Infrared spectrometry for solid phase analysis: Corrosion rusts

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Corrosion Science 50 (2008) 1228–1234 www.elsevier.com/locate/corsci

Infrared spectrometry for solid phase analysis: Corrosion rusts J.P. Labbe´ a, J. Le´dion b, F. Hui b,* b

a SERAM, 151, boulevard de l’Hoˆpital, 75013 Paris, France ENSAM-LIM UMR CNRS 8006, 151, boulevard de l’Hoˆpital, 75013 Paris, France

Received 25 January 2007; accepted 30 August 2007 Available online 8 February 2008

Abstract For the quantitative analysis of iron oxides powders by infrared spectrometry, the importance of grinding, symmetry (three spectra per sample) and calibration are emphasized (five independent teams of two persons for nine phases, 60 spectra). Simple semi-quantitative problems (three FeOOH phases) are solved by a standard Cramer system. When maghemites are present, it is preferable to adjust the spectrum profile through a least squares method (425–587 cm1 range). Contrary to Beer’s law, the Bouguer–Lambert formalism gives absolute values (lmoles Fe). Slope measurements are useful for the most difficult substance (Fe3O4). Ó 2008 Elsevier Ltd. All rights reserved. Keywords: A. Iron oxides; A. Oxyhydroxides; B. Infrared spectrometry; C. Rust; C. Corrosion

1. Introduction Infrared absorption spectrometry (IRAS) has been used for years as a simple and versatile analytical tool, and the pioneer contribution of Lecomte [1] still remains a standard reference. Nevertheless, its development in the field of corrosion is greater for organic inhibitors than for inorganic surface products, owing to the limited number of standard spectra for inorganic compounds [2,3]. Yet, the possibilities of this technique are important. In comparison with X-ray photoelectron spectroscopy (XPS), minute crystal structure variations can be evidenced, with a great sensitivity for 3 anion groups such as CO2 3 or PO4 for instance [4,5]. IRAS also compares favourably to X-ray diffraction, as amorphous organic and inorganic substances are easier to measure [6]. Mo¨ssbauer spectrometry is also a very interesting quantitative, non-destructive technique for iron corrosion, even with amorphous substances [7–10], if a sufficient amount of substance is available. Organic matter escapes such measurements as well as the previously referred anions. An IR spectrum is obtained in about 30 min. IRAS

*

Corresponding author. Tel.: +33 1 44 24 62 15; fax: +33 1 44 24 62 90. E-mail address: [email protected] (F. Hui).

0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.08.023

is generally considered but as a qualitative method [11], the sample being examined by transmittance or reflection. In fact, quantitative informations can also be obtained on various mixtures of corrosion products, inhibiting layers, scaling compounds [4]. The technique of abrasion with a microspatula which was developed in our laboratory [5] is convenient for quantitative results with small amounts of matter, transmittance being more efficient than reflection measurements [12]. In the iron rust system, many spectra have been registered, mostly for qualitative identifications [12–16]. Even in recent papers, spectra are often given in the absorbance mode, the wavelength being limited to 25 lm [17,18] (400 cm1). The methods of preparation and characterization of the various FeOOH, Fe2O3, Fe3O4 phases have been summarized [19]. Special papers give interpretation of IR spectra [20,21]. Quantitative attempts are seldom mentioned. A normalization of a-FeOOH and a-Fe2O3 spectra at a concentration of 1 mg cm2 is quoted [22], as well as an estimation of the optical pathlength by interfringe measurement to apply Beer’s law [23]. Misawa remains the pioneer [24–27] who, as early as 1971, tried a true measurement on rusts as a mixture of a, c, d-FeOOH and Fe3O4 or a + c + amorphous FeOOH phases. The possibilities of spectrometers were somewhat limited at that period and the hypotheses that the 580 and

J.P. Labbe´ et al. / Corrosion Science 50 (2008) 1228–1234

470 cm1 absorptions are mostly due to Fe3O4 and d-FeOOH, respectively [24] are not satisfactory (see below). Nonetheless, he mentioned the effect of grinding, mostly on the wavenumber of a maximum. The true possibilities of such measurements have not therefore been fully investigated. This paper describes the determination of several components in the far IR range. 2. Quantitative measurements 2.1. Bouguer–Lambert law Quantitative measurements are usually treated by Beer’s law symbolism, valid for continuous media (liquid solutions in organic chemistry). The absorbance (A) depends on C (concentration of substance) and l (optical pathlength), the constant factor (ek) being the absorptivity at a given wavelength: Ak = ek‘C. For solid dispersed media, it is more suitable to use Bouguer–Lambert variables, expressing C from the amount of matter m (mass or mole) in the volume v = S‘ (S: area of a cylindrical pellet). For the same amount of substance, A is greater if S is smaller (micropellet technique); ‘ is not to be taken into account: Ak ¼ ek m=S

ð1Þ

2.2. Grinding conditions Moreover, ek is no more a constant, except with very small particles (long grinding times) [28]. The experimental study [29] on calcite with particle sizes ranging from 0.001 to 26 lm dispersed in KBr confirmed the absorbance of a given amount of CaCO3 to increase with grinding time towards a limit which is obtained later when the intrinsic absorptivity of the band is higher. For example, if the particle size decreases from 2 to 1 lm, the absorbance measured at 1430 cm1 (A1430) is doubled (very strong band), whereas A875 increases by 30% (medium band) and A710 is almost constant (weak band). Great care therefore should be taken in quantitative IR analysis, as already mentioned in the case of simple goethite–lepidocrocite mixtures [30]. This is why strictly the same grinding conditions (in the same mortar, with the same person . . .) are advisable, particularly when several ‘‘dilutions” are required to ensure a proper sampling operation from an important amount of initial sample. Whenever possible, 5 mm diameter pellets were used in this work (rather important S value, yet smaller than beam diameter). For minute amounts of substance, 3 mm diameter pellets were preferred. With even smaller pellet sizes a beam condenser should be used, as the energy gathered by the spectrometer in the vicinity of 200 cm1 is then very small. 2.3. Isotropy Another requirement underlined by theoretical considerations is isotropy, seldom fulfilled, as a pellet is never

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exactly cylindrical but rather wedge-shaped, and the infrared beam partly polarized. The practical consequence is an important variation (up to 25%) of a band absorbance value (even as measured with a conventional linear base line) when the pellet is rotated by small increments in its plane (the curve is symmetrical relative to the vertical plane containing the beam). This is why three spectra were registered for each pellet, rotating it each time by 120° in its plane to obtain a constant mean value with a 6% uncertainty [31]. A second consequence of anisotropy is the possible preferential orientation (from the pressure applied) of crystals relative to the pellet plane. Such is the case for c-FeOOH which exhibits completely different spectra with platy or rod-like crystals [19]. The interpretation of the various bands is known [21], but they appear with different intensities. A simple means of treating such a difficulty is to consider both types of crystals as different entities. In the following, they will be referred to as LA (platy) and LB (rod-like). 3. Experimental 3.1. Pellet preparation As grinding is one of the most important operations, mortars were cut from a-Al2O3 monocrystals (shaped with a jeweller saw and diamond polished). For quantitative measurements, The time for all grindings was standardized (10 min). A weighed amount of substance was ground in the presence of 23 ± 2 mg CsBr to obtain 5 mm diameter pellets, (5 mg for 3 mm diameter pellets). The residue sticking to the walls of the mortar was gathered by grinding an additional very small amount of CsBr and placed into the mould. This ensured quantitative transfer within 99 ± 1%. The absorptivities referred to in this paper are mean values from 20 pellets prepared by five independent teams of two persons, giving 60 spectra for each phase. 3.2. Infrared spectra CsBr pellets were examined (4000–200 cm1) with a 45-BRUCKER Fourier transform spectrometer (16 cm1 resolution, 1200 scans). All spectra were recorded in the absorbance mode, giving a linear scale for quantitative measurements. A special 1.5 mm thick sample holder received each pellet in an almost cylindrical hole retaining the pellet through a slight friction and maintaining its whole surface into the beam. To reduce the important effect of water bands (especially near 200 cm1), the pathlength in the ambient atmosphere was reduced to 6 mm by a proper cylindrical extension provided by the Brucker company. A single beamsplitter is to be used: changing a KBr for a polyethylene beamsplitter does not allow a continuous measurement, not only of absorbance, but also of slope variations after 700 cm1.

2.10 3.57 12.2 10.5 5.11 13.3 1.58 0 1.43 LA and LB: c-FeOOH, platy and rod-like crystals, respectively.

275 286

2.31 5.54 5.65 6.30 3.34 6.92 2.09 0 2.53 2.72 7.01 1.65 2.57 2.24 3.88 2.90 0 3.07

300 330

3.78 8.44 2.07 3.12 2.85 0.524 3.57 0.30 13.1 5.19 7.52 15.5 13.6 6.70 3.72 4.31 3.05 6.48

360 500

6.90 10.4 11.1 6.96 8.09 4.90 4.33 1.78 4.53 6.03 8.91 10.9 6.82 4.65 1.02 5.42 1.64 12.1

525 550

5.75 7.32 10.5 5.41 3.24 1.05 8.69 2.45 15.3 5.75 5.95 7.91 5.14 3.32 2.83 10.4 5.34 12.9

575 680

5.03 4.34 2.00 1.53 10.0 1.44 6.06 3.08 1.60 3.00 2.20 3.17 2.00 4.20 0.65 2.40 2.20 0.30

750 800

1.94 1.82 0.83 0.98 3.24 8.65 1.08 1.90 0.21 1.23 1.79 0.64 7.00 1.80 8.62 0.52 1.69 0.18

900 1020

0.67 1.68 8.01 5.40 0.27 0 0.29 1.28 0 0 1.56 0.47 0.61 0 0 0.15 1.07 0

1100 400

6.33 8.95 5.90 7.59 12.5 17.8 6.24 2.51 1.36 7.14 10.8 5.28 5.93 13.5 14.4 6.57 2.26 2.70

425 450 475 750 800 900 1020

Shortest tangent (cm1)

4.1.4. Fe3O4 and c-Fe2O3 from slope measurements Among the constituents of rusts, magnetite (MG) is quoted to play an important role [24,25]. However, the pure substance exhibits but a very weak absorption, even at its maximum (570 cm1), which makes it impossible to measure in this way. Fortunately, a very high slope exists on its absorption curve (3160 cm3 mol1), contrary to

Table 1 Absorptivity values (m2 mol1 Fe Þ for various rust phases

4.1.3. Feroxyhyte (d-FeOOH) The crystallinity of d-FeOOH (visible by its 430 cm1 peak in addition to the broad 475 cm1 band characterizing an amorphous phase) is always limited. Our best sample (n = 0.33) was obtained by violent oxidation (H2O2) [19] of FeCl2 at pH 8, but may be improved in the future. A d-phase obtained after corrosion will be calculated as a mixture of such a d-FeOOH standard and ‘‘amorphous FeOOH”.

7.54 12.1 6.72 5.41 12.0 8.13 6.27 2.29 7.94

4.1.2. a-, and c-FeOOH From water measurements, they correspond to the FeOOH, nH2O formula, the values of n being zero for the a phase, 0.15 for LA and 0.24 for LB.

7.50 11.9 10.5 6.87 10.5 7.37 4.90 2.03 10.9

This is why it was preferred to limit pH variations when precipitating by using a weak base: ammonia was used in this study (pkA = 9.2), giving n = 1.9.

– – 3.17 2.00 – – – – –

Long tangent: 1300–250 cm1

FeOOH; nH2 O ! ð1–2xÞFeOOH þ xFe2 O3 þ ðn þ xÞH2 O

– – – – – 7.92 – – –

4.1.1. Amorphous phases The method of preparation is important to obtain a correct standard, which is usually obtained by slowly adding KOH to ferric nitrate and strictly controlling the pH after each addition [19]: the local increase of pH around the drop is important when the buffer capacity gets low. We indeed checked the fact that, with an excess of strong base, the dehydrating properties result in an excess of the 575 cm1absorptivity, which means that a mixture is obtained, containing both amorphous FeOOH and c-Fe2O3, the relative amounts of which were obtained from the absorptivity values of the Table 1, visualized in Fig. 1

– – – – – 8.05 – – –

Fig. 2 shows the spectra of four phases from which the absorptivities were calculated, isolating the characteristic part (Fe system) by a long tangent (1800–200 cm1). A short tangent can also be useful to isolate the contribution of a single species (showing one or two sharp peaks) in simple problems. Table 1 gives the absorptivities ek (measured with short and long tangents) at various wavenumbers for pure substances, visualized in Fig. 1. The exact meaning of amorphous, a, c and d-FeOOH determinations is to be found below, together with the interest of slope measurements to obtain Fe3O4 and c-Fe2O3 values.

– – 7.54 4.98 – – – – –

4.1. Absorptivity values

Amorphous d-FeOOH LA LB b-FeOOH a-FeOOH c-Fe2O3 Fe3O4 a-Fe2O3

4. Results

316 3160 970

J.P. Labbe´ et al. / Corrosion Science 50 (2008) 1228–1234 Mean slope (cm3 mol1)

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J.P. Labbe´ et al. / Corrosion Science 50 (2008) 1228–1234

12

content, comprised between 0 (pure c-Fe2O3) and 33 mol Fe % (Fe3O4), therefore corresponds to a linear variation of the slope between the limits 316 cm3 mol1 (pure c-Fe2O3) and 3160 cm3 mol1 (pure Fe3O4). Any maghemite phase can then be described as a mixture of these two limits, both from its slope and its profile (e.g., the ratio of c-Fe2O3 absorbances at 575 and 425 cm1 is twice that of Fe3O4). When the phases are poorly crystallized, which is most frequently the case in actual corrosion products, the 635 cm1 peak, characteristic of c-Fe2O3, is weaker and should be used only for crystallinity evaluation. The sample used in this study can be written c-Fe2O3, mH2O with m = 0.58.

AK

8

AM

Absorptivity (m2. mol-1)

4

FX 0

G

16

LA

12

LB

8

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4.1.5. Ferric hydroxy-carbonates They are recognised and measured [31] by three (very strong) peaks in the mas(CO3)d(HOH) range (1450 cm1, 1550 cm1, 1650 cm1) and two small bands linked to carbonates (ms: 1070 cm1; d, out of plane: 850 cm1).

4 0

H

4.2. Semi-quantitative results

12

MH 8 4

MG 0 1200

1000

600

800

400

200

Wavenumber(cm-1) Fig. 1. Absorptivity of nine phases in the iron system: AM: amorphous FeOOH AK: b-FeOOH FX: d-FeOOH G: a-FeOOH LA and LB: platy and rod-like c-FeOOH, respectively H: a-Fe2O3 MG: Fe3O4 MH: cFe2O3.

ABSORBANCE

MG

MH

FX

G ST

1200

1000

LT

800

600

400

200

Wavenumber(cm-1) Fig. 2. Spectrum profiles of Fe3O4 (MG) and c-Fe2O3 (MH) as compared to a-FeOOH (G) and d-FeOOH (FX).

c-Fe2O3(316 cm3 mol1: Fig. 1; Table 1), allowing its reliable measurement. In fact, maghemite phases (MH) are often the true species found in rusts [32,33]. Their ferrous

Atmospheric corrosion was to be evaluated in a 70-yearold sewerage system. The depth in the rust crust is therefore linked to the time elapsed. This is why three levels were distinguished in this crust (air–rust interface, bulk material, metal level). Fe3O4 or maghemites (Fig. 1) do not play a significant part in these products. Also, d-FeOOH can be excluded from the lack of 330 cm1 broad band and b-FeOOH in the same manner at 680 cm1. A three phase system is therefore present, with an amorphous compound (AM), goethite (G) and lepidocrocite L. In a quantitative problem, LA is to be distinguished from LB: in our case LA, calculated from the 1020 cm1 band, would absorb more strongly at 540 cm1. LB is chosen as an hypothesis, confirmed later by the overall spectrum profile. LB, G and AM determination is very simple from the observation of an isobestic zone at 483 ± 3 cm1, where their absorptivities are almost equal: 7.2 ± 0.2 m2 mol1. The A483 value is a spectral measurement of the sum LB + G + AM. Isolating the 1020 cm1 peak by a short tangent from 950 to 1150 cm1 gives a linear variation of the ratio r = A1020/A483 = k  LB/(LB + G + AM) as a function of LB rate. The origin is a first point of the line, a second corresponding to pure LB (maximum r value). The LB percentage can therefore be written as r = 100 r/rmax. The same type of calculation gives LB from the 750 cm1 absorbance; the mean value LB ¼0.5 (L1020 + L750) is shown on Table 3. The corresponding mean value for G is: G ¼0.5 (A900 + A800), taken from A900 and A800 bands, and the amorphous part by difference (AM = 100  LB  G). In fact, as will be seen on Fig. 1, LB is not identical from both wavelengths (systematically smaller at 750 cm1 than at 1020 cm1), the same being true for G800 relative to G900. The reason for this lies in the fact that the lattice order is not perfectly isotropic. For instance the in-plane OH bending mode at 900 cm1 has a transition moment in the ab

J.P. Labbe´ et al. / Corrosion Science 50 (2008) 1228–1234

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Table 2 IR analysis of three coupons in a sewerage system LB (%)

G (%)

AM % Cryst. of % Cryst. of Observations (%) LB G (360 cm1) (400 cm1)

H1 15 ± 3 15 ± 3 70 H2 50 ± 3 19 ± 3 31 H3 57 ± 1 11 ± 3 32

84 51 38

19 46 11

Air–rust interface Bulk Rust–metal interface

plane [20] whereas for the 800 cm1 out of plane bending it is parallel to the c axis of the particles. Nevertheless, as they vary systematically in the same manner, their mean values LB and G remain a correct image of LB and G contents. The most sensitive way to obtain crystallinity variations consists in calculating A360/A483 and A400/A483 to measure LB and G from a 2  2 system of equations. The crystallinity rates are LB/LB and G/G (expressed as % in Table 2). The results are as follows: LB decreases in the course of time (H3 > H2 > H1) but is never zero, whereas its crystallinity increases continuously, and the amorphous content is far higher at the air–rust interface, from G alteration. 4.3. General problem of complex rust systems The difficulty rises sharply if a great number of phases are to be measured, and a few important facts are to be kept in mind: (a) As hematite (a-Fe2O3) is usually unimportant in corrosion problems, the main oxide phases are maghemites, calculated as a mixture of pure c-Fe2O3 and Fe3O4, the latter measured through its very great slope (corrected from the former when necessary). The main drawback is that the absorbance of sharp peaks as measured with a short tangent is greatly altered in the presence of dark compounds, and calculating goethite and lepidocrocites at 800–900 and 1200–750 cm1 gets hazardous. (b) Akaganeite (b-FeOOH), easy to identify (strong broad band at 680 cm1), is linked to the presence of

Cl ions, and generally observed as the main phase in sea-water and archaeological rusts [34], which results in simpler systems. (c) The best way of measuring several phases including oxides is to apply statistical methods, optimizing the spectrum profile in a given range of wavelengths. Most methods minimize the sum of the square of differences measured at a number of wavelengths between measured and estimated spectral data, be it the standard sum of squares (using pure compounds as standards), the q-matrix method (from known mixtures) or the inverse least squares (assuming errors exclusively in reference concentrations). Partial least squares and principal component regression models are more general: the ‘‘factors” are not constrained to follow Beer’s law (each factor being an analyte). The importance of calibration should never be underestimated [35]. A small program was developed to calculate several phases including maghemites, as xi (lmole Fe). A rough estimation gives the starting point: d-FeOOH from the A1100–A1200 difference, a-FeOOH from the A900 value, Fe3O4 from the slope and c-Fe2O3 from the A575 measurement (corrected for the first three phases). At each wavelength these values are multiplied by the corresponding absorptivities and added. The differences from the experimental spectrum are then used at 25 wavelengths to obtain a standard deviation r1. After incrementing one by one the xi values by a given Dxi (which could be fixed at various levels to compare efficiency and operating time), the best set of xi amounts is found by the lowest r before the relative amounts get constant. Results appear on Table 3. 5. Discussion Solving a Cramer system of equations is a general method to measure a few ferric oxyhydroxide phases when oxides are absent. In all systems described by three phases in which the isobestic point method is valid, additional information on crystallinity can be obtained from the 400

Table 3 Validation of spectrum profiles from 25 points (425–587 cm1) on 7 mixtures of a- and d-FeOOH, c-Fe2O3, Fe3O4 Sample

A B C D E F G

Phase

Prepared Found Prepared Found Prepared Found Prepared Found Prepared Found Prepared Found Prepared Found

Absolute amounts (lmol Fe)

Percentages

d-FeOOH

a-FeOOH

c-Fe2O3

Fe3O4

Sum

rm

d-FeOOH

a-FeOOH

c-Fe2O3

Fe3O4

0.050 0.00 0.059 0.05 0.130 0.13 0.267 0.23 0.260 0.33 0.517 0.58 0.534 0.52

0.223 0.23 0.211 0.21 0.042 0.04 0.560 0.61 0.055 0.12 0.297 0.25 0.285 0.18

0.307 0.36 0.589 0.59 0.329 0.32 0.225 0.21 0.319 0.32 0.061 0.11 0.236 0.31

0.654 0.61 0.337 0.30 0.188 0.18 0.070 0.03 0.648 0.67 0.264 0.20 0.066 0.08

1.23 1.20 1.20 1.15 0.689 0.67 1.12 1.08 1.28 1.44 1.14 1.14 1.12 1.09

/ 0.01890 / 0.02426 / 0.01315 / 0.01708 / 0.00916 / 0.01039 / 0.01713

4 0 5 4 19 19 24 21 20 23 45 51 48 48

18 19 18 18 6 6 50 56 4 8 26 22 25 17

25 30 49 51 48 48 20 19 25 22 5 10 21 28

53 51 28 26 27 27 6 3 51 47 23 18 6 7

rm: minimum standard deviation value.

J.P. Labbe´ et al. / Corrosion Science 50 (2008) 1228–1234

and 360 cm1 bands of a-FeOOH and c-FeOOH (Fe–OH stretching in the lattice) [20]. In the sewerage system studied by this method, the atmospheric corrosion is uniform and slow. Whatever the mechanism (e.g. corrosion of a uniform type, building-up of an efficient barrier after a certain time), the overall result after 70 years is a non-zero value of LB at the air–rust interface (oldest rust), at the place where the mechanism of dissolution–reprecipitation as amorphous FeOOH (before a-FeOOH crystallization) takes place [24,26,27]. The variation expected therefore shall take years. The important increase of LB crystallinity as time passes (H1> H2 > H3) is also consistent with a tendency of the system to stabilize. The corresponding increase of G (and its crystallinity: H2 > H3) is completely altered at the air–rust interface (H1). The increase of the amorphous phase together with the decrease of both G % and its crystallinity means that a portion of a-FeOOH is involved in the first part of the dissolution–amorphous reprecipitation mechanism, the long term a-FeOOH increase being totally masked at this particular place. An important result is that no special treatment is necessary, and the sewerage system shall last quite a number of years. The presence of small amounts of silicates (broad 1000–1200 cm1 band in H1) is not a serious difficulty to measure these ferric phases, as these bands remain very small. The effect of particle size does exist, but remains limited in our conditions. Complex systems containing maghemites are treated by statistical calculations, allowing the spectrum profile to be optimized in a given range of wavenumbers. The reason for this is the important negative slope observed in the overall spectrum. Two consequences are to be expected. First, such dark substances absorb a part of the incident energy outside the range of their absorption bands. Such a screening effect is very important in the near IR, less in the vicinity of 400 cm1, which is equivalent to a decrease of the incident flux U0 in the absorbance measurement (A = lg U0/U): all absorbances are smaller. Second, the necessity of measuring through a tangent also limits the value obtained on a steep slope. Fortunately, such a drawback also offers an advantage. As already mentioned, the slope indeed is a reliable measurement for Fe3O4, which allows all maghemites to be characterized by a mixture of Fe3O4 and c-Fe2O3 (Table 2). This means that an absolute value of MG can be introduced into the program, and maintained constant in the statistical calculation of the other phases. In the difficult case of nearly pure substances, the standard deviation is observed to keep decreasing even when the amount of the main component is by far exceeded (e.g. multiplied by 2 in the case of FX (0.95 lmol) containing G (0.03 lmol), MH (0.07 lmol) and MG (0.07 lmol). Relative contents, of course, keep almost constant, as the overall profile of the spectrum is not very different from that of the pure component. In such a case, the absolute value given by the slope is a great help to obtain a correct sum of the components (absolute value, as shown on Table 2). The total iron can then be known even if the substance cannot be weighed (unknown water content, too small amount of material).

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Absolute measurements from Eq. (1) are therefore valid, not only relative proportions usually referred to by using ‘‘Beer’s law” with ‘‘concentrations”. Of course, the accuracy of such absolute measurements is rather low, a discrepancy of 0.1 lmol Fe being attained in unfavourable cases (Table 3, G) for a total amount of about 1 lmol Fe. The limiting factors of such calculations are to be kept in mind: first, wavenumbers greater than 600 cm1are to be avoided, as the rather sharp 635 cm1peak of c-Fe2O3is linked to its crystallinity. Second, limiting the range of measurements to 425 cm1avoids errors due to the quality of spectra when the energy is too low: the spectrum tangent is poorly defined around 220 cm1and all absorbance measurements are more hazardous in the far infrared. 6. Conclusions The main phases of rusts are susceptible to be determined from their IR spectra, Bouguer–Lambert law allowing absolute amounts of matter to be estimated down to the microgram level (pitting corrosion), by using the micropellet technique. Simple systems are easy to calculate. Additional properties such as crystallinity rates were obtained (by the isobestic point method) in a grey cast-iron network. When a noticeable slope is observed on the spectrum, maghemites are present, calculated as a sum of c-Fe2O3 and Fe3O4, the slope value being used to measure the latter. Least squares methods in a limited range (425–585 cm1) give a correct estimation of complex systems. The quality of calibration standards is critical. The grinding conditions are to be kept identical for standard mixtures and sample under examination, as well as other conditions (spectrometer and its settings, three spectra per sample). A good knowledge of spectrum profiles for pure substances is a help to simplify the problem by evaluating the number of phases present as well as to detect possible interferences (e.g. silicates) in actual corrosion problems. Possible improvements are linked to the quality of spectra in the vicinity of 200 cm1. At present, it is advisable to register up to 50 lm (200 cm1) to obtain good results at 400 cm1. References [1] J. Lecomte, Spectroscopie dans l’infrarouge, Handbuch der Physik, vol. XXVI, Springer-Verlag, Berlin, 1958. [2] Sadtler Res. Lab. Inc., Philidelphia, PA, Sadtler Standard Spectra, 1967. [3] R.A. Nyquist, R.O. Kagel, IR Spectra of Inorganic Compounds, Academic Press, New York, London, 1971. [4] J.P. Labbe´, B. Be´diang, J. Le´dion, Analyse qualitative et quantitative des me´langes calcite–aragonite par spectrome´trie d’absorption infrarouge, Analysis 12 (1984) 514–522. [5] J.P. Labbe´, A. Que´merais, M. Michel, G. Daufin, Fouling of inorganic membranes during whey filtration: analytical methodology, J. Membr. Sci. 51 (1990) 293–307. [6] M. Dupeyrat, J.P. Labbe´, M. Michel, F. Billoudet, G. Daufin, Mouillabilite´ et interactions solide-liquide dans l’encrassement de divers mate´riaux par du lactose´rum et du lait, Le Lait 67 (1987) 465– 486.

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[7] A. Vertes, I. Czako-Nagy, Mo¨ssbauer spectroscopy and its application to corrosion studies, Electrochim. Acta 34 (1989) 721–758. [8] A.R. Orlowe, J.M.R. Genin, The mechanism of oxidation of ferrous hydroxide in sulphated aqueous media: importance of the initial ratio of the reactants, Corros. Sci. 32 (1991) 965–984. [9] P.H. Refait, J.M. Ge´nin, The transformation of chloride containing green rust one into sulphated green rust two by oxidation in mixed Cl and SO2 4 aqueous media, Corros. Sci. 36 (1994) 55–65. [10] W. Meisel, G. Krysa, Relative Mo¨ssbauer konstanten von eisen verbindungen zur quantitativen analyse von gemischen, Z. Anorg. Chem. 395 (1973) 31–36. [11] H. Leidheiser Jr., S. Music, The atmospheric corrosion of iron as studied by Mo¨ssbauer spectroscopy, X-ray diffraction and Fourier transform-infrared analysis of the rust formed by corrosion of steel in aqueous solutions, Corros. Sci. 22 (1982) 1089–1096. [12] G.W. Poling, Infrared reflection studies of the oxidation of copper and iron, J. Electrochem. Soc. 116 (1969) 958–963. [13] S. Music, M. Gotic, S. Popovic, X-ray diffraction and Fourier transform-infrared analysis of the rust formed by corrosion of steel in aqueous solutions, J. Mater. Sci. 28 (1993) 5744–5752. [14] A. Raman, B. Kuban, A. Razvan, The application of infrared spectroscopy to the study of atmospheric rust systems – I: standard spectra and illustrative applications to identify rust phases in natural atmospheric corrosion products, Corros. Sci. 32 (1991) 1295–1306. [15] A.M.G. Pacheco, M.G.I.B. Teixera, M.G.S. Ferreira, Initial stages of chloride induced atmospheric corrosion of iron: an infrared spectroscopic study, Br. Corros. J. 25 (1990) 57–59. [16] W. Wolski, Zur untersuchung der jungen ku¨nstlichen roste, Werkstoffe und Korr. 37 (1986) 543–549. [17] A.V. Ramesh Kumar, R. Balasubramaniam, Corrosion product analysis of corrosion resistant ancient indian iron, Corros. Sci. 40 (1998) 1169–1178. [18] V.Y. Chen, H.J. Tzeng, L.I. Wei, H.C. Shih, Mechanical properties and corrosion resistance of low-alloy steels in atmospheric conditions containing chloride, Mater. Sci. Eng. A 398 (2005) 47–59. [19] U. Schwertmann, R.M. Cornell, Iron Oxides in the Laboratory, VCH Verlagsgeselshaft mbH, Weinheim, New York, Basel, Cambridge, 1991. [20] P. Cambier, Infrared study of goethites of varying crystallinity and particle size – I: interpretation of OH and lattice vibration frequencies, Clay Miner. 21 (1986) 191–200. [21] D.G. Lewis, V.C. Farmer, Infrared absorption of surface hydroxyl groups and lattice vibrations in lepidocrocite and boehmite, Clay Miner. 21 (1986) 93–100.

[22] S.A. Fysh, P.M. Fredericks, Fourier transform infrared studies of aluminous goethites and hematites, Clay Clay Miner. 31 (1983) 377– 381. [23] J.E. Katon, A.J. Sommer, IR microscopy routine sampling methods extended to the microscopic domain, Anal. Chem. 64 (1992) 931–940. [24] T. Misawa, T. Kyuno, W. Suetaka, S. Shimodeira, The mechanism of atmospheric rusting and the effect of Cu and P on the rust formation of low alloy steels, Corros. Sci. 11 (1971) 35–48. [25] T. Misawa, K. Hashimoto, S. Shimodaira, The mechanism of formation of iron oxide and oxyhydroxides in aqueous solutions at room temperature, Corros. Sci. 14 (1974) 131–149. [26] T. Misawa, K. Asami, K. Hashimoto, S. Shimodaira, The mechanism of atmospheric rusting and the protective amorphous rust on low alloy steel, Corros. Sci. 14 (1974) 279–289. [27] M. Yamashita, H. Miyuki, Y. Matsuda, H. Nagano, T. Misawa, The long term growth of the protective rust layer formed on weathering steel by atmospheric corrosion during a quarter of a century, Corros. Sci. 36 (1994) 283–299. [28] G. Duykaerts, Contribution a` l’analyse quantitative par les spectres d’absorption infrarouge des poudres – I: examen the´orique de la question, Spectrochim. Acta 7 (1955) 25–31. [29] J. Bonhomme, Contribution a` l’analyse quantitative par les spectres d’absorption infrarouge des poudres – II: e´tude expe´rimentale, Spectrochim. Acta 7 (1955) 32–44. [30] L.J. Janik, M. Rampack, Div. Soils Div. Rep., CSIRO Austr., The effect of surface area on the quantitative estimation of goethite and lepidocrocite by infrared spectroscopy 1 (1975) 1–8. [31] B. Chaufer, M. Rabiller-Baudry, A. Bouguen, J.P. Labbe´, A. Que´merais, Spectroscopic characterization of zirconia coated by polymers with amine groups, Langmuir 16 (2000) 1852–1860. [32] A.K. Singh, T. Ericsson, L. Ha¨ggstro¨m, J. Gulman, Mo¨ssbauer and X-ray diffraction phase analysis of rusts from atmospheric test sites with different environments in Sweden, Corros. Sci. 25 (1985) 931– 945. [33] J.M. Blengino, M. Keddam, J.P. Labbe´, L. Robbiola, Physicochemical characterization of corrosion layers formed on iron in a sodium carbonate-bicarbonate containing environment, Corros. Sci. 37 (1995) 621–643. [34] A. Beaudouin, M.C. Clerice, J. Francßoise, J.P. Labbe´, M.A. LoeperAttia, L. Robbiola, Corrosion d’objets arche´ologiques en fer apre`s de´chloruration par la me´thode au sulfite alcalin: caracte´risation physicochimique et retraitement e´lectrochimique, Metal 95, James and James Ltd., 1997, pp. 170–177. [35] D.M. Haaland, Practical FTIR, Academic Press, 1990.