J. MATER. CHEM., 1992,2(1), 49-55 Investigation of the Local Structure around Iron dispersed in Vinyl Chloride-Vinylidene Chloride (VC-VdC) Copolymer Coatings on Mild Steel using Glancing-angle X-ray Absorption Spectroscopy Stefania Pizzini,” Kevin J. Roberts,*”+ Ian S. Dring,b Peter J. Moreland,b Richard J. Oldmanb and James Robinson“ “Department of Chemistry, Strathclyde University, 295 Cathedral Street, Glasgow G7 7 XL, UK blCl Chemicals and Polymers, The Heath, Runcorn, Cheshire WA74QD, UK “Department of Physics, University of Warwick, Coventry CV4 7AL, UK The chemistry occurring in vinyl chloride-vinylidene chloride polymer coatings on mild steel has been investigated by probing the local environment around Fe ions using glancing-angle X-ray absorption spectroscopy.Measurements made as a function of penetration depth reveal that close to the air interface the Fe species are present as octahedrally co-ordinated Fe”’ and have a local structure similar to that observed in disordered y-FeOOH. Although this structure is found to be independent of the pH of the coating formulation, Fe ion transport from the metal substrate is apparently enhanced at acidic pH formulations. Deeper into the coating the Fe has a structure typical of a mixed tetrahedral-octahedral environment such as Fe,O,. Keywords: Copolymer coating ; Glancing-angle X-ray absorption spectroscopy; /on transport The first commercial water-borne paints made as alternatives to conventional solvent-borne paints were based generally upon acrylic polymers or copolymers.Their high permeability to water and oxygen resulted in poor anti-corrosive perform- ance and made them unsuitable as competitive primers. How- ever, in the 1980s a new water-borne vinyl chloride-vinylidene chloride (VC-VdC) copolymer (Haloflex 202$)was developed’ which gave an excellent anti-corrosive performance consistent with the outstanding barrier properties of the chlorine-con- taining polymer.’ Through the use of ethylene oxide-propyl- ene-ethylene oxide block copolymers and other formulation additives to aid colloid stability and latex particle coalescence, these properties were transposed to a paint with an exceptional anti-corrosive performance. However, the chemical nature of these chlorine-containing vinyl (acrylic) copolymers (Fig.I) leads to a dehydrochlorination process at alkaline pH. The latex is therefore formulated into a paint at pH 4-6,compared with the traditional range of pH 8-9, whereby the dehydrochlorination process is significantly suppressed. The acidically formulated paints exhibit some interesting anti-corrosive properties as detailed el~ewhere.~ For example, the extent of flash rusting during the ‘wet’ film condition decreases with decreasing pH from 7 to 4.Associated with this reduction in pH is the formation of a darkening effect at the metal/polymer interface in the dry film. Moreland and Padget related the enhanced protective performance of the paint to the presence of this thin 1-2 pm interfacial layer arising from an insignificant steel substrate corrosion process during ‘wet’ painting.Transmission electron microscopy (TEM) studies4 indicate that this layer is composed of platelets of ca. 0.05-0.2 pm in size dispersed between ca. 0.2 pm latex particles. Beyond this interfacial layer the same studies reveal that often the latex particles are ‘decorated’ (Fig. 2). X-Ray diffraction (XRD) measurements showed this interfacial layer to include a phase which is isostructural with the mineral pyroaurite [Mg,Fe2(0H),6C03.4H20] and in which divalent substrate cations substitute for Mg. The properties of anion exchange and buffering capacity expected from this type of t Also at SERC Daresbury Laboratory, Warrington WA4 4AD, UK $ ‘Haloflex’ is a trademark belonging to the ICI group of companies.HCI H H H CCI H4\0 OR ...I # A B C Fig. 1 Schematic formula of the acrylate-modified vinyl chloride- vinylidene chloride copolymer (A =vinyl chloride; B =vinylidene chloride; C =acrylate ester) compound are consistent with a good anti-corrosive perform- ance and hence Moreland and Padget3 considered its presence was possibly playing a key role. Information on the local co-ordination and the oxidation state of Fe species dispersed in such polymer coatings can be obtained from X-ray Absorption Spectroscopy (XAS) measurements. XAS (see Fig. 3) of condensed samples exhibit an oscillatory structure which can extend up to lOOOeV or central atom I I ---I I I 0 backscattering atoms 0 100 200 v1-total Fig.3 EXAFS spectroscopy: (a) the absorption process generates photoelectron waves (A) which are backscattered (B) by the nearest neighbours and which give rise to interference which varies with the energy of the incident photons; (b) this interference is observed as a periodic modulation on the high-energy side of the absorption thresh- old; (c) the intensity of the emitted photoelectric wave essentially represents a Fourier series of backscattered contributions from the various near-neighbour shells. Owing to the short-range nature of the process this is damped rapidly by the crystal lattice which results in EXAFS oscillations, typically, containing significant contributions from only the first three co-ordination shells of the structure. Fourier transformation of the fine structure yields a radial distribution func- tion from the absorbing atom and identification of the characteristic phase shift of the backscattered wave can be used to identify the atom type more, beyond the absorption-edge threshold.In metals and semiconductors the features appearing close to the absorption edge have been attributed to transitions to localised electronic states. The strong oscillations just beyond the absorption edge (XANES) can be explained in terms of the multiple scatterings of photoelectrons by the atoms in a local cluster around the absorbing atom, and can give information on the geometrical arrangement of the near neighbours of the absorbing atom, The structure observed from 30-40eV beyond the edge, the extended X-ray absorption fine structure (EXAFS), is due to the interference between the outgoing photoelectron wave and the wave backscattered from neighbour atoms (e.g.ref. 5 and 6). Interference effects are determined by the distance, the chemical type and the number of atoms around the absorbing atom. Since only elastically scattered electrons can interfere, and the elastic mean free path of electrons is short, the analysis of the EXAFS spectra provides information on the local atomic structure of the absorbing atom. Previous investigations7 have shown the potential of XAS as a probe of the structural and chemical properties of the Fe species transported through polymer coatings.In studies of polymer coating on vacuum evaporated Fe thin films, Oldman7 found that for a polymer formulation prepared at pH 2, the Fe species incorporated in the polymer were found to be present as Fe" ions with a local structure similar to hydrated FeCl,. For the pH 4.5 formulation, an Fe"' com-pound was found and the nearest-neighbour structure indi- cated the presence of a structure typical of an oxide. EXAFS investigations on some related systems have been reported in the literature. Barrett and co-w~rkers~~~ demonstrated the J. MATER. CHEM., 1992, VOL. 2 enhanced surface sensitivity afforded by the glancing angle XAS technique by investigating the early stages of the thermal corrosive oxidation of stainless steel in an atmospheric environment.The uncorroded surface showed an oxide layer in which Fe is present as Fe304. After oxidation for 4 min at 1000°C the material was shown to develop an iron-rich protective layer in which iron is present predominantly as Fe,O,. Kerkar et al." investigated in situ Fe thin films passivated in aqueous solutions and found that their structure was typical of disordered y-FeOOH. In this paper we describe the results of XAS measurements carried out on Haloflex polymer coatings of different thick- nesses and pH formulations, coated onto mild-steel substrates. The aim of this investigation is to clarify the structural properties of iron species which become dispersed in the polymer after coating onto 'real' surfaces such as mild steel plates.XAS measurements above the Fe K-absorption-edge, carried out for several incident angles corresponding to increasing penetration of the X-ray beam in the sample, allow the local structure of Fe species to be obtained as a function of depth. An important stage of this work has been the acquisition and analysis of XAS data recorded from bulk Fe oxides and oxyhydroxides. A comparison of the XAS spectra with model crystal structures has helped the interpretation of the data and the identification of bulk-like Fe species dispersed in the polymer coatings. Materials and Methods Model Compounds The model compounds for this investigation were an Fe foil 5pm thick and FeO, a-Fe203, Fe304, a-FeOOH and y-FeOOH powders. FeO crystallises with an NaC1-like cubic structure.Each Fe" ion is surrounded by six oxygens in octahedral co-ordination." The structure of a-Fe203 (haematite) can be regarded as a slightly distorted hexagonal close packing (h.c.p.) of 0 ions with the metallic cations in some of the octahedral interstices.12 At low and medium temperatures Fe,O, (mag-netite) has an inverse cubic spinel structure (Fe;' Fe;' Fe;' Oi-). Oxygens are in an almost perfect cubic close packing (c.c.P.) with the metal ions lying in tetrahedral and octahedral interstices. Fe"' ions at A sites are tetrahedrally co-ordinated to oxygen and Fe" and Fe"' ions at B sites are octahedrally co-ordinated to oxygen.' a-FeOOH is ortho- rhombic with 0 and OH in an almost perfect h.c.p. and each Fe atom is surrounded by an octahedron of 0 atoms.14 y-FeOOH is also orthorhombic and its structure is built up of well defined layers parallel to the (100) face.Each layer is made up of octahedra surrounding Fe atoms and linked together by sharing corners. The octahedra are nearly regular and have four corners occupied by 0 atoms and two by OH groups.l5 The powders for the EXAFS measurements were finely ground, dispersed in polypropylene and pressed into 1.3 cm diameter discs. To provide a suitable model for iron films oxidised in an aqueous environment we compared our data with that pro- duced by Kerkar et a1.I' in which a thin Fe film was electrochemically passivated in an aqueous solution contain- ing 0.1 mol dm-3 of sodium perchlorate at a potential of +0.8 V us.SCE. VC-VdC Polymer Coatings The substrates for the polymer coatings were mild-steel blocks (nominal composition C, 0.08-0.13%; Mn, 0.3-0.6%; P, J. MATER. CHEM., 1992, VOL. 2 0.04%; S, 0.05%; balance Fe) of 45 mm x 10mm x 1 mm in size, polished using diamond paste down to 0.25 pm diameter. The steel surfaces were coated with Haloflex 202 latex films (containing no additives or pigments) of different thickness and pH. The data presented here refer to the conditions, sample (1) pH 1.5, thickness 50 pm; sample (2)pH 1.5, thick-ness 2 pm; sample (3) pH 7.0, thickness 2pm. The pH 1.5 represented the as-received latex and maximum corrosion activity, while pH 7 represented a much milder corrosion condition. A thickness of 50 pm is typical for paint coating, but a coating of 2 pm allowed more opportunity for measure- ments at the metal/polymer interface. The pH of the nominally acidic formulations was adjusted using ammonium hydroxide solution.The coatings were allowed to dry for 7 days in a dust-free environment before the measurements were carried out. XAS Measurements XAS measurements of the model compounds and polymer coatings were carried out above the Fe K-absorption edge on stations 8.1 and 9.2, respectively, of the Synchrotron Radiation Source (SRS) at Daresbury Laboratory. Double- crystal order-sorting Si (220) and (1 11) monochromators, respectively, were used to obtain spectra in angular scanning mode.The XAS spectra of the model compounds were recorded in transmission geometry [Fig. 4(a)] with the exception of the electrochemically passivated film which was recorded in situ by Kerkar et a!." using fluorescence detection (see ref. 16). To achieve surface sensitivity, the polymer coatings were examined using glancing angle XAS (see e.g. ref. 17, 18) with depth-dependent information provided by varying the glanc- ing angle. Owing to the low concentration of Fe expected in the polymer films, the spectra for these samples were recorded using fluorescence detection as this mode provides optimum signal-to-noise for data acquisition. The X-ray beam was collimated by fine slits and, in order to have a precise alignment with respect to the X-ray beam direction, the samples were mounted on a precision goniometer." The experimental set-up is shown schematically in Fig.qb). XANES spectra of the three Haloflex polymer-coated samples were recorded at incident grazing angles 4 which varied between 0.1 and 10" with the fluorescence signal monitored by a wide-angle ionisation chamber positioned above the sample surface. EXAFS spectra of polymer coating (2)were recorded for monochromator \ collimator slit Fig. 4 The experimental facilities for the collection of X-ray absorption spectra on the SRS:(a) transmission geometry used for the analysis of bulk model compounds where the X-ray intensity transmitted through the sample (IT)is measured with respect to the incident beam intensity (Io);(b) fluorescence geometry where the X-ray fluor- escence yield of a dilute analyte (I,) is measured with respect to I, at low 9 # =0.1 O, using a multi-element Ge solid-state detector.The energy resolution of the detector was such that Fe fluorescence could be accurately discriminated from the background signal. Since Fe species are highly dispersed at the surface of the polymer films, acquisition times of ca. 5 h had to be used to obtain EXAFS data of acceptable quality. Data Analysis The analysis of the experimental EXAFS spectra were carried out using standard procedures. The spectra were background subtracted using a smooth cubic-spline function to fit the atom-like background above the absorption edge. The near- edge background-subtracted spectra were normalised to the absorption step height, which was obtained by extrapolating the background absorption before and after the edge.The EXAFS function ~(k)is defined as: where k is the photoelectron wavevector, p(k)is the absorption coefficient and po is the background absorption i.e. the absorption of an isolated atom. In the so-called plane-wave approximation the EXAFS function can be expressed by: where the summation is made over i co-ordination shells at an average distance ri from the absorbing (central) atom and Niis the number of atoms at distance ri.Fi(k)is the magnitude of the backscattering amplitude for each type of atom. A is the electron mean free path, ci is root-mean-square deviation around the average distance ri and it appears in a Debye- Waller-like factor describing structural and thermal disorder.$i is a phase-shift term introduced by the fact that the wavefunction of the ejected photoelectron is modified by the potentials of the absorbing and backscattering atoms. The central Fe atom and backscattering atoms (Fe, 0) phase shifts were calculated ab initio and were not iterated in the least-squares fitting process. Fits to the experimental EXAFS data were obtained using EXCURVE90 (see e.g. ref. 20) which is a least-squares fitting program based on the curved-wave theory. For each neighbouring atom shell, the parameters allowed to vary were the distances from the central atom (ri),the co-ordination numbers (NJ,the disorder param- eter (20i2)and an inner potential-energy shift (E,) which is used to define the photoelectron wavevector k according to the relation k =[(2rn/h2)(E-Eo)]'/2 (3) where E is the kinetic energy of the photoelectron, rn is its mass and h is Planck's constant.In the fitting of the model compounds, the neighbour-shell distances were allowed to vary with respect to the crystallo- graphic values while the co-ordination numbers were fixed to the crystallographic ones. The accuracy in the determination of the structural param- eters is 1-3% for the shell distances and 10-30% for the co- ordination numbers. Inaccuracy in the determination of the co-ordination numbers results largely from the strong corre- lation with the disorder term.Results and Discussion EXAFS and XANES Studies of Model Compounds Fig. 5 shows the experimental and least-squares-fitted EXAFS spectra for a-Fe,03, FeO, Fe30,, a-FeOOH and y-FeOOH. 52 J. MATER. CHEM., 1992, VOL. 2 10 Table 1 The co-ordination numbers, N, neighbour-shell distances and 5 Debye- Waller factors' are compared with the crystallographic radial 5 distribution functions 2s 0 As 0 siteb atom Ncryst rcrysl/A N r/A 2a21A2 x x -5-5 a-Fe,O,-1 0 0 3 I .960 3 1.94 0.010-1 0 0 3 2.088 3 2.10 0.012I I -1 5 46810 4 6810 Fe 1 2.885 1 2.87 0.0 12 klA-' Fe 3 2.996 3 2.96 0.0 12klA -' Fe 3 3.364 3 3.36 0.015 4 6 0 3 3.382 3 3.34 0.015 4 0 3 3.601 3 3.64 0.015 2 Fe 6 3.700 6 3.64 0.0202 0 3 3.807 3 3.74 0.020 v2s 0 $0 Fe 1 3.985 1 3.99 0.020 -2 x -2 Fe304-4 T 0 4 1.886 1.3 1.91 0.0204 -6 0 0 6 2.058 4.0 2.07 0.034 4 0 Fe 6 2.968 4.0 2.99 0.03 1 -4 6 8 10 4 6 8 10 12 0 Fe 6 3.480 8.0 3.47 0.030 klA-' klA-' T Fe 12 3.480-I --T 0 12 3.493 10 T Fe 4 3,635 1.3 1.91 0.020 0 0 2 3.563 5 0 0 6 3.659 T 0 12 4.5272 so 0 0 12 4.675 x T 0 12 4.748 -5 T 0 12 5.397 0 Fe 12 5.140 8 5.14 0.024 -1 0 T Fe 12 5.452 7 5.44 0.0264 6 8 10 1-2 0 Fe 8 5.452 k/A-' T Fe 12 5.935Fig.5 Experimental (-) and least-squares-fitted (---) EXAFS 0 Fe 12 5.935spectra for (a) a-Fe,O,; (b)FeO; (c) Fe,O,; (d) a-FeOOH and (e) y-FeOOH FeO 0 6 2.166 6 2.15 0.035 The results of the least-squares fits are summarised in Table 1 Fe 12 3.063 12 3.06 0.022 0 8 3.752 8 3.73 0.026where they are compared with the crystallographic radial Fe 6 4.332 6 4.39 0.0 16 distribution functions, 0 24 4.843 The Fe K-near-edge spectra recorded for Fe metal and for Fe 24 5.306 24 5.24 0.030 the model oxides and oxyhydroxides are shown in Fig.6. For Fe 12 6.126 12 6.23 0.015 each compound, the energy separation of the measured pre- a-FeOOH edge feature (A) and the edge-feature (B, corresponding to the 0 2 1.953 3 1.94 0.007 steepest point up the absorption edge) are summarised in 0 1 1.954 Table 2. The amplitudes of feature A (C for Fe foil), normalised 0 2 2.089 3 2.10 0.007 to edge step height are also reported. 0 1 2.093 Fe 2 3.010 2 2.99 0.010The results summarised in Table2, show that the energy Fe 2 3.28 1 2 3.22 0.010 separation of the edge features A and B and the height of the Fe 4 3.459 4 3.39 0.010 pre-edge feature A in the near-edge spectra of Fe oxides and 0 2 3.589 4 3.55 0.012 oxyhydroxides can be used as fingerprints of the oxidation 0 2 3.666 0 1 3.753 2 3.74 0.015state of the Fe cation and of the local geometry of the Fe 0 1 3.767species, For Fe203, ol-Fe00H and y-FeOOH, where Fe is y-FeOOHpresent as Fe", the energy separation between A and B is ca.0 1 I .905 1.91 0.00310eV. In FeO, where iron is present as Fe", and in Fe304 0 2 2.023 2 1 1.99 0.003where both Fe"' and Fe" are present, the energy separation is 0 1 2.027 1 2.03 0.003 ca. 7 eV. The pre-edge feature A arises from the ls+3d dipole 0 2 2.133 2 2.13 0.003 forbidden transition," which in Fe compounds becomes par- Fe 4 3.070 4 3.04 0.015 tially allowed due to the hybridisation with p orbitals of oxygen Fe 2 3.080 2 3.08 0.015 atoms.The amplitude of the pre-edge feature is dependent on 0 2 3.622 the geometry of the co-ordination to oxygen atoms. It is 0.005-0 2 2 3.686 3.6870.008 for FeO, Fe,O, and both forms of FeOOH, where Fe 0 Fe 2 3.870 2 3.97 0.015 is in an octahedral environment. In Fe304, where the mixing of p and d states is enhanced by the tetrahedral co-ordination, a Calculated from the least-squares fits to the Fe K-edge spectra. In the amplitude of feature A is greater (0.016). This is in agreement the least-squares fits, the co-ordination numbers were fixed to the with the observation of Dring et dZ2 crystallographic values.T =tetrahedral site, 0=octahedral site. In Fig. 7(b) the first derivative functions of the near-edge Depth-dependent XANES Studies of Polymer Coatings on spectra are compared with the derivative near-edge spectra Mild Steel recorded for Fe304, y-FeOOH and for the thin Fe film Fig. 7(a) shows the near-edge spectra recorded for polymer passivated in aqueous perchlorate solution. lo coating (1) on mild steel, for incident angles 0.1, 1, 5 and 10 '. The near-edge spectra for polymer coating (2) and (3) J. MATER. CHEM., 1992, VOL. 2 f t I -20 0 20 40 60 80 100 energy/eV Fig.6 Fe K-near-edge spectra measured for the model compounds (a)a-FeOOH; (b) y-FeOOH, a-Fe203; (d)FeO; (e) Fe304 and 0Fe Table 2 Energy separation of features A and B and height of feature A in the near-edge spectra of the model compounds (Fig.5) compound ion E, -E,/eV A height/k Cb Fe Fe 0.46 FeO Fen 7.5 0.008 Fe304 Fez03 FeII/Fem Feu1 7.2 10.4 0.016 0.008 a-Fe00H Feu1 10.0 0.007 y-FeOOH Feu1 10.4 0.005 a Normalized to the edge-stop height. Position of the inflection point at the absorption edge. formulations were recorded for angles increasing from 0.1 to 5 ", In Fig. 8 the heights of the pre-edge feature A in these near-edge spectra are plotted as a function of incident angle. These are compared with the height of the pre-edge feature modelled by summing in several proportions the spectra of Fe metal and y-FeOOH. A qualitative analysis of the near-edge spectra recorded for the polymer coatings deposited on steel may therefore give some preliminary information on the valency and the co- ordination geometry of Fe species.The near-edge spectra recorded for polymer coating (1) as a function of incident angle (Fig. 7) indicate that the local environment of Fe species dispersed in the polymer film changes with depth, i.e. Fe species close to the air interface have a different local co- ordination from that exhibited by the Fe species close to the steel substrate. The spectra recorded for angles 0.1, 1 and 2 O show some resemblance to those of or-FeOOH and y-FeOOH and this indicates that, closer to the surface, Fe might be octahedrally co-ordinated to oxygen atoms.For an incident angle of 0.1 ",the energy separation between the edge-features A and B is 10.4 eV. A comparison with the results in Table 2 suggests that iron may be present in the surface coatings as Fe"'. Significant changes in the near-edge spectra are observed for incident angles greater than 2", i.e. where X-rays probe Fe species closer to the steel substrate. These changes are seen better in the first-derivative spectra [Fig. 7(b)].Note in particular the variation in the near-edge structure at ca. 8 eV beyond the absorption edge. In this energy region, the deriva- tive spectrum for an incident angle of 10" shows features typical of Fe,O, and this indicates that, close to the steel substrate, Fe species might be present in a mixed tetrahedral- octahedral environment.The presence of tetrahedral sites close to the steel substrate is also supported by the enhance- ment of the amplitude of the pre-edge feature A in the spectra recorded for increasing incident angles. In summary, the near- edge spectra suggest that close to the air interface, Fe species -20 0 20 40 60 80 100 energ y/eV (b 1 1 . EKm . . 1I.,, -10 0 10 20 30 40 energylevFig. 7 (a) Fe K-near-edge spectra for polymer coating (1) on mild steel measured for incident angles (i) 0.1; (ii) 1; (iii) 2; (iv) 5 and (v)lo";(b) the first derivatives of the spectra in (a)(iii)-(vii) are compared with the derivative spectra of y-FeOOH (i); Fe304 (viii); and with a spectrum for the Fe film passivated in aqueous perchlorate solution (ii) I 1 0.64 Jt I I I I l o 0.2 0.1 0.6 o a 1.0 4/degrees Fig.8 Plotted height of the pre-edge feature A, shown in Fig. 6 and 7, in the Fe K-near-edge spectra recorded for the polymer ccating (2) (m) and (3) (@) at various values of 4. These are compared with the height of A obtained adding-in different proportions the spectra of metallic Fe and y-FeOOH dispersed-polymer coating (1) (i.e. 50 pm thick, formulated at a pH of 1.5) are FeIn cations with an octahedral environment typical of FeOOH. Closer to the steel interface Fe species are present as a mixture of octahedral and tetrahedral sites. The near-edge spectra recorded for small incident angles (<0.7') for polymer coating (2) (the spectra are presented elsewhere'*) are very close to those measured for similar angles for the 50 pm thick film.This indicates that the Fe species close to the polymer/air interfaces are present as Fe"' cations in an octahedral environment and that this does not depend on coating thickness. When the X-ray penetration depth is increased, i.e. the measurements made at greater incident angles (4= 1 and 1.5 "), the near-edge spectra change dramatically and resemble closely the spectrum of metallic Fe. The height of the pre-edge feature A increases as the penetration depth is increased and for 4 = 1O this reaches the value found for metallic iron. This indicates that for this angle, the X-ray beam penetrates the steel substrate. Fig. 8 shows that for 4=0.7-1" the height of the edge feature (A or C) crosses the value expected for metallic Fe, indicating that the X-ray beam starts penetrating the substrate.The X-ray path from the surface to the steel substrate and back is ca. 180 pm for d, =0.95" and 120 pm for d, =0.7". From these data, and using tabulated values of the mass absorption coefficient of Fe23 it can be calculated that the average Fe loading in this coating is between 1.7 and 2.5 wt.%. Fig. 8 shows that for the thin polymer coating formulated at a pH of 7, sample (3), the height of the pre-edge feature reaches the value expected for Fe metal for a lower angle than for the film with pH 1.5. This indicates that the substrate is reached by X-rays for a smaller incident angle, and therefore that the Fe concentration in this coating is lower than in the film with acidic formulation.EXAFS Studies of the Environment around Iron close to the Metal/Polymer Interface More detailed information on the local structure of Fe species dispersed in polymer coatings close to the air interface was obtained from the EXAFS data recorded for coating (1) in comparison with those obtained for the Fe film passivated in sodium perchlorate aqueous solution. lo The first derivative near-edge spectrum [Fig. 7(b)] shows that the passivated Fe film is very similar to the spectra recorded for polymer coatings (1) and (2) for the smallest incident angles. Kerkar et a1." showed that the structure of the passivated iron film can be regarded as a disordered y-FeOOH.The similarities in the near-edge spectra suggest that Fe species dispersed in the polymer surface coatings, close to the air interface, might have a similar structure. The results of the EXAFS data, shown in Fig. 9, are in agreement with the conclusions from the near-edge data. The results of the least-squares fits to the EXAFS data (Table 3) for the surface coatings prepared at pH 1.5 show that Fe is octahedrally co-ordinated to oxygen. The structural param- eters for the nearest-neighbour oxygen shells are, however, not sufficient to identify the structure in which Fe is involved. These distances are characteristic of the octahedral co-ordi- nation in both crystalline a-FeOOH, y-FeOOH and Fe203 (Table 1).The results of the analysis for the Fe co-ordination shells at greater distances, the presence of a nearest-neighbour shell at 3.89 A and perhaps more significantly the absence of Fe-Fe correlations in the range 3.3-3.5 8, indicate, however, that Fe species might be involved in a compound similar to y-FeOOH. This, however, does not imply that the layer formed at the metal/polymer interface has the same passiv- ation properties as that observed by Kerkar et a/." on electrochemically treated steel, only that the local structures of the two layers appear to be rather similar. It is noteworthy that the co-ordination numbers calculated for the distant shells are largely reduced with respect to those typical of bulk iron oxyhydroxides. This is clearly shown by the Fourier transforms of the spectra of sample 2 and of y-FeOOH (Fig. 10).The reduction in amplitude of the near- J. MATER. CHEM., 1992, VOL. 2 6 3 %o5 x -3 -6 4 6 8 10 kla - I 4 6 8 10 k1A-l Fig. 9 Experimental (-) and least-squares-fitted fluorescence EXAFS spectra: (a) polymer coating (2); (b) Fe film passivated in perchlorate solution Table 3 Co-ordination numbers N, neighbour-shell distances r and disorder terms 20' calculated from the least-squares fits of the fluorescence EXAFS data of polymer coating (2) for q5 =0.1O and for the Fe film passivated in aqueous solution Fe dispersed in polymer coating 0 1.o 1.86 0.004 0 3.0 2.00 0.003 0 1.o 2.08 0.004 Fe I .5 2.96 0.020 Fe 1.1 3.89 0.020 0 I.2 1.85 electrochemically passivated Fe film 0.006 0 2.0 1.94 0.003 0 2.4 2.06 0.004 Fe 1.4 3.00 0.017 Fe 1.o 3.80 0.013 1.2 E 0.9 i;c K2 0.6 0.0 0 2 4 6 ria Fig. 10 Fourier transform of the Fe K-edge EXAFS spectrum of polymer coating (2) (-) in comparison with y-FeOOH (---) J.MATER. CHEM., 1992, VOL. 2 neighbour shells beyond the first could be due to either structural disorder or the amorphous nature (small particle size) of the iron compound dispersed in the polymer. The results of this study, coupled with those of Moreland and Padget3 show that the chemistry at the polymer/metal interface is far from straightforward. Moreland and Padget,3 based on TEM investigations, concluded that a compound isostructural to pyroaurite, observed close to the steel sub- strate, would be responsible for the corrosion inhibition properties of the Haloflex 202 polymer coatings.In pyroaurite both Fe and Mg cations occupy octahedral sites. By compari- son, the results obtained here show that, whereas close to the air interface Fe species are present in an octahedral environ- ment, close to the substrate interface Fe is present in a mixed tetrahedral-octahedral environment typical of Fe304. The variation in the structure and composition of the Fe species in (1) from a disordered y-FeOOH, close to the air interface, to an Fe304-like structure, deeper in the film, can be compared with the reaction pathways of Fe in aqueous solution postulated by Misawa et a1.24These authors pro- posed several reaction pathways which depend on pH and generally involve Fe"-Fe"' mixed-valence intermediates. In acidic solutions the following pathway is proposed: Fe2++(FeI@, Fe:I)(" -2X)+ green complex +Fe 0, (OH) -2x amorphous ferric oxyhdroxide +(aging)+ (a-Fe00H) The results of the XAS measurements on the polymer coating (1) are consistent with this reaction pathway.The presence of Fe" was observed by Oldman7 and a mixed-valence Fe"-Fe"' compound and an amorphous ferric oxyhydroxide, possibly y-FeOOH, were observed in this work. The mixed-valence species is found close to the polymer/steel interface where the oxygen supply is scarce whilst the final corrosion product (before subsequent ageing leading to the formation of crystal- line a-FeOOH), an amorphous Fe" oxyhydroxide, possibly of y-form, is found close to the air/polymer interface where the oxygen supply is greater.However, the chemical pathways proposed by Misawa et for the transition from Fe" to Fe"' also allow for the precipitation of a pyroaurite-type material. Since both amorphous and crystalline materials are present it is highly likely that more than one Fe species is formed, which can engage in anion exchange and buffering. Further ageing should lead to crystalline a-FeOOH but this stage is not reached in the case of Fe dispersed in the dry polymer coatings. Our studies did not show any significant role played by chloride, which is a natural component of the Haloflex latex, in the formation of the interfacial film although earlier work7 in a different kind of experiment which involved total dissol- ution of Fe, showed formation of hydrated FeC12.Conclusions The results of the investigation of VC-VdC polymer coatings on mild steel shows that glancing-angle X-ray absorption measurements made as a function of incident angle allow depth-specific structural information on the local structure of Fe within polymer coatings to be obtained. Close to the air interface Fe species are present as Fe"' and have a local structure typical of disordered y-FeOOH. The local environ- ment of Fe is very similar to that of Fe in thin films passivated in aqueous solutions.Deeper in the polymer, Fe is present in a compound characterised by the mixed tetrahedral-octa- hedral co-ordination typical of Fe304. The identity of the oxyhydroxide compound close to the air interface is not found to depend on the pH of the polymer, as similar spectra were recorded for polymer coating formulations prepared at pH 1.5 and pH 7. The concentration of Fe species transported in the pH 7 formulation is found to be considerably smaller than that found in the acidic formulations. The Authors thank SERC and ICI Chemicals and Polymers for the financial support of this work, SERC Daresbury Laboratory for provision facilities and beam time on the SRS and M. Kerkar for his assistance in recording the in situ XAS spectra of the passivated iron film.References 1 A. J. Burgess, D. Caldwell and J. C. Padget, J. Oil Colour Chem. Assoc., 1981, 64, 175. 2 J. C. Padget and P. J. Moreland, J. Coating Technol., 1982, 55, 39. 3 P. J. Moreland and J. C. Padget, ACS Syrnp. Ser. 322, Polymeric Materials for Corrosion Control, ed. R. A. Dickie, F. L. Floyd, American Chemical Society, Washington, DC, 1986. 4 P. J. Moreland, I. M. Fraser and G. T. Finlan, personal communi- cation, 199 1. 5 E. A. Stern and S. M. Heald, in Handbook on Synchrotron Radiation, ed. E. E. Koch, North-Holland, Amsterdam, 1983. 6 D. E. Koningsberger and R. Prins, in X-Ray Absorption: Prin- ciples, Applications, Techniques of EXAFS, SEXAFS and XANES, Wiley, New York, 1988. 7 R. J. Oldman, J.Phys. Paris Colloque, 1986, CS 47, 321. 8 N. T. Barrett, P. N. Gibson, G. N. Greaves, K. J. Roberts and M. Sacchi, Physica B 1989, 158, 690. 9 N. T. Barrett, P. N. Gibson, G. N. Greaves, P. Mackle, K. J. Roberts and M. Sacchi, J. Phys. D, 1989, 22, 542. 10 M. Kerkar, J. Robinson and A. J. Forty, Faraday Discuss. Chern. SOC.1990, 31. 11 E. R. Jette and F. Foote, J. Chem. Phys, 1933, 1, 29. 12 R. L. Blake, R. E. Hessevick, T. Zoltai and L. W. Finger, Am. Mineral., 1966, 51, 123. 13 M. E. Fleet, Acta Crystallogr. Sect. B 1981, 37, 917. 14 A. Szytula, A. Burewicz, Z. Dimitrijewic, S. Krasnicki, H. Rzany, J. Todorovic, A. Wanic and W. Wolski, Phys. Status Solidi, 1968, 26, 429. 15 H. Christensen and A. N. Christensen, Acta Chem. Scand. Ser. B, 1978, 32, 87. 16 J. B. Hastings, in EXAFS Spectroscopy, Techniques and Appli- cations, ed. B. K. Teo and D. C. Joy, Plenum Press, New York, 1980, p. 171. 17 S. P. Pizzini, K. J. Roberts, G. N. Greaves, N. T. Barrett, I. D. Dring and R. J. Oldman, in Synchrotron Light: Applications and Related instrumentation 11, World Scientific, Singapore, New Jersey, London, Hong Kong, 1990, p. 67. 18 S. Pizzini, PhD Thesis, University of Strathclyde, 1990. 19 S. Pizzini, K. J. Roberts, G. N. Greaves, N. Harris, P. Moore, E. Pantos and R. J. Oldman, Rev. Sci. Instrum., 1989, 60,2525. 20 S. J. Gurman, J. Phys. C, 1988, 21, 3699. 21 L. A. Grunes, Phys. Rev. B, 1983,27, 21 11. 22 I. S. Dring, D. H. Hall, R. J. Oldman, J. L. Casci, W. N. E. Meredith and R. P. Tooze, Physica B, 1989, 167. 23 International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. IV. 24 T. Misawa, K. Hashimoto and S. Shimodaira, Corros. Sci., 1974, 14, 131. Paper 1/03376J; Received 4th July, 1991