Influences of Mo on stress corrosion cracking susceptibility of newly developed FeCrMnNiNC-based lean austenitic stainless steels

Materials Characterization 119 (2016) 200–208

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Influences of Mo on stress corrosion cracking susceptibility of newly developed FeCrMnNiNC-based lean austenitic stainless steels Heon-Young Ha a,⁎, Won-Gyu Seo b, Jun Young Park a, Tae-Ho Lee a, Sangshik Kim b,⁎ a b

Ferrous Alloy Department, Korea Institute of Materials Science, 797 Changwondaero, Seongsangu, Changwon, Gyeongnam, 642-831, Republic of Korea Department of Materials Engineering and Convergence Technology, ReCAPT, Gyeongsang National University, Chinju, Republic of Korea

a r t i c l e

i n f o

Article history: Received 25 April 2016 Received in revised form 6 August 2016 Accepted 8 August 2016 Available online 10 August 2016 Keywords: Lean duplex stainless steel Pitting corrosion Stress corrosion cracking Molybdenum

a b s t r a c t In this paper, newly developed high interstitial alloys, Febalance18Cr10Mn1Ni0.4N0.15CxMo (x = 0, 0.91, and 1.76 wt%), were briefly introduced, and susceptibility to stress corrosion cracking of the alloys was discussed based on slow strain rate test results and fractographic observation. It was found that the economical developed alloys exhibited superior mechanical properties and resistance to pitting corrosion to those of AISI 316L stainless steel. In addition, it was revealed that the addition of Mo ranging from 0 to 1.76 wt% affected tensile properties only in a limited manner, but it increased the resistance to the stress corrosion cracking of the alloys. The improved resistance to stress corrosion cracking was attributed to the enhanced pitting corrosion resistance and suppressed intergranular decohesion of the matrix by addition of Mo. © 2016 Elsevier Inc. All rights reserved.

1. Introduction FeCrMnNC-based alloying system with reduced Ni content (less than 2 wt%) has been investigated as one of the promising substitutes for conventional FeCrNi-based AISI 300-series austenitic stainless steels [1–14]. The interstitial alloying elements, N and C, are economical and strong austenite stabilizers [1,2,4–16]. The N and C in solid solution state are effective in increasing mechanical strength without much reduction in elongation and wear resistance [1,5,6,9,11,12,14,17,18], and they are also beneficial to improve resistance to localized corrosion including pitting and crevice corrosion [7,8,18,19]. Moreover, the combined addition of N and C contributes to reducing the production cost. In order to produce high nitrogen stainless steel (HNS), a pressurized melting process that ensures appreciable N content in the FeCrMnbased alloy is generally employed, which causes an increase in the production cost [20–24]. This is one of the major obstacles for the widespread application of the HNSs, hence the combined addition of N and C to FeCrMn-based alloy has been proposed. Addition of C can effectively suppress formation of delta ferrite during the solidification because the C is a strong austenite former, which results in retaining the high N content in the FeCrMn-based matrix without using the pressurizing melting facility. That is, it is theoretically possible to obtain N content up to approximately 0.4 wt% in the FeCrMnC-based alloy via the conventional melting process [6,8,9,13]. Along with aforementioned advantages, various types of FeCrMnNC-based alloys, that is, high interstitial alloys (HIAs), have been developed. ⁎ Corresponding authors. E-mail addresses: [email protected] (H.-Y. Ha), [email protected] (S. Kim).

http://dx.doi.org/10.1016/j.matchar.2016.08.006 1044-5803/© 2016 Elsevier Inc. All rights reserved.

In order to increase the resistance to localize corrosion of stainless steels, small amount of Mo has been frequently used [25–33]. Mo is a ferrite stabilizer and improves the resistance against the detrimental attack of chlorides, and thus AISI 316 type stainless steels (316, 316L, and 316LN) with approximately 2 wt% Mo is therefore preferred over 304 stainless steel in coastal and de-icing salt situations [30,31]. In particular, the beneficial effect of N on enhancement of the resistance to pitting corrosion of stainless steels is reported to be more pronounced by alloying Mo [32,34]. Thus, Mo is used in stainless steels 2–4 wt%, sometimes up to 6 wt%, for excellent corrosion-resistant properties. However, because Mo is expensive alloying element and it easily forms sigma phase which is brittle intermetallic phase degrading impact toughness and corrosion resistant of the matrix [35], the use of Mo in the stainless steels is carefully controlled to avoid the problems. In this paper, we designed Febalance18Cr10Mn1Ni0.4N0.15CxMo (x = 0, 0.91, and 1.76 wt%) alloys in which the Mo content was controlled in order to increase the corrosion resistance, and successfully fabricated flawless ingot (10 kg) without pressurized melting process. In addition, employing appropriate thermomechanical process, it was possible to make hot-rolled plate with single austenite phase free from sigma phase, carbides and nitrides. It was confirmed that the addition of 0.15 wt% C was beneficial to stabilize the N element in the Fematrix, and the combined addition of N and C together with Mn effectively stabilized the austenite phase instead of Ni. In the present investigation, the mechanical and corrosion properties of the developed alloys was investigated and the results was compared with those of the AISI 316L stainless steel. In addition, the stress corrosion cracking (SCC) resistance of the developed alloy was examined, and the influences of Mo on the SCC

H.-Y. Ha et al. / Materials Characterization 119 (2016) 200–208

behavior of the NC-bearing Febal18Cr10Mn1Ni-based alloys were discussed. Although the SCC behavior of N-bearing stainless steels has been investigated by many researchers [36–40], comparatively little is known about the SCC behavior of NC-bearing stainless steel. Because the interstitial alloying elements in high content in the FeCrMn-based stainless steel probably cause the brittle fracture of the matrix [41–43], it is important to assess the SCC behavior of NC-bearing stainless steels. In addition, it is well known the beneficial effect of Mo on the pitting corrosion resistance of stainless steels, however, the effect of Mo on the SCC resistance was controversial [33,34–46]. Shu et al. [33] investigated the SCC behavior of ferritic stainless steels (Febalance 19Cr1.6Mo0.5Cu-based and Febalance15Cr0.5Cu-based alloys) containing Cu and Mo, and concluded that the simultaneously added Cu and Mo in tested steels caused the SCC. In contrast, Hamada et al. [45] investigated the sensitization behavior of 308 (UNS30800, Febalance(19–21)Cr(10 − 12)Ni2Mn1Si) and 316 (UNS31600, Febalance(16–18)Cr(10–14)Ni2Mn0.75Si) stainless steels for detecting the susceptibility to intergranular SCC, and concluded that the 316 stainless steel was almost immune to sensitization because of the addition of Mo. Therefore, in this paper, newly developed HIAs (Febalance18Cr10Mn1Ni0.4 N0·15CxMo (x = 0, 0.91, and 1.76 wt%) alloys) were briefly introduced, and the SCC susceptibility of the alloys with different Mo contents was closely investigated. The effect of Mo on the SCC susceptibility was discussed based on slow strain rate test (SSRT) results and micrographic and the fractographic observation. 2. Materials and experimental procedure 2.1. Materials The investigated alloys were three types of NC-bearing Febal18Cr10Mn1Ni-based alloys with Mo up to 1.76 wt%. Alloy ingots (10 kg) were fabricated using a commercial vacuum induction melting furnace (VIM 4 III-P, ALD) under N2 pressure of 1 bar. After homogenization at 1250 °C for 2 h under Ar atmosphere, the ingots were hotrolled into sheet 4 mm in thickness. For each alloy, specimens (1 cm × 10 cm × 0.4 cm and 1 cm × 1 cm × 0.4 cm) were taken from hot-rolled plate, and then solutionized at 1150–1200 °C for 1 h depending on the Mo content followed by water-quenching. The temperature for solution treatment was determined based on the thermodynamic calculation and microstructural observation. The chemical compositions of the alloys measured using an optical emission spectroscopy (QSN 750-II, PANalytical) and an inductively coupled plasma atomic emission spectroscopy (Optima 8300DV, PerkinElmer) are given in Table 1. For micrographic observation, the specimens were polished to 1 μm using a diamond suspension and etched in a solution of 15 mL HCl and 85 mL ethanol for 15–30 s depending on the Mo content. The microstructure was investigated using a light optical microscope (EPIPHOT, Nikon) and a scanning electron microscope (SEM; JSM-5800, JEOL). 2.2. Experimental procedure Mechanical properties and pitting corrosion resistance of the developed alloys were evaluated and compared with those of AISI 316L stainless steel. The AISI 316L stainless steel investigated in the

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present study was a commercial alloy, whose chemical composition was Febalance-17.35Cr-0.97Mn-12.04Ni-2.36Mo-0.49Si-0.019C-0.026N0.003S-0.009P, in wt%. Tensile specimens (ASTM E8M) for each alloy were tensile-tested at 25 °C with a nominal strain rate of 1.67 × 10− 3 s − 1 using a servohydraulic machine (INSTRON 5882, Canton). The pitting corrosion resistance of the alloys was assessed through a potentiodynamic polarization test in a 0.6 M (3.5 mass%) NaCl solution at 25 °C at a potential sweep rate of 2 mV s− 1 using a potentiostat (Reference 600, GAMRY). The tensile and polarization tests were repetitively conducted 3–5 times, and good reproducibility was confirmed. The SSRT method was utilized to evaluate the SCC susceptibility using a constant extension rate test machine (CERT-1, MTDI) in accordance with ASTM G129. For the SSRT, the smooth tensile specimens with the dimensions shown in Fig. 1 were prepared from 4 mm thick plate and their longitudinal direction was parallel to the rolling direction. The detailed description on the experimental procedure is given in our previous paper [19]. The testing cell consisted of a specimen as a working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE) positioned in a salt bridge. The SSRTs in a 2 M NaCl solution at 50 °C were conducted under an anodic applied potential of 0.05 V versus corrosion potential (Ecorr) at a strain rate of 10−6 s−1. In order to exclude the influences of H and/or H2 on the SCC during the SSRT period, the anodic potential above the Ecorr level was needed as the applied potential. In addition, it was possible to accelerate the SCC by applying the anodic potential. However, the applied potential should be lower than the repassivation potential (Erp) in order to minimize the influence of pure pitting corrosion on SCC [37,47–49]. For these reasons, the applied potential for the SSRT was determined as + 0.05 V versus Ecorr. The Ecorr values of the alloys were measured by polarization test in the same solution to that of the SSRT, 2 M NaCl solution at 50 °C. The solution was circulated by using a peristaltic pump at a speed of 20 mL min−1 in order to maintain the concentration of [Cl−] and/or pH in the solution during the SSRT. As a reference data, the SSRTs were also performed on each specimen in laboratory air at the same strain rate of 10−6 s−1. The relative humidity for the laboratory air was controlled to be 45 to 55%. The SCC susceptibility was then quantified based on the SSRT results obtained in air and aqueous chloride environments. After the SSRT, the corrosion and the fracture surface of tested specimens were observed by SEM to understand the mode of fracture and the SCC mechanisms. The cross sections of the tested specimens in aqueous environments were observed by an optical microscope to identify the mechanism associated with propagation and arrest of crack formed on the specimen surface. In order to understand the susceptibility to SCC of the alloys, the pitting and general corrosion behavior of the alloys were investigated through potentiodynamic polarization tests. In order to measure the pitting potential (Epit) and Erp simultaneously, a cyclic polarization test was conducted in the 2 M NaCl solution at 50 °C. The potential was swept from − 0.1 V versus Ecorr to the potential value at which the current density exceeded 0.1 mA cm− 2, and then reversely swept to the initiation potential with a potential sweep rate of 2 mV s− 1. In addition, the general corrosion behavior of the alloys was measured using linear potentiodynamic polarization test in a 0.6 M NaCl + 0.02 M HCl solution (pH 1.38) at 25 °C, at a potential sweep rate of 2 mV s− 1.

Table 1 Chemical composition of the investigated alloys (in wt%) measured using an optical emission spectroscopy and an inductively coupled plasma atomic emission spectroscopy. Alloy

Fe

Cr

Mn

Mo

Ni

C

N

Si

P

S

O

0Mo 1Mo 2Mo

balance balance balance

18.959 18.643 18.412

9.900 9.886 9.913

– 0.914 1.760

1.084 1.073 1.070

0.182 0.162 0.154

0.344 0.377 0.410

0.133 0.129 0.132

0.003 0.002 0.002

0.002 0.001 0.002

0.007 0.006 0.008

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3. Results and discussion 3.1. Microstructure

Fig. 1. Schematic illustration of a slow strain rate test specimen.

Then, intergranular corrosion (IGC) behavior of the alloys was evaluated using a double-loop electrochemical potentiokinetic reactivation (DLEPR) test in a 0.5 M H2SO4 + 0.01 M KSCN solution. The potential was positively swept from −0.05 V versus Ecorr up to 0.33 VSCE in passive potential region and then reversely swept to the initial potential with a potential sweep rate of 1.67 mV s−1. Before the DLEPR test, the specimens were immersed in the solution for 300 s to determine the Ecorr. From the DLEPR curve, degree of sensitization value was calculated. After the DLEPR test, the corroded surfaces were observed using the SEM. For the polarization and DLEPR tests, the specimens were mounted in cold epoxy resin and ground using SiC paper to 2000-grit, and the tested area was controlled to 0.13 cm2 (circle with a diameter of 4 mm) using electroplating tape (3 M™ Electroplating Tape 470). All the electrochemical tests for each condition were repetitively performed at least 5 times in order to confirm the reproducibility. After the test, it was also confirmed that the crevice corrosion did not occurred on the specimens.

The equilibrium phase diagrams of the alloys were calculated using Thermo-Calc.™ (version 4.1, TCFE 7 database). It is shown that the ferrite formation temperature slightly decreases as the Mo content increases. Based on Fig. 2, the solutionization temperature to make an austenite single phase without carbides and nitrides was determined to be 1150–1200 °C depending on the Mo content. After solutionization for 1 h, the alloys were quenched in water. Fig. 3(a), (b) and (c) are the SEM micrographs of the 0Mo, 1Mo, and 2Mo alloys, respectively. They show that the three alloys have an austenite single phase with annealing twin boundaries. The average grain sizes for the 0Mo, 1Mo and 2Mo specimen were 165, 164 and 167 μm, respectively, suggesting that the addition of Mo did not significantly affect the size of grain. Fig. 3(a-1), (b-1) and (c-1) are the SEM micrographs of the 0Mo, 1Mo, and 2Mo alloys taken at a high magnification, respectively. Fig. 3 demonstrates that the NC-bearing Febal18Cr10Mn1Ni-based alloys have clean grain boundaries free from the Cr-carbides and/or Cr-nitrides after proper solutionization. 3.2. Tensile properties The engineering stress-strain curves are presented in Fig. 4(a) and the tensile properties are summarized in Table 2. The developed alloys exhibit average yield strength of 473.7–507.0 MPa, tensile strength of

Fig. 2. Equilibrium phase diagrams of the (a) 0Mo, (b) 1Mo, and (c) 2 Mo alloys calculated using Thermo-Calc.™ (version 4.1, TCFE 7 database) software.

Fig. 3. SEM micrographs of the solutionized (a)(a-1) 0Mo, (b)(b-1) 1Mo, and (c)(c-1) 2 Mo alloys taken at low and high magnifications, respectively.

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Fig. 4. (a) Stress-strain curves of the 0Mo, 1Mo and 2Mo alloys measured in air at 25 °C at a nominal strain rate of 1.67 × 10−3 s−1. (b) Potentiodynamic polarization curves of the 0Mo, 1Mo, 2Mo alloys, and AISI 316L stainless steel measured in a 0.6 M NaCl solution at 25 °C at a potential sweep rate of 2 mV s−1.

951.0–987.3 MPa, and elongation of 68.7–74.7%. It is noted that the tensile strength of the alloys increases and the elongation slightly decreases as the Mo content increases. The mechanical properties of the developed alloys were compared with the AISI 316L stainless steel, which is Mo-bearing (2–3 wt%) Febal17Cr12Ni-based austenitic stainless steel exhibiting improved resistance to general and localized corrosion than the conventional FeCrNi austenitic stainless steels such as AISI 304 stainless steel. The yield strength, tensile strength and elongation values of the standard AISI 316L stainless steel in annealed state were reported to be 170 MPa, 485 MPa and 40%, respectively [50]. Thus, it is revealed that the developed alloys have remarkably superior tensile properties to AISI 316L stainless steel, which is primarily attributed the solid solution hardening effects of N and C [1,6,10,11]. 3.3. Resistance to pitting corrosion The resistance to pitting corrosion of the developed alloys and AISI 316L stainless steel was assessed using a potentiodynamic polarization Table 2 Tensile properties of the alloys measured in air at 25 °C at a nominal strain rate of 1.67 × 10−3 s−1. Alloy

Yield strength, MPa

Tensile strength, MPa

Elongation, %

0Mo 1Mo 2Mo

473.7 ± 3.1 507.0 ± 11.1 491.3 ± 15.8

951 ± 5.3 975.0 ± 12.8 987.3 ± 12.6

74.7 ± 2.3 70.7 ± 3.5 68.7 ± 0.6

test in a 0.6 M NaCl solution at 25 °C, and the results are presented in Fig. 5(b). All the alloys exhibited passive state under an open circuit condition, and the Ecorr values of the alloys including AISI 316 L stainless steel were approximately −0.4 VSCE. The polarization behavior of the alloys was almost similar to each other, but the Epit values were different. Fig. 5 confirms that the alloying Mo is beneficial to pitting corrosion resistance of the NC-bearing Febal18Cr10Mn1Ni-based alloys. The average Epit value of the 0Mo alloy was 0.359 VSCE and increased to 0.682 VSCE for the 2Mo alloy. In addition, Fig. 5 demonstrates that the 1Mo and 2Mo alloys have equivalent or superior resistance to pitting corrosion in comparison with that of AISI 316L stainless steel. It should be noted that the 1Mo and 2Mo alloys contains only 1 wt% Ni plus 0.91 and 1.76 wt% Mo in the Febal18Cr10MnNC-based matrix, respectively, in contrast, the AISI 316 L stainless steel contains 12 wt% Ni and 2.4 wt% Mo in the Fe17Crbased matrix. Thus, it is concluded that the developed alloys are more economical than AISI 316L stainless steel. In addition, the improved resistance to pitting corrosion of the NC-bearing Febal18Cr10Mn1Ni-based alloys are primarily attributed to the N anc C in solid solution state. It is well known the beneficial effect of alloying N on resistance to pitting corrosion of stainless steels [21–24,34,51]. Recently, the authors found that the C in solid solution also can improve the localized corrosion resistance of stainless steels by strengthening the passive film with higher Cr concentration [7,8,19]. The results from the tensile (Fig. 4(a)) and polarization tests (Fig. 4(b)) support that the N and C in solid solution state of Febal18Cr10Mn1Ni-based alloys are effective to increase mechanical properties and resistance to pitting corrosion as well. Notable point is

Fig. 5. (a) Potentiodynamic polarization curves of the 0Mo, 1Mo, and 2 Mo alloys measured in a 2 M NaCl solution at 50 °C at a potential sweep rate of 2 mV s−1. (b) Variation of Epit and Erp values as a function of the Mo content.

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Table 3 Slow strain rate test results of the alloys at a strain rate of 10−6 s−1 in air at 25 °C and a 2 M NaCl solution at 50 °C under anodic applied potential. Alloy Environment

Yield strength, MPa

Tensile strength, MPa

Elongation, RTE, % %

0Mo

440.0 ± 31.5 410.4 ± 48.7

986.7 ± 63.5 583.6 ± 103.4

70.9 ± 7.9 16.7 ± 9.3

75.4 ±

463.0 ± 14.1 428.3 ± 45.0

1009.0 ± 63.6 649.3 ± 111.5

58.1 ± 3.4 18.1 ± 5.6

71.0 ±

455.5 ± 10.6 451.0 ± 33.9

1005.5 ± 13.4 993.0 ± 12.7

71.0 ± 2.4 74.8 ± 0.9

−5.4 ±

1Mo

2Mo

Air at 25 °C 2 M NaCl at 50 °C Air at 25 °C 2 M NaCl at 50 °C Air at 25 °C 2 M NaCl at 50 °C

13.7

9.0

4.9

that the developed HIAs exhibiting superior mechanical properties and pitting corrosion resistance to AISI 316L stainless steel are much more economical than the conventional AISI 316L stainless steel. Thus it can be concluded that the NC-bearing Febal18Cr10Mn1Ni-based alloys are promising substitutes for the conventional AISI 300-series stainless steels. Fig. 5(a) exhibits the cyclic polarization curves of the 0Mo, 1Mo and 2Mo alloys measured in a 2 M NaCl solution at 50 °C, and the average Epit and average Erp values of the alloys are presented in Fig. 5(b) as a function of the Mo content. The Ecorr of the alloys were approximately −0.28 VSCE, and the three alloys exhibited passivity under open circuit condition in the 2 M NaCl solution at 50 °C. Fig. 5(a) confirms that the alloys

exhibit similar polarization behavior, but a notable difference is observed in the Epit and Erp values. Similar to the results from Fig. 4(b), the Epit values measured in the 2 M NaCl solution at 50 °C was also elevated by the alloying Mo. The average Epit value of the 0Mo alloy was − 0.089 VSCE and that of the 2Mo alloy was 0.350 VSCE. In addition, a rise in the Erp value was also observed with increase in the Mo content. As the Mo content increased from 0 to 1.76 wt%, the Erp value increased from −0.208 to 0.126 VSCE. The results from Figs. 4(b) and 5 demonstrate that the resistance to pitting corrosion is enhanced as the Mo content increases. Based on the polarization curves obtained in the 2 M NaCl solution at 50 °C (Fig. 5), the applied potential for the SSRT was determined as 0.05 V versus the Ecorr value of each alloy, which is in the passive potential range below the Epit and Erp values.

3.4. SCC susceptibility Table 3 summarizes the SSRT results of the 0Mo, 1Mo and 2Mo alloys tested in air at 25 °C and the 2 M NaCl solution at 50 °C under an anodic applied potential of 0.05 V versus Ecorr. The strain rate was 10−6 s−1. This table shows that the variation in yield strength, tensile strength and elongation of the investigated alloys in air at 25 °C is within the error range, suggesting that the effect of Mo on the tensile properties of the investigated alloys in air at a slow strain rate of 10−6 s−1 is not significant.

Fig. 6. SEM fractographs of (a)(a-1) 0Mo, (b)(b-1) 1Mo and (c)(c-1) 2Mo specimens tested in air at a nominal strain rate of 10−6 s−1, taken at low and high magnifications, respectively.

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It is important to investigate the fracture morphology, including the site of crack initiation (i.e., surface or internal) and the fracture mode (i.e., cleavage or intergranular cracking) of the SSRTed specimen in order to understand the tensile deformation, as well as SCC behavior. Fig. 6 shows the low- and high-magnification SEM fractographs of the specimens tested in air at 25 °C at a nominal strain rate of 10−6 s−1. The high-magnification SEM fractographs (Fig. 6(a-1)–(c-1)) were taken from the middle part of the specimens. The low-magnification fractographs (Fig. 6(a)–(c)) show that the tensile fracture is initiated in the middle of the specimen, which is typical fracture mode for tensile test, and also present the intergranular fracture mode with fine dimples on the intergranular facets. With increasing the Mo content from 0 to 1.76 wt%, no notable difference was detected on the fracture surface. The SSRT results (Table 3) demonstrate that the 0Mo alloy is significantly susceptible to SCC in the 2 M NaCl solution at 50 °C under an anodic applied potential. It is also noted that the addition of Mo can improve the resistance to SCC of NC-bearing Febal18Cr10Mn1Ni-based alloys. As previously reported [19,36,52,53], the SCC susceptibility can be expressed by the reduction ratio of tensile elongation (RTE) in SCCcausing environment with respect to that in inert environment (i.e., air in this study). The average RTE value is 75.4 ± 13.7% for the 0Mo alloy and 71.0 ± 9.0% for the 1Mo alloy, respectively, accompanied with the reduction in yield strength and particularly tensile strength values for both alloys. It was found that the addition of 0.91 wt% Mo was not significantly effective in improving the SCC susceptibility of the NC-bearing Febal18Cr10Mn1Ni-based alloys in the aqueous chloride environment. The SCC susceptibility of the NC-bearing Febal18Cr10Mn1Ni-based alloys was, however, notably reduced with the addition of 1.76 wt% Mo. Indeed, the 2Mo alloy was almost immune to SCC in anodic dissolution condition. Fig. 7 exhibits SEM images of the corroded surfaces (Fig. 7(a)–(c)) and fractured surfaces (Fig. 7(a-1)–(c-2)) of the SSRTed specimens. A

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couple of stress corrosion cracks on the specimen surface are observed in the 0Mo and 1Mo specimens (Fig. 7(a) and (b), respectively) with little sign of plastic deformation. Notable point was that the cracks were formed along the grain boundaries. On the surface of the 2Mo specimen, on the other hand, a considerable amount of plastic deformation was noted, while the cracks were hardly observed. The examination of fractured surface strongly suggested the improved resistance to SCC through the addition of 1.76 wt% Mo. The low-magnification SEM fractographs in Fig. 7(a-1)–(c-1) also exhibit the beneficial effect of Mo addition on SCC resistance. For the 0Mo and 1Mo specimens, the fracture initiated at the surface and propagated in intergranular manner. In contrast, with the addition of 1.76 wt% Mo, the fracture initiated in the middle of specimen, and the intergranular fracture mode was not observed. Higher magnification SEM fractographs of the 0Mo, 1Mo, and 2Mo specimens are shown in Fig. 7(a-2), (b-2) and (c-2), respectively. It is clearly shown that intergranular fracture mode is completely suppressed by the 1.76 wt% addition of Mo, exhibiting a dimpled rupture mode for the 2Mo specimen. The fractographic analysis strongly suggested that the SCC mechanism in the investigated alloys was related to the intergranular decohesion in aqueous chloride environment under anodic dissolution potentials. In order to understand the relationship between intergranular decohesion and the SCC susceptibility, the area fraction of the intergranular fracture on the fracture surface is plotted as a function of RTE. Fig. 8 shows the linear relationship between the degree of intergranular fracture and the SCC susceptibility. This figure further confirms that the primary SCC mechanism in the investigated alloys was related to the intergranular decohesion. 4. Discussion The improved SCC resistance of the Febalance18Cr10Mn1Ni0.4N0.15CxMo (x = 0, 0.91, and 1.76 wt%) alloys

Fig. 7. SEM micrographs of the corroded surfaces of the (a) 0 M, (b) 1Mo and (c) 2Mo alloys, and low and high magnification SEM fractographs of (a-1)(a-2) 0Mo, (b-2)(b-2) 1Mo and (c1)(c-2) 2Mo specimens, respectively, after the slow strain rate test in a 2 M NaCl solution at 50 °C at a strain rate of 10−6 s−1 under an anodic applied potential of 0.05 V versus Ecorr.

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Fig. 8. Area fraction of intergranular fracture on the fracture surface as a function of reduction ratio of tensile elongation.

with increase in the Mo content is attributed to two possibilities; enhanced pitting corrosion resistance and increased grain boundary cohesion by alloying Mo. Figs. 4(b) and 5 present that the alloying Mo shifts the Epit value to the higher potentials, which demonstrates that the alloying Mo decreases the pit initiation probability. In addition, Fig. 5 exhibits that the Mo addition also elevates the Erp value, reflecting that the initiated pit is more prone to repassivate in the alloy containing higher Mo content. The reason for the change in the Erp value can be found by investigating the general corrosion behavior of the alloys. The increased Erp value indicates suppressed dissolution rate of the metal inside the pit resulting in pit extinction. After passive film breakdown and pit initiation, the confined solution inside the pit became acidified through the hydrolysis reaction of the metal. Thus, in order to simulate the environment in the pit cavity, the acidified NaCl solution was selected to investigate the bare metal dissolution rate. Fig. 9 shows the polarization curves of the three alloys measured in a 0.6 M NaCl + 0.02 M HCl solution (pH 1.38) at 25 °C. Using the acidified NaCl solution at lower pH than 2 [51,54], it is also possible to simultaneously investigate the resistance to general and pitting corrosion by measuring the critical dissolution current density (icrit) and the Epit of the alloys, respectively. From Fig. 10, it is found that the Ecorr is slightly elevated as the Mo content increased from −0.573 VSCE for the 0Mo alloy to −0.520 VSCE for the 2Mo alloy. Typical active-passive transition behavior for stainless steel was observed in the three alloys as shown in Fig. 9(a). The influence of Mo was pronounced in the Epit values and the icrit values, as presented in Fig. 9(b) and (c), respectively. The average Epit values of the 0Mo, 1Mo and 2Mo alloys were 0.124, 0.216, and 1.123 VSCE,

respectively, exhibiting the beneficial role of Mo in the resistance against pitting corrosion, and the 2Mo alloy was almost immune to the pitting corrosion in this corrosive solution. In addition, the icrit of the alloy was reduced as a function of Mo content as shown in Fig. 9(c). The average icrit values of the 0Mo, 1Mo, and 2 Mo alloys were 1.411, 0.459, and 0.355 mA cm−2, respectively. This result demonstrated that the alloying Mo reduced the active dissolution rate of the bare metal inside the pits resulting in suppressing the pit propagation, and consequently, rise in the Erp value. The SCC of the alloys is generally initiated in association with the corrosion pits formed on the surface, and the propagation of stress corrosion cracks can be accelerated by the pit propagation (i.e., bare metal dissolution inside the pit). Therefore, it is considered that the reduced SCC susceptibility of the investigated alloys is due to the reduced probability of pit initiation and the increased repassivation kinetics through the addition of Mo. In addition, the effect of Mo on the grain boundary cohesion needs to be discussed. It is considered that the intergranular decohesion of the investigated alloys during the SSRT is not attributed to the formation of Cr-depletion region at the grain boundaries. Firstly, Cr-carbides and/or Cr-nitrides were not observed at the grain boundaries of the alloys as demonstrated in the microstructure analyses (Fig. 3). Secondly, the DLEPR test results also supported that the grain boundaries of the alloys were not sensitized. Fig. 10(a) presents the DLEPR test results measured in the 0.5 M H2SO4 + 0.01 M KSCN solution at 25 °C. Ecorr values of the 0Mo, 1Mo, and 2Mo alloys in this solution were − 0.533, − 0.499, and − 0.481 VSCE, respectively, in which the Mo effect on increasing the Ecorr values was also shown. The three alloys exhibited typical active-passive transition behavior in this solution. From the Ecorr, the alloys actively dissolved until reaching the passivation potential, and they began to passivate at approximately − 0.25 VSCE. The maximum current density in the activation polarization curve (i.e., the icrit) was observed at approximately −0.35 VSCE, and the values were affected by the Mo content. With increase in the Mo content from 0 to 1.76 wt% in the alloy, the icrit value decreased from 30.450 to 4.321 mA cm−2. In the reactivation polarization curve, the maximum current density (ir) values were also affected by the Mo content, which was also lowered with alloying Mo. Degree of sensitization of the alloys can be evaluated by the ratio between the two current density values (ir/icrit) [55–57]. The ir/icrit values are plotted as a function of the Mo content in Fig. 10(b). It is clear that the ir/icrit value linearly decreases as the Mo content increases. At this point, it should be noted that the i r/i crit values obtained are very small, even the maximum value is 0.00613 (0Mo alloy). For the conventional IGC related to the grain boundary sensitization by forming Cr-carbides and/or Cr-nitrides in stainless steels, particularly Fe balance 18Cr10Mn-based stainless steels, the i r /icrit value will be

Fig. 9. (a) Potentiodynamic polarization curves of the 0Mo, 1Mo, and 2Mo alloys measured in a 0.6 M NaCl + 0.02 M HCl solution (pH .38) at 25 °C at a potential sweep rate of 2 mV s−1. Variation of (b) Epit and (c) icrit values as a function of the Mo content.

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Fig. 10. (a) Double-loop electrochemical potentiokinetic reactivation test results of the alloys measured in a 0.5 M H2SO4 + 0.01 M KSCN solution at 25 °C at a potential sweep rate of 1.67 mV s−1. (b) Variation of the degree of sensitization (ir/icrit) of the alloys as a function of the Mo content. (c) Low- and (b) high-magnification SEM images of the 0Mo alloy surface after the double-loop electrochemical potentiokinetic reactivation test.

exceed at least 0.05 [57–60]. Thus, it is reasonable to conclude that the conventional IGC associated with the corrosion of Cr-depletion region does not occur in the developed alloys. The observation of the corroded surface after the DLEPR test (Fig. 13) confirms that the grain boundaries of the 0Mo alloy, which exhibits the highest ir/icrit value, is not sensitized. In the fractograph analysis for the 0Mo and 1Mo alloys (Fig. 7(a-1) and (b-1)), the corrosion damage on the intergranular facet was not significantly observed. Thus, it is presumed that the intergranular decohesion phenomenon in the alloys containing Mo content below 1 wt% is due to the inherent effect of Mo on the grain boundary energy. It was reported that the Mo significantly improves the grain boundary cohesion in Fe, which was proven by investigating the surface thermodynamic energy of Fe using first-principles calculations [61–63]. Thus, alloying Mo is expected to decrease the probability of embrittlement of the Fe-based alloys. In conclusion, it is considered that the addition of 1.76 wt% Mo to NC-bearing Febal18Cr10Mn1Ni-based alloys largely decreases the SCC susceptibility by enhancing the pitting corrosion resistance and suppressing the intergranular decohesion of the matrix.

1. The developed Febalance18Cr10Mn1Ni0.4N0.15C-based alloys containing 0.91–1.76 wt% Mo were more economical than AISI 316L stainless steel and exhibited superior tensile properties and pitting corrosion resistance to the AISI 316L stainless steel. Thus it can be concluded that the developed alloys are promising substitutes for the conventional AISI 300-series stainless steels. 2. The addition of Mo ranging from 0 to 1.76 wt% affected tensile properties measured in air at nominal strain rate of 1.67 × 10−3 s−1 only in a limited manner, and no notable difference was detected on the fracture surface. However, SSRT revealed that alloying 1.76 wt% Mo significantly increased the resistance to the stress corrosion cracking of the alloys in aqueous chloride solution. It was noted that the alloys containing less than 1 wt% Mo exhibited intergranular fracture mode, but the alloy with 1.76 wt% Mo did not show the intergranular fracture surface. 3. The addition of 1.76 wt% Mo to FeCrMnNiNC-based stainless steel largely decreased the SCC susceptibility by enhancing the pitting corrosion resistance and repassivation kinetics, which were investigated by polarization test in aqueous chloride solution. In addition, it was found that the alloying Mo suppress the intergranular decohesion of the matrix.

5. Conclusions Acknowledgment In

this paper, newly developed HIAs (Febalance18Cr10Mn1Ni0.4N0.15CxMo (x = 0, 0.91, and 1.76 wt%) alloys) were briefly introduced, and the SCC susceptibility of the alloys with different Mo contents was closely investigated. The effect of Mo on the SCC susceptibility was discussed based on slow strain rate test (SSRT) results and micrographic and the fractographic observation. The followings summarized the findings of this paper.

This work has been supported by the Engineering Research Center (ERC) Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-0009451). This study was also supported financially by Fundamental Research Program of the Korea Institute of Materials Science (KIMS).

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