Corrosion Science 50 (2008) 2493–2497 2493–2497
Contents lists available at ScienceDirect
Corrosion Science j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o r s c i
Quantitative analysis analysis of iron oxides using Fourier transform infrared spectrophotometry H. Namduri, S. Nasrazadani
*
College of Engineering, University of North Texas, Denton, TX-76207, USA
a r t i c l e
i n f o
Article history: history: Received 14 August 2007 Accepted 17 June 2008 Available online 4 July 2008
Keywords: A. Steel B. IR spectroscopy C. Rust C. Oxidation C. Passivity
a b s t r a c t
In this study, a systematic approach based on the application of Fourier transform infrared spectrophotometry (FTIR) was taken, in order to quantitatively analyze the corrosion products formed in the secondary cycle of pressurized water reactors (PWR). Binary mixtures of iron oxides were prepared with known compositions containing pure commercial magnetite (Fe3O4), maghemite (c-Fe2O3), and hematite (a-Fe2O3) for calibration purposes. Calcium oxide (lime) was added to all samples as a standard reference in obtaining the c alibration curves. Using regression analysis, relationships were developed for intensity versus concentration for absorption bands corresponding to each of the phases in their corresponding FTIR spectrum. Correlation coefficients, R2, of 0.82, 0.87, and 0.86 were obtained for maghemite–magnetite, hematite–magnetite, and hematite–maghemite systems, respectively. The calibration curves generated were used to quantify phases in multi-component unknown field samples that were obtained from different components (moisture separators, condensers, and high and low pressure heaters) of the two units (units 1 and 2) of the secondary cycle of the Comanche Peak PWR. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Formation and transformation of iron oxides is of interest to a wide variety of industries including steel making, power generating, paint systems, pharmaceutical, and petrochemical, to name a few. The thermal hydraulic performance and integrity of the steam generators in nuclear power plants may be compromised due to the presence of corrosion deposits. The amount of iron transported in a steam generator is dependent on the composition of iron oxide formed in the feed train. Deposits that contain well crystallized magnetite and maghemite are more stable than deposits containing a combination of oxides and oxyhydroxides [1] [1].. Therefore, it is very important to quantitatively understand the composition of the deposits. Standard methods for identifications and characterizations of iron oxides have traditionally used either X-ray diffraction (XRD) or Mössbauer spectroscopy (MS) [2] [2].. XRD has been widely used in characterizing corrosion products. Although these techniques have served industry well in the past, they suffer from shortcomings that could be replaced by Fourier transform infrared spectrophotometry (FTIR). MS is a technique that utilizes a live radioactive source, which makes the technique relatively unsafe from an operational point of view, since it poses a potential health risk to the operator. The main limitations of MS technique include the level *
Correspondi ng author. Tel.: +1 940 565 4052; fax: +1 940 565 2666. E-mail address:
[email protected] [email protected] (S. (S. Nasrazadani).
0010-938X/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.06.034
of operator expertise requirements and the complexity of spectral interpretation. Even though XRD is easier to operate and interpret, its spectra is limited in its ability in differentiating magnetite and maghemite. On the other hand, FTIR instrumentation is simple and spectra interpretation is relatively easy [3] [3].. In the past, it has been established that the FTIR technique can be routinely used to efficiently differentiate and quantify different iron oxides and oxyhydroxides. oxyhydroxides. To the best knowledge of the authors, no attempts have been made in using FTIR for quantitative analysis of iron oxides formed in the power plants. The main objective of this research is to quantify iron oxide phases formed in the secondary side of the steam generator units at Comanche peak steam electric station (CPSES). Such analysis will allow better interpretation and control of the corrosion process. 2. Literature review
Magnetite is a well-known form of iron oxide that forms at room temperature in crevices between steel plates and at elevated temperature inside boiler tubes, heat exchangers etc. The oxidation product of Fe3O4 is either c-Fe2O3 or a-Fe2O3 depending on the oxidation temperature and/or possibly the crystallite size of the starting magnetite [4–5] [4–5].. Studies performed by Nasrazadani and Raman [4] [4] have have shown that transformation of magnetite to hematite goes through the formation of maghemite. The production of maghemite begins with
2494
H. Namduri, S. Nasrazadani / Corrosion Science 50 (2008) 2493–2497
nucleation and growth of goethite or lepidocrocite, followed by dehydration to hematite, and then reduction to magnetite. The deposition rate of hematite is an order of magnitude greater than magnetite. As seen from the Pourbaix diagram of iron, it is important that the reducing conditions be maintained in the steam generators during operation, so as to facilitate formation of magnetite. Turner and Klimas showed that lowering the concentration of hematite relatively to magnetite in the fee dwater will significantly lower the rate of tube bindle fouling [6]. Theoretical studies by Jobe showed that hematite has a very low solubility and a much smaller dissolution rate than magnetite and lepidocrocite in the presence of 5 ppb of dissolved oxygen. Formation of thin layer of maghemite/magnetite is known to act as a very good passive film [7]. FTIR spectra of iron oxides are well established. It is been observed that the absorption band at a high wavenumber region is due to OH stretching, and at lower wavenumber as a result of Fe-O lattice vibration. FTIR spectrum of magnetite exhibits two strong infrared absorption bands at 570 cm ¡1 (t1) and 390 cm¡1(t2) [8]. According to Ishii et al, these bands can be assigned to the Fe-O stretching mode of the tetrahedral and octahedral sites for the ¡1 and the Fe-O stretching mode of the octahet1 band at 570 cm dral sites for the t2 band at 390 cm ¡1, provided that Fe3+ ion displacements at tetrahedral sites are negligible [8]. FTIR spectrum of magnetite exhibits two other absorption bands at 268 cm¡1and 178 cm¡1 which were beyond the detection limit of our instrument. Maghemite, a defective form of magnetite, has absorption bands at 630 cm ¡1, 590 cm¡1, and 430 cm¡1. Table 1 summarizes possible FTIR peaks for different iron oxides. Legodi and his group performed quantitative analysis on calcium carbonate present in different cement blends using FTIR [9]. Reig and group performed quantitative FTIR analysis on calcium
Table 1 Infrared bands of different iron oxides [4,13]
Iron oxide/hydroxide
Wave numbers of bands (cm¡1)
Magnetite (Fe3O4) Maghemite (c-Fe2O3) Hematite (a-Fe2O3) Goethite (a-FeOOH) Lepidocrocite ( c-FeOOH)
Broad bands at 570 and 400 cm¡1 700, 630–660, 620 range (Fe-O range) 540, 470 and 352 cm¡1 1124, 890 and 810 cm ¡1 for OH stretch 1018 cm ¡1 (in plane vibration) and 750 cm¡1 (out of plane vibration)
carbonate and silica (quartz) using the constant ratio method. The group used potassium ferricynaide as standard and successfully showed the accuracy of quantifying the conc entration of silica and quartz in geological samples using FTIR [10]. The same group also successfully showed that FTIR can be used to quantify butyl acetate and toluene in binary and ternary mixtures using constant method ratio. They used valeronitrile as the standard and they also showed that the above method is independent of optical path length [11]. The Xu group showed that FTIR can be efficiently used for quantifying minerals. They used a multifunctional analysis, which is based on Beer’s law to quantify different minerals present in oil wells. In this method, the absorbance at a specific wave number is equal to the sum of the absorbance of all sample components at that wavenumber [12]. 3. Experimental procedure
Commercially available powders of magnetite (puratronic 99.999% purity), maghemite (99+% purity), and hematite (99.99% purity) were obtained. Three binary sets of sample mixtures with known concentrations of maghemite and magnetite, hematite and magnetite, and hematite and maghemite were prepared. All the samples were added to KBr powder and compressed into pellets using hydraulic press. Magnitude of compression applied in KBr pellet preparation needs to be kept constant to avoid variance in absorbance intensity from one sample to the next. Nicolet Avatar 370 DTGS FTIR was used to quantify iron oxides. FTIR spectra collection was done for 32 scans with 2 cm¡1 resolutions. Three equivalent runs of each of the three sets of the samples were made on the FTIR spectrometer. The average values of background subtracted peak intensity results were used for obtaining calibration curve. Once all of the spectra for the samples were obtained, three calibration curves were drawn for the three sets of samples. To set the calibration curve for known amounts of iron oxide (magnetite, maghemite and hematite) in each mixture, I /I o ratio was used. The intensity of the iron oxide peak (magnetite 570 cm¡1, hematite540 cm¡1 and maghemite 630 cm¡1) is represented by I and I o represents, the intensity of the 3640 cm¡1 peak of CaO. This calibration curve was used to quantify the amount of iron oxides present in the field samples collected from the secondary side of CPSES. The most readily available samples of the secondary system were obtained from the feedwater heaters (FW HTR-low pressure and high pressure feedwater heaters), condenser, and moisture separator-reheater (MSR), as these components are rou-
Fig. 1. Simplified schematic of secondary system sample locations.
H. Namduri, S. Nasrazadani / Corrosion Science 50 (2008) 2493–2497
tinely opened during outages and represent major temperature locations of the system as illustrated by Fig. 1. The CaO absorption band was used primarily as a reference because it does not interfere with any of the iron oxide phases. Even though an absorption band of 3640 cm¡1 is close to the OH band, it has a very distinct peak and can be easily discerned (Fig. 2). CaCO3 (894–865 cm ¡1 peak) has been previously used as a standard reference in quantifying the amount of limestone in different cement blends [9]. A linear fit was used to obtain the calibration curve.
2495
4. Results and conclusions
Fig. 3 shows FTIR spectra of single phases of hematite, magnetite, and maghemite. A sharp peak at 3640 cm¡1 belonging to calcium oxide is shown in all of the spectra of iron oxides. The peak intensity of CaO was fairly constant in all the spectra. Calibration curves were obtained for combinations of two phases of iron oxides. Correlation factors of 0.822 (magnetite added to maghemite, Fig. 4), 0.8584 (maghemite added to hematite, Fig. 5), and 0.8708 (magnetite added to hematite, Fig. 6) were obtained.
Fig. 2. FTIR spectra of 100% CaO showing 3640 cm ¡1 peak.
Fig. 3. FTIR spectra of 100% hematite, maghemite and magnetite.
2496
H. Namduri, S. Nasrazadani / Corrosion Science 50 (2008) 2493–2497 Table 2 FTIR intensities for different known concentrations of iron oxides used in calibration curves ( I = intensity of iron oxide mixture, and I o = intensity of 3640 cm¡1 peak of CaO)
1.20
Maghemite/Magnetite -1 Maghemite 630 cm
1.00
0.80 O
I / 0.60 I y = 0.0032x + 0.6477 2
R = 0.8222
0.40
0.20
0.00 0
20
40
60
80
100
Maghemite Conc. (%) Fig. 4. FTIR calibration for mixture containing magnetite and maghemite.
Hematite (%)
Magnetite (%)
Maghemite (%)
I /I o
100 – – 80 60 40 20 80 60 40 20 – – – –
– 100 – 20 40 60 80 – – – – 20 40 60 80
–
1.30 1.13 1.02 1.01 0.98 0.82 0.81 1.13 1.02 0.91 0.95 0.86 0.81 0.75 0.76
100 – – – – 20 40 60 80 80 60 40 20
1.50 1.40
Hematite/Magnetite -1 Hematite 540 cm
1.30 1.20
y = 0.0059x + 0.6335 2 R = 0.8708
1.10 O
I / I 1.00 0.90 0.80 0.70 0.60
0
10
20
30
40
50
60
70
80
90
100
Hematite Conc. (%) Fig. 5. FTIR calibration for mixture containing hematite and magnetite.
1.50 1.40 1.30
most intense peak of maghemite) was used for I values in the case of mixtures containing magnetite and maghemite. No peak interferences of any phases were observed in all of the mixtures since FTIR spectra of all iron oxides are well resolved and spectra resolution of most FTIR instruments is 2 cm ¡1. These calibration curves were then used to quantify the iron oxide phases present in the field samples collected from the secondary side of unit 1 and unit 2 of CPSES. The percentage concentrations of the iron oxides present in the selected field samples is given in Table 3. The samples from the moisture separator mainly show hematite and magnetite. The main feedwater heater sample showed 96% magnetite and about 4% maghemite. The high-pressure feedwater heater sample showed mostly hematite; whereas, low-pressure feedwater heater sample showed hematite and magnetite. The presence of magnetite and hematite is expected in feedwater systems due to the transformation of hydroxides and other hydrated iron species, which move through the fee dwater system into stable iron oxides (Schikorr reaction). The two samples form the main condenser mainly consisted of hematite with traces of magnetite and maghemite. Detection limits determination for iron oxides quantification using FTIR was not done in this study and is planned for future work. 5. Summary and conclusions
1.20 O1.10 I / I
1.00 y = 0.0046x + 0.7849 2 R = 0.8584
0.90 0.80 0.70
0
10
20
30
40
50
60
70
80
90
100
Hematite Conc. (%) Fig. 6. FTIR calibration for mixture containing maghemite and hematite.
A confidence limit of 95% was used in the calculations. The average values of I /I o for three runs made with mixtures with different concentrations of iron oxides are shown in the Table 2. Hematite peak at 540 cm ¡1 intensity (the most intense peak for hematite) was used for the I value for mixtures containing hematite and magnetite, and maghemite and hematite. The peak at 630 cm¡1 (the
A quantitative determination of iron oxides can be quickly performed relatively accurately using FTIR technique. The technique involves taking mid infrared measurements with samples in the form of KBr pellets. By using a standard reference like CaO, normalization can be performed. The peak of 3450 cm ¡1 is free from interference with any of the major iron oxide peaks considered in this study. This method makes it a very suitable method in quickly determining the concentrations of major iron oxides in the power industry. The FTIR technique was reconfirmed to be a valuable tool to differentiate between Fe3O4 and c-Fe2O3. It is also been shown that this technique can be used in quantifying iron oxides. It has also been shown that the infrared active mode of calcium oxide can be efficiently used in the quantification process. The FTIR quantification method performed in this study can be further finetuned and extended to other major metallic oxides including: chromium oxide, nickel oxide, lead oxide, and silicon dioxide. This will prove valuable for studying corrosion deposits formed in nuclear power plants.
2497
H. Namduri, S. Nasrazadani / Corrosion Science 50 (2008) 2493–2497 Table 3 Concentration of field sample collected from different components of secondary side of unit 1 and unit 2 steam generator system of CPSES
Field samples
Sample description
Oxides present
I o
Maghemite (I )
Hematite (I )
I /I o
Final concentrations
1 2 3 4 5 6
Moisture separator Main feedwater heater Main condenser, hotwell Main condenser High pressure feedwater heater Low pressure feedwater heater
Magnetite and hematite Magnetite and maghemite Magnetite and hematite Maghemite and hematite Maghemite and hematite Magnetite and hematite
3.02 4.29 3.08 0.28 0.75 5.81
– 2.81 – – – –
2.85 – 4.01 0.49 1.03 5.37
0.94 0.66 1.3 1.74 1.37 0.92
52% Hematite and 48% magnetite 96% magnetite and 4% maghemite Mostly hematitea Mostly hematiteb Mostly hematiteb 52% magnetite and 48% hematite
a b
Amount of magnetite was below the detection limit. Amount of maghemite was below the detection limit.
Acknowledgement
Authors would like to thank both Mr. Jim Stevens and Mr. Robert Theimer for providing field samples. References [1] Domingo, Clementa, The pathways to spinel iron oxides by oxidation of iron (II) in basic media, Materials Research Bulletin 26 (1991) 47–55. [2] Blesa, A.J.G. Maroto, S.I. Passaggio, F. Labenski, C. Saragovi-Badler, Moessbauer study of the behaviour of synthetic corrosion products of nuclear power plants, Radiation Physical Chemistry 11 (1978) 321–326. [3] Brundle Richard, Charles Evans, Wilson, “Encyclopedia of Materials Characterization”, Butterworth–Heinemann, 1992, ISBN 0–7506-9168-9. [4] S. Nasrazadani, A. Raman, Application of IR spectra to study the rust systems, Corrosion Science 34 (8) (1993) 1335–1365. [5] Nasrazadani, Namduri, Steven, Theimer, Fellers, “Application of FTIR in the Analysis of Iron Oxides and Oxyhydroxides Formed in PWR Secondary System”, 2003 Steam Generator Secondary Side Management Conference, February 10–12, 2003.
[6] Turner, Klimas, “The Effect of Alternative Amines on the Rate of Boiler Tube Fouling”, Final Report, TR-108004, EPRI Report, September 1997. [7] David Jobe, “The calculated solubilities of hematite, magnetite and lepidocrocite in steam generator feed trains”, AECL, 1997. [8] M. Ishii, M. Nakahira, Infrared absorption spectra and cation distribution in (Mn,Fe)3O4, Solid State Communications 11 (1972) 209–212. [9] Legodi, D. De Waal, J.H. Potgieter, Quantitative determination of CaCO3 in cement blends by FT-IR, Society for Applied Spectroscopy 55 (3) (2001) 361– 365. [10] Reig, J.V.G. Adelantado, M.C.M. Moya Moreno, FTIR quantitative analysis of calcium carbonate(calcite) and silica (quartz) mixtures using the constant ratio method. Application to geological samples, Talanta 58 (2002) 811–821. [11] Reig, J.V. Gimeno Adelantado, V. Peris Martinez, M.C.M. Moya Moreno, M.T. Domenech Cerbo, FT-IR quantitative analysis of solvent mixtures by the constant ratio method, Journal of Molecular Structure 480–481 (1999) 529–534. [12] Xu, B.C. Cornilsen, D.C. Popko, B. Wei, W.D. Pennington, J.R. Wood, Quantitative mineral analysis by FTIR spectroscopy, The Internet Journal of Vibrational Spectroscopy 5 (4) (2001) 1–12. [13] R.M. Cornell, U. Schwertmann, The Iron Oxides, Weinheim, New York, 1996.
