The RBS spectra from Frit 21 borosilicate glasses doped with 5 wt % UO/sub 2/, SrO, or Cs/sub 2/O show that: during the initial stages of leaching (0 to 3 h) there is a substantial (300 to 500%) enhancement in the concentration of U, Sr, Ca, and Ti in the outer surface layer and that this enhancement is accompanied by a large depletion of Na, Si, and Cs; and upon further leaching under static conditions (24 h) the leached surface layer composition is indistinguishable from the unleached surface. Other borosilicate glasses such as PNL 76-68 may eventually show the same behavior if the final equilibrium pH value is greater than 9. The technique of Rutherford backscattering depth profile analysis can be a powerful tool for investigating the initial stages of glass corrosion

Boatner, L. A.

Naramoto, H.

Sales, B. C.

White, C. W.

UNT Digital Library

Note: This is a draft of a paper being submitted for publication. Con-tents of this paper should not be quoted nor referred to withoutpermission of the authors.DISCLAIMER -RUTHERFORD BACKSCATTERING INVESTIGATION OF THE CORROSION OFBOROSILICATE GLASSB. C. Sales, L. A. Boatner, H. Naramoto, and C. W. WhiteBy acceptance of thii article, thtpublisher or recipient acknowledgesthe U.S. Government's riflht toretain a nonexclusive, royeltv-freelicense in and to any copyrightcovering the article. CONF-811122—46DE82 004053SOLID STATE DIVISIONOAK RIDGE NATIONAL LABORATORYOperated byUNION CARBIDE CORPORATIONfor theU.S. DEPARTMENT OF ENERGYOak Ridge, TennesseeOctober 1981"RUTHERFORD- BACKSCAT-fERi-NG-J-NVESHGATION- OF- THE •• CORROSIW-GF —-- - - -BOROSILICATE GLASS*B. C. SALES, L. A. BOATNER, H. NARAMOTO, AND C. W. WHITESolid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830INTRODUCTIONOne of the major reservations regarding the use of glass as a primary- nuclear waste form is its ability tc retain hazardous radioactive nuclei whenexposed to ground water at temperatures above about 75°C. It is now generallyaccepted that the removal of radioactive nuclei from glass in an aqueous-- environment proceeds via two main mechanisms, i.e., either by a diffusion pro-• cess (which may be driven or undriven) or by direct matrix dissolution - which,in its initial phase, also results from a diffusion related process [1]. The- term "leaching" is generally employed in describing the overall process result-ing in the remo/al of nuclei (whether they are radio-active or not) from glass- that is exposed to an aqueous environment. Predictions of the long-term leach-"- ing characteristics of glass must necessarily be based on the extrapolation of- data from experiments whose duration is much shorter than the half-lives of- some of the more dangerous radioactive nuclei. Obviously the ability to- reliably extrapolate the results of such studies depends on how well one under-— stands the basic mechanisms of glass corrosion and leaching. Although thequalitative understanding of the leaching of simple borosilicate glasses is- good, more quantitative data and reliable predictive theories are needed. In- particular it is not sufficient for a theory to account only for the concen-- tration of ions found in the leachant solution during an experimant. The— chemical structure of the leached glass surface must also be considered sincethis layer greatly affects the subsequent leaching characteristics of the- material. For example, some results indicate that under certain conditions asurface layer several microns thick can form which may protect the bulk of the- glass from further leaching [2]. In the present study, the technique of depth— profile analysis by means of Rutherford backscattering (RBS) has been used to- study the evolution of the leaching process and associated changes in chemicalcomposition of the leached borosilicate glass surface layer.- EXPERIMENTAL- Rutherford Backscattering Depth Profile AnalysisThe technique of Rutherford backscattering depth profile analysis is" described in detail in several excellent review articles [3,4]. Basically, a~ beam of *He+ (E « 2 MeV in the present case) bombards the surface of the sampleunder investigation. The 4He + ions then elastically scatter from nuclei of theelements composing the sample and emerge at an energy which depends only on the~ mass of the scattering atom, the angle of the detector (which is fixed at a~ scattering angle near 180°), and the depth of the scattering atom below the~ surface. For example, 4He + ions will scatter off of atoms on the surface Into~ a detector at a scattering angle of 180° with an energy E s « ({M-m)(*Hii))2 • 2MeV where m 1s the mass of the *He+ ions, and M the mass of the scattering atom.. The *He+ Ions, which scatter from the same element below the surface, will" emerge with an energy less than E s. This additional energy loss 1s roughly'Research sponsored by the Division of Materials Sciences, Office of Basicr Energy Sciences. U. S._ Department-Of .Energy under -contractM-Uwith Union Carbide Corporation..2.proportional to the depth, of the atom below the surface. Hence, the totalRutherford backscattering spectra can be viewed as the superposition of con-centration versus depth profiles for each element in the sample. Theseprofiles, however, will be shifted in energy for each element by an amountwhich depends on the value of Es.In principle, analysis of the energies of the 4He + ions scattered by the ele-ments in a glass allows one to determine quantitatively the concentration ofeach element as a function of death in a 1-2 micron surface layer. The depthresolution is approximately 150 A, and the sensitivity of RBS to a particularelement is proportional to its atomic number squared. Hence in a simple boro-silicate glass composed mainly of the elements Si, Na, B, and 0, one can detectconsiderably less than 1 at- % of the heavier waste elements such as Sr and Cs,or U and the other actinides. The large atomic numbers of these particularwaste elements also simplifies the depth-versus-concentration calculations.SAMPLE PREPARATIONSeveral prototype borosilicate glasses were prepared by melting theappropriate amounts of oxides or carbonates in a platinum crucible at 1160cC.The molten glass was poured into a mold of spectroscopically pure carbon,annealed at 550°C for 2 h, and then slowly cooled to room temperature. Samplesmeasuring ^ 0.5 x 1.0 x 0.2 cm were cut from the resulting glass billets. Onesurface of each sample was polished to a 1.0 micron finish with A1203 powder.The samples were then leached in distilled water at 90°C in a teflon containerfor times ranging from 0 to 48 h. The ratio of the surface area of the glassto the volume of water was fixed at 0.1 cm"1. After drying the samples in agentle stream of pure nitrogen gas, the polished surface of each sample wascoated with a 100 A thick film of aluminum to prevent charging of the specimenduring the backscattering measurements.RESULTSVarious compositions of borosilicate glass were investigated including thetwo frequently studied compositions Frit 21 and Frit 131 which are used asreference glasses at the Savannah River facility [5]. Since the RBS spectrafrom all of the compositions investigated exhibited the same qualitativebehavior, only data taken with the Frit 21 glass will be presented in thepresent work. The composition of Frit 21 consists of: 52.5 wt. % SiO?, 10" wt. % B2O3, 10 wt. % TiO2, 18.5 wt. % Na2O, 4 wt. % Li20 and 5 wt. % CaO.Using Frit 21 as the host material, several glass samples were prepared anddoped with 5 wt. % of one of the following waste oxides: SrO, CS2O, UO2, ^ 0 3 ,or Gd203« The glass samples prepared from the five glass billets were thenleached in distilled water at 90°C for 0, 0.5, 2-3, and 22-24 h. Leaching ofthese glasses for longer than ^ 24 h at 90°C produced extensive flaking of glassscales which formed on the surface and made any RBS measurements very difficult.The backscattering spectra, which consist of the total backscattering yield in" counts versus energy, is shown in Fig. 1 for the leached and unleached uranium-doped glass. The RBS spectrum of the unleached glass indicates that the U con-centration is uniform to a depth of 1-2 microns and the U concentration calcu-lated from the spectrum is consistent with the amount of UO2 added to the glass.After leaching the glass for 0.5 h, there is a three-fold increase in the ura-nium concentration within 150 A of the surface. The near-surface concentrationsof Ti and Ca also increased. After leaching the sample for 3 h, the U concen-tration increased even further and this buildup extended below the surface to4 depth of about 0.5 pm. A similar increase was observed for the Ti and Caconcentrations, while there was a concurrent depletion of both Na and Si froma 0.5 jim thick surface region. After leaching for 24 h, however, the composi-tion of the leached surface layer was virtually identical to the unleached.surface. This appears to be due to the large increase in the pH of-the water -which attained a value of 10.1 after 24 h. For a pH > 9, the entire glass net-work begins to dissolve, and evidently the rate at which this dissolution occursis more rapid than the rate of formation of the surface layer. Backscatteringyield from the uranium alone is shown in Fig. 2 and the striking increase in theU concentration near the surface, which occurs during the initial stages ofleaching, is clearly evident. The evolution of the surface layer in Gd203 andNd£03 doped glasses was similar to that observed with the UO2 doped glass.Backscattering spectra from leached and unleached samples of Frit 21 dopedwith 5 wt. % SrO are shown in Fig. 3. After leaching for 0.5 h there is anoticeable increase in the concentrations of Sr and Ti near the surface as wellas a decrease in the near-surface concentration of Na. After 2.5 h of leachingthe surface concentrations of Ti and Sr increased by factors of 5 and 2,respectively, over the unleached values. This build up extended 0.2-0.3microns below the surface. Within this outer layer the Ca concentration in-creased while the Na and Si concentrations decreased. As in the case of the U- ;doped glass, after 24 h of leaching the composition of the leached surfacelayer in the strontium-doped material was the same as that of the unleachedsurface and the solution pH was high (̂  10). • ,Shown in Fig. 4 are backscattering spectra from leached and unleached samplesof Frit 21 doped with 5 wt. % C.S2O. Upon leaching, the behavior of the elements,,which comprise Frit 21 is similar to that observed in the UO2 and SrO dopedglasses. The Cs behavior is, however, quite different from that of either U or •Sr. In particular, after leaching for 2 h, the Cs concentration near the sur- ;face decreases by a factor of 3. This depleted Cs layer extends about 0.5 urn ;below the surface. Again, after leaching for 24 h, the composition of the :leached surface~layer was indistinguishable from the unleached surface. ::Karim et al. have previously used the techniques of RBS and XPS (x-ray pho-toelectron spectroscopy) to study the leaching characteristics of three boro-silicate glasses doped with UO3. In all three glass compositions (which in- -eluded PNL 76-68) they found an increase of ̂  50% in the U concentration near )the surface after leaching in distilled water for 3-24 h at 75°C. The uranium ibuild up was qualitatively similar to that found after leaching U doped Frit 21for 0.5 h (Fig. 1). In contrast to the results presented here, however, evenafter 24 h of leaching there was no evidence of a return of the elemental sur- 'face concentrations to the unleached values. Part of this difference canpossibly be explained by the lower leaching temperature (75°C versus 90°C)employed by Karim et al [6]. The primary difference, however, is probably due :to the large inherent variations in leachability among different borosilicateglass compositions.A simple qualitative measure of the leaching process is represented by thetime dependence of the pH of the leachant solution. A pH of about 9 usually •represents the dividing line between stage 1 and stage 2 leaching [1]. During |.stage 1 (pH < 9) the selective removal of Na and Cs ions via an ion exchange ;reaction dominates but there is also some dissolution of the SiOg and B2O3 glass •matrix. It is also during stage 1 that the U, Sr, Ca, Ti, and rare-earth con-centrations "pile up" near the surface. During stage 2 (pH > 9) the dissolutionof the SlOo and B2O3 matrix dominates. The dissolution apparently occurs at arate that is so rapid that it precludes observation of the selective leaching ;effects. This results in a leached surface whose composition is virtuallyidentical to the unleached surface. Experiments are currently underway totest this hypothesis. ; LThe change of the solution pH as a function of time depends on the compo- :'.sition and leaching characteristics of the glass. It has been shown, for : "example, that 1n a closed glass-water system the solution eventually reaches a 'steady state value determined by the composition of the glass [7]. For a flass *with the composition Na2u • B2O3 • 2SiO2 the equilibrium pH value 1s 11-11.5 ;while for Na20'*-B2O3 r 8SIO2 this value is 8.5-9 [8]. As nottd btfart, *ft«r__Jl l g 7 T ~ f l n p p r f with " p f ± h p r I I i B a , J ^ pwas between 10 and 10.5 - well in to stage 2. Under the same experimental con-. d i t ions the leachant solut ion fo r 76-68 glass only attained a pH of 6.0-6.5.Hence fo r the 76-68 glass, one might expect the return of the unleached surface._ composition only a f ter longer leaching periods when the pli is greater than 9.... CONCLUSIONSThe principal findings of the present work may be summarized as follows:_ 1. The RBS spectra from Frit 21 borosilicate glasses doped with5 wt. % UOj, SrO, or CS2O show that:a. During the initial stages of leaching (0-3 h) there is asubstantial (300-500%) enhancement in the concentration ofU, Sr, Ca, and Ti in the outer surface layer and that this_ enhancement is accompanied by a large depletion of Na, Si,and Cs.b. Upon further leaching under static conditions (24 h) theleached surface layer composition is indistinguishablefrom the unleached surface.2. Other borosilicate glasses such as PNL 76-68 may eventually show the *same behavior if the final equilibrium pH value is greater than 9.3. The technique of Rutherford backscattering depth profile analysis can- be a powerful tool for investigating the initial stages of glasscorrosion.- ACKNOWLEDGMENTSIt is a pleasure to acknowledge the able technical assistance of H. E.-.Harmon and S. Corbato.- REFERENCES- 1. D. E. Clark, C. G. Pantano, Jr. and L. L. Hench in: Corrosion of Glass(Magazines for Industry, Inc., New York 1979).- 2. F. K. Altenhein, W. Lutze and 6. Malow in: Scientific Basis for NuclearWaste Management, Vol. 3, J. G. Moore ed. (Plenum Press, New York 1981)pp. 363-370.— 3. L. C. Feldman, CRC Reviews in Solid State and Materials Science, May 1981,pp 143-167.- 4. W. K. Chu, J. W.Mayer, M-A. Nicolet, T. M. Buck, G. Amsel and F. Eisen,Thin Solid Films 17, 1 (1973).- 5. M. J. Plodinec in: Scientific Basis for Nuclear Waste Management, Vol. 2,Clyde J. M. Northrup, Jr., ed. (Plenum Press, New York 1980) pp. 223-229.- 6. D. P. Karim, P. P. Pronko, T. L. M. Marcuso, D. 0. Lam and A. P. Paulikas— in: Scientific Basis for Nuclear Waste Management, Vol. 3, J. G. Moore ed.- (Plenum Press, New York 1981) pp. 397-404.•: 7. S. M. Budd and J. Frackiewicz, Phys. and Chem. of Glasses _3» H 6 (1962).- — 8 . K. B. Harvey in: Proceedings of Oak Ridge National Laboratory Conference- on the Leachability of Radioactive Solids, Gatlinburg, Tennessee,December 9-12, 1980, p 86.FIGURE CAPTIONSFig 1. Energy spectra of 2 MeV 4He + ions backscattered from leached andunieached Frit 21 borosiincate glass doped with 5 wt. % UO2. The glass wasleached in distilled water at 90°C for 0, 0.5, 3 and 24 h.Fig. 2. High energy portion of backscattering spectra shown in Fig. 1(unleached: f, leached 0.5 h: A, leached 3 h : E ) . The backscattering yield isfrom uranium only.Fig. 3. Energy spectra of 2 MeV 4He + ions backscattered from leached andunleached Frit 21 borosilicate glass doped with 5 wt. % SrO. The glass wasleached in distilled water at 90°C for 0, 0.5, 2.5, and 24 h.Fig. 4. Energy spectra of 2 MeV W " ions backscattered from leached andunleached Frit 21 borosilicate ;lass doped with 5 wt. % C.S2O. The glass wasleached in distilled water at SO°C for 0, 0.5, 2, and 22.li.,1 1 1 1 I i i i I I i INo" •«> . • ; CaIiSi238U\2A h©oo X... I3.0 ho(J• • • • • • - . . . •0.5 h• • • - • • . .0.0 hI I I I I I I I 1 1 1 1 VI.0.5 1.0 1.5Energy (MeV)765coU 321-----s1.651 1 1•••mmmmm*••mm1 1 fi4;iifI1.8511Im.m«1f------1.95Energy (MeV)1 i i I I I I IoIAlINo1 'iCa _.SioooCM3ou " * • - • - . .- • IISr2.5 h0.5 h0.0 hr i i i I I I I I.-....J [....0.5 1.0 1.5Energy (MeV)ooo1/1cooU1 1 1o1-.• ' • . . .* * ' ^ -V••| | ,i INo11 1Al1iii"*"•*•"••.••V -1tSiJ||]V•I...w:-.iiJSi/ • •• • — — . . • .t1Co1- " • • .*• t• • • • • . » . - - :! 1' 1 'TiNI |1Cs— .—^1i0 1 —22 h12.0 h10.5 h0.0 h0.5 1.0 1.5Energy (MeV)

Rutherford Backscattering Investigation of the Corrosion of Borosilicate Glass

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Rutherford Backscattering Investigation of the Corrosion of Borosilicate Glass

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