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Infra-red spectra of surface films on Fe anodically oxidized in a neutral solution
Corrosion Science, 1970. Vol. 10, pp. 111 to 112. Pergamon Press. Printed in Great Britain
SHORT COMMUNICATION INFRA-RED SPECTRA OF SURFACE FILMS ON Fe ANODICALLY OXIDIZED IN A NEUTRAL SOLUTION* H. EBIKO and W. SUi~TAKA The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal, Japan THE COMPOUNDS constituting the surface films on electrochemically passivated Fe have been investigated by means of electrochemical and electron optical techniques, but the obtained results have not been in general agreement. Infra-red spectroscopy can be considered as a potential tool for studying the surface films, but no paper in this field of research has appeared in the literature as far as the authors are aware. In the present communication, the infra-red spectra of surface films on anodieaUy oxidized Fe will briefly be reported and discussed. Eleetropolished sheets (25 x 50mm) of high purity Fe (kindly supplied by Battelle Memorial Institute) were potentiostatically oxidized at 0 or 800mV for 4h in a boric acid-borax aqueous solution. A Hg/HgzCbJ3.3N KC1 half cell was used as the reference electrode. The boric acidborax aqueous solution was prepared after the procedure proposed by Nagayama e t al. 1 and its pH was 8.42 at 25 4- I°C. The infra-red spectra were obtained by means of the optical system described in a previous paper, z After anodic oxidation the specimens were immediately washed with a jet of methanol, dried in v a c u o , and mounted on the sample holder of the speetrophotometer. After measurement of the spectrum, the specimens were further washed with a jet of water so as to remove substances soluble in water, rinsed in methanol and dried in v a c u o . The spectra of the specimens was then re-determined. The important parts of the obtained spectra are listed in Table 1, in which letters used have their usual meanings. The absorption bands appearing at 1260, 1105, 860 and 815cm -1, which disappeared on washing with water and changed equally in intensity by changing the specimen, can be assigned to a complex borate ion. These bands did not correspond to the absorptions of borax, other sodium borates nor bode acid, and may be therefore attributed to a complex borate of Fe. The continuous absorptions in the high wavenumber region indicate the surface films contain compounds which are semi-conductive in nature. The absorption bands at 630 and 450cm -1 arise probably from the lattice vibrations of the semi-conductors. In changing the oxidizing potential from 0 to 800mV, the relative intensity of the band at 450cm -x decreased whereas the band at 630cm -x increased in intensity. This fact suggests that the semi-conductor having the absorption band at 450cm -~ would be a compound of Fe in a low oxidation state and that of 630cm -~ a compound of Fe in a high oxidation state. Since FeD has a strong absorption band at 410cm -x and FenD4 at 570cm -~ and the shift of strong absorption bands to higher wavenumbers *Manuscript received 11 August 1969. 111
112
H . EBIKO a n d W . SU~"rAKA TABLE 1.
INFRA-RED ABSORPTION MAXIMA OF SURFACE FILMS ON F e PASSIVATED IN BORIC ACID--BORAX SOLUTION
Applied potential OmV
800mV
Washed with methanol water cm-' 450 Sb 640 ?
c m -~
450 Sb 630 m 815 m 860 sh 1105 m 1260 m 1340 wb *Continuous absorption.
Washed with methanol water
,
em-x 450 mb 635 rn 820 S 855 sh ,
1105 S 1262 S
cm-1
450 mb 630 rn
t"
t"
were often f o u n d in such the reflection m e a s u r e m e n t as was e m p l o y e d in this work, ~ the b a n d s at 630 a n d 450cm -1 m a y be a t t r i b u t e d to FesO 4 a n d FeO, respectively. The spectra o f lattice v i b r a t i o n o f b u l k substances, however, can n o t safely be consulted for identifying c o m p o u n d s in thin film, because crystal structures o f c o m p o u n d s in thin film m a y be different from those in b u l k state. F u r t h e r investigations are required, before F e O a n d Fe304 are c o n c l u d e d as m a i n constituent o f the semic o n d u c t o r films m e n t i o n e d above. The detailed results and discussion, as well as the results o b t a i n e d a b o u t the surface films on F e oxidized in a basic or an acidic solution, will shortly be r e p o r t e d . REFERENCES 1. M. N A G A Y ~ and M. COHEN,J. electrochera. Soc. 109, 781 (1962). 2. H. EmKO, N. TAaATAand W. SU~TAKA,J. spectrosc. Soc. Japan 18, 275 (1969).
SHORT COMMUNICATION INFRA-RED SPECTRA OF SURFACE FILMS ON Fe ANODICALLY OXIDIZED IN A NEUTRAL SOLUTION* H. EBIKO and W. SUi~TAKA The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal, Japan THE COMPOUNDS constituting the surface films on electrochemically passivated Fe have been investigated by means of electrochemical and electron optical techniques, but the obtained results have not been in general agreement. Infra-red spectroscopy can be considered as a potential tool for studying the surface films, but no paper in this field of research has appeared in the literature as far as the authors are aware. In the present communication, the infra-red spectra of surface films on anodieaUy oxidized Fe will briefly be reported and discussed. Eleetropolished sheets (25 x 50mm) of high purity Fe (kindly supplied by Battelle Memorial Institute) were potentiostatically oxidized at 0 or 800mV for 4h in a boric acid-borax aqueous solution. A Hg/HgzCbJ3.3N KC1 half cell was used as the reference electrode. The boric acidborax aqueous solution was prepared after the procedure proposed by Nagayama e t al. 1 and its pH was 8.42 at 25 4- I°C. The infra-red spectra were obtained by means of the optical system described in a previous paper, z After anodic oxidation the specimens were immediately washed with a jet of methanol, dried in v a c u o , and mounted on the sample holder of the speetrophotometer. After measurement of the spectrum, the specimens were further washed with a jet of water so as to remove substances soluble in water, rinsed in methanol and dried in v a c u o . The spectra of the specimens was then re-determined. The important parts of the obtained spectra are listed in Table 1, in which letters used have their usual meanings. The absorption bands appearing at 1260, 1105, 860 and 815cm -1, which disappeared on washing with water and changed equally in intensity by changing the specimen, can be assigned to a complex borate ion. These bands did not correspond to the absorptions of borax, other sodium borates nor bode acid, and may be therefore attributed to a complex borate of Fe. The continuous absorptions in the high wavenumber region indicate the surface films contain compounds which are semi-conductive in nature. The absorption bands at 630 and 450cm -1 arise probably from the lattice vibrations of the semi-conductors. In changing the oxidizing potential from 0 to 800mV, the relative intensity of the band at 450cm -x decreased whereas the band at 630cm -x increased in intensity. This fact suggests that the semi-conductor having the absorption band at 450cm -~ would be a compound of Fe in a low oxidation state and that of 630cm -~ a compound of Fe in a high oxidation state. Since FeD has a strong absorption band at 410cm -x and FenD4 at 570cm -~ and the shift of strong absorption bands to higher wavenumbers *Manuscript received 11 August 1969. 111
112
H . EBIKO a n d W . SU~"rAKA TABLE 1.
INFRA-RED ABSORPTION MAXIMA OF SURFACE FILMS ON F e PASSIVATED IN BORIC ACID--BORAX SOLUTION
Applied potential OmV
800mV
Washed with methanol water cm-' 450 Sb 640 ?
c m -~
450 Sb 630 m 815 m 860 sh 1105 m 1260 m 1340 wb *Continuous absorption.
Washed with methanol water
,
em-x 450 mb 635 rn 820 S 855 sh ,
1105 S 1262 S
cm-1
450 mb 630 rn
t"
t"
were often f o u n d in such the reflection m e a s u r e m e n t as was e m p l o y e d in this work, ~ the b a n d s at 630 a n d 450cm -1 m a y be a t t r i b u t e d to FesO 4 a n d FeO, respectively. The spectra o f lattice v i b r a t i o n o f b u l k substances, however, can n o t safely be consulted for identifying c o m p o u n d s in thin film, because crystal structures o f c o m p o u n d s in thin film m a y be different from those in b u l k state. F u r t h e r investigations are required, before F e O a n d Fe304 are c o n c l u d e d as m a i n constituent o f the semic o n d u c t o r films m e n t i o n e d above. The detailed results and discussion, as well as the results o b t a i n e d a b o u t the surface films on F e oxidized in a basic or an acidic solution, will shortly be r e p o r t e d . REFERENCES 1. M. N A G A Y ~ and M. COHEN,J. electrochera. Soc. 109, 781 (1962). 2. H. EmKO, N. TAaATAand W. SU~TAKA,J. spectrosc. Soc. Japan 18, 275 (1969).