Improved corrosion resistance of 316L stainless steel by nanocrystalline and electrochemical nitridation in artificial saliva solution

Improved corrosion resistance of 316L stainless steel by nanocrystalline and electrochemical nitridation in artificial saliva solution

Accepted Manuscript Title: Improved corrosion resistance of 316L stainless steel by nanocrystalline and electrochemical nitridation in artificial sali...

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Accepted Manuscript Title: Improved corrosion resistance of 316L stainless steel by nanocrystalline and electrochemical nitridation in artificial saliva solution Author: Lv Jinlong Liang Tongxiang PII: DOI: Reference:

S0169-4332(15)02432-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.09.267 APSUSC 31515

To appear in:

APSUSC

Received date: Revised date: Accepted date:

27-8-2015 22-9-2015 28-9-2015

Please cite this article as: L. Jinlong, L. Tongxiang, Improved corrosion resistance of 316L stainless steel by nanocrystalline and electrochemical nitridation in artificial saliva solution, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.09.267 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved corrosion resistance of 316L stainless steel by nanocrystalline and electrochemical nitridation in artificial saliva solution

a

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Lv Jinlonga,b, Liang Tongxianga,b Beijing Key Laboratory of Fine Ceramics, Institute of Nuclear and New Energy Technology,

State Key Lab of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084, China

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b

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Tsinghua University, Zhongguancun Street, Haidian District, Beijing 100084, China

Abstract: The fluoride ion in artificial saliva significantly changed semiconductor characteristic

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of the passive film formed on the surface of 316L stainless steels. The electrochemical results showed that nanocrystalline α'-martensite improved corrosion resistance of the stainless steel in a

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typical artificial saliva compared with coarse grained stainless steel. Moreover, comparing with

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nitrided coarse grained stainless steel, corrosion resistance of the nitrided nanocrystalline stainless

Ac ce pt e

steel was also improved significantly, even in artificial saliva solution containing fluoride ion. The present study showed that the cryogenic cold rolling and electrochemical nitridation improved corrosion resistance of 316L stainless steel for the dental application. Keywords: Stainless steel; EIS; Raman spectroscopy; Passive films; XPS

* Corresponding author. Tel.: +86 10 89796090; fax: +86 10 69771464. E-mail address: [email protected]

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1. Introduction The conventional polycrystalline stainless steel with micrometer scale has been well used in orthopedics, orthodontics and dentistry [1] because of its high corrosion

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resistance and low cost. However, pitting and crevice corrosion could occur in stainless steels under the environment with the chloride ion [2] or fluoride ion [3].

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Harmful ions could be released into solution. For example, the toxicity of chromium

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was related to its valence state and the Cr3+ was the actual agent of toxicity. The releasing metal ions of the traditional microcrystalline 304 stainless steel implanted in

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body had been found to cause toxic effects on body tissues [4]. The element Cr in 304

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stainless steel was used to improve its corrosion resistance, while element Ni was used to stabilize the austenitic phase. European standard for release rate of Ni and Cr

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is far below the limit of 0.2 μg/(cm2 week) [5]. Mutlu et al. [6] found that the Fe, Cr,

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Ni, Cu and Mo elements were released from the stainless steel foams, however, there was not a higher metal ion release than reference levels of these metal ions in body fluids for 17-4 PH stainless steel [6]. The fluoride ions often exist in oral environments. Both NaF and other fluoride compounds are frequently used as prophylactic products in dental treatments to prevent plaque formation and caries development [7, 8]. However, Lee et al. [9] found that the corrosion rate of all the alloys in the presence of fluoride ions increased up to 1000 times, moreover, a remarkable decrease in the breakdown potential of the passive film was also recorded. Kocijan et al. [10] studied the evolution of the passive films on 2205 duplex stainless steel and AISI 316L stainless steel in artificial saliva in the presence and absence of

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fluoride ions. They found that the range of the passive range increased in the 2205 duplex stainless steel compared to the AISI 316L stainless steel and they also found that the passive films on both materials predominantly contained Cr-oxides. In the

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fluoridated acidified saliva a low concentration of fluoride induced the forming of HF which would dissolve the surface passive film [11]. Moreover, Nie et al. [12] have

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investigated the effect of grain size on the corrosion behaviour of 304 stainless steel in

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an artificial saliva solution. They found that nanocrystalline structure was more resistant to corrosion than microcrystalline one. In artificial saliva, bulk

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nanocrystalline and amorphous Ni50.2Ti49.8 alloys showed significantly higher

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pitting corrosion potential than microcrystalline Ni50.2Ti49.8 alloy [13]. The study revealed that thicker and more protective passive films were formed with

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increase of nitrogen. High nitrogen content could improve the biocorrosion resistance

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for high nitrogen nickel-free stainless steels [14]. The electrochemical nitridation was used to modify the surface of stainless steel in a nitrate-bearing solution at room temperature using the electrochemical method [15]. The nitrided AISI446 stainless steel obtained by an electrochemical nitridation technique showed excellent corrosion resistance in simulated PEMFC environments [16]. To the best of our knowledge, however, the electrochemical nitridation technique has not ever been evaluated in nanocrystalline 316L stainless steels in artificial saliva environment. Therefore, first, the nanocrystalline 316L stainless steel was obtained by cryogenic cold rolling at liquid-nitrogen temperature. Then, the effects of nanocrystalline, fluoride ion and electrochemical nitridation on corrosion resistance of 316L stainless steel in artificial

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saliva environment were investigated.

2. Experimental 2.1. Material preparation and characterization

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The AISI 316L stainless steel with chemical composition (wt.%) C 0.025, Cr 17.01, Ni 12.03, Mn 1.40, P 0.028, S 0.003, Si 0.40, Mo 2.05 and balance Fe was

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chosen. The as-received samples were annealed at 1050 °C for 1 h and

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water-quenched. Some samples were rolled to 97% thickness reduction at cryogenic temperature (cryogenic cold rolling was carried out after the sample was immersed in

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liquid-nitrogen for enough long time). Transmission electron microscope (TEM,

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JEM-2100F) was employed to study the microstructures of the samples.

2.2. Electrochemical nitridation, XPS analysis and electrochemical

d

test

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The samples were sealed in holders with acid resistant epoxy resin in order to

expose to the electrolyte a planar area of 1 cm2. All the samples were abraded with 1000, 2000 and 3000 grit silicon carbide paper and polished with 1.5 μm alumina powder. The polished samples were ultrasonically cleaned finally in acetone and ethanol. The electrochemical cell employed in this study was made of glass beaker with the three electrodes. A platinum sheet and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes, respectively. All potentials will be referred to the SCE. The electrochemical tests were performed using a CHI Instruments CHI660E electrochemical workstation (Chenhua instrument Co. Shanghai, China) controlled by a computer and software. The electrochemical

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nitridation of the stainless steel was carried out at room temperature in a solution of 0.1 M HNO3 + 0.5 M KNO3. In electrochemical nitridation process, the sample was stabilized at open circuit potential (OCP) for 5 min and then a cathodic potential of

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-0.7 VSCE was applied for 10 h. After electrochemical nitridation, samples were washed with deionized water, rinsed with acetone. The surface compositions of two

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electrochemically nitrided 316L stainless steels were measured by XPS. The XPS

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experiments were performed using PHI Quantera SXM (ULVAC-PHI, INC). Photoelectron emission was excited by monochromatic Al Kα radiation. The vacuum

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of the specimen chamber was 6.7×10-8 Pa. The C 1s peak from adventitious carbon at

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284.8 eV was used as a reference to correct the charging shifts. The electrochemical tests of the stainless steels were conducted at 37 ◦C in

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artificial saliva solution. The composition of artificial saliva (g/L) contained NaCl 0.4,

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KCl 0.4, CaCl2·2H2O 0.795, NaH2PO4·2H2O 0.78, Na2S·9H2O 0.005, CO(NH2)2 1. Moreover, the artificial saliva with NaF (1 g/L) was also used to study the effect of fluoride ion on the corrosion resistance. In electrochemical impedance spectroscopy (EIS) measurement, the passive film was formed at 0.1 VSCE for 1 h in the solution, then a sinusoidal voltage perturbation of 5 mV and applied frequency (f) ranging from100 kHz to 0.01 Hz were used. Based on Mott-Schottky theory [17], the space charge capacitance of the n-type and p-type semiconductor is given by equation (1) and (2), respectively

-2 C -2 = C H-2 + C SC =

2 kT (E - E fb ) ε S ε0 qN D e

(1)

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-2 C -2 = CH-2 +CSC =

-2 kT (E - E fb ) εS ε0 qN A e

(2)

where ε0 is the vacuum permittivity (8.854×10-12 F m-1), εs is the dielectric constant of

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the sample, e is the electron charge (1.6×10-19 C), k is the Boltzmann constant (1.38×10-23 J K-1), ND and NA is the donor and acceptor concentration, respectively. T

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is the absolute temperature and Efb is the flat-band potential. For p-type

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semiconductor, C-2 versus E should be linear with a negative slope which is inversely proportional to the acceptor concentration. On the other hand, n-type semiconductor

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yields a positive slope which is inversely proportional to the donor concentration. The

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dielectric constant εs is assumed as 12 for the passive films on stainless steels [17]. The 50 mV/s in the Mott–Schottky test is selected and this sweeping rate is fast

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3. Results and discussion

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enough to satisfy the assumption of “frozen-in defect structure” in passive film [18].

The solution annealed 316L stainless steel has austenitic phase in Fig. 1a. The

microstructure of solution annealed sample is consisted of nearly equiaxed grains with an average grain size of 40 μm. After the cryogenic cold rolling, the austenite stainless steel is changed completely to α'-martensite phase in Fig. 1b. The TEM micrograph in Fig.1b shows the nanocrystalline α'-martensite phase with a high density of dislocations. The dislocation slip could be the main mechanism of grain refinement of the 316L stainless steels [19]. The high degree of cold reduction resulted in predominantly dislocation cell-type α'-martensite in Fe–16Cr–10Ni alloy [20]. The lath-type α'-martensite is formed with the increasing of cold rolling level, then

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dislocation-cell-type α'-martensite occurs with further higher cold reduction. The dislocation-cell type α'-martensite produced a ring diffraction pattern in inset of Fig.1b. The dislocation cell type α'-martensite contains higher dislocation density

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compared with lath-type α'-martensite. The dislocation cell is expected to provide a high density of heterogeneous nucleation sites [21] and promotes to form

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nanocrystalline stain-induced α'-martensite. The corrosion resistance of stainless steel

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is related to chemical composition and its microstructure. It is worthwhile to note that the coarse grained austenite phase and nanocrystalline α'-martensite do not have a

an

preferred orientation from diffraction pattern of TEM and XRD (not shown here).

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Therefore, the effect of orientation on corrosion resistance can be excluded in present research. Fig. 1c shows the surface XPS N1s and Mo3p3/2 spectra for

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electrochemically nitrided coarse grained and nanocrystalline 316L stainless steels.

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The evaluation of possible species on the surface could be performed on the basis of well characterized standards from Handbook of X-ray photoelectron spectroscopy [22]. The peak at 399.8 eV and 397 eV is attributed to NH3 and nitrides, respectively. Therefore, the CrN or Cr2N could be formed. The grain refinement of 316L stainless steels significantly inhibits form of NH3 during room temperature electrochemical nitridation. Therefore, grain refinement of 316L stainless steels promotes to form more nitrogen chromium compounds. Fig. 2 shows the linear polarization plots of solution annealed (SA) and cryogenic cold rolling (CR) 316L stainless steels in the standard artificial saliva. Two samples have distinct passive regions. The average corrosion potentials estimated from the

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linear polarization curves are −0.45 VSCE and −0.38 VSCE for SA and CR samples, respectively. Near the breakdown potential, the polarization curve showed significant fluctuating for SA sample, suggesting that competition process between the forming

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and breakdown of passive film in the solution. An increase in current density with increasing potential can occur if the increase in potential is not accompanied by a

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corresponding thickening of the passive film [23].

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Moreover, the CR sample exhibits lower passivated current density than SA sample in the solution. The lower current density value indicates a thicker or compact

an

passive film which can provide effective corrosion resistance for CR sample in the

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solution [24]. The results of the linear polarization plots obtained in the standard artificial saliva indicates that the differences between the SA and CR samples are

d

significant. This can be attributed to the fact that the passive films formed on the

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surface of two samples are different.

Generally, the EIS test is carried out to determine the stability of the passive film.

In this plot, the real impedance is plotted vs. the imaginary impedance at each frequency. As the frequency increases, the impedance of the capacitor decreases [25, 26]. For this purpose, the impedance spectra were measured at 0.1 VSCE after potentiostatic test in the standard artificial saliva at 37 °C. Typical Nyquist and Bode plots are illustrated in Fig. 3a and b, respectively. The diameter of the semicircle of Nyquist plots indicates corrosion resistance. In Fig.3a, the diameter of the semicircle of Nyquist plots in CR sample is larger than that in SA sample in the standard artificial saliva at 37 °C. Moreover, the Bode plots in Fig. 3b depict one time constant

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at middle frequencies for two samples, however, the maxima of the phase angle of CR sample is higher than that of SA sample, which indicates that the passive film in the former is closer to ideal capacitor behaviour.

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The negative and positive slopes in the Mott–Schottky plots in Fig.4a indicate the p-type semiconducting and n-type semiconducting of the passive films, respectively.

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The inner Cr enriched passive film which behaves as a p-type semiconductor should

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be formed [27], while outer Fe enriched passive film which behaves as n-type semiconductor properties could be attributed to the presence of compounds, such as

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Fe2O3 [28]. The donor and acceptor concentrations in the passive film calculated from

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Mott-Schottky plots are presented in Fig. 4b. The values of donor and acceptor concentrations are all of the order of 1020 to 1021 cm−3 which agrees well with those

d

reported for austenitic stainless steels. Comparing with SA sample, the donor and

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acceptor concentrations in CR sample significantly decrease. In addition, the donor concentration is more than the acceptor concentration for two samples in the solution. Sikora et al. [29, 30] reported the point defect model (PDM) and assumed that oxygen vacancies and cation interstitials in passive film imparted n-type semiconductor properties, while cation vacancies imparted p-type semiconductor properties. The difference between donor and acceptor concentrations in two samples are obvious. The chromium oxides are stable in acerbic solution, while iron oxides are easily dissolved in the acidic solution, therefore, fewer chromium vacancies in the inner layer and more oxygen vacancies in outer layer are formed in Fig. 4b. However, nanocrystalline α'-martensite significantly reduces the doping concentration of passive

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film on its surface. The decrease of the doping concentration in passive film means the restraint of electron transfer, inhibition of electrochemical reaction and slow-down of passive film dissolving. The results of Mott-Schottky plots in Fig.4 further support

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results of potentiodynamic polarization curves in Fig. 2 and EIS in Fig. 3. According to Wang and Li [31,32], comparing with coarse-grained 304 stainless steel, the

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corrosion resistance of nanocrystalline 304 stainless steel in NaCl solution was also

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enhanced. The improved corrosion resistance of nanocrystalline α'-martensite may be attributed to the following two reasons. Firstly, more diffusion paths due to grain

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refinement facilitate diffusion of element and promote to form thicker passive film,

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which improve corrosion resistance of nanocrystallised stainless steel [33]. Secondly, diffusion rate of chromium along dislocations in α'-martensite phase is faster than that

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in austenite phase [34]. The faster diffusion rate of chromium promotes it to react with

Ac ce pt e

oxygen to form oxide containing more chromium, which improves corrosion resistance of the stainless steel.

Fig. 5 shows the linear polarization plots of SA and CR 316L stainless steels in the

standard artificial saliva containing fluorine ion. Comparing with samples in the standard artificial saliva in Fig. 2, the passive regions shorten narrows and breakdown potential decreases significantly, although the corrosion potential increases. Moreover, the passivation current density increases significantly due to more fluorine ions. The lower breakdown potential and higher passivation current density suggest that corrosion resistance of the passive film decreases due to more fluorine ions. Moreover, the CR sample exhibited lower passivated current density and higher breakdown

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potential than SA sample in the artificial saliva containing fluorine ion. These results indicate that the corrosion resistance of 316L stainless steel is also improved due to nanocrystalline obtained by cryogenic cold rolling in the standard artificial saliva

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containing fluorine ion. Typical Nyquist and Bode plots of 316L stainless steels after being passivated at

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0.1 VSCE for 1 h in the standard artificial saliva containing fluoride ion are illustrated

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in Fig. 6a and b, respectively. In Fig.6a, the diameter of the semicircle of Nyquist plots in CR sample is also larger than that in SA sample in the solution environment.

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The Bode plots in Fig. 6b also show one time constant for two samples. Comparing

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with the samples passivated in artificial saliva without fluorine ion in Fig. 3a, the diameter of the semicircle of Nyquist plots of the samples in artificial saliva with

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fluorine ion significantly decreases in Fig.6a, especially for SA sample. This shows

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that the fluoride ion in artificial saliva deteriorates significantly corrosion resistance of 316L stainless steel [35].

The Mott–Schottky plots in Fig.7a indicate the p-type semiconducting and n-type semiconducting of the passive films in the standard artificial saliva containing fluoride ion. The donor and acceptor concentrations in the passive film calculated from Mott-Schottky plots are presented in Fig. 7b. Comparing with SA sample, The donor and acceptor concentrations in CR sample reduce. In addition, the donor concentration is lower than acceptor concentration for two samples in the solution. The result is opposite to situation in Fig. 4b. Therefore, the adding fluoride ion to artificial saliva significantly changes semiconductor characteristic of the passive film

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formed on the surface of 316L stainless steel. According to PDM [36], fluoride ions could be incorporated in a passive film by occupying oxygen vacancies. The absorbed fluoride ions would fill oxygen vacancies

vacancy: oxygen vacancy pairs by the following reaction:

χ 2

V••

(3)

O

cr

'

Null = VMχ +

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and the system responded to the loss of oxygen vacancies by generating cation

where VMx and V.. represented cation vacancy and oxygen vacancy, respectively. '

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o

The newly generated oxygen vacancies could in turn reacted with additional fluoride

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ions at the film/solution interface to generate more oxygen and cation vacancies.

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Therefore, the absorption of fluoride ions and cation vacancy generation were autocatalytic. More oxygen vacancies in the passive film after adding more fluoride

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ions to artificial saliva in Fig. 7b will facilitate the forming of more cation vacancies,

Ac ce pt e

(i.e., acceptors) in the passive film. An increase in the generation rate of cation vacancies will promote the breakdown of passive film and deteriorate the corrosion resistance of the stainless steels. The high acceptor or donor concentrations will result in high the passive current density which facilitates the dissolving of passive film [37, 38].

Fig. 8 shows anodic polarization curves of electrochemically nitrided SA and CR

samples in the artificial saliva containing fluorine ion. The results of potentiodynamic test show that electrochemically nitrided SA sample has similar corrosion potential with electrochemically nitrided CR sample, However, the latter has lower passive current than the former in the artificial saliva containing fluorine ion. Moreover, the

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passive range of polarization curves in electrochemically nitrided CR sample is wider than that in electrochemically nitrided SA sample. It is obviously seen that electrochemically nitrided CR sample has higher corrosion resistance than

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electrochemically nitrided SA sample in the solution. Typical Nyquist and Bode plots of electrochemically nitrided SA and CR 316L

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stainless steels after being passivated at 0.1 VSCE for 1 h in the standard artificial

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saliva containing fluorine ion are illustrated in Fig. 9a and b, respectively. In Fig.9a, the diameter of the semicircle of Nyquist plot in electrochemically nitrided CR sample

an

is larger than that in electrochemically nitrided SA sample in the environment. These

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data of EIS test in Fig.9a further support the results from potentiodynamic polarization curves in Fig.8. The Bode plots in Fig. 9b also show one time constant

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for two samples. Moreover, comparing Fig.6a with Fig.9a, it can be found that

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electrochemical nitridation improved the corrosion resistance of the 316L stainless steels in artificial saliva with fluorine ion, especially for CR sample. The Mott–Schottky plots of the passive films formed on the surface of

electrochemically nitrided SA and CR 316L stainless steels in artificial saliva containing fluoride ion are showed in Fig. 10a. They indicate the p-type semiconducting and n-type semiconducting of the passive films for electrochemically nitrided SA and CR 316L stainless steels in the artificial saliva containing fluoride ion. The donor and acceptor concentrations in the passive film calculated from Mott-Schottky plots are presented in Fig. 10b. The donor concentration is lower than the acceptor concentration for two samples in the solution. Comparing with

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electrochemically nitrided SA sample, the donor and acceptor concentrations in passive film for electrochemically nitrided CR sample significantly decrease. Comparing Fig.7b with Fig.10b, it can be found that electrochemical nitridation

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decreases donor and acceptor concentrations in passive film on 316L stainless steels in artificial saliva with fluorine ion.

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Comparing with 304 stainless steel, more molybdenum in 316L stainless steel is

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used to improve its corrosion resistance. Olsson et al. [39] thought that molybdenum did not significantly change the composition of the passive film of stainless steel. The

an

molybdenum was believed to have a role in the de-protonation of the hydroxide by

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acting as an electron acceptor, which would help to create oxygen abundance in the inner regions of the passive layer and help to form more protecting CrO3/Cr2O3 oxides.

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Moreover, they proposed one possible mechanism of synergism between molybdenum

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and nitrogen in the duplex stainless steel. In situ electrochemical techniques showed that the nitrogen was incorporated in the form of (Fe, Cr)-nitrides in the passive film on Fe–20Cr alloy and Cr was enriched in the passive film on the alloy with more nitrogen [40]. Shen et al.[41] also found a decrease in the donor and acceptor densities with increasing of nitrogen content in 316LN stainless steels. Lee et al. [42] also found that more nitrogen in stainless steel decreased the donor densities and the number of metastable pits, while the absorption of chloride ions on the passive film had the opposite effect. Alloying austenitic stainless steel with nitrogen increased its microstructure homogeneity and decreased the concentration of charge carriers, which beneficially affected the protecting and electronic properties of the passive film on its

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surface [43]. The Raman spectroscopy technique was used to further analyze compositions of passive films on the samples. The passive film is very complex and its analysis is

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rather difficult using Raman Spectroscopy, since different bonds may have the same frequency, even though their atoms and structures are completely different. However,

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Raman spectra of passive films in steels have been investigated. Fig. 11 shows the

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Raman spectra of the passive films in electrochemically nitrided SA and CR 316L stainless steels in artificial saliva containing fluoride ion. The broad peak near 615

an

cm-1 indicates the bending vibration of hematite, while the peak at 664 cm-1 originates

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from magnetite [44]. The defects or dopants in magnetite are much more than those in hematite, therefore, it can be concluded that the nitrided CR sample with more

d

hematite will have a relatively higher corrosion resistance than nitrided SA sample

Ac ce pt e

with a relatively less hematite [45].

4.Conclusions

The effects of grain refinement, martensitic transformation of 316L stainless steel

in cryogenic temperature and electrochemical nitridation on its corrosion resistance in artificial saliva environment in the presence and absence of fluoride ions were investigated. The main conclusions were as follows: 1. The nanocrystalline α'-martensite improved corrosion resistance of 316L stainless steel in a typical artificial saliva environment. 2. Comparing with nitrided coarse grained 316L stainless steel, corrosion resistance of the nitrided nanocrystalline 316L stainless steel was also improved significantly, even

Page 15 of 34

in artificial saliva solution containing fluoride ion. 3. The present study showed that the cryogenic cold rolling and electrochemical nitridation improved corrosion resistance of 316L stainless steel for the dental

ip t

application, especially in artificial saliva solution containing fluoride ion. 4. The adding fluoride ion to artificial saliva significantly changed semiconductor

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characteristic of the passive films formed on the surface of 316L stainless steel.

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5. The nitrided cold rolled 316L stainless steel with more hematite had a relatively higher corrosion resistance than nitrided solution annealed 316L stainless steel.

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Acknowledgments

China (Grant No. 91326203)

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References

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This work was financially supported by National Natural Science Foundation of

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ip t

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carbonate/bicarbonate solution. Electrochim. Acta. 2014;117:351–59. [24] S. Fajardo, D.M. Bastidas, M. Criado, J.M. Bastidas. Electrochemical study on the corrosion behaviour of a new low-nickel stainless steel in carbonated alkaline solution in the presence of

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austenitic stainless steel alloyed with nitrogen in an acid solution. Corros. Sci.2011;53 (6): 2176–83. [44] T.K. Yeh, Y.C. Chien, B.Y. Wang, C.H. Tsai. Electrochemical characteristics of zirconium

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formed thin iron oxide films.

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[45] J. Wielant, V. Goossens, R. Hausbrand, H.E. Terryn. Electronic properties of thermally

Figure captions

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Fig.1. The microstructures of (a) solution annealed (SA) and (b) cryogenic cold rolling (CR) 316L

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stainless steels, respectively. (c) XPS spectra of N1s and Mo3p3/2 for electrochemically nitrided SA and CR 316L stainless steels.

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artificial saliva at 37 °C.

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Fig. 2. The polarization plots of SA and cryogenic CR 316L stainless steels in the standard

Fig. 3 (a) The Nyquist plots and (b) Bode plots for SA and CR 316L stainless steels after being passivated at 0.1 VSCE for 1 h in the standard artificial saliva at 37 °C. Fig. 4 (a) The Mott-Schottky plots for SA and CR 316L stainless steels after being passivated at 0.1 VSCE for 1 h in the standard artificial saliva at 37 °C. (b) The donor and acceptor concentrations in two samples.

Fig. 5. The linear polarization plots of SA and CR 316L stainless steels in the standard artificial saliva containing NaF (1g/L) at 37 °C. Fig. 6 (a) The Nyquist plots and (b) Bode plots for SA and CR 316L stainless steels after being passivated at 0.1 VSCE for 1 h in the standard artificial saliva containing (1g/L) NaF at 37 °C.

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Fig. 7 (a) The Mott-Schottky plots for SA and CR 316L stainless steels after being passivated at 0.1 VSCE for 1 h in the standard artificial saliva containing (1g/L) NaF at 37 °C. (b) The donor and acceptor concentrations in two samples.

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Fig. 8. The polarization plots of electrochemically nitrided SA and CR 316L stainless steels in the standard artificial saliva containing NaF (1g/L) at 37 °C.

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Fig. 9 (a) The Nyquist plots and (b) Bode plots for electrochemically nitrided SA and CR 316L

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stainless steels after being passivated at 0.1 VSCE for 1 h in the standard artificial saliva containing (1g/L) NaF at 37 °C.

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Fig. 10 (a) The The Mott-Schottky plots for electrochemically nitrided SA and CR 316L stainless

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steels after being passivated at 0.1 VSCE for 1 h in the standard artificial saliva containing NaF (1g/L) at 37 °C. (b) The donor and acceptor concentrations in two samples.

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Fig. 11. The Raman spectra for electrochemically nitrided SA and CR 316L stainless steels after

°C.

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being passivated at 0.1 VSCE for 10 h in the standard artificial saliva containing (1g/L) NaF at 37

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*Highlights (for review)

1. Nanocrystalline α'-martensite improved corrosion resistance of stainless steel in artificial saliva. 2. The cryogenic cold rolling and electrochemical nitridation improved corrosion resistance of 316L stainless steel for the dental application. 3. Adding fluoride ion to saliva changed semiconductor characteristic of passive film on 316L

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stainless steel.

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