Effect of alloyed molybdenum on corrosion behavior of plasma immersion nitrogen ion implanted austenitic stainless steel

Effect of alloyed molybdenum on corrosion behavior of plasma immersion nitrogen ion implanted austenitic stainless steel

Corrosion Science 74 (2013) 106–115 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/c...

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Corrosion Science 74 (2013) 106–115

Contents lists available at SciVerse ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Effect of alloyed molybdenum on corrosion behavior of plasma immersion nitrogen ion implanted austenitic stainless steel P. Saravanan a,⇑, V.S. Raja a, S. Mukherjee b a b

Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India FCIPT, IPR, Gandhinagar, India

a r t i c l e

i n f o

Article history: Received 14 September 2012 Accepted 9 April 2013 Available online 24 April 2013

a b s t r a c t Plasma immersion ion implantation (PIII) of nitrogen has been performed on two austenitic stainless steels (with and without Mo addition) at three different temperatures namely, 250, 380 and 500 °C for 3 h. Grazing angle X-ray diffraction (GXRD) was carried out on the surface of the steels (both PIII treated and untreated). GXRD results suggest that PIII is more effective in Mo containing stainless steel (SS). The electrochemical corrosion studies examined through both by DC polarization and EIS technique in 3.5 wt.% NaCl reveals that, 3 h N-implantation at 250 and 380 °C improves the corrosion and pitting resistance of both the austenitic stainless steels under investigation. The effect N implantation on pitting resistance is seen more in the presence of Mo, than when it is not present in the SS. It is further emphasized that the pitting resistance of the alloys significantly deteriorates, when they are implanted at 500 °C. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Plasma immersion ion implantation (PIII) developed in 1988 [1] involving both the implantation and diffusion of nitrogen, seems to be effective in the temperature range 250–500 °C for stainless steels [2–7]. By this process it is possible to enhance the wear and corrosion resistance of the stainless steel (SS) simultaneously [4,8,9]. When austenitic stainless steel (18Cr–8Ni SS) was nitrided using plasma immersion ion implantation technique, formation of a single high nitrogen face-centered cubic (fcc) phase cN with high nitrogen concentration of about 32 at% has been reported [10]. The cN phase stemmed from the supersaturation of nitrogen in the austenite c matrix was found to possess improved wear and corrosion resistance [11–16]. Mukherjee et al. [17] reported that, in addition to expanded austenite (c0 ), CrN was present in the nearsurface region of 17.5 nm and beyond 17.5 nm depth only expanded austenite was seen in PIII modified austenitic SS, this was supported by earlier work carried out on 316 SS [18]. Another group of researchers [19] investigated the influence of nitrogen dose on mechanical properties of 316L stainless steel at different nitrogen fluence keeping the implantation energy, the current density and the specimen temperature as constant. The Grazing angle X-ray diffraction (GXRD) results showed the ferrous nitrides formation (e-FeN and e-Fe3N) and residual stresses within

⇑ Corresponding author. Present address: R&D Center for Iron and Steel, Steel Authority of India Limited, Ranchi, India. Tel.: +91 8986880260; fax: +91 6512411064. E-mail address: [email protected] (P. Saravanan). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.04.030

the nitrided layer are connected with nitrogen distribution. Nitride formation mainly depends on the solubility of nitrogen in the metal matrix, which in turn depends on the alloy composition [20]. Published literatures, related to the study on the effect of implantation parameters and alloy composition on corrosion behavior is scanty. Hence in this paper, attempt was made to bring out the relationship among the implantation temperature, alloy composition, phase formation and the corrosion behavior of the basic austenitic stainless steel namely, 18Cr–8Ni SS. 2. Experimental work 2.1. Materials and microstructure The effect of Mo on PIII treatment was studied by comparing two basic austenitic stainless steels namely, Type 304L SS and Type 316L SS (Table 1). To examine the resistance of the alloy to intergranular attack (IGC), specimens were electrolytically etched as per ASTM A 262 A in a solution of 10% oxalic acid at room temperature. Samples were etched for 90 s at a current of 1 A/cm2 by keeping them as an anode and a platinum sheet as a cathode. After etching, the microstructures of the specimens were observed in an optical microscope. As the samples exhibited step-structure (Fig. 1), indicative of the alloys intergranular corrosion resistance, they were used for further studies. 2.2. Plasma immersion ion implantation (PIII) For plasma immersion ion implantation, the samples were polished systematically using silicon carbide emery papers starting

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P. Saravanan et al. / Corrosion Science 74 (2013) 106–115 Table 1 Composition (wt.%) of the austenitic stainless steels alloys. Material

Type 304L SS Type 316L SS

Elements C

Mn

S

P

Si

Ni

N

Cr

Mo

0.03 0.03

2.0 1.94

0.03 0.003

0.045 0.006

0.75 0.30

8.2 14.85

0.01 0.067

18.0 17.23

– 2.42

2.4. Electrochemical corrosion studies Corrosion behavior of both untreated and PIII treated/implanted steels was studied using potentiodynamic polarization and AC impedance techniques. The electrical connections were provided by soldering on insulated copper wire on the backside (untreated) of the specimens. Thick araldite coating was applied excluding the implanted surface, which examined for its corrosion behavior. Care was taken to avoid formation of crevices at the metal coatings interface. 2.4.1. Potentiodynamic polarization Electrochemical corrosion behavior of untreated and PIII treated steels was studied using potentiodynamic polarization technique with a scan rate of 0.5 mV/s. The set up consisted of a PARC EG&G Potentiostat/Galvanostat Model 273 driven by M352 SoftCorrIII software and an electrochemical corrosion cell. Experiments were carried out using a platinum sheet as a counter electrode and saturated calomel electrode (SCE) as a reference electrode. The electrolyte used was 3.5 wt.% NaCl. On stabilization of the open circuit potential (OCP) for 30 min, potentiodynamic polarization scan was started at a starting potential of 300 mV cathodic to the OCP and the scan was stopped a few millivolts after the sample reached Epit.

Fig. 1. Optical microstructure of steels after 10% oxalic acid etching as per ASTM A 262A: (a) Type 304L SS and (b) Type 316L SS.

from 120 to 600 grade and finally using alumina powder of 0.25 lm. Details of the experimental set-up for PIII have been provided elsewhere [21]. The system was firstly evacuated to a base pressure of 0.9 Pa. From this, the operating pressure of 1.33– 13.3 Pa was achieved by introducing nitrogen into the system. Before the start of implantation, the surface oxide layer was removed by sputtering with argon gas for 30 min. The substrate was negatively biased to a voltage of 1 kV, and the implantation was done at 100 mA current with the dosage of 1.8  1019 ions/cm2 for 3 h. Desired temperature of substrate was maintained by means of a heater.

2.4.2. Electrochemical impedance analysis Passive layer characteristics were studied by electrochemical impedance spectroscopy (EIS) for different time duration in 3.5 wt.% NaCl. The experiments were carried out at the open circuit potential with AC amplitude of 10 mV rms sine wave in the frequency range 100 kHz–0.001 Hz. Such AC amplitude was used by several authors in the published literature [15,22]. Though a relatively low value could be better to maintain a reasonably good steady state, such a marginally high value will help to obtain good signal/noise ratio, especially in chloride medium. The first experimental data reported here as 0 h immersion corresponds to a prior immersion of the specimen in the environment for 30 min to stabilize the corrosion potential. As the specimens were continuously exposed till the completion of EIS studies, no such stabilization time was needed for successive EIS studies on immersion/test exposure. The impedance set up consisted of a Solartron 1273 potentiostat and a Solartron 1255 frequency response analyzer driven by Z plot software. The impedance spectra were analyzed with Z-view equivalent circuit software. 3. Results and discussion 3.1. Phase analysis

2.3. Phase analysis In order to know the phase changes occurring only at the surface of the PIII treated samples GXRD with a angle of 2° was employed The samples were scanned from 30 to 100o using Expertpro diffractometer with Cu Ka radiation (k = 1.54 A°) as a source.

The GXRD patterns of the nitrogen implanted Type 304L SS (Fig. 2a) show multiphase structure in the surface layer depending upon the implantation temperature. The diffraction analysis shows the presence of transformed ferrite/martensite (a0 (110)) on the surface of the untreated steel. The formation of a0 might be due to mechanical strain occurred during standard metallographic sample preparation prior to GXRD analysis [23,24]. The a0 disappears with

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Fig. 2. Grazing angle X-ray diffractrograms (GXRD) of PIII treated steels for different temperatures: (a) Type 304L SS and (b) Type 316L SS.

nitrogen ion implantation at 250 °C and 380 °C, stabilizing the fcc phase [25]. GXRD pattern of the nitrogen implanted specimens (250 °C) showed two new broad peaks just ahead of (1 1 1)c and (2 0 0)c planes of the untreated austenite peaks which may indicate the formation of a new phase. It was observed that the new phase peaks appear at lower angles as compared with those of (1 1 1)c and (2 0 0)c planes of austenite phase. Similar peaks were observed by other investigators and were considered to be due to interaction of nitrogen with the alloy matrix [18,24,26,27]. Some authors attributed this to an expansion of austenite lattice 0 (c ) [28,29], while some authors referred it as S phase [30–32] on the contrary, Haen et al. [33] reported the formation of different types of nitrides such as FeN, c0 -Fe4N, Fe3NiN in Type 304L SS due to nitriding.

It is difficult to distinguish nitrides from an expanded austenite lattice, as both of them were reported to exhibit similar crystal structure [33]. It is presumed that expansion of austenite causes lattice strain (uniform strain or non-uniform strain) as more N is added to the lattice. Notably expansion leads to peak shift towards lower Bragg’s angle and strain causes peak broadening in XRD spectra. However, if the N level goes beyond the solubility limit, it can lead to some nitride phase formation and associated new peaks in XRD spectra. Further, the low scattering of N atom inhibits measurable intensities for the reflections of planes even if the crystal symmetry is changed. Hence, in order to make the analysis simple, the shift in the peak position is considered to be due to various metal-nitrogen stoichiometry of the phase, whether or not N addition leads to nitride formation or just an expanded austenite. Following this argument it is hypothesized that nitrogen

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implantation results in MxNy phase where in M represents metal component of the alloy (having one or more metallic elements), x is the number of metal atoms, N corresponds to the nitrogen and y is the number of nitrogen atoms. The x/y ratio defines the number of metal atoms associated with each nitrogen atom (or stoichiometry of metal to nitrogen) and is reflected in the subsequent changes in lattice parameters or the peak positions in the GXRD pattern. The advantage of such a method of analysis is that it enables suggesting the formation of nonstoichiometric phases. The d value of PIII implanted Type 304L SS were compared with the literatures/JCPDS file to identify the phases formed and tabulated in Table 2. Detailed discussion regarding the phase changes have been presented elsewhere [34]. From the table it is clear that with increase in implantation temperature, the stoichiometry of metal to nitrogen changes from (Fe3Ni)N (at 250 °C) to (FeNi)N + Fe2N (at 380 °C) and finally to (FeCrNi)N and/or FeN (at 500 °C). It is also to be noted that at higher implantation temperature, say 500 °C, a0 is formed along with the nitrides, which was reported as nitrogen induced bct phase [35,36]. GXRD spectra of Type 316L SS is shown in Fig. 2b. The identified phases for Type 316L SS based on GXRD pattern of is also tabulated in Table 2. It is obvious from the Table 2 that PIII treated Type 316L SS shows a single stable nitride MxNy (Where, M = FeNi or FeMo) at all the treatment temperatures. These observations are different from that of Type 304L SS specimens, where dual nitride phases are seen for all the temperatures (say, (FeNi)N and Fe2N nitrides for 380 °C).

Fig. 3. Polarization plot of untreated and PIII treated Type 304L SS for different temperatures in 3.5 wt.% NaCl.

3.2. Electrochemical corrosion studies 3.2.1. Potentiodyamic polarization Electrochemical polarization studies were carried for untreated and PIII treated chosen austenitic stainless steels in 3.5 wt.% NaCl solution. Fig. 3 shows the typical polarization plots of untreated and PIII treated Type 304L SS. The polarization plots for Type 316L SS are represented in Fig. 4. Electrochemical parameters for Type 304L SS and Type 316L SS are tabulated in Tables 3 and 4 respectively. Both the alloys show a metastable passivity, which is clearly observed in the polarization plots as fluctuation in the passive region. The anodic polarization curves for both untreated and treated Type 304L SS and Type 316L SS alloys do not exhibit any active–passive transition, suggesting instantaneous passivity [37]. From the Tables 3 and 4, it is evident that PIII treated steels (Type 304L and Type 316L SS) at 250 °C and 380 °C show lower ipass and higher Epit as compared to that of the untreated steel. However, on further increasing the temperature to 500 °C, a considerable decrease in Ecorr, and Epit is observed in both the steels suggesting increased susceptibility for pitting. It is to be noted that

Fig. 4. Polarization plot of untreated and PIII treated Type 316L SS for different temperatures in 3.5 wt.% NaCl.

the 250 °C PIII treated Type 304L SS shows lower ipass (3  107 A/ cm2) than that of the untreated steel (4  107 A/cm2), but its pitting potential [4mVSCE)] is lower than that of the untreated steel (80 mVSCE). In case of PIII treated Type 316L SS better corrosion resistance (lower ipass) and pitting resistance (higher Epit) than that of untreated steel has been observed even at 250 °C. A marginal

Table 2 Possible phases (from JCPDS file) on the surface of the PIII treated austenitic stainless steels at different temperatures. Implantation temperatures (°C)

Alloys Type 304L SS

Type 316L SS

‘d’ in nm

Possible phases

‘d’ in nm

Possible Phases

250

0.219 0.217 0.208

Fe3NiN(1 1 1) Fe4N(0 1 1) c(1 1 1)

380

0.227 0.212 0.201 0.224 0.205

FeNiN(1 1 1) Fe2N(0 1 1) a0 (1 1 0) FeNiN(1 1 1) a0 (1 1 0)

0.219 0.217 0.215 0.208 0.228 0.212 0.209 0.228 0.212 0.209

Fe3NiN(1 1 1) Fe4N(0 1 1) Fe3Mo3N(5 1 1) c(1 1 1) FeNiN(1 1 1) and Fe3Mo3N(4 2 2) Fe2N(0 1 1) c(1 1 1) FeNiN(1 1 1) and Fe3Mo3N(4 2 2) Fe2N(0 1 1) c(1 1 1)

500

110

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Table 3 Polarization parameters for 3 h PIII treated and untreated Type 304L SS in 3.5 wt.% NaCl. Treatment condition

Ecorr, mV (vs SCE)

ipass (A/cm2) (at 0 mV (vs SCE))

Epit, mV (vs SCE)

Scatter band of Epit, mV (vs SCE)

Untreated 250 °C 380 °C 500 °C

225 184 144 289

4.0  107 3.0  107 1.42  107

80 4 245 220

74–86 1–9 239–248 (214)–(229)

Table 4 Polarization parameters for 3 h PIII treated and untreated Type 316L SS in 3.5 wt.% NaCl. Treatment condition

Ecorr, mV (vs SCE)

ipass (A/cm2) (at 0 mV (vs SCE))

Epit, mV (vs SCE)

Scatter band of Epit, mV (vs SCE)

Untreated 250 °C 380 °C 500 °C

200 190 183 173

1.49  107 1.33  107 9.47  108 1.67  107

276 408 800 0

268–282 398–416 789–817 (8)–(+6)

decrease in ipass (1.42  107 A/cm2) accompanied by a significant increase in the pitting potential (4–245 mVSCE) is observed in Type 304L SS on increasing implantation temperature from 250 °C to 380 °C, whereas, a significant reduction in ipass by an order of magnitude and a significant positive shift in the pitting potential was observed in Type 316L SS. After corrosion studies the samples were observed in optical microscope to understand the pitting behavior of the alloys. The optical micrographs of Type 304L SS and Type 316L SS after polarization studies are shown in Figs. 5 and 6 respectively. From figures it is clear that both the alloys undergo pitting attack irrespective of implantation temperature. It is interesting to note that 500 °C implanted steels undergo a grain boundary attack along with pitting, which may be indicative of CrN phase formation at that temperature [23,38,39].

3.2.2. Electrochemical impedance spectroscopy study To understand the physical characteristics of the passive film, both the SS were subjected to electrochemical impedance study (EIS) in 3.5 wt.% NaCl for different periods of immersion. Prior to EIS studies OCP vs time was monitored and recorded up to the equilibrium potential. Fig. 7 and 8 present the OCP vs time plot for untreated and PIII treated 304L SS and 316L SS respectively. Gross variation in OCP with time, notwithstanding the fluctuations, can be related to the surface/interface modifications that are expected to occur on the alloy surface after immersion in a chemical environment. An old air-formed film may dissolve and a new film compatible with the aqueous environment may form, causing a drift in OCP. However from the plots shown in Figs. 7 and 8, it seems that OCP reaches steady state very quickly, indicating that such a change has occurred either very quickly; or the resulting change has not given rise to a much shift in potentials [40,41]. Small fluctuations noticed in the plots are due to film breakdown due to chlorides and reformation by the alloys. Fig. 9a shows the EIS plot (Bode plot) for untreated Type 304L SS and Fig. 9b–d represent Type 304L SS implanted at 250, 380 and 500 °C respectively. The untreated alloy shows single time constant for the entire period of immersion. The 250 °C treated Type 304L SS shows single one time constant up to 96 h of immersion and later breaks into two time constants (as seen by the two phase angle peaks in Bode plots). Finally, for 912 h of immersion, one of the two time constants disappears and only one time constant is seen. It is possible that the alloy passivates strongly at the beginning of immersion and gives rise to a dominant single time constant. The presence of two time constants may be attributed to the differential corrosion rates exhibited by the alloy surface, such as those related to pitting, which may become dominant as the immersion time progresses. It seems that at 912 h of immersion, the passive film is destroyed completely and the alloy undergoes active dissolution. Bode plots for the steel treated at 380 °C for (Fig. 9c) show a similar trend as that of 250 °C treated alloys. However, the two time constants become visible only after 792 h of immersion. The Bode plots obtained at different immersion periods for Type 304L SS specimen PIII treated at 500 °C also exhibits similar trends, but its second time constant disappears (destruction of passive

Fig. 5. Optical micrograph of untreated and PIII treated Type 304L SS specimen after polarization studies.

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Fig. 6. Optical micrograph of untreated and PIII treated Type 316L SS specimen after polarization studies.

Fig. 7. Change in OCP as a function of time for untreated and PIII treated Type 304L SS in 3.5 wt.% NaCl.

Fig. 8. Change in OCP as a function of time for untreated and PIII treated Type 316L SS in 3.5 wt.% NaCl.

film) within a very short interval of immersion time, indicating poor corrosion resistance at this treatment temperature (Fig. 9d). EIS plot for the untreated Type 316L SS is shown in Fig. 10a, and Figs. 10b to d represents for implanted Type 316L SS. The Bode plots for 250 °C PIII treated Type 316L SS show initially a single time constant. With the progress of immersion, the single time constant resolves or splits into two and to three on further immersion (Fig. 10b). Like 250 °C PIII treated, 380 °C treated Type 316L SS also shows single and double time constants behavior on prolonged immersion (600 h), but it doesn’t show any third time constant even after 2424 h immersion. Hence it can be said that strong passive film is formed at 380 °C implanted condition as compared to that of 250 °C implanted condition. This was supported by earlier work carried out by Li and co-workers, where they also reported the formation three layered nitride at the implanted surface of Type 316L SS [25]. The 500 °C PIII treated steel shows two time constants even at 0 h immersion, one which disappear

in a short interval of time of immersion, suggesting a weaker passive layer. To obtain the impedance parameters, the EIS plots were fitted with three different of equivalent circuits as shown in Fig. 11. Both the untreated alloys show single time constant therefore, only one parallel combination of resistance and capacitance is chosen (Fig. 11a). Two parallel combinations of resistance and capacitance are chosen in the equivalent circuit for PIII treated Type 304L SS alloy as it showed double time constants in the EIS spectra (Fig. 11b). Three parallel combinations of resistance and capacitance are chosen in the equivalent circuit for PIII treated Type 316L SS (showing three time constant) as shown in Fig. 11c. Here Rs represents the solution resistance and Qdl and Rp [Rp  |Z (x = 0)|] respectively the constant phase element and the polarization resistance of the passive alloy (possibly containing Cr2O3). Figs. 11b and c show the equivalent circuit with two time and three constants, where in Qf is the constant phase element of the passive film, Rf is the film

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Fig. 9. Bode plot of: (a) untreated, (b) 250 °C, (c) 380 °C and (d) 500 °C PIII treated Type 304L SS for different time of immersion in 3.5 wt.% NaCl.

resistance (in case of Type 316L SS, Rf1 film resistance for first film and Rf2 for the second) and Rct is the charge transfer resistance. The total resistance (Rt) is given as the sum of all the resistance except Rs (Rt = Rf + Rct for Type 304L SS and Rt = Rf1 + Rf2 + Rct for 316L SS), which is obtained by fitting. Hence for treated alloy the total resistance is equal to the polarization resistance (Rt  Rp), where as in case of untreated alloy it is only Rp. The polarization resistance (Rp) values of untreated and PIII treated Type 304L SS and Type 316L SS were plotted against the immersion time (Fig. 12). From Fig. 12, it is clearly seen that Rp values of both the steels decreases with increase in immersion period. Out of three treatment temperatures, 380 °C PIII treated steel shows higher Rp value than that of the other temperatures confirming the polarization results. The 500 °C PIII treated steel has lower Rp value than that of untreated steels for varying immersion time, suggesting that corrosion resistance of the steels deteriorates when treated at this temperature.

4. Effect of implantation temperature Nitrogen presence greatly enhances the localized corrosion resistance. Several studies, in past, have been devoted to understand this subject. Nitrogen has been found to influence localized corrosion in the following manner: (1) nitrogen in solid solution is dissolved and produces NHþ 4 , increase the pH of the pit [42]; (2) concentrated nitrogen at the passive film/alloy surface stabilizes the film, and prevents attack of anions (Cl) [43,44]; (3) produced nitrate ions improve the resistance to pitting corrosion [45]; (4) nitrogen addition stabilizes the austenitic phase [46]; (5) anodic segregation of nitrogen at actively dissolving surfaces [47]; (6) synergistic effects of Mo and N [48]; and (7) removal of free chlorine [49]. The present study does not, however, concern with analyzing the mechanism through which nitrogen enhances the localized corrosion resistance of the implanted SS. Instead, it examines why the change in implantation temperature. Implantation

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Fig. 10. Bode plot of: (a) untreated, (b) 250 °C, (c) 380 °C and (d) 500 °C PIII treated Type 316L SS for different time of immersion in 3.5 wt.% NaCl .

temperature could influence (a) the amount of nitrogen in the implanted layer and/or (b) the phases formed. The effect of implantation temperature can be studied by comparing the results of polarization and impedance studies and correlating it with the phase formation. Out of the three implantation conditions, 250 and 380 °C implantation temperatures enhance pitting resistance (higher pitting potential) and corrosion resistance (as observed by lower passive current density and high Rp or Rt) for two alloys. This can be attributed to the formation of favorable nitrides and also the difference in the nitrogen concentration on the alloy surface at these temperatures [9,50]. The thickness of nitrided layer and the amount of each of these phases present in it depend on the nitrogen fluence and the treatment temperature [7,11]. Since nitrogen fluence in the present case is not varied, only the effect of treatment temperature needs to be considered. The increased corrosion resistance at 380 °C as compared to that observed at 250 °C may be due to formation of nitrides with higher N content. As seen from GXRD pattern diffusion of N is more at higher implantation temperature, leading

to decrease in x/y ratio of MxNy phase. Hence N content will be more along the thickness at 380 °C than that at 250 °C. The corrosion resistance offered by the implanted layer on increasing the implantation temperature to 500 °C is found to be the least as compared to that observed at other treatment temperatures namely, 250 °C and 380 °C. Since samples treated at 500 °C have higher diffusion of N than that of other temperatures, so thickness is not a reason for the decrease in corrosion resistance. Hence, composition and chemical state of nitrides will be the main reason for corrosion resistance deterioration. Earlier Lei and his co-work [51], reported that formation of hcp phase along with the iron nitrides (in 1Cr18Ni9Ti SS) when treated above 500 °C and this was supported by other researchers [35,36,52], but he could not give a clear reason for decrease in corrosion resistance. If we compare phases formed at 500 °C implantation for both the alloys, it is clear that Mo suppress the formation of a0 in 316L SS. But still corrosion resistance deteriorates at this implantation temperature (500 °C) for 316L SS, hence a0 is not main responsible for decrease in corrosion resistance of the alloys under investigation. Lei et al. [53] later

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Fig. 11. Equivalent circuit chosen for fitting EIS spectra: (a) Untreated steels, (b) PIII treated Type 304L SS and (c) PIII treated Type 316L SS.

in their work showed the chemical composition of the cN phase as a formula with atomic fraction (Fe0.60,Cr0.22,Ni0.18)2N. They also reported that cN phase has weaker Cr–N ionic-type bonds and stronger Fe–N ionic-type bonds, compared with the stoichiometric nitrides, such as CrN, Cr2N, Fe4N, and Fe23N phases. This was supported by other researchers [22,29,44] and suggested Cr–N formed along with the iron nitrides at higher temperatures (500 °C) might be responsible for the decrease in corrosion resistance. Later Ram Mohan and his co-workers [24] confirmed this by XPS analysis of PIII treated Type 304L SS, that Cr to be present in nitride layer as CrFeNiN, which could not be identified by GXRD. Supporting the above discussion optical microstructure (Figs. 6 and 7) of the corroded 500 °C implanted samples also shows a grain boundary attack, which is an indicative of Cr depletion and intergranular attack [23,38,39]. Hence it can be concluded that the decrease in corrosion resistance of the alloys at 500 °C implanted conditions was mainly due to the formation of weaker Cr–N type nitride. 5. Effect of Mo on PIII treated austenitic SS The effect of Mo on PIII treated austenitic stainless steel can be understood by comparing the results of both the chosen steels. From GXRD results it is clear that Mo suppress the formation of a0 phase at higher implantation temperature by forming a stable nitride namely, Fe3Mo3N along with (FeNi)N. This argument is supported by optical microstructure (Figs. 6 and 7) of the specimens obtained after polarization studies, where in 380 and 500 °C implanted 304L SS showed the evidence of martensitic structure. The electrochemical polarization study depicts that Mo containing steel (Type 316L SS) shows better corrosion and pitting resistance. This is supported by the EIS studies, where Type 316L SS showed higher passivity than that of 304L SS and also having one additional time constant indicating multiple layered passive film characteristics. Many researchers detailed the synergetic effect of Mo and N in increasing the corrosion resistance of stainless steel [48,54–59]

Fig. 12. Variation of Rp with immersion time for the PIII treated alloys in 3.5 wt.% NaCl: (a) Type 304L SS and (b) Type 316L SS.

and mechanisms proposed for the action of Mo largely revolve around the presence of Mo in the passive film [55,56]. Some reported that Cr and Mo tend to be enriched in the passive film of the N alloyed stainless steels. Mo enrichment in the outer layer was believed to occur along with N in the form of nitrides [48], as in this case it is Fe3Mo3N nitride. They suggested that during contact with environment the oxidized species of Mo, such as MoO2, MoO42 and hydrated form of MoO(OH)2, might enhance the formation of a homogeneous and more protective passive film [57,58]. For example, as indicated by Clayton et al. [59] MoO42- and CrO42 species formed on the surface of the alloy could provide a bipolar film and enhance the resistance to breakdown of passivity in the Cl- ion media. Hence, it can be concluded that formation Fe3Mo3N nitride phase assist in improving pitting and corrosion resistance of Type 316L SS than that of Type 304L SS. 6. Conclusion From phase analysis it is clear that nitrogen uptake increases with increase in implantation temperature and Mo suppress the 0 formation of a at higher implantation temperatures (500 °C). From electrochemical studies, it can be said that 250 °C and 380 °C implantation temperatures enhance pitting and corrosion resistance of the chosen steel under investigation. This is attributed to the formation of favorable nitrides and also the difference in

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