Nitrogen Surface Enrichment of Austenitic Stainless Steel ISO 5832-1: SHTPN vs Low-temperature Plasma Nitriding

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Specimens of ISO 5832-1stainless steel were submitted to low temperature plasma nitriding treatment and Solution Heat Treatment after Plasma Nitriding (SHTPN) process aiming to obtain a S-phase, chromium precipitates-free, surface layer. The following techniques were used for analysis: scanning electron microscopy (SEM), optical microscopy (OM), microhardness, X-ray diffraction (XRD) and wavelength dispersive spectroscopy (WDS) microanalysis. Corrosion resistance was evaluated by means of potentiodynamic anodic polarization and open-circuit measurements of corrosion potential. Results indicated that the S-phase layer with N concentrations of about 0.9 wt% and 2 µm thick was formed during low temperature plasma nitriding, while layers with concentratios of about 0.45 wt% of N and up to 200 µm thick resulted from the SHTPN process. Results proved the increase in localized corrosion resistance caused by nitrogen in solid solution for both processes, as well as the deleterious effect caused by the precipitation of chromium nitride.

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DOI: http://dx.doi.org/10.1590/1516-1439.352014

Nitrogen Surface Enrichment of Austenitic Stainless Steel ISO 5832-1:

SHTPN _vs_ Low-temperature Plasma Nitriding

_Ricardo Fernando dos Reis_ _a_ - _, Rodrigo Lupinacci Villanova_ _a_ _,_

_Kaiuã Dubyna Costa_ _a_ _, Gabriela Costa Durante_ _a_

_aLaboratório de Materiais, Departamento Acadêmico de Mecânica – DAMEC,_
_Universidade Tecnológica Federal do Paraná – UTFPR, Av. Sete de Setembro, 3165,_

_Rebouças, CEP 80230-901, Curitiba, PR, Brazil_

Received: November 12, 2014; Revised: April 24, 2015

Specimens of ISO 5832-1stainless steel were submitted to low temperature plasma nitriding treatment
and Solution Heat Treatment after Plasma Nitriding (SHTPN) process aiming to obtain a S-phase,
chromium precipitates-free, surface layer. The following techniques were used for analysis: scanning
electron microscopy (SEM), optical microscopy (OM), microhardness, X-ray diffraction (XRD) and
wavelength dispersive spectroscopy (WDS) microanalysis. Corrosion resistance was evaluated by
means of potentiodynamic anodic polarization and open-circuit measurements of corrosion potential.
Results indicated that the S-phase layer with N concentrations of about 0.9 wt% and 2 µm thick was
formed during low temperature plasma nitriding, while layers with concentratios of about 0.45 wt% of
N and up to 200 µm thick resulted from the SHTPN process. Results proved the increase in localized
corrosion resistance caused by nitrogen in solid solution for both processes, as well as the deleterious
effect caused by the precipitation of chromium nitride.

Keywords: _SHTPN, plasma nitriding, ISO 5832-1, corrosion behaviour_

1. Introduction

Austenitic stainless steels have a wide variety of
applications because of their excellent corrosion resistance;
however, depending on the medium they are exposed to,
they are susceptible to crevice and/or localized corrosion.
This fact, allied to low hardness and tribological properties,
limits their applicability. Several techniques are being used
to improve the surface characteristics of these steels, like
plasma nitriding at low temperatures 1-4, ion implantation 5, and
high temperature gas nitriding (HTGN) 6 . More recently, the
so-called “Solution Heat Treatment after Plasma Nitriding”
(SHTPN) process was proposed as a new method for surface
enrichment of nitrogen in solid solution, with good results
regarding improvement of corrosion resistance of stainless
steels 7,8 .

Plasma nitriding at relatively high temperatures (723 K or
more) increases the surface hardness with consequent higher
wear resistance of these steels; however, it can lead to the
formation of CrN phases. Such precipitation reduces the
dissolved chromium content of the metal matrix, impairing
the formation of the passive layer characteristic of stainless
steels, leading to the decrease in corrosion resistance 9, 10 .
Most of the beneficial effects of nitrogen in stainless steels
are associated to its presence in solid solution (SS) 11 . Hence,
the use of plasma nitriding is limited to the low temperature
range, where only the formation of the S-phase (also called
expanded austenite - γN) occurs 1, 12, 13 .

The S-phase formation on austenitic stainless steels by
advanced nitriding treatments has been the subject of several
studies 1-13 . Theo focus of this work is not the study of the
S-phase itself, rather the SHTPN process as an alternative
to low temperature plasma nitriding.

The SHTPN method takes place in two steps: first, plasma
nitriding at high temperature (PN) (above 873 K) is carried
out, in order to produce a thick nitrided layer (regardless the
formation of nitrides), followed by the second step, where
the material undergoes a solution heat treatment (SHT).
This second step causes the diffusion of nitrogen and the
consequent formation of a nitrogen-rich, precipitate-free,
thick nitrided layer.

A linear correlation between the S-phase microhardness
and the nitrogen concentration in solid solution was determined
in a previous work 7 . Such correlation can be described by
Equation 1, which allows estimating the nitrogen content in
solid solution from microhardness measurements.

_%N_ = 0,0105 _HV_ - 2,4116 (1)

The corrosion resistance can be maintained or even
increased by treatments that raise the surface nitrogen content
in solid solution, as can be depicted from the PREN (Pitting
Resistance Equivalent Number). This parameter is relative
to the pitting corrosion in media containing chlorides and
can be calculated from Equation 2:

( ) ( ) (2)

The aim of this work it to compare results obtained by
low temperature plasma nitriding and SHTPN, regarding
the formation of S-phase and corrosion resistance of
ISO 5832‑1 14 austenitic stainless steel.

2. Experimental Procedures

ISO 5832-1 14 stainless steel cylindrical specimens of
15.0 mm height and 15.8 mm diameter were cut from a bar.
After cutting, the samples were ground with emery papers
and polished with 1 µm alumina suspension. Its chemical
composition is shown in Table 1. The starting microstructure
was fully austenitic, with 210 HV hardness. Initial PREN
value, according to the Equation 2, was 28.14.

Before the beginning of the process, the samples were
submitted to ultrasound cleaning in ethanol during one hour,
in order to eliminate contaminants from the preparation step.

The samples were plasma nitride in a pulsed DC glow
discharge. Nitriding parameters for both low temperature
and SHTPN processes are summarized in Table 2.

For the low temperature plasma nitriding process, three
temperatures were chosen, aiming to obtain a chromium
precipitate-free, S-phase layer as thick as possible. The
nitriding time was the same used for the SHTPN process 7 .
Six samples for each process were produced.

The SHTPN process was carried out as described
elsewhere 7 .

After nitriding, samples were cooled down to 573 K by
forced convection in the same gas mixture, and then cooled
to room temperature under vaccum.

Samples treated by both processes were longitudinally
cut and mounted in resin. They were then prepared using
the adequate metallographic process, and the morphology,
thickness, and microhardness profiles of the resulting layers
were evaluated. The morphology evaluation of the nitrided
layer was carried out by means of optical microscopy
(OM) and scanning electron microscopy (SEM). Prior to
microscopic observation, all samples were electrolytically
etched in aqueous solution of oxalic acid (10%). The
thickness of nitrided layers was measured with the aid of an
image analyzer software (ImagePro-Plus  ) coupled to the
optical microscope. Microhardness measurements (Vickers
indenter) were obtained with the use of a microhardness

tester Schimadzu model HMV 2. X-ray diffraction was used
to identify the phases present after treatments, and a Philips
diffractometer was used to obtain the diffractograms with
the following parameters: Cu kα radiation (λ = 1.54060),
current of 30 mA, voltage of 40 kV, 2θ-scan step of 0.05°,
and scanning angle range of 20 to 120°.

Quantitative measurements of the nitrogen concentrations
were conducted for the solubilized samples (SHTPN) by means
of wavelength dispersive spectrometry (WDS) microanalysis.
Such measurements were made in cross-sections of the
samples, from the surface to the core. Further details are
described elsewhere 7 . Ferrite content was also measured
with the aid of a ferrite content measurement instrument
(Feritscope  ), with a detection limit of 0.1%.

The corrosion behavior and the effect of nitrogen
in solid solution of treated samples were evaluated by
electrochemical tests (potentiodynamic anodic polarization
test and open circuit corrosion potential – Ecorr _vs_ time).
Prior to electrochemical testing and after nitriding treatments,
the samples were ultrasound cleaned in ethanol during one
hour in order to eliminate any contaminant from handling,
and then cleaned again with acetone. Just before setting
the corrosion test cell, samples were smoothly ground
with 1200 mesh emery paper in order to standardize the
initial passivation condition of the tested surfaces. For the
electrochemical tests, a potentiostat‑galvanostat IviumStat
(Ivium Technologies) connected to a microcomputer was used.
The electrolyte was a 0.5M NaCl aqueous solution at room
temperature, the reference electrode was a Ag/AgCl type, and
the counter electrode was made of graphite. Before the start
of potentiodynamic tests, samples remained immersed in the
electrolyte during one hour, until the corrosion potential was
reached (Ecorr _vs_ time). The potential scan rate used for the
potentiodynamic anodic polarization tests was 0.167 mV/h
(0.6 V/h), in accordance to the ASTM G61-86 15 Standard.
The tests ended when the anodic corrosion current density
reached the pre-established value of 100 µA/cm 2 . Three
samples of each condition (SHTPN and PN 673) were
tested. In order to check the effect the high temperature
plasma nitriding process over localized corrosion resistance,
a potentiodynamic anodic polarization test was conducted on
a PN 1,023 sample, which is characterized by the presence
of chromium-nitride precipitates (see Reis, et al. 7 ).

Table 1. ISO 5832-1 (SC) austenitic stainless steel chemical composition (wt %).

C Mn Si Cr Ni Mo N Fe

0.017 1.750 0.350 17.800 14.300 2.760 0.077 Balance

SC – Starting condition

Table 2. Summary of the processes parameters.

Plasma nitriding (PN) Solution heat treatment (SHT)

Temperature

_T_ (K)

Time

_t_ (h)

Pressure

Pressure Temperature

Gas mixture
_P_ (Pa) _T_ (K)

_T_ (K)

Time
_t_ (min)

Treatment

media

623 3 533 (4 Torr) 20% N2 80% H2 -- -- -673 3 533 (4 Torr) 20% N2 80% H2 -- -- -723 3 533 (4 Tor) 20% N2 80% H2 -- -- -1,023 3 1,333 (10 Torr) 90% N2 10% H2 1,473 45 Commercial salt

baths

3. Results and Discussion

_3.1. SHTPN_

The results obtained via the SHTPN process are summarized
in Table 3. These results are similar to those presented and
discussed in a previous work “Nitrogen Surface Enrichment
of Austenitic Stainless Steel ISO 5832-1” 7 . Thus, they won’t
be discussed in this work.

Figure 1 shows the microstructure of a specimen solubilized
after nitriding at 1,023 K. It can be seen from the picture
that it’s a precipitates-free structure. In order to verify the
possible existence of chromium-based precipitates, a long
lasting electrolytic etching was made onto the surface. As
can be observed from Figure 1, there are no signs of attack
in the grain boundary regions. XRD analyses indicate that
the microstructure is 100% austenitic, and ferrite was not
detected by means of ferrite content analysis 7 .

According to the values of the Tables 1 and 3, considering
the Equation 2, it appears that the PREN value was raised
from 28.14 (SC) to 34.11 (SHTPN), indicating improvement
in the pitting corrosion resistance.

_3.2. Low-temperature plasma nitriding_

Figure 2 shows micrographs of samples nitrided at 623,
673 and 723 K.
A thin layer on the surface was formed on the surface
of samples for all processing temperatures. From XRD
analyses, no formation of compound layer was observed,
and the diffraction patterns show that there is a relative
increase in the amount of S-phase (γN) with increasing
treatment temperatures. The obtained diffractograms are
show in Figure 3.

XRD indicates the formation of a monophasic S-phase
layer for the sample nitrided at 723 K (PN 723). However,
the observation of the sample micrograph (Figure 2c), reveals
that the area around grain boundaries at the surface layer
region was attacked (dark aspect). Even though no chromium
compounds were identified by XRD, such attack at grain
boundaries is consistent with the presence of chromium
precipitates (CrxNy or CrwCz) in those areas. It is believed
that the amount and/or size of such precipitates are below
the detection limits of the XRD apparatus.

Table 3. Summary of results obtained for the SHTPN process 7 .

Table 4 shows the thickness of the nitrided cases, measured
in cross-sections of the samples. The case thickness increased
with process temperature, with lower values than those
reported by Gontijo et. al _._ 16 for AISI 304L and AISI 316L
steels at the same treatment conditions. These differences
can be explained due to the lower diffusion coefficient
and greater solubility of nitrogen (because of the bigger
molybdenum content) in the ISO 5832-1 14 steel.

Due to the small thickness of the formed layers, it wasn’t
possible to obtain the microhardness profile. So, the surface
microhardness was measured (see Table 4), even though the
layer thickness wouldn’t be enough to meet the requirements
of the ASTM standard 17 . Since the layer thickness affects
the hardness and the measured values on the treated samples
reflects the interaction between the layer hardness and part of
the substrate, higher hardness values are obtained when the
portion of the substrate, which contributes to the measurement,
decreases with increasing thickness of the modified layer.

However, for the present work, it is considered that
the portion contributing to this effect is smaller when the
increased hardness of the material is taken into account.
This hardness increase is caused by the solubilization of
nitrogen in the S-phase and, for samples nitrided at 723 K,
by the presence of chromium compounds.

Figure 1. Microstructure of sample SHTPN. Long time electrolytic
etching: oxalic acid 10%.

Thickness of nitrided case
Processing

(µm)

(µm) % N (wt)

(µm) % N (wt) % Ferrite

(UHL)
Plasma Nitriding Solution heat treatment Total UHL*

1,023 K / 3 h 1,473 K / 45 min 400 200 0.45 0

- _UHL_ uniform hardness layer

Table 4. Summary of results obtained for the low temperature nitrding process.

Microhardness
Present phases

(HV0.1)

% N (wt)
% Ferrite
Estimated in SS

Nitriding

Thickness of the

nitrided case

(µm)

623 K / 3h 1.21 ± 0.36 γ and γN 360 ± 12 0.59 0
673 K / 3h 2.15 ± 0.55 γ and γN 544 ± 14 0.90 0
723 K / 3h 5 98 ± 0 97 γ and CrN 773 ± 12 -- 0

Figure 2. Micrograph of ISO 5832-1 steel nitrided during 3 hours at: (a) 623 K (PN 623), (b) 673 K (PN 673), (c) 723 K (PN 723).
Electrolytic etching: oxalic acid 10%.

Figure 3. Diffractogram of the ISO 5832-1 steel nitrided during 3
hours at: 623 (PN 623), 673 (PN 673) and 723 K (PN 723).

For samples nitrided at 423 and 573 K, the nitrogen
concentration in the hardened layer can be estimated form
Equation 1, provided that the hardness increase is caused
by nitrogen in solid solution (SS). These values, as well as
the main results obtained from low temperature nitriding
regarding the production of the S-phase layer area shown
i bl 4

From the results obtained for the low temperature
nitriding process, the condition that led to the formation
of a thick, chromium precipitate-free S-phase layer was
PN 673 (nitrided at 673 K). So, this sample was chosen for
the corrosion tests.

Based on the steel chemical composition (Table 1),
the measured nitrogen content (Table 4), and Equation 2,
it can be stated that the low temperature plasma nitriding
led to an increase in PREN, as well as the SHTPN process.
PREN changed from 28.14 to 36.53 for samples nitrided
at 623 K (PN 623), and from 28.14 to 41.31 for samples
nitrided at 673 K (PN 673). Again, there is in an indicator
of the corrosion resistance improvement for this sample.

_3.3. Electrochemical tests_

3.3.1. Open circuit corrosion potential (Ecorr vs Time)

The open circuit corrosion potential (Ecorr _vs_ time)
was measured for the following test conditions: starting
condition (SC), SHTPN, PN 673, PN 1,023. Obtained results
are shown in Figure 4.

Except for the sample PN 1,023, the results indicate
that the open circuit potential is less noble at the beginning
of the test, and increases with time, stabilizing in a more
noble potential. This shows that the material is passivating
in this medium, i.e., a passive layer is being formed on the

f f h i l d i h

Measured open circuit potentials show that there is a little
variation among tested conditions – starting condition, low
temperature nitriding, and SHTPN. The greatest potential was
measured for the sample PN 673, which is consistent with
the grater surface nitrogen concentration in solid solution
for this condition. The SHTPN process didn’t caused major
changes in the open circuit potential when compared to the
starting condition (SC).

A major discrepancy on the results was verified for
the sample nitrided at 1,023 K (PN 1,023), where the open
circuit potential decreases to values far below the others. This
decrease in the potential can be explained by the formation
of chromium-nitrides precipitates in the compound layer (see
Figure 4 of the article Reis et. al. for further details 7 ), thus
reducing the passivation of the material and its resistance
to uniform corrosion.

3.3.2. Potentiodynamic anodic polarization

The potentiodynamic anodic polarization test allow
comparing the susceptibility to localized corrosion as a
function of the nitrogen surface enrichment caused by the
different treatments that the samples were submitted.

Figure 5 shows the curves obtained in the potentiodynamic
polarization tests for the ISO 5832-1 14 steel in the following
conditions: SC, SHTPN, low temperature plasma nitriding
(PN 673), and high temperature plasma nitriding (PN 1,023).

The electrochemical behavior verified for the starting
condition (SC) is consistent with materials that have a
defined critical pitting corrosion potential (Ec). This potential
corresponds to the value where the breakdown of the passive
layer occurs, and localized corrosion starts to happen. This
potential is characterized by a sharp increase in the corrosion
current density.

The critical pitting corrosion potential wasn’t reached
for conditions PN 673 and SHTPN. The sudden increase
in current density observed close to 1,400 mV (Ag/AgCl)
should be associated to the water dissociation, according
to the reaction shown in Equation 3. When this reaction
occurs at the sample surface, it is impossible to say if the
increase in current density is due to passivity breakdown or
the reaction itself, which limits the test to potentials of this
magnitude. However, it becomes evident that conditions PN
673 and SHTPN have higher pitting corrosion resistance
than the starting condition (potentials greater than 1,300 mV
(Ag/AgCl)).

2 _H_ 2 _O_ → _O_ 2 + 4 _H_ + + 4 _e_ - (3)

Table 5 summarizes the results obtained in the
potentiodynamic anodic polarizations tests.

The gain of localized corrosion resistance for the
conditions PN 673 and SHTPN can be associated to the
presence of nitrogen in solid solution and the increase in
PREN, as discussed before.

Differently from what was observed in the previous
test (Ecorr _vs_ time), where a better behavior in terms of
uniform corrosion was observed for the PN 673 condition,
it was impossible to distinguish the behavior in terms of
localized corrosion (critical pitting corrosion potential) of
the low temperature plasma nitriding (PN 673) and SHTPN

di i h l d i i ifi i

Figure 4. Comparative between the open circuit corrosion potential
for the studied conditions.

Figure 5. Potentiodynamic anodic polarization curves.

Table 5. Critical pitting corrosion potentials obtained from the
potentiodynamic anodic polarization curves.

Condition _EC_ (mV (Ag/AgCl))

SC 686.8 ± 0.6
NP 673 - 1,300
SHTPN > 1,300

of the localized corrosion resistance, as observed by the
increase in the critical pitting corrosion potential. The main
difference between studied conditions is the thickness of the
nitrogen-rich layer. For the SHTPN condition, this thickness
is about 150-250 times greater than those produced by low
temperature plasma nitriding (see Tables 3 and 4). Because
of this significant difference, an increase in the lifespan of
treated components submitted to wear is expected. Further
investigations regarding the cavitation erosion resistance
of samples treated under the same conditions of this work
are being conducted.

As expected, the localized corrosion resistance of the
samples nitrided at high temperature (PN 1,023) decreased
significantly, since the critical pitting corrosion potential
d d f b 686 8 (A /A Cl i di i )

about 291.2 mV (Ag/AgCl) for this condition. This reduction
is due to the precipitation of chromium-nitride compounds,
which reduces the chromium content in solid solution in the
steel matrix close to the precipitates, impairing the localized
corrosion resistance 7 .

Besides that, an increase in current density was also
observed for this condition, confirming the low corrosion
resistance of high temperature nitrided samples.

4. Conclusions

- The surface S-phase (γN) layer was formed for both
SHTPN and low temperature plasma nitriding processes,
but with significant differences between them, especially
regarding their thicknesses. For the SHTPN process,
thicknesses of up to 200 µm were obtained, while for
low temperature nitriding an estimated thickness of
about 2 µm was produced for the PN 673 condition.

- The nitrogen concentration in the modified layer was
also different for different processes. A concentration
of 0.4 wt% of nitrogen was measured for the SHTPN
process, while the estimated value for low temperature
nitriding it was 0.59 wt% (PN 673 condition).

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Acknowledgements

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