Effect of nitrogen on the corrosion behavior of austenitic stainless steel in chloride solutions

Effect of nitrogen on the corrosion behavior of austenitic stainless steel in chloride solutions

Materials Letters 59 (2005) 3311 – 3314 www.elsevier.com/locate/matlet Effect of nitrogen on the corrosion behavior of austenitic stainless steel in ...

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Materials Letters 59 (2005) 3311 – 3314 www.elsevier.com/locate/matlet

Effect of nitrogen on the corrosion behavior of austenitic stainless steel in chloride solutions Fathy M. Bayoumi*, Wafaa A. Ghanem Central Metallurgical Research and Development Institute (CMRDI), P.O.Box: 87, Helwan, Cairo, Egypt Received 15 February 2005; accepted 12 May 2005 Available online 16 June 2005

Abstract The effect of partial replacement of nickel with nitrogen on the mechanism of localized corrosion resistance and repassivation for nitrogen-bearing stainless steel was investigated using anodic potentiodynamic polarization technique. The solutions used for this study contained 0.0, 0.05 and 0.33 M Fe3+ for solutions I, II and III, respectively, in a total Cl ion concentration 1 M. The pitting attack was found to be retarded by nitrogen addition and the samples were able to passivate as the nitrogen increase. Addition of nitrogen allows the decrease in the wt.% of Ni, but to a certain limit. Nitrogen is adsorbed on the interface of the metal oxide and results in the repulsion of Cl ions. Moreover, it reacts with H+ ions in the solution leading to higher pH, which explains the retardation effect of nitrogen to corrosion. D 2005 Elsevier B.V. All rights reserved. Keywords: Localized corrosion; Pitting corrosion; Anodic polarization; Nitrogen stainless steel

1. Introduction Nitrogen is considered an important alloying addition to austenitic stainless steel in terms of corrosion resistance. It promotes passivity, widens the passive range in which pitting is less probable, improves stress corrosion cracking resistance in some media, and enhances the resistance to intergranular corrosion [1 –5]. Moreover, nitrogen dissolved in austenitic stainless steel was found to increase its strength [6,7]. The following mechanisms have been suggested to explain how nitrogen operates: (1) nitrogen in solid solution is dissolved and produces NH4+, depressing oxidation inside a pit [1,6,8,9]; (2) concentrated nitrogen at the passive film/ alloy surface stabilizes the film, and prevents attack of anions (Cl) [10 – 13]; (3) produced nitrate ions improve the resistance to pitting corrosion [14]; (4) nitrogen addition stabilizes the austenitic phase [15]; and (5) nitrogen blocks the kink, and controls the increase of electric current for pit production [16]. * Corresponding author. Fax: +20 2 501 0639. E-mail addresses: [email protected] (F.M. Bayoumi), [email protected] (W.A. Ghanem). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.063

In order to develop a resources-saving stainless steel with excellent localized corrosion resistance, 2 key technologies were adopted. The first key is high nitrogen alloying. Nitrogen enrichment improves pitting and crevice corrosion resistance without increasing chromium or molybdenum content. Therefore, adding nitrogen may contribute to reduce necessary chromium and molybdenum content. Moreover, as nitrogen is austenite former, adding nitrogen lowers the nickel content required for forming a single austenitic phase. Lowering the nickel in stainless steel lowers the costs of production. The aim of this paper is to study the effect of partial replacement of nickel with nitrogen on the pitting corrosion behavior of austenitic stainless steel.

2. Experimental The chemical composition of the samples is described in Table 1. The samples were prepared, cold worked, and normalized by Steel laboratory (CMRDI). Austenitic stainless steel (sample 1) was used as the blank material for all the samples. In samples 2– 5 the nitrogen content was

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Table 1 The chemical composition of the sample used, wt.% Sample #

C

Si

Mn

Cr

W

Ni

N

Fe

1 2 3 4 5

0.42 0.41 0.35 0.39 0.40

3.07 3.07 3.07 3.07 3.07

1.61 1.61 1.61 1.61 1.61

19.22 19.22 19.22 19.22 19.22

1.70 1.70 1.70 1.70 1.70

8.56 7.06 5.13 3.71 0.13

0.00 0.256 0.321 0.373 0.45

Bal. Bal. Bal. Bal. Bal.

Bal—balance (i.e. that complete 100%).

gradually increased while the nickel content was gradually decreased. 2.1. Electrochemical measurements Experiments were designed to investigate the role of partial replacement of nickel by nitrogen on the corrosion behavior of austenitic stainless steel. The solutions used in this investigation were: solution I, 1 M NaCl; solution II, 0.85 M NaCl + 0.05 M FeCl3; and solution III, 0.01 M NaCl + 0.33 M FeCl3. The overall chloride concentration was maintained constant. Potentiodynamic polarization curves were used to test corrosion behavior of the samples in different solutions at 25 -C. PGSTAT30, Autolab computerized potentiostat was used for testing. The current was measured as a function of the potential, which changed with scan rate 0.5 mV/s. The electrode potentials were measured with respect to Silver/silver chloride reference electrode in a saturated KCl solution. The counter electrode was a platinum wire, while the sample was used as the working electrode.

which is considered as the pitting potential, occurred at å 0.5 V. When the N/Ni reaches 3.462, that is the case of the highest nitrogen and lowest nickel, the passivation occurred by a slight decrease in current, followed by a break in the current then a breakdown of the passivation. The breakdown potential E p, as estimated by the extrapolation method, occurred at 0.35 V. A general view to the potentiodynamic curves of Fig. 1 reveals that sample 3 has more positive zero current potential and the lowest anodic current suggesting more resistance to corrosion The current for passivation decreases as the fraction of nitrogen to nickel increase but to a certain limit. This indicates that nitrogen promotes passivation of nitrogen stainless steel and the passivation requires a certain proportion of nickel to be present, i.e., the passivation due to nitrogen is supported by a presence of a critical percentage of nickel. The initiation of pitting is clearly retarded by the presence of nitrogen, the incubation time being prolonged, so that the extent of attack is greatly decreased. However, it may be noted that the current oscillations (specially in sample 3) indicating pit formation and repassivation. These experiments strongly suggest the important role of nitrogen for repassivation. Not so much the pit initiation is retarded by nitrogen, but the stable pit growth is suppressed by immediate repassivation. The hypothesis about the effect of the negatively charged Ny segregated beneath the passive film on the adsorption mechanism [17] of pit initiation. The accelerated dissolution rate of the passive film at the spot, where the aggressive anions are adsorbed, causes thinning of the film and approaching of the segregated Ny and the adsorbed anions (Cl). Upon breakthrough of the passive film, the repulsive interaction of segregated Ny and adsorbed Cl certainly will lead to desorption of the Cl, since the segregated species cannot move away rapidly. Moreover, the Ny will be further enriched by anodic segregation. The removal of the aggressive anions may be the reason for the fast repassivation of the pits in a wide range of potentials.

3. Results and discussion 3.2. Corrosion in FeCl3 solution 3.1. Corrosion in 1 M NaCl Figs. 2 and 3 show potentiodynamic polarization curves for the samples corroded in presence 0.05 and 0.33 Fe3+ ions, respectively. 2.0 1 2 3 4 5

1.8 1.6 1.4

Potential, V (Ag/AgCl)

Fig. 1 shows potentiodynamic polarization curves for different samples of nitrogen stainless steel in 1 M NaCl solution at 25 -C. Sample 1, where there is no nitrogen and the nickel is 8.56 wt.%, the curve indicates anodic dissolution, the Cl ions prevent passivation to occur. In samples 2 – 5, where some percentages of nitrogen are added at the same time the percentages of nickel are decreased, there is a general tendency for passivation. The break in the current as the potential increases explains this passivation. The passivation current was taken as the average current of the passive region. There is no passivation observed for sample 1 where the N/Ni ratio is almost 0 (n å 0 and Ni = 8.56 wt.%). When the N/Ni ratio is 0.036, sample 2, a small break in the anodic current is observed. If the ratio N/Ni is raised to 0.063 the zero current potential is shifted to more positive values than the other samples and the passivation current is estimated to be å 10 3 A. This passive region extended to higher potential which is shifted to more positive. The break in this passive region occurred at almost 1.2 V. The relatively high passivation current of this sample may be attributed to the higher conductivity of the passive film. When the ratio N/Ni reaches 0.101, a break in current occurred immediately after the zero current potential. The passive current is almost 2  10 5 A. The breakdown potential,

1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 10-9

10-8

10-7

10-6

10-5

10-4

10-3

Current density, A cm

10-2

10-1

100

-2

Fig. 1. Potentiodynamic polarization curves for different samples of nitrogen stainless steel in 1 M NaCl solution.

F.M. Bayoumi, W.A. Ghanem / Materials Letters 59 (2005) 3311 – 3314

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2.0 1.8

1 2 3 4 5

1.6

Potential, V (Ag/AgCl)

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 10-9

10-8

10-7

10-6

10-5

10-4

Current density, A cm

10-3

10-2

10-1

100

-2

Fig. 2. Potentiodynamic polarization curves for different samples of nitrogen stainless steel in 0.05 M FeCl3 solution, the total Cl 1 M.

The total Cl ions content was adjusted to 1 M by adding corresponding amounts of NaCl to The FeCl3 solution. Generally, samples 1 and 2 showed no passivation. Samples 3, 4 and 5 showed passivation as indicated by the break in the current in the anodic region. Sample 3, where the N/Ni ratio is 0.063 the average passivation current is å 10 3 A and the breakdown potential occurred at 1.2 V, it also has the lowest anodic current. When the ratio is N/Ni 0.101 the current break at an average passivation current 5  10 4 A and the breakdown potential, E p, is 0.35 V. If the ratio N/Ni reached 3.462, i.e. almost no nickel, the high wt.% of nitrogen could only lower the current, after the zero current potential, to a value of 5  10 5 A, but immediately the current increase again. It is noteworthy to observe that the mode of passivation in sample 3 where there is a medium addition of nitrogen and nickel, is different from that for sample 5 where the nitrogen is greatest and the nickel is almost non. In sample 3, the

anodic current is almost the lowest and the passivation takes place over a wider range of potential. In sample 5, the current decreases greatly with increasing the potential and the breakdown of passivation occurs rapidly. Few authors have tried to analyze the solution after corrosion test of nitrogen steel [18]. Their results suggest that upon dissolution of the steels mainly NH4+ is formed from the dissolved nitrogen. Most probably nitrogen from the metal surface, segregated in a negatively charged state, is directly converted according to the chemical reaction

N3 þ 4Hþ Y NHþ 4

ð1Þ

The negatively charged segregated state of nitrogen on iron was well established by AES, LEED and XPS [19 – 21]. The enriched N can be found at the oxide/metal interface and at the top of the passive layer [20,22,23].

2.0

1.6 1.4

8.56

5.13 3.17

4.5

1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 10-8

7.06

0.13

5.0

Corrosion Rate, mm/year

Potential, V (Ag/AgCl)

% of Nickel

1 2 3 4 5

1.8

NaCl 0.05 FeCl3 0.33 FeCl3

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

10-7

10-6

10-5

10-4

10-3

10-2

10-1

-3

Current density, A cm

Fig. 3. Potentiodynamic polarization curves for different samples of nitrogen stainless steel in 0.33 M FeCl3 solution, the total Cl 1 M.

0.0

0.1

0.2

0.3

0.4

0.5

% of Nitrogen Fig. 4. The effect of partial replacement of nickel by nitrogen on the corrosion rate of stainless steel in different solutions.

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The stability of a corroding pit depends on the potential drop, the composition of the pit electrolyte and the pH drop. pH shifts have often served to explain the stability of a corroding pit. Especially in non-buffered solutions the hydrolysis of metal ions leads to acidification according to:

Mezþ þ H2 O ¼ MeOHðz1Þ þ Hþ

3. The presence of a critical value of nitrogen and nickel lowers the general corrosion of the stainless steel in presence of Fe3+ ions, but almost has no effect in 1 M NaCl.

ð2Þ

The presence of hydrolysable cations whether as corrosion products or in the electrolyte may lower the pH to a value reaching down to 2.8. The precipitation of a hydroxide would be prevented, but passivation should still be possible. The pH shift of the pitting potential

Acknowledgment The author expresses her thanks and gratitude to all members in Steel Laboratory of CMRDI for supplying the materials. Special thanks are due to Prof. Dr Kamal AbdRabbo and Said Nabil.

Ep ¼ Ep0  0:059 pH for iron Ep0 ¼ 0:58 V

ð3Þ

is generally much too small to shift the pitting potential above the potentiostatically applied value. So the metal surface cannot reach the active range of the polarization curve, except when the potential is set very close to the pitting potential, a case which may often occur in technical corrosion cases. The effect of nitrogen in the steel on the chemistry in a growing pit can be mainly by raising the pH. This can be explained in terms of reaction (1) i.e. the formation of ammonium ion NH4+, which buffers pH in its formation. However, in the cases that the hydrolysis reactions deliver much more H+ ions to the solution the nitrogen can tie up:

Fe Y Fe2þ Y FeOOH þ 3Hþ

ð4Þ

FeCl3 þ 3H2 O Y FeðOHÞ3 þ 3HCl

ð5Þ

So the effect of N in a growing pit is rather limited. Fig. 4 shows the effect of partial replacement of the nickel by nitrogen on the general corrosion rate, mm/year, estimated from the polarization data. The figure clearly reveals that the corrosion rate is independent of the partial replacement of Ni by N in the solution containing only 1 M NaCl. However, the solution containing FeCl3 showed noteworthy results. As the nitrogen content is increased and the nickel is decreased, the corrosion rate decreases to a minimum critical value. Beyond this critical value, the corrosion rate starts to increase a gain regardless of increasing the wt.% of nitrogen with the decrease of the percentage of nickel. This suggests a combined effect of both nitrogen and nickel on protection against general corrosion of the stainless steel. This requires further study to investigate the exact mechanism behind this behavior.

4. Conclusions 1. The pitting attack was found to be retarded by nitrogen addition and the samples were able to passivate as the nitrogen content is increased. 2. Addition of nitrogen allows the decrease of the wt.% of Ni, but to a certain limit.

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