presence of a significant ratio of unreacted hexavalent chromium in the chromate conversion coatings which decays with the time of exposure in air-equilibrated ...
Corrosion Science, Vol. 34, No. l, pp. 41-49, 1993 Printed in Great Britain.
0010-938X/93 $5.00 + 0.00 © 1992 Pergamon Press Ltd
THE MECHANISM OF CORROSION INHIBITION BY C H R O M A T E CONVERSION COATINGS FROM X-RAY ABSORPTION NEAR E D G E SPECTROSCOPY (XANES) M. W. KENDIG,*A. J. DAVENPORT'~and H. S. ISAACS~ *Rockwell International Science Center, 1049 Camino dos Rios, Thousand Oaks, CA 91360, U.S.A. t Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973, U.S.A.
Abstract--X-Ray absorption spectra of chromate conversion coatings on AI 2024-T3 and 3003-H14 have been obtained as a function of processing time and environmental exposure. The spectra reveal the presence of a significant ratio of unreacted hexavalent chromium in the chromate conversion coatings which decays with the time of exposure in air-equilibrated 0.5 M NaCI. These results suggest that the unique corrosion-protective property of chromated A1 lies in the presence of a source of the hexavalent chromium species as an active inhibiting species. INTRODUCTION CORROSION protection of light metal alloys such as those of AI often include the application of a chromate conversion coating produced by the reaction of the alloy with an acidic solution containing dichromate and fluoride ions. i-3 The oxidation of the AI in the presence of the complexing F - produces electrons to reduce the hexavalent Cr of the dichromate ion (Cr2072-) and form a protective hydrated, 3-valent Cr(OH)3 film: 6 e - + 8H + + Cr2 O22A1 - 6 e -
> 2Cr(OH)3 + H 2 0
(1)
~ 2A13+ (in the presence of F - ) . 2
(2)
Concerns over the environmental hazards presented by hexavalent chromium (Cr(VI)) demand the replacement of the chromate conversion coatings by more environmentally benign materials. 4-5 Currently there are no entirely suitable replacements for chromate conversion coatings. C h r o m a t e d surfaces provide a superior degree of corrosion protection. In order to replace the current process that uses Cr(VI), it is important to understand first the mechanism by which chromate conversion coatings protect light alloys. Katzman et al. 1 have hypothesized that the chromate conversion coatings retain a certain quality of Cr(VI) due to the incompleteness of reaction (1). The residual Cr(VI) in the film serves as a source for dynamic repair of breaks and defects in the film by reaction (1). The objective of this study is to examine this hypothesis by determining to what extent Cr(VI) remains in chromate conversion-coated AI and to what extent it effuses or otherwise reacts upon exposure to a corrosive environment. Non-destructive surface analytical methods sensitive to the valency of Cr include X-ray photo-electron microscopy (XPS) 2 and glancing angle X-ray absorption near Manuscript received 15 April 1992; in amended form 20 May 1992. 41
42
M.W. KENDIG,A. J. DAVENPORTand H. S. ISAACS
edge spectroscopy (XANES).6-9 Of these methods, XPS presents some drawbacks to a precise evaluation of the ratio of Cr(VI) to the total Cr speciation since a photodecomposition reaction of the Cr(VI) occurs in the U H V environment, as most recently characterized by Schrott and coworkers,10 as well as Halada and Clayton. la Previously, Asami et al. hypothesized a photodecomposition by the XPS source in the U H V from an observed color change in a sample exposed to XPS analysis. 2 Despite its drawbacks, XPS has been used to estimate the quantity of Cr(VI) in conversion coatings at 9% ,2 while wet chemical analyses have placed the relative concentration of Cr(VI) as high as 60% 3 in certain conversion coatings produced by an accelerated process. X A N E S has proven to be an effective technique for the study of inhibiting species in oxide films, and can be performed under ambient pressures so that the film is unaffected by exposure to a vacuum. In addition, films are less susceptible to photoreduction by the higher energy photons. Using X A N E S , the valence state of species present at very low concentrations (equivalent to a few monolayers) can be determined in air, in an electrochemical cell or under a polymer coating. It has already been used to study the role of chromate 6 and cerium ions 7-9 as corrosion inhibitors. In addition X A N E S has been used to study the valence state of chromium in films on aluminum formed by anodizing in chromate solution 12-t3 as well as in anodized films on aluminum grown in chromium-free solutions then subsequently sealed in a dichromate solution. 14 Open circuit exposure of aluminum with an air-formed film to chromate solutions yields 3-valent chromium in the film. As the oxide film on aluminum thickens by anodizing, the proportion of 6-valent chromium in the film increases. 6 Anodizing aluminum in chromate solutions yields a mixture of 3- and 6-valent chromium, the latter being found in the outer part of the film. 7"12-13 Sealing an anodized aluminum film in dichromate yields entirely 6-valent chromium. 14 It has been proposed that the reduction of chromium from the 6-valent state to the 3-valent state takes place at flaws in the film, which are more numerous for thinner films so the proportion of 3-valent chromium is expected to be higher for thinner films. 6 The availability of bright, broadband X-ray radiation available at synchrotrons makes X A N E S a readily performed analysis of the speciation of Cr in chromate Crbearing films on aluminum. It has been used in this work to evaluate the relative quantity of Cr(VI) in chromate conversion coatings. EXPERIMENTAL METHOD Chromate conversion coated (Alodine 1200S) AI 3003-H14 panels were obtained from Q-Panel (Cleveland, Ohio). These materials were chosen since they provide a highly reproducible source of chromated conversion-coatedmaterial. The panels were exposedto aerated 0.5 M NaC1for times up to 29 days. To examine the mechanismof filmformation in determining the chemistryof chromate conversion coatings, Y' x 3" couponsof A12024-T3were exposed for varyingtimes (1, 3, 5, 10, 20 and 30 rain) to a standard production bath (Alodine 1200S) at Rockwell's Space Systems Division.15 These times represented a range that is both shorter and longer than the manufacturingspecification of 5 rain. The XANES were obtained at Beamline X-11A at the National SynchrotronLight Source (Brookhaven National Laboratory)usinga harmonicallyrejecting Si(111) two-crystalmonchromator. Ionization chamber currents record the incident, 10, fluorescent, If, and transmitted, It, intensities. The absorption coefficient, Z, for a fluorescencespectrum is defined as I~/lo while that for a transmission spectrum equals log 11o/1f.16 Spectra were obtained at the Cr K edge (5989 eV). The spectra for the coated metals were obtained using the fluorescence mode with an incident beam at a glancing angle of 50 mrads. This relatively high angle allowed complete penetration of the film. The spectra, hence, represented
Corrosion inhibition by chromate conversion coatings | .4
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.... - I 0
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..... 0
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Cr 3.
-
, ..... 20
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40
50
(eV)
Fl6. 1. Normalized transmission spectra for Cr metal, Cr203 and K2CrO4.
integration of the entire film. Standard spectra were obtained for Cr203 and K2Cr207, These latter spectra were obtained in transmission. For transmission measurements of standard compounds, a reference sample and an additional ionization chamber were used for an internal energy calibration. Spectra are often normalized with respect to the absorption edge. The intensities are divided by the intensity of the absorption edge to define the edge-normalized spectra. The edge intensity is determined by a linear extrapolation of the intensity above 100 eV back to Eo. EXPERIMENTAL RESULTS AND DISCUSSION X A N E S is characterized by two regions. Below a characteristic energy, the absorption edge, relatively little absorption intensity above the baseline occurs. As the energy increases above the absorption edge, incident X-rays are strongly absorbed. The absorption edge corresponds to expulsion of a core electron from a bound state to the continuum. This process contrasts with pre-edge transitions which do not produce a transition from relatively low absorption to high absorption, but rather produce a sharp m a x i m u m or peak in absorption. These processes are due to electronic transitions from one bound state to another bound but higher energy electronic state. Figure 1 shows the near edge spectra measured in transmission for the 0, 3+ and 6+ oxidation states of Cr present in the metal and the compounds chromium (III) oxide and potassium chromate, respectively. The first m a x i m u m in the derivative of the edge for the metallic sample defines the E 0. All energies cited here are shifts relative to E0. The hexavalent chromium has a distinct pre-edge peak at about 4 eV above the edge for Cr metal, whereas the Cr(III) oxide shows no preedge peak, but rather an absorption edge at 10 eV above the 5989 eV edge of the metal. The pre-edge peak is due to a transition which is symmetry-allowed for the non-centrosymmetric tetrahedral coordination found for 6-valent chromium compounds, but is not allowed for octahedral 3-valent chromium. Linear combination of spectra for the Cr(III) and Cr(V1) species allows a quantitative evaluation of the apparent ratio of Cr(VI) to total Cr in an oxide film. Figure 2 shows normalized spectra calculated from weighted sums of the Cr(III) and Cr(VI) spectra of Fig. l. Note that the pre-edge peak heights for these edgenormalized spectra increase with increasing amounts of Cr(VI) as expected. The preedge peak height relative to the edge intensity plotted as a function of the percentage of the Cr(VI) spectrum defines a calibration curve as shown in Fig. 3. The calibration curve of Fig. 3 is subsequently used to evaluate the Cr(VI) fraction from the pre-edge
44
M.W. KENDIG, A. J. DAVENPORTand H. S. ISAACS
peak height of edge-normalized spectra. The open symbols were calculated by a linear combination of the spectra of pure K2CrO4 and Cr20 3. The solid symbols are data taken from mixed powders of K2CrO 4 and Cr203 with known compositions. Figure 4 shows an actual spectrum for a Q-panel ® surface superimposed on a spectrum from a weighted sum of Cr203 and K2CrO4. Note that there is a relatively good agreement between the respective spectra for the region around 5 eV corresponding to the Cr(VI) absorption, as well as above the edge for energies greater than 40 eV. There are, however, some significant differences between the coating spectrum and the standards at the 15 eV edge. The edge for the film occurs at a slightly higher energy than for Cr203. The spectrum for the film has less structure between 20 and 40 eV as compared to the weighted sums of the standards. It can be concluded that the Cr(III) compound of the film is not identical to the crystalline Cr203 standard. The lack of structure resembles more closely the spectrum of Cr(OH)3--a structure containing hydrogen bonded CrO 6 octahedra with little long range order. 17 Alternatively it could be suggested that the spectrum for the Cr(III) species could be that for CrO 6 octahedra modified by an A106 environment. This would seem unlikely in light of XPS work which showed that the surface of the chromate film contains little A1. 2 Although detailed characterization of the chemistry of the Cr(III) species is beyond the scope of the present report, it is clear that it is not the crystalline chromium oxide, Cr203. Based on the preceding discussion, the intensity of the pre-edge peak provides a measure of the quantity of Cr(VI) in the film, whereas the edge height depends on the total quantity of Cr in the film. Before attempting to quantify the Cr speciation in chromate conversion coatings, the reproducibility of these quantities for a set of welldefined samples must be established. The issue of both the reproducibility of the spectrum and the variation between samples was addressed by evaluating a number of different chromated A13003-H14 Q-panel ®samples. The Cr near edge spectra for these samples have been superimposed in Fig. 5. As can be seen, little variation exists between the spectra both in terms of the pre-edge peak height and the edge height. This demonstrates good sample-to-sample consistency. Figure 6 shows selected XANES for AI Alloy 2024-T3 samples processed for different times in a chromate coating bath. As the processing time increases, the edge intensity (above 100 eV) of the spectrum increases and then reaches a limit at 5 min (see Fig. 7). Figure 6 also shows the presence of the pre-edge peak at 4 eV above E0,
"~
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Fx6.2. Normalized spectra obtained from weighted sums of spectra of Cr(IlI) and Cr(VI) standard compounds. The curves are for different percentages of the Cr(VI) spectrum added to the Cr(III) spectrum as indicated in the legend.
Corrosion inhibition by chromate conversion coatings
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% Cr(Vl) FIG. 3. Normalized pre-edge intensity (× 100) as a function of the percentage of Cr(VI). The open circles are calculated from linear combinations of spectra for Cr(III) and Cr(VI) compounds. The solid points represent observed data for mixed standards having the indicated percentage of Cr(VI).
indicating the presence of about 10-20% Cr(VI) based on an evaluation using the calibration curve of Fig. 3. This value for the AI 2024-T3 samples is significantly greater than the percentage Cr(VI) observed in the 3003-H14 material. Although the alloy composition may play some role in determining this difference, the data presented here remain inconclusive, since the 3003 was produced on a spray coilcoating line whereas the 2024 samples were dip-processed. Hence, the differences in processing as well as alloy composition could play a role in the Cr(VI) fraction, and the dominant effect has not been determined. The calculated ratio, R, of Cr(VI) to total Cr and the total Cr in terms of edge height as a function of processing time for the A12024-T3 in Alodine 1200S appear in I
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~ '~
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Edge-normalized spectra for a blank A13003-H14 Q-panel®and a weighted sum of the spectra for Cr(III) oxide and potassium dichromate.
46
M . W . KENDIG, A. J. DAVENPORTand H. S. ISAACS
0.3o ~.)
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u
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Four superimposed X A N E spectra for A1 3003-H14 Q-Panel ® blanks illustrating reproducibility.
Fig. 7. The sample processed for the shortest time (the i min curve in Fig. 6) shows a significantly lower concentration of Cr(VI) relative to the total Cr in the film (Fig. 7). This suggests that more Cr(VI) is converted to Cr(III) at the coating/substrate interface in the earliest stages of film formation. However, complete conversion of Cr(VI) to hydrated Cr(III) oxide (reactions 1 and 2) never occurs, particularly after the initial stage of film deposition. For processing times >5 min, the film thickness as determined by the total Cr (edge) and the residual quantity of Cr(VI) (as measured from the pre-edge intensity) remain more or less constant, as summarized in Fig. 7. It should be noted that the currently used manufacturing process specifies a 5 min immersion. To evaluate any changes in the film, and conversion or effusion of the Cr(VI) upon exposure to a corrosive environment, different samples of the chromated AI 3003-H14 Q-Panels ® (Fig. 5) were exposed to air-equilibrated 0.5 M NaCI for varying times between 0 and 29 days. Two or more spectra were obtained for the resulting surfaces from which the edge height and % Cr(VI) were evaluated. The results in terms of edge height and R as a function of exposure time appear in Fig. 8. There is a small decrease in the edge height over the 29 days of exposure, but a
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XANES for 2024-T3 as a function of process time in the chromating bath.
Corrosion inhibition by chromate conversion coatings 30
,
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,
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FIG. 7. Ratio of the Cr 6+ pre-edge peak to the edge height and edge height (proportional to total Cr) for AI 2024-T3 as a function of process time in the chromating bath.
proportionately much larger and more rapid decrease of the fraction of Cr(VI), as determined from the pre-edge intensity, occurs with time of exposure to 0.5 M NaCI. Assuming an exponential decay of the Cr species in the film as a first approximation, the data for both the Cr(VI) (pre-edge) and the total Cr (edge) in Fig. 8 are fitted to an equation of the form: Y = Y0 e -'/~
(3)
where y is either the edge height or the percentage of Cr(VI). From a least squares fit of an exponential curve to the data of Fig. 8 using equation (3), the time constants, r, for the edge degradation and the percentage Cr(VI) loss (pre-edge decrease) are 16
i
141 i k ~
,21
u
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i
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10 L
O
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o
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®
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FIG. 8. Ratio of the Cr 6÷ pre-edge peak to the edge height, and the edge height for the chromated AI 3003-H14 Q-Panel® as a function of the time of exposure to air-equilibrated 0.5 M NaCI.
48
M.W. KENDIG, A. J. DAVENPORTand H, S. ISAACS
0.0074 and 0.0016 days, respectively. H e n c e , the time required for the C r ( V I ) to decrease by a certain p r o p o r t i o n a l a m o u n t is significantly less than the time required to get the same p r o p o r t i o n a l change in the quantity of film as m e a s u r e d by the total Cr f r o m the edge height. T h e m e c h a n i s m of the decrease in C r ( V I ) possibly relates to the slight solubility of the hexavalent Cr which, w h e n released u p o n exposure to an a q u e o u s e n v i r o n m e n t , partly reacts with the AI substrate at defects in the film according to equations (1) and (2), to repair and maintain it. CONCLUSIONS The results presented here provide several conclusions. 1. T h e ratio of hexavalent Cr to total Cr in conversion coatings is approximately 2 0 % , suggesting that C r ( V I ) remains a m a j o r constituent of the c h r o m a t e conversion coatings on Al and Al alloys. 2. With e n v i r o n m e n t a l exposure to air-equilibrated 0.5 M NaC1, the relative quantity of hexavalent Cr in the film decreases approximately exponentially. 3. M o r e C r ( V I ) is converted to C r ( I I I ) at the coating/substrate interface in the earliest stages of film formation. 4. B o t h the total a m o u n t of c h r o m i u m and the ratio of C r ( V I ) : C r ( I I I ) in c h r o m a t e conversion coatings on A1 2024-T3 f o r m e d by the bath used in this w o r k reach a limit after about 5 min of processing. 5. T h e C r ( I I I ) in the conversion coating is not crystalline Cr203, but has a closer resemblance to an a m o r p h o u s h y d r a t e d Cr(OH)3. 6. T h e c h r o m a t e conversion coating appears to be an active coating in that the slightly soluble hexavalent Cr in the film provides a continuous timed-release source of an inhibitor for repairing the film at defect sites. Acknowledgements--This work was supported in part by Rockwell International Science Center Indepen-
dent Research and Development Program and in part by the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC02-76CH00016. The authors gratefully acknowledge use of the XllA beam line at the NSLS and the support of the XllA Participating Research Team. REFERENCES 1. H. A. KATZMAN,G. M. MALOUF, R. BAUERand G.W. STUPIAN,Appl. Surf. Sci. 2, 416 (1979). 2. K. ASAMI,M. OKI, G. E. THOMPSON,G. C. WOODand V. ASHWORTrL Electrochim. Acta. 32, 337 (1987). 3. T. DROZDAand E. MALECZKI,J. Radioanal. Nucl. Chem. Lett. 95, 339 (1985). 4. J. K. HOWELL, Plating 60, 1033 (1973). 5. T. S. SEHMBHI,C. BARNESand J. J. B. WARD, Trans. Inst. Met. Finish 62, 55 (1984). 6. J. K. HAWKINS, H. S. ISAACS, S. M. HEALD, J. TRANQUADA, G. E. THOMPSON and G. C. WOOD, Corros. ScL 27,391 (1987). 7. A. J. DAVENPORTand H. S. lSAACS,Corros. Sci. 31,105 (1990). 8. A. J. DAVENPORT,H. S. ISAACSand M. W. KENDIG, Corros. Sci. 32,653 (1991). 9. A. J. DAVENPORT,H. S. ISAACSand M. W. KENDIG, J. electrochem. Soc. 136, 1837 (1989). 10. A. G. 5CHROTr, G. S. FRANKEL, A. J. DAVENPORT,H. S. ISAACS,C. V. JAHNESand M. RUSSAK,Surf. Sci. 250, 139 (1991), 11. G. P. HALADAand C. R. CLAYTON,J. electrochem. Soc. 138, 2921 (1991). 12. S. W. M. CHUNG,J. ROBINSON,G. E. THOMPSON,G. C. WOODand H. S. ISAACS,Phil. Mag. B. 63,557
(1991). 13. S. W. M. CHUN6,J. ROalNSON,G. E. THOMPSON,G. C. WOODand K. SHIMIZU,In Proc. Syrup. X-ray Methods in Corrosion and Interracial Electrochemistry (eds A. J. DAVENPORTand J. G. GORDON).The Electrochemical Society, in press.
Corrosion inhibition by chromate conversion coatings
49
14. J. S. WAINWRIGHT,M. R. ANTONIO and O. J. MURPHY, In Proc. Symp. X-ray Methods in Corrosion and Interfacial Electrochemistry (eds A. J. DAVENPORTand J. G. GORDON). The Electrochemical Society, in press. 15. The authors wish to acknowledge Eric Eichinger who provided the Al 2024-T3 samples having different processing time in an Alodine 1200S chromating bath. 16. A. J. DAVENPORTand H. S. ISAACS, Corrosion '91, Paper No. 74, NACE (1991); in Surface and Interface Characterisation in Corrosion (Ed. S. SIJAn). NACE, Houston, in press. 17. M. KERKAR, J. ROBINSONand A. J. FORTY, Faraday Discuss. Chem. Soc. 89, 31 (1990).
