- Email: [email protected]
Characterization of AISI 4340 corrosion products using Raman spectroscopy
Corrosion Science 74 (2013) 414–418
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
Characterization of AISI 4340 corrosion products using Raman spectroscopy E. Hazan ⇑, Y. Sadia, Y. Gelbstein Department of Materials Engineering, Ben-Gurion University, Beer-Sheva 84105, Israel
a r t i c l e
i n f o
Article history: Received 29 September 2012 Accepted 2 May 2013 Available online 13 May 2013 Keywords: A. Low alloy steel B. Raman spectroscopy C. Atmospheric corrosion C. High temperature corrosion C. Oxidation C. Rust
a b s t r a c t Most of the currently available corrosion product characterization techniques require extensive samples preparation methods and cannot be employed under aqueous conditions. Raman spectroscopy possesses the advantage of characterization of the corrosion products under practical operation conditions, including aqueous, without any sample preparation procedures. The present study examines the corrosion characteristics of AISI-4340 steel at aqueous and atmospheric environments using Raman spectroscopy. FeO, Fe2O3, and Fe3O4 were observed as the main corrosion products. The measured oxides thicknesses ratio were compared to the theoretic value obtained from Tammann equation. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction AISI-4340 is a widely used low alloyed martensitic steel which provides a combination of high strength, ductility and toughness, required in many industrial and military applications and in particular in the aeronautic and marine industries, where high mechanical properties are required [1]. Despite of all of these advantages, this steel, like many other steels, suffers from susceptibility to corrosion, which is one of the most common failure mechanisms in steels. At the aeronautic industry, atmospheric and aqueous corrosion are among the most common failure mechanisms. Under humid conditions, where aqueous corrosion is the main failure mechanism, the expected corrosion products are magnetite: Fe3O4, hematite: c-Fe2O3, goethite: a-FeOOH and lepidocrocite: c-FeOOH [2]. Upon atmospheric corrosion, in oxygen rich atmospheres, the expected corrosion products in pure iron are FeO, Fe3O4, a-Fe2O3 and c-Fe2O3 [2]. In atmospheric oxidation, the time dependent reaction rate is parabolic above 200 °C and logarithmic below this temperature. When atmospheric corrosion occurs above 570 °C, three distinct oxide layers appear on the surface of the sample in the following sequence: wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3), which is the outer layer, keeping a FeO:Fe3O4:Fe2O3 = 100:4:1 thickness ratio of approximately FeO:Fe3O4:Fe2O3 = 100:4:1 as can be seen in Fig. 1 [2]. At temperatures below 570 °C, the wustite oxide layer is not thermodynamically stable and the adjacent oxide layer to the ⇑ Corresponding author. Tel.: +972 8 6880321; fax: +972 8 6569129. E-mail address: [email protected] (E. Hazan). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.05.002
un-oxidized sample should be the magnetite layer. The oxidation kinetics of pure iron and steels in air had been studied extensively [3–10] at different temperatures and environmental conditions. The oxidation rate at isothermal conditions above 200 °C is expressed by the Tammann-type parabolic equation [5,9], Eq. (1).
x2 ¼ kx t þ x20
ð1Þ
where x is the total thickness of the oxide layers (in cm), t is the oxidation time (in s), x0 is the initial oxide layer thickness at t = 0 (in cm) and kx is the parabolic rate coefficient (in cm2 s1), which depends on the oxidation temperature according to the following equation: 0
kx ¼ kx exp
Q RT
ð2Þ
0
while kx is constant which depends on the oxide composition (in cm2 s1), Q is the activation energy for iron oxidation (in J mol1), R is the ideal gas constant (in J K1 mol1) and T is the absolute temperature (in K). The overall scaling rate constant (kx) at oxidation of pure iron can be express follows (Eq. (3)) [9].
kx ¼ 6:1 exp
169452 RT
ð3Þ
Characterizing the structure, layer thickness and stoichiometry of the corrosion products can be vital for understanding the corrosion mechanism. There are many well-known characterization methods for investigating the structure and composition of steel’s corrosion products, such as XPS (X-Ray photoelectron
E. Hazan et al. / Corrosion Science 74 (2013) 414–418
415
Fig. 1. Oxidation layers of pure iron.
spectroscopy) [11], XRD (X-Ray diffraction) [12,13], IR (Infra-Red) [13] and Raman spectroscopy [14–16]. Raman spectroscopy uses the inelastic scattering of electromagnetic radiation by molecules. Monochromatic light of a laser interacts with phonons, the vibrational modes in the crystal lattice. The scattering shifts the energy of the scattered light. The shifts in energy yield the Raman spectrum that is specific for each material since the phonon modes are specific for each material [14,17,18]. The coupling of the Raman measurement system with optical microscopy offers a simple way to investigate the thickness and structure of the corrosion products. Moreover, Raman spectroscopy needs no special sample preparation and can work through an interface of water [17,18]. It was chosen in the current work to focus on the Raman Spectroscopy as a method for characterizing the corrosion products of AISI-4340. Raman also offers the advantage of identifying hydrogen bonds, which is difficult with standard characterization methods such as XPS (X-Ray photoelectron spectroscopy), and AES (Auger electron spectroscopy) [18]. Unlike pure iron, very little investigation has been done on the characterization and identification of corrosion products in AISI-4340 steel [19]. In this paper, the Raman spectra of possible corrosion products in AISI-4340 steel at different corrosive environments and its influence on the oxidation rate and thickness are reported.
2. Experimental An AISI-4340 steel plate with dimensions of 5 cm 5 cm 1 cm and chemical composition given in Table 1, was immersed in 3%NaCl solution for 72 h at 25 °C. Five other AISI-4340 steel samples were introduced into an atmospheric tube furnace (Thermcraft) at 840 °C for 1, 4, 8, 16, 30 h and air-cooled. All of the plates were then cut using a diamond disc saw (Struers Secotom 15) and one face of each plate was grinded with silicon carbide papers up to 4000 grit and surface polished to 1 lm finish using diamond paste for a metallurgical examination using an optical microscope (Zeiss-Axiovert 25). Between each polishing procedure, the sample was cleaned with acetone and ethanol. The Raman spectra were recorded using a Jovin-Yvon LabRam HR 800 micro-Raman system, equipped with a liquid-N2-cooled detector. For the excitation, a He–Ne 633 nm laser had been used. In order to prevent thermal transformations of iron oxides to the most stable phase, i.e. hematite, the laser power on the sample was about 5 mW. Each sample was examined three times for reproducibility. The measurements were taken with a confocal microscope with a
Fig. 2. Raman spectrum of the outer, lepidocrocite, oxide layer, following an immersion in 3%NaCl solution for 72 h at room temperature, compared to the spectrums reported previously for pure iron [14–16,20–23].
100 lm aperture and a 100 lens, using a 600 g mm1 grating. Typical measurement times were 1–3 min. Raman photons were collected with a cooled CCD. 3. Results and discussion All of the heat-treated and the aqueous exposed samples showed a uniform outer corrosion layer over the entire specimens as was expected. Analyzing the Raman spectrum of the outer corrosion layer of the aqueous exposed sample, clearly indicated a lepidocrocite (c-FeOOH) structure with the most intense peaks at 249 cm1 and 1312 cm1 with an additional representative peaks at 212 cm1 (shoulder), 380 cm1, 528 cm1 and 660 cm1 [14– 16,20–23], Fig. 2. In addition, a medium intensity peak was observed at 1598 cm1. The existence of the 1312 cm1 and the 1598 cm1 peaks reviles a typical signature of carbon materials with a background intensity in between those two peaks. Carbon materials could appear at the sample preparation procedure by decomposition of the organic materials such as acetone and ethanol [24]. The oxide layers formed at the surface of the samples exposed to an atmospheric corrosion for 16 h are shown in Fig. 3. In order to identify the corrosion products, Raman sampling was taken at different points along the oxide layers, Fig. 3. The Raman spectra of the outer and the two inner oxide layers are shown in Figs. 4
Table 1 Chemical composition (in wt.%) of AISI 4340 steel. Alloy
C
P
Mn
S
Si
Cr
Ni
Mo
Fe
AISI 4340
0.38
0.016
0.69
0.004
0.26
0.8
0.69
0.22
Bal.
Fig. 3. Optical microscope image of AISI-4340 oxide layers (detection points are marked with white dots) following an atmospheric corrosion at 840 °C for 16 h.
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and 5, respectively. Analysis of the spectra clearly revealed the existence of three different iron oxide corrosion layers, with the structures of hematite (the most outer layer), magnetite and wustite (the closest to the sample) [21–23]. Hematite is characterized by two strong intense peaks at 226 cm1 and 292 cm1 and additional medium intense peaks at 245 cm1, 296 cm1 (shoulder), 412 cm1, 495 cm1, 611 cm1, 806 cm1 and 1326 cm1 [21– 23], Fig. 4. It is noteworthy, in relation to Fig. 5, that both the magnetite and the wustite oxides exhibit the same typical Raman peaks at 305 cm1, 529 cm1 and 662 cm1. Nevertheless, there is a difference in the spectra’s baseline trends below a wave-number of 600 cm1, which can be attributed to the FeO decomposition under illumination, as was previously reported by Thibeau et al. [23]. According to Hanesch [14], the most intense peak is at 595 cm1 with additional medium intense peaks at 320 cm1 and 670 cm1. This intense peak at 595 cm1 was revealed probably because of the very weak laser power (0.01 mW). As was expected, no variations of the Raman spectra and the oxides sequence were observed following the other investigated oxidation periods. All of the samples exposed to atmospheric corrosion exhibited an intermediate layer in-between the wustite and the magnetite layers. This layer was found to compose of small magnetite islands (bright phase) embedded in a wustite matrix (darker gray), which were identified using the Raman microscope, as can be seen in Fig. 6. wustite is unstable below 570 °C and only magnetite and hematite can coexist at thermodynamic equilibrium. Since wustite has a range of stoichiometry (Fe1xO where 0.05 < x < 0.16), probably there is an oxygen gradient from the oxide zone adjacent to the
sample toward the zones at the vicinity of the magnetite layer. Upon cooling, following the various heat treatments, the stoichiometry of the wustite composition is changed into a lower oxygen content initiating a precipitation of magnetite at the nearest areas to the magnetite layer. This effect is probably responsible for the observed intermediate two-phase layer in-between the wustite and the magnetite layers [3]. The thickness ratio of pure iron oxides is expected to be approximately FeO:Fe3O4:Fe2O3 = 100:4:1. In the case of the investigated AISI-4340 steel, the overall oxide thickness, as calculated from Tammann equation (theoretical thickness), Eq. (1), and as measured by optical microscopy is presented in Fig. 7. It can be seen from Fig. 7 that although the theoretical calculation using Tammann equation (Eq. (1)) predicts a monotonic overall oxide thickness increase with increasing of the heat treatment period, the observed thickness increased up to 8 h of atmospheric oxidation, and decreased upon prolonged heat treatment. The decreased overall thickness at prolonged heat treatment durations can be attributed to the spalling of the oxide layers beyond a critical thickness, due to the high mechanical stresses involved. According to Fig. 7, the oxidation rate of AISI-4340 steel, during the first 4 h is higher than that during the second 4 h. This can be due to the square-root dependence of the oxide thickness in time exposure. The thickness ratios of the corrosion products at all of the examined samples exposed to atmospheric corrosion, as were measured using an optical microscope, with the thickest oxide layer (wustite) normalized to 100, according to the predicted thickness ratio, are listed in Table 2. The overall scaling rate constant, kx, of oxidation
Fig. 4. Raman spectrum of the outer oxide layer (identified as hematite).
Fig. 5. Raman spectrum of the middle layer (bottom) and the closest layer to the sample (top), identified as magnetite and wustite, respectively.
E. Hazan et al. / Corrosion Science 74 (2013) 414–418
417
4. Conclusion
Fig. 6. Optical microscope image of a mixture layer between the wustite and magnetite layers. This mixture layer is composed of small magnetite islands (bright phase) embedded in a wustite matrix (darker gray).
The oxide layers of AISI-4340 steel were successfully characterized by Raman spectroscopy following heat treatments at 840 °C for 1, 4, 8, 16 and 30 h and following an exposure to aqueous corrosion. The samples exposed to atmospheric corrosion exhibit an intermediate two-phase layer, composed of both the magnetite and wustite oxides, at the interface between these two oxide layers. This result was attributed to the thermo-dynamical instability of the wustite layer at room temperature and to the slow cooling of the oxygen poor wustite following heat treatments. Upon atmospheric corrosion, the thickness of the overall oxide layer was found as higher than theoretically estimated based on Tammann equation for pure iron for the shorter exposure periods, as was attributed to the compositional differences of the investigated steel compared to pure iron. According to Tammann equation, the overall scaling rate constant of AISI-4340 steel at early stages of high temperature oxidation is 1.736 107 cm2 s1. The ratio of the oxide layers thickness of AISI-4340 steel also found to be different from that of pure iron, observed by a higher growth rate of the outer layers (hematite and magnetite) compared to the inner layer (wustite), during long exposure periods. Alloying elements in AISI-4340 steel have no influence on the characteristic spectrum of the oxide layers formed on this steel. At longer periods, the thickness of the overall oxide layer was found as lower than the theoretically estimated values due to spelling of the layer beyond a certain critical thickness, resulting from mechanical failures. Acknowledgments
Fig. 7. Overall oxide layer thickness of AISI-4340 steel, as was measured by optical microscopy (white) and calculated using Tammann equation (black).
The authors would like to thank Dr. Leila Zeiri from the nanotechnology center at BGU for her assistance in the Raman measurements and analyses. References
of pure iron at 840 °C is 6.798 108 cm2 s1 while that of AISI4340 steel was calculated to be 1.736 107 cm2 s1. The increase in the overall scaling rate can be attributed to the presence of alloying elements in the steel, which react more easily with oxygen. As can be seen from the results listed in Table 1, the thickness ratio of the oxide layers does not follow the theoretic ratio of FeO:Fe3O4:Fe2O3 = 100:4:1, calculated using Tammann equation (Eq. (1)) for pure iron. The differences can be attributed to the different influence of the various alloying elements in the AISI-4340 steel on the growth rate of the oxide layers, compared to pure iron. In addition, as was described previously, practical considerations such as the separation of the oxide layers, due to mechanical stresses, thermal stresses, fatigue and creep beyond a certain critical thickness of the oxide layers, are influential factors.
Table 2 Thickness ratio of corrosion products of AISI-4340 steel. Exposure time (h)
Hematite (Fe2O3)
Magnetite (Fe3O4)
Wustite (Fe1xO)
1 4 8 16 30
3 2.2 4.6 7.4 5
– 8.8 10 14.8 22.8
100 100 100 100 100
[1] J.E. Pemberton, A.L. Guy, Properties and Selection: Irons Steels and HighPerformance Alloys, ASM Handbook, vol. 1, ASM International, 1990. pp. 210– 217. [2] O. Kubaschewski, B.E. Hopkins, Oxidation of Metals and Alloys, second ed., Butterworths, London, 1967. [3] D. Gene‘ve, D. Rouxel, P. Pigeat, B. Weber, M. Confente, Surface composition modification of high-carbon low-alloy steels oxidized at high temperature in air, Appl. Surf. Sci. 254 (2008) 5348–5358. [4] D. Gene‘ve, D. Rouxel, B. Weber, M. Confente, Segregation across the metal/ oxide interface occurring during oxidation at high temperatures of diluted iron based alloys, Mater. Sci. Eng. A 435–436 (2006) 1–11. [5] L. del Campo, R.B. Pérez-Sáez, M.J. Tello, Iron oxidation kinetics study by using infrared spectral emissivity measurements below 570 °C, Corros. Sci. 50 (2008) 194–199. [6] N. Bertrand, C. Desgranges, D. Gauvain, D. Monceau, D. Poquillon, Low temperature oxidation of pure iron: growth kinetics and scale morphologies, Mater. Sci. Forum 461–464 (2004) 591–598. [7] V.B. Trindade, R. Borin, B.Z. Hanjari, S. Yang, U. Krupp, H. Christ, Hightemperature oxidation of pure Fe and the ferritic steel 2.25Cr1Mo, Mater. Res. 8 (2005) 365–369. [8] K. Gemma, M. Kawakami, C. Kobayashi, N. Itoh, M. Tomida, Kinetics of oxidation of pure iron near the eutectoid temperature of wüstite, Mater. Sci. 25 (1990) 4555–4561. [9] R.Y. Chen, W.Y.D. Yeun, Review of the high-temperature oxidation of iron and carbon steels in air or oxygen, Oxid. Met. 59 (2003) 433–468. [10] J. Paidassi, The kinetics of the air oxidation of iron in the range 700–1250°C, Acta Metallurgica 6 (1958) 184–194. [11] T. Hanawa, S. Hiromoto, A. Yamamoto, D. Kuroda, K. Asami, XPS characterization of the surface oxide film of 316L stainless steel samples that were located in quasi-biological environments, Mater. Trans. 43 (2002) 3088–3092. [12] Y. Zhao, H. Ren, H. Dai, W. Jin, Composition and expansion coefficient of rust based on X-ray diffraction and thermal analysis, Corros. Sci. 53 (2011) 1646– 1658. [13] A.V.R. Kumar, R. Balasubramaniam, Corrosion product analysis of corrosion resistant ancient Indian iron, Corros. Sci. 40 (1998) 1169–1178.
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[14] M. Hanesch, Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies, Geophys. J. Int. 177 (2009) 941–948. [15] J. Dünnwald, A. Otto, An investigation of phase transitions in rust layers using Raman spectroscopy, Corros. Sci. 29 (1989) 1167–1176. [16] R.A. Antunes, I. Costa, Dalva Lúcia Araújo de Faria, Characterization of corrosion products formed on steels in the first months of atmospheric exposure, Mater. Res. 6 (2003) 403–408. [17] J.R. Ferraro, K. Nakamoto, C.W. Brown, Introductory Raman Spectroscopy: Basic Theory, Academic Press, 2003. [18] J.E. Pemberton, A.L. Guy, Materials Characterization, ASM Handbook, vol. 10, ASM International, 1986. pp. 271–296, 998–1034 and 1035–1066. [19] C.J. Semino, J.R. Galvele, Passivity breakdown of high purity iron and AISI 4340 steel in 0.5 M NaCl solution, Corros. Sci. 16 (1976) 297–306.
[20] A.P. Lee, J. Webb, D.J. Macey, W. van Bronswijk, A.R. Savarese, G.C. de Witt, In situ Raman spectroscopic studies of the teeth of the chiton Acanthopleura hirtosa, J. Biol. Inorg. Chem. 3 (1998) 614–619. [21] D.L.A. de Faria, S. Venâncio Silva, M.T. de-Oliveira, Raman microspectroscopy of some iron oxides and oxyhydroxides, J. Raman Spectrosc. 28 (1997) 873– 878. [22] S. Pineau, R. Sabot, L. Quillet, Ch. Caplat, I. Dupont-Morral, Ph. Refait, Formation of the Fe(II–III) hydroxysulphate green rust during marine corrosion of steel associated to molecular detection of dissimilatory sulphite-reductase, Corros. Sci. 50 (2008) 1099–1111. [23] R.J. Thibeau, C.W. Brown, R.H. Heidersbach, Raman spectra of possible corrosion products of iron, Appl. Spectrosc. 32 (1978) 532–535. [24] A.C. Ferrari, J. Robertson, Resonant Raman spectroscopy of disordered, amorphous, and diamond-like carbon, Phys. Rev. B 64 (2001) 075414.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
Characterization of AISI 4340 corrosion products using Raman spectroscopy E. Hazan ⇑, Y. Sadia, Y. Gelbstein Department of Materials Engineering, Ben-Gurion University, Beer-Sheva 84105, Israel
a r t i c l e
i n f o
Article history: Received 29 September 2012 Accepted 2 May 2013 Available online 13 May 2013 Keywords: A. Low alloy steel B. Raman spectroscopy C. Atmospheric corrosion C. High temperature corrosion C. Oxidation C. Rust
a b s t r a c t Most of the currently available corrosion product characterization techniques require extensive samples preparation methods and cannot be employed under aqueous conditions. Raman spectroscopy possesses the advantage of characterization of the corrosion products under practical operation conditions, including aqueous, without any sample preparation procedures. The present study examines the corrosion characteristics of AISI-4340 steel at aqueous and atmospheric environments using Raman spectroscopy. FeO, Fe2O3, and Fe3O4 were observed as the main corrosion products. The measured oxides thicknesses ratio were compared to the theoretic value obtained from Tammann equation. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction AISI-4340 is a widely used low alloyed martensitic steel which provides a combination of high strength, ductility and toughness, required in many industrial and military applications and in particular in the aeronautic and marine industries, where high mechanical properties are required [1]. Despite of all of these advantages, this steel, like many other steels, suffers from susceptibility to corrosion, which is one of the most common failure mechanisms in steels. At the aeronautic industry, atmospheric and aqueous corrosion are among the most common failure mechanisms. Under humid conditions, where aqueous corrosion is the main failure mechanism, the expected corrosion products are magnetite: Fe3O4, hematite: c-Fe2O3, goethite: a-FeOOH and lepidocrocite: c-FeOOH [2]. Upon atmospheric corrosion, in oxygen rich atmospheres, the expected corrosion products in pure iron are FeO, Fe3O4, a-Fe2O3 and c-Fe2O3 [2]. In atmospheric oxidation, the time dependent reaction rate is parabolic above 200 °C and logarithmic below this temperature. When atmospheric corrosion occurs above 570 °C, three distinct oxide layers appear on the surface of the sample in the following sequence: wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3), which is the outer layer, keeping a FeO:Fe3O4:Fe2O3 = 100:4:1 thickness ratio of approximately FeO:Fe3O4:Fe2O3 = 100:4:1 as can be seen in Fig. 1 [2]. At temperatures below 570 °C, the wustite oxide layer is not thermodynamically stable and the adjacent oxide layer to the ⇑ Corresponding author. Tel.: +972 8 6880321; fax: +972 8 6569129. E-mail address: [email protected] (E. Hazan). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.05.002
un-oxidized sample should be the magnetite layer. The oxidation kinetics of pure iron and steels in air had been studied extensively [3–10] at different temperatures and environmental conditions. The oxidation rate at isothermal conditions above 200 °C is expressed by the Tammann-type parabolic equation [5,9], Eq. (1).
x2 ¼ kx t þ x20
ð1Þ
where x is the total thickness of the oxide layers (in cm), t is the oxidation time (in s), x0 is the initial oxide layer thickness at t = 0 (in cm) and kx is the parabolic rate coefficient (in cm2 s1), which depends on the oxidation temperature according to the following equation: 0
kx ¼ kx exp
Q RT
ð2Þ
0
while kx is constant which depends on the oxide composition (in cm2 s1), Q is the activation energy for iron oxidation (in J mol1), R is the ideal gas constant (in J K1 mol1) and T is the absolute temperature (in K). The overall scaling rate constant (kx) at oxidation of pure iron can be express follows (Eq. (3)) [9].
kx ¼ 6:1 exp
169452 RT
ð3Þ
Characterizing the structure, layer thickness and stoichiometry of the corrosion products can be vital for understanding the corrosion mechanism. There are many well-known characterization methods for investigating the structure and composition of steel’s corrosion products, such as XPS (X-Ray photoelectron
E. Hazan et al. / Corrosion Science 74 (2013) 414–418
415
Fig. 1. Oxidation layers of pure iron.
spectroscopy) [11], XRD (X-Ray diffraction) [12,13], IR (Infra-Red) [13] and Raman spectroscopy [14–16]. Raman spectroscopy uses the inelastic scattering of electromagnetic radiation by molecules. Monochromatic light of a laser interacts with phonons, the vibrational modes in the crystal lattice. The scattering shifts the energy of the scattered light. The shifts in energy yield the Raman spectrum that is specific for each material since the phonon modes are specific for each material [14,17,18]. The coupling of the Raman measurement system with optical microscopy offers a simple way to investigate the thickness and structure of the corrosion products. Moreover, Raman spectroscopy needs no special sample preparation and can work through an interface of water [17,18]. It was chosen in the current work to focus on the Raman Spectroscopy as a method for characterizing the corrosion products of AISI-4340. Raman also offers the advantage of identifying hydrogen bonds, which is difficult with standard characterization methods such as XPS (X-Ray photoelectron spectroscopy), and AES (Auger electron spectroscopy) [18]. Unlike pure iron, very little investigation has been done on the characterization and identification of corrosion products in AISI-4340 steel [19]. In this paper, the Raman spectra of possible corrosion products in AISI-4340 steel at different corrosive environments and its influence on the oxidation rate and thickness are reported.
2. Experimental An AISI-4340 steel plate with dimensions of 5 cm 5 cm 1 cm and chemical composition given in Table 1, was immersed in 3%NaCl solution for 72 h at 25 °C. Five other AISI-4340 steel samples were introduced into an atmospheric tube furnace (Thermcraft) at 840 °C for 1, 4, 8, 16, 30 h and air-cooled. All of the plates were then cut using a diamond disc saw (Struers Secotom 15) and one face of each plate was grinded with silicon carbide papers up to 4000 grit and surface polished to 1 lm finish using diamond paste for a metallurgical examination using an optical microscope (Zeiss-Axiovert 25). Between each polishing procedure, the sample was cleaned with acetone and ethanol. The Raman spectra were recorded using a Jovin-Yvon LabRam HR 800 micro-Raman system, equipped with a liquid-N2-cooled detector. For the excitation, a He–Ne 633 nm laser had been used. In order to prevent thermal transformations of iron oxides to the most stable phase, i.e. hematite, the laser power on the sample was about 5 mW. Each sample was examined three times for reproducibility. The measurements were taken with a confocal microscope with a
Fig. 2. Raman spectrum of the outer, lepidocrocite, oxide layer, following an immersion in 3%NaCl solution for 72 h at room temperature, compared to the spectrums reported previously for pure iron [14–16,20–23].
100 lm aperture and a 100 lens, using a 600 g mm1 grating. Typical measurement times were 1–3 min. Raman photons were collected with a cooled CCD. 3. Results and discussion All of the heat-treated and the aqueous exposed samples showed a uniform outer corrosion layer over the entire specimens as was expected. Analyzing the Raman spectrum of the outer corrosion layer of the aqueous exposed sample, clearly indicated a lepidocrocite (c-FeOOH) structure with the most intense peaks at 249 cm1 and 1312 cm1 with an additional representative peaks at 212 cm1 (shoulder), 380 cm1, 528 cm1 and 660 cm1 [14– 16,20–23], Fig. 2. In addition, a medium intensity peak was observed at 1598 cm1. The existence of the 1312 cm1 and the 1598 cm1 peaks reviles a typical signature of carbon materials with a background intensity in between those two peaks. Carbon materials could appear at the sample preparation procedure by decomposition of the organic materials such as acetone and ethanol [24]. The oxide layers formed at the surface of the samples exposed to an atmospheric corrosion for 16 h are shown in Fig. 3. In order to identify the corrosion products, Raman sampling was taken at different points along the oxide layers, Fig. 3. The Raman spectra of the outer and the two inner oxide layers are shown in Figs. 4
Table 1 Chemical composition (in wt.%) of AISI 4340 steel. Alloy
C
P
Mn
S
Si
Cr
Ni
Mo
Fe
AISI 4340
0.38
0.016
0.69
0.004
0.26
0.8
0.69
0.22
Bal.
Fig. 3. Optical microscope image of AISI-4340 oxide layers (detection points are marked with white dots) following an atmospheric corrosion at 840 °C for 16 h.
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and 5, respectively. Analysis of the spectra clearly revealed the existence of three different iron oxide corrosion layers, with the structures of hematite (the most outer layer), magnetite and wustite (the closest to the sample) [21–23]. Hematite is characterized by two strong intense peaks at 226 cm1 and 292 cm1 and additional medium intense peaks at 245 cm1, 296 cm1 (shoulder), 412 cm1, 495 cm1, 611 cm1, 806 cm1 and 1326 cm1 [21– 23], Fig. 4. It is noteworthy, in relation to Fig. 5, that both the magnetite and the wustite oxides exhibit the same typical Raman peaks at 305 cm1, 529 cm1 and 662 cm1. Nevertheless, there is a difference in the spectra’s baseline trends below a wave-number of 600 cm1, which can be attributed to the FeO decomposition under illumination, as was previously reported by Thibeau et al. [23]. According to Hanesch [14], the most intense peak is at 595 cm1 with additional medium intense peaks at 320 cm1 and 670 cm1. This intense peak at 595 cm1 was revealed probably because of the very weak laser power (0.01 mW). As was expected, no variations of the Raman spectra and the oxides sequence were observed following the other investigated oxidation periods. All of the samples exposed to atmospheric corrosion exhibited an intermediate layer in-between the wustite and the magnetite layers. This layer was found to compose of small magnetite islands (bright phase) embedded in a wustite matrix (darker gray), which were identified using the Raman microscope, as can be seen in Fig. 6. wustite is unstable below 570 °C and only magnetite and hematite can coexist at thermodynamic equilibrium. Since wustite has a range of stoichiometry (Fe1xO where 0.05 < x < 0.16), probably there is an oxygen gradient from the oxide zone adjacent to the
sample toward the zones at the vicinity of the magnetite layer. Upon cooling, following the various heat treatments, the stoichiometry of the wustite composition is changed into a lower oxygen content initiating a precipitation of magnetite at the nearest areas to the magnetite layer. This effect is probably responsible for the observed intermediate two-phase layer in-between the wustite and the magnetite layers [3]. The thickness ratio of pure iron oxides is expected to be approximately FeO:Fe3O4:Fe2O3 = 100:4:1. In the case of the investigated AISI-4340 steel, the overall oxide thickness, as calculated from Tammann equation (theoretical thickness), Eq. (1), and as measured by optical microscopy is presented in Fig. 7. It can be seen from Fig. 7 that although the theoretical calculation using Tammann equation (Eq. (1)) predicts a monotonic overall oxide thickness increase with increasing of the heat treatment period, the observed thickness increased up to 8 h of atmospheric oxidation, and decreased upon prolonged heat treatment. The decreased overall thickness at prolonged heat treatment durations can be attributed to the spalling of the oxide layers beyond a critical thickness, due to the high mechanical stresses involved. According to Fig. 7, the oxidation rate of AISI-4340 steel, during the first 4 h is higher than that during the second 4 h. This can be due to the square-root dependence of the oxide thickness in time exposure. The thickness ratios of the corrosion products at all of the examined samples exposed to atmospheric corrosion, as were measured using an optical microscope, with the thickest oxide layer (wustite) normalized to 100, according to the predicted thickness ratio, are listed in Table 2. The overall scaling rate constant, kx, of oxidation
Fig. 4. Raman spectrum of the outer oxide layer (identified as hematite).
Fig. 5. Raman spectrum of the middle layer (bottom) and the closest layer to the sample (top), identified as magnetite and wustite, respectively.
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Fig. 6. Optical microscope image of a mixture layer between the wustite and magnetite layers. This mixture layer is composed of small magnetite islands (bright phase) embedded in a wustite matrix (darker gray).
The oxide layers of AISI-4340 steel were successfully characterized by Raman spectroscopy following heat treatments at 840 °C for 1, 4, 8, 16 and 30 h and following an exposure to aqueous corrosion. The samples exposed to atmospheric corrosion exhibit an intermediate two-phase layer, composed of both the magnetite and wustite oxides, at the interface between these two oxide layers. This result was attributed to the thermo-dynamical instability of the wustite layer at room temperature and to the slow cooling of the oxygen poor wustite following heat treatments. Upon atmospheric corrosion, the thickness of the overall oxide layer was found as higher than theoretically estimated based on Tammann equation for pure iron for the shorter exposure periods, as was attributed to the compositional differences of the investigated steel compared to pure iron. According to Tammann equation, the overall scaling rate constant of AISI-4340 steel at early stages of high temperature oxidation is 1.736 107 cm2 s1. The ratio of the oxide layers thickness of AISI-4340 steel also found to be different from that of pure iron, observed by a higher growth rate of the outer layers (hematite and magnetite) compared to the inner layer (wustite), during long exposure periods. Alloying elements in AISI-4340 steel have no influence on the characteristic spectrum of the oxide layers formed on this steel. At longer periods, the thickness of the overall oxide layer was found as lower than the theoretically estimated values due to spelling of the layer beyond a certain critical thickness, resulting from mechanical failures. Acknowledgments
Fig. 7. Overall oxide layer thickness of AISI-4340 steel, as was measured by optical microscopy (white) and calculated using Tammann equation (black).
The authors would like to thank Dr. Leila Zeiri from the nanotechnology center at BGU for her assistance in the Raman measurements and analyses. References
of pure iron at 840 °C is 6.798 108 cm2 s1 while that of AISI4340 steel was calculated to be 1.736 107 cm2 s1. The increase in the overall scaling rate can be attributed to the presence of alloying elements in the steel, which react more easily with oxygen. As can be seen from the results listed in Table 1, the thickness ratio of the oxide layers does not follow the theoretic ratio of FeO:Fe3O4:Fe2O3 = 100:4:1, calculated using Tammann equation (Eq. (1)) for pure iron. The differences can be attributed to the different influence of the various alloying elements in the AISI-4340 steel on the growth rate of the oxide layers, compared to pure iron. In addition, as was described previously, practical considerations such as the separation of the oxide layers, due to mechanical stresses, thermal stresses, fatigue and creep beyond a certain critical thickness of the oxide layers, are influential factors.
Table 2 Thickness ratio of corrosion products of AISI-4340 steel. Exposure time (h)
Hematite (Fe2O3)
Magnetite (Fe3O4)
Wustite (Fe1xO)
1 4 8 16 30
3 2.2 4.6 7.4 5
– 8.8 10 14.8 22.8
100 100 100 100 100
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