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Interpretation of Mössbauer spectra of passive films on metals
SURFACE SCIENCE 38 (1973) 249-25 1 r%,North-Holland Publishing Co.
INTERPRETATION OF PASSIVE
OF M&3SBAUER
SPECTRA
FILMS ON METALS
Recieved 16 February 1973
The problems of current research on passive films are mainly two: a determination of the electrodic mechanism leading to the maximum in the current-potential re~aiionship at which passivation occurs; and an eiucidation of the structure of the passive film grown at potentials substantially positive to that of the current-potential maximum. Information on the second matter pertains to the understanding of the degree of protection of metals provided by these films. The first of these problems has been approached with e~~ipsometryl) with fair success. However, the progress of the e~l~~son~etrj~~nvest~gatjon of the nature of the passive film finally grown remains partial. Correspondingly, the results of electron diffraction investigations must now be regarded as ambiguous because of the difficulty that the electron beam causes heating and this may bring about chemical changes to the fim s). Examinations of the passive film formed on Fe at pH 5.8 by means of Mijssbauer spectroscopy have been carried out (a) on the m-situ film at room temperature; (b) on the same film, but dried at 298*K; (c) on both of the above type films in the temperature range of 4 to 298°K. The following results were found :
Sample
In-situ Dried 3 hr Dried 5 hr
Temperature (“K)
Isomer shift (mm/set)
Quadtupole split (mmisec)
298 298 298
0.70*0.01 0.6610.0t 0.61 iO.01
1.0210.07 1.00+0.10 0.8O$AiO
The low temperature Mijssbauer spectra showed superparamagnetic behaviour characteristics of antiferromagnetic materials. Such results alfow calculation of the size of the particles making up the film as well as giving a value of 470+ fO kUe for the internal magnetic field of both the in-situ and dry samples. The essential results from this investigation are: 249
250
W. E.O’GRADY
AND
J.O’M.BOCKRIS
(1) The values of the isomer shift and quadrupole split found for the in-situ film formed at 300 mV versus the normal H electrode correlate well with the values for Fe3+ found in the literatures). (2) On the basis of the hydration number versus isomer shift work of Hazony et al. 4), it is estimated that the passive film contains about two water molecules per ion atom. (3) The low temperature results indicate a particle size of about 150 8, in diameter for the in-situ film; about 80 A for the dried film. (4) The dried film has the same Miissbauer parameters as those of yFe,O,. (5) In respect to the structure of the in-situ passive film, the results are not consistent with those for any known stoichiometric, iron oxide, including hydrates. The parameters of this film do, however, match values for the isomer shift and quadrupole splitting published for iron-containing polymers5y6), the structure of which contains di-oxy and di-hydroxy bridging bonds between the iron atoms.
---oJp-$l+;*;__ I I
,
0
H
0 H
Fig. 1.
Thus, the Miissbauer spectra of the in-situ passive film on iron is most consistent with a structure containing groups of this kind in chains linked together by water. It could be described somewhat as a polymeric film of hydrated iron oxide. A representation is attempted in fig. 1. The Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A.
O’GRADY
W. E. and
The School of Physical Sciences, The Flinders University, Adelaide, South Australia 5042
J. O’M. BOCKRIS
MtiSSBALJER
SPECTRA
OF PASSIVE
FILMS ON METALS
251
References 1) J. O’M. Bockris, M. Genshaw and V. Brusic, Symp. Faraday Sot. 4 (1970) 177; H. Wroblowa, V. Brusic and J. O’M. Bockris, J. Phys. Chem. 75 (1971) 2823. 2) K. L. Chopra, Thin Film Phenomena (McGraw-Hill, New York, 1969) p.46. 3) N. N. Greenwood, in: Physical Chemistry, an Advanced Treatise, Vol. 4, Eds. H. Eyring, D. Henderson and W. Yost (Academic Press, New York, 1970) p. 634. 4) Y. Hazony, R. C. Axtmann and J. W. Hurley, Chem. Phys. Letters 2 (1968) 440. 5) R. Prados and M. L. Good, J. Inorg. Nucl. Chem. 33 (1971) 3733. 6) J. L. Mackey and R. L. Collins, J. Inorg. Nucl. Chem. 29 (1967) 655.
INTERPRETATION OF PASSIVE
OF M&3SBAUER
SPECTRA
FILMS ON METALS
Recieved 16 February 1973
The problems of current research on passive films are mainly two: a determination of the electrodic mechanism leading to the maximum in the current-potential re~aiionship at which passivation occurs; and an eiucidation of the structure of the passive film grown at potentials substantially positive to that of the current-potential maximum. Information on the second matter pertains to the understanding of the degree of protection of metals provided by these films. The first of these problems has been approached with e~~ipsometryl) with fair success. However, the progress of the e~l~~son~etrj~~nvest~gatjon of the nature of the passive film finally grown remains partial. Correspondingly, the results of electron diffraction investigations must now be regarded as ambiguous because of the difficulty that the electron beam causes heating and this may bring about chemical changes to the fim s). Examinations of the passive film formed on Fe at pH 5.8 by means of Mijssbauer spectroscopy have been carried out (a) on the m-situ film at room temperature; (b) on the same film, but dried at 298*K; (c) on both of the above type films in the temperature range of 4 to 298°K. The following results were found :
Sample
In-situ Dried 3 hr Dried 5 hr
Temperature (“K)
Isomer shift (mm/set)
Quadtupole split (mmisec)
298 298 298
0.70*0.01 0.6610.0t 0.61 iO.01
1.0210.07 1.00+0.10 0.8O$AiO
The low temperature Mijssbauer spectra showed superparamagnetic behaviour characteristics of antiferromagnetic materials. Such results alfow calculation of the size of the particles making up the film as well as giving a value of 470+ fO kUe for the internal magnetic field of both the in-situ and dry samples. The essential results from this investigation are: 249
250
W. E.O’GRADY
AND
J.O’M.BOCKRIS
(1) The values of the isomer shift and quadrupole split found for the in-situ film formed at 300 mV versus the normal H electrode correlate well with the values for Fe3+ found in the literatures). (2) On the basis of the hydration number versus isomer shift work of Hazony et al. 4), it is estimated that the passive film contains about two water molecules per ion atom. (3) The low temperature results indicate a particle size of about 150 8, in diameter for the in-situ film; about 80 A for the dried film. (4) The dried film has the same Miissbauer parameters as those of yFe,O,. (5) In respect to the structure of the in-situ passive film, the results are not consistent with those for any known stoichiometric, iron oxide, including hydrates. The parameters of this film do, however, match values for the isomer shift and quadrupole splitting published for iron-containing polymers5y6), the structure of which contains di-oxy and di-hydroxy bridging bonds between the iron atoms.
---oJp-$l+;*;__ I I
,
0
H
0 H
Fig. 1.
Thus, the Miissbauer spectra of the in-situ passive film on iron is most consistent with a structure containing groups of this kind in chains linked together by water. It could be described somewhat as a polymeric film of hydrated iron oxide. A representation is attempted in fig. 1. The Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A.
O’GRADY
W. E. and
The School of Physical Sciences, The Flinders University, Adelaide, South Australia 5042
J. O’M. BOCKRIS
MtiSSBALJER
SPECTRA
OF PASSIVE
FILMS ON METALS
251
References 1) J. O’M. Bockris, M. Genshaw and V. Brusic, Symp. Faraday Sot. 4 (1970) 177; H. Wroblowa, V. Brusic and J. O’M. Bockris, J. Phys. Chem. 75 (1971) 2823. 2) K. L. Chopra, Thin Film Phenomena (McGraw-Hill, New York, 1969) p.46. 3) N. N. Greenwood, in: Physical Chemistry, an Advanced Treatise, Vol. 4, Eds. H. Eyring, D. Henderson and W. Yost (Academic Press, New York, 1970) p. 634. 4) Y. Hazony, R. C. Axtmann and J. W. Hurley, Chem. Phys. Letters 2 (1968) 440. 5) R. Prados and M. L. Good, J. Inorg. Nucl. Chem. 33 (1971) 3733. 6) J. L. Mackey and R. L. Collins, J. Inorg. Nucl. Chem. 29 (1967) 655.