CORROSION RESISTANCE OF PLASMA NITRIDED AND NITROCARBURIZED AISI 316L AUSTENITIC STAINLESS STEEL F.A.P. Fernandes 1*, J. Gallego 2, G.E. Totten 3, C.A. Picon 2, L.C. Casteletti 1 1 Department of Materials Engineering, São Carlos School of Engineering, University of São Paulo, Av. Trabalhador Sãocarlense, n. 400, 13566-590, São Carlos, SP, Brazil. *e-mail: codoico@gmail.com 2 Engineering Faculty of Ilha Solteira, São Paulo State University, Av. Brasil, n. 56, 15385-000, Ilha Solteira, SP, Brazil. 3 Department of Mechanical and Materials Engineering, Portland State University, Post Office box 751, 97207-0751, Portland, OR, USA. Abstract: The usage of coatings in surface engineered components is increasing due to the need for improved hardness, corrosion and wear resistance. Plasma nitriding and nitrocarburizing of austenitic stainless steels can produce layers of expanded austenite (Sphase). This interesting phase is supersaturated with respect to nitrogen and is characterized by high hardness and wear resistance. In this study plasma nitriding and nitrocarburizing of AISI 316L stainless steel were conducted at 400, 450 and 500 C. The plasma treated AISI 316L steel samples were characterized by optical microscopy, X-ray diffraction and corrosion tests. Corrosion characterization was performed by potentiodynamic polarization in 3.5% NaCl solution. After plasma treatment, it was observed that the layer thickness increases with temperature. The treatments at 400 C produced homogenous and precipitate-free S-phase layers while at 450 and 500 C X-ray diffraction indicates the presence of iron carbide and/or chromium and iron nitrides. The potentiodynamic polarization curves show that corrosion resistance is higher for the samples treated at 400 C relative to the untreated substrate. A change in the dominant corrosion mechanism was also observed after nitriding or nitrocarburizing from localized pitting corrosion to general corrosion. Key words: Nitriding; Nitrocarburizing; X-ray diffraction; Corrosion. 1 INTRODUCTION Surface coatings are one of the most versatile ways to improve the performance of components with respect to wear and/or corrosion. Thermochemical treatments such as nitriding, carburizing and nitrocarburizing at low temperatures are widely used surface engineering technologies to improve surface hardness and wear resistance of stainless steels without compromising their good corrosion resistance 1-3. It is well known that when such treatments are performed at a temperature sufficiently low, a nitrogen expanded austenite, or S-phase can be produced on the surface of an austenitic stainless steel or other face-centered cubic (fcc) alloys 4. This very promising coating can only be achieved if the treatment temperature is lower than 500ºC 3,4. The S-phase is a metastable phase with a supersaturation of nitrogen and/or carbon which remains in solid solution. It has been reported that nitrogen in solid solution as an alloying element promotes passivity by widening the passive range in which pitting is less probable which improves stress corrosion cracking and also enhances intergranular corrosion resistance 5-7. The aim of this study is to evaluate the influence of treatment temperature on the morphology, microstructure, microhardness and corrosion resistance properties of the plasma nitrided and nitrocarburized AISI 316L steel samples. 1
2 MATERIALS AND METHODS AISI 316L austenitic stainless steel (ASS) samples 22mm in diameter and 3mm thick of were cut and then prepared by conventional metallographic techniques to obtain a polished surface. The chemical composition of the steel was (in wt%): C, 0.019; Mn, 1.47; Si, 0.401, Cr, 16.26; Ni, 10.5; Mo, 2.02; N, 0.067; Cu, 0.47 and Fe, balance. Prior to plasma treatment, the samples were cleaned by argon sputtering (on work pressure and temperature of 50ºC less than the treatment temperature for 30 min) inside the plasma chamber. Plasma nitriding (PN) and nitrocarburizing (PNC) were performed using the dc method with the following gas mixtures: 80 vol. % H2 and 20 vol. % N2, for nitriding and 77 vol. % H2, 20 vol. % N2 and 3 vol. % CH4 for nitrocarburizing. The treatments were performed at a pressure of 500Pa during 5h at temperatures of 400, 450 and 500ºC. Optical microscopy analyses was performed on the cross-section of the samples using a Zeiss microscope with the interference contrast technique on samples etched with nitromuriatic acid. X-ray diffraction (XRD) patterns were obtained on the surface of the samples using Geirgerflex Rigaku equipment with a scanning angle from 30 to 100. The tests were performed using copper radiation (Cu-Kα) and continuous scanning with a speed of 2.min-1. The electrochemical cell used to obtain the potentiodynamic polarization curves utilized a saturated calomel (SCE) reference electrode and a platinum auxiliary electrode. The electrolyte employed was a 3.5% aqueous NaCl solution. For monitoring the potential and current, an Autolab model VGSTAT-302 potentiostat was employed. The polarization curves of the nitrided and nitrocarburized samples were obtained with a scanning speed of 1mV.s-1 from -1.0 to 1.125V. 3 RESULTS AND DISCUSSION The optical micrographs of the cross-sections of the AISI 316L (ASS) samples which were plasma nitrided and nitrocarburized at temperatures of 400, 450 and 500 C are shown in Figure 1. For nitrided (Figs. 1a, 1b and 1c) and nitrocarburized (Figs. 1d, 1e, and 1f) samples, the micrographs clearly show the austenitic matrix beneath each layer for all treatment conditions. Figure 1, Optical cross sections of plasma (a-c) nitrided and (d-f) nitrocarburized ASS samples at (a, d) 400ºC, (b, e) 450ºC and (c, f) 500ºC. 2
The treatments performed at 400 and 450 C (Figs. 1a, 1b, 1d and 1e) produced homogeneous and precipitate-free layers under the optical microscope. These layers appear to be bright and featureless and posses all of the characteristics of the nitrogen supersaturated S- phase. It is also worth to noting that the layers produced at 500 C (Fig. 1c and 1f) yielded the appearance of a dark region just above the S-phase. This region, according to the literature 1-3 is indicative of S-phase decomposition and occurs due to chemical bonding between carbon and/or nitrogen and the alloying elements of the material forming carbides and/or nitrides. Figure 2 shows the XRD patterns of plasma nitrided and nitrocarburized ASS steel. For comparison, the substrate diffraction pattern is shown for both PN (Fig. 3a) and PNC (Fig. 3b). Narrow diffraction peaks are observed for the ASS substrate which are consistent to the austenite phase (Fe-γ). Figure 2, X-ray diffraction patterns of plasma (a) nitrided and (b) nitrocarburized ASS samples at 400, 450 and 500ºC. PN and PNC at 400ºC produced the Fe-γ (111) reflection and broadened peaks dislocated to lower diffraction angles. These peaks were labeled as S 1, S 2...S 5 and are an intrinsic characteristic of the S-phase which confirms the presence of a homogeneous and precipitate-free S-phase layer. The peaks related to the S-phase are always broadened because of an enormous quantity of interstitial elements introduced on the surface of the sample originating from a high defect density and residual stress. The treatments performed at 450ºC also yielded the appearance of peaks shifted to lower diffraction angles which correspond to the S-phase. Nevertheless, in addition to the S- phase, the XRD reveals evidence of nitride precipitation such as CrN, Cr 2 N and Fe 2 N which were not detected by optical microscopy. Increasing treatment temperature increases mobility of chromium and iron due to chemical bonding with nitrogen. At 500ºC distinct patterns are observed for PN and PNC. After PN and PNC at 500ºC, it is estimated that the S-phase is decomposed increasing nitrogen and carbon compounds depending on the treatment. The increased amount of these compounds enables their observation under the optical microscope (Fig. 1c and 1f). Therefore, nitriding at 500ºC has resulted in chromium (CrN, Cr 2 N) and iron (Fe 2 N, Fe 3 N and Fe 4 N) nitride precipitation and for nitrocarburizing, iron carbide (Fe 3 C) is produced in addition to these nitrides. Thus, morphological analysis from Fig. 1 are in agreement with XRD analysis. In Figure 3, the potentiodynamic polarization curves obtained in 3,5% NaCl solution for the plasma nitrided (Fig. 3a) and nitrocarburized (Fig. 3b) ASS samples are shown. For both treatments, the curves obtained for the samples treated at 400, 450 and 500 C are compared with the untreated material. All plasma treated specimens and the substrate 3
exhibited a very similar cathodic region ranging from -1.00V to -0.25V. Moreover, the Tafel region of the treated samples was also close to that obtained for the untreated steel resulting in a slightly increased corrosion potential (E corr ) for nitrided and nitrocarburized samples as shown in Tab. 1. Table 1 presents the quantitative electrochemical parameters collected from the polarization curves in Fig. 3. It shows the corrosion potential (E corr ), corrosion current (I corr ) and the current density at a 1.2V potential (I 1.2V ) which means the current at end of the test at the highest potential. The ASS substrate yielded a typical polarization curve with passivation and pitting corrosion 8. The breakdown of passivity occurs at about 300mV which leads to an abrupt increase in current density reaching 32mA.cm -2 (Tab. 1). The corrosion currents (I corr ) of the untreated and all plasma treated ASS samples are of the same magnitude- about 10-8 A.cm -2 (Fig. 3 and Tab. 1). Examination of the currents of the curves at a potential larger than 600mV reveals an increase with increasing treatment temperature for both nitriding and nitrocarburizing treatments (Fig. 3). Figure 3, Potentiodynamic polarization curves of plasma (a) nitrided and (b) nitrocarburized ASS samples at 400, 450 and 500 C. Samples nitrided and nitrocarburized at 400 C yielded the lowest current densities after the Tafel region which resulted in similar surfaces after polarization. Inspection of the surfaces of these samples reveals a clean and smooth surface without any corrosion damage. The polarization curves obtained for the samples treated at 450 C exhibited a sudden increase in current density after the Tafel region until about 380mV where it stabilizes and reaches a value close to 1mA.cm -2 at the end of the test. Microscopic examination of these surfaces reveal similar corroded surfaces that are rough in appearance. The samples treated at 500 C exhibit polarization curves where the current densities also increase abruptly after the Tafel region but without stabilization. The current increases to a value close to that observed for the ASS substrate (Fig. 3) producing a rough corroded area with very small pits and orange debris. The very low currents exhibited by samples treated at 400 C is probably related to the presence of the nitrogen supersaturated S-phase like that shown by the XRD patterns (Fig. 2). The nitrogen in solid solution plays an important role in improving electrochemical properties mainly by forming ammonium ions which restricts the decrease of ph at active sites on the surface, thus avoiding pit nucleation and growth 1,5,6. This leads to a change of the corrosion mechanism from localized pitting corrosion to a general form in which the dissolution rate depends on the treatment temperature. 4
Table 1, Electrochemical parameters from the polarization curves of the plasma nitrided and nitrocarburized ASS samples. Sample E corr, mv I corr, 10-8 xa.cm -2 I 1.2V, 10-3 xa.cm -2 ASS-Sub. -323 2.479 32.678 PN 400-252 5.365 0.5339 PN 450-231 5.237 1.201 PN 500-280 2.441 17.401 PNC 400-243 1.611 0.6657 PNC 450-237 3.650 0.9351 PNC 500-280 1.099 35.187 At 450 and 500 C, the XRD analysis (Fig. 2) indicated that both PN and PNC treatments have produced carbon and/or nitrogen compounds on the surface of the samples in addition the S-phase. The occurrence of these compounds is favorable because chromium and iron atoms acquire mobility as the temperature is increased allowing chemical bond formation between substitutional and interstitial elements 1,4. The increase of current densities of the polarization tests as the temperature was raised from 450 to 500 C is related to the massive precipitation of nitrides as observed by XRD experiments (Fig. 2). These results and observations suggest that both PN and PNC at 400 C considerably improves the corrosion resistance of ASS in 3.5% NaCl aqueous solution. 4 CONCLUSIONS From these data it can be concluded that plasma nitriding and nitrocarburizing of AISI 316L stainless steel produces layers in which the thickness increases with temperature. The treatments at 400 C produced homogenous and precipitate-free, S-phase layers while at 450 and 500 C XRD indicates the presence of iron carbide and/or chromium and iron nitrides depending on the treatment type and temperature. Potentiodynamic polarization curves show that corrosion resistance decreases as temperature increases. A change in the dominant corrosion mechanism was also observed after nitriding or nitrocarburizing from localized pitting corrosion to general corrosion. Thus, the results suggest that both nitriding and nitrocarburizing at 400 C considerably improves the corrosion resistance of ASS in 3.5% NaCl solution. Acknowledgements The authors acknowledge CAPES for the scholarship granted to F.A.P. Fernandes. REFERENCES [1] C.X. Li and T. Bell: Corros. Sci., 2004, 46, 1527-1547. [2] A. Fossati, F. Borgioli, E. Galvanetto and T. Bacci: Corros. Sci., 2006, 48, 1513-1527. [3] F.A.P. Fernandes, S.C. Heck, R.G. Pereira, C.A. Picon, P.A.P. Nascente and L.C. Casteletti: Surf. Coat. Tech., 2010, 204, 3087-3090. [4] H. Dong: Int. Mater. Rev., 2010, 55, 65-98. [5] R.C. Newman and T. Shahrabi: Corros. Sci., 1987, 27, 827-838. [6] R.F.A. Jargelius Pettersson: Corros. Sci., 1999, 41, 1639-1664. [7] N. Padhy, S. Ningshen, B.K. Panigrahi and U. Kamachi Mudali: Corros. Sci., 2010, 52, 104-112. [8] Y. Sun: Corros. Sci., 2010, 52, 2661-2670. 5