characterization of conversion-coated aluminum using fourier transform infrared and raman...

Characterization of Conversion-Coated Aluminum Using Fourier Transform Infrared and Raman Spectroscopies

A N G E L A M. A H E R N , * P A U L R. S C H W A R T Z , and L O R I A. S H A F F E R Alcoa Technical Center, 100 Technical Drive, Alcoa Center, Pennsylvania 15069-0001

Fourier transform infrared and Raman spectroscopies have been employed to define the molecular composition of chromium phosphate conversion coatings on aluminum. Attenuated total reflectance at 55 ° can be employed to probe the structure of conversion coatings present on aluminum at relatively high coating weights (>23 mg Cr/m2). Both reflection-absorption infrared and surface-enhanced Raman spectroscopic techniques can discern the presence of conversion coatings at coverages as low as 9 mg Cr/mL On the basis of the vibrational spectra from these techniques, we have determined that hydrated chromium phosphate is the major component in these conversion coatings on aluminum. Reflection-absorption infrared and surface-enhanced Raman spectroscopies also provide a means to determine the molecular structure of the nascent oxide layer on aluminum as a function of processing conditions. Specular reflection, attenuated total reflectance at 35 ° , diffuse reflectance, and Raman spectroscopic methods, in general, lack the surface sensitivity necessary to probe thin (-< 23 mg/m ~) inorganic films on aluminum.

Index Headings: FT-IR; Raman spectroscopy; Aluminum; Conversion coating; Chromium phosphate.

I N T R O D U C T I O N

Conversion coatings on aluminum provide resistance against corrosion from exposure to water as well as en- hancing the adhesion between the aluminum surface and organic coatings. 1 Although amorphous phosphate conversion coatings are widely employed, the molecular structures of such coatings have not been determined definitively. This investigation focuses on chromium phosphate conversion coatings on rolled-aluminum surfaces. Since all aluminum surfaces have a nascent aluminum oxide surface layer, the region of interest is ac- tually the conversion coating/aluminum oxide interface. A detailed molecular description of this interface is vital in determining a comprehensive mechanism of organic coating adhesion to aluminum, and would provide a means by which one could design surfaces with superior per- formance characteristics.

Chromium phosphate conversion coatings are usually characterized by elemental surface spectroscopies such as x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). 2,3 Although these spectroscopies provide the elemental composition of the coating and the thickness of the coating, the results often cannot be used to determine the molecular bonding of these elements to one another. The composition of conversion coatings has also been investigated by ion scattering spectrometry (ISS) and secondary ion mass spectrometry

Received 16 March 1992. * Au thor to whom correspondence should be sent.

(SIMS). 4 These surface techniques yield information regarding the molecular structure of the coatings through identification of constituent ions scattered from the surface. These surface techniques can adulterate the surface during analysis, and when employed for depth profiling purposes, they become destructive analytical techniques. Furthermore, the sensitivity of such techniques to surface contaminants and the requirement for ultra-high vacuum environments make these techniques less than ideal for process monitoring. Vibrational spectroscopic techniques can provide the molecular structure of the coating as well as a measure of interaction between coating and substrate. Such analyses are performed under ambient conditions and are usually nondestructive (the vapor deposition of silver overlayers for surface-enhanced Raman may be considered destructive).

We have employed both Fourier transform infrared (FT-IR) and Raman spectroscopic techniques to probe the structure of the chromium phosphate/aluminum oxide interface) A secondary goal of this endeavor is to determine which surface vibrational spectroscopy or spectroscopies are most effective for probing the structure of an inorganic pretreatment layer on inorganic substrates. Attenuated total reflectance (ATR) and reflection-absorption infrared spectroscopy (RAIRS) are two FT-IR techniques which can distinguish the presence of conversion coatings on rolled-aluminum substrates. Dif- fuse reflectance infrared spectroscopy (DRIFTS) and specular reflection techniques were found to be insen- sitive to the presence of these thin chromium phosphate coatings. Raman spectroscopy also lacks the surface sensitivity necessary to probe the structure of conversion coatings. Surface-enhanced Raman spectroscopy (SERS) is as sensitive as RAIRS to thin conversion coatings on aluminum. Both RAIRS and SERS provide molecular information regarding the structure of the oxide layer on aluminum.

Results from this investigation have been compared to previous FT-IR studies of conversion coatings on aluminum. 6,7 These studies have involved specular reflection and ATR methods which are less than ideal to employ.

E XPE R IME NT AL

All FT-IR spectra, except the RAIR spectra, were acquired with a Digilab FTS-60 spectrometer. The RAIR spectra were obtained with a Digilab FTS-15/90. Both spectrometers are equipped with liquid nitrogen-cooled MCT (mercury-cadmium-telluride) detectors. The ac- cessories employed include: a Harrick DRA-Praying Mantis diffuse reflectance accessory; a Spectra-Tech

1412 Volume 46, Number 9, 1992 0003-7028/92/4609-141252.00/0 APPLIED SPECTROSCOPY © 1992 Society for Applied Spectroscopy

Model 301 continuously-variable ATR accessory with a 45 ° KRS-5 crystal; a Perkin-Elmer specular reflection accessory; and a Digilab grazing angle accessory with a wire grid polarizer. All spectra were obtained with 4 cm -1 resolution and represent the average of 5000 scans. RAIR spectra were acquired with p-polarized (i.e., polarized parallel to the plane of incidence) incident radiation.

All Raman and SER spectra were obtained with a Spex Model 1403 double monochromator equipped with a Ha- mamatsu R928 PMT detector and a DM3000 data acquisition system, and with 300 mW (measured at the source) of 514.5-nm radiation with the use of a Coherent Innova 70 argon-ion laser. Samples were irradiated at an incident angle of 60 ° , and the 90 ° scattered light was collected. Unless otherwise stated, all spectra are single scans obtained at a scan rate of 2 cm-1/5 s. Slit settings were 200-300-300-200 #m for all scans. A 9-point poly- nomial smoothing functional was applied to all SER spectra.

Samples for SERS were coated with a layer of silver to be used as the SER substrate. Silver island films were deposited at a rate of 0.05/~/s to a thickness of 50/~ by electron beam evaporation in an oil-free, cryopumped vacuum system having a base pressure of 2 × 10 -7 Torr. The film thickness and the deposition rate were con- trolled with a quartz crystal microbalance. Various silver deposition rates (from 0.05 to 5.0/~/s) were employed to produce SER substrates. The reproducibility of SER spectra increases as the silver deposition rate decreases. Schlegel and Cotton have shown that the most effective vapor-deposited SER substrates (i.e., those that produce high-quality and reproducible SER spectra) are pro- duced when the deposition rate is slow (roughly 0.02 to 0.08 •/s). s

All aluminum sheet metal samples were specially pre- pared (i.e., rolled and coated) for this study in full-scale production mills.t All samples are 5XXX alloy with vary- ing conversion-coating weights and tempers.

Spectra of Cr20~ (Aldrich) and CrPO4"4H20 (Alfa/ Johnson Matthey) were acquired to aid in assigning bands in the vibrational spectra of conversion-coated aluminum. A DRIFT spectrum of Cr203 was obtained with the use of the instrumentation and accessory mentioned above. The sample was diluted with potassium bromide. The spectrum represents the average of 256 scans at 8 cm -1 resolution. A Raman spectrum of CrP04" 4H20 was obtained with the instrumentation and sample geometry described above. Changes in the acquisition parameters from those listed above include the laser power (500 mW measured at the source) and the slit settings (300-500- 500-300 urn). The sample was diluted with a minimal amount (< 1% by weight) of potassium bromide and pressed into a pellet.

RESULTS AND DISCUSSION

FT-IR. ATR. ATR spectra were acquired for incident angles (i.e., angle formed by the IR beam with a vector

t Details concerning the composition of the conversion coating bath and coating conditions (including experimental coating conditions) are proprietary. For general information on coating compositions and methods of applying conversion coatings to aluminum, see Ref. 1.

0.08

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~ 0.o2 . 0

0.00

4000 3500 3000 2500 2000 1600 1200 Wavenumbers

I I I I I I I I I I I

I ~ ~ /

I I f I I i I I I I L

800 400

FIG. 1. Thirty-five degree ATR spectrum of chromium phosphate conversion-coated metal. Coating weight is 23 mg Cr/m 2.

normal to the base of the crystal) of 35 ° and 55 °. These two angles provide deep and shallow penetration of the IR beam into the sample surface. 5a A typical 35 ° ATR spectrum of conversion-coated metal is shown in Fig. 1. The chromium phosphate coating weight for this sample is 23 mg Cr/m 2. A similar spectrum was also obtained for the reference metal (no conversion coating present), and a spectral subtraction of the reference metal spectrum from that in Fig. 1 approximated a straight line. Since no peaks were evident in the difference spectrum, no molecular information was obtained for the conversion-coated surface with the use of 35 ° ATR spectroscopy. This technique only distinguishes the presence of aluminum oxide.

ATR at the shallower depth of penetration (at 55 °) resulted in marginal success. A 55 ° ATR spectrum of conversion-coated metal is shown in Fig. 2. When the analogous reference metal spectrum is subtracted from that in Fig. 2, the difference spectrum in Fig. 3 is obtained. This spectrum exhibits a band at ~1100 cm -1 which has been attributed previously to the phosphate group in chromium phosphate conversion coatings on aluminum2 This method, however, is not sensitive to coating weights below about 23 mg Cr/m 2. Difference spectra obtained for 55 ° ATR spectra of conversion-coated metal with weights of 9 and 14 mg Cr/m 2 did not evidence any peaks, within the signal-to-noise ratio of the measurement, attributable to the conversion coating.

These data suggest that ATR/FT-IR spectroscopy, in general, is not a sensitive technique with which to probe the interfacial chemistry of thin (i.e., <23 mg Cr/m 2) inorganic films on aluminum substrates. One possible reason for this lack of sensitivity is the relatively poor contact that is often achieved between the sheet metal and the internal reflection element.

DRIFT and Specular Reflection. Since mill-finished aluminum surfaces are rough and are not ideal reflectors, DRIFTS would seem to be a natural choice for probing the structure of such surfaces. However, DRIFTS was no more successful than 35 ° ATR spectroscopy in pro- ducing meaningful and interpretable spectra. The DRIFT spectra of all samples investigated were nearly identical regardless of changes in coating weight or temper. No spectral features attributable to the conversion coating

APPLIED SPECTROSCOPY 1413

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FIG. 2. Fifty-five degree ATR spectrum of 23 mg Cr/m 2 conversion- coated metal.

a)

were observed in the raw data (spectra as obtained) or in the difference spectra.

The results from the specular reflection analyses were similar to those from DRIFT spectroscopy. The specular reflection spectra for all samples investigated were vir- tually identical to one another, and no IR absorptions could be assigned definitively to vibrations of the conversion coating. Since spectra were acquired within strin- gent scan parameters, such as 4.0 cm -1 resolution and the co-addition of several thousand interferograms, we conclude that both DRIFT and specular reflection techniques lack the surface sensitivity required to obtain meaningful, vibrational spectroscopic data for thin inorganic films on aluminum. Neither DRIFTS nor specular reflection is very sensitive to the nascent aluminum oxide layer and can provide no information on the oxide structure as a function of temper or other surface treat- ments.

RAIRS. The surface morphology of rolled aluminum presents a few difficulties for doing RAIRS. Figure 4 shows optical micrographs of the surface and cross-sec-

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0,040

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Fro. 3. ATR difference spectrum calculated by subtracting the 55 ° ATR spectrum of the reference metal from the analogous spectrum of conversion-coated metal.

A. 0.035 =.

o e 0.030 t . -

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, . 0

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FIG. 4. Optical photomicrographs of (a) the surface and (b) cross- section profile of conversion-coated aluminum.

tional profile of conversion-coated aluminum. The surface is dominated by grooves indicative of the rolling direction of the metal. These grooves can be spaced a few tens of microns apart and can be from ~ 2 to 20 ~m deep. Because of this topography, sample orientation with respect to the IR beam affects the quality (signal- to-noise ratio) and reproducibility of the spectra. We have found that orienting the samples with the roll grain parallel to the direction of the incident beam results in high-quality and reproducible spectra. This result is probably due to the narrower angular light scattering dispersion of incident radiation for the sample oriented with the roll grain parallel to the plane of incidence. 1°

The surface morphology of these aluminum samples results in the breakdown of the characteristic interaction of polarized radiation with smooth metallic surfaces. 11 At a high angle of incidence, the incident and reflected electric vectors of p-polarized radiation at a smooth metallic surface constructively interfere. This constructive

1414 Volume 46, Number 9, 1992

= = ,, ~

o ,p 4000 3500 3000 2500 2000 1500

Wavenumbers

I I I I I 1 0 8 6 - -~ ' 628,

a

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1000 500

FIG. 5. RAIR spectra of (a) reference (uncoated) aluminum and (b) chromium phosphate-coated (23 mg Cr/m 2) aluminum.

interference results in a sizable component of the electric vector oriented normal to the metal surface which can interact with a thin surface film on the metal." For s-polarized (i.e., polarized perpendicular to the plane of incidence) radiation, the incident and reflected electric vectors nearly cancel, which precludes absorption of the radiation by a thin film on the metal substrate. This selective interaction of polarized light with the surface is realized at smooth aluminum surfaces (mirrors) and at metal surfaces coated with relatively thick (e.g., several mg/cm 2) organic coatings. ~°,~2 However, this polar- ization phenomenon breaks down at thinly coated or uncoated aluminum surfaces, as those under study here, where the inherent roughness of rolled metal becomes a dominant factor. 1° It is possible to observe both p- and s-polarized spectra from these thinly coated surfaces. The p-polarized spectra are usually more intense and of a higher signal-to-noise ratio, and only p-polarized spectra were employed in this investigation.

The RAIR spectra of conversion-coated metal and the reference (uncoated) metal are shown in Fig. 5. These spectra are very similar with respect to peak positions and intensities. The broad band in each spectrum between ~3410 and 3260 cm -~ is assigned to O-H stretching vibrations of hydroxyl groups or water molecules. The group of bands between ~ 2960 and 2850 cm -~ is assigned to C-H stretching modes, indicating the presence of some aliphatic hydrocarbon material on the surface of the metal. A possible identification of the hydrocarbon is residual rolling lubricant. Both spectra exhibit a band at 1887 cm -1, which is assigned to an A1-H stretching mode. 13 The band at ~1645 cm -~ in both spectra in Fig. 5 is attributed to physisorbed water molecules, whereas bands at ~ 1600, 1430, and 1360 cm -~ are assigned to bending vibrations of water molecules coordinated to octahedral A1 ions (AI°~), tetrahedral A1 ions (Alt~t~), or pairs of adjacent sites (Alt~t'~-A1 t~t~ or Al°¢~-Al°~)." The strong 1068-cm -~ band in Fig. 5b is tentatively assigned to an A1-O mode or an A10-H bending mode. ~5 The band at 934 cm -~ in both spectra is assigned to an A1-O-AI asymmetric stretching mode? 6 The bands between 835 and 630 cm -~ in both spectra are assigned to modes of isolated and condensed (i.e., identical groups bonded through

TABLE I. RAIR spectral frequencies and assignments for conversion- coated metal and reference metal.

Frequencies (cm ~)

Conversion- coated Reference metal metal Assignments

3410-3260 3400-3260 2961 2957

} } 2917 2848 1887 1887 1642 ~1645

~1600 1611 1435 1425

1362 1360 1320 1321 1269 1268

1086

1068 934 934 835 834 729 731 628 630

O-H stretching mode CH~ and CH2 C-H stretching modes

A1-H stretching mode ~a Physisorbed H~O" H20 coordinated to A1 °¢~ ~4 H20 coordinated to A1 ...... A1 ~ or

Al~t~_Alo~ ~ H20 coordinated to A1 t~t'~ TM

Contribution from phosphate group of conversion coating 9

AI-O or AIO-H bending mode ~5 AI-O-A1 asymmetric stretch 16 Condensed Al04 tetrahedra ~ Isolated Al04 tetrahedra ~7 Condensed Al06 octahedra ~7

common oxygen atoms) A104 tetrahedra and AIOs oc- tahedraY These data are summarized in Table I. The only significant difference between the two spectra is in the region at approximately 1100 cm -1. A spectral subtraction dramatically emphasizes this difference, as shown in Fig. 6. The difference spectrum shown in Fig. 6 is nearly identical to a previously published spectrum 9 of a chromium phosphate conversion coating. (From the reference, it is impossible to determine how the FT-IR spectrum was acquired. It is not clear whether the substrate was aluminum for the FT-IR work.)

The prevalent depression in the spectra in Fig. 5 at 1100 cm -1 is most probably a dispersion effect, is These

anomalies in the spectra do not restrict the usefulness of the spectra, as demonstrated by the high-quality difference spectrum that can be calculated (Fig. 6). Such dispersion effects would be deleterious for quantitative applications. In this study, we are concerned with the question of what species are present, not how much of each is present.

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FIG. 6. RAIR difference spectrum calculated by subtracting the spectrum of the reference metal (Fig. 5a) from the spectrum of the conversion-coated metal (Fig. 5b).


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FIG. 7. RAIR difference spectra of chromium phosphate-coated aluminum at weights of (a) 9 mg Cr/m 2, (b) 14 mg Cr/m 2, and (c) 23 mg Cr/m 2.

RAIR spectroscopy provides a method with which to monitor compositional changes (on the molecular level) in the coating as a function of coating weight. Figure 7 shows the RAIR difference spectra for conversion coatings of weights of 9, 14, and 23 mg Cr/m 2. Comparison of the three spectra reveals that, as the conversion-coating weight increases, the band centered at 1055 cm -~ decreases or remains constant in intensity while the shoulder at 1113 cm -~ increases in intensity until it is the dominant feature in the difference spectrum (Fig. 7c). At the low weight, the spectrum exhibits a strong AI-O mode or A10-H mode at 1055 cm -~ with a weak, higher-frequency phosphate shoulder. With an increase in weight to 14 mg Cr/m 2, the spectrum now exhibits a well-resolved phosphate mode at 1112 cm -1 with a lower- frequency AI-O shoulder. When the weight reaches 23 mg Cr/m 2, the spectrum exhibits a single phosphate absorption with no significant spectral features attributable to the native oxide layer.

Comparison of the RAIR spectra of a conversion-coated sheet of different tempers provides a means to discern how (or whether) temper influences the molecular composition of the aluminum surface. The temper of a metal is a measure of hardness and strength2 s The two tempers we have investigated are as-rolled and annealed. The spectra in Fig. 8 are for conversion-coated metals which differ only in temper. Some of the differences include a decrease in the relative intensity (relative to the intensity of the 1078-cm -1 peak) of the O-H and C-H stretching modes (3500-2800 cm -~ region) in the spectrum of the annealed metal (Fig. 8a), compared to the as-rolled metal (Fig. 8b). This result may indicate the presence of less bound water and residual lubricant on the surface of annealed metal. The spectrum of the annealed metal lacks the AI-H stretching mode present at 1882 cm -1 in the spectrum of the as-rolled metal. The spectrum in Fig.

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FIG. 8. RAIR spectrum of conversion-coated aluminum (a) annealed and (b) as-rolled. Conversion coating present at 11 mg Cr/m 2.

8a, annealed metal, also evidences a decrease in the intensity of the 1650-1550 cm -1 region relative to the intensity of the 1079-cm -1 band as compared to the intensity of the same modes in the spectrum in Fig. 8b, as- rolled metal. This intensity difference, along with that observed in the O-H stretching region, suggests that annealed metal has fewer bound and physisorbed water molecules than does as-rolled metal. The spectrum in Fig. 8a exhibits different band intensities in the 1700- 1250 cm -1 region and lacks a well-resolved 1260-cm -1 band when compared to the spectrum in Fig. 8b. The spectrum of annealed metal also lacks a band at ~ 830 cm -1 that is present for the as-rolled metal. The absence of this mode would suggest that annealed metal has a significantly lower concentration of (or lacks) condensed A104 tetrahedra as compared with that of as-rolled metal.

These data clearly demonstrate that RAIRS is a pow- erful surface spectroscopy with which to probe the ms- lecular structure of conversion coatings on aluminum. This technique can be employed to monitor such coatings down to weights of about 9 mg Cr/m 2. Furthermore, RAIRS can be utilized to define the influence of an- nealing on the molecular composition of surfaces.

Raman and SERS. Raman spectra of the samples investigated here do not exhibit any bands, and do not provide any molecular information regarding how, or whether, the aluminum surface is modified. Raman spectroscopy lacks the sensitivity to probe the structure of such thin layers on aluminum.

The SER spectra, by comparison, are vibrationally rich. Figure 9 shows the SER spectra for the reference metal and conversion-coated metal. The reference metal SER spectrum evidences a band at 236 cm -~ due to a Ag- aluminum oxide mode. Bands at 626, 668, and 772 cm -~ are attributed to condensed AIOo octahedra, isolated A104 tetrahedra, and condensed A104 tetrahedra, respective- ly. 17 The bands at 928 and 1056 cm -~ are assigned to an A1-O-A1 stretching mode and an AI-O or A10-H bending mode. ~,~s The two dominant bands at 1358 and 1592 cm -~


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]° ~¢~ 500 1000 1500 ,om-,, / / (x0.42)

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a

I I I 500 1000 1500 2000

Raman shift (cm "1)

FIG. 9. SER spectra of (a) 9 mg Cr/m 2 conversion-coated aluminum and (b) reference aluminum sample. The inset is a Raman spectrum of CrP04" 4H20.

TABLE I I . S E R spectra l frequencies and a s s i g n m e n t s for conversion- coated metal and reference metal.

Frequencies (cm ')

Conversion- coated Reference metal metal Assignments

524

602

794

960

236

626 668 772

804 928 954

1056

Ag-aluminum oxide mode Cr-O symmetric stretching mode

[Cr(III) compounds] 2~ Cr-O-Cr symmetric stretching mode

[Cr(VI) compounds] 2~ Cr-O asymmetric stretching mode

[Cr(III) compounds] 21 Condensed A106 octahedra ~7 Isolated A104 tetrahedra 17 Condensed A104 tetrahedra ~ Cr-O-Cr asymmetric stretching mode

[Cr(VI) compounds] 21.23

AI-O-A1 asymmetric stretching mode 16

Phosphate stretching mode A1-O and A10-H bending mode 15

1268 1382 1358 Graphitic carbon ~° 1584 1582 Graphitic carbon 2°

are attributed to graphitic carbon. 2° The source of the graphitic carbon is most probably laser-induced degra- dation of residual rolling lubricant on the surface.

A conversion-coated surface yields a dramatically different SER spectrum than that of the uncoated aluminum. The broad bands at 524 and 602 cm -1 in Fig. 9a are attributed to Cr-O and Cr-O-Cr stretching modes of chromium (III) compounds. 21,22 The weak broad band at 794 cm -1 is assigned to the Cr-O-Cr asymmetric stretching mode usually indicative of chromium (VI) compounds along with some intensity due to the underlying aluminum oxide layer. 21 The broad band at 960 cm -1 is assigned to the phosphate group; however, some intensity in this region could derive from the aluminum oxide. The weak modes at 1382 and 1584 cm -~ are due to graphitic carbon. The SERS assignments for the reference and conversion-coated metal are summarized in Table II.

Hydrated chromium (III) phosphate was used as a model compound in an effort to assign the modes in the SER spectra of the conversion-coated aluminum. The Raman spectrum of CrPO4" 4H20 (inset to Fig. 9) is dominated by a set of strong bands between 800 and 900 cm-L Weaker features appear at 350, 388, and 552 cm -1, which are attributed to Cr-O bending and stretching modes (identical to those modes of Cr203), and a weak feature appears at 1012 cm -~ due to the phosphate group. 2~ The set of bands between 800 and 900 cm -~ are similar to modes observed in Raman spectra of chromium (VI) oxides and should not appear in the spectra of chromium (III) compounds. We suggest that these bands appear in the Raman spectrum of CrPO4.4H20 due to some interaction (e.g., a photothermal reaction) with the incident radiation resulting in a change of oxidation state of the chromium. Further investigation would be needed to confirm this hypothesis. The SER spectrum of conversion coatings evidences a broad, weak band at 794 cm -~,

which is due to a chromium (VI) oxide mode; however, there is no evidence that chromium (VI) oxides are present prior to exposure to laser radiation. The SER spectra of chromium phosphate conversion coatings exhibit more chromium (III) oxide character than chromium (VI) oxide character. Chromium (VI) oxide bands have been previously observed in SER spectra of chromium (III) compounds. 2s The reasoning for this apparent disparity is that the chromium (III) oxide surfaces investigated were of a higher oxidation state than bulk Cr203 with charge compensation by the adsorption of 02- or 022- .

The decrease in the relative intensity of the graphitic carbon modes from the spectrum of the uncoated metal to that of the conversion-coated metal (Fig. 9) may suggest the presence of less organic material on the conversion-coated surface. At least two scenarios are consistent with these data. Since both samples were subjected to the same processing conditions, except that the reference sample was not conversion coated, we can infer that the coating process also provides an additional cleaning step, or that the conversion coating protects any residual organic material from deleterious interactions with the laser radiation.

The spectrum in Fig. 9a is for a conversion coating at a weight of 9 mg Cr/m 2. We have obtained SER spectra for coating weights of 14 and 23 mg Cr/m 2 (not shown). As the conversion coating increases, the SER spectra do not change significantly (with respect to number of peaks, relative intensities, or peak positions) compared with that in Fig. 9a. The only noticeable change is in the relative intensities of the graphitic carbon bands. As the coating weight increases, the graphitic carbon band intensities decrease. Figure 10 shows the SER spectra in the C-H stretching region for 0, 9, 14, and 23 mg Cr/m 2 conversion-coated metal. As conversion coating weight increases, the signal associated with aliphatic C-H stretching modes disappears into the background. These data are consistent with the two scenarios presented above


I F

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Raman shift (cm "1)

FIG. 10. SER spectra of (a) 0 mg Cr/m 2, (b) 9 mg Cr/m 2, (c) 14 mg Cr/m 2, and (d) 23 mg Cr/m 2 conversion-coated aluminum.

for the changes in the SER signal from graphitic carbon, but they cannot be used to distinguish between the two possibilities. The presence of residual hydrocarbon species on the surface corroborates the RAIRS results (see above).

Previous Vibrational Spectroscopic Investigations and the Composition of Conversion Coatings. Previous IR investigations of phosphate coatings often have focused attention on steel as the substrate. 24-26 Coating weights were > 500 mg/m 2 and were iron or zinc phosphate conversion coatings. Chromium phosphate conversion coatings on aluminum have been studied previously. 6,7 Coat- ing weights as low as 130 mg/m 2 have been probed with the use of ATR spectroscopy. 6 These studies have yielded two different compositions for chromium phosphate coatings on aluminum. Suetaka has found that the major component of the coating is CrP04 with hydrated ALP04 as a secondary component. 7 Nimon and Korpi determined that in fresh (wet) coatings hydrated CrP04 is the major component. After the coating is dried, hydrated CrP04 and hydrated ALP04 are major components, present in equal amounts. Both fresh and dried coatings have hydrated Cr203 as a minor component. 6

The IR spectrum of hydrated Cr203 that Nimon and Korpi used to assign bands in the spectra of conversion coatings on aluminum differ from those previously published 27 and those acquired in our laboratory. The differences in Nimon and Korpi's reference spectrum may have resulted in incorrect assignments of certain conversion-coating modes to hydrated Cr203. G Nimon and Korpi's spectrum of Cr203 evidences medium to strong bands between ~ 1550 and 1450 cm -1 that are absent in our spectrum shown in Fig. 11. Our spectrum evidences strong modes between 700 and 500 cm -1 that appear as weak modes in Nimon and Korpi's spectrum. Water of hydration should result in bands at ~3450 and ~1640

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4000 3500 3000 2500 2000 1600 1200 Wavenumbers

FIG. 11. DRIFT spectrum of Cr203.

I I I I I [ I I I I

800

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400

cm -1. We cannot comment on possible assignments for the band at ~ 1550-1450 cm -1 in the previously published spectrum. Nimon and Korpi also assign a 625-cm -1 band to a vibration of the phosphate group. Although both CrP04 and ALP04 have absorptions in this region, the spectrum of Cr20~ also evidences a strong band at ~625 cm -1. We contend that it would be difficult to rule out that some intensity in this region derives from Cr203.

The RAIRS difference spectrum in Fig. 6 compares favorably with a reference spectrum of CrP04- 4H20. Hy- drated ALP04 produces an IR spectrum with an additional medium-intensity band at 833 cm -1. Since this band is absent in our spectrum, we conclude that these conversion coatings lack any appreciable A1PO4. The SER spectrum in Fig. 9a also confirms that the major constituent of the conversion coating is CrP04. The spectrum evidences many bands attributed to Cr-O modes along with the phosphate stretching mode. The SER spectra we have obtained do not compare favorably with a Raman spectrum of hydrated A1PO4 and, consequently, do not substantiate the presence of ALP04 in the conversion coatings. On the basis of the RAIRS and SERS data, we conclude that these thin amorphous phosphate conversion coatings are predominantly hydrated CrPO4.

CONCLUSIONS

This investigation clearly demonstrates the strength of RAIRS as a surface FT-IR spectroscopy with which to probe the molecular composition of conversion coatings on aluminum sheet. We have determined that RAIRS is sensitive to conversion-coating coverages as low as 9 mg Cr/m 2. RAIRS can also be employed to characterize the surface chemistry of aluminum as a function of heat treatment (temper). Other surface FT-IR techniques, such as 35 ° ATR, DRIFT, and specular reflection, lack the sensitivity to be employed to investigate such thin inorganic coatings on aluminum substrates. ATR at 55 ° can be used to study relatively high coverages of conversion coatings (23 mg Cr/m 2) on aluminum. Raman spectroscopy is not sensitive enough to provide molecular information regarding the conversion coating or the native oxide layer. SER spectroscopy can be employed to


p r o b e t h e m o l e c u l a r s t r u c t u r e of a l u m i n u m oxide layers a n d c o n v e r s i o n coa t ings on a l u m i n u m subs t r a t e s . S E R S can d i s ce rn t h e p r e s e n c e o f c o n v e r s i o n coa t ings d o w n to 9 m g C r / m 2. B o t h R A I R a n d S E R s p e c t r a e v i d e n c e b a n d s a t t r i b u t a b l e on ly to h y d r a t e d CrPO4.

ACKNOWLEDGMENTS

The authors would like to thank R. Hoffman and N. Panseri for their expert assistance with the deposition of the silver overlayers. The authors would also like to acknowledge the assistance of W. Tanner in acquiring some of the Raman and SER spectra. Thanks also go to J. Guthrie, A. Dennis, J. Greenwald, J. Weir, and D. Festa for providing valuable feedback on this manuscript.

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characterization of conversion-coated aluminum using fourier transform infrared and raman...

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