Quantitative Analysis for Corrosion Studies by the Mossbauer Effect Donald D. J o y e and Robert C. Axtmann Department of Chemical Engineering, Princeton University, Princeton, N. J. 08540
The development of quantitative methods that utilize the Mossbauer Effect has lagged because of manifold experimental difficulties. The corrosion of thin samples of iron, however, is a nearly ideal system for nondestructive, quantitative assay by a Mossbatter method. In this work a constant acceleration spectrometer gave the ratio of the amount of unreacted iron to that of corrosion product formed in an atmosphere of H20, HCI, and air. The spectrometer furnished an internal calibration for the method while a simple gravimetric technique provided an absolute calibration. tive interpretation of the Mossbauer spectra ai!ual*itaed in a provisional identification of the corrosion product as FezOa.2Hz0. At room temperature the ratio of the Mtissbauer efficiency of iron to that of the corrosion product is 1.61 0.22.
tremely thin iron foils of natural isotopic composition were exposed to an atmosphere containing HC1, HzO, and air. The weight per cent of uncorroded metal was determined by comparison of the Mossbauer signals from pure iron and from the corrosion product after calibration by a simple gravimetric procedure. In principle, kinetic studies could have been performed on a single sample without its removal from either the corrosive environment or from the spectrometer, if the correspondence between the Mossbauer spectra and the gravimetric procedure had already been established. In this work, however, the same Mossbauer data were employed both for the calibration and for the weight loss determination so that multiple samples were required.
THE FIRST OBSERVATION of recoil-free emission of nuclear gamma rays was reported in 1958 ( I ) , yet in DeVoe and Spijkerman's 1966 review on the Mossbauer Effect (2), not one of the 160 articles cited is to a quantitative method. To date the principal chemical applications of the new spectrometry have been in structural analysis and in the characterization of chemical bonding. Contemplation of the experimental details involved in Mossbauer spectrometry reveals some of the obstacles that may have impeded the development of quantitative procedures : Recoil-less emission of gamma rays has been observed with reasonable efficiency in only a handful of elements, all with atomic numbers, A 2 26. Even for these elements, isotopically enriched samples are frequently required to prevent nonresonant gamma-ray absorption from obscuring the resonant absorption. The effect is, for practical purposes, limited to the solid state or to gases trapped in clathrate compounds (3). In the usual transmission technique, thin samples are required for the effect to be observed; extremely thin samples are necessary to obviate tedious corrections. Backscattering or reflection techniques (4) can be employed with thick samples and, indeed, may have application in corrosion studies although they were not used in the present study. Except for iron and tin, cryogenic techniques are usually necessary although the apparatus may be quite crude if liquid nitrogen temperatures are sufficient as in the case of iodine. The absolute efficiency of the Mossbauer absorption cannot, as yet, be computed ab initio so that any quantitative method must rely for calibration on another technique. In the work reported here, nearly ideal conditions existed from the standpoint of the difficulties listed above, Ex-
EXPERIMENTAL
*
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(1) R. L. Mossbauer, 2.Physik, 151,124 (1958). (2) J. R. DeVoe and J. J. Spijkerman, ANAL,C H ~ M 38. (5), 382R
(1966). (3) Y.Hazony, P. Hillman, M. Pasternak, and S. L. Ruby, Phys. Letters, 2, 337 (1962). (4) N. Hershkowitz and J. C. Walker, Nucf. Instr. Methods, 53, 273 (1967).
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ANALYTICAL CHEMISTRY
Sample Preparation and Treatment. Square coupons, approximately 1.25 inch on a side, and weighing ca. 0.09 gram, were cut from a sheet of 99.99Z iron that had been rolled to 0.0005 inch by the A. D. Mackay Co. of New York City. The coupons were degreased in a 3 : l acetone-isopropanol solution, pickled in 37.5 w/o HCl for 30 seconds with agitation after 15 seconds, washed, dried, and weighed. The samples were suspended from a hole punched in one corner in 500-ml erlenmeyer flasks that contained 50 ml of 6 w/o HC1 solution by means of wires that passed through a rubber stopper. The hole in the stopper was loosely stuffed with cotton; the ambient temperature was 23.5" f 0.5" C ; and the HC1 vapor concentration was approximately 60 ppm on a volume basis. The corrosion product was removed after the spectrometer measurements by agitating the samples for 30-60 seconds in 200 ml of a 10% ammonium nitrate solution that had been heated to a few degrees below the boiling point. The samples were then washed, dried, and reweighed to determine the amount of remaining iron. The loss of unreacted iron during this treatment was less than 0.0005 gram. Mossbauer Equipment. A constant acceleration spectrometer that has recently been reported by Hazony (5) was used. The essential components are a (modified) Elron Industries, Ltd. Mossbauer Transducer, an Elron Transducer Driving Unit (Model MD-l), a (modified) Radiation Instruments Development Laboratories multi-channel analyzer (Model 35-12), a thin NaI crystal-RCA 6810A photomultiplier tube detector with a single channel analyzer to select the 14.4-keV photopeak, and a 0.75-mCi 6 7 Csource ~ diffused into a copper foil by the New England Nuclear Corporation. The gammaray beam passed through a lead collimator that limited the size of the beam to B cross section smaller than that of the sample. The aluminum cover on the NaI crystal filtered the 6.3-keV X-ray from iron. Such a precise and stable spectrometer was not required for the present purposes. The values of the splittings and isomeric shifts of the Mossbauer peaks were irrelevant to the quantitative measurements and could be determined more accurately than was necessary for the identification of the corrosion product. The relative intensities of four simul(5)
Y.Hazony, Rev. Sei. Instr. 38,1760(1967).
too
150
200
250
300
CHANNEL NUMBER Figure 1. Mossbauer spectra of iron (solid line) and of the corrosion product (dashed line) which was provisionally identified as Fez03.2HzO
taneously determined peaks, two from the unreacted iron and two from the corrosion product, were sufficient for the quantitative determination of the iron (cf. Equation 1 below). Thus any constant acceleration spectrometer that sweeps rapidly through the appropriate velocity range would have been adequate. Such devices are available commercially and include models that are intended primarily for lecture demonstrations. Constant-velocity spectrometers which sweep incrementally at a slow pace over the velocity range are less well-suited to the present measurement unless the detector sub-system is extremely stable. The 0.75-mCi 57C0source permitted determinations in from 2.5-4 hours for l o 5 counts per velocity channel with each of the 250 channels that were actually used being equivalent to a width of 1.6 X mm/second. The experimental time for the same statistical precision is inversely proportional to source strength, so that a 7.5-mc source, a practical size, would have enabled equivalent determinations in about 20 minutes. Had a proportional counter containing 90% Kr-lox CHI been used as a detector, an improved signal-to-noise ratio would have resulted with a corresponding reduction (30-40x) in the time per determination. We have found, however, that nuclear channels with proportional counters are considerably less reliable than those employing scintillation counters. Data Reduction. The memory of the multichannel analyzer which stored 14.4-keV gamma counts us. velocity was read onto computer cards by an IBM 526 card punch. An IBM 7094 computer fitted the Mossbauer peaks to Lorentzian curves by means of a modification of the basic nonlinear least squares algorithm due to Marquardt (6).
RESULTS AND DISCUSSION Qualitative Mossbauer Spectra. Unreacted iron gives the familiar spectrum of six lines whose intensities are in the ratio 3:2:1:1:2:3 and which arise from the magnetic hyperfine interaction of 57Fe nuclei in a ferromagnetic material. In this experiment only the two middle lines of this spectrum were (6) D.W.Marquardt,J. SOC.Zndt. Appl. Math., 11,431 (1963).
actually scanned as shown in Figure 1. The well known (7) separation between these lines afforded a calibration of the velocity scale of the spectrometer and gave 1.61 X mm/second-channel which is equivalent to 7.73 X 10-lo eV/ channel. A corrosion product spectrum was obtained with material that had been scraped from a piece of corroded iron and redispersed on a plastic backing. This spectrum, also shown in Figure 1, reveals a well resolved doublet whose reasonably symmetrical appearance suggests a single compound with a quadrupole splitting, A = 0.64 mm/second. The midpoint of the doublet determines the isomer shift, 6 = 0.62 mm/ second relative to a sodium nitroprusside absorber at room temperature. Quantitative Mossbauer Spectra. Figure 1 shows that the higher velocity quadrupole peak, labelled c, of the corrosion product overlaps peak d of the pure iron spectrum. During the quantitative measurements of the corroded samples the spectrometer scanned over peaks a, 6 , c, and d and the computer program analyzed for the intensities of all four. Because the intensities of peaks a and d are known to be equal, this fact and the known positions of the four peaks were used as constraints on the program to reduce the computer time required for convergence. The intensity of the full, pure iron spectrum (not shown in Figure 1) is proportional to the number of S7Fe atoms in the uncorroded portion of the sample. A constant of proportionality e l includes the Mossbauer efficiency for pure iron and instrumental factors such as the detector geometry and efficiency and the duration of the experiment. Likewise ec may be defined as a proportionality constant that relates the intensity of the corrosion product spectrum to the number of S7Fe atoms in the corrosion product. e, includes the appropriate Mossbauer efficiency, expected to be different from that of pure iron, plus the identical instrumental factors that are contained in El. (7) H. Brafman, M. Greenspahn, and R. J. Herber, Nucl. Znsfr. Methods, 42,245 (1966). VOL 40, NO. 6, MAY 1960
871
Table I.
eo,
Sample XL2 XL3 X U XL5 XL6 XL7
the Ratio of M k b a u e r Efficiency in Iron to That in Corrosion Product Peak areas (arb. units) Gga ( A D) ( B C) eo G,b 0.917 0.882 0.841 0.879 0.780 0.856
+
+
4.72 3.38 2.73 2.46 2.74 2.85
1.854 1.886 1.721 1.268 3.19 1.471 = 1.61
(eo),,
1.39 1.44 1.80 1.60 1.45 1.96 =t0.22
0.905 0.870 0.856 0.879 0.762 0.879
Determined gravimetrically. b Determined from Equation 2 via the MBssbauer intensities and 0
(eo>av.
If we define EO 3 e f / e c then this quantity is the ratio of the two Mossbauer efficienciesand is constant for any series of experiments at the same temperature. eo is independent of time although the statistical precision with which it is determined is a function of the total number of counts accumulated and hence of time. The times of the individual experiments in a series thus need not be exactly the same since the spectra of both substances, iron and corrosion product, are taken simultaneously. The relationship between the fraction of uncorroded iron, G, and the peak intensities is
where A , B, C, and D represent the areas under peaks a, 6 , c, and d. The coefficient six arises because ( A 0)represents only ‘I6of the total Mossbauer absorption in the pure iron (cf. first paragraph in the Results and Discussion Section) while ( B C ) represents the total absorption of the corrosion product. In precision work it may be desirable to determine a more accurate value of the coefficient by carefully measuring the intensity ratio of the iron peaks. Particularly in thin, rolled samples the measured intensities deviate from the theoretical ones, presumably because of a nonrandom orientation of directions of magnetization. Such effects would be quite small in relationship to the other errors in the present method, however. Substituting eo into Equation 1 and rearranging gives
+
+
eo was determined from the gravimetric analyses for G and Equation 2 from six different samples that had been exposed to the corrosive atmosphere from 47 to 240 hours. The results are given in Table I. EO).^ = 1.61 0.22 and, as expected, is not equal to unity. The indicated standard deviation of these measurements, amounting to *13.7% from the mean, is reasonable: the statistical uncertainty in the ratio ( A D)/(B C) as given by the computer fit of the spectra averaged about 7.5 %; the weighing error ( < O S mg) as reflected in the ratio (1 - G)/G varied with the samples but averaged approximately 8%; and there was an undetermined but presumably random error due to the inhomogeneous growth of the film of corrosion product. The calibration of the Mossbauer method, then, consisted of the determination of from values for G,, the gravimetrically measured fraction of pure iron remaining. Once this determination had been made, the peak intensities from this and subsequent experiments can be used to give values of Gm,the Mossbauer measurement of remaining fraction, without further recourse to gravimetry. The last column of Table I shows the values of Gm computed by inserting and the peak intensities from the present series of experiments into a rearrangement of Equation 2. Identification of the Product. The reddish-brown corrosion product was insoluble in boiling HzO, boiling NaOH, and boiling, concentrated “Os. A silver nitrate test for chloride was negative. The isomer shift and quadrupole splitting reported in Section A above fall within the range of data for ferric compounds reported in Gol’danski’s compendium (8),and are close to, but not coincidental with, the values for Fe203and F e ( 0 3 . On heating, two samples of the compound gave off amounts of water consistent with the formula Fez03-2H10.
*
+
+
ACKNOWLEDGMENT
The authors are grateful to Yehonathan Hazony for a number of valuable suggestions. David E. Earls assisted materially with the experimental details and John W. Hurley, Jr., contributed to the computer analysis of the spectra. RECEIVED for review November 28, 1967. Accepted February 12, 1968. Work supported by the U. S. Atomic Energy Commission, The use of the facilities of the Princeton University Computer Center was supported in part by Grant GP-579 from the National Science Foundation. This paper is based in part on the B.S.E. thesis of D. D. Joye, Princeton University, 1967. (8) V. I. Gol’danski, “The Mossbauer Effect and Its Applications in Chemistry,” Consultants Bureau, N e w York, 1964.
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ANALYTICAL CHEMISTRY
