with the following equation;× MPY = 0.13 × icorr ×(µA cm−2) × weight (g)/ density (g cm− ). equivalent weight (g)/ density (g cm−3).

Figure 10. Resistance to general corrosion of the DSSs: (a) Linear potentiodynamic polarization curves Figure 10. Resistance to general corrosion of the DSSs: (a) Linear potentiodynamic polarization of the DSS-N and DSS-NC measured in a 0.2 M HCl solution at 25 ◦C with a potential sweep rate of curves of1 the DSS-N and DSS-NC measured in a 0.2 M HCl solution at 25 °C with a potential sweep 2 mV s− , and (b) the enlarged polarization curves showing active–passive transition behavior. (c) The rate of 2 mV s−1, and (b) the enlarged polarization curves showing active–passive transition behavior. average and standard deviation values (scatter band) of the corrosion rate (icorr) and critical anodic (c) The average and standard deviation values (scatter band) of the corrosion rate (icorr) and critical current density (icrit), which were obtained from ﬁve repeated polarization tests. anodic current density (icrit), which were obtained from five repeated polarization tests. Table 4. The corrosion characteristics of the DSS-N and DSS-NC measured through polarization tests Tablein a 0.2 4. MThe HCl corrosion at 25 ◦C (Figurecharacteristics 10). of the DSS-N and DSS-NC measured through polarization tests in a 0.2 M HCl at 25 °C (Figure 10). 2 Polarization Resistance, Corrosion Rate, 2 Alloy Ecorr,VSCE icorr, µA cm− 2 icrit, µA cm− Ohm cm− Milli-Inches per Year

DSS-N Ecorr0.405, icorr, μA 117.29 Polarization resistance, 355.35 54.18icrit 158.32, μA − Corrosion rate, milli-inches DSS-NC 0.377 61.44 517.25 28.38 104.94 Alloy − −2 −2 Ohm cm per year −2 VSCE cm cm 3.5. Galvanic Corrosion Between the Two Constituent Phases DSS-NThe dissolution −0.405 rate117.29 of the dual-phase 355.35 matrix is related to the galvanic 54.18 corrosion rate between 158.32 the twoDSS-NC constituent −0.377 phases. 61.44 Thus, for the DSSs, 517.25 the galvanic corrosion rate was 28.38 quantified by measuring 104.94 the corrosion depth between the α and γ phases [38,43,46]. The polished DSS specimens were immersed 3.5.in the Galvanic 0.2 M HClCorrosion solution Between and thethe corrosionTwo Constituent surface Phases was examined using a surface profiler. Figure 11a,b shows three-dimensional images of the corroded surfaces of the DSS-N and DSS-NC, respectively. The dissolution rate of the dual-phase matrix is related to the galvanic corrosion rate between A towering structure composed of the α (bluish color) and γ (golden-red color) phases was observed, the two constituent phases. Thus, for the DSSs, the galvanic corrosion rate was quantified by showing that the α and γ phases exhibited different corrosion rates in the strong acid. In both DSSs, measuring the corrosion depth between the α and γ phases [38,43,46]. The polished DSS specimens the more corroded phase was the α, suggesting that the α phase was more active than the γ phase. were immersed in the 0.2 M HCl solution and the corrosion surface was examined using a surface Therefore, the dissolution depth of the active phase (i.e., α phase), as compared to that of the noble profiler. Figure 11a,b shows three-dimensional images of the corroded surfaces of the DSS-N and phase (i.e., γ phase), can be correlated with the galvanic corrosion rate of the dual-phase matrix. DSS-NC, respectively. A towering structure composed of the α (bluish color) and γ (golden-red Analysis on the contour maps revealed that the corrosion depth for the DSS-N was 0.131 µm and that color) phases was observed, showing that the α and γ phases exhibited different corrosion rates in for the DSS-NC was 0.114 µm (Figure 11c). the strong acid. In both DSSs, the more corroded phase was the α, suggesting that the α phase was more active than the γ phase. Therefore, the dissolution depth of the active phase (i.e., α phase), as compared to that of the noble phase (i.e., γ phase), can be correlated with the galvanic corrosion rate of the dual-phase matrix. Analysis on the contour maps revealed that the corrosion depth for the DSS-N was 0.131 µm and that for the DSS-NC was 0.114 µm (Figure 11c).

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Figure 11. Three-dimensional images of the corroded surfaces of the (a) DSS-N and (b) DSS-NC. Figure 11. Three-dimensional images of the corroded surfaces of the (a) DSS-N and (b) DSS-NC. The The images were obtained after immersion in a 0.2 M HCl solution at 25 ◦C for 40 min. (c) Variation in images were obtained after immersion in a 0.2 M HCl solution at 25 °C for 40 minutes. (c) Variation the corrosion depth of the two DSSs. in the corrosion depth of the two DSSs. 4. Discussion 4. Discussion This study examined the microstructure and alloying element partitioning for the DSS-N and This study examined the microstructure and alloying element partitioning for the DSS-N and DSS-NC. Then for the two DSSs, the Epit and Erp were measured through polarization tests in 2–5 M DSS-NC. Then for the two DSSs, the Epit and Erp were measured through polarization tests in 2–5 M NaCl solutions. Weight loss ratio was obtained from immersion tests in a 6% FeCl3 + 1% HCl NaCl solutions. Weight loss ratio was obtained from immersion tests in a 6% FeCl3 + 1% HCl solution, and the Ecorr, icorr, and icrit were measured by the polarization tests in a 0.2 M HCl solution. solution, and the Ecorr, icorr, and icrit were measured by the polarization tests in a 0.2 M HCl solution. In addition, the galvanic corrosion rate was quantified by measuring the corrosion depth between the In addition, the galvanic corrosion rate was quantified by measuring the corrosion depth between twothe constituent two constituent phases phases after after immersion immersion in the in HClthe HCl solution. solution. TheThe two two DSSs DSSs had had dual-phase dual-phase microstructures without without non-metallic non-metallic inclusions inclusions and and precipitates, precipitates, asas indicated indicated in Figuresin Figures2–4 .2–4. In addition, In addition, the twothe DSSstwo DSSs possessed possessed similar similar grain sizesgrain andsizes phase and fractions.phase Thefractions. investigation The oninvestigation the alloying on element the alloying partitioning element (Figure partitioning5) indicated (Figure that C-substitution 5) indicated brought that a slightC-substitution deviation brought in the Cr-partitioning a slight deviation in the in DSS,the Cr-partitioning and the γ of DSS-NC in the containedDSS, and the Nwith γ of meaningfulDSS-NC amountcontained of C N (0.26N-0.15C with meaningful wt%), amou whilent of that C (0.26N-0.15C of the DSS-N wt%), had while N only that (0.40 of the wt%). DSS-N Thus, had it N could only be concluded(0.40 wt%). that Thus, the onlyit could variable be concluded to determine that the the only corrosion variable behavior to determine was chemical the corrosion compositions behavior of thewas constituent chemical phases.compositions of the constituent phases. TheThe pitting pitting corrosion corrosion resistanceresistance ofof thethe DSSsDSSs was quantified quantified by by E Epitpit andand Erp E rpobtainedobtained from from the the − polarizationpolarization tests tests in in 2–5 2–5 M M NaCl NaCl solutions solutions atat 50 ◦°C.C. The solutions solutions with with Cl Cl −concentrationsconcentrations higher higher than than 2 M2 M were were required required for for these these tests, tests, inin orderorder to provoke the the stable stable pitting pitting corrosion corrosion in inthe the investigated investigated alloys. Higher Epit indicates higher resistance to the stable pit initiation, and higher Erp means greater alloys. Higher Epit indicates higher resistance to the stable pit initiation, and higher Erp means greater tendencytendency for for the the pit pit extinction extinction (i.e.,(i.e., repassivation)repassivation) [47,48]. [47,48]. Figures Figures 66 and and 77 revealedrevealed that that the the DSS-NC DSS-NC exhibited higher Epit and Erp than the DSS-N. In addition, the immersion tests (Figure 8) also exhibited higher Epit and Erp than the DSS-N. In addition, the immersion tests (Figure8) also indicated indicated that the DSS-NC was more resistant against pitting corrosion than the DSS-N. Therefore, that the DSS-NC was more resistant against pitting corrosion than the DSS-N. Therefore, it was it was concluded that the DSS-NC possessed higher resistance to stable pitting corrosion and concluded that the DSS-NC possessed higher resistance to stable pitting corrosion and repassivation repassivation tendency than the DSS-N. tendency than the DSS-N. The sites for pitting corrosion were observed (Figure 9) and it was found that the pits grew to The sites for pitting corrosion were observed (Figure9) and it was found that the pits grew to the the less corrosion resistant phase (i.e., α phase). The bare surface of the matrix in the pit cavity is α lessexposed corrosion to the resistant acidified phase solution (i.e., thatphase). is locally The confined bare surface within of the the occlu matrixded in pit, the and pit cavityhence uniform is exposed tocorrosion the acidified takes solution place upon that isthe locally matrix. confined For this within reason, the the occluded uniform pit,corrosion and hence rate uniformof the matrix corrosion is takesdirectly place correlated upon the with matrix. the pit Forgrowth this rate. reason, Slower the growth uniform of the corrosion pit embryo rate implies of the matrixless probability is directly correlatedof stable withpit formation the pit growth and higher rate. tendency Slower growth for pit ofextinction. the pit embryo In the impliescase of dual-phase less probability matrix, of the stable pituniform formation corrosion and higher occurred tendency in the form for pit of extinction.galvanic corrosion, In the case as supported of dual-phase by the matrix, pit propagating the uniform corrosionbehavior occurred shown in in Figure the form 9. Therefore, of galvanic it was corrosion, concluded as supportedthat the galvanic by the corrosion pit propagating rate of the behavior DSS shownmatrix in was Figure associated9. Therefore, to the itpit was growth concluded rates of that the thetwo galvanic DSSs. corrosion rate of the DSS matrix was associatedFrom to the the polarization pit growth (Figure rates of 10) the and two the DSSs. immersion tests (Figure 11) in the 0.2 M HCl solution, quantitativeFrom the polarizationinformation about (Figure the 10 galvanic) and the corrosi immersionon rates tests of (Figurethe matrices 11) in of the the 0.2 two M HClDSSs solution, was quantitativeobtained. The information strong acid about with theCl− galvanicwas required corrosion for these rates tests of theto simulate matrices the of thelocally two confined DSSs was obtained.solution inside The strong the pit acidcavity. with As Clshown− was in requiredFigure 10, for rise these in the tests Ecorr and to simulate decreases the in the locally icorr and confined icrit solutionwere observed inside the in pitthe cavity. DSS-NC As in shown comparison in Figure wi th10 ,those rise inof thethe E DSS-N.corr and This decreases observation in the firstly icorr and indicated that the C-substitution for part of N made the DSS matrix noble. Furthermore, the icorr of icrit were observed in the DSS-NC in comparison with those of the DSS-N. This observation firstly indicated that the C-substitution for part of N made the DSS matrix noble. Furthermore, the icorr of the Metals 2019, 9, 930 11 of 13

2 2 DSS-NC (61.4 µA cm− ) was only half of that of the DSS-N (117.3 µA cm− ), which implied that the C-substitution lowered pit growth rate. Uniform dissolution of the dual-phase matrix inside the pit cavity occurred in the form of galvanic corrosion between the two constituent phases, as shown in Figure 11. In the DSSs, the less noble phase was the α, and the dissolution rate of the α relative to the γ of the DSS-NC was lower than that of the DSS-N (Figure 11c). Therefore, it was concluded that the DSS-NC exhibited lower galvanic corrosion rate than the DSS-N, and hence the growth rate of the pit embryo was lowered in the DSS-NC, which consequently shifted the potentials for the stable pit initiation (i.e., Epit) and the pit extinction (i.e., Erp) to the higher values. The observations in Figure 11 implied that the difference in the Ecorr values of two constituent phases was reduced in the DSS-NC in comparison with the DSS-N. The difference in the nobility between the α and γ was caused by the element partitioning (Figure5). It was noted that the C-substitution particularly changed the N, C, and Cr partitioning in comparison with the DSS-N, which resulted in decrease in the galvanic corrosion rate between the two constituent phases. In order to understand the difference in the galvanic corrosion rate between the two DSSs, estimating the respective Ecorr values of the α and γ phases for each DSS from the alloying element combination needs to be done. For this, the abilities of alloying elements, including N and C, to change the nobility of the SS matrix should be separately quantified. Further work on this topic is now underway, and if the roles of the various alloying elements for SSs in making matrix noble are quantitatively established, it would be helpful to design new DSSs with well-balanced nobility between the two constituent phases, leading to high corrosion resistance.

5. Conclusions The eﬀects of C-substitution for part of N on the pitting corrosion resistance and repassivation behavior of type UNS S32205 DSS (Fe-22Cr-5Ni-3Mo-1Mn-based DSS, in wt%) were investigated. For this investigation, normal UNS S32205 DSS containing N only (DSS-N) and the C-substituted DSS (DSS-NC) was fabricated. The followings summarize the ﬁndings of this study.

(1) The microstructural analyses revealed that the two DSSs after solution treatments had dual-phase microstructures without non-metallic inclusions and precipitates. They had similar initial microstructure, such as grain sizes and phase fractions. Cr partitioning was promoted by C-substitution for N in the DSS, and the γ phase of DSS-NC contained N with a meaningful amount of C (0.26N-0.15C wt%) while that of the DSS-N had N only (0.40 wt%).

(2) Polarization tests performed in 2–5 M NaCl solutions and immersion tests in a 6% FeCl3 + 1% HCl solution revealed that the DSS-NC possessed higher resistance to stable pitting corrosion and repassivation tendency than the DSS-N. Furthermore, the corrosion pits initiated and propagated to less a corrosion resistant phase (i.e., α phase). (3) Polarization tests and corrosion depth measurements conducted in a HCl solution indicated that the DSS-NC exhibited a lower galvanic corrosion rate between the α and γ phases than the DSS-N. Therefore, the growth rate of pit embryo was lowered in the DSS-NC, which shifted the potentials for the stable pit initiation and the pit extinction to the higher values.

Author Contributions: Conceptualization, H.-Y.H., T.-H.L. and N.K.; methodology, H.-Y.H., J.M. and J.H.J.; software, J.H.J.; validation, H.-Y.H., J.M. and J.H.J.; investigation, H.Y., S.-D.K., T.-H.L. and H.-Y.H.; resources, T.-H.L., H.-Y.H. and J.M.; data curation, J.M. and S.-D.K.; writing—original draft preparation, H.Y. and H.-Y.H.; writing—review and editing, T.-H.L. and N.K. Funding: This study was financially supported by Fundamental Research Program (grant number POC3380) of the Korea Institute of Materials Science (KIMS). This study was also supported by the Ministry of Trade, Industry & Energy (MI, Republic of Korea) under Strategic Core Materials Technology Development Program (No. 10067375) and the Basic Science Research Program of the National Research Foundation (NRF) funded by the Ministry of Science & ICT (NRF-2019R1C1C1010246). Conflicts of Interest: The authors declare no conflict of interest. Metals 2019, 9, 930 12 of 13

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