Corrosion and mechanical properties of duplex-treated 301 stainless steel

Corrosion and mechanical properties of duplex-treated 301 stainless steel

Surface & Coatings Technology 205 (2010) 1557–1563 Contents lists available at ScienceDirect Surface & Coatings Technology j o u r n a l h o m e p a...

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Surface & Coatings Technology 205 (2010) 1557–1563

Contents lists available at ScienceDirect

Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Corrosion and mechanical properties of duplex-treated 301 stainless steel M. Azzi, M. Benkahoul, J.E. Klemberg-Sapieha ⁎, L. Martinu Department of Engineering Physics, Ecole Polytechnique, C.P. 6079, Succ. Centre Ville, Montreal, Quebec, H3C 3A7, Canada

a r t i c l e

i n f o

Available online 7 September 2010 Keywords: Duplex treatment Plasma nitriding Cr–Si–N Magnetron sputtering Corrosion EIS

a b s t r a c t Plasma nitriding is a widely used technique for increasing the surface hardness of stainless steels, and consequently, for improving their tribological properties. It is also used to create an interface between soft stainless steel substrates and hard coatings to improve adhesion. This paper reports on the mechanical and corrosion properties of AISI301 stainless steel (SS) after a duplex treatment consisting of plasma nitriding followed by deposition of Cr bond coat and CrSiN top layer by magnetron sputtering. Mechanical properties of the deposited films, such as hardness (H) and reduced Young's modulus (Er), were measured using depthsensing indentation. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were carried out to evaluate resistance to localized and to general corrosion, respectively. The corrosion behavior has been correlated with the microstructure and composition of the surface layers, determined by complementary characterization techniques, including XRD, SEM, and EDS. The CrSiN layers exhibited an H value of 24 GPa, whereas the nitrided layer was shown to present a gradual increase of H from 5 GPa (in the nitrogen-free SS matrix) to almost 14 GPa at the surface. The electrochemical measurements showed that the nitriding temperature is a critical parameter for defining the corrosion properties of the duplex-treated SS. At a relatively high temperature (723 K), the nitrided layer exhibited poor corrosion resistance due to the precipitation of chromium nitride compounds and the depletion of Cr in the iron matrix. This, in turn, leads to poor corrosion performance of the duplex-treated SS since pores and defects in the CrSiN film were potential sites for pitting. At relatively low nitriding temperature (573 K), the nitrided interface exhibited excellent corrosion resistance due to the formation of a compound-free diffusion layer. This is found to favor passivation of the material at the electrode/electrolyte interface of the duplex-treated SS. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Stainless steels (SS) are generally chosen for their excellent corrosion resistance. This is due to the thin oxide layer which forms naturally on the surface, known as the passive layer. This layer is composed of a mixture of Fe-oxide and Cr-oxide. The latter is responsible for the excellent corrosion properties of the material [1]. Without Cr in the solid solution of the SS matrix, the material would have poor corrosion properties, very similar to those of plain carbon steels. It has been suggested that a Cr content of 11 wt.% in steel is the minimum requirement to obtain stainless steel [1]. Pitting, one of the most severe forms of localized corrosion, is one of the major concerns related to the use of stainless steels [2]. A number of research groups investigated different techniques for improving the pitting resistance of SS. Shahryari et al. [3] successfully improved the pitting resistance of biomedical grade 316LVM stainless steel by cyclic potentiodynamic polarization in sodium nitrate. The authors explained this improvement by the enrichment of the outer part of the oxide layer with hexavalent chromium Cr(VI). However,

⁎ Corresponding author. Tel.: + 1 514 340 5747; fax: + 1 514 340 3218. E-mail address: [email protected] (J.E. Klemberg-Sapieha). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.155

once scratched, the reformed passive layer will have the initial, low pitting resistance. Therefore, these modified materials should be carefully handled. Weng et al. [4] demonstrated that ion implantation of 304 SS surface by molybdenum, titanium, yttrium and nitrogen improved their corrosion resistance. Stainless steels have, in general, low hardness and wear resistance, which restrict their use to applications where no tribological processes are present. Plasma nitriding is one of the methods used to improve their mechanical and tribological properties [5,6]. It produces a very hard nitrided layer, strongly adherent to the substrate. The structure and mechanical properties of nitrided steels has been extensively studied over the past three decades [6–11]. Mingolo et al. [6] showed that nitriding 316L SS at 673 K results in the formation of a very hard (14 GPa), supersaturated, metastable phase, called expanded austenite, γN. They described this phase by a special triclinic (t) crystalline structure, with a distortion of lattice angle due to the high nitrogen content (45 at.%). At higher nitriding temperatures, they reported less nitrogen content in the solid solution due to the precipitation of iron and chromium nitrides. Borgioli et al. [7–9] studied the effect of temperature, pressure, and time of nitriding treatment on the characteristics of 316 L SS. The nitriding process has been also used to form bond coats between hard coatings and soft metallic substrates. Snyders et al. [12]


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showed that a simple nitriding treatment significantly improved the adhesion of the Diamond Like Carbon (DLC) coatings on 316L SS substrates. Similar findings were reported by Chen [13] and Podgornik [14]. Another method for improving the mechanical and tribological characteristics of SS surface is the application of hard coatings. Hard chrome plating has been the most widely used technique for corrosion and wear applications for almost 90 years. It consists of depositing a thick layer of Cr onto metal components such as hydraulic actuators or shafts, using an electrolytic bath containing Cr(VI), which is known to be a human carcinogen. In 2007, a Restriction of Hazardous Substances (RoHS) Directive was issued banning several toxic substances in Europe, including Cr(VI). This stimulated considerable activity aimed at systematic replacement of wet technologies with high performance dry coating using “clean” methods. In this context, the vacuum-based coating and spray coating techniques have received considerable attention as environmentallyfriendly and cost-effective approaches to the replacement of hard chrome [15–17]. Among coating materials which have been investigated are metals and alloys (Cr, CrAl) [18], nitrides (TiN, CrN) [19,20], carbides (TiC, WC, CrC) [21], and nano-composites (TiSiCN–SiCN, WCCo, CrC–CrNi) [22–25]. Here, we investigate the mechanical and corrosion properties of duplex-treated 301 SS. Duplex treatment consists of plasma nitriding followed by deposition of a Cr bond coat and a hard CrSiN top layer. The effect of the plasma nitriding process on the mechanical and electrochemical properties of the coating/substrate system is its principal focus. 2. Experimental 2.1. Duplex treatment process The duplex treatment process was carried out in a sputtering system equipped with two magnetrons holding Cr and Si targets and facing the rotating substrate holder. The targets were connected to DC pulse generators (Advanced Energy Pinnacle II power supply), while the substrate holder was capacitively connected to a radio-frequency (RF, 13.56 MHz) power supply. The RF induced negative bias voltage, Vb, was controlled by adjusting the level of the RF power. 301 austenitic stainless steel (SS) was used as substrate material with the following nominal wt.% composition: C b 0.15, Cr: 16–18, Ni: 6–8, Mn b 2, Si b 1, P b 0.045 and Fe balance. Substrates of square shape (25 mm × 25 mm × 1 mm) were first polished to obtain mirrorlike finish and then ultrasonically cleaned in acetone and isopropanol. Then, they were mounted on a sample holder, and the vacuum chamber was pumped to a base pressure of 1.3 × 10− 4 Pa. Prior to duplex treatment, the SS substrates were sputter cleaned in Ar plasma at 1 Pa for 10 min at Vb = −600 V. The nitriding process was carried out in a pure N2 plasma at 4.2 Pa for a duration of 4 h while Vb was held at −600 V (180 W R.F. power). Low temperature (LT) and high temperature (HT) nitriding treatments were performed at 573 K and 723 K, respectively. Subsequently, a 0.5 μm thick Cr bond coat was deposited in a pure Ar plasma at a pressure of 0.7 Pa, Vb = −200 V, and T = 523 K, by activating only the Cr target, using a pulse frequency (fp) of 300 kHz, a duty cycle (Dc) of 88%, and a target current (ICr) of 0.70 A. Finally, a 2 μm thick CrSiN (Si 2.3 at.%) top coat was deposited in a 1:1 N2/Ar atmosphere at a pressure of 1.2 Pa, using both targets (ICr = 0.7 A and ISi = 0.4 A) and the same fp, Dc, Vb, and T values as in the bond coat process. 2.2. Surface characterization X-ray diffraction (XRD) analysis was performed using a Rotaflex X-ray diffractometer with CuKα radiation (0.154 nm wavelength), under 40 kV voltage and 40 mA current. XRD patterns were acquired

in grazing incidence configuration (1.5° glancing angle) to obtain diffraction signal from only surface layers. The hardness (H) and reduced Young's modulus (Er) of the films were measured by depthsensing indentation using a tribo-indentor (Hysitron) equipped with a Berkovich pyramidal tip. The applied loads ranged between 1 and 10 mN. For each sample, H and Er were obtained from a minimum of 25 indentations for each load. Scanning electron microscopy (SEM) observations were made using a field emission scanning electron microscope (FESEM, Philips XL30) equipped with an energy dispersive spectrometer EDS for elemental analysis. Roughness of samples was determined by a Dektak 3030 profilometer. 2.3. Corrosion measurements Corrosion tests were performed in aerated and unstirred NaCl 1 wt.% aqueous solution (pH = 7) at room temperature using a threeelectrode cell type described elsewhere [26]. NaCl solution was used as stainless steels are known to be susceptible to pitting corrosion in chloride-containing solutions [2,3]. The cell consists of a Teflon container with a circular opening at the center of one vertical face, where the sample was mounted and exposed to the electrolyte. A copper strip provided electrical connection to the sample. In this setup, only a circular area of 10 mm diameter (0.78 cm2) of the specimen was exposed to the electrolyte. A Standard Calomel Electrode (SCE) and a graphite rod were used as reference and counter electrodes, respectively. A potentiostat Autolab PGSTAT302 (Echochemie) equipped with a frequency response analyzer was used for electrochemical measurements. Corrosion tests consisted of stabilizing the electrodes in the electrolyte for 1 h, during which the open corrosion potential (OCP) was monitored. Next, electrochemical impedance spectroscopy (EIS) was carried out at the OCP, in the frequency range of 5 × 10− 2 Hz–105 Hz, with a potential perturbation of 10 mV. The acquired impedance spectra were presented as Nyquist diagrams and interpreted in terms of equivalent electrical circuits. Then, potentiodynamic polarization was performed at a scan rate of 1 mV/s, starting 200 mV below the OCP, in the cathodic zone, to a potential at which an anodic current density of 1 mA/cm2 was reached, i.e. after pitting had been initiated [2]. The corrosion current density, io, was found using the Tafel interpolation technique. The breakdown potential, Eb, was identified on the polarization curve as the potential at which an irreversible increase in the current took place, as a result of the breakdown of the passive layer. The corrosion test was repeated 3 times for each sample to ensure reproducibility of the results. 3. Results and discussions In this section, we first characterize the nitrided SS in terms of microstructure, mechanical, and electrochemical properties, then we evaluate the performance of the duplex-treated SS. 3.1. Characterization of the nitrided SS 3.1.1. Microstructure and mechanical properties of the nitrided SS SEM plan view micrograph of the LT nitrided is SS presented in Fig. 1. The microstructural features of the SS such as grains and twins are clearly visible as if the surface is chemically etched. This resulted in a significant increase in the surface roughness (Ra); the Ra value for the LT nitrided SS was 87 nm as compared to 10 nm for the nontreated SS. The same, but to a lesser extent, was observed on the HT nitrided SS with Ra value of 39 nm. The difference in Ra will be shown to depend upon the microstructure of the nitrided zone. M. Rahman et al. [10] reported a similar observation on 316 L stainless steel substrates; they compared the microstructures of chemically etched surfaces (without nitriding) with nitrided surfaces (without chemical etching) and found similar microstructures.

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Fig. 1. SEM micrograph of LT nitrided SS.

Fig. 2a shows the EDS spectra acquired from the surface of nonnitrided and nitrided SS. The formation of a nitrided layer at the SS surface is confirmed by the Nitrogen Kα peak at 40 eV. EDS was also performed on the cross-section of the nitrided samples to measure the thickness of the nitrided layer. Nitrogen Kα peak intensity vs. depth is shown in Fig. 2b. The intensity decreases with depth indicating lower nitrogen content. The nitriding depth for the LT nitrided sample is


found to be almost 6 micrometers, while it is only 4 μm for the HT nitrided one. XRD patterns acquired from LT and HT nitrided samples are shown in Fig. 3. The pattern of bare SS is shown for comparison. The main phase in untreated SS is fcc austenite (γ) with 0.356 nm crystal lattice parameter. After LT nitriding, all γ peaks shifted towards lower angles indicating an expansion in the austenitic matrix (increase in crystallographic d-spacing). This is due to nitrogen diffusion and the formation of interstitial solid solution phase called “expanded austenite, γN” or “S phase”. After HT nitriding, the XRD shows that CrN phase has precipitated at the nitrided zone, and that the fcc austenitic structure has been replaced by the bcc ferritic one (Fe α) with 0.286 nm crystal lattice parameter. Similar results were obtained by Mingolo [6] and Borgoli [7] who found that only diffusion phase is formed at low temperature, while a compound layer is formed at high temperature. Hardness measurements were carried out on the surface and cross-section of nitrided SS using nano-indentation. Results obtained from surface measurements show a three-fold increase in surface hardness of SS after nitriding; H values of HT and LT nitrided samples were 15 GPa compared to 4.8 GPa for the untreated sample. This increase in surface hardness was shown to improve significantly the tribological properties of SS [5]. Hardness measurements obtained from cross-section, in terms of H vs. depth, are presented in Fig. 4. For both LT and HT nitrided samples, H values were 14 GPa near the surface, slightly lower than the value measured at the surface. This difference could be attributed to edge effects in cross-section measurement. For the LT sample, H starts to decay at a depth of 6 μm and reaches 6 GPa at approximately 8 μm, while for the HT sample, a more rapid decay in H was observed, indicating lower nitriding depth. These mechanical measurements are in good agreement with EDS line scan results on nitriding thickness. The lower thickness value for the HT nitrided layer is not what we expect if it is controlled by the N diffusion coefficient in SS: D = Do * exp ð−E = RT Þ where D is the diffusion coefficient (m2/s), Do is the diffusion coefficient at infinite temperature, E is the activation energy for diffusion (J kg− 1), T is the temperature (K), and R is the gas constant (J T− 1 kg− 1). This discrepancy is explained by the formation of new phases at high temperatures such as Fe α and CrN, which retards the diffusion of nitrogen. We attribute the increase of Ra to the expansion of the SS crystalline cell after plasma nitriding. This expansion generates

Fig. 2. (a) EDS spectra acquired from untreated and nitrided SS surfaces, (b) EDS line scan on cross-section of LT and HT nitrided SS. Inset to (b): Cross-section micrograph showing the line scan location.

Fig. 3. XRD patterns of untreated and nitrided SS at low (LT) and high (HT) temperatures.


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Fig. 4. Hardness profiles of LT and HT nitrided SS.

compressive stresses in the nitrided layer which are partly accommodated by the formation of protrusions at the grain boundaries causing an etching-like effect, the extent of which was found to be less important in HT nitrided SS compared to LT nitrided SS. This difference is due to the precipitation of the CrN compound out of the SS matrix in the HT nitrided sample, leading to less distortion of the crystalline structure, contributing to stress relief.

3.1.2. Electrochemical characterization of nitrided SS The corrosion properties of nitrided SS were evaluated through two electrochemical techniques: potentiodynamic polarization and EIS. Fig. 5 shows the polarization curves of the untreated and nitrided SS. The resulting corrosion potential, Ec, corrosion current density, io, breakdown potential, Eb, and passive current density, ip, measured at 200 mV, are presented in Table 1. The measured Eb of untreated SS is in agreement with the literature [2,27,28], with the onset of pitting at approximately 330 mV. SEM images of the exposed area after polarization show large pits close to the O-ring area (see Fig. 6a and b) indicating high susceptibility to crevice corrosion. The LT nitrided sample exhibited improved pitting resistance. This is demonstrated by the gradual increase of the current after pitting has been initiated, as compared to the steep increase in the case of untreated SS. SEM images of the exposed area after polarization (see Fig. 6c and d) shows

Fig. 5. Potentiodynamic polarization curves of untreated and nitrided SS in NaCl 1 wt.%.

less corrosion attack with very few pits located in the center of the exposed area and no susceptibility to crevice corrosion since the Oring area was intact. This could be attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation [29–31]. Another possible reason for corrosion enhancement is the presence of compressive stresses at the nitrided surface which act against the formation and growth of pits. The slight increase in the value of io, however, may be due to the increase in the surface roughness which results in a larger electrode/electrolyte contact area [32]. The HT nitrided sample exhibited very poor corrosion resistance with very low Ec, very high io and almost no passivation as pits were found to form at the open circuit potential condition. SEM images of the exposed area after polarization (Fig. 6e and f) shows that the entire exposed area was fully covered with pits. EIS was performed to characterize the electrochemical performance of the samples in the absence of polarization, i.e. at the open circuit potential condition. Fig. 7 shows the EIS spectra of the untreated and nitrided samples after 1 h immersion in NaCl 1 wt.%. The spectra are presented in Nyquist diagram, in which the impedance, Z(f), is presented as a complex number: Z ðf Þ = Reðf Þ + iTImðf Þ where f is the potential perturbation frequency (Hz), i is the imaginary unit, Re(f) and Im(f) are the real and imaginary parts of the impedance (Ω cm2). It should be mentioned that high impedance magnitude results in low current amplitude. HT nitriding significantly reduced the impedance as compared to untreated SS, while LT nitriding is shown to provide impedance values slightly lower than untreated SS. In order to derive quantitative results, the EIS spectra were modeled using the equivalent electrical circuit shown in the inset of Fig. 7. It consists of a resistor Rs which corresponds to the ionic resistance of the solution between the working electrode and the reference electrode, a capacitor Cdl which represents the charge buildup at the working electrode/electrolyte interface, and a resistor Rp representing the polarization resistance, which is inversely proportional to the corrosion rate io. To obtain a better fit, the capacitance Cdl is replaced by the constant phase element (CPE) with the following impedance equation: n

ZCPE = 1 = ½Yo ðjωÞ  where Yo is the CPE parameter (Fsecn − 1 cm− 2), ω is the angular frequency (rd/s), and n is the CPE exponent which represents the degree of deviation from a pure capacitor. For n = 1, the CPE is an ideal capacitor, while for n b 1, it is a non-ideal capacitor. This deviation has been shown to be related to surface roughness and inhomogeneity of the electrode [33,34]. The equivalent circuit parameters obtained through modeling are presented in Table 2. Rs values were very close for all tested samples since Rs represents the electrolyte resistance. The HT nitrided sample exhibited a very low Rp value indicating easy charge transfer at the electrode/electrolyte interface, and consequently poor corrosion properties. The LT nitrided sample had a much higher corrosion resistance represented by a high Rp value. The results of EIS are consistent with those obtained in polarization measurements, with high Rp values correlating with low io values. It is well known that stainless steels owe their excellent corrosion properties to the presence of Cr in solid solution with Fe [1]. The former significantly improves the properties of the passive thin oxide film formed at their surface. This, in turn, increases the corrosion resistance of SS. For Cr content of less than 11 wt.% in steel, the oxide layer would fail to protect the surface against chloride attack. During plasma nitriding, the precipitation of the CrN phase (at HT) depletes the surface of Cr in solid solution, and therefore reduces the content of Cr-oxide in

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Table 1 Corrosion potential, Ec, corrosion current density, io, breakdown potential, Eb, and passive current density, ip, of untreated and nitrided 301 SS. Sample

Corrosion Potential, Ec [mV] Corrosion current density, io [A/cm2] Breakdown potential, Eb [mV]

Bare 301 LT nitrided SS HT nitrided SS LT duplex-treated SS HT duplex-treated SS

− 50 − 24 − 397 − 164 − 304

1 × 10− 8 2 × 10− 8 8 × 10− 7 3 × 10− 8 4 × 10− 7

the passive layer, leading to poor corrosion resistance. In contrast, the formation of the expanded austenite γN (at LT), without the consumption of Cr, leads to the formation of an oxide layer similar to

330 320 − 350 N1000 mV Not possible to read on the polarization curve

Passive current density @ 200 mV, ip [A/cm2] 1.8 × 10− 8 2.2 × 10− 8 No passivation 3.6 × 10− 7 6.5 × 10− 5

that formed on the non-nitrided SS surface. Further, the formation of a diffusion layer increases the resistance to localized corrosion due to the compressive stresses at the surface, acting against pit formation.

Fig. 6. SEM micrographs of exposed areas after polarization test of untreated SS (a) and (b), LT nitrided SS (c) and (d), and HT nitrided SS (e) and (f).


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Fig. 7. Measured and fit EIS spectra of untreated and nitrided SS (LT and HT) after one hour immersion in NaCl 1 wt.%. Inset: Randles circuit used for EIS spectra modeling.

3.2. Characterization of the duplex-treated SS 3.2.1. Microstructure and mechanical properties of nitrided SS SEM images of LT duplex-treated SS (not presented here for brevity) show that the deposited layers (Cr and CrSiN) followed the morphology of the nitrided surface, with the austenitic structure being clearly visible after deposition. XRD analysis shows a single phase of bcc Cr in the bond coat and a single phase of fcc CrN in the top coat. This was confirmed by EDS measurements of almost 50 at.% of nitrogen, 2.3 at.% Si, and 47.7 at.% Cr. At this concentration, the Si atoms were found soluble in the CrN matrix in agreement with refs. 19 and 20. The CrSiN top coat had an H value of 24 GPa, while the Cr bond coat had a value of 12 GPa. 3.3. Electrochemical properties of duplex-treated SS Potentiodynamic polarization and EIS were performed on LT and HT duplex-treated SS. The corresponding polarization curves are shown in Fig. 8. The LT duplex-treated sample exhibited pit-free behavior; when the potential scan was reversed at approximately 1000 mV, the current was lower than the one measured in the forward scan, indicating an improvement in the passivation at the electrode/electrolyte interface when sample was exposed to high oxidizing condition (1000 mV). SEM images of the exposed area after the polarization test show no signs of pitting. The progressive increase in current is the result of the transition from trivalent to hexavalent state of Cr ion in the oxide layer [35]. For the HT duplex-treated SS, the ip value is two orders of magnitude higher, and the Ec is lower than that of LT sample (see Table 1), indicating a more electrochemically active surface. Further, the curve shows an irreversible current increase, indicating the formation of pits after polarization. Indeed, many pits were visible at the surface. The difference in electrochemical behavior is attributed to poor corrosion properties of the HT nitrided layer, since the Cr/CrSiN layers were deposited under the same condition. This demonstrates that the

Fig. 8. Potentiodynamic polarization curves of LT and HT duplex-treated SS in NaCl 1 wt.%.

corrosion properties of a coating/substrate system depend not only upon the top layer but also upon the electrochemical properties of the entire system. This is due to the fact that the liquid can infiltrate through pores and defects in the coatings and reach sub-coating layers. If these have a tendency to passivate (like the LT nitrided layer), the system will perform well and act passively. On the contrary, if one of the coating layers is susceptible to pitting (like the HT nitrided layer), the whole system may perform poorly in a corrosive environment. 4. Conclusion A duplex treatment consisting of plasma nitriding followed by deposition of 0.5 μm thick Cr bond layer and 2 μm thick CrSiN top coat, was applied to 301 SS using sputtering system. Following systematic microstructural, mechanical, and corrosion characterization, we found that: - Plasma nitriding resulted in the formation of a few-μm thick, hard nitrided layer with a three-fold increase in surface hardness H (15 GPa vs. 4.8 GPa) and a significant increase in surface roughness Ra. - The corrosion behavior of nitrided SS depend strongly on the nitriding temperature; at 723 K, the precipitation of CrN phase resulted in poor corrosion resistance, while at 573 K, the formation of a diffusion layer was found to confer better resistance to pitting. - The corrosion behavior of the duplex-treated SS is strongly affected by the corrosion characteristics of the nitrided interlayer. If this has the tendency to passivate, the coating system performs well and acts passively. On the contrary, if it is susceptible to pitting, the coating system will have poor corrosion resistance. Acknowledgements The authors wish to thank Mr. Francis Turcot for his technical assistance, Dr. Philippe Robin and Dr. Salim Hassani for helpful discussions. This work has been supported by NSERC and CRIAQ within the CRDPJ328038-05 project.

Table 2 Characteristics of the equivalent circuit derived from the EIS spectra modeling. Specimen 301 LT nitrided SS HT nitrided SS

Rs [Ω.cm2] (% error) 56 (0.8) 58 (0.8) 56 (1.1)

CPE (Yo) [Fsecn−1 cm− 2] (% error) −4

(1.0%) 0.22 × 10 0.35 × 10− 4 (1.1) 0.28 × 10− 3 (1.9)

CPE (n) (% error)

Rp [Ω cm2] (% error)


0.94 (0.3) 0.90 (0.3) 0.78 (0.8)

666 × 103 (14.0) 215 × 103 (8.1) 9 × 103 (5.5)

6.6 × 10− 2 9.8 × 10− 2 7.6 × 10− 2

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