CORROSION RESISTANCE OF PLASMA NITRIDED AND ...
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CORROSION RESISTANCE OF PLASMA NITRIDED AND NITROCARBURIZED
AISI 316L AUSTENITIC STAINLESS STEEL
F.A.P. Fernandes1*
, J. Gallego2, G.E. Totten
3, C.A. Picon
2, L.C. Casteletti
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1Department of Materials Engineering, São Carlos School of Engineering, University of São
Paulo, Av. Trabalhador Sãocarlense, n. 400, 13566-590, São Carlos, SP, Brazil. *e-mail:
[email protected] 2Engineering Faculty of Ilha Solteira, São Paulo State University, Av. Brasil, n. 56, 15385-
000, Ilha Solteira, SP, Brazil. 3Department of Mechanical and Materials Engineering, Portland State University, Post Office
box 751, 97207-0751, Portland, OR, USA.
Abstract: The usage of coatings in surface engineered components is increasing due to the
need for improved hardness, corrosion and wear resistance. Plasma nitriding and
nitrocarburizing of austenitic stainless steels can produce layers of expanded austenite (S-
phase). This interesting phase is supersaturated with respect to nitrogen and is characterized
by high hardness and wear resistance. In this study plasma nitriding and nitrocarburizing of
AISI 316L stainless steel were conducted at 400, 450 and 500°C. The plasma treated AISI
316L steel samples were characterized by optical microscopy, X-ray diffraction and corrosion
tests. Corrosion characterization was performed by potentiodynamic polarization in 3.5%
NaCl solution. After plasma treatment, it was observed that the layer thickness increases with
temperature. The treatments at 400°C produced homogenous and precipitate-free S-phase
layers while at 450 and 500°C X-ray diffraction indicates the presence of iron carbide and/or
chromium and iron nitrides. The potentiodynamic polarization curves show that corrosion
resistance is higher for the samples treated at 400°C relative to the untreated substrate. A
change in the dominant corrosion mechanism was also observed after nitriding or
nitrocarburizing from localized pitting corrosion to general corrosion.
Key words: Nitriding; Nitrocarburizing; X-ray diffraction; Corrosion.
1 INTRODUCTION
Surface coatings are one of the most versatile ways to improve the performance of
components with respect to wear and/or corrosion. Thermochemical treatments such as
nitriding, carburizing and nitrocarburizing at low temperatures are widely used surface
engineering technologies to improve surface hardness and wear resistance of stainless steels
without compromising their good corrosion resistance1-3
.
It is well known that when such treatments are performed at a temperature sufficiently
low, a nitrogen expanded austenite, or S-phase can be produced on the surface of an
austenitic stainless steel or other face-centered cubic (fcc) alloys4. This very promising
coating can only be achieved if the treatment temperature is lower than 500ºC3,4
. The S-phase
is a metastable phase with a supersaturation of nitrogen and/or carbon which remains in solid
solution.
It has been reported that nitrogen in solid solution as an alloying element promotes
passivity by widening the passive range in which pitting is less probable which improves
stress corrosion cracking and also enhances intergranular corrosion resistance 5-7
.
The aim of this study is to evaluate the influence of treatment temperature on the
morphology, microstructure, microhardness and corrosion resistance properties of the plasma
nitrided and nitrocarburized AISI 316L steel samples.
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2 MATERIALS AND METHODS AISI 316L austenitic stainless steel (ASS) samples 22mm in diameter and 3mm thick
of were cut and then prepared by conventional metallographic techniques to obtain a polished
surface. The chemical composition of the steel was (in wt%): C, 0.019; Mn, 1.47; Si, 0.401,
Cr, 16.26; Ni, 10.5; Mo, 2.02; N, 0.067; Cu, 0.47 and Fe, balance.
Prior to plasma treatment, the samples were cleaned by argon sputtering (on work
pressure and temperature of 50ºC less than the treatment temperature for 30 min) inside the
plasma chamber. Plasma nitriding (PN) and nitrocarburizing (PNC) were performed using the
dc method with the following gas mixtures: 80 vol. % H2 and 20 vol. % N2, for nitriding and
77 vol. % H2, 20 vol. % N2 and 3 vol. % CH4 for nitrocarburizing. The treatments were
performed at a pressure of 500Pa during 5h at temperatures of 400, 450 and 500ºC.
Optical microscopy analyses was performed on the cross-section of the samples using
a Zeiss microscope with the interference contrast technique on samples etched with
nitromuriatic acid. X-ray diffraction (XRD) patterns were obtained on the surface of the
samples using Geirgerflex Rigaku equipment with a scanning angle from 30 to 100°. The
tests were performed using copper radiation (Cu-Kα) and continuous scanning with a speed
of 2°.min-1
.
The electrochemical cell used to obtain the potentiodynamic polarization curves
utilized a saturated calomel (SCE) reference electrode and a platinum auxiliary electrode. The
electrolyte employed was a 3.5% aqueous NaCl solution. For monitoring the potential and
current, an Autolab model VGSTAT-302 potentiostat was employed. The polarization curves
of the nitrided and nitrocarburized samples were obtained with a scanning speed of 1mV.s-1
from -1.0 to 1.125V.
3 RESULTS AND DISCUSSION The optical micrographs of the cross-sections of the AISI 316L (ASS) samples which
were plasma nitrided and nitrocarburized at temperatures of 400, 450 and 500°C are shown in
Figure 1. For nitrided (Figs. 1a, 1b and 1c) and nitrocarburized (Figs. 1d, 1e, and 1f) samples,
the micrographs clearly show the austenitic matrix beneath each layer for all treatment
conditions.
Figure 1, Optical cross sections of plasma (a-c) nitrided and (d-f) nitrocarburized ASS
samples at (a, d) 400ºC, (b, e) 450ºC and (c, f) 500ºC.
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The treatments performed at 400 and 450°C (Figs. 1a, 1b, 1d and 1e) produced
homogeneous and precipitate-free layers under the optical microscope. These layers appear to
be bright and featureless and posses all of the characteristics of the nitrogen supersaturated S-
phase. It is also worth to noting that the layers produced at 500°C (Fig. 1c and 1f) yielded the
appearance of a dark region just above the S-phase. This region, according to the literature1-3
is indicative of S-phase decomposition and occurs due to chemical bonding between carbon
and/or nitrogen and the alloying elements of the material forming carbides and/or nitrides.
Figure 2 shows the XRD patterns of plasma nitrided and nitrocarburized ASS steel.
For comparison, the substrate diffraction pattern is shown for both PN (Fig. 3a) and PNC
(Fig. 3b). Narrow diffraction peaks are observed for the ASS substrate which are consistent to
the austenite phase (Fe-γ).
Figure 2, X-ray diffraction patterns of plasma (a) nitrided and (b) nitrocarburized ASS
samples at 400, 450 and 500ºC.
PN and PNC at 400ºC produced the Fe-γ (111) reflection and broadened peaks
dislocated to lower diffraction angles. These peaks were labeled as S1, S2...S5 and are an
intrinsic characteristic of the S-phase which confirms the presence of a homogeneous and
precipitate-free S-phase layer. The peaks related to the S-phase are always broadened because
of an enormous quantity of interstitial elements introduced on the surface of the sample
originating from a high defect density and residual stress.
The treatments performed at 450ºC also yielded the appearance of peaks shifted to
lower diffraction angles which correspond to the S-phase. Nevertheless, in addition to the S-
phase, the XRD reveals evidence of nitride precipitation such as CrN, Cr2N and Fe2N which
were not detected by optical microscopy. Increasing treatment temperature increases
mobility of chromium and iron due to chemical bonding with nitrogen.
At 500ºC distinct patterns are observed for PN and PNC. After PN and PNC at 500ºC,
it is estimated that the S-phase is decomposed increasing nitrogen and carbon compounds
depending on the treatment. The increased amount of these compounds enables their
observation under the optical microscope (Fig. 1c and 1f). Therefore, nitriding at 500ºC has
resulted in chromium (CrN, Cr2N) and iron (Fe2N, Fe3N and Fe4N) nitride precipitation and
for nitrocarburizing, iron carbide (Fe3C) is produced in addition to these nitrides. Thus,
morphological analysis from Fig. 1 are in agreement with XRD analysis.
In Figure 3, the potentiodynamic polarization curves obtained in 3,5% NaCl solution
for the plasma nitrided (Fig. 3a) and nitrocarburized (Fig. 3b) ASS samples are shown. For
both treatments, the curves obtained for the samples treated at 400, 450 and 500°C are
compared with the untreated material. All plasma treated specimens and the substrate
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exhibited a very similar cathodic region ranging from -1.00V to -0.25V. Moreover, the Tafel
region of the treated samples was also close to that obtained for the untreated steel resulting
in a slightly increased corrosion potential (Ecorr) for nitrided and nitrocarburized samples as
shown in Tab. 1.
Table 1 presents the quantitative electrochemical parameters collected from the
polarization curves in Fig. 3. It shows the corrosion potential (Ecorr), corrosion current (Icorr)
and the current density at a 1.2V potential (I1.2V) which means the current at end of the test at
the highest potential. The ASS substrate yielded a typical polarization curve with passivation
and pitting corrosion8. The breakdown of passivity occurs at about 300mV which leads to an
abrupt increase in current density reaching 32mA.cm-2
(Tab. 1).
The corrosion currents (Icorr) of the untreated and all plasma treated ASS samples are
of the same magnitude- about 10-8
A.cm-2
(Fig. 3 and Tab. 1). Examination of the currents of
the curves at a potential larger than 600mV reveals an increase with increasing treatment
temperature for both nitriding and nitrocarburizing treatments (Fig. 3).
Figure 3, Potentiodynamic polarization curves of plasma (a) nitrided and (b) nitrocarburized
ASS samples at 400, 450 and 500°C.
Samples nitrided and nitrocarburized at 400°C yielded the lowest current densities
after the Tafel region which resulted in similar surfaces after polarization. Inspection of the
surfaces of these samples reveals a clean and smooth surface without any corrosion damage.
The polarization curves obtained for the samples treated at 450°C exhibited a sudden
increase in current density after the Tafel region until about 380mV where it stabilizes and
reaches a value close to 1mA.cm-2
at the end of the test. Microscopic examination of these
surfaces reveal similar corroded surfaces that are rough in appearance.
The samples treated at 500°C exhibit polarization curves where the current densities
also increase abruptly after the Tafel region but without stabilization. The current increases
to a value close to that observed for the ASS substrate (Fig. 3) producing a rough corroded
area with very small pits and orange debris.
The very low currents exhibited by samples treated at 400°C is probably related to the
presence of the nitrogen supersaturated S-phase like that shown by the XRD patterns (Fig. 2).
The nitrogen in solid solution plays an important role in improving electrochemical properties
mainly by forming ammonium ions which restricts the decrease of pH at active sites on the
surface, thus avoiding pit nucleation and growth1,5,6
. This leads to a change of the corrosion
mechanism from localized pitting corrosion to a general form in which the dissolution rate
depends on the treatment temperature.
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Table 1, Electrochemical parameters from the polarization curves of the plasma nitrided and
nitrocarburized ASS samples.
Sample Ecorr, mV Icorr,
10-8
xA.cm-2
I1.2V,
10-3
xA.cm-2
ASS-Sub. -323 2.479 32.678
PN 400 -252 5.365 0.5339
PN 450 -231 5.237 1.201
PN 500 -280 2.441 17.401
PNC 400 -243 1.611 0.6657
PNC 450 -237 3.650 0.9351
PNC 500 -280 1.099 35.187
At 450 and 500°C, the XRD analysis (Fig. 2) indicated that both PN and PNC
treatments have produced carbon and/or nitrogen compounds on the surface of the samples in
addition the S-phase. The occurrence of these compounds is favorable because chromium and
iron atoms acquire mobility as the temperature is increased allowing chemical bond
formation between substitutional and interstitial elements1,4
. The increase of current densities
of the polarization tests as the temperature was raised from 450 to 500°C is related to the
massive precipitation of nitrides as observed by XRD experiments (Fig. 2).
These results and observations suggest that both PN and PNC at 400°C considerably
improves the corrosion resistance of ASS in 3.5% NaCl aqueous solution.
4 CONCLUSIONS From these data it can be concluded that plasma nitriding and nitrocarburizing of AISI
316L stainless steel produces layers in which the thickness increases with temperature. The
treatments at 400°C produced homogenous and precipitate-free, S-phase layers while at 450
and 500°C XRD indicates the presence of iron carbide and/or chromium and iron nitrides
depending on the treatment type and temperature. Potentiodynamic polarization curves show
that corrosion resistance decreases as temperature increases. A change in the dominant
corrosion mechanism was also observed after nitriding or nitrocarburizing from localized
pitting corrosion to general corrosion.
Thus, the results suggest that both nitriding and nitrocarburizing at 400°C
considerably improves the corrosion resistance of ASS in 3.5% NaCl solution.
Acknowledgements The authors acknowledge CAPES for the scholarship granted to F.A.P. Fernandes.
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