Corrosion behavior of AISI302 steel treated by elevated temperature nitrogen plasma immersion ion implantation

Corrosion behavior of AISI302 steel treated by elevated temperature nitrogen plasma immersion ion implantation

Scripta Materialia 53 (2005) 1427–1432 www.actamat-journals.com Corrosion behavior of AISI302 steel treated by elevated temperature nitrogen plasma i...

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Scripta Materialia 53 (2005) 1427–1432 www.actamat-journals.com

Corrosion behavior of AISI302 steel treated by elevated temperature nitrogen plasma immersion ion implantation Shaoqun Jiang, Xinxin Ma *, Yue Sun, Mingren Sun School of Materials Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang 150001, PR China Received 22 January 2005; received in revised form 9 August 2005; accepted 11 August 2005 Available online 28 September 2005

Abstract The corrosion behavior of AISI302 steel implanted with nitrogen at elevated temperature was investigated by electrochemical impedance spectroscopy. Equivalent circuits for explaining the impedance characteristics are proposed. The thick passive layer containing Cr2O3 and the expanded austenite layer in the sub-surface worked together, resulting in the high corrosion resistance.  2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Stainless steels; Ion implantation; Corrosion

1. Introduction Cr18Ni9 austenitic stainless steels, which have excellent corrosion resistance, make up the highest proportion of austenitic stainless steels in use. However, their relatively poor wear resistance and low hardness are drawbacks and preclude many possible industrial applications. In the treatment of austenitic stainless steels, it is important to improve the surface mechanical properties without degrading the corrosion resistance. Elevated temperature plasma immersion ion implantation (PIII) is an effective surface modification technique. The tribological behavior of Cr18Ni9 austenitic stainless steels treated by elevated temperature nitrogen PIII can be improved significantly [1,2]. Mechanisms of tribological behavior have been investigated systematically [3,4]. To date though, very little work has been done on the corrosion behavior of the Cr18Ni9 steels treated by elevated temperature PIII, especially corrosion behavior studies based on the alternating current (AC) impedance spectroscopy principles. In this paper, the corrosion behavior of elevated temperature treated *

Corresponding author. Tel.: +86 451 86418835; fax: +86 451 86413922. E-mail address: [email protected] (X. Ma).

AISI302 steel (also called 1Cr18Ni9 steel) was studied. The corrosion mechanisms are discussed based on the analyses of the phase structures and the element states. 2. Experimental A commercial AISI302 austenitic stainless steel plate with a thickness of 0.6 mm and surface roughness Ra of 0.10 lm was cut into discs of 14 mm in diameter. Each sample was ultrasonically cleaned in acetone before loading onto a sample holder in the DLZ-01 PIII facility which was described elsewhere [5]. Before the treatment process, 2 kV Ar plasma pre-cleaning was performed. The system was pumped to a base pressure less than 5 · 103 Pa and then back-filled with nitrogen to the working pressure of 0.22 Pa. By adjusting the frequency and pulse width of the implantation parameters, the samples were kept at a certain temperature. The samples were treated at 390 C by using a negative pulsed voltage of 35 kV, 500 Hz and 60 ls. The processing time was 4 h and the implantation dose was about 1.3 · 1019 N+/cm2. The temperature of sample during the implantation process was measured by a Raytek GP infrared temperature monitor. After the elevated temperature PIII, the chemical states of elements in the implanted layers were measured by

1359-6462/$ - see front matter  2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.08.015

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X-ray photoelectron spectroscopy (XPS). In the process, Ar ions were used to sputter and denude the sample at a sputtering speed of 4 nm/min. X-ray diffraction (XRD) with Cu Ka radiation at glancing angles of 1, 5 and normal angles was used to determine the phases present at various depths of modified layers. At 1, 5 and normal angles, information depths were about 80 nm, 400 nm and 4580 nm (depth for 80% signal), respectively. One of the main disadvantages with stainless steels is localized corrosion when they are exposed to chloride solutions. Therefore, 3 wt.% NaCl solution was used for the corrosion tests. The corrosion behavior of the samples in 3 wt.% NaCl solution was studied by electrochemical impedance spectroscopy (EIS). A three-electrode electrochemical cell with double-carbon bars as a counter electrode and a saturated calomel electrode as the reference electrode was employed for all the measurements made with the untreated or treated samples as the working electrode. An electrode area of 1 cm2 of the samples was exposed for the electrochemical measurements. The measurements at the open circuit potential condition were started after 10 min of immersion of the samples in a 3 at.% NaCl solution in order to obtain the necessary stability of the system for an AC impedance measurement. The experiments were carried out at the open circuit potential with an amplitude of 10 mV in the frequency range 98 mHz to 30 kHz. All experiments were recorded with a frequency response analyser (Solartron 1255) and a potentiostat (EG&G 273). The impedance spectra were analyzed with the simulation Boukamp equivalent circuit software. After the electrochemical tests, the samples were cleaned with distilled water, dried and examined by PMG3 microscope. In order to give sufficient information on the corrosion mechanism, the sample implanted at 420 C and 35 kV was also measured by EIS. 3. Results and discussion 3.1. Composition of implanted layer Fig. 1 gives the compositional depth profiles of O, Cr, Fe, Ni and N in the implanted layer. It can be found that

the oxygen content is very high near the surface of the implanted sample. The oxygen content drops quickly when the depth is over 25 nm. Oxygen in the modified layer stems from absorbed oxygen and the original oxide film on the sample surface recoiled into the substrate by energetic ion bombardment. The chromium content within 40 nm is slightly higher than that in the substrate. When the depth is over 40 nm, the chromium content remains at about 18 wt.%. The nitrogen content increases firstly and then is about 20 at.% with increasing depth. It means that implantation at elevated temperature increases the thickness of implanted layer. Fig. 2 shows the deconvoluted XPS spectra of Cr 2p3/2 and N 1s of the implanted sample at the depth of 20 nm from the surface. The peaks were deconvoluted using appropriate peak positions and half width of its various constituents. In Fig. 2a, the chemical states of chromium correspond to Cr, Cr2O3 and Cr(OH)3 at the binding energies of 574.1 eV, 576.0 eV and 577.2 eV, respectively [6–8]. The amount of Cr is the highest, corresponding to the maximum content (78.19%) of the total chromium. The chromium in Cr2O3 is about 17.18% of the total chromium. The remainder exists in Cr(OH)3. In Fig. 2b, the deconvoluted N 1s spectrum shows two distinct species of nitrogen corresponding to binding energies of 399.0 eV and 397.1 eV [9,10], respectively. The peak at 399.0 eV can be attributed to absorbed nitrogen and the peak at 397.1 eV can be taken as the weakly bonded nitrogen atom with austenite (c). This kind of weakly bound atom derives from the solution of nitrogen and accounts for a dominant proportion (about 89.60%). Since the solubility of nitrogen in c is very low, the formation of supersaturated nitrogen solution is favored. A previous study [11] showed that the different chemical states of nitrogen correspond to CrN and Cr2N at the binding energies of 396.7 eV and 397.4 eV, respectively. Thus the deconvoluted N 1s spectrum here proves that no chromium nitride precipitates at the depth of 20 nm of the implanted layer. This also indicates that no chromium nitride precipitates and chromium almost presents as Cr2O3 in the deconvoluted XPS spectra of Cr 2p3/2 and N 1s of the implanted sample at depths less than 20 nm from the surface.

100 90

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Depth (nm) Fig. 1. Compositional depth profiles of the implanted layer.

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Fig. 2. XPS spectra of Cr 2p3/2 (a) and N 1s (b) at 20 nm depth of the implanted layer.

γ (220)

γ(111)

O1s

γ (200)

Intensity (a.u.)

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γN

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γN

5

o

γN

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1 1200

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o

0

Binding energy (eV) Fig. 3. Normalized differential survey spectrum of the untreated and treated samples at the depth of 20 nm.

Compared with austenitic stainless steels nitriding at high temperature [12], the nitrogen PIII at elevated temperature is a non-equilibrium process, and the relative low treatment temperature (390 C) is not favorable for precipitation of nitrides. Fig. 3 shows the normalized differential survey spectrum of untreated and treated samples at the depth of 20 nm. It can be found that the content of oxygen increases after implantation. According to the literature [13], oxygen penetrated into the depth more than 20 nm, indicating that elevated temperature nitrogen PIII increases the thickness of the layer containing Cr2O3. 3.2. Phase structure in the implanted layer XRD patterns are shown in Fig. 4. The elevated temperature implanted sample exhibits phase changes. We found the high nitrogen face-centered cubic (fcc) phase with a ser-

30

40

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2θ (°) Fig. 4. X-ray diffraction patterns acquired from implanted sample.

ies of broad peaks (labeled nitrogen expanded austenite cN) on the left of the primary austenite peaks [14–18]. The content of nitrogen in the cN is above 20% and the fcc phase expands to triclinic slightly. The nitrogen expanded austenite (cN) has high strength and excellent corrosion resistance [14–16]. It can improve wear resistance and corrosion resistance of AISI302. The intensity value of cN changes with the accretion of incident angle. The intensity value of cN at the incident angle of 5 is stronger than that at the incident angle of 1 and normal angle. This suggests that cN forms in a wide depth range, most in the sub-surface layer. Furthermore, the phase structure and chemical content vary greatly with depth. This indicates different nitrogen contents and surface properties exist at different depths. Our results are in good agreement with another groupÕs work [14]. The position of cN peak and the thickness of modified layer depend on the process parameters, e.g.

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35 kV shows that the second time constant feature is more evident. The most noticeable change in the EIS spectrum of implanted AISI302 takes place in the high frequency range. This indicates that ion implantation changes the outer zone of the surface layer. By fitting of the impedance spectrum data for the nonimplanted and implanted AISI302 samples obtained with the Boukamp simulation program, we propose two different models: a Randles circuit with a Faradaic impedance ZF representing the polarization resistance for the non-implanted sample (Fig. 6a) and a two-layer circuit for the implanted sample (Fig. 6b). The selection of the circuit was a compromise between a reasonable fitting of the experimental values and a minimum of components in the equivalent circuit. Re is the NaCl solution resistance, Cf the doublelayer capacitance, Rf the polarization resistance, Cf1 the capacitance of the thick Cr2O3 layer after treatment, Rf1 the resistance of the thick Cr2O3 layer after treatment, Cd the double-layer capacitance and Rt the charge transfer resistance. Hong et al. [19] suggested the Randles circuit for AISI304 stainless steel in similar media. During the implantation process, some defects can form on the surface of the AISI302 sample, which leads to a non-ideal Cr2O3 passive layer. The electrolyte can penetrate into the subsurface layer and results in the formation of a double-layer

voltage, temperature. High voltage implantation process increases nitrogen incorporation and lattice expansion. Elevated temperature implantation can facilitate thermal diffusion and increases nitrogen retention in the substrate. This, however, does not indicate that lattice expansion and the modified layer thickness are monotonically enhanced with increasing implanting temperature/voltage. In order to get superior modified layer, a suitable implanting temperature and voltage are very important. 3.3. Corrosion behavior of the implanted layer Fig. 5 shows the Bode diagrams of the implanted AISI302 after 20 min exposure. The spectrum of a reference sample (non-implanted) is also included in this figure for comparison. It appears that the impedance modulus of the sample implanted at 390 C/35 kV is higher than that of the non-implanted one (Fig. 5a). The phase angle spectra (Fig. 5b) shows a single time constant feature for the nonimplanted sample. However, for the implanted sample a second peak, which corresponds to a second constant time, is observed. The two-layer circuit character is a common characteristic, and similar results were also obtained using adjacent experimental parameters. The EIS results from the sample implanted at 420 C with an applied voltage of

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Fig. 5. Bode plots for non-implanted and implanted AISI302 after 20 min immersion in 3 wt.% NaCl solution: (a) modulus of the impedance and (b) phase angle versus frequency.

Cf

Cd

Rf1

Rt

Re

Re Rf a

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b

Fig. 6. Electrical equivalent circuits for the interpretation of experimental Bode diagrams of non-implanted (a) and implanted AISI302 (b) in 3 wt.% NaCl solution.

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at the interface of the cN phase layer. Thus this two-layer model assumes that the passive film does not totally cover the steel and can not be considered as a homogeneous layer but rather as a defective layer. In fact, neither solid surfaces in the active range nor passive films on steel can be considered to be ideally homogeneous [20]. The measured capacitive response is not generally ideal as like a pure capacitor. A constant phase element (CPE) is then introduced for the spectrum fitting, instead of an ideal capacitance element. The impedance expression of CPE is given by [21–24] n

Z Q ¼ 1=½AðjxÞ 

ð1Þ

where Q represents CPE, A and n are frequency-independent fit parameters and x(=2pf) the angular frequency. The factor n, defined as a power, is an adjustable parameter that lies between 0 and 1. For n = 0, Q is an ideal resistance, and for n = 1, Q describes an ideal capacitor with A equal to the capacitance C. When n = 0.5, Q represents a Warburg impedance with diffusion character and for 0.5 < n < 1, Q describes a frequency dispersion of time constants due to local inhomogeneities in the dielectric material. It is generally believed that Q is related to some type of heterogeneity of electrode surface as well as to the fractal nature (roughness or porosity) of the surface [24].

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The fitting parameters are summarized in Tables 1 and 2 for the equivalent circuits. The polarization resistance obtained for the implanted sample (Rpol  1108.820 kX cm2) is greatly increased compared with that obtained for the non-implanted sample (Rpol  288.909 kX cm2), indicating a large increase in the protectiveness of the surface layer. This change is caused by the formation of a thicker Cr2O3 passive layer on the surface and the formation of cN in the sub-surface layer and deeper after implantation treatment, which improves the corrosion resistance of this steel. Fig. 7 shows the surface morphology of the samples with and without treatment. The untreated sample was severely eroded in the corrosion test. The corrosion pits are clearly visible and can be observed on the entire surface. On the other hand, the pits on the treated specimen can hardly be observed. This is consistent with the result of Bode, i.e., the treated sample possesses better corrosion resistance. Table 1 Fitting parameters for simulated spectrum of non-implanted sample Re (X cm2)

Rf (kX cm2)

Cf (F/cm2)

nf

Rpol  jZ(x ! 0)j (kX cm2)

9.400

288.900

2.147 · 104

0.901

288.909

Table 2 Fitting parameters for simulated spectrum of implanted sample Re (X cm2) 6.871

Rf1 (X cm2) 2257.690

Cf1 (F/cm2) 5

5.517 · 10

nf1 0.833

Cd (F/cm2) 6.943 · 10

5

Rt (kX cm2)

nd

Rpol  jZ(x ! 0)j(kX cm2)

1106.600

0.928

1108.820

Fig. 7. Surface morphology of non-implanted sample (a) and implanted sample (b) after electrochemical impedance spectroscopy.

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4. Conclusion The elevated temperature nitrogen PIII greatly improves the corrosion resistance of AISI302 stainless steel. The treated sample shows a lower corrosion rate than that of untreated sample. No pits were found on the eroded surface of the sample implanted at 390 C/35 kV. The implantation process increases the thickness of the passive layer containing Cr2O3, thereby yielding an expanded austenite layer (cN) in the sub-surface layer. The corrosion reactions were mitigated, resulting in excellent corrosion resistance. The electrochemical impedance spectrum obtained from the untreated AISI302 sample shows a one-time-constant feature which fits into a Randles circuit with a Faraday impedance ZF representing the polarization resistance; whereas the implanted AISI302 sample shows a twotime-constant feature and fits into a two-layer circuit. Acknowledgements The authors would like to thank Prof. Jin Hu and Prof. Weidong Fei at the Harbin Institute of Technology and Dr. Xiaodong Li at the University of South Carolina for their support. References [1] Jones AM, Bull SJ. Surf Coat Technol 1996;83:269. [2] Tian XB, Zeng ZM, Tang BY, Kwok TK, Chu PK. Surf Coat Technol 2001;128–129:226.

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