Biomaterials 22 (2001) 641}647
In vitro corrosion resistance of plasma source ion nitrided austenitic stainless steels M.K. Lei *, X.M. Zhu Surface Engineering Laboratory, Department of Materials Engineering, Dalian University of Technology, Dalian 116021, People+s Republic of China Department of Materials Science and Engineering, Dalian Railway Institute, Dalian 116028, People+s Republic of China Received 25 January 2000; received in revised form 16 April 2000; accepted 12 July 2000
Abstract Plasma source ion nitriding has emerged as a low-temperature, low-pressure nitriding approach for low-energy implanting nitrogen ions and then di!using them into steel and alloy. In this work, a single high nitrogen face-centered-cubic (f.c.c.) phase (c ) formed on , the 1Cr18Ni9Ti and AISI 316L austenitic stainless steels with a high nitrogen concentration of about 32 at % was characterized using Auger electron spectroscopy, electron probe microanalysis, glancing angle X-ray di!raction, and transmission electron microscopy. The corrosion resistance of the c -phase layer was studied by the electrochemical cyclic polarization measurement in Ringer's , solutions bu!ered to pH from 3.5 to 7.2 at a temperature of 373C. No pitting corrosion in the Ringer's solutions with pH"7.2 and 5.5 was detected for the c -phase layers on the two stainless steels. The high pitting potential for the c -phase layers is higher, about 500 , , and 600 mV, above that of the two original stainless steels, respectively, in the Ringer's solution with pH"3.5. The corroded surface morphologies of the c -phase layers observed by scanning electron microscopy are consistent with the results of the electrochemical , polarization measurement. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Plasma source ion nitriding; Austenitic stainless steel; Corrosion; Surface modi"cation
1. Introduction The surgical implants are usually made of metallic materials, such as austenitic stainless steel, cobalt}chromium alloys, and titanium and its alloys. Among all the metallic materials, the austenitic stainless steels are the most popular materials because of their relatively low cost, ease of fabrication and reasonable corrosion resistance . However, the austenitic stainless steels are prone to localized attack in long-term applications due to the aggressive biological e!ects. The corrosion products include iron, chromium, nickel and molybdenum, etc. ions which can accumulate in tissues surrounding the implant or be transported to distant parts of the body. Tracana et al.  demonstrated that metallic ions resulting from the in vitro corrosion of austenitic stainless steels cause alteration of the expression of human lymphocyte-surface antigens and inhibit the immune response as assessed by lymphocyte proliferation. The * Corresponding author. Tel.: #86-411-4707255; fax: #86-4114708116. E-mail address: [email protected]
presence of these ions in vivo not only caused toxic e!ects in mouse testicular seminiferous epithelium but also alterations in the spleen cellular population . Problems can also occur on hip joint prostheses coupling between the metallic femoral head and the polymetric acetabular cup due to the low friction and wear resistance of austenitic stainless steels. Leitao et al.  suggested that there is a close relationship between the macrophages, lymphocytes and the worn metal particles around the implant, although the mechanism of aseptic loosening is not fully understood. In order to improve the combining wear and corrosion resistance for austenitic stainless steels, di!erent surface modi"cation techniques, such as the plasma thermochemical di!usion treatment , ion implantation , and thin "lms deposition [6,7], have been used in the recent years. However, the request for austenitic stainless steels in biomedical applications is not satis"ed because the modi"ed surfaces with improved wear resistance usually show deteriorated corrosion resistance and/or the deposited "lms no longer have improved wear and corrosion resistance as they peel out from the stainless-steel substrates.
0142-9612/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 2 2 6 - X
M.K. Lei, X.M. Zhu / Biomaterials 22 (2001) 641}647
Plasma-based low-energy ion-implantation technique including plasma source-ion nitriding/carburizing and plasma source low-energy ion-enhanced deposition of thin "lms has been developed from a combination of two new techniques based on conventional plasma-based ion implantation and low-energy ion beam implantation . The engineering surfaces for improvement in wear and corrosion resistance were obtained on the plasma source ion nitrided Fe}Cr}Ni (18-8 type) austenitic stainless steels [9}11]. A high nitrogen face-centered-cubic (f.c.c.) phase (c ) which stems from the supersaturation of , nitrogen in the austenite (c) matrix possesses about ten times higher hardness and correspondingly increased wear resistance, and a superior pitting corrosion resistance in 1% NaCl solution and an equivalent general corrosion resistance in 0.5 mol l\ H SO solution, com pared with the original austenitic stainless steel. Though it is found that the c -phase layer on the , nitrided austenitic stainless steel has a great potential as a new engineering surface in biomedical applications, there is no corresponding study for corrosion resistance of the c phase in vitro and/or in vivo. In this paper, we , present the experimental results by electrochemical polarization measurement in Ringer's solutions bu!ered to pH from 3.5 to 7.2 for the c phase on the 1Cr18Ni9Ti , and AISI 316L austenitic stainless steels to verify the possibility and further spread to their biomedical applications.
2. Experimental Commercial 1Cr18Ni9Ti and AISI 316L austenitic stainless steels were selected as the sample material. The composition (wt%) of the 1Cr18Ni9Ti stainless steel (Dalian Steel Factory, China) was C 0.10, Mn 1.50, Si 0.80, Cr 18.00, Ni 9.00, Ti 0.80, P)0.035, S)0.030 and Fe as the balance. The composition of AISI 316L stainless steel (Sandvik Steel, Sweden) met ASTM standard speci"cation for surgical implant materials (F55-82). Samples (25 mm diameter;6 mm length) were cut from the bars of the stainless steels. All samples were "nely ground through 180, 400 and 800 grit silicon carbide paper, polished using 1 lm diamond paste, and "nally cleaned in acetone followed by air drying. In the plasma source ion nitriding process, the samples were placed in an electron}cyclotron resonance (ECR) microwave plasma and biased with a pulsed negative potential of !2 kV at a process temperature of 3803C regulated by an auxiliary heater. The temperature of the samples was constantly measured during the nitriding process by a thermocouple attached within 2 mm of the surface being nitrided. The plasma source ion nitriding device, which is described elsewhere [8,9], was pumped down to a base pressure of 1.5;10\ Pa by a di!usional/mechanical pump package. The typical operating
Table 1 Typical ECR microwave plasma source ion nitriding parameters Microwave power Plasma density Electron temperature Base pressure Nitriding pressure Pulsed negative bias Voltage Repetition rate Length Nitrogen ion implantation dose rate Nitriding process temperature Nitriding time
300 W 5;10 }1.5;10 cm\ 7}10 eV 1.5;10\ Pa (5}10);10\ Pa 2 kV 100}1000 Hz 50}500 ls 0.63 mA cm\ 3803C 4h
parameters applied during the nitriding process are listed in Table 1. The nitrogen concentration pro"les of the nitrided surfaces of the 1Cr18Ni9Ti and AISI 316L stainless steels were made to a depth of 400 nm by Auger electron spectroscopy (AES) using a RIBER SIA-100 surface analysis system and to a depth of 50 lm using a JXA-733 electron probe microanalyzer (EPMA). The near-surface structure of the nitrided samples was investigated using glancing angle X-ray di!raction on a RIGAKU D/MAX-3A di!ractometer using Cu K radiation. An ? incidence angle of 103 was used for all the samples. The microstructural characteristics of the nitrided layers were also investigated using a HITACHI-800 transmission electron microscope (TEM). Electrochemical cyclic polarization curves were determined at 37$0.53C using a EG & G PAR model 352 potentiostat/galvanostat interfaced with a computer and a recorder. The test solutions which were prepared from the analytical grade agents and distilled water have the following composition (g l\): 8.50 NaCl, 0.20 KCl, and 0.20 NaHCO . The pH values of the solutions were adjusted to 3.5, 5.5 and 7.2 either with HCl or NaOH. A conventional three-electrode cell was used. The counter electrode was a platinum sheet and all the potentials were reported with respect to a saturated calomel electrode (SCE). The working electrode was inpressed with a starting potential of 100 mV below the open-circuit potential and scanned towards the positive direction at a scanning velocity of 0.5 mV s\ till the current density reached a value of 1000 lA cm\ after passing through the pitting potential. Then the sweep direction was reversed to obtain the dynamic polarization hysteresis from which the pitting protection potential was arrived at. Three or four individual samples for the nitrided and unnitrided stainless-steel samples were, respectively, tested in each solution. The corroded surface morphologies were observed using a PHILIPS XL30 scanning electron microscope (SEM).
M.K. Lei, X.M. Zhu / Biomaterials 22 (2001) 641}647
3. Results 3.1. Composition and microstructure Fig. 1 shows the nitrogen concentration depth pro"les by AES and EPMA on the nitrided surfaces of the 1Cr18Ni9Ti and AISI 316L stainless steels, respectively, at a process temperature of 3803C. The AES gives good depth resolution while the EPMA provides semiquantitative compositional pro"le at considerably greater depths. The thickness of all the nitrided layers was about 13 lm for the two stainless steels, and the similar peak nitrogen concentrations, about 32 at%, were also obtained [Fig. 1(a) and (b)]. The crystal structure of the nitrided layers was investigated using X-ray di!raction, as shown in Fig. 2. A single c phase on the nitrided surfaces was obtained for the , two samples [Fig. 2(a) and (b)]. The anisotropic distribution of disordered nitrogen in f.c.c. lattice of the c phase resulted in its complicated structure, leading to complete disappearance of some peaks of high-index planes . No apparent di!erence in the di!raction patterns was detected between the two c -phase layers. This indicates , that c phase of similar structure was formed on the two , stainless steels. The formation of a single c phase on the ,
Fig. 2. Glancing X-ray di!ractograms for the plasma source ion nitrided stainless steels at 3803C: (a) 1Cr18Ni9Ti SS; (b) AISI 316L SS.
nitrided surfaces has been con"rmed unambiguously by TEM observation. Fig. 3 shows a typical TEM image and selected area di!raction (SAD) pattern of the nitrided layer on the 1Cr18Ni9Ti stainless steel. The great strain in f.c.c. lattice from the high nitrogen supersaturation caused the formation of stacking faults and dislocations. The dense and wide microstructure suggested a very low nucleation rate of the c phase [Fig. 3(a)]. The , SAD pattern of the c phase showed a [2 5 1 ] zone , without any superlattice spots. This implies absence of the other nitrogen-containing relative phases, such as nitrogen-induced hexagonal-close-packed (h.c.p.) martensites (e and e ), ordering c phase, CrN, and a phase , ,  [Fig. 3(b)]. 3.2. Corrosion resistance
Fig. 1. Summary of data from AES and EPMA depth pro"les of nitrogen in the plasma source ion nitrided stainless steels at 3803C: (a) 1Cr18Ni9Ti SS; (b) AISI 316L SS.
Figs. 4}6 show the potentiodynamic cyclic polarization curves from the nitrided and unnitrided 1Cr18Ni9Ti and AISI 316L stainless steels in the Ringer's solutions with the pH"7.2}3.5 at 373C. Because the Cl-ions in the Ringer's solutions are known to act as aggressive species for pitting corrosion in the stainless steels, the two original stainless steels underwent a transition course: from self-passivation to pitting corrosion. Meanwhile, by
M.K. Lei, X.M. Zhu / Biomaterials 22 (2001) 641}647
Fig. 4. Cyclic potentiodynamic polarization curves from the nitrided 1Cr18Ni9Ti and AISI 316L stainless steels in Ringer's solution with pH"7.2 at 373C.
Fig. 3. TEM images of the outer surface layer on the plasma source ion nitrided 1Cr18Ni9Ti stainless steel at 3803C: (a) bright-"eld micrograph; (b) selected area di!raction pattern of the c phase showing , [2 5 1 ] zone.
decreasing the pH value in the solutions, the pitting potentials of the stainless steels corresponded to the reduction, although their corrosion potentials did not obviously change. The AISI 316L stainless steel has the higher pitting potential and protection potential than those of the 1Cr18Ni9Ti stainless steel because of its composition with the lower carbon and higher chromium contents, and the other additive alloy elements. For the two c -phase layers on the two stainless steels in the , solutions with pH"7.2 and 5.5, no pitting corrosion was detected due to the transpassivation solution of the passive "lms on the surfaces (Figs. 4 and 5). The c -phase , layer on the two stainless steels possess similar pitting corrosion resistance in the weakly acidic and neutral Ringer's solutions. When the pH value became 3.5, a similar passivation-pitting corrosion transition to those of the two unnitrided stainless steels occurred for the two c -phase layers, however, the high pitting potential is ,
Fig. 5. Cyclic potentiodynamic polarization curves from the nitrided 1Cr18Ni9Ti and AISI 316L stainless steels in Ringer's solution with pH "5.5 at 373C.
higher, about 500 and 600 mV, above that of the two original stainless steels, respectively (Fig. 6). The transpassivation solution also appeared before the reversed scanning, the protection potential did not be detected for the c phase layers. Furthermore, the c phase obtained , , on the AISI 316L stainless steel has a high pitting corrosion resistance in comparison to that on the 1Cr18Ni9Ti stainless steel. Though the peak nitrogen concentrations of the two c -phase layers on the two , stainless steels are similar, their pitting corrosion resistance is clearly di!erent in the highly acidic Ringer's solution. This indicates that the composition of the stainless-steel matrix a!ects the pitting corrosion resistance of the c phase in the highly acidic solution, whereas these , e!ects become immaterial in the weakly acidic and neutral solutions.
M.K. Lei, X.M. Zhu / Biomaterials 22 (2001) 641}647
Fig. 6. Cyclic potentiodynamic polarization curves from the nitrided 1Cr18Ni9Ti and AISI 316L stainless steels in Ringer's solution with pH"3.5 at 373C.
Typical corroded surface morphologies in the Ringer's solutions for the nitrided and unnitrided 1Cr18Ni9Ti and AISI 316L stainless steels were obtained by SEM. Some apparent pits were observed on the original stainless steels in the solutions with pH"7.2}3.5. Fig. 7(a) shows the typical pit produced on the unnitrided 1Cr18Ni9Ti stainless steel in the solution with pH"7.2. The maximum diameter of the pits was about 120 lm. No pits could be found for the c -phase layer on the nitrided , 1Cr18Ni9Ti stainless steel in the solution with pH"7.2, and the surface after the transpassivation solution is shown in Fig. 7(b). The corroded surface morphologies are consistent with the results of the electrochemical polarization measurement.
Fig. 7. SEM photographs of the corroded surfaces from the 1Cr18Ni9Ti stainless steel in Ringer's solution with pH"7.2 at 373C: (a) unnitrided; (b) nitrided.
4. Discussion For combining improvement in the wear and corrosion resistance for the austenitic stainless steels, plasma nitriding into the AISI 316L stainless steel was initially performed at a low process temperature of 4003C . A c and c mixed-phase layer formed on the nitrided , stainless steel. The hardened surface on the stainless steel has a pitting corrosion resistance equivalent to that of the original stainless steel in 1% NaCl solution at room temperature. The later low-temperature plasma nitriding processes for austenitic stainless steels could produce the c -phase layer on the nitrided surfaces with more or less , improved pitting corrosion resistance in the NaCl solutions at room temperature, compared to the original stainless steels [13,14]. The nitrogen ion implantation of the AISI 316L stainless steel with an implantation dose of 10 ions cm\ increased the pitting corrosion resistance in the Hank's solution at room temperature, but was deteriorated for the implanted stainless steel with a lower
or higher implantation dose . Plasma source ion nitriding formed a modi"ed layer on the 1Cr18Ni9Ti and AISI 316L stainless steels with the combining wear and corrosion resistance. The microhardness of the c -phase , layers on the stainless steels, about 2000 HK (0.1 N load), which could be compared with those of several hard thin "lms, such as TiO , TiN and diamond-like (DLC) "lms deposited on the stainless steels or titanium alloys [7,15], is by about two times higher than that produced on the modi"ed surfaces by low-temperature plasma nitriding [13,14] and ion implantation . The thickness of the hardened layers, about 13 lm, is two and three orders of magnitude thicker than that of the ion-implanted layers on the stainless steel. This c -phase layer on the , 1Cr18Ni9Ti stainless steel improved its wear resistance under long-term high load-bearing conditions . Furthermore, the single nitrogen-di!used layer for the c phase has a good adhesion on the stainless-steel sub, strate, unlike the "lms deposited on stainless steels, which
M.K. Lei, X.M. Zhu / Biomaterials 22 (2001) 641}647
may peel out from the substrate during load-bearing. The good pitting corrosion resistance of the c -phase layer in , the Ringer's solutions with pH"7.2}3.5 at 373C is equivalent to or higher than that measured at room temperature for the surfaces modi"ed by low-temperature plasma nitriding [13,14] and ion implantation . To explain the high pitting corrosion resistance of the c phase, the related dissolving reaction for the c phase, , , which is dependent on the supersaturation of nitrogen in f.c.c. lattice of the c-phase matrix, may also be carried out in the Ringer's solutions with pH value from 3.5 to 7.2 as  [N]#4H>#3ePNH>
Su$cient NH> ions from the high nitrogen concentra tion in the c phase have a local neutralizing e!ect in the , acidic pits on the corroded surfaces of the stainless steels, increasing the pitting corrosion resistance in the Ringer's solutions. The solution of disordered nitrogen in single c phase favored reaction (1). In fact, reaction (1) can also , take place for the other nitrogen-containing relative phases, therefore, certain pitting corrosion resistance for the c phase [5,10] and nitrogen-induced e and e mar, , tensites  was found on the nitrided 1Cr18Ni9Ti stainless steel. The limited dissolution of nitrogen which is restricted due to the stronger bonds of metal-N than those of the c phase leads to the low concentration of , NH> on the surfaces, leading to insu$cient improve ment in the pitting corrosion resistance. No bene"cial e!ect on the pitting corrosion resistance is observed for the stable CrN phase which extremely inhibits the dissolution of nitrogen in the nitrided layer, except of the lacking Cr in the matrix of austenitic stainless steels. The corrosion resistance of the modi"ed austenitic stainless steels is signi"cantly dependent on the microstructure of the surfaces. The c -phase layer has been reported on the , nitrogen-modi"ed stainless steels by low-temperature nitriding [13,14,18], nitrogen ion beam implantation , and nitrogen plasma immersion ion implantation , however, its general corrosion resistance in the H SO solutions is often di!erent, even con#icting. Ruling out the di!erences in the process characteristics of the several nitrogen-modi"ed techniques, the low-temperature plasma nitriding of AISI 304 stainless steel also has con#icting results [14,18]. We suggested that the di!erent composition and microstructure of the c phase were , indeed obtained in the processes [10}12]. For applications in biomedical materials it is important to know what results can be obtained during in vivo exposure of the c -phase layer. Further experiments are , needed to investigate the behavior of tissues, cells or biological environments in contact with the c phase on , the nitrided stainless steels by plasma source ion nitriding. The current results unquestionably indicate that this c -phase layer formed on the stainless steels possesses the ,
combining corrosion and wear resistance. The high pitting corrosion resistance in the Ringer's solutions with pH"7.2}3.5 at 373C is also satis"ed for the applications in local acidi"cation of physiological solution caused by the surgical implantation and the corresponding in#ammation of the surrounding tissue.
5. Conclusions Plasma-based low-energy ion-implantation technique based on conventional plasma-based ion implantation and low-energy ion beam implantation produced an engineering surface on the 1Cr18Ni9Ti and AISI 316L austenitic stainless steels for biomedical applications. A single high nitrogen f.c.c. phase (c ) with a high nitro, gen concentration of about 32 at % was obtained on the two stainless steels, respectively. No pitting corrosion in the Ringer's solutions with pH"7.2 and 5.5 at 373C was detected for the c -phase layers on the stainless steels. , The high pitting potential for the c -phase layers is , higher, about 500 and 600 mV, above that of the two original stainless steels, respectively, in the Ringer's solution with pH"3.5 at 373C. The corroded surface morphologies of the c -phase layers observed by SEM are , consistent with the results of electrochemical polarization measurement.
Acknowledgements We acknowledge the contributory discussions and technical assistance of Professor Z.L. Zhang, Drs. Y. Huang, Y.H. Li, Z.P. Zhang, X.P. Zhu, and Professor Z.W. Yu, and Mrs L.J. Yuan and X.L. Xu in this research. This work is supported by the National Science Foundation of China under Grant No. 59771060.
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