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Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels
International Journal of Minerals, Metallurgy and Materials Volume 16, Number 5, October 2009, Page 517
Materials
Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels Hua-bing Li, Zhou-hua Jiang, Yan Yang, Yang Cao, and Zu-rui Zhang School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China (Received 2008-12-10)
Abstract: Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels (HNSS) were investigated by electrochemical and immersion testing methods in chloride solution, respectively. The chemical constitution and composition in the depth of passive films formed on HNSS were analyzed by X-ray photoelectron spectrum (XPS). HNSS has excellent pitting and crevice corrosion resistance compared to 316L stainless steel. With increasing the nitrogen content in steels, pitting potentials and critical pitting temperature (CPT) increase, and the maximum, average pit depths and average weight loss decrease. The CPT of HNSS is correlated with the alloying element content through the measure of alloying for resistance to corrosion (MARC). The MARC can be expressed as an equation of CPT=2.55MARC29. XPS results show that HNSS exhibiting excellent corrosion resistance is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen. Key words: high nitrogen austenitic stainless steel; pitting corrosion; crevice corrosion; nitrogen; critical pitting temperature; synergistic effect
[This work was financially supported by the National Natural Science Foundation of China and Baosteel Group Corporation (No.50534010).]
1. Introduction High nitrogen stainless steels have received much attention in recent years due to their excellent mechanical properties and corrosion resistance. Nitrogen is considered as an important alloying addition to austenitic stainless steels replacing partly or entirely nickel without sacrificing mechanical and corrosion resistance properties [1-2]. The beneficial effect of nitrogen on the corrosion resistance has been investigated extensively. It is generally accepted that the resistance of stainless steels to pitting corrosion, crevice corrosion, and stress corrosion cracking has been obviously improved in some environments with increasing nitrogen in steels [3-5]. Moreover, nitrogen alloying, especially together with molybdenum, improves greatly the resistance to local corrosion due to the synergistic effect of nitrogen and molybdenum [6-7]. Typical mechanisms have been suggested to explain the beneficial roles of nitrogen in stainless steels as the following: (1) nitrogen in solid solution is disCorresponding author: Hua-bing Li, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.
solved and produces NH 4 , depressing oxidation inside a pit [8-9]; (2) concentrated nitrogen and chromium at the surface of passive films stabilizes the film [10-11]; (3) nitrogen addition stabilize the austenitic phase [12]; (4) nitrogen dissolves to form nitrites or nitrates which act as local inhibitor [13-14]; (5) synergistic effects of nitrogen and molybdenum buffer the pH and stabilize molybdates, and molybdates assist the formation of ammonium [6-7]. However, there is still a controversy on the mechanisms, passive film properties, and local corrosion resistance [15-16]. The present work is to investigate the pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels (HNSS) by electrochemical and immersion testing methods in the chloride solution. A commercial 316L stainless steel (316LSS) was selected for comparison. The chemical constitution and composition in the depth of passive films formed on HNSS were analyzed by X-ray photoelectron spectrum (XPS). The effect of nitrogen on the corrosion Also available online at www.sciencedirect.com
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
resistance of HNSS in the chloride solution and the related mechanisms were discussed.
2. Experimental The chemical composition of materials used in the Table 1.
experiment is shown in Table 1. The HNSS specimens designated as A1, A2, and A3 were manufactured with a vacuum induction furnace and an electro-slag remelting furnace under nitrogen atmosphere [17]. A type of 316LSS was also included for comparison.
Chemical composition of steels used in the experiment
wt%
Steels
C
Si
Cr
Mn
Mo
Ni
S
P
Al
O
N
Fe
A1
0.049
0.19
19.56
19.4
2.29
0.003
0.03
0.04
0.0050
0.82
Bal.
A2
0.022
0.19
19.84
18.9
2.26
0.002
0.03
0.02
0.0042
0.88
Bal.
A3
0.058
0.19
19.55
19.5
2.26
0.003
0.03
0.04
0.0048
0.96
Bal.
316LSS
0.014
0.60
17.14
0.8
2.28
12.58
0.007
0.013
0.03
0.0050
Bal.
All the specimens were solution annealed at 1150qC for 1 h followed by water quenching. The anodic polarization curves of A1, A2, A3 and 316LSS were performed in the 3.5wt% NaCl solution. The effects of pH values in the 3.5wt% NaCl solution and the concentration of chloride ions on the pitting corrosion resistance of A2 were also investigated. The pH values of the solution were adjusted by adding HCl and NaOH. The scan rate and test temperature in the above experiments were all controlled at 20 mVmin1 and 30qC, respectively. The critical pitting temperatures (CPT) of HNSS were determined by measuring polarization curves at different temperatures and plotting the breakdown potential versus temperature. The abrupt transition with increasing temperature from transpassive corrosion to pitting corrosion indicates the CPT [18]. Anodic polarization tests were carried out at a scanning rate of 5 mVs1 at the temperature ranging from 30 to 70qC in the 3.5wt% NaCl solution. The breakdown potential (Epit) was determined from the potential at which the current density exceeds 100 μAcm2. All electrochemical measurements were carried out using a potentiostat PARSTAT 2273, which was comprised of three electrodes. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The solution was de-aerated with high purity nitrogen before testing for half an hour and kept under nitrogen atmosphere during testing. The research on the crevice corrosion of HNSS and 316LSS was performed by using a multiple crevice assembly in a 10wt% FeCl3+0.05 mol/L HCl solution. Details on the multiple crevice assembly were described in Ref. [19]. The maximum and average pit depths were measured using a laser scanning confocal microscope (LSCM). The average weight loss was investigated after the specimens were cleaned by ultrasonic cleaning equipment. XPS analysis was performed to obtain the chemical
composition of the passive films of A1, A2 and A3 steels with a VG ESCALAB250 instrument. The passive film was formed by a potential sweep in the anodic direction to the passive region (400 mV) and holding there for 1 h. The Al KD X-ray source was used for photo-electron excitation. The calibration of the binding energy of Au 4f7/2 was found to be 84.0 eV. The depth profiles of the chemical composition were obtained by sputtering argon ions. The C1s electron peak at 284.6 eV was used as the reference for peak binding energy calculation and peak identification to correct the charge-shifting.
3. Results and analysis 3.1. Anodic polarization curves in the 3.5wt% NaCl solution The anodic polarization curves of A1, A2, A3 and 316LSS in the 3.5wt% NaCl solution are shown in Fig. 1. All the steels investigated exhibit obvious passive behavior. The HNSS specimens have a much wider passive potential area compared to 316LSS, and their pitting potentials (Eb10) are about 1 V higher than that of 316LSS. The results indicate that the combined addition of nitrogen and manganese replacing nickel does not degrade the pitting corrosion resistance of the steels, and the pitting corrosion resistance of HNSS is more excellent than that of 316LSS. Furthermore, the pitting potentials and passive potential area of high nitrogen austenitic steels increase with the nitrogen content increasing in steels as shown in Fig. 2. The results agree with that nitrogen alloying can improve obviously the pitting corrosion resistance of the stainless steels in chloride solution [10]. 3.2. Effects of pH value and the concentration of chloride ions on pitting resistance The anodic polarization curves of A2 steel in the 3.5wt% NaCl solutions with the pH values ranging from 1 to 9 are shown in Fig. 3. It can be seen that
H.B. Li et al., Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels
there are a wider passive region of all curves, and the passive regions increase with the pH value of the solution increasing. The occurrence of secondary passivation behaviors is observed in the solution with pH 3, 5.61, 7, and 9 except for that of pH 1.
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ting potentials of A2 steel do not decrease greatly with the pH value of the solution decreasing, and the pitting potential in the solution with pH 1 is about 0.87 V. The above results indicate that the stability of the passive film against pitting corrosion increases with the pH value of the solution increasing, and A2 steel exhibits excellent pitting corrosion resistance in the 3.5wt% NaCl solution with pH values ranging from 1 to 9.
Fig. 1. Anodic polarization curves of A1, A2, A3 and 316LSS in the 3.5wt% NaCl solution. Fig. 4. Effect of pH value on the pitting potential (Eb10) of A2 steel.
Fig. 5 shows the anodic polarization curves of A2 steel in solutions with different NaCl contents. The steel exhibits a wider passive region in all solutions, and its pitting potentials (Eb10) are all above 1 V and decrease a little with increasing the NaCl content in the solution as shown in Fig. 6. The results indicate that A2 steel has excellent pitting corrosion resistance in the solution with high content chloride. Fig. 2. Effect of nitrogen on the pitting potential (Eb10) of HNSS in the 3.5wt% NaCl solution.
Fig. 5. Anodic polarization curves of A2 steel in solutions with different NaCl contents.
3.3. Critical pitting temperature (CPT) Fig. 3. Anodic polarization curves of A2 steel in the solution with different pH values.
Fig. 4 shows the effect of pH value on the pitting potential of A2 steel. With the pH value of the solution increasing, the pitting potentials increase. The pit-
Plots of pitting potentials of HNSS with different nitrogen contents in the 3.5wt% NaCl solution versus temperature are shown in Fig. 7. The CPT values of A1, A2, and A3 steels that can be obtained from the abrupt transition with increasing temperature from
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
transpassive corrosion to pitting corrosion are 60, 63 and 66qC, respectively. With increasing nitrogen in the steels, the CPT increases as shown in Fig. 8, this is consistent with the anodic polarization curves in the 3.5wt% NaCl solution. So a conclusion can be obtained that the beneficial effect of nitrogen alloying on pitting corrosion resistance is clearly apparent. The CPT values of 304 and 316 stainless steels in the 3.5wt% NaCl solution are 23.5qC and 28qC, respectively [20]. Due to their CPT values higher than those of 304 and 316 stainless steels (316SS), high nitrogen steels show better pitting corrosion resistance compared to 304 and 316SS.
Fig. 6. Effect of NaCl content on the pitting potentials (Eb10) of A2 steel.
Fig. 7. Effect of temperature on pitting potentials (Eb100) of A1, A2, and A3 steels in the 3.5wt% NaCl solution: (a) A1 steel; (b) A2 steel; (c) A3 steel.
corrosion as shown in the following equation [3, 21]: PREN=[%Cr]+3.3[%Mo]+(16-30)[%N]
Fig. 8.
Effect of nitrogen on CPT.
The pitting resistance equivalent number (PREN) is usually used to describe the effect of nitrogen, molybdenum, and chromium on the pitting and crevice
(2)
The most commonly used formulae of measures for pitting resistance are PREN20N and PREN30N. The disadvantage of these formulae might be that the negative effect of manganese and the synergistic factor of nitrogen and molybdenum were not taken into account. Several different formulae had been proposed to describe the pitting corrosion resistance of austenitic stainless steels [21]. The measure of alloying for resistance to corrosion (MARC) is a new formula which has recently been introduced by Speidel as shown in the following equation. MARC can describe the pitting and crevice corrosion behaviors best, and is also used to describe the relation between CPT and the
H.B. Li et al., Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels
alloying element content for several commercial alloys [22]: MARC=[%Cr]+3.3[%Mo]+20[%N] 0.5[%Mn]+20[%C]0.25[%Ni]
(3)
As shown in Fig. 9, CPT depends on the alloy composition. The CPT of high nitrogen austenitic stainless steels is correlated with the alloying element content through the MARC as shown in the following equation. The relation of CPT and MARC of 22Cr and 316 stainless steels in the 3.5wt% NaCl solution also meets the equation well as shown in Fig. 9. Obviously, chromium, molybdenum, carbon, and nitrogen improve the pitting corrosion resistance while manganese and nickel reduce the corrosion resistance slightly. CPT=2.55 MARC29
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out in the 10wt% FeCl30.05 mol/L HCl solution. Crevice corrosion areas are observed on the surfaces of all the specimens. The striking crevice corrosion is caused for 316LSS, and some larger and deeper pits can be found on the surround of crevice corrosion area as shown in Fig. 10(d). The results show that the specimen of 316LSS is attacked seriously by crevice corrosion and pitting corrosion.
(4)
3.4. Crevice corrosion Fig. 10 shows the appearance of specimens A1, A2, A2, and 316LSS after crevice corrosion tests carried
Fig. 10.
Fig. 9.
Relation between CPT and MARC.
Appearance of crevice corrosion of A1, A2, A3 steels, and 316LSS: (a) A1 steel; (b)A2 steel; (c) A3 steel; (d) 316LSS.
The maximum and average pit depths of 316LSS are deeper than those of HNSS, and the average weight loss of 316LSS is also higher than that of HNSS as shown in Table 2. With increasing the nitrogen content in steels, the maximum, average pit depths and average weight loss of high nitrogen steels
decrease. The results indicate that HNSS specimens have excellent crevice corrosion resistance than 316LSS, and increasing the nitrogen content in steels improves significantly the crevice corrosion resistance.
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
Table 2.
Maximum, average pit depths and average weight loss of A1, A2, A3, and 316LSS Average pit depth / μm Average weight loss / (gcm2)
Steels
Nitrogen content / wt%
Maximum pit depth / μm
A1
0.82
396.0
278.0
0.00547
A2
0.88
379.2
273.7
0.00436
A3
0.96
254.4
172.4
0.00362
316LSS
1461.0
1119.8
0.08955
3.5. XPS results and analysis Excellent corrosion resistance of stainless steels is attributed to the protective passive film, which prevent metal substratesu from reacting with corrosive environment. So it is very important to investigate the structure and composition of passive films for understanding profoundly the nature of the effect of nitro-
gen on the corrosion resistance. In order to explore the chemical constitution and composition profiles in the depth of passive films formed on HNSS, XPS analysis was performed. Fig. 11 shows the X-ray photoelectron spectra obtained from the passive films formed on A1 steel.
Fig. 11. X-ray photoelectron spectra obtained from the passive films formed on A1 steel.
The Fe2p signal shows the presence of Femet, Fe2+, and Fe3+ oxide. Chromium is present in forms of Crmet, CrN, Cr2O3, Cr(OH)3, and CrO 24 . The fitting results of the spectra of Mo3d appear that Mo is present in Momet, MoO2, MoO(OH)2, and MoO 24 . The spectra of N1s indicates nitrogen presents in forms of CrN, N2(free), N(atom), NH3, and NH 4 . The results are in good agreement with many XPS results of the passive film, confirming the present of NH3 or NH 4 in the passive film of nitrogen containing austenitic stainless steels [8-9]. The present of nitrides CrN or Cr2N in the
passive film of nitrogen alloying stainless steels was reported by other authors [7-8]. The formation of CrN in passive films is due to the anodic segregation [7]. Nitrogen is present on the surface in the form of CrN in the investigation, and the occurring of the following reaction is possible. This will result in the formation of NH3 ligands, or perhaps NH 4 via further protonation, which lowers the pH in the pits and mitigates the environment. 2CrN+3H 2 O o Cr2 O 3 +2NH 3Ligand
(4)
H.B. Li et al., Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels
XPS depth profiles of the passive films formed on HNSS show that chromium enriches in the inner layer
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of the passive film, and nitrogen enriches on surface of the passive film as shown in Fig. 12.
Fig. 12. XPS depth profiles of the passive films formed on HNSS in the 3.5wt% NaCl solution: (a) A1 steel; (b) A2 steel; (c) A3 steel.
Chromium enriching in the form of Cr oxides, hydroxides or nitrides due to the selective dissolution of iron improves the stability of the passive film. Nitrogen in solid solution is dissolved and produces NH3 and NH 4 , which effectively buffer local pH, promote the repassivation, and depress oxidation inside a pit. This may shift the electrode reaction to a higher pH level and favor the formation of MoO 24 and CrO 24 . Both MoO 24 and CrO 24 are responsible for producing bipolar films consisting of a cation selective outer layer containing MoO 24 and CrO 24 and an intrinsically anion selective inner layer. The bipolarity of the duplex film is considered to be largely responsible for the development of an interfacial barrier layer composed mainly of Cr2O3. This is achieved by enhancing the deprotonation of Cr(OH)3 and resistance to the ingression of Cl and OH. Both of these properties should bring a greater resistance to the breakdown of passivity in Cl media and increase local corrosion resistance [7]. So HNSS exhibiting excellent corrosion resistance in chloride solution is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen.
4. Conclusions (1) The pitting corrosion resistance property of
HNSS is superior to that of 316LSS in chloride solution. HNSS has good corrosion resistance in the solution with the pH values ranging from 1 to 9 and the solution with high chloride content. Pitting potentials and critical pitting temperature (CPT) increase with increasing the nitrogen content in steels. The CPT of HNSS is correlated with the alloying element content through the measure of alloying for resistance to corrosion (MARC). The MARC can be expressed as an equation of CPT=2.55 MARC29. (2) HNSS exhibits excellent crevice corrosion than 316LSS. With increasing the nitrogen content in steels, the average pit depth and average weight loss decrease, and the resistance to crevice corrosion of HNSS improves. (3) HNSS exhibiting excellent corrosion resistance is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen.
References [1] F.M. Bayoumi and W.A. Ghanem, Effect of nitrogen on the corrosion behavior of austenitic stainless steel in chloride solutions, Mater. Lett., 59(2005), No.26, p.3311. [2] P.J. Uggowitzer, R. Magowski, and M.O. Speidel, Nickel free high nitrogen austenitic steel, ISIJ Int., 36(1996),
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No.7, p.901. [3] Y. Katada, N. Washizu, and H. Baba, Localized corrosion behavior of high nitrogen austenitic of high nitrogen steel, Mater. Sci. Forum, 475-479(2005), p.225. [4] H. Baba and Y. Katada, Effect of nitrogen on crevice corrosion in austenitic stainless steel, Corros. Sci., 48(2006), No.9, p.2510. [5] H. Baba, T. Kodama, and Y. Katada, Role of nitrogen on the corrosion behavior of austenitic stainless steels, Corros. Sci., 44(2002), No.10, p.2393. [6] C.O.A. Olsson, The influence of nitrogen and molybdenum on passive films formed on the austenoferritic stainless steel 2205 studied by AES and XPS, Corros. Sci., 37(1993), No.3, p.467. [7] Y.C. Lu, M.B. Ives, and C.R. Clayton, Synergism of alloying elements and pitting corrosion resistance of stainless steels, Corros. Sci., 35(1993), No.1-4, p.89. [8] I. Olefjord and L. Wegrelius, The influence of nitrogen on the passivation of stainless steels, Corros. Sci., 38(1996), No.7, p.1203. [9] S.D. Chyou and H.C. Shih, X-ray photoelectron spectroscopy and auger electron spectroscopy studies on the passivation behavior of plasma nitrided low-ally steel in nitric acid, Mater. Sci. Eng. A, 184(1991), No.2, p.241. [10] H.J. Grabke, The role of nitrogen in the corrosion of iron and steels, ISIJ Int., 36(1996), No.7, p.777. [11] Y.C. Lu, R. Bandy, C.R. Clayton, et al., Surface enrichment of nitrogen during passivation of a high resistance stainless steel, J. Electrochem. Soc., 130(1983), No.8, p.1774. [12] Y. Sun, X.Y. Li, and T. Bell, X-ray diffraction characterisation of low temperature plasma nitrided austenitic stainless steels, J. Mater. Sci., 34(1999), No.19, p.4793.
[13] M.U. Kamachi, R.K. Dayal, T.P.S. Gill, et al., Influence of nitrogen addition on microstructure and pitting corrosion resistance of austenitic weld metals, Mater. Corros., 37 (1986), No.12, p.637. [14] H.P. Leckie and H.H. Uhlig, Environmental factors affecting the critical potential for pitting in 18-8 stainless steel, J. Electrochem. Soc., 113(1996), No.12, p.1262. [15] A.S. Vanini, J.P. Audouard, and P. Marcus, The role of nitrogen in the passivity of austenitic stainless steels, Corros. Sci., 36(1996), No.11, p.1825. [16] C.O.A. Olsson and D. Landolt, Passive film on stainless steelChemistry, structure and growth, Electrochim. Acta, 48(2003), No.9, p.1093. [17] H.B. Li, Z.H Jiang, M.H Shen, et al., Manufacturing high nitrogen austenitic stainless steels by nitrogen gas alloying and adding nitrided ferroalloys, J. Iron Steel Res. Int., 14(2007), No.3, p.63. [18] E.A. Abd El Meguid and A.A. Abd El Latif, Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions, Corros. Sci., 49 (2007), No.2, p.263. [19] Y. Katada and H. Baba, Long duration exposure tests of Mn-free high nitrogen stainless steels in seawater, CAMP-ISIJ, 20(2007), No.6, p.1142. [20] W.W. Wu, Y.M Jiang, J.X. Liao, et al., Influence of Cl on critical pitting temperature for 304 and 316 stainless steels, Corros. Sci. Prot. Technol. (in Chinese), 19(2007), No.1, p.16. [21] G. Mori and D. Bauernfeind, Pitting and crevice corrosion of superaustenitic stainless steels, Mater. Corros., 55(2004), No.3, p.164. [22] H.J.C Speidel and M.O. Speidel, Nickel and chromium-based high nitrogen alloys, Mater. Manuf. Processes, 19(2004), No.1, p.95.
Materials
Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels Hua-bing Li, Zhou-hua Jiang, Yan Yang, Yang Cao, and Zu-rui Zhang School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China (Received 2008-12-10)
Abstract: Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels (HNSS) were investigated by electrochemical and immersion testing methods in chloride solution, respectively. The chemical constitution and composition in the depth of passive films formed on HNSS were analyzed by X-ray photoelectron spectrum (XPS). HNSS has excellent pitting and crevice corrosion resistance compared to 316L stainless steel. With increasing the nitrogen content in steels, pitting potentials and critical pitting temperature (CPT) increase, and the maximum, average pit depths and average weight loss decrease. The CPT of HNSS is correlated with the alloying element content through the measure of alloying for resistance to corrosion (MARC). The MARC can be expressed as an equation of CPT=2.55MARC29. XPS results show that HNSS exhibiting excellent corrosion resistance is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen. Key words: high nitrogen austenitic stainless steel; pitting corrosion; crevice corrosion; nitrogen; critical pitting temperature; synergistic effect
[This work was financially supported by the National Natural Science Foundation of China and Baosteel Group Corporation (No.50534010).]
1. Introduction High nitrogen stainless steels have received much attention in recent years due to their excellent mechanical properties and corrosion resistance. Nitrogen is considered as an important alloying addition to austenitic stainless steels replacing partly or entirely nickel without sacrificing mechanical and corrosion resistance properties [1-2]. The beneficial effect of nitrogen on the corrosion resistance has been investigated extensively. It is generally accepted that the resistance of stainless steels to pitting corrosion, crevice corrosion, and stress corrosion cracking has been obviously improved in some environments with increasing nitrogen in steels [3-5]. Moreover, nitrogen alloying, especially together with molybdenum, improves greatly the resistance to local corrosion due to the synergistic effect of nitrogen and molybdenum [6-7]. Typical mechanisms have been suggested to explain the beneficial roles of nitrogen in stainless steels as the following: (1) nitrogen in solid solution is disCorresponding author: Hua-bing Li, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.
solved and produces NH 4 , depressing oxidation inside a pit [8-9]; (2) concentrated nitrogen and chromium at the surface of passive films stabilizes the film [10-11]; (3) nitrogen addition stabilize the austenitic phase [12]; (4) nitrogen dissolves to form nitrites or nitrates which act as local inhibitor [13-14]; (5) synergistic effects of nitrogen and molybdenum buffer the pH and stabilize molybdates, and molybdates assist the formation of ammonium [6-7]. However, there is still a controversy on the mechanisms, passive film properties, and local corrosion resistance [15-16]. The present work is to investigate the pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels (HNSS) by electrochemical and immersion testing methods in the chloride solution. A commercial 316L stainless steel (316LSS) was selected for comparison. The chemical constitution and composition in the depth of passive films formed on HNSS were analyzed by X-ray photoelectron spectrum (XPS). The effect of nitrogen on the corrosion Also available online at www.sciencedirect.com
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
resistance of HNSS in the chloride solution and the related mechanisms were discussed.
2. Experimental The chemical composition of materials used in the Table 1.
experiment is shown in Table 1. The HNSS specimens designated as A1, A2, and A3 were manufactured with a vacuum induction furnace and an electro-slag remelting furnace under nitrogen atmosphere [17]. A type of 316LSS was also included for comparison.
Chemical composition of steels used in the experiment
wt%
Steels
C
Si
Cr
Mn
Mo
Ni
S
P
Al
O
N
Fe
A1
0.049
0.19
19.56
19.4
2.29
0.003
0.03
0.04
0.0050
0.82
Bal.
A2
0.022
0.19
19.84
18.9
2.26
0.002
0.03
0.02
0.0042
0.88
Bal.
A3
0.058
0.19
19.55
19.5
2.26
0.003
0.03
0.04
0.0048
0.96
Bal.
316LSS
0.014
0.60
17.14
0.8
2.28
12.58
0.007
0.013
0.03
0.0050
Bal.
All the specimens were solution annealed at 1150qC for 1 h followed by water quenching. The anodic polarization curves of A1, A2, A3 and 316LSS were performed in the 3.5wt% NaCl solution. The effects of pH values in the 3.5wt% NaCl solution and the concentration of chloride ions on the pitting corrosion resistance of A2 were also investigated. The pH values of the solution were adjusted by adding HCl and NaOH. The scan rate and test temperature in the above experiments were all controlled at 20 mVmin1 and 30qC, respectively. The critical pitting temperatures (CPT) of HNSS were determined by measuring polarization curves at different temperatures and plotting the breakdown potential versus temperature. The abrupt transition with increasing temperature from transpassive corrosion to pitting corrosion indicates the CPT [18]. Anodic polarization tests were carried out at a scanning rate of 5 mVs1 at the temperature ranging from 30 to 70qC in the 3.5wt% NaCl solution. The breakdown potential (Epit) was determined from the potential at which the current density exceeds 100 μAcm2. All electrochemical measurements were carried out using a potentiostat PARSTAT 2273, which was comprised of three electrodes. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The solution was de-aerated with high purity nitrogen before testing for half an hour and kept under nitrogen atmosphere during testing. The research on the crevice corrosion of HNSS and 316LSS was performed by using a multiple crevice assembly in a 10wt% FeCl3+0.05 mol/L HCl solution. Details on the multiple crevice assembly were described in Ref. [19]. The maximum and average pit depths were measured using a laser scanning confocal microscope (LSCM). The average weight loss was investigated after the specimens were cleaned by ultrasonic cleaning equipment. XPS analysis was performed to obtain the chemical
composition of the passive films of A1, A2 and A3 steels with a VG ESCALAB250 instrument. The passive film was formed by a potential sweep in the anodic direction to the passive region (400 mV) and holding there for 1 h. The Al KD X-ray source was used for photo-electron excitation. The calibration of the binding energy of Au 4f7/2 was found to be 84.0 eV. The depth profiles of the chemical composition were obtained by sputtering argon ions. The C1s electron peak at 284.6 eV was used as the reference for peak binding energy calculation and peak identification to correct the charge-shifting.
3. Results and analysis 3.1. Anodic polarization curves in the 3.5wt% NaCl solution The anodic polarization curves of A1, A2, A3 and 316LSS in the 3.5wt% NaCl solution are shown in Fig. 1. All the steels investigated exhibit obvious passive behavior. The HNSS specimens have a much wider passive potential area compared to 316LSS, and their pitting potentials (Eb10) are about 1 V higher than that of 316LSS. The results indicate that the combined addition of nitrogen and manganese replacing nickel does not degrade the pitting corrosion resistance of the steels, and the pitting corrosion resistance of HNSS is more excellent than that of 316LSS. Furthermore, the pitting potentials and passive potential area of high nitrogen austenitic steels increase with the nitrogen content increasing in steels as shown in Fig. 2. The results agree with that nitrogen alloying can improve obviously the pitting corrosion resistance of the stainless steels in chloride solution [10]. 3.2. Effects of pH value and the concentration of chloride ions on pitting resistance The anodic polarization curves of A2 steel in the 3.5wt% NaCl solutions with the pH values ranging from 1 to 9 are shown in Fig. 3. It can be seen that
H.B. Li et al., Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels
there are a wider passive region of all curves, and the passive regions increase with the pH value of the solution increasing. The occurrence of secondary passivation behaviors is observed in the solution with pH 3, 5.61, 7, and 9 except for that of pH 1.
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ting potentials of A2 steel do not decrease greatly with the pH value of the solution decreasing, and the pitting potential in the solution with pH 1 is about 0.87 V. The above results indicate that the stability of the passive film against pitting corrosion increases with the pH value of the solution increasing, and A2 steel exhibits excellent pitting corrosion resistance in the 3.5wt% NaCl solution with pH values ranging from 1 to 9.
Fig. 1. Anodic polarization curves of A1, A2, A3 and 316LSS in the 3.5wt% NaCl solution. Fig. 4. Effect of pH value on the pitting potential (Eb10) of A2 steel.
Fig. 5 shows the anodic polarization curves of A2 steel in solutions with different NaCl contents. The steel exhibits a wider passive region in all solutions, and its pitting potentials (Eb10) are all above 1 V and decrease a little with increasing the NaCl content in the solution as shown in Fig. 6. The results indicate that A2 steel has excellent pitting corrosion resistance in the solution with high content chloride. Fig. 2. Effect of nitrogen on the pitting potential (Eb10) of HNSS in the 3.5wt% NaCl solution.
Fig. 5. Anodic polarization curves of A2 steel in solutions with different NaCl contents.
3.3. Critical pitting temperature (CPT) Fig. 3. Anodic polarization curves of A2 steel in the solution with different pH values.
Fig. 4 shows the effect of pH value on the pitting potential of A2 steel. With the pH value of the solution increasing, the pitting potentials increase. The pit-
Plots of pitting potentials of HNSS with different nitrogen contents in the 3.5wt% NaCl solution versus temperature are shown in Fig. 7. The CPT values of A1, A2, and A3 steels that can be obtained from the abrupt transition with increasing temperature from
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International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009
transpassive corrosion to pitting corrosion are 60, 63 and 66qC, respectively. With increasing nitrogen in the steels, the CPT increases as shown in Fig. 8, this is consistent with the anodic polarization curves in the 3.5wt% NaCl solution. So a conclusion can be obtained that the beneficial effect of nitrogen alloying on pitting corrosion resistance is clearly apparent. The CPT values of 304 and 316 stainless steels in the 3.5wt% NaCl solution are 23.5qC and 28qC, respectively [20]. Due to their CPT values higher than those of 304 and 316 stainless steels (316SS), high nitrogen steels show better pitting corrosion resistance compared to 304 and 316SS.
Fig. 6. Effect of NaCl content on the pitting potentials (Eb10) of A2 steel.
Fig. 7. Effect of temperature on pitting potentials (Eb100) of A1, A2, and A3 steels in the 3.5wt% NaCl solution: (a) A1 steel; (b) A2 steel; (c) A3 steel.
corrosion as shown in the following equation [3, 21]: PREN=[%Cr]+3.3[%Mo]+(16-30)[%N]
Fig. 8.
Effect of nitrogen on CPT.
The pitting resistance equivalent number (PREN) is usually used to describe the effect of nitrogen, molybdenum, and chromium on the pitting and crevice
(2)
The most commonly used formulae of measures for pitting resistance are PREN20N and PREN30N. The disadvantage of these formulae might be that the negative effect of manganese and the synergistic factor of nitrogen and molybdenum were not taken into account. Several different formulae had been proposed to describe the pitting corrosion resistance of austenitic stainless steels [21]. The measure of alloying for resistance to corrosion (MARC) is a new formula which has recently been introduced by Speidel as shown in the following equation. MARC can describe the pitting and crevice corrosion behaviors best, and is also used to describe the relation between CPT and the
H.B. Li et al., Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels
alloying element content for several commercial alloys [22]: MARC=[%Cr]+3.3[%Mo]+20[%N] 0.5[%Mn]+20[%C]0.25[%Ni]
(3)
As shown in Fig. 9, CPT depends on the alloy composition. The CPT of high nitrogen austenitic stainless steels is correlated with the alloying element content through the MARC as shown in the following equation. The relation of CPT and MARC of 22Cr and 316 stainless steels in the 3.5wt% NaCl solution also meets the equation well as shown in Fig. 9. Obviously, chromium, molybdenum, carbon, and nitrogen improve the pitting corrosion resistance while manganese and nickel reduce the corrosion resistance slightly. CPT=2.55 MARC29
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out in the 10wt% FeCl30.05 mol/L HCl solution. Crevice corrosion areas are observed on the surfaces of all the specimens. The striking crevice corrosion is caused for 316LSS, and some larger and deeper pits can be found on the surround of crevice corrosion area as shown in Fig. 10(d). The results show that the specimen of 316LSS is attacked seriously by crevice corrosion and pitting corrosion.
(4)
3.4. Crevice corrosion Fig. 10 shows the appearance of specimens A1, A2, A2, and 316LSS after crevice corrosion tests carried
Fig. 10.
Fig. 9.
Relation between CPT and MARC.
Appearance of crevice corrosion of A1, A2, A3 steels, and 316LSS: (a) A1 steel; (b)A2 steel; (c) A3 steel; (d) 316LSS.
The maximum and average pit depths of 316LSS are deeper than those of HNSS, and the average weight loss of 316LSS is also higher than that of HNSS as shown in Table 2. With increasing the nitrogen content in steels, the maximum, average pit depths and average weight loss of high nitrogen steels
decrease. The results indicate that HNSS specimens have excellent crevice corrosion resistance than 316LSS, and increasing the nitrogen content in steels improves significantly the crevice corrosion resistance.
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Table 2.
Maximum, average pit depths and average weight loss of A1, A2, A3, and 316LSS Average pit depth / μm Average weight loss / (gcm2)
Steels
Nitrogen content / wt%
Maximum pit depth / μm
A1
0.82
396.0
278.0
0.00547
A2
0.88
379.2
273.7
0.00436
A3
0.96
254.4
172.4
0.00362
316LSS
1461.0
1119.8
0.08955
3.5. XPS results and analysis Excellent corrosion resistance of stainless steels is attributed to the protective passive film, which prevent metal substratesu from reacting with corrosive environment. So it is very important to investigate the structure and composition of passive films for understanding profoundly the nature of the effect of nitro-
gen on the corrosion resistance. In order to explore the chemical constitution and composition profiles in the depth of passive films formed on HNSS, XPS analysis was performed. Fig. 11 shows the X-ray photoelectron spectra obtained from the passive films formed on A1 steel.
Fig. 11. X-ray photoelectron spectra obtained from the passive films formed on A1 steel.
The Fe2p signal shows the presence of Femet, Fe2+, and Fe3+ oxide. Chromium is present in forms of Crmet, CrN, Cr2O3, Cr(OH)3, and CrO 24 . The fitting results of the spectra of Mo3d appear that Mo is present in Momet, MoO2, MoO(OH)2, and MoO 24 . The spectra of N1s indicates nitrogen presents in forms of CrN, N2(free), N(atom), NH3, and NH 4 . The results are in good agreement with many XPS results of the passive film, confirming the present of NH3 or NH 4 in the passive film of nitrogen containing austenitic stainless steels [8-9]. The present of nitrides CrN or Cr2N in the
passive film of nitrogen alloying stainless steels was reported by other authors [7-8]. The formation of CrN in passive films is due to the anodic segregation [7]. Nitrogen is present on the surface in the form of CrN in the investigation, and the occurring of the following reaction is possible. This will result in the formation of NH3 ligands, or perhaps NH 4 via further protonation, which lowers the pH in the pits and mitigates the environment. 2CrN+3H 2 O o Cr2 O 3 +2NH 3Ligand
(4)
H.B. Li et al., Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels
XPS depth profiles of the passive films formed on HNSS show that chromium enriches in the inner layer
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of the passive film, and nitrogen enriches on surface of the passive film as shown in Fig. 12.
Fig. 12. XPS depth profiles of the passive films formed on HNSS in the 3.5wt% NaCl solution: (a) A1 steel; (b) A2 steel; (c) A3 steel.
Chromium enriching in the form of Cr oxides, hydroxides or nitrides due to the selective dissolution of iron improves the stability of the passive film. Nitrogen in solid solution is dissolved and produces NH3 and NH 4 , which effectively buffer local pH, promote the repassivation, and depress oxidation inside a pit. This may shift the electrode reaction to a higher pH level and favor the formation of MoO 24 and CrO 24 . Both MoO 24 and CrO 24 are responsible for producing bipolar films consisting of a cation selective outer layer containing MoO 24 and CrO 24 and an intrinsically anion selective inner layer. The bipolarity of the duplex film is considered to be largely responsible for the development of an interfacial barrier layer composed mainly of Cr2O3. This is achieved by enhancing the deprotonation of Cr(OH)3 and resistance to the ingression of Cl and OH. Both of these properties should bring a greater resistance to the breakdown of passivity in Cl media and increase local corrosion resistance [7]. So HNSS exhibiting excellent corrosion resistance in chloride solution is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen.
4. Conclusions (1) The pitting corrosion resistance property of
HNSS is superior to that of 316LSS in chloride solution. HNSS has good corrosion resistance in the solution with the pH values ranging from 1 to 9 and the solution with high chloride content. Pitting potentials and critical pitting temperature (CPT) increase with increasing the nitrogen content in steels. The CPT of HNSS is correlated with the alloying element content through the measure of alloying for resistance to corrosion (MARC). The MARC can be expressed as an equation of CPT=2.55 MARC29. (2) HNSS exhibits excellent crevice corrosion than 316LSS. With increasing the nitrogen content in steels, the average pit depth and average weight loss decrease, and the resistance to crevice corrosion of HNSS improves. (3) HNSS exhibiting excellent corrosion resistance is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen.
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