201 (2006) 4424–4429www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Corrosion performance of the plasma nitrided 316L stainless steel

Linda Gil a,⁎, Sonia Brühl c, Lorena Jiménez a, Ovídio Leon a, Rafael Guevara, Mariana H. Staia b

a Departamento de Ingeniería Metalúrgica, Universidad Nacional Experimental Politécnica (UNEXPO), Puerto Ordaz, Venezuelab Escuela de Ingeniería Metalúrgica y Ciencia de los Materiales, Facultad de Ingeniería, Universidad Central de Venezuela,

Apartado 49141, Caracas 1042-A, Venezuelac Universidad Tecnológica Nacional, Facultad Regional Concepción del Uruguay, Argentina

Available online 22 September 2006

Abstract

The AISI 316L stainless steel has been widely used in artificial knee or hip joints, as well as internal fixation devices. It is well known that thismaterial has a good corrosion resistance and acceptable biocompatibility properties. Ion nitriding is a well established process for steel hardeningthat can also be applied to this kind of steels with the aim of enhancing its hardness but without reducing its corrosion resistance. In this work, theeffects of ion nitriding on the corrosion performance of a 3l6L stainless steel was evaluated in a 0.9% sodium chloride solution by usingelectrochemical tests such as potentiodynamic polarization and linear polarization in both nitrided and untreated AISI 316L steel condition.Surface characterization before and after corrosion testing was performed using scanning electron microscopy (SEM) with an energy dispersive X-ray analysis (EDS). It was shown that an ion nitriding treatment in a 25% N2–75% H2 atmosphere performed at a temperature of 410 °C improvesthe surface hardness of the AISI 316L stainless steel. However, under the experimental conditions carried out in this research, the nitrided steel isas prone to localized corrosion as the untreated one. It is considered that this behavior is mainly due to the presence of CrN, which precipitatesduring processing, contributing to the depletion of chromium from the adjacent matrix and leading to a galvanic corrosion mechanism.© 2006 Published by Elsevier B.V.

Keywords: Plasma nitriding; 316 L stainless steel; Corrosion resistance; Medical implants

1. Introduction

In recent years, many attempts have been made to improve thecorrosion properties and biocompatibility of metals and alloys usedin orthopedic surgery [1]. The most common materials suitable fororthopedic implants are stainless steel, cobalt and nickel-alloyenriched with chromium or molybdenum, and titanium or titaniumalloys. These alloys were chosen because they possess goodmechanical properties, sufficient corrosion resistance and biotoler-ance in implanted environments [1]. There are three types ofmaterials that constitute a prosthesis: the support materials, insuringmechanical fixation, the friction materials, insuring the sliding ofarticulating surfaces (joint coupling), and the anchorage materialspermitting the prosthesis to fix to the bone [2]. Support materialsare exclusively made of metals such as stainless steels or titaniumalloys [2]. Medical implants that required high strength up to theonset of substrate plastic deformation, such as bone plates or screw,are made of materials as AISI 316L stainless steels. Unlike other

⁎ Corresponding author. Tel.: +58 286 9618724; fax: +58 286 9617850.E-mail address: lindagil@cantv.net (L. Gil).

0257-8972/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.surfcoat.2006.08.081

stainless steel, AISI 316L is not ferromagnetic which permits theimplanted patients to be examined by using magnetic resonance[2]. The austenitic chromium–nickel–molybdenum steel achievesits “stainless” characterization through the formation of thin andadherent chromium rich oxide film [3]. However, in surface defectslike fissures or pits the local corrosion rate may be drasticallyincreased, due to an incomplete repassivation of the surface causedby lack of oxygen. For implants made of stainless steel in vivopitting corrosion has been observed [3]. Corrosion of metallicmaterials in implants may affect the body tissue by cell reaction toelectrical current, change the pH and release of metallic ion fromthe implant. For stainless steel, the biological environments reactsby formation of connective tissue between metal surface and bodytissue [3]; besides that, allergic reaction to nickel and chromium arereported. Although the metallic orthopedic implants may haveexcellent bulk properties such as ideal strength and elasticity, it hasrelatively poor surface properties, e.g. poor wear resistance andlimited biocompatibility. It is necessary to make a compromisebetween bulk properties and surface properties. In the case of hipreplacement, thewear debris from the implant is one of the essentialfactors for the aseptic loosening which is a frequent cause of failure

Table 1Nitriding parameters

Nitriding time 8 hTemperature 410 °CPressure 6.5 hPaAtmosphere 75% H2+25% N2

Voltage 680 VDuty cycle ton / toff 70–200 μsCurrent density ≈ 1 mA cm−2

Fig. 1. Optical micrographs showing the etched microstructure of: (a) nitridedAISI 316L (b) untreated AISI 316 L (etchant: Glyceregia).

4425L. Gil et al. / Surface & Coatings Technology 201 (2006) 4424–4429

of the prosthetic implants [4]. Life expectancy of an orthopedicimplant with mobile parts as a hip implant for instance isapproximately 10 years. Such a short utile life span is duemainly tothe mechanical wear and corrosion caused by the organisms on themobile parts of the prosthesis, which demand their replacement [5].

Considerable research has been carried out related to bio-materials in the natural tissue environments. It is at the bone/biomaterials interfaces that the biocompatibility of the materialsbecomes the determining factor for the success of the implant,both in terms of acceptance of the implant by the organism and itsfunctional suitability. It is generally accepted that the improvementof wear, corrosion resistance and biocompatibility of the implantsby surface engineering is an optimal option [5]. Nitriding is,nowadays, a well-established industrial practice for improvingwear and corrosion resistance of steel components. However, ifaustenitic stainless steels are treated at temperatures generally usedfor nitriding of low steels or tools steels (about 450 °C or higher),they suffer a significant decrease of corrosion resistance due to theprecipitation of substantial amounts of chromium nitride, whichdepletes chromium from solid solution prejudicing the formationof the protective film [6]. Nitriding techniques are effective inimproving both surface hardness and corrosion resistance onlywhen they are performed at temperatures below 450 °C. This hasbeen correlated to the formation of a metastable single phase,known as supersaturated or expanded austenite γN [5,6], S phase[7–10], or m phase [11–13], which provided to a high hardnessand a good corrosion resistance. Many nitriding techniques wereused to produce this phase, from plasma nitriding [6,13–18], ionbeen implantation [19,20], glow-discharge nitriding [6], plasmaassisted nitriding [21]. Even if the first report on the so-called Sphase by Ichii et al. [22] dates nearly two decades ago and theworking conditions to form this phase are now outlined, itsstructure is still amatter of debate and has not completely clarified.The phases present in the modified layer depend on both thephysical parameters and the used nitriding technique.

In this work, the effects of ion nitriding on the corrosionperformance of a 3l6L stainless steel was evaluated in a 0.9%sodium chloride solution at pH 6.3, 37 °C, to simulate thenatural tissue environment, by using electrochemical tests suchas potentiodynamic polarization and linear polarization in bothnitrided and untreated AISI 316L steel conditions.

2. Experimental procedure

AISI 316L stainless steel disc samples were cut from anannealed AISI 316L steel bar (diameter: 60 mm) and progres-sively polished with SiC emery papers down to 1000 grit level.

The ion nitriding process was performed using an industrialequipment with a DC pulsed power supply, and a chamber of600 mm diameter and 1200 mm high; the nitriding temperaturewas measured by means of a thermocouple in contact with thesamples and the processing conditions are presented in Table 1.

Before nitriding, in order to remove the oxide film from thesurface, the samples were subject to a sputtering process during2 h using a gas mixture of 60% Ar and 40% H2. The thicksamples were ground and mechanically polished by differentgrades of SiC emery papers (240, 600, 800, and 1000), and thenmanually polished by 1.0 μm and 0.5 μm alumina suspensionand degreased finally. A glyceregia reagent was used forrevealing the microstructural features. The microstructure of theuntreated and treated samples before and after corrosion testingwas examined by means of an optical microscope. Scanningelectron microscopy (SEM) technique coupled with energydispersion spectroscopy (EDS) analysis techniques were alsoemployed. The glancing-angle XRD measurements at 2° in-coming angle to the surface were performed with CuKα radi-ation in order to identify the microstructure of the nitrided zone.Knoop microhardness profile measurements (HK0.01) werecarried out on the modified layer and on the matrix, using aMicromet tester, series 2100 (Buhler Ltd, USA).

The electrochemical techniques used in the present workinclude the linear polarization and potentiodynamic polariza-tion. The anodic potentiodynamic polarization measurements

Table 2EDS analysis of: (a) nitrided and (b) untreated AISI 316L steel

Element Weight% Atomic%

a) Plasma nitrided 316L stainless steelN K 7.87 25.08Si K 0.72 1.15Cr K 17.33 14.87Mn K 1.74 1.41Fe K 64.53 51.56Ni K 7.81 5.93

b) Untreated 316L stainless steelSi K 0.73 1.25Cr K 18.23 19.32Mn K 1.42 1.43Fe K 68.49 67.56Ni K 11.13 10.44

4426 L. Gil et al. / Surface & Coatings Technology 201 (2006) 4424–4429

were performed using Gamry PC4/750 Potentiostat/GalvanostatModel DHC2 (Gamry Instruments, Inc. USA). An electrolytesolution of NaCl (9 g/l of H20) at pH=6.3 and a temperature of37 °C, was used with the aim to simulate the natural tissueenvironment. A stabilization period of 3600 s was employedbefore starting the measurement. The electrode potential wasraised from −0.25 mV to 1.6 mV with the scanning rate of0.5 mV/s, and the current that flowed through the coating-substrate system was recorded. An Ag/AgCl electrode was usedas the reference electrode and graphite as the counter electrode.It has to be reminded that the experiment was carried in such away that the electrolyte was in contact only with the coatedsurface. The contact area in all cases was 0.54 cm2. The linearpolarization experiments were carried out by scanning thepotential ±10 mVabout to open circuit potential (Ecorr) at rate of1 mV s−1.

Fig. 2. SEM micrographs of the nitrided AISI 316L stainless steel cros

3. Results and discussion

3.1. Microstructure and morphology

After nitriding treatment, the surface of the sample shows apeculiar morphology, as shown in Fig. 1a. The surface appearsto be plasma etched, and this etching (also observed by otherauthors [5,13,23]) can be ascribed to both the cathodic sput-tering performed before the nitriding process and the treatmentitself. The surface of the treated sample, as delineated by thisplasma etching, still shows an austenitic structure with thecharacteristic twins, as commonly revealed by chemical etchingwith glyceregia on untreated samples (Fig. 1b). Moreover, asindicated by Borgioli et al. [6] the slip bands are delineatedwithin the grains of the treated sample, which cannot beascribed to the plasma etching and, therefore, they suggestedthat the formation of the nitrided layer is accompanied by highcompressive stresses which cause a plastic deformation in thematerial. These features of the nitrided surface were reportedpreviously in the literature by other authors [5,13,23,24].Table 2 shows the EDS average analysis performed on top ofboth nitrided and unitrided sample, respectively where it couldbe observed that the nitrogen content on the surface of thenitrided sample is approximately 25 at.%.

In the literature, different investigations have reported thatwhen the nitriding process is carried out between 310 °C and420 °C, the nitrided layer corresponds to a supersaturated solidsolution of nitrogen in iron γ (f.c.c.) also called expandedaustenite (γN), which presents high hardness and very goodcorrosion resistance [6,13–18].

In Fig. 2, the SEM micrograph of the sample cross-sectionshows the microstructure of the nitrided sample treated at410 °C, where two layers are identified whose total thickness is

s-section sample and EDS microanalysis of outer and inner layers.

Fig. 4. Knoop microhardness profile of AISI 316l L nitrided sample at 410 °C.

4427L. Gil et al. / Surface & Coatings Technology 201 (2006) 4424–4429

of approximately 6 μm. In this figure, the average compositioncorresponding to each layer of the nitrided zone is alsoindicated. The EDS microanalysis of the two layers shows adifferent nitrogen concentration between them, the higherconcentration of 22.5 at.% N corresponds to the outer layerand only 9.3 at.% N was determined for the inner one.

Czerwiec et al. [21] have also reported a bi-layered com-pound zone and have attributed its existence to the in situcleaning by ion bombardment in Ar–H2 plasma prior to thenitriding process. Typical X-ray diffraction patterns of 410 °Cthe plasma nitrided and untreated 316 stainless steel are shownin Fig 3. Both γ(111) and γ(200) peaks of austenite were observedin case of the untreated 316 stainless steel sample. However, forthe plasma nitrided sample, a set of peaks which do not matchany existing ASTM X-ray diffraction index was observed.These peaks have been associated with a metastable phasecalled “expanded austenite, γN” or “S phase”. Both peaks ap-peared at lower angles when compared to the austenite phasewhich corresponds to the untreated sample, indicating anexpansion of the lattice, due to the incorporation of nitrogen inthe interstitial position of the fcc austenite structure [6–13]. TheX-ray diffraction shows that besides the γN phase, a weak peakof CrN nitride was detected in the layer.

The hardness depth profile of the sectioned nitrided sampleshown in Fig. 4 was determined by carrying out Knoop hard-

Fig. 3. XRD pattern of the (a) nitrided AISI 316L sample (b) untreated AISI316L sample.

ness test. Near the surface of the nitrided layer, at 1 μm from thesurface, the value obtained which was approximately 17.7 GPa(1800 HKN0.01) corresponds to the average hardness values ofthe two layers, and could be attributed to both the formation of ahard γN phase, which is a supersaturated solid solution ofnitrogen in the austenite, and to the presence of chromiumnitride.

3.2. Corrosion behavior

Fig. 5 shows the potentiodynamic polarization curves of bothnitrided and untreated AISI 316L samples in an aerated solutionof NaCl 9 g /1 (physiological serum) at pH=6.3 and atemperature of 37 °C. The anodic polarization curve of thenitrided AISI 316L stainless steel can be divided into tworegions. In the first region, the dissolution of the nitrided AISI316L steel was kinetically limited and the anodic current in-creased slowly with potential, showing a “passive-like” be-havior. Finally, there is a transpassive region beginning at acritical potential (Epit) where the rapid increased in the currentvalue is due to breakdown of the passive film. This phenom-enon is commonly known as pitting corrosion [25] and the

Fig. 5. Potentiodynamic polarization curves of the nitrided 316L stainless steel(– · –) and of the untreated 316L stainless steel (–) in aerated neutral 0.9 wt.%NaCl solution.

Table 3Electrochemical parameters estimated from the polarization tests performed inan aerated 0.9 wt.% NaCl solution

Sample Ecorr

(V)Epit

(V)icorr(nA/cm2)

Rp

(KΩ)Epit−Ecorr

(V)

Untreated 316L steel −1.65 1.78 49 1.52 3.43Nitrided 316L steel −1.24 2.50 27 72.84 3.74

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potential at which a rapid increase of the current density occursis usually termed the “pitting potential” or “breakdown poten-tial” (Epit or Ebk). The anodic potentiodynamic polarizationcurves obtained in the present investigation were similar tothose previously published for the nitrided 316L steel inchloride-containing solutions [13,14,25,26].

Table 3 presents the corrosion results as determined from theabove experiments. These results show that the nitrided AISI316L exhibited almost the same corrosion resistance as the316L untreated stainless steel. As a result of the nitriding pro-cess, the corrosion potential of the untreated 316L stainless steel(−1.65 V) was shifted towards a slightly more noble value(−1.24 V), the corrosion current density was decreased from49 nA/cm2 to 27 nA/cm2 and the polarization resistance, Rp,was increased from 1.52 to 72.84 (KΩ). Nevertheless, it has

Fig. 6. SEM micrographs showing: (a) the corroded surface morphology of th

been demonstrated that the nitriding treatment, carried out in theexperimental conditions described in the present investigation,did not bring the expected improvement in the corrosion re-sistance of the AISI 316L steel and this system could not beused for biomedical applications.

This behavior was mainly due to the formation of CrN in thecompound layer, which contributed to the depletion ofchromium from the adjacent matrix. If the chromium contentis below 11–12% in this area, the steel is said to be sensitizedand then, when exposed to an aggressive medium, is attackedmainly due to a galvanic corrosion mechanism.

This behavior is corroborated by experimental findingsreported previously [13,26] which indicated that the presence ofmixed phase (nitrides and γN phase) produces average corrosionpotentials which are similar to those of the original stainlesssteel.

Pitting tendencies can also be assessed from the valuescorresponding to the “pitting potential” (Epit) indicated inTable 3. If the value of Epit is close to that of Ecorr, little polar-ization is required to initiate the pits formation. Thus, sampleswhich are more prone to exhibit pitting will have a relatively lowEpit−Ecorr difference values. Table 2 summarizes the Epit−Ecorr

difference values for nitrided and untreated samples. The resultspresented in this table indicate that the nitrided AISI 316L

e nitrided AISI 316L stainless steel; (b) EDS microanalysis inside a pit.

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sample is as prone to localized corrosion as the untreated sampleunder experimental conditions carried out in this research.

3.3. Post-tests analysis of samples

Superficial corroded samples were examined using a scan-ning electron microscope and their morphologies are presentedin Fig. 6. Abundant pitting corrosion occurs for the nitrided316L stainless steel samples. This result is consistent with thesuggestion that CrN is precipitated in the matrix of γN expandedaustenite. Inhomogeneous microstructures enhanced corrosiondue to electrolytic cell reaction between second phase particlesand the matrix, whereas homogeneous structures are relativelyfree of internal cell reactions. Therefore, when CrN precipitatesresult in substantial chromium segregation in 316L stainlesssteel, local galvanic cells will be set up which further enhancethe pitting corrosion [13–18,26–29].

4. Conclusions

❖ Plasma source ion nitriding of 316L austenitic stainless steelat a process temperature of 410 °C for a nitriding time of 8 hproduced a nitrogen expanded austenite (γN) layer with athickness of 6 μm, with a nitrogen content varying fromapproximately 22 to 25 at.%.

❖ Nevertheless, it has been demonstrated that the nitridingtreatment, carried out in the experimental conditions de-scribed in the present investigation, did not bring the ex-pected improvement in the corrosion resistance of the AISIsteel.

❖ Abundant pitting corrosion occurs for the nitrided 316Lstainless steel samples (Fig. 6). This result indicates that CrNis precipitated in the matrix of the γN expanded austenite,contributing to substantial chromium segregation towardsthe grain boundaries and producing, therefore, local galvaniccells which further enhances the pitting corrosion.

Acknowledgments

The authors wish to acknowledge the financial supportreceived from Fondo Nacional de Ciencia, Tecnología eInnovación (FONACIT) through the project S1-2001000759,project UCV F-2001000600 and to CDCH-UCV through theprojects PI 08-17-5120-2003 and P.G. 08-00-5791-2005.

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