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Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment time
Surface & Coatings Technology 200 (2006) 3511 – 3517 www.elsevier.com/locate/surfcoat
Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment time A. Fossati*, F. Borgioli, E. Galvanetto, T. Bacci Dipartimento di Ingegneria Civile, Universita` di Firenze, via S. Marta 3, 50139 Firenze, Italy Received 11 August 2004; accepted in revised form 25 October 2004 Available online 8 December 2004
Abstract In the present paper, the results relative to low-temperature glow-discharge nitriding treatments of AISI 316L austenitic stainless steel are reported. This process is able to produce surface-modified layers essentially composed of a metastable phase called S-phase. Nitriding treatments were carried out at 703 K for times ranging from 0 to 5 h. Treated samples were characterized by means of morphological analysis, surface and profile microhardness measurements and electrochemical tests in NaCl aerated solutions in order to investigate the influence of treatment time on the microstructure, the hardness and the corrosion resistance properties; moreover, the results were compared with those observed for untreated samples. The thickness of the modified layers increases as the nitriding time increases. The surface hardness of treated samples is higher than the untreated AISI 316L stainless steel samples and increases as the treatment time increases up to a value of about 1450 HK0.1. The pitting corrosion resistance of nitrided samples increases as the treatment time increases; after a 5-h nitriding treatment, the anodic current values are reduced of about 3 magnitude orders in comparison with the untreated samples. Low-temperature glow-discharge nitriding treatment is an effective technique to increase the pitting corrosion resistance and the surface microhardness of AISI 316L austenitic stainless steel. D 2004 Elsevier B.V. All rights reserved. Keywords: Glow-discharge nitriding; Austenitic stainless steel; AISI 316L; S-phase; Pitting corrosion
1. Introduction The austenitic stainless steels are widely used in many industrial fields because of their very high general corrosion resistance; nevertheless, they can suffer pitting or crevice corrosion in specific environments and their low hardness and wear resistance can limit the number of possible industrial applications. Traditional gas nitriding carried out at relatively high temperature (773 K or above) can increase the surface hardness and the wear resistance, but generally decreases the corrosion resistance [1]. At these temperatures, the chromium diffusion coefficient is quite large, then the chromium atoms can move, preferentially bind nitrogen
* Corresponding author. Tel.: +39 55 479 6399; fax: +39 55 479 6504. E-mail address: [email protected] (A. Fossati). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.122
atoms and lead to the formation of the very stable and hard CrN [2,3]; the Cr-depleted austenitic matrix zones cannot therefore form a uniform and protective passivity film [4] and are subject to active corrosion. Nowadays, many innovative technologies, like magnetron sputtering [5], plasma source ion nitriding [6], plasma immersion ion implantation [7], low energy ion implantation [8] and plasma nitriding [9], have been utilised to perform low-temperature nitriding treatments and produce surface-modified layers avoiding the precipitation of large amounts of chromium nitrides. Low-temperature nitriding allows to obtain a modified layer essentially composed by a metastable phase known as supersaturated or expanded austenite gN [7,10–14], or S-phase [5,9,15,16], or m-phase [17–19] which, due to its large nitrogen concentration, has proved to have higher hardness and higher pitting corrosion resistance [20–22] in comparison with the untreated material.
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The aim of this work is to study the influence of the treatment time on the microstructure, the morphology, the mechanical properties and the corrosion behaviour in NaCl aqueous solutions of the AISI 316L austenitic stainless steel nitrided by low-temperature d.c. glow-discharge process.
2. Experimental procedure Prismatic samples (40184 mm) were obtained from an AISI 316L steel annealed bar (A 60 mm) by cutting, grinding and polishing up to 6-Am diamond suspension. Before the plasma nitriding process, the samples were ultrasonically cleaned in an acetone bath for 10 min. Glowdischarge treatments were performed in a laboratory plasma equipment, similar to industrial ones, using a d.c. power supply. The furnace system had an axial symmetry. Samples were fastened by means of screws at each face of the prismatic sample holder. The screws and the sample holder were realized by the same steel bar from which were obtained the samples. The samples and the sample holder, placed in the centre of the treatment chamber, worked as cathode and were completely surrounded by a cylindrical metal screen made up of AISI 304 stainless steel which was grounded and worked as anode. The anode–cathode distance was about 60 mm. Gas composition (80% N2 and 20% H2) was fixed during the sputtering step and the nitriding treatments. The treatment temperature was measured by a thermocouple inserted in the sample holder and controlled by varying the discharge current from the d.c. current supply. Nitriding treatments were performed at 703 K at a working pressure of 103 Pa for times in the range of 0–5 h. The current density necessary to maintain constant the temperature during the nitriding treatment was 2.6F0.1 mA cm2, while the measured voltage drop between the electrodes was 175F5 V. Before the nitriding treatments samples were warmed up to 653 K by means of a cathodic sputtering in order to remove the natural passive film and enable a homogeneous and correct nitriding process. After the sputtering step, the pressure and the temperature were increased up to their nominal values; the 0-h treatment consisted in the cathodic sputtering up to 653 K and just in the raising in temperature up to 703 K, then the power supply was turned off and the chamber evacuated. The microstructure of treated and untreated samples was examined by light and scanning electron microscopy. XRD analysis was performed in order to identify the phases present in the surface layers. Diffraction patterns were collected in Bragg–Brentano configuration (Cu-Ka radiation). Surface microhardness and microhardness profile measurements (Knoop indenter) were carried out using a load of 100 and 10 gf, respectively. Treated and untreated samples were tested in 5% NaCl aerated solutions at room temperature by means of electrochemical methods obtaining polarization and free corrosion
potential–time curves. Before the corrosion tests, samples were ultrasonically cleaned in acetone for 10 min. The corrosion tests were performed using a Pyrex cell equipped with a Ag/AgCl (KCl saturated) reference electrode and a platinum grid as counter electrode. Linear voltammetries were performed at a potential scan rate of 0.3 mV s1 after a delay period of 1 day; during the delay, the free corrosion potential–time curves were recorded. The sample surface area exposed to the electrolyte was 1 cm2. Corrosion tests were carried out at least three times for each sample type in order to assess the reproducibility. After the corrosion tests, microscopy analysis was carried out in order to evaluate the corrosion morphology and the damaging amount.
3. Results and discussion 3.1. XRD analysis Diffraction spectra of nitrided and untreated samples are shown in Fig. 1. In the case of untreated samples, austenite and aV-martensite peaks are visible. This phase, which is formed during the grinding and polishing operations, is highly metastable [23]. XRD analysis carried out on the surface of AISI 316L steel samples warmed up to 473 K by means of the cathodic sputtering reveals the total absence of this kind of martensitic phase. In the case of nitrided samples, XRD analysis shows that the nitriding process produced modified layers on the austenitic substrate. For all the kinds of nitrided samples, a set of diffraction peaks is detected which is positioned at different diffraction angles, depending on the different nitriding times; the diffraction peak set shifts towards lower diffraction angles as the nitriding time increases, up to a maximum shift value which is reached after a
Fig. 1. X-ray diffraction patterns of the AISI 316L austenitic stainless steel samples untreated and nitrided at 703 K for different times.
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3.2. Morphological analysis
Fig. 2. Surface morphology of sample glow-discharge nitrided at 703 K for 5 h.
treatment of 3.5 h. Accordingly to the literature data, these peaks are attributed to the S-phase. In the case of the shortest treatments (0–2 h), the austenitic substrate peaks together with S-phase peaks are still visible due to the small thickness of the modified layers. S-phase is generally considered a supersaturated solid solution of nitrogen in austenite, even if the crystallography has not been well clarified. As already observed by other authors, the shift of the S-phase peak positions depends on the nitrogen concentration. Particularly if the nitrogen concentration increases, the S-phase peaks move towards smaller diffraction angles [8]. This suggests that longer nitriding time increases the nitrogen surface concentration of the AISI 316L austenitic stainless steel nitrided samples. CrN diffraction peaks are not clearly detectable in our instrumental configuration; it has to be noted that a large amount of CrN precipitates is typical of traditional gas nitriding carried out at higher temperatures and it is the main responsible for the loss of corrosion resistance of austenitic stainless steels.
In Fig. 2, the typical surface morphology of nitrided samples is shown. This morphology, observed also by other authors [2,14,19], has been extensively described in a previous paper and can be ascribed to both the cathodic sputtering and the treatment itself [24]; the morphology features are enhanced as the nitriding time increases. In Fig. 3a and b, some typical micrographs of the crosssections of nitrided samples are reported. After glyceregia etching, the nitrided layer of any kind of nitrided samples appears to be a homogeneous layer separated from the matrix by a strong etched line. The thickness of the modified layer increases as the treatment time increases. The mean thickness of the modified layers for each nitriding time is reported in Table 1. It has to be noted that in the case of samples nitrided for 0 h, a modified layer is already detectable which shows an appreciable thickness. This allows to deduce that the cathodic sputtering up to 653 K and the short period of raising in temperature are able to produce a thin S-phase layer in a very short time (less than 15 min). It has to be pointed out that the S-phase layer is really a modification of the austenitic substrate, and it is not a coating, as also previously observed [24]; in Fig. 3a and b, it is clearly visible that the grain boundaries of austenite parent phase continue into the modified layer without any interruption. 3.3. Microhardness Knoop microhardness measurements were carried out on the surface of untreated and nitrided samples; the microhardness values are reported in Fig. 4. Nitrided samples show higher surface hardness in respect of the untreated samples. The microhardness values increase as the treatment time increases. In the case of the samples nitrided for 5 h,
Fig. 3. Microstructure of the modified layers formed on the AISI 316L austenitic stainless steel samples by glow-discharge nitriding treatment at 703 K for 2 h (a) and 5 h (b) (etchant glyceregia).
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Table 1 Modified layer thickness of AISI 316L samples glow-discharge nitrided at 703 K for different times Nitriding time (h)
Thickness (Am)
0 1 2 3.5 5
3F1 4F1 5F1 7F1 10F1
the microhardness value is over six times higher than that of untreated AISI 316L stainless steel samples. The increase of the measured microhardness values in the case of the samples treated for longer times can be ascribed to two different causes. At a fixed test load, the surface hardness measure of a layered structure mainly depends on the hardness of the different layers and on their own thickness. Particularly, the measured hardness value increases as the thickness and the hardness of the modified layers increase. Moreover, the influence of the substrate hardness on the measured hardness value of a surfacemodified sample decreases as the thickness of the modified layer increases. Morphological analysis revealed thicker modified layers in the case of samples nitrided for longer times, so that the influence of the substrate low hardness on the measured hardness of nitrided samples is more and more negligible as the nitriding time increases. Moreover, it can be hypothesized that the hardness value of S-phase depends on the nitrogen concentration. If nitrogen concentration increases, a larger reticular distortion and thus a higher hardening effect can be expected. As a matter of fact, XRD analysis revealed a higher reticular distortion in the case of samples treated for longer times, so that the S-phase present on the surface layer of these types of samples is expected to have a higher hardness in comparison with the S-phase present on the surface layer of samples nitrided for shorter times. Thus, due to the synergy between higher thickness and higher hardness of the modified layers, it appears reasonably
Fig. 4. Surface microhardness values of AISI 316L austenitic stainless steel samples untreated and glow-discharge nitrided at 703 K for different times.
explained the increase of the measured microhardness values of the nitrided samples with the increase of the nitriding time. The highest microhardness value (about 1450 HK0.1) was obtained in the case of 5-h nitriding treatment. In this case, the thickness of the modified layer is large enough to make completely negligible the influence of the austenitic substrate. Thus, the above-mentioned value could be considered a good estimate of the hardness values of the S-phase, when the highest nitrogen concentration is obtained. This microhardness value is in agreement with the highest hardness values measured by other authors [2,9,11,15]. This suggests that low-temperature glowdischarge nitriding is an effective nitriding technique of AISI 316L austenitic stainless steel. Knoop microhardness measurements were carried out on the cross-sections of the treated samples; the microhardness profiles are shown in Fig. 5. As the treatment time increases, the thickness of the hardened layers increases, in agreement with morphological observations. Next to the surface samples nitrided for 5 h show microhardness values in agreement with those measured on their surface. In the case of the samples treated for shorter times, the highest measured hardness values near the surface, at 2.5-Am depth, are lower in comparison with the other nitrided sample types. This can be ascribed either to the small thickness of the layer or to the lower nitrogen concentration through the layer, as suggested by XRD analysis. 3.4. Corrosion behaviour The typical polarization curves of nitrided samples are reported in Fig. 6; as reference, the typical linear voltammetry of the untreated AISI 316L stainless steel samples, obtained in the same experimental conditions, is also reported. The linear voltammetry of untreated samples shows the typical behaviour of a passive material that is sensibly
Fig. 5. Knoop microhardness profiles of AISI 316L austenitic stainless steel samples nitrided at 703 K for different times.
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Table 2 Average pitting potentials E pit V and highest anodic current density values from polarization curves in 5% NaCl aerated solution in the case of AISI 316L samples untreated and nitrided at 703 K for different times
Fig. 6. Polarization curves of AISI 316L austenitic stainless steel samples untreated and glow-discharge nitrided at 703 K for different times in 5% NaCl aerated solution.
subject to pitting corrosion if the potential is higher than a threshold; the pitting branch starts at relatively low anodic potential values (about +100 mV) due to the high chloride concentration. At the highest polarization potentials, the anodic current values are quite large and are limited only by concentration polarization phenomena. The morphological analysis carried out on the surface of the corroded samples reveals the presence of many deep pits with a vertical development due to the position of the samples during the corrosion test (Fig. 7). Nitrided samples are also subject to pitting corrosion, but the anodic current values are smaller than those of the untreated samples. In Table 2, the pitting potential values E pitV measured from the polarization curves and defined as the potential at which the anodic current values overcome definitively the threshold of 100 AA cm2 are reported. At the highest polarization potentials, the anodic current values of the nitrided samples are sensibly reduced in respect of the untreated AISI 316L stainless steel samples and are limited
Fig. 7. Corrosion morphology of untreated AISI 316L austenitic stainless steel samples after a linear voltammetry in 5% NaCl aerated solution carried out up to +1000 mV Ag/AgCl.
Sample type
E pit V (mV (Ag/AgCl))
Highest current density (AA cm2)
Untreated Nitrided—0 h Nitrided—1 h Nitrided—2 h Nitrided—3.5 h Nitrided—5 h
~+220 ~+80 ~+300 ~+350 ~+580 N+1000
~6.2104 ~5.5103 ~2.4103 ~4.5102 ~1.8102 ~6.2101
only by concentration polarization phenomena. As the nitriding time increases, the highest anodic current density values become smaller (see Table 2). Particularly after the 5h nitriding treatment, the highest anodic current values are reduced of about 3 orders of magnitude in comparison with those of the untreated samples. Just the 0-h treatment is able to reduce of 1 order of magnitude the maximum anodic current values. Free corrosion potential–time curves are in agreement with linear voltammetries. Fig. 8 shows the typical curves for untreated and nitrided samples. Nitrided samples appear to be always nobler in comparison with untreated AISI 316L stainless steel samples. Moreover, the curves of treated samples, specially in the case of samples nitrided for longer times, show slower, smaller and less frequent potential fluctuations in respect of untreated samples. Potential fluctuations can be ascribed to pit nucleation phenomena. Thus, nitrided samples, also in free corroding conditions, appear to be less subject to pitting corrosion in comparison with the untreated stainless steel samples. Morphological analysis carried out on the surface of treated samples after the corrosion tests is in agreement with electrochemical tests and free corrosion potential–time curves. Fig. 9 shows the typical corrosion morphology of nitrided samples at the end of the polarization tests.
Fig. 8. Typical free corrosion potential–time curves of AISI 316L austenitic stainless steel samples untreated and nitrided at 703 K for different times in 5% NaCl aerated solution.
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Fig. 9. Corrosion morphology of AISI 316L austenitic stainless steel samples nitrided at 703 K for 0 h (a), 3.5 h (b) and 5 h (c) after a linear voltammetry in 5% NaCl aerated solution carried out up to +1000 mV Ag/AgCl.
Nitrided samples show a sensibly lower damaging amount in respect of untreated samples. As the nitriding time increases, the pit dimensions and the pit density decrease; at the end of the corrosion tests, samples nitrided for 5 h appear almost untouched. Finally, it has to be noted that also the 0-h nitriding treatment is able to reduce sensibly the pitting damaging amount occurred during the polarization test (Fig. 9a). The high pitting corrosion resistance of nitrided samples and the different corrosion behaviour of different nitrided sample types can be reasonably explained considering two reasons: the nitrogen surface concentration and the thickness of the modified layers. It is well known in literature that just a little amount of nitrogen in austenitic stainless steels allows to increase sensibly their pitting corrosion resistance [21,22]. In the past, many mechanisms were proposed which could explain the beneficial effect of the interstitial nitrogen [20]. The following mechanism is one of the most accepted. If an austenitic stainless steel containing interstitial nitrogen is subject to pitting corrosion, interstitial nitrogen atoms are released. Then nitrogen atoms can react with H+ to form NH4+ following the present reaction [20]: N þ 4Hþ þ 3e YNHþ 4
ð1Þ
The local pH increase due to this cathodic reaction allows to facilitate the repassivation [21]. In literature, it is reported that the nitrogen interstitial concentration in the S-phase can reach values of about 30 at.% [6,8,25]. In the above-mentioned repassivation mechanism, a larger amount of nitrogen is expected to increase the repassivation capability for austenitic stainless steels, then it makes sense to expect a higher pitting corrosion resistance for S-phase, due to its very high nitrogen concentration, in comparison with the untreated austenitic stainless steel samples. Moreover, solid state diffusion laws and XRD analysis suggest that samples nitrided for longer times can reach higher surface nitrogen concentrations. Thus, due to their larger nitrogen surface concentration, a higher pitting corrosion resistance can be expected for samples nitrided for longer times.
As well as the surface nitrogen content, the thickness of nitrided layers appears to be responsible for the pitting resistance properties of treated samples. Thicker corrosion resistant layers are expected to be more effective in protecting the substrate due to the decrease of the probability of local expositions of the substrate to the aggressive environment. Thus, owing to the reduction of surface defects, a thicker S-phase layer is expected to be more protective for the substrate. Morphological analysis showed that samples nitrided for a longer time have thicker modified layers, and thus they are expected to be more corrosion resistant in respect of samples nitrided for shorter times. Following this, the synergy between higher nitrogen surface concentration and thicker modified layers can reasonably explain the higher pitting corrosion resistance of samples nitrided for longer times.
4. Conclusions Glow-discharge nitriding treatments of AISI 316L austenitic stainless steel samples were carried out at 703 K for times in the range of 0–5 h. It was observed that the low-temperature nitriding treatments are able to produce modified layers on the austenitic substrate. The modified layers are essentially constituted of a metastable phase already described in literature and known as S-phase. The thickness of the modified layers increases as the nitriding time increases. XRD analysis shows an increase of S-phase diffraction peak shift towards lower diffraction angles as the nitriding time increases. This shift can be ascribed to the increase of nitrogen surface concentration as the nitriding time increases. The surface microhardness values and the thickness of the hardened layers increase as the nitriding time increases. After the 5-h treatment, the microhardness value is about 1450 HK0.1. This value is over six times that of the untreated AISI 316L samples and can be considered as a
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good estimate of the hardness values of S-phase when the nitrogen concentration maximum is obtained. Electrochemical corrosion tests carried out in 5% NaCl aerated solution show a higher pitting corrosion resistance for all the treated samples in comparison with the untreated samples: all the nitrided samples, at the end of the polarization tests, show a smaller damaging amount in comparison with the untreated alloy. The pitting corrosion resistance of treated samples increases as the nitriding time increases. The low-temperature d.c. glow-discharge nitriding treatment is an effective technique to increase the surface hardness and the pitting corrosion resistance of the AISI 316L austenitic stainless steel.
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[9] E. Menthe, U.A. Bulak, J. Olfe, A. Zimmermann, K.-T. Rie, Surf. Coat. Technol. 133–134 (2000) 259. [10] M. Samandi, B.A. Shedden, D.I. Smith, G.A. Collins, R. Hutchings, J. Tendys, Surf. Coat. Technol. 59 (1993) 261. [11] N.X. Randall, N. Renevier, H. Michel, P. Collignon, Vacuum 48 (1997) 849. [12] T. Czerwiec, N. Renevier, H. Michel, Surf. Coat. Technol. 131 (2000) 267. [13] M.P. Fewell, D.R.G. Mitchell, J.M. Priest, K.T. Short, G.A. Collins, Surf. Coat. Technol. 131 (2000) 300. [14] X. Xu, Z. Yu, L. Wang, J. Qiang, Z. Hei, Surf. Coat. Technol. 162 (2003) 242. [15] E. Menthe, K.-T. Rie, J.W. Schultze, S. Simson, Surf. Coat. Technol. 74–75 (1995) 412. [16] T. Bacci, F. Borgioli, E. Galvanetto, G. Pradelli, Surf. Coat. Technol. 139 (2001) 251. [17] K. Marchev, C.V. Cooper, J.T. Blucher, B.C. Giessen, Surf. Coat. Technol. 99 (1998) 225. [18] K. Marchev, M. Landis, R. Vallerio, C.V. Cooper, B.C. Giessen, Surf. Coat. Technol. 116–119 (1999) 184. [19] V. Singh, K. Marchev, C.V. Cooper, E.I. Meletis, Surf. Coat. Technol. 160 (2002) 249. [20] R.F.A. Jargelius-Pettersson, Corros. Sci. 41 (1999) 1639. [21] H. Baba, T. Kodama, Y. Katada, Corros. Sci. 44 (2002) 2393. [22] K. Osozawa, N. Okato, Proc. USA–Japan Seminar: Passivity and Its Breakdown on Iron and Iron-Based Alloys, Honolulu, NACE, 1976, p. 135. [23] Y. Zhiwei, X. Xiaolei, W. Liang, Q. Jianbing, H. Zukun, Surf. Coat. Technol. 153 (2002) 125. [24] F. Borgioli, A. Fossati, E. Galvanetto, T. Bacci, Surf. Coat. Technol. (in press). ¨ ztqrk, D.L. Willianmson, Surf. Coat. Technol. 158–159 (2002) [25] O. O 288.
Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment time A. Fossati*, F. Borgioli, E. Galvanetto, T. Bacci Dipartimento di Ingegneria Civile, Universita` di Firenze, via S. Marta 3, 50139 Firenze, Italy Received 11 August 2004; accepted in revised form 25 October 2004 Available online 8 December 2004
Abstract In the present paper, the results relative to low-temperature glow-discharge nitriding treatments of AISI 316L austenitic stainless steel are reported. This process is able to produce surface-modified layers essentially composed of a metastable phase called S-phase. Nitriding treatments were carried out at 703 K for times ranging from 0 to 5 h. Treated samples were characterized by means of morphological analysis, surface and profile microhardness measurements and electrochemical tests in NaCl aerated solutions in order to investigate the influence of treatment time on the microstructure, the hardness and the corrosion resistance properties; moreover, the results were compared with those observed for untreated samples. The thickness of the modified layers increases as the nitriding time increases. The surface hardness of treated samples is higher than the untreated AISI 316L stainless steel samples and increases as the treatment time increases up to a value of about 1450 HK0.1. The pitting corrosion resistance of nitrided samples increases as the treatment time increases; after a 5-h nitriding treatment, the anodic current values are reduced of about 3 magnitude orders in comparison with the untreated samples. Low-temperature glow-discharge nitriding treatment is an effective technique to increase the pitting corrosion resistance and the surface microhardness of AISI 316L austenitic stainless steel. D 2004 Elsevier B.V. All rights reserved. Keywords: Glow-discharge nitriding; Austenitic stainless steel; AISI 316L; S-phase; Pitting corrosion
1. Introduction The austenitic stainless steels are widely used in many industrial fields because of their very high general corrosion resistance; nevertheless, they can suffer pitting or crevice corrosion in specific environments and their low hardness and wear resistance can limit the number of possible industrial applications. Traditional gas nitriding carried out at relatively high temperature (773 K or above) can increase the surface hardness and the wear resistance, but generally decreases the corrosion resistance [1]. At these temperatures, the chromium diffusion coefficient is quite large, then the chromium atoms can move, preferentially bind nitrogen
* Corresponding author. Tel.: +39 55 479 6399; fax: +39 55 479 6504. E-mail address: [email protected] (A. Fossati). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.122
atoms and lead to the formation of the very stable and hard CrN [2,3]; the Cr-depleted austenitic matrix zones cannot therefore form a uniform and protective passivity film [4] and are subject to active corrosion. Nowadays, many innovative technologies, like magnetron sputtering [5], plasma source ion nitriding [6], plasma immersion ion implantation [7], low energy ion implantation [8] and plasma nitriding [9], have been utilised to perform low-temperature nitriding treatments and produce surface-modified layers avoiding the precipitation of large amounts of chromium nitrides. Low-temperature nitriding allows to obtain a modified layer essentially composed by a metastable phase known as supersaturated or expanded austenite gN [7,10–14], or S-phase [5,9,15,16], or m-phase [17–19] which, due to its large nitrogen concentration, has proved to have higher hardness and higher pitting corrosion resistance [20–22] in comparison with the untreated material.
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The aim of this work is to study the influence of the treatment time on the microstructure, the morphology, the mechanical properties and the corrosion behaviour in NaCl aqueous solutions of the AISI 316L austenitic stainless steel nitrided by low-temperature d.c. glow-discharge process.
2. Experimental procedure Prismatic samples (40184 mm) were obtained from an AISI 316L steel annealed bar (A 60 mm) by cutting, grinding and polishing up to 6-Am diamond suspension. Before the plasma nitriding process, the samples were ultrasonically cleaned in an acetone bath for 10 min. Glowdischarge treatments were performed in a laboratory plasma equipment, similar to industrial ones, using a d.c. power supply. The furnace system had an axial symmetry. Samples were fastened by means of screws at each face of the prismatic sample holder. The screws and the sample holder were realized by the same steel bar from which were obtained the samples. The samples and the sample holder, placed in the centre of the treatment chamber, worked as cathode and were completely surrounded by a cylindrical metal screen made up of AISI 304 stainless steel which was grounded and worked as anode. The anode–cathode distance was about 60 mm. Gas composition (80% N2 and 20% H2) was fixed during the sputtering step and the nitriding treatments. The treatment temperature was measured by a thermocouple inserted in the sample holder and controlled by varying the discharge current from the d.c. current supply. Nitriding treatments were performed at 703 K at a working pressure of 103 Pa for times in the range of 0–5 h. The current density necessary to maintain constant the temperature during the nitriding treatment was 2.6F0.1 mA cm2, while the measured voltage drop between the electrodes was 175F5 V. Before the nitriding treatments samples were warmed up to 653 K by means of a cathodic sputtering in order to remove the natural passive film and enable a homogeneous and correct nitriding process. After the sputtering step, the pressure and the temperature were increased up to their nominal values; the 0-h treatment consisted in the cathodic sputtering up to 653 K and just in the raising in temperature up to 703 K, then the power supply was turned off and the chamber evacuated. The microstructure of treated and untreated samples was examined by light and scanning electron microscopy. XRD analysis was performed in order to identify the phases present in the surface layers. Diffraction patterns were collected in Bragg–Brentano configuration (Cu-Ka radiation). Surface microhardness and microhardness profile measurements (Knoop indenter) were carried out using a load of 100 and 10 gf, respectively. Treated and untreated samples were tested in 5% NaCl aerated solutions at room temperature by means of electrochemical methods obtaining polarization and free corrosion
potential–time curves. Before the corrosion tests, samples were ultrasonically cleaned in acetone for 10 min. The corrosion tests were performed using a Pyrex cell equipped with a Ag/AgCl (KCl saturated) reference electrode and a platinum grid as counter electrode. Linear voltammetries were performed at a potential scan rate of 0.3 mV s1 after a delay period of 1 day; during the delay, the free corrosion potential–time curves were recorded. The sample surface area exposed to the electrolyte was 1 cm2. Corrosion tests were carried out at least three times for each sample type in order to assess the reproducibility. After the corrosion tests, microscopy analysis was carried out in order to evaluate the corrosion morphology and the damaging amount.
3. Results and discussion 3.1. XRD analysis Diffraction spectra of nitrided and untreated samples are shown in Fig. 1. In the case of untreated samples, austenite and aV-martensite peaks are visible. This phase, which is formed during the grinding and polishing operations, is highly metastable [23]. XRD analysis carried out on the surface of AISI 316L steel samples warmed up to 473 K by means of the cathodic sputtering reveals the total absence of this kind of martensitic phase. In the case of nitrided samples, XRD analysis shows that the nitriding process produced modified layers on the austenitic substrate. For all the kinds of nitrided samples, a set of diffraction peaks is detected which is positioned at different diffraction angles, depending on the different nitriding times; the diffraction peak set shifts towards lower diffraction angles as the nitriding time increases, up to a maximum shift value which is reached after a
Fig. 1. X-ray diffraction patterns of the AISI 316L austenitic stainless steel samples untreated and nitrided at 703 K for different times.
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3.2. Morphological analysis
Fig. 2. Surface morphology of sample glow-discharge nitrided at 703 K for 5 h.
treatment of 3.5 h. Accordingly to the literature data, these peaks are attributed to the S-phase. In the case of the shortest treatments (0–2 h), the austenitic substrate peaks together with S-phase peaks are still visible due to the small thickness of the modified layers. S-phase is generally considered a supersaturated solid solution of nitrogen in austenite, even if the crystallography has not been well clarified. As already observed by other authors, the shift of the S-phase peak positions depends on the nitrogen concentration. Particularly if the nitrogen concentration increases, the S-phase peaks move towards smaller diffraction angles [8]. This suggests that longer nitriding time increases the nitrogen surface concentration of the AISI 316L austenitic stainless steel nitrided samples. CrN diffraction peaks are not clearly detectable in our instrumental configuration; it has to be noted that a large amount of CrN precipitates is typical of traditional gas nitriding carried out at higher temperatures and it is the main responsible for the loss of corrosion resistance of austenitic stainless steels.
In Fig. 2, the typical surface morphology of nitrided samples is shown. This morphology, observed also by other authors [2,14,19], has been extensively described in a previous paper and can be ascribed to both the cathodic sputtering and the treatment itself [24]; the morphology features are enhanced as the nitriding time increases. In Fig. 3a and b, some typical micrographs of the crosssections of nitrided samples are reported. After glyceregia etching, the nitrided layer of any kind of nitrided samples appears to be a homogeneous layer separated from the matrix by a strong etched line. The thickness of the modified layer increases as the treatment time increases. The mean thickness of the modified layers for each nitriding time is reported in Table 1. It has to be noted that in the case of samples nitrided for 0 h, a modified layer is already detectable which shows an appreciable thickness. This allows to deduce that the cathodic sputtering up to 653 K and the short period of raising in temperature are able to produce a thin S-phase layer in a very short time (less than 15 min). It has to be pointed out that the S-phase layer is really a modification of the austenitic substrate, and it is not a coating, as also previously observed [24]; in Fig. 3a and b, it is clearly visible that the grain boundaries of austenite parent phase continue into the modified layer without any interruption. 3.3. Microhardness Knoop microhardness measurements were carried out on the surface of untreated and nitrided samples; the microhardness values are reported in Fig. 4. Nitrided samples show higher surface hardness in respect of the untreated samples. The microhardness values increase as the treatment time increases. In the case of the samples nitrided for 5 h,
Fig. 3. Microstructure of the modified layers formed on the AISI 316L austenitic stainless steel samples by glow-discharge nitriding treatment at 703 K for 2 h (a) and 5 h (b) (etchant glyceregia).
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Table 1 Modified layer thickness of AISI 316L samples glow-discharge nitrided at 703 K for different times Nitriding time (h)
Thickness (Am)
0 1 2 3.5 5
3F1 4F1 5F1 7F1 10F1
the microhardness value is over six times higher than that of untreated AISI 316L stainless steel samples. The increase of the measured microhardness values in the case of the samples treated for longer times can be ascribed to two different causes. At a fixed test load, the surface hardness measure of a layered structure mainly depends on the hardness of the different layers and on their own thickness. Particularly, the measured hardness value increases as the thickness and the hardness of the modified layers increase. Moreover, the influence of the substrate hardness on the measured hardness value of a surfacemodified sample decreases as the thickness of the modified layer increases. Morphological analysis revealed thicker modified layers in the case of samples nitrided for longer times, so that the influence of the substrate low hardness on the measured hardness of nitrided samples is more and more negligible as the nitriding time increases. Moreover, it can be hypothesized that the hardness value of S-phase depends on the nitrogen concentration. If nitrogen concentration increases, a larger reticular distortion and thus a higher hardening effect can be expected. As a matter of fact, XRD analysis revealed a higher reticular distortion in the case of samples treated for longer times, so that the S-phase present on the surface layer of these types of samples is expected to have a higher hardness in comparison with the S-phase present on the surface layer of samples nitrided for shorter times. Thus, due to the synergy between higher thickness and higher hardness of the modified layers, it appears reasonably
Fig. 4. Surface microhardness values of AISI 316L austenitic stainless steel samples untreated and glow-discharge nitrided at 703 K for different times.
explained the increase of the measured microhardness values of the nitrided samples with the increase of the nitriding time. The highest microhardness value (about 1450 HK0.1) was obtained in the case of 5-h nitriding treatment. In this case, the thickness of the modified layer is large enough to make completely negligible the influence of the austenitic substrate. Thus, the above-mentioned value could be considered a good estimate of the hardness values of the S-phase, when the highest nitrogen concentration is obtained. This microhardness value is in agreement with the highest hardness values measured by other authors [2,9,11,15]. This suggests that low-temperature glowdischarge nitriding is an effective nitriding technique of AISI 316L austenitic stainless steel. Knoop microhardness measurements were carried out on the cross-sections of the treated samples; the microhardness profiles are shown in Fig. 5. As the treatment time increases, the thickness of the hardened layers increases, in agreement with morphological observations. Next to the surface samples nitrided for 5 h show microhardness values in agreement with those measured on their surface. In the case of the samples treated for shorter times, the highest measured hardness values near the surface, at 2.5-Am depth, are lower in comparison with the other nitrided sample types. This can be ascribed either to the small thickness of the layer or to the lower nitrogen concentration through the layer, as suggested by XRD analysis. 3.4. Corrosion behaviour The typical polarization curves of nitrided samples are reported in Fig. 6; as reference, the typical linear voltammetry of the untreated AISI 316L stainless steel samples, obtained in the same experimental conditions, is also reported. The linear voltammetry of untreated samples shows the typical behaviour of a passive material that is sensibly
Fig. 5. Knoop microhardness profiles of AISI 316L austenitic stainless steel samples nitrided at 703 K for different times.
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Table 2 Average pitting potentials E pit V and highest anodic current density values from polarization curves in 5% NaCl aerated solution in the case of AISI 316L samples untreated and nitrided at 703 K for different times
Fig. 6. Polarization curves of AISI 316L austenitic stainless steel samples untreated and glow-discharge nitrided at 703 K for different times in 5% NaCl aerated solution.
subject to pitting corrosion if the potential is higher than a threshold; the pitting branch starts at relatively low anodic potential values (about +100 mV) due to the high chloride concentration. At the highest polarization potentials, the anodic current values are quite large and are limited only by concentration polarization phenomena. The morphological analysis carried out on the surface of the corroded samples reveals the presence of many deep pits with a vertical development due to the position of the samples during the corrosion test (Fig. 7). Nitrided samples are also subject to pitting corrosion, but the anodic current values are smaller than those of the untreated samples. In Table 2, the pitting potential values E pitV measured from the polarization curves and defined as the potential at which the anodic current values overcome definitively the threshold of 100 AA cm2 are reported. At the highest polarization potentials, the anodic current values of the nitrided samples are sensibly reduced in respect of the untreated AISI 316L stainless steel samples and are limited
Fig. 7. Corrosion morphology of untreated AISI 316L austenitic stainless steel samples after a linear voltammetry in 5% NaCl aerated solution carried out up to +1000 mV Ag/AgCl.
Sample type
E pit V (mV (Ag/AgCl))
Highest current density (AA cm2)
Untreated Nitrided—0 h Nitrided—1 h Nitrided—2 h Nitrided—3.5 h Nitrided—5 h
~+220 ~+80 ~+300 ~+350 ~+580 N+1000
~6.2104 ~5.5103 ~2.4103 ~4.5102 ~1.8102 ~6.2101
only by concentration polarization phenomena. As the nitriding time increases, the highest anodic current density values become smaller (see Table 2). Particularly after the 5h nitriding treatment, the highest anodic current values are reduced of about 3 orders of magnitude in comparison with those of the untreated samples. Just the 0-h treatment is able to reduce of 1 order of magnitude the maximum anodic current values. Free corrosion potential–time curves are in agreement with linear voltammetries. Fig. 8 shows the typical curves for untreated and nitrided samples. Nitrided samples appear to be always nobler in comparison with untreated AISI 316L stainless steel samples. Moreover, the curves of treated samples, specially in the case of samples nitrided for longer times, show slower, smaller and less frequent potential fluctuations in respect of untreated samples. Potential fluctuations can be ascribed to pit nucleation phenomena. Thus, nitrided samples, also in free corroding conditions, appear to be less subject to pitting corrosion in comparison with the untreated stainless steel samples. Morphological analysis carried out on the surface of treated samples after the corrosion tests is in agreement with electrochemical tests and free corrosion potential–time curves. Fig. 9 shows the typical corrosion morphology of nitrided samples at the end of the polarization tests.
Fig. 8. Typical free corrosion potential–time curves of AISI 316L austenitic stainless steel samples untreated and nitrided at 703 K for different times in 5% NaCl aerated solution.
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Fig. 9. Corrosion morphology of AISI 316L austenitic stainless steel samples nitrided at 703 K for 0 h (a), 3.5 h (b) and 5 h (c) after a linear voltammetry in 5% NaCl aerated solution carried out up to +1000 mV Ag/AgCl.
Nitrided samples show a sensibly lower damaging amount in respect of untreated samples. As the nitriding time increases, the pit dimensions and the pit density decrease; at the end of the corrosion tests, samples nitrided for 5 h appear almost untouched. Finally, it has to be noted that also the 0-h nitriding treatment is able to reduce sensibly the pitting damaging amount occurred during the polarization test (Fig. 9a). The high pitting corrosion resistance of nitrided samples and the different corrosion behaviour of different nitrided sample types can be reasonably explained considering two reasons: the nitrogen surface concentration and the thickness of the modified layers. It is well known in literature that just a little amount of nitrogen in austenitic stainless steels allows to increase sensibly their pitting corrosion resistance [21,22]. In the past, many mechanisms were proposed which could explain the beneficial effect of the interstitial nitrogen [20]. The following mechanism is one of the most accepted. If an austenitic stainless steel containing interstitial nitrogen is subject to pitting corrosion, interstitial nitrogen atoms are released. Then nitrogen atoms can react with H+ to form NH4+ following the present reaction [20]: N þ 4Hþ þ 3e YNHþ 4
ð1Þ
The local pH increase due to this cathodic reaction allows to facilitate the repassivation [21]. In literature, it is reported that the nitrogen interstitial concentration in the S-phase can reach values of about 30 at.% [6,8,25]. In the above-mentioned repassivation mechanism, a larger amount of nitrogen is expected to increase the repassivation capability for austenitic stainless steels, then it makes sense to expect a higher pitting corrosion resistance for S-phase, due to its very high nitrogen concentration, in comparison with the untreated austenitic stainless steel samples. Moreover, solid state diffusion laws and XRD analysis suggest that samples nitrided for longer times can reach higher surface nitrogen concentrations. Thus, due to their larger nitrogen surface concentration, a higher pitting corrosion resistance can be expected for samples nitrided for longer times.
As well as the surface nitrogen content, the thickness of nitrided layers appears to be responsible for the pitting resistance properties of treated samples. Thicker corrosion resistant layers are expected to be more effective in protecting the substrate due to the decrease of the probability of local expositions of the substrate to the aggressive environment. Thus, owing to the reduction of surface defects, a thicker S-phase layer is expected to be more protective for the substrate. Morphological analysis showed that samples nitrided for a longer time have thicker modified layers, and thus they are expected to be more corrosion resistant in respect of samples nitrided for shorter times. Following this, the synergy between higher nitrogen surface concentration and thicker modified layers can reasonably explain the higher pitting corrosion resistance of samples nitrided for longer times.
4. Conclusions Glow-discharge nitriding treatments of AISI 316L austenitic stainless steel samples were carried out at 703 K for times in the range of 0–5 h. It was observed that the low-temperature nitriding treatments are able to produce modified layers on the austenitic substrate. The modified layers are essentially constituted of a metastable phase already described in literature and known as S-phase. The thickness of the modified layers increases as the nitriding time increases. XRD analysis shows an increase of S-phase diffraction peak shift towards lower diffraction angles as the nitriding time increases. This shift can be ascribed to the increase of nitrogen surface concentration as the nitriding time increases. The surface microhardness values and the thickness of the hardened layers increase as the nitriding time increases. After the 5-h treatment, the microhardness value is about 1450 HK0.1. This value is over six times that of the untreated AISI 316L samples and can be considered as a
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good estimate of the hardness values of S-phase when the nitrogen concentration maximum is obtained. Electrochemical corrosion tests carried out in 5% NaCl aerated solution show a higher pitting corrosion resistance for all the treated samples in comparison with the untreated samples: all the nitrided samples, at the end of the polarization tests, show a smaller damaging amount in comparison with the untreated alloy. The pitting corrosion resistance of treated samples increases as the nitriding time increases. The low-temperature d.c. glow-discharge nitriding treatment is an effective technique to increase the surface hardness and the pitting corrosion resistance of the AISI 316L austenitic stainless steel.
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