научная статья по теме STUDY ON CORROSION PROTECTION OF DIFFERENT STAINLESS STEELS BY NANOCRYSTALLINE PLASMA ELECTROLYSIS Химия

Текст научной статьи на тему «STUDY ON CORROSION PROTECTION OF DIFFERENT STAINLESS STEELS BY NANOCRYSTALLINE PLASMA ELECTROLYSIS»

ФИЗИКОХИМИЯ ПОВЕРХНОСТИ И ЗАЩИТА МАТЕРИАЛОВ, 2008, том 44, № 4, с. 432-437

ФИЗИКО-ХИМИЧЕСКИЕ ПРОБЛЕМЫ ЗАЩИТЫ МАТЕРИАЛОВ ОТ КОРРОЗИИ И ВОЗДЕЙСТВИЯ =

ОКРУЖАЮЩЕЙ СРЕДЫ

УДК 620.193

STUDY ON CORROSION PROTECTION OF DIFFERENT STAINLESS STEELS BY NANOCRYSTALLINE PLASMA ELECTROLYSIS

© 2008 M. Kh. Aliev, A. Sabour, T. Shahrabi

Faculty of Engineering, Tarbiat Modares University, Tehran, Iran, P.O.Box: 14115-143

e-mail: sabour02@yahoo.com Received June 1, 2007

The effect of nitrogen and carbon bombardment on the corrosion behavior of three different stainless steels (AISI 304, AISI 316L and AISI 430) is discussed in this work. Nitrogen and Carbon was entered into the lattice in order to generate a rich region near the surface by a relatively new method called plasma electrolysis. Different ion bombardment doses under different applied voltages and thus different electrical fields (230, 245, 260 volts) at 330 mS/Cm of electrical conductivity of specific electrolyte have been tested to optimize the ion bombardment doses for each steel. The corrosion measurements were carried out in sodium chloride solution by using potentiodynamic scanning (PDS) and electrochemical impedance spectroscopy (EIS). The experimental results showed that the effect of N and C ion bombardment mainly depends on the microstructure and/or composition of the stainless steels. Less compact structures and/or less amount of alloying elements (as occurs with the body centered cubic, ferritic AISI 430) achieve bigger changes with this modification, whereas on stainless steels with a larger amount of alloying elements and/or more compact structures (like the face centered cubic, austenitic AISI 316L) ion bombardment slightly modifies the corrosion behavior.

PACS: 81.65.Kn

INTRODUCTION

It is well known that the austenitic and ferritic stainless steels suffer different forms of localized corrosion [1], which is very dangerous leading to degradation/failure of structural stainless steels in service [2]. The damage is mostly due to halide ions, particularly chloride ions [1, 3-5]. Corrosion is mostly a surface phenomenon, so the corrosion resistance is closely related to the composition and structure of surface films on metals [6]. Therefore, surface modification techniques are suitable to improve corrosion properties of a material [1].

The result of ion bombardment into materials is the formation of a near-surface alloy of graded composition that has no well defined interface with respect to the substrate, in contrast to a deposition layer [7]. A graded alloy can be produced from the surface to the unchanged underlying bulk alloy so that both the surface and the bulk can be independently optimized.

The addition of N into steels is known to enhance the corrosion resistance to localized corrosion [8]. In this work, N ion bombardment was performed on three stainless steels (AISI 304, AISI 316L, AISI 430) in order to evaluate the influence of the microstructure and/or steel composition on the improvement of the localized corrosion resistance.

Carbon bombardment was carried out on AISI 304 and AISI 430 stainless steels in order to confirm whether the improvement of the corrosion resistance was due to chemical effects produced by the nitrogen entrance

or due to physical effects produced by ion bombardment.

METHODS AND MATERIALS

Three different commercial stainless steels (AISI 304, AISI 316L and AISI 430), whose chemical compositions are given in Table 1, were used in this work.

First, all (5 x 20 cm (dia)) cylinder shape specimens were ground to a surface finish of SiC # 600 emery paper and then bombarded with different applied voltages (230, 245, 260 volts) by plasma electrolytic nitrid-ing(PEN) method in sodium nitride electrolyte similar to our other works [9.10]. All of these steels were bombarded with N and nanocrystalline compound layer was formed on the surface of the sample as obtained by our

Chemical composition of AISI 304, AISI 430 and AISI 316L stainless steels (wt %)

Si Mn Ni Cu Cr P Mo

AISI304 0.44 1.52 8.19 0.29 18.30 0.031 0.21

AISI316L 0.36 1.12 10.64 0.34 16.98 0.033 2.13

AISI 430 0.32 0.3 0.21 0.03 16.53 0.020 0.01

Co C S V Sn Ti Nb

AISI304 0.12 0.064 0.002 - 0.013 - -

AISI 316L 0.16 0.033 0.001 0.09 0.010 0.013 0.021

AISI 430 0.02 0.064 0.001 - - - -

other works [10.11]. Additionally, the same experimental conditions were used on AISI 304 and AISI 430 bombarded with C by plasma electrolytic carburising in Glycerol electrolyte (PEC).

The electrochemical tests were carried out in two different solutions. AISI 304 and AISI 430 were tested in a 0.5 M sodium chloride solution, whereas AISI 316L was tested in a saturated sodium chloride solution due to their higher resistance to localized corrosion. The EIS experiments (lasted 5 weeks) were carried out at open circuit potential with an amplitude of 5 mV in the frequency range 10 mHz to 100 KHz. Potentiodynamic Scanning (PDS) was also done on the surface of bombarded samples from -0.3 volts vs. open circuit potential to nobel direction up to 1.5 volts vs. saturated calomel electrode. The scan rate of these tests were 0.5 mV/Sec and all these tests were done after approximately 2 hours of immersion in order to reach fix potential. All experiments were recorded with a frequency response detector Solartron 1025 and a potentiostat/galvanostat EG&G 273A. The impedance spectra were analysed with the simulation Zview2 software.

Phase, 90

80

70

60

50

40

30

20

10

0

90 80 70 60 50 40 30 20 10

degrees

(a)

230 volts non bombarded 245 volts 260 volts

_l_L_

(b)

-DO

non bombarded 230 volts 245 volts 260 volts

RESULTS AND DISCUSSION

Fig. 1 shows the Bode diagrams of all the N bombarded samples after 1 h of immersion.

N bombardment has an important effect on AISI 304 (Fig. 1a) steel, in a way that this effect increases as the bombardment dose increase. The higher bombardment dose (by 260 volts) completely modifies the corrosion mechanism of this material with the apparition of a second time constant clearly defined. However, lower bombardment doses mean fewer modifications of the corrosion mechanism, although the intermediate bombardment dose is enough to achieve lower corrosion rates than the non-bombarded sample.

In contrast, for the lower bombardment dose (by 230 volts), bombardment seems to be harmful, as can be observed in the low frequency range. This negative effect of N bombardment for this sample could be due to the slight chemical effect of the bombarde N, which is not enough to hide the physical damage produced during the bombardment process. Fitting of the impedance data with the Zview2 simulation program proposes two different models: a Randles circuit for the non-bombarded sample and the one bombarded with 230 volts and a double layer circuit for the most bombarded samples (Fig. 1a).

The effect of N bombardment on AISI 316L is lower than on AISI 304 as is shown in the Bode diagram (Fig. 1b). The lower phase values in the low frequency range of the non-bombarded sample revealed a higher corrosion rate. The higher bombarded samples (by 260 and 245 volts) showed a similar behaviour whereas the lower bombarded sample (by 230 volts) seems to have better behaviour, displaying bigger phase values than the other implanted samples. All the samples, including

90 80 70 60 50 40 30 20 10

0 0

Frequency, Hz

Fig. 1. Bode diagrams of N-bombarded (a) AISI 304 stainless steel, (b) AISI 316L stainless steel and (c) AISI 430 stainless steel with different bombardment doses after 1 h of immersion.

the non-bombarded one, showed a two time-constant mechanism and fit into a double layer circuit (Fig. 1b).

Even though all implantation doses modify the corrosion rate, this change is not enough to produce a change of the corrosion mechanism. In 316L stainless steel, lower phase values are shown in the low-intermediate frequency range by all the bombarded samples. This can be interpreted as evidence of the physical damage produced during the bombardment process. The chemical effect of the bombarded nitrogen on AISI 316L is lower than on AISI 304, as a consequence of the extremely resistant passive layer. Even if 230 volts is the lowest applied voltage for bombardment dose, the radiation damage is also smaller, modifying in less quantity the passive layer.

(c)

non bombarded 260 volts 230 volts 245 volts

0.01

100 10000 1000000

1

E, V (SCE) 1.6 г

O

-O.4 2.O

l.5

l.O

O.5

O

O.5

-l.O

(c)

43O raw 230 volts 245 volts — 260 volts

-l2 -l0

-б -4

O

log i (log(A/cm2))

Fig. 2. Potentiodynamic diagrams of N-bombarded ( a) AISI 304 stainless steel, (b) AISI 316L stainless steel, and (c) AISI 430 stainless steel with different bombardment doses after 5 weeks of immersion.

The biggest differences on the corrosion mechanism are observed in AISI 430 (Fig. 1c). The effect of N bombardment is so important that all bombardment doses completely modify the corrosion mechanism. Higher implantation doses mean bigger changes in corrosion behaviour. Thus, the most bombarded sample (by 260 volts) completely changes the spectra showing a three time-constant mechanism. A triple layer circuit fit with spectra, whereas lower implantation doses fit into a double layer circuit. By contrast, for the non-bombarded sample a monolayer circuit is fitted, as occurred on AISI 304.

Changes in the electrochemical response for both N bombarded AISI 304 and AISI 430 stainless steels can be due to a spinel oxide sublayer in the outermost region of the passive layer, which improve the corrosion resistance.

0.01

Phase, 9O SO TO 60 5O 4O 30 2O lO O

9O

SO

TO

60

5O

4O

3O

2O

lO

O O

9O SO TO 60 5O 4O 3O 2O lO

degrees

(a)

-00

-260 volts non bombarded 245 volts 23O volts

l l00 l0000 lOOOOOO

(b) Ф

non bombarded 245 volts

- 260 volts

- 230 volts

.Ol l l00 l0000 lE+06 l0 l000 lOOOOO

(c)

l00 l0000 lE+06 l0 l000 lOOOOO

Frequency, Hz

Fig. 3. Bode diagrams of N-bombarded (a) AISI 30

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