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Depth profile analysis of thin passive films on stainless steel by glow discharge optical emission spectroscopy
Corrosion Science 51 (2009) 1554–1559
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Depth profile analysis of thin passive films on stainless steel by glow discharge optical emission spectroscopy M. Uemura a, T. Yamamoto a, K. Fushimi a, Y. Aoki a, K. Shimizu b, H. Habazaki a,* a b
Graduate School of Engineering, Hokkaido University, N13-W8, Sapporo 060-8628, Japan Univeristy Chemical Laboratory, Keio Univeristy, 4-1-1 Hiyoshi, Yokohama 223-8521, Japan
a r t i c l e
i n f o
Available online 25 November 2008
Keywords: A. Stainless steel GDOES C. Passive films
a b s t r a c t Thin passive films formed on highly corrosion-resistant type-312L stainless steel, containing 20 mass% chromium and 6 mass% molybdenum, in 2 mol dm 3 HCl solution at 293 K have been analyzed by glow discharge optical emission spectroscopy (GDOES). The stainless steel does not suffer pitting corrosion even in this aggressive solution, showing a wide passive potential region. The depth profiles obtained clearly show a two-layer structure of the air-formed and passive films: an outer iron-rich layer and an inner layer highly enriched in chromium. Alloy-constituting molybdenum is deficient in the inner layer of the passive films and is enriched in the outer layer, particularly at the active dissolution potential. The molybdenum species in the outer layer may retard the active dissolution of stainless steel, promoting the formation of stable passive films highly enriched in chromium. Chloride ions are present only at the outermost part of the passive films, not penetrating into the interior part. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Glow discharge optical emission spectroscopy (GDOES) has been widely used for depth profiling of relatively thick films, several tens of lm in thickness, such as galvanized or painted steels, due to its high sputtering rates of more than 1 lm min 1 [1]. It has recently been revealed the enormous potential of glow discharge optical emission spectroscopy (GDOES) for depth profiling of extremely thin films, either conducting or non-conducting, less than 10 nm in thickness [2]. Even the orientation of thiourea molecules adsorbed on a mirror-finished copper surface has been clearly resolved by rf-GDOES [3]. Such significant features arise from the nature of rf-GD sputtering, samples being sputtered very stably, from the commencement of analysis, with Ar+ ions of very low energies, <50 eV, wide incident angles and high current densities of the order of 100 mA cm 2 [4]. In this work, rf-GDOES, with excellent depth resolution, has been applied for depth profiling of thin passive films formed on type-312L stainless steel, which is highly resistant to pitting corrosion in acidic and neutral chloride-containing solutions. The stainless steel contains 6 mass% of molybdenum, which may primarily contribute to the improved resistance to localized corrosion, although low sulphur content [5] and presence of nitrogen [6] in the steel also reduce the pitting susceptibility.
* Corresponding author. Tel.: +81 11 706 6575; fax: +81 11 706 6575. E-mail address: [email protected] (H. Habazaki). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.11.017
The beneficial role of molybdenum in enhancing the stability of passive state and in improving the pitting corrosion resistance of stainless steels have been extensively investigated using surface analytical methods [7–15] and electrochemical measurements [16–21]. The beneficial role of molybdenum has been explained by adsorption of molybdenum species [22], by formation of molybdenum compounds [23], by synergistic interaction of molybdenum and other alloying elements [7,24] and by suppressing active dissolution by formation of molybdenum compounds [8]. Due to absence of consistency for the role of molybdenum in improved corrosion resistance of stainless steels, further studies are needed to obtain a better understanding of the role of molybdenum [25]. In neutral chloride solutions, accumulation of molybdenum species in the passive films was generally observed [26,17]. A bipolar model [27] is proposed to explain the improved pitting resistance. The presence of molybdate ions in the outer part of the passive films changes its anionic selectivity to a cationic one and induces the formation of a bipolar layer that promotes the migration of O2 ions and the formation of chromium oxide [23]. In acidic solutions, including the inside of pits formed in neutral chloride solutions, molybdenum reduces the active dissolution rate of the metallic substrate, promoting passivation. Molybdenum is passive in the active region of stainless steels, reducing the dissolution rate by forming MoO2-containing film [15]. In a separate paper, the present authors reported results of depth profiling of passive films formed on type-312L stainless steel in neutral chloride solution by GDOES. The results disclosed that molybdenum accumulated in the outer part of the passive films,
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which thickened with increase in the polarization potential in NaCl solution [28]. In this work, thin passive films formed in aggressive HCl solution were analyzed by GDOES to get further insights into the role of molybdenum in enhancement of passivity.
XPS spectra of the as-polished specimen was also measured using a Shimadzu ESCA-3000 system with Mg Ka excitation (hm = 1253.6 eV). Binding energies of the photoelectrons were calibrated by a method described elsewhere [29].
2. Experimental
3. Results and discussion
A type-312L stainless steel plate of 2 mm in thickness was used in this work. The composition of the type-312L stainless steel is shown in Table 1. The surface of the specimens was polished mechanically with SiC papers up to No.1500 and finally mirror-finished with diamond slurry of 3 lm. The preparation of a mirrorfinished surface was essential for obtaining depth profiles with excellent depth resolution. Potentiostatic and potentiodynamic polarization curves of the specimens were obtained in deaerated 2 mol dm 3 HCl solution at 293 K. The potentiodynamic polarization was performed at a potential sweep rate of 1 mV s 1 and the potentiostatic polarization was carried out at several potentials for 1.8 ks. After potentiostatic polarization, the specimens were washed in deaerated distilled water under an argon atmosphere. Then the specimens were dried and immediately analyzed by GDOES. The air-exposure time before GDOES analysis was less than 5 min. Depth profiles of the specimens polarized potentiostatically and as-polished were obtained using a Jobin-Yvon 5000RF GDOES instrument in an argon atmosphere of 800 Pa with application of RF of 13.56 MHz and power of 40 W. Light emissions of characteristic wavelengths were monitored throughout the analysis with a sampling time of 1 ms to obtain depth profiles. The wavelengths of the spectral lines used were 121.567 nm (hydrogen), 130.217 nm (oxygen), 134.724 nm (chloride), 425.433 nm (chromium), 385.991 nm (iron), 341.477 nm (nickel), 224.700 nm (copper) and 317.035 nm (molybdenum). The signals were detected from a circular area of approximately 4 mm in diameter. For quantification, type-304, type-316L and type-312L stainless steels were used as reference materials.
Fig. 1 shows the potentiodynamic and potentiostatic polarization curves of type-312L stainless steel in 2 mol dm 3 HCl solution at 293 K. In this solution, no passivation occurred on type-304 and type-316L stainless steels. However, the type-312L stainless steel is passivated at about 0.15 V vs. Ag/AgCl, showing a wide passive region to 1.0 V vs. Ag/AgCl where transpassive dissolution commences. No pitting corrosion occurs in this aggressive acid solution. During the potentiostatic polarization at 0.18 V vs. Ag/ AgCl, the current density increased gradually with polarization time, probably due to breakdown of an air-formed oxide film and increased surface roughness. In the passive potential region, the
a Alloy/film interface Fe
Cu
Intensity / arb.unit
Cr
H Ni O Mo
Table 1 Composition of the type 312L stainless steel used (mass%). C
Si
Mn
P
S
Ni
Cr
Mo
Cu
N
0
Fe
20
40
60
80
100
Sputtering time / ms
SUS312L 0.009 0.48 0.31 0.026 0.0006 17.95 20.16 6.29 0.77 0.207 53.79
b
Current Density, i / A m-2
10 10 10 10
3
Alloy/film interface
2
Intensity / arb.unit
10
1
0
-1
Fe
Cr
H
Cu
Ni
O 10 10 10
-2
-3
Cl
Mo
-4
-0.5
0.0
0.5
1.0
1.5
Potential, E / V vs. Ag / AgCl Fig. 1. Potentiodynamic and potentiostatic polarization curves of type-312L stainless steel in 2 mol dm 3 HCl solution at 293 K.
0
20
40
60
80
100
Sputtering time / ms Fig. 2. GDOES depth profiles of type-312L stainless steel specimens (a) as-polished and (b) polarized at 0.5 V vs. Ag/AgCl in 2 mol dm 3 HCl for 1.8 ks.
M. Uemura et al. / Corrosion Science 51 (2009) 1554–1559
current densities after the potentiostatic polarization for 1.8 ks are in the order of 1 mA m 2 or less, indicating high stability of the passive state. The passive current density increases slightly with increase in polarization potential. The marked increase in current density at 1.0 V vs. Ag/AgCl is associated with the transpassive dissolution of chromium. Fig. 2 shows depth profiles of an as-polished specimen (Fig. 2a) and a specimen passivated at 0.5 V vs. Ag/AgCl (Fig. 2b). The thickness of the air-formed film should be only a few nm, but enrichment of chromium species in the inner part of the film and even accumulation of nickel in the underlying alloy substrate are clearly resolved. In addition, significant enrichment of copper at the alloy/ film interface region is obvious. Molybdenum is present in the outer part of the film but highly deficient in the inner layer. The hydrogen profile is also directly measured using GDOES. The hydrogen species are present mainly in the outer part of the air-formed film, where the film material should be formed at the film surface by outward diffusion of cations. The concentration of hydrogen species in the inner layer is low. The depth profile of the air-formed film on type-304 stainless steel revealed the presence of hydrogen species throughout the thickness of the air-formed film. The findings may indicate that the molybdenum-containing outer layer on type-312L stainless steel prevents penetration of OH anions into the inner layer possibly due to the cation-selective nature of the outer layer [27]. The chemical state of the constituting elements in the airformed film cannot be identified by GDOES. Such supplemental information was obtained by XPS analysis. As shown in Fig. 3, oxidized species of iron, chromium, nickel and molybdenum are present in the air-formed film. From the peak intensity of each oxidized species with respect to the corresponding metallic peak intensity,
c
Fe 2p
716
712
Feo
708
704
585
Binding Energy / eV
Intensity / arb.unit
Nio Ni2+
865
860
855
850
Binding Energy / eV
Cro
580
575
570
565
945
Binding Energy / eV
d
Ni 2p3/2
Intensity / arb.unit
b
Cu 2p3/2
Cr3+
Intensity / arb.unit
Intensity / arb.unit
Fe3+ Fe2+
e
Cr 2p3/2
Mo 3d
Mo4+
Mo6+
235
Cu0, Cu+
940
230
f
Moo
225
935
930
925
Binding Energy / eV
Cu L3M4,5M4,5
Intensity / arb.unit
a
oxidized iron and chromium species are the major cationic components in the air-formed film. Hexavalent molybdenum is the major molybdenum species in the air-formed film as shown in the Mo3d spectrum. Although the copper content in type-312L is only 0.77 mass%, a relatively intense peak appears in the Cu 2p spectrum, due to enrichment at the alloy/film interface. The peak binding energy corresponds to metallic copper or monovalent copper species. The Cu L3M4,5M4,5 Auger peak shows that metallic copper, not monovalent copper, is predominant. Thus, metallic copper is enriched immediately beneath the air-formed film, due to preferential oxidation of other metallic elements. High diffusivity of copper atoms in a metallic substrate may also contribute to the accumulation of copper at the alloy surface [30]. The depth profile of the specimen passivated at 0.5 V vs. Ag/ AgCl (Fig. 2b) shows that hydrogen species are present almost throughout the film thickness. Due to the reduced molybdenum content in the outer layer of the passive film, OH- ions are now able to penetrate into the inner layer. Since the sensitivity of GDOES for detecting various elements is higher than the sensitivities of XPS and AES, a trace amount of chloride ions are detected but only at the outermost surface of the passive film. No chloride ions penetrated into the interior of the passive film, as found in previous XPS and AES studies [8,9]. To show more quantitatively the depth distribution of the components of the stainless steel, mass fraction of the alloy component is depicted as a function of sputtering time (Fig. 4). It is obvious for all specimens that the surface films consist of two layers: an outer iron-rich layer and an inner layer highly enriched in chromium species. The concentration of chromium in the inner layer increases with increase in polarization potential, except at 1.0 V vs. Ag/AgCl, at which transpassive dissolution of chromium
Intensity / arb.unit
1556
220
Binding Energy / eV
910
Cuo
920
930
KineticEnergy / eV
Fig. 3. XPS spectra of as-polished type-312L stainless steel. (a) Fe 2p3/2, (b) Ni 2p3/2, (c) Cr 2p3/2, (d) Mo 3d, (e) Cu 2p3/2, and (f) Cu L3M4,5M4,5 Auger electrons.
1557
d
1.0 Alloy/film interface as-polished
0.8
Cr 0.6
Fe
0.4
Ni 0.2
Cu 0.0 0.00
Mo
0.05
0.10
Mass fraction of alloy component
a
Mass fraction of alloy component
M. Uemura et al. / Corrosion Science 51 (2009) 1554–1559
0.15
1.0
Cr
0.8 V vs Ag/AgCl
0.6
Fe
0.4
Ni 0.2
0.05
-0. 18 V vs Ag/AgCl
0.6
Fe
0.4
Ni 0.2
Cu
Mo 0.0 0.00
0.05
0.10
Mass fraction of alloy component
Mass fraction of alloy component
e Alloy/film interface
Cr
Mass fraction of alloy component
0.15
0.15
1.0 Alloy/film interface
0.8
Cr
1.0 V vs Ag/Ag Cl
0.6
Fe
0.4
Ni 0.2
Mo 0.0 0.00
0.05
Sputtering time / s
c
0.10
Sputtering time / s
1.0 0.8
Mo
Cu 0.0 0.00
Sputtering time / s
b
Alloy/film interface
0.8
0.10
Cu 0.15
Sputtering time / s
1.0
Cr
Alloy/film interface
0.8
0.2 V vs Ag/AgCl
0.6
Fe
0.4
Ni 0.2
Mo
Cu 0.0 0.00
0.05
0.10
0.15
Sputtering time / s Fig. 4. GDOES depth profiles of type-312L stainless steel specimens (a) as-polished and polarized at (b)
commences. The oxidized nickel found by XPS is mainly present in the outer iron-rich layer and is deficient in the inner layer. Similarly, molybdenum is deficient in the chromium-enriched inner layer and is mainly present in the outer layer of the film. The molybdenum content also slightly decreases in the underlying alloy beneath the surface films. At the active potential of 0.18 V vs. Ag/AgCl, the enrichment of molybdenum in the outer layer of the film is significant, but the molybdenum content in the inner layer is still low. Molybdenum is known to reduce the active dissolution rate of stainless steels [8,16,31] and iron-chromium alloys [32]. Molybdenum is passive in the active region of stainless steels in acid solutions, and tetravalent molybdenum is present in the surface film [33]. Thus, the tetravalent molybdenum enriched in the outer layer of the film retards the dissolution of stainless steel and promotes passivation.
0.18 V, (c) 0.2 V, (d) 0.8 V and (e) 1.0 V vs. Ag/AgCl in 2 mol dm
3
HCl.
The enrichment of copper immediately beneath the air-formed film is more significant than that beneath the passive films. The concentration of copper reaches 6 mass% immediately beneath the air-formed film, decreasing with increase in polarization potential, since copper is oxidized and dissolved in the solution under anodic polarization. Assuming that the sputtering rate is constant throughout a passive film and independent of film composition, the relative thicknesses of the surface films formed at several potentials can be compared by the sputtering time of the surface films. Fig. 5 shows the sputtering time for the surface films as a function of polarization potential. The sputtering time for the outer iron-rich layer is also shown in this figure. XPS analysis of the air-formed film indicated that its thickness is approximately 3 nm. Thus, the thinnest passive film formed at 0.2 V vs. Ag/AgCl is estimated to be
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M. Uemura et al. / Corrosion Science 51 (2009) 1554–1559
80
Sputtering time / ms
60
Film
40
as-polished 20
Outer iron-rich layer
3
in oxide film / arb.unit
Normalized Intensity of Cr or Mo
4
2
Cr
1
Mo as-polished
0
0.0
0.5
1.0
0
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential, E / V vs. Ag / AgCl
Potential, E / V vs. Ag / AgCl Fig. 5. Sputtering time for the surface film as well as for the outer iron-rich layer as a function of polarization potential.
Fig. 6. Integrated intensities of chromium and molybdenum in the surface film, normalized by those in the air-formed film, as a function of polarization potential.
2.3 nm in thickness. Further increase in polarization potential increases the film thickness to make the field strength in the passive film constant. At the polarization potential of 1.0 V vs. Ag/AgCl, the film thickness increases greatly. In this transpassive region, the film formed is no longer highly protective as in the passive region. The thinning of the outer iron-rich layer in the passive region is also obvious in Fig. 5. Iron-rich oxide or hydrated oxyhydroxide is not chemically stable in acidic chloride solutions and dissolves readily. Thus, the outer layer becomes thin in 2 mol dm 3 HCl solution. The thickness of the outer layer with respect to the total thickness of the passive film is 0.2–0.25. Thus, the major part of the passive film formed on type-312L stainless steel in 2 mol dm 3 HCl is a chromium-enriched inner layer. In passive films formed in 0.9 mol dm 3 NaCl solution, it was found that the outer iron-rich layer became thick with increase in polarization potential and the molybdenum concentration increased in the outer layer [28]. In the present HCl solution, the outer layer of the passive films is thin and the molybdenum concentration is relatively low. The bipolar model proposed by Sakashita and Sato [27] may not be applicable to the present passive films. The integrated intensity of molybdenum and chromium in the surface films obtained by GDOES depth profiles are plotted as a function of polarization potential (Fig. 6). The intensity has been normalized by the integrated intensity of chromium and molybdenum in the air-formed film. Since the integrated intensity is roughly proportional to the amount of these elements in the surface film, this potential dependence should represent the change in the amount of these elements in the surface films as a function of potential. The intensity of molybdenum increases in the active region of 0.18 V vs. Ag/AgCl. The amount of molybdenum is still large at 0 V vs. Ag/AgCl, though this potential is in the passive region of stainless steel. This is due to the fact that this potential is still in the passive region of molybdenum [32]. The amounts of chromium in the surface films formed at all potentials are larger than that in the air-formed film. The amount of chromium at 0.8 V vs. Ag/AgCl is approximately 3.8 times as large as that at 0.2 V vs. Ag/AgCl. This is almost in agreement with the fact that the charge passed during polarization at 0.8 V vs. Ag/AgCl (21 C m 2) is 3.5 times as large as that at 0.2 V vs. Ag/AgCl (6.0 C m 2). In the passive region, chro-
mium-enriched passive films should be developed without significant dissolution of chromium. The present study clearly demonstrates that molybdenum species in surface films on type-312L stainless steel are present always in the outer part of the film, not in the inner chromium-enriched layer. Through XPS analysis it is well known that air exposure of polarized specimens results in the change in the chemical state of molybdenum species, i.e., oxidation of Mo4+ to Mo6+ species. The air exposure could not be avoided in our GDOES system, but efforts were made to minimize the exposure time before depth profiling in this work. Since a distinct difference was found in the concentrations of molybdenum in the outer layer and inner layer of the surface films at all potentials, it is likely that the depth distribution before exposure to air is similar to that obtained in this study. Thus, the outer layer containing molybdenum species, formed in the active region of stainless steel, reduces the dissolution rate of the stainless steel and promotes the accumulation of chromium in the inner layer. The findings in this work support the role of molybdenum in improving the passivity and resistance to localized corrosion proposed by Hashimoto [34]. The stability of the passive film is mainly attributed to the inner chromium-enriched oxide film. Once breakdown of the passive film occurs chemically or mechanically, molybdenum species formed on the surface may reduce the dissolution and assist repassivation, resulting in suppression of the initiation and growth of pits. 4. Conclusions (1) Depth profiles of thin passive films as well as an air-formed film on highly corrosion-resistant type-312L stainless steel can be obtained at sufficient depth resolution by GDOES. (2) The air-formed film consists of two layers, comprising an outer iron-rich layer and an inner layer highly enriched in chromium species. Molybdenum species are present in the outer layer and deficient in the inner layer. (3) The outer layer becomes thin after polarization at passive potentials in 2 mol dm 3 HCl solution, due to dissolution of the iron-rich oxy-hydroxide layer. Enrichment of chromium
M. Uemura et al. / Corrosion Science 51 (2009) 1554–1559
is more significant in the inner layer with increasing polarization potential. (4) Marked enrichment of tetravalent molybdenum species occurs in the outer part of the film, not in the inner layer, at the active dissolution potential, suppressing the dissolution rate of the stainless steel. (5) Chloride ions do not penetrate into the passive film, being present only at the outermost surface of the passive film.
Acknowledgments Thanks are due to Mr. T. Adachi, Nippon Yakin Kogyo Co., Ltd. for supplying the type-312L stainless steel specimens. The present work was supported in part by Grants-in-Aid for Scientific Research (A) No. 19206077 and for Exploratory Research, No.19656184 from the Japan Society for the Promotion of Science as well as by the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] R. Payling, Glow discharge optical emission spectrometry, Spectroscopy 13 (1998) 36–43. [2] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson, Radiofrequency GDOES: a powerful technique for depth profiling analysis of thin films, Surf. Interface Anal. 35 (2003) 564–574. [3] K. Shimizu, R. Payling, H. Habazaki, P. Skeldon, G.E. Thompson, Rf-GDOES depth profiling analysis of a monolayer of thiourea adsorbed on copper, J. Anal. At. Spectrom. 19 (2004) 692–695. [4] Y.C. Ye, R.K. Marcus, Effects of limiting orifice (anode) geometries on charged particle characteristics in an analytical radiofrequency glow discharge as determined by Langmuir, current and voltage probes, J. Anal. At. Spectrom. 12 (1997) 33–41. [5] Z. Szklarska-Smialowska, Pitting Corrosion of Metals. 1986, Houston: NACE. [6] I. Olefjord, L. Wegrelius, The influence of nitrogen on the passivation of stainless steels, Corros. Sci. 38 (1996) 1203–1220. [7] K. Sugimoto, Y. Sawada, The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions, Corros. Sci. 17 (1977) 425–445. [8] K. Hashimoto, K. Asami, K. Teramoto, An X-ray photoelectron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steel in 1 N HCl, Corros. Sci. 19 (1979) 3–14. [9] A. Schneider, S. Hofmann, R. Kirchheim, Auger-spectroscopic investigations into pitting corrosion of FeCr, FeMo and FeCrMo Alloys, Werkst. Korros.-Mater. Corros. 42 (1991) 169–178. [10] E. Devito, P. Marcus, XPS study of passive films formed on molybdenumimplanted austenitic stainless-steels, Surf. Interface Anal. 19 (1992) 403–408. [11] 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 (1995) 467–479. [12] M.F. Montemor, A.M.P. Simoes, M.G.S. Ferreira, M.D. Belo, The role of Mo in the chemical composition and semiconductive behaviour of oxide films formed on stainless steels, Corros. Sci. 41 (1999) 17–34.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Depth profile analysis of thin passive films on stainless steel by glow discharge optical emission spectroscopy M. Uemura a, T. Yamamoto a, K. Fushimi a, Y. Aoki a, K. Shimizu b, H. Habazaki a,* a b
Graduate School of Engineering, Hokkaido University, N13-W8, Sapporo 060-8628, Japan Univeristy Chemical Laboratory, Keio Univeristy, 4-1-1 Hiyoshi, Yokohama 223-8521, Japan
a r t i c l e
i n f o
Available online 25 November 2008
Keywords: A. Stainless steel GDOES C. Passive films
a b s t r a c t Thin passive films formed on highly corrosion-resistant type-312L stainless steel, containing 20 mass% chromium and 6 mass% molybdenum, in 2 mol dm 3 HCl solution at 293 K have been analyzed by glow discharge optical emission spectroscopy (GDOES). The stainless steel does not suffer pitting corrosion even in this aggressive solution, showing a wide passive potential region. The depth profiles obtained clearly show a two-layer structure of the air-formed and passive films: an outer iron-rich layer and an inner layer highly enriched in chromium. Alloy-constituting molybdenum is deficient in the inner layer of the passive films and is enriched in the outer layer, particularly at the active dissolution potential. The molybdenum species in the outer layer may retard the active dissolution of stainless steel, promoting the formation of stable passive films highly enriched in chromium. Chloride ions are present only at the outermost part of the passive films, not penetrating into the interior part. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Glow discharge optical emission spectroscopy (GDOES) has been widely used for depth profiling of relatively thick films, several tens of lm in thickness, such as galvanized or painted steels, due to its high sputtering rates of more than 1 lm min 1 [1]. It has recently been revealed the enormous potential of glow discharge optical emission spectroscopy (GDOES) for depth profiling of extremely thin films, either conducting or non-conducting, less than 10 nm in thickness [2]. Even the orientation of thiourea molecules adsorbed on a mirror-finished copper surface has been clearly resolved by rf-GDOES [3]. Such significant features arise from the nature of rf-GD sputtering, samples being sputtered very stably, from the commencement of analysis, with Ar+ ions of very low energies, <50 eV, wide incident angles and high current densities of the order of 100 mA cm 2 [4]. In this work, rf-GDOES, with excellent depth resolution, has been applied for depth profiling of thin passive films formed on type-312L stainless steel, which is highly resistant to pitting corrosion in acidic and neutral chloride-containing solutions. The stainless steel contains 6 mass% of molybdenum, which may primarily contribute to the improved resistance to localized corrosion, although low sulphur content [5] and presence of nitrogen [6] in the steel also reduce the pitting susceptibility.
* Corresponding author. Tel.: +81 11 706 6575; fax: +81 11 706 6575. E-mail address: [email protected] (H. Habazaki). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.11.017
The beneficial role of molybdenum in enhancing the stability of passive state and in improving the pitting corrosion resistance of stainless steels have been extensively investigated using surface analytical methods [7–15] and electrochemical measurements [16–21]. The beneficial role of molybdenum has been explained by adsorption of molybdenum species [22], by formation of molybdenum compounds [23], by synergistic interaction of molybdenum and other alloying elements [7,24] and by suppressing active dissolution by formation of molybdenum compounds [8]. Due to absence of consistency for the role of molybdenum in improved corrosion resistance of stainless steels, further studies are needed to obtain a better understanding of the role of molybdenum [25]. In neutral chloride solutions, accumulation of molybdenum species in the passive films was generally observed [26,17]. A bipolar model [27] is proposed to explain the improved pitting resistance. The presence of molybdate ions in the outer part of the passive films changes its anionic selectivity to a cationic one and induces the formation of a bipolar layer that promotes the migration of O2 ions and the formation of chromium oxide [23]. In acidic solutions, including the inside of pits formed in neutral chloride solutions, molybdenum reduces the active dissolution rate of the metallic substrate, promoting passivation. Molybdenum is passive in the active region of stainless steels, reducing the dissolution rate by forming MoO2-containing film [15]. In a separate paper, the present authors reported results of depth profiling of passive films formed on type-312L stainless steel in neutral chloride solution by GDOES. The results disclosed that molybdenum accumulated in the outer part of the passive films,
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which thickened with increase in the polarization potential in NaCl solution [28]. In this work, thin passive films formed in aggressive HCl solution were analyzed by GDOES to get further insights into the role of molybdenum in enhancement of passivity.
XPS spectra of the as-polished specimen was also measured using a Shimadzu ESCA-3000 system with Mg Ka excitation (hm = 1253.6 eV). Binding energies of the photoelectrons were calibrated by a method described elsewhere [29].
2. Experimental
3. Results and discussion
A type-312L stainless steel plate of 2 mm in thickness was used in this work. The composition of the type-312L stainless steel is shown in Table 1. The surface of the specimens was polished mechanically with SiC papers up to No.1500 and finally mirror-finished with diamond slurry of 3 lm. The preparation of a mirrorfinished surface was essential for obtaining depth profiles with excellent depth resolution. Potentiostatic and potentiodynamic polarization curves of the specimens were obtained in deaerated 2 mol dm 3 HCl solution at 293 K. The potentiodynamic polarization was performed at a potential sweep rate of 1 mV s 1 and the potentiostatic polarization was carried out at several potentials for 1.8 ks. After potentiostatic polarization, the specimens were washed in deaerated distilled water under an argon atmosphere. Then the specimens were dried and immediately analyzed by GDOES. The air-exposure time before GDOES analysis was less than 5 min. Depth profiles of the specimens polarized potentiostatically and as-polished were obtained using a Jobin-Yvon 5000RF GDOES instrument in an argon atmosphere of 800 Pa with application of RF of 13.56 MHz and power of 40 W. Light emissions of characteristic wavelengths were monitored throughout the analysis with a sampling time of 1 ms to obtain depth profiles. The wavelengths of the spectral lines used were 121.567 nm (hydrogen), 130.217 nm (oxygen), 134.724 nm (chloride), 425.433 nm (chromium), 385.991 nm (iron), 341.477 nm (nickel), 224.700 nm (copper) and 317.035 nm (molybdenum). The signals were detected from a circular area of approximately 4 mm in diameter. For quantification, type-304, type-316L and type-312L stainless steels were used as reference materials.
Fig. 1 shows the potentiodynamic and potentiostatic polarization curves of type-312L stainless steel in 2 mol dm 3 HCl solution at 293 K. In this solution, no passivation occurred on type-304 and type-316L stainless steels. However, the type-312L stainless steel is passivated at about 0.15 V vs. Ag/AgCl, showing a wide passive region to 1.0 V vs. Ag/AgCl where transpassive dissolution commences. No pitting corrosion occurs in this aggressive acid solution. During the potentiostatic polarization at 0.18 V vs. Ag/ AgCl, the current density increased gradually with polarization time, probably due to breakdown of an air-formed oxide film and increased surface roughness. In the passive potential region, the
a Alloy/film interface Fe
Cu
Intensity / arb.unit
Cr
H Ni O Mo
Table 1 Composition of the type 312L stainless steel used (mass%). C
Si
Mn
P
S
Ni
Cr
Mo
Cu
N
0
Fe
20
40
60
80
100
Sputtering time / ms
SUS312L 0.009 0.48 0.31 0.026 0.0006 17.95 20.16 6.29 0.77 0.207 53.79
b
Current Density, i / A m-2
10 10 10 10
3
Alloy/film interface
2
Intensity / arb.unit
10
1
0
-1
Fe
Cr
H
Cu
Ni
O 10 10 10
-2
-3
Cl
Mo
-4
-0.5
0.0
0.5
1.0
1.5
Potential, E / V vs. Ag / AgCl Fig. 1. Potentiodynamic and potentiostatic polarization curves of type-312L stainless steel in 2 mol dm 3 HCl solution at 293 K.
0
20
40
60
80
100
Sputtering time / ms Fig. 2. GDOES depth profiles of type-312L stainless steel specimens (a) as-polished and (b) polarized at 0.5 V vs. Ag/AgCl in 2 mol dm 3 HCl for 1.8 ks.
M. Uemura et al. / Corrosion Science 51 (2009) 1554–1559
current densities after the potentiostatic polarization for 1.8 ks are in the order of 1 mA m 2 or less, indicating high stability of the passive state. The passive current density increases slightly with increase in polarization potential. The marked increase in current density at 1.0 V vs. Ag/AgCl is associated with the transpassive dissolution of chromium. Fig. 2 shows depth profiles of an as-polished specimen (Fig. 2a) and a specimen passivated at 0.5 V vs. Ag/AgCl (Fig. 2b). The thickness of the air-formed film should be only a few nm, but enrichment of chromium species in the inner part of the film and even accumulation of nickel in the underlying alloy substrate are clearly resolved. In addition, significant enrichment of copper at the alloy/ film interface region is obvious. Molybdenum is present in the outer part of the film but highly deficient in the inner layer. The hydrogen profile is also directly measured using GDOES. The hydrogen species are present mainly in the outer part of the air-formed film, where the film material should be formed at the film surface by outward diffusion of cations. The concentration of hydrogen species in the inner layer is low. The depth profile of the air-formed film on type-304 stainless steel revealed the presence of hydrogen species throughout the thickness of the air-formed film. The findings may indicate that the molybdenum-containing outer layer on type-312L stainless steel prevents penetration of OH anions into the inner layer possibly due to the cation-selective nature of the outer layer [27]. The chemical state of the constituting elements in the airformed film cannot be identified by GDOES. Such supplemental information was obtained by XPS analysis. As shown in Fig. 3, oxidized species of iron, chromium, nickel and molybdenum are present in the air-formed film. From the peak intensity of each oxidized species with respect to the corresponding metallic peak intensity,
c
Fe 2p
716
712
Feo
708
704
585
Binding Energy / eV
Intensity / arb.unit
Nio Ni2+
865
860
855
850
Binding Energy / eV
Cro
580
575
570
565
945
Binding Energy / eV
d
Ni 2p3/2
Intensity / arb.unit
b
Cu 2p3/2
Cr3+
Intensity / arb.unit
Intensity / arb.unit
Fe3+ Fe2+
e
Cr 2p3/2
Mo 3d
Mo4+
Mo6+
235
Cu0, Cu+
940
230
f
Moo
225
935
930
925
Binding Energy / eV
Cu L3M4,5M4,5
Intensity / arb.unit
a
oxidized iron and chromium species are the major cationic components in the air-formed film. Hexavalent molybdenum is the major molybdenum species in the air-formed film as shown in the Mo3d spectrum. Although the copper content in type-312L is only 0.77 mass%, a relatively intense peak appears in the Cu 2p spectrum, due to enrichment at the alloy/film interface. The peak binding energy corresponds to metallic copper or monovalent copper species. The Cu L3M4,5M4,5 Auger peak shows that metallic copper, not monovalent copper, is predominant. Thus, metallic copper is enriched immediately beneath the air-formed film, due to preferential oxidation of other metallic elements. High diffusivity of copper atoms in a metallic substrate may also contribute to the accumulation of copper at the alloy surface [30]. The depth profile of the specimen passivated at 0.5 V vs. Ag/ AgCl (Fig. 2b) shows that hydrogen species are present almost throughout the film thickness. Due to the reduced molybdenum content in the outer layer of the passive film, OH- ions are now able to penetrate into the inner layer. Since the sensitivity of GDOES for detecting various elements is higher than the sensitivities of XPS and AES, a trace amount of chloride ions are detected but only at the outermost surface of the passive film. No chloride ions penetrated into the interior of the passive film, as found in previous XPS and AES studies [8,9]. To show more quantitatively the depth distribution of the components of the stainless steel, mass fraction of the alloy component is depicted as a function of sputtering time (Fig. 4). It is obvious for all specimens that the surface films consist of two layers: an outer iron-rich layer and an inner layer highly enriched in chromium species. The concentration of chromium in the inner layer increases with increase in polarization potential, except at 1.0 V vs. Ag/AgCl, at which transpassive dissolution of chromium
Intensity / arb.unit
1556
220
Binding Energy / eV
910
Cuo
920
930
KineticEnergy / eV
Fig. 3. XPS spectra of as-polished type-312L stainless steel. (a) Fe 2p3/2, (b) Ni 2p3/2, (c) Cr 2p3/2, (d) Mo 3d, (e) Cu 2p3/2, and (f) Cu L3M4,5M4,5 Auger electrons.
1557
d
1.0 Alloy/film interface as-polished
0.8
Cr 0.6
Fe
0.4
Ni 0.2
Cu 0.0 0.00
Mo
0.05
0.10
Mass fraction of alloy component
a
Mass fraction of alloy component
M. Uemura et al. / Corrosion Science 51 (2009) 1554–1559
0.15
1.0
Cr
0.8 V vs Ag/AgCl
0.6
Fe
0.4
Ni 0.2
0.05
-0. 18 V vs Ag/AgCl
0.6
Fe
0.4
Ni 0.2
Cu
Mo 0.0 0.00
0.05
0.10
Mass fraction of alloy component
Mass fraction of alloy component
e Alloy/film interface
Cr
Mass fraction of alloy component
0.15
0.15
1.0 Alloy/film interface
0.8
Cr
1.0 V vs Ag/Ag Cl
0.6
Fe
0.4
Ni 0.2
Mo 0.0 0.00
0.05
Sputtering time / s
c
0.10
Sputtering time / s
1.0 0.8
Mo
Cu 0.0 0.00
Sputtering time / s
b
Alloy/film interface
0.8
0.10
Cu 0.15
Sputtering time / s
1.0
Cr
Alloy/film interface
0.8
0.2 V vs Ag/AgCl
0.6
Fe
0.4
Ni 0.2
Mo
Cu 0.0 0.00
0.05
0.10
0.15
Sputtering time / s Fig. 4. GDOES depth profiles of type-312L stainless steel specimens (a) as-polished and polarized at (b)
commences. The oxidized nickel found by XPS is mainly present in the outer iron-rich layer and is deficient in the inner layer. Similarly, molybdenum is deficient in the chromium-enriched inner layer and is mainly present in the outer layer of the film. The molybdenum content also slightly decreases in the underlying alloy beneath the surface films. At the active potential of 0.18 V vs. Ag/AgCl, the enrichment of molybdenum in the outer layer of the film is significant, but the molybdenum content in the inner layer is still low. Molybdenum is known to reduce the active dissolution rate of stainless steels [8,16,31] and iron-chromium alloys [32]. Molybdenum is passive in the active region of stainless steels in acid solutions, and tetravalent molybdenum is present in the surface film [33]. Thus, the tetravalent molybdenum enriched in the outer layer of the film retards the dissolution of stainless steel and promotes passivation.
0.18 V, (c) 0.2 V, (d) 0.8 V and (e) 1.0 V vs. Ag/AgCl in 2 mol dm
3
HCl.
The enrichment of copper immediately beneath the air-formed film is more significant than that beneath the passive films. The concentration of copper reaches 6 mass% immediately beneath the air-formed film, decreasing with increase in polarization potential, since copper is oxidized and dissolved in the solution under anodic polarization. Assuming that the sputtering rate is constant throughout a passive film and independent of film composition, the relative thicknesses of the surface films formed at several potentials can be compared by the sputtering time of the surface films. Fig. 5 shows the sputtering time for the surface films as a function of polarization potential. The sputtering time for the outer iron-rich layer is also shown in this figure. XPS analysis of the air-formed film indicated that its thickness is approximately 3 nm. Thus, the thinnest passive film formed at 0.2 V vs. Ag/AgCl is estimated to be
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80
Sputtering time / ms
60
Film
40
as-polished 20
Outer iron-rich layer
3
in oxide film / arb.unit
Normalized Intensity of Cr or Mo
4
2
Cr
1
Mo as-polished
0
0.0
0.5
1.0
0
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential, E / V vs. Ag / AgCl
Potential, E / V vs. Ag / AgCl Fig. 5. Sputtering time for the surface film as well as for the outer iron-rich layer as a function of polarization potential.
Fig. 6. Integrated intensities of chromium and molybdenum in the surface film, normalized by those in the air-formed film, as a function of polarization potential.
2.3 nm in thickness. Further increase in polarization potential increases the film thickness to make the field strength in the passive film constant. At the polarization potential of 1.0 V vs. Ag/AgCl, the film thickness increases greatly. In this transpassive region, the film formed is no longer highly protective as in the passive region. The thinning of the outer iron-rich layer in the passive region is also obvious in Fig. 5. Iron-rich oxide or hydrated oxyhydroxide is not chemically stable in acidic chloride solutions and dissolves readily. Thus, the outer layer becomes thin in 2 mol dm 3 HCl solution. The thickness of the outer layer with respect to the total thickness of the passive film is 0.2–0.25. Thus, the major part of the passive film formed on type-312L stainless steel in 2 mol dm 3 HCl is a chromium-enriched inner layer. In passive films formed in 0.9 mol dm 3 NaCl solution, it was found that the outer iron-rich layer became thick with increase in polarization potential and the molybdenum concentration increased in the outer layer [28]. In the present HCl solution, the outer layer of the passive films is thin and the molybdenum concentration is relatively low. The bipolar model proposed by Sakashita and Sato [27] may not be applicable to the present passive films. The integrated intensity of molybdenum and chromium in the surface films obtained by GDOES depth profiles are plotted as a function of polarization potential (Fig. 6). The intensity has been normalized by the integrated intensity of chromium and molybdenum in the air-formed film. Since the integrated intensity is roughly proportional to the amount of these elements in the surface film, this potential dependence should represent the change in the amount of these elements in the surface films as a function of potential. The intensity of molybdenum increases in the active region of 0.18 V vs. Ag/AgCl. The amount of molybdenum is still large at 0 V vs. Ag/AgCl, though this potential is in the passive region of stainless steel. This is due to the fact that this potential is still in the passive region of molybdenum [32]. The amounts of chromium in the surface films formed at all potentials are larger than that in the air-formed film. The amount of chromium at 0.8 V vs. Ag/AgCl is approximately 3.8 times as large as that at 0.2 V vs. Ag/AgCl. This is almost in agreement with the fact that the charge passed during polarization at 0.8 V vs. Ag/AgCl (21 C m 2) is 3.5 times as large as that at 0.2 V vs. Ag/AgCl (6.0 C m 2). In the passive region, chro-
mium-enriched passive films should be developed without significant dissolution of chromium. The present study clearly demonstrates that molybdenum species in surface films on type-312L stainless steel are present always in the outer part of the film, not in the inner chromium-enriched layer. Through XPS analysis it is well known that air exposure of polarized specimens results in the change in the chemical state of molybdenum species, i.e., oxidation of Mo4+ to Mo6+ species. The air exposure could not be avoided in our GDOES system, but efforts were made to minimize the exposure time before depth profiling in this work. Since a distinct difference was found in the concentrations of molybdenum in the outer layer and inner layer of the surface films at all potentials, it is likely that the depth distribution before exposure to air is similar to that obtained in this study. Thus, the outer layer containing molybdenum species, formed in the active region of stainless steel, reduces the dissolution rate of the stainless steel and promotes the accumulation of chromium in the inner layer. The findings in this work support the role of molybdenum in improving the passivity and resistance to localized corrosion proposed by Hashimoto [34]. The stability of the passive film is mainly attributed to the inner chromium-enriched oxide film. Once breakdown of the passive film occurs chemically or mechanically, molybdenum species formed on the surface may reduce the dissolution and assist repassivation, resulting in suppression of the initiation and growth of pits. 4. Conclusions (1) Depth profiles of thin passive films as well as an air-formed film on highly corrosion-resistant type-312L stainless steel can be obtained at sufficient depth resolution by GDOES. (2) The air-formed film consists of two layers, comprising an outer iron-rich layer and an inner layer highly enriched in chromium species. Molybdenum species are present in the outer layer and deficient in the inner layer. (3) The outer layer becomes thin after polarization at passive potentials in 2 mol dm 3 HCl solution, due to dissolution of the iron-rich oxy-hydroxide layer. Enrichment of chromium
M. Uemura et al. / Corrosion Science 51 (2009) 1554–1559
is more significant in the inner layer with increasing polarization potential. (4) Marked enrichment of tetravalent molybdenum species occurs in the outer part of the film, not in the inner layer, at the active dissolution potential, suppressing the dissolution rate of the stainless steel. (5) Chloride ions do not penetrate into the passive film, being present only at the outermost surface of the passive film.
Acknowledgments Thanks are due to Mr. T. Adachi, Nippon Yakin Kogyo Co., Ltd. for supplying the type-312L stainless steel specimens. The present work was supported in part by Grants-in-Aid for Scientific Research (A) No. 19206077 and for Exploratory Research, No.19656184 from the Japan Society for the Promotion of Science as well as by the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] R. Payling, Glow discharge optical emission spectrometry, Spectroscopy 13 (1998) 36–43. [2] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson, Radiofrequency GDOES: a powerful technique for depth profiling analysis of thin films, Surf. Interface Anal. 35 (2003) 564–574. [3] K. Shimizu, R. Payling, H. Habazaki, P. Skeldon, G.E. Thompson, Rf-GDOES depth profiling analysis of a monolayer of thiourea adsorbed on copper, J. Anal. At. Spectrom. 19 (2004) 692–695. [4] Y.C. Ye, R.K. Marcus, Effects of limiting orifice (anode) geometries on charged particle characteristics in an analytical radiofrequency glow discharge as determined by Langmuir, current and voltage probes, J. Anal. At. Spectrom. 12 (1997) 33–41. [5] Z. Szklarska-Smialowska, Pitting Corrosion of Metals. 1986, Houston: NACE. [6] I. Olefjord, L. Wegrelius, The influence of nitrogen on the passivation of stainless steels, Corros. Sci. 38 (1996) 1203–1220. [7] K. Sugimoto, Y. Sawada, The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions, Corros. Sci. 17 (1977) 425–445. [8] K. Hashimoto, K. Asami, K. Teramoto, An X-ray photoelectron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steel in 1 N HCl, Corros. Sci. 19 (1979) 3–14. [9] A. Schneider, S. Hofmann, R. Kirchheim, Auger-spectroscopic investigations into pitting corrosion of FeCr, FeMo and FeCrMo Alloys, Werkst. Korros.-Mater. Corros. 42 (1991) 169–178. [10] E. Devito, P. Marcus, XPS study of passive films formed on molybdenumimplanted austenitic stainless-steels, Surf. Interface Anal. 19 (1992) 403–408. [11] 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 (1995) 467–479. [12] M.F. Montemor, A.M.P. Simoes, M.G.S. Ferreira, M.D. Belo, The role of Mo in the chemical composition and semiconductive behaviour of oxide films formed on stainless steels, Corros. Sci. 41 (1999) 17–34.
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