Improvement of Corrosion Resistance of Plasma Nitrided Stainless Steel

M. Moradshahi1, S. Amiri1, M. Basiri1, Sh. Lahooti2, H. Habibi1
1) Applied Plasma Laboratory, Center of Nuclear Fusion Research, Atomic Energy Organization of Iran, Tehran
2) Department of Nuclear Materials, Center of Nuclear Medicine and Agriculture, Iran, Karaj
Abstract. In this study specimens of 321 austenite stainless steel were plasma nitrided at different temperatures. Then
their corrosion behavior was studied by anodic polarization and surface techniques such as SEM, EDX and XRD. The
results showed that plasma nitriding at low temperature (400°C) improve the corrosion resistance of 321 S.S. due to
formation of N-Expanded austenite phase (γN) in surface layer.
Keywords: Plasma nitriding, Corrosion, Passivation, expanded austenite and Pitting corrosion.
PACS: 81.65.Lp and 52.77.-j
INTRODUCTION
It is well known that austenite stainless steels are prone to pitting corrosion in acidic environments and solutions
having halide ions. This, restricted their use in many applications especially in offshore installations and chemical
industries[1]. In fact corrosion depends strongly on the microstructure and composition of the material at the near
surface region. Thus surface modification is necessary for improving corrosion resistance of austenite SS. It is well
known that nitriding of austenite SS at low temperatures leads to formation of expanded austenite (γN) phase that has
a good wear and corrosion resistance[2-5]. In this work we aim to investigation of corrosion behavior and
mechanism of plasma nitrided SS at different temperatures.
EXPERIMANTAL
The equipment used for plasma nitriding was a cylindrical DC diode glow discharge unit with a vacuum chamber
with 80 cm height and 25 cm diameter in which two steel plates were inserted as cathode and anode. A resistive
heater under the cathode heated the samples and cathode temperature was controlled by an electrically isolated
thermocouple(Fig. 1). Samples (20×20×3 mm) of austenitic (AISI321) stainless steel were obtained and polished by
grit SiC papers and 1μm Al2O3 pastes. Then the samples were put on the cathode after ultrasonically cleaning in
ethanol and the chamber was evacuated to a pressure of 7×10-6 torr. Before nitriding step, the samples were
sputtered in Ar/H2 (80:20) gas mixture and temperature of 400°C using 1200 V DC voltage to remove the passive
oxide film from the surface. Afterward, plasma nitriding was performed by applying 600 V to N2-H2 (80:20) gas
mixture at pressure of 600 mtorr and temperature of 400°C and 500°C for 20 h. Experimental conditions are listed in
table 1.
Corrosion tests were performed in a glass three electrode cell at 25°C. SCE, platinum and working samples
served as reference, counter and working electrodes respectively. Polarization curves were determined with a
potential scan rate of 2 mv/s after about 1 h immersion in 1 N H2SO4 electrolyte (ASTM:G5-94). SEM, EDX, and
XRD were used to observe and analyze the samples after each experiment.
FIGURE 1. Development apparatus for nitriding of Al alloys. 1) Vacuum chamber 2) Electrodes 3) Specimens 4) Heater 5)
Feed through 6) Window 7) thermocouple 8) Pressure gauge 9) Vacuum valve 10) Diffusion pump 11) Rotary pump 12) Power
supply 13) Gas supply 14) Flow meter
TABLE 1. Conditions of plasma nitriding experiments.
Sample Pressure(mtorr) Treatment time(hr) Temperature(ºC)
StN1 600 20 400
StN2 600 20 500
RESULTS AND DISCUSSION
cross sectional micrographs of nitrided samples were shown in Fig.2 after etching by Marble reagent for 10 s.
According to these pictures, increasing of temperature from 400°C to 500ºC results in considerable increment in
nitride layer thickness from about 3.5μm to about 10 μm as measured by SEM. Superior corrosion properties of the
treated layer are demonstrated by its greater resistance to chemical etching compared with that of the bulk material.
Results of EDX analysis indicate that nitrogen concentration in nitride layer increases from 18.5 at% to about 27
at% with increasing of temperature. Therefore the yield and rate of nitriding process improved at higher
temperatures. Results of corrosion experiments were shown in Fig.3 and table 2. According to polarization data,
corrosion potential (Ecorr) was slightly shifted to a noble direction in treated samples. Regarding to StN1 sample,
passivation current density (ipass) is slightly higher than, but close to, that for the untreated sample. However the
passivation range of the polarization curve expanded in StN1 sample. It indicates that StN1 sample is less
susceptible to localized corrosion. But about StN2 sample it can be seen that corrosion rate was increased 17 times
as high as untreated sample. Indeed, plasma nitriding at this temperature degrade the corrosion resistance of
austenite SS. SEM observations of corroded samples (Fig.4a-c) showed that the corrosion mechanism of untreated
321 SS was localized pitting corrosion and materials outside the pits suffered very limited corrosion. StN2 sample
was also severely attacked by crevice and intergranular corrosion during polarization test. While corrosion of StN1
sample was more likely to be in a general form. So, if the real corrosion area inside the pits trench rather than the
whole testing area were used calculated the current densities, the resultant actual corrosion densities would be far
greater than those shown in Fig.3 suggesting that low temperature plasma nitriding considerably improves corrosion
resistance of 321 SS in 1 N H 2SO4 solution. This can attributed to formation of γN phase in sample treated at 400

CONCLUSIONS
It is concluded form this study that as compared with the untreated steel, corrosion rate of low temperature
plasma nitrided sample was reduced and the mechanism of corrosion was changed from pitting to general that is
very less harmful in industry. But nitriding at high temperatures (>450ºC) resulted in severe intergranular corrosion
because of depletion of Cr as chromium nitrides precipitated in grain boundaries.
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