Characterization of a chromate-inhibited primer by Doppler broadening energy spectroscopy.

A chromate-inhibited primer has been characterized by Doppler broadening energy spectroscopy (DBES), scanning electron microscopy (SEM), and Raman spectroscopy. The S-parameter obtained by DBES exhibited a three-layer depth profile, which is attributed to the presence of a "skin" layer in the polymer matrix of the primer and a concentration gradient in the inorganic phases distributed through the primer. The primer was also examined by DBES following immersion in solution. The change observed in the S-parameter upon immersion indicated water filling the molecular free volumes in the primer. The ability to monitor the depth of water uptake as a function of immersion time illustrates that the primer does not need to be fully saturated for chromate release to occur.

Keywords: Surface analysis, corrosion, corrosion protection, application characteristics, physical properties, aerospace, aircraft, aluminum

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For many decades, chromate-based paint systems have been used to provide corrosion protection to a range of metals. However, due to their Cr(VI) content, these coatings pose a substantial hazard to the environment and to human health. (1) There is currently a significant research effort directed at finding effective, chromate-free alternatives. A detailed understanding of the mechanisms by which chromate-based paints inhibit corrosion is essential in order to achieve this objective.

In general, a chromate-based paint system consists of a conversion or anodized coating, a chromate-inhibited primer layer, and a polymeric topcoat. (2-4) The paint system therefore acts as a physical barrier to moisture and corrosive species, and also provides a reservoir of inhibitor in the form of chromate that is available to heal defects in the paint system. (3,5-7) Chromate inhibits corrosion by leaching from the paint system, diffusing through surface moisture, and reacting with actively corroding sites, thereby resulting in passivation. (5,8) While chromate has been shown to leach from conversion coatings, (9,10) the majority of chromate that leaches from the paint system comes from the primer. (11)

The leaching of chromate from the primer for subsequent surface passivation involves a number of processes, including uptake of water into the primer, dissolution of the chromate inhibitor, and diffusion of the dissolved inhibitor out of the primer and into the defect site. While there have been many studies on the properties of chromate conversion coatings, only a small number of studies on the physicochemical characterization of primers have been reported. Using electrochemical impedance spectroscopy (EIS), Fedrizzi et al. (8) observed a change in coating capacitance upon uptake of water by the coating. Similarly, Carbonini et al. (12) found that coating capacitance curves exhibited several distinct regions, which they attributed to water uptake, inhibitor dissolution, and blocking of pores within the primer by re-precipitation of the inhibitor.

Investigations into the uptake of water by primers have also been carried out using the positron-based techniques of positron annihilation lifetime spectroscopy (PALS) and Doppler broadening energy spectroscopy (DBES). These techniques are particularly relevant to the uptake of water by primers, since they probe the free volume or void space within a material. Madani et al. (13) used PALS to characterize both dry and water-saturated epoxy systems incorporating a selection of nonchromate corrosion inhibitors. The same authors have studied voids at the interface between a SrCr[O.sub.4] inhibitor phase and an epoxy matrix using PALS. (14) While Cao et al. (15) examined primer/topcoat systems using DBES, the mean implantation depth of the positrons was insufficient to provide any information about the primer layer.

We have recently reported on the characterization of a chromate-inhibited primer by electron microprobe analysis, X-ray microdiffraction, Raman spectroscopy, and PALS. (16) Here, we extend these studies with a report on the characterization of a chromate-inhibited primer by DBES. This technique was used to provide information on the free volume or void space as a function of distance into the primer layer. Measurements were made both before and after immersion in solution, which provided insights into the structure of the primer layer and changes to the structure upon uptake of water. The results of DBES were also correlated with those from analysis of the primer by scanning electron microscopy (SEM) and Raman spectroscopy.

[FIGURE 1 OMITTED]

EXPERIMENTAL

Sample Preparation

The metal substrate used was aluminum alloy 2024-T3, containing 4.31 wt% Cu, 1.5 wt% Mg, 0.6 wt% Mn, 0.08 wt% Si, and 0.17 wt% Fe, as determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Panels of the alloy measuring 12.7 x 7.6 cm were wiped with acetone and processed through alkaline cleaning, deoxidizing, and conversion coating steps according to the conditions described elsewhere. (16) The panels were then primed with PRC Desoto PR143 chromate-inhibited primer (BAE Systems, Salisbury, Australia), which is qualified under Mil-P-23377 specification. This primer is composed of SrCr[O.sub.4], BaS[O.sub.4], and Ti[O.sub.2] (anatase) particles embedded in an epoxy polyamide matrix. (16) Coating thicknesses were determined using an eddy current technique (Fischer Instrumentation Ltd. Isoscope MP3). Calibration was done using 11.3 [micro]m and 48.4 [micro]m calibration foils on a piece of rolled AA2024-T3 from the same batch of material as used for the samples. The thickness of the primer coatings was determined to be 24.2 [+ or -] 3.3 [micro]m.

[FIGURE 2 OMITTED]

Doppler Broadening Energy Spectroscopy

For DBES experiments, a mono-energetic slow positron beam was used. The DBES spectra were recorded at room temperature as a function of positron energy from ~0 to 30 keV. DBES spectra were measured using a conventional fast-fast spectrometer in collinear geometry with a Ge detector, with an energy resolution of 1.24 keV for the measurement of the Doppler broadening of the 511 keV positron annihilation line. The energy windows used to define the S-parameter, which represents the fraction of annihilations taking place in the low momentum region, were |[E.sub.[gamma]]-511 keV| < 0.8 keV ([p.sub.L]/[m.sub.0]c [less than or equal to] 3 x [10.sup.-3], where [m.sub.0] is the electron mass). The S-parameter provided a measure of the concentration of defects or voids in the coating, which could be measured as a function of distance from the outer surface. Samples were characterized by DBES following 0, 6, and 12 hr of immersion in 5% w/v NaCl aqueous solutions at pH 7.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) was performed using a Leica Thermal Field Emission SEM (360FE SEM) on polished sections of the primer. The preparation of sectioned specimens is described elsewhere. (16) Specimens were imaged at an accelerating voltage of 15 kV, a probe current of 1.5 nA, and a working distance of 22 mm at normal angle. A pure Cu reference specimen (99.9999%) was used for calibration in quantitative energy dispersive X-ray analysis (EDS).

Raman Spectroscopy

Polished sections of the primer were analyzed by Raman spectroscopy using a Renishaw inVia confocal microscope system. Specimens were illuminated through a 50x objective with 514 nm excitation from an [Ar.sup.+] laser at an incident power of ~1 mW and a spot size of approximately 1 [micro]m. Spectra were collected over the range 570 to 1210 [cm.sup.-1] at 2 [micro]m intervals across the sample. Each spectrum was averaged over 10 scans, corresponding to a total collection time per spectrum of 10 sec. Depth profiles were generated from the background-subtracted intensities of the chosen spectral peaks. The spectral resolution of all spectra was 1 [cm.sup.-1].

[FIGURE 3 OMITTED]

RESULTS AND DISCUSSION

A secondary electron image of a section of the chromate-inhibited primer is shown in Figure 1. It can be seen that the inorganic phases are present as particles or platelets of various sizes distributed through the polymer matrix. EDS analysis of the primer layer indicated the presence of Sr, Cr, Ba, S, Ti, O, and C, which is consistent with the presence of SrCr[O.sub.4], BaS[O.sub.4], and Ti[O.sub.2] (anatase), as well as the organic binding medium.

The distributions of the inorganic phases were examined by Raman spectroscopy. A Raman spectrum averaged over a 100 [micro]m x 25 [micro]m region in the primer is shown in Figure 2. Peaks corresponding to SrCr[O.sub.4], BaS[O.sub.4], Ti[O.sub.2] (anatase), and the polymer matrix can be identified in the spectrum. (16) Individual spectra collected over the mapped area were used to construct depth profiles of the inorganic phases in the primer layer, as shown in Figure 3. Peaks at 866, 987, and 638 [cm.sup.-1] were used to derive the depth profiles of SrCr[O.sub.4], BaS[O.sub.4], and Ti[O.sub.2] (anatase), respectively. Figure 3 shows that these three inorganic phases were distributed in a similar manner as a function of depth through the primer layer. In particular, the phases were dispersed relatively evenly through the primer, except for a region near the surface of the primer in which the concentration of these phases appeared to be lower.

The S-parameter obtained from DBES measurements on the chromate-inhibited primer is presented in Figure 4. It can be seen that the S-parameter initially increased, reaching a maximum value of 0.5 at a mean implantation depth of 0.2 [micro]m into the primer. The S-parameter then decreased, reaching a steady value of 0.47 by a mean implantation depth of ~5 [micro]m. This behavior has been observed previously for high-solid polyurethane topcoats (15,17) and can be attributed to the presence of a "skin" layer, an intermediate layer, and the bulk. This follows from the fact that the S-parameter is proportional to the concentration of voids in the material. Hence, the initial increase observed in the S-parameter indicates an increase in the number of voids just below the surface of the primer, which is typical of polymers with a low-defect "skin" layer at their surface. (18,19) The intermediate region, in which the S-parameter decreases with increasing depth into the primer, is most likely due to an increase in the concentration of inorganic phases, since these materials typically contain a small number of voids resulting in a smaller S-parameter. (20,21) Finally, the S-parameter reaches a steady value in the bulk region, where the concentration of inorganic phases has reached a constant level. The depth profiles of the inorganic phases obtained by Raman spectroscopy (Figure 3) show lower concentrations in the first ~5 [micro]m near the surface, and are therefore consistent with the S-parameter results.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Changes in the S-parameter ([DELTA]S = [S.sub.immersed] - [S.sub.unimmersed]) after immersion of the primer for periods of 6 and 12 hr are shown in Figure 5. A decrease in the S-parameter was observed upon immersion, with a larger decrease observed for the primer immersed for 12 hr. Since the S-parameter relates to the concentration of voids in the primer, the observed change ([DELTA]S < 0) is consistent with water filling some of the voids in the primer. Clearly, the presence of the "skin" layer does not prevent water from penetrating into the primer, although it probably influences the rate of water uptake. It can be seen that the magnitude of change in the S-parameter was largest near the surface of the primer, indicating that even after 12 hr of immersion the solution had not yet penetrated the bulk of the primer. Detailed studies on the release of chromate from the same primer under immersion conditions revealed that chromate was present in the solution after times much less than 12 hr. (16) This suggests that the primer does not need to be fully saturated for chromate release to occur. Thus, as water penetrates the primer, a greater reservoir of chromate becomes available for release (and subsequent passivation). These results are important where the design of primer systems with a "skin" layer and a controlled gradient of inhibitor are sought in order to tailor release kinetics.

CONCLUSIONS

The distribution of free volume or voids within a chromate-inhibited primer was characterized via the S-parameter obtained from DBES measurements. The S-parameter exhibited a three-layer depth profile, indicative of a low-defect "skin" layer in the polymer matrix and a concentration gradient in the inorganic solids embedded in the matrix. This result was in agreement with the concentration gradients obtained for SrCr[O.sub.4], BaS[O.sub.4], and Ti[O.sub.2] (anatase) by Raman spectroscopy. The "skin" layer and concentration gradients may play a role in the water absorption profile and subsequent kinetics of chromate release. Changes observed in the S-parameter upon immersion indicated greater uptake of water by the primer and deeper penetration into the primer with time. Clearly, as water penetrates further into the primer more chromate becomes available for release and subsequent repair.

ACKNOWLEDGMENTS

The authors would like to thank Mr. Andrew Stonham of BAE Systems, Salisbury, Australia, for organizing the painting of panels. BAE Systems is acknowledged for providing support for part of this work through the Corrosion Prediction Modelling Project.

References

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F.H. Scholes, ([double dagger]) S.A. Furman, A.E. Hughes, and A.J. Hill -- CSIRO Manufacturing & Infrastructure Technology*

F. Tuomisto and K. Saarinen -- Helsinki University of Technology ([dagger])

S.J. Pas -- Westfalische Wilhelms-Universitat**

* Private Bag 33, Clayton South MDC, 3169, Australia.

([dagger]) Laboratory of Physics, P.O. Box 1100, FIN-02015 HUT, Finland.

** Institut fur Physikalische Chemie, Corrensstrasse 30, D-48149 Munster, Germany.

([double dagger]) Author to whom correspondence should be addressed. Email: [e-mail].