1

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

1

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:

codoico@gmail.com 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.

2

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.

3

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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