Langmuir 1987, 3, 905-910
905
Photoelectrochemical Studies of Passive Films on Zirconium and Amorphous Iron-Zirconium Alloyst A. R. Newmark and U. Stimming* Electrochemistry Laboratory, Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027 Received September 4,1986. In Final Form: December 16, 1986 The photoelectrochemicalproperties of passive films on zirconium of varying thicknesses and on amorphous ironzirconium alloys of varying compositions have been investigated. Photocurrents were measured as a function of wavelength and potential. A tailing of the photocurrent is observed in photocurrent spectra down to photon energies well below the band-gap energy. From photocurrent spectra, band-gap energies of 4.8 and 3.3 eV for the passive films on Zr and Fe-Zr alloys, respectively, have been determined. The spectra together with the potential dependence of the photocurrent imply that localized states play an important role in the photoexcitation process of these films. Cathodic photocurrents at potentials above the flat-band potential are observed on the thinnest passive films formed on zirconium. Reverse tunneling is discussed as a possible explanation for this, which emphasizes the importance of localized states in thin passive films. Introduction Photoelectrochemistry has been shown to be a useful in situ technique for studying the optical and electronic properties of passive films.' From a detailed analysis, some conclusions on structural properties are possible as well. In particular, the use of photoelectrochemistry in differentiating between crystalline and noncrystalline f i s is actively being p u r ~ u e d . Due ~ ~ ~to the large number of localized states in amorphous and highly disordered materials, their photoresponses usually extend to much lower photon energies than those of their crystalline counterparts. The potential dependence of the photocurrent observed in noncrystalline films is usually in clear contrast to the relationship found for crystalline films. The study of passive films on glassy metals has recently become of interest, due to their increased resistance to corrosion, in some casesa4 For these reasons, the passive films formed on glassy Fe-Zr alloys are being investigated by photoelectrochemical techniques. In order to compare the photoelectrochemical behavior of the films formed on these alloys to that on the pure metal, the passive films formed on zirconium are also being studied. Photoelectrochemical data already exist for the passive films formed on iron.CharlesbyloJ1 has studied passive films on zirconium under dark conditions and under illumination with ultraviolet light. From electron diffraction studies Charlesby found that the thin oxide films formed on zirconium are largely amorphous in nature. Charlesby also found a strong dependence of the photocurrent on the field, which Vermilyea12attributed to the lowering of an escape barrier of an electron trapped in a localized electronic state. This effect of an electron surmounting a barrier which has been lowered by the electric field is known as the Poole-Frenkel effect. Model calculations for a photoexcitation involving localized states allowing for the Poole-Frenkel effect, as well as tunneling of the electron through the barrier, have been carried out.13J4 The result is an exponential dependence of the photocurrent on the square root of the electric field, for intermediate fields (104_106 V/cm) where the Poole-Frenkel process dominates. An enhancement of the photocurrent due to tunneling is found for higher fields (>lo5V/cm). Presented at the symposium on "Corrosion", 191st National Meeting of the American Chemical Society, New York, NY, April 13-18, 1986.
A noncrystalline structure of the passive films formed on the glassy ironzirconium alloys may be expected since the alloys themselves are amorphous. A noncrystallinity of the passive f i i formed on zirconium is also not unlikely for thin films, since they usually lack long-range order normal to the surface. By photoelectrochemical techniques the properties of passive films formed on both of these materials have been studied. Experimental Section Zirconium samples were cut from zirconium (Aesar 99.8%)foil of either 0.1- or 1-mm thickness and etched in a solution of nitric and hydrofluoric acids. They were then rinsed in water and allowed to air dry before being mounted in their holder. A Teflon electrode holder, which exposed one side of the sample to the electrolyte, was used most of the time. Due to the exposure to air during drying, a thin oxide film was probably present on the surface of the sample prior to anodic oxidation. Amorphous ironzirconium alloys of varying composition were obtained from the National Bureau of Mines where they were prepared by sputtering onto 1-mm-thicktitanium substrates. The alloy films ranged from 5 to 7 wm in thickness. A detailed description of the preparation of these samples is given elsewhere.15 The working electrode when mounted in its holder had an exposed area of 0.5 cm2. A thin gold foil was used as the counter electrode and the reference electrode was mercury/mercurous sulfate/O.l M Na2S04,but all potentials are given with respect to the normal hydrogen electrode (NHE). (1) Stimming, U. Electrochim. Acta 1986, 31, 415. (2) Leitner, K.; Schultze, J. W.; Stimming, U. J.Electrochem. SOC. 1986,133, 1561. (3) Danzfuss, B.; Stimming, U. J. Electroanal. Chem. 1984, 164, 89. (4) Hashimoto, K. In Passivity of Metals and Semiconductors; Froment, M., Ed.; Elsevier: Amsterdam, 1983; p 235 and references therein. (5) Wilhelm, S. M.; Hackerman, N. J.Electrochem. SOC.1981, 126, 1668. (6) Abrantes, L. M.; Peter, L. M. J. ElectroanaL SOC.1983,150,593. (7) Froelicher, M.; Hugot-LeGoff, A.; Jovancicevic, V. In Passiuity of Metals and Semiconductors; Froment, M., Ed.; Elsevier: Amsterdam, 1983; p 491. (8)Stimming, U. In Passiuity of Metals and Semiconductors; Froment, M., Ed.; Elsevier: Amsterdam, 1983; p 477. (9) Searson, P.; Stimming, U.; Latanision, R.M. In Surfaces, Znhibition, and Passivation; McCafferty, E., Brodd, R. J., Eds.; The Electrochem. SOC.:Princeton, NJ, 1986; p 175. (10) Charlesby, A. Acta Metall. 1953, 1, 340. (11) Charlesby, A. Acta Metall. 1953, 1, 348. (12) Vermilyea, D. A. Acta Metall. 1954,2, 346. (13) Stimming, U.; Newmark, A. R.Proceedings of TELAVZ'84 Conference, Telavi, U.S.S.R.,1984, in press. Newmark, A. R.; Stimming, U. Electrochim. Acta, in press. (14) Newmark, A. R.;Stimming, U. J. Electroanal. Chem. 1986,204, 197. (15) McCormick, L. D.; Wheeler, N. S.; Molock, C. R.; Chien, C. L. J. Electrochem. SOC.1984, 131, 530.
0743-7463/81/2403-0905$01.50/00 1987 American Chemical Society
906 Langmuir, Vol. 3,No. 6, 1987
Newmark and Stimming
too,
nl
Zr in 1 M No,SO,
--
0.2rd/cm2
l.Od/cm* 2.0rd/cm2 (II
I
-
N
*
6
0
2E
X
v
F"
Q
I ' d
,
I
0
4
8
12
U (V vs nhe) Figure 1. Charge density as a function of electrode potential for the f i i formation on Zr in 1M NaZSO4at various current densities.
Figure 2. Photocurrent spectra for films formed on Zr in 1 M Na2SO4at Vox= 3 V at various electrode potentials.
Experiments were performed in either 1 M NazSOl or 1 M NaClO, as stated and Nz was continuously bubbled through the solution to maintain an oxygen-free atmosphere. All solutions were prepared with high-purity water (Millipore, 16 MQ). Film Formation. All of the photoelectrochemicalexperiments performed with Zr were carried out on films which were formed potentiostatically by polarizing the electrode at an oxidation potential, Vox,for either 10 or 40 min, depending on the experiment. To study the film formation,however, galvanostatic experiments were performed, where the voltage increase was recorded with time. Most of the results obtained on the Fe-Zr alloys were on preexisting air-formed films. To compare the air-formed films to ones formed electrochemically, etching experiments were performed. The sample, mounted in the Teflon holder, was etched in a solution of the same composition as that used for the Zr. The etching was done very rapidly so as not to destroy the alloy but only remove the air-formed film. After it was rinsed well with water, the electrode was placed in the cell and was polarized at -0.5 V for 10 min. (The current was approximately 0.1 mA/cm2.) Photoelectrochemical Apparatus and Experiments. A 1OOO-W xenon lamp was used as the light source (Oriel) together with a monochromator (Instruments SA,H10). Experimentswere usually conducted in a range of wavelengths between 180 and 800 nm. In most cases an exit slit of 0.5 mm was used on the monochromator,giving a band width of A4 nm at each wavelength. The light was then passed through a light chopper (0.3 Hz)and focused through a quartz window in the cell onto the working electrode. Chopped light enables currents under dark and illuminated conditions to be recorded, the difference being the photocurrent. Photocurrents were measured as a function of wavelength at various electrode potentialsand also as a function of electrode potential at given wavelengths. The variation of light intensity with wavelength was detected by using a thermopile and is accounted for in the results presented by conversion of the photocurrent, iPb,into a quantum efficiency, q , which relates to the incident number of photons.
the voltage increase was recorded with time. This was done for 0.2, 1.0, and 2.0 mA/cm2, and a linear relationship was obtained between the charge and the electrode potential in each case, as shown in Figure 1. This implies that a field-induced mechanism governs the growth of the film. Assuming that the film formed is zirconium dioxide and the roughness factor of the etched electrode is 2,16a growth rate of about 2.5 nm/V is found at the lowest current density. Similar growth rates have been found for the passive film on zirconium by other investigators.10J7 Figure 2 shows photocurrent spectra obtained a t various potentials for a film formed on zirconium in 1M Na2S04 at an oxidation potential of 3 V. A sharp increase in the photocurrent is observed at about 5 eV and a tailing exists down to a little below 3 eV. This type of spectral behavior was also exhibited by films formed on zirconium at 6 and 10 V. The potentials shown in Figure 2 range from 2 to -0.4 V. Potentials down to -1.1 V were studied as well and similar spectra were obtained at the higher photon energies. At the lower photon energies, however, cathodic photocurrents were observed, as shown in Figure 3. The onset of the cathodic photocurrent is at higher photon energies for lower potentials. For example, for U = -1.1 V the photocurrent becomes cathodic a t about 4.9 eV, whereas for U = -0.6 V cathodic photocurrents are first observed at about 4.4 eV. In Figure 4 spectra are shown for air-formed oxide films on Fe33Zr67in 1 M NaC104 and FeS0Zrmin 1 M Na2S04. The spectrum of the film formed on the FeSZrs7 sample seem to exhibit a very similar spectral behavior as the film on the Fe&rm sample. The increase in the photocurrent is much smoother on these films as compared to the films formed on zirconium and the tailing continues down to just above 2 eV. No influence has been found regarding the
Results To study the film formation on zirconium, a constant current was applied to the electrode in 1M Na2S04and
(16) Schultze, J. W.; Stimming, U.; Weise, J. Ber. Bunsenges. Phys. Chem. 1982,86, 276. (17) Cox, B. J.Electrochem. SOC.1970, 117,654.
hv /eV
Langmuir, Vol. 3, No. 6, 1987 907 200,
Fe,
-+
in 1M Na,SO,
Zr,
/
before etching first time etched second time etched third time etched
/ /
-0.od
I
3
4
5
hu /eV Figure 3. Photocurrent spectra for the same film formed on Zr as shown in Figure 2 but for lower electrodepotentials and at lower
photon energies.
A
Fe, Fe,
Zr, Zr,
in 1M Na,SO, in 1M NaClO,
I
hu /eV Figure 4. Photocurrent spectra recorded at U = 1.3 V for films formed on FewZrwin 1M Na2S04and Fea3Zrs7in 1 M NaC104.
O : - and C104-. difference in the anions S Passive films could not be formed electrochemically on the Fe-Zr amorphous alloys due to the preexisting airformed f i i . An experiment was performed on one sample to see the effect of etching and compare the photoelectrochemical behavior of an electrochemically formed film to the existing film. A sample of the amorphous iron-
2
3
4
5
hv /eV Figure 5. Photocurrent spectra recorded at U = 1.3 V before etching and for three subsequent etching processes for films formed on FewZrwin 1 M Na2S04.
zirconium alloy of composition 5050 in 1M Na2SOl was studied. A spectrum was recorded on the air-formed film a t U = 1.3 V. After the sample was etched and reduced, as described earlier, cyclic voltammetry was performed, increasing the upper limit with each sweep to study the oxide formation. The electrode was then polarized a t U = 1.3 V for 10 min and another spectrum was recorded a t U = 1.3 V. This process was repeated 3 more times. Figure 5 is a plot of the quantum efficiency vs. the photon energy for the air-formed film and three of the four subsequent electrochemically formed f i s . After the first two etching and film formation processes the photocurrent decreased, but after the third process it was more or less the same magnitude as the time before. This decrease in the photocurrent was probably due to incomplete removal of the air-formed film. After the first two etching procedures, there was probably still some oxide on the surface, thus a thicker film giving rise to a larger photocurrent. An important aspect of these spectra is that their qualitative behavior is the same. Similar values for the band-gap energies were obtained from all four spectra, using eq 1, which will be discussed later. Thus the airformed films and the electrochemically formed ones seem to exhibit the same photoelectrochemical response. The charge acquired due to the anodic polarization of the electrode seemed to be fairly low. For the sample etched and reduced for the third time, the charge due to oxide formation was only about 3 mC/cm2, corresponding to a growth rate of about 1.1nm/V, assuming the film density is 5.5 g/cm3 (an average of the oxides formed on iron and zirconium) and a roughness factor of 1.5. Such a growth rate seems to be low for the rate of formation of passive films. Two possible explanations for this are that either the parameters chosen to obtain this value are inappropriate or there was still some oxide left on the surface prior to polarization. An interesting result came out of these etching experiments. After the etching procedure was repeated a fourth
Newmark and Stimming
908 Langmuir, Vol. 3, No. 6, 1987
30,
I
Fe,
Zr,
in 1M Na,SO,
Zr in 1M Na,SO,
U,, = 6V
D
/ m
m
third time etched fourth time etched the difference
0
dm
/
m
5
-
3
-
1
-
0
/o
X
i=u
h v /eV Figure 6. Photocurrent spectra for the same film as in Figure 5 after etching a third and fourth time and the difference spectrum.
time, a second hump appeared in the spectrum. By comparing this last spectrum with the one recorded previously and taking the difference, another spectrum was obtained, as shown in Figure 6. The evaluation of the band-gap energy (see below) of the new spectrum yielded a value of 3.5 eV. This value i s very close to those obtained for the passive fiims formed on titaniumS2Since titanium is the substrate of the Fe-Zr alloy, pinholes probably developed after the fourth etching procedure and when the electrode was oxidized so was the exposed titanium. Since the photoelectrochemical response is an average over the electrode surface, the response of the passive film formed on Ti and on the Fe-Zr alloy was observed. Thus the formation of pinholes was detected by the change in the photoelectrochemical behavior of the films.
Discussion Spectral Dependence. The absorption of a photon by a crystalline semiconducting material is governed by selection rules which characterize the type of electronic transition that occurs. The following equation has been used to describe the absorption behavior of thin films, where the absorption coefficient can be assumed to be proportional to the photocurrent? i,hhV
0:
(hv -
(1)
where Eg is the band-gap energy and n describes the type of transition. For direct transitions n = ' I 2and for indirect transitions n = 2.18 Equation 1 has also been observed for amorphous materials as an empirical relationship (the selection rules are not believed to still hold19) with n = 2 being observed most often. The value of E,, which would be the intercept of a plot of (iphhV)'lnvs. hv, then becomes (18)Johnson, E. J. In Semiconductors and Semimetals; Willardson, R. K., Beer, A. C., Eds.; Academic: New York, 1967;Vol. 3. (19)Mott,N. F.;Davis, E. A. Electronic Processes in Noncrystalline Solids; Clarendon: Oxford, 1979.
hv /eV Figure 7. Plot according to eq 1 with n = 2 for a film formed on Zr in 1M Na2S04at Vox= 6 V at various electrode potentials.
somewhat ambiguous for amorphous materials, which are characterized by large numbers of localized states within their band gap. The observance of eq 1 for amorphous materials has been attributed to the change of the density of states function with energy a t the band edges.20 A plot according to eq 1with n = 2 is shown in Figure 7 for a film formed a t U,, = 6 V on zirconium for various potentials, U. The intercept gives a value of about 4.8 eV, which is slightly lower than the value of the band-gap energy of 5 eV found by Clechet et al. from photoelectrochemical data.21 Similar values of the intercept were obtained for the films formed on zirconium a t U,, = 10 V, for all potentials investigated, and at U,, = 3 V, for the higher potentials. Plots according to eq 1with n = 'I2 did not yield straight lines. For the lower potentials of the film formed at U,, = 3 V, a potential-dependent intercept was found. The lower the potential, the higher the value of the intercept, with the highest value being 5.0 eV for U = -1.1 V. This remarkable change in the intercept with potential is possibly due to a potential-dependent contribution of a cathodic photocurrent to the overall photocurrent. As was seen in Figure 3, a cathodic photocurrent is observed, which is larger a t lower potentials. The cathodic photocurrent detracts from the anodic photocurrent, giving a lower net photocurrent. This observation of a cathodic photocurrent a t potentials above the flatband potential can be explained by a reverse tunneling of photoexcited electrons against the field from localized states to the electrolytez2 For a given potential, the lower the photoh energy the larger the impact of the reversetunneling process on the net photocurrent, thus effecting the intercept of the square root plot. If a reverse-tunneling process is in fact occurring, then the assumption that the (20)Tauc, J. In Amorphous and Liquid Semiconductors; Tauc, J., Ed.; Plenum: New York, 1974. (21)Clechet, P.;Martin, J.; Oliver, R.; Vallouy, C. C. R. Acad. Sci. C 1976,282,887. (22)Stimming, U.Langmuir 1987,3, 423.
Langmuir, Vol. 3, No. 6, 1987 909
Photoelectrochemical Studies of Passive Films
Fe, Zr, ., Fe, Zr,
in 1M Na,SO, in 1M NaCIO, E
0 A
A
m
A 0
. A
0
5
hu /eV Figure 8. Plot accordingto eq 1with n = 2, using the data shown
in Figure 4.
Figure 9. Poole-Frenkel plot as described by eq 2 for films of varying thicknessesformed on Zr, at various photon energies; the absolute value sign of 7 is used to represent anodic and cathodic
photocurrents. absorption coefficient is proportional to the photocurrent may not be valid anymore under those conditions. This stants of the material and free space, respectively. The phenomenon of reverse tunneling is discussed in more Poole-Frenkel process and the probability of electron detail elsewhere.22 tunneling have been developed in the solid-state literaThe data for the films on both compositions of the Fe-Zr t ~ r e ~and " ~applied ~ to the photoexcitation process in alloys were also evaluated by using eq 1. Straight lines amorphous passive films.14 There have been various syswere obtained with n = 2 for both curves, giving the same tems where it has been found to apply, for instance, in intercept of about 3.3 eV, as shown in Figure 8. This value ion-implanted hafniumz4and titanium%passive films, on is in between that found for the passive film formed on thin passive films on titanium: and on irong and on bulk zirconium (4.8 eV) and on iron (at 1.9 e v ) , suggesting that amorphous iron-titanium oxide^.^ the film formed is neither an iron oxide nor a zirconium The electric field across a passive film is equal to the oxide. It is conceivable, however, to draw two other potential drop across the f i i , U - U,, divided by the film straight lines for each composition film giving two more thickness for insulating films or the depletion layer intercepts in the vicinity of 4.5 and 2 eV. It is interesting thickness a t a given potential for semiconducting films, to note that the value of these two intercepts roughly where U, is the flat-band potential. Figure 9 shows a corresponds to the values of the band-gap energies found Poole-Frenkel plot for films formed on zirconium a t oxfor the passive films formed on pure iron and on pure idation potentials of 3, 6, and 10 V a t various photon zirconium. energies, assuming U, = -1.5 V29for all films and thickPotential Dependence. The potential dependence of nesses of 11,19, and 29 nm, respectively. For a given field the photocurrent in noncrystalline passive films has been across the film and a given photon energy, the photocurattributed to the Poole-Frenkel e f f e ~ t , ~ .which ~ ~ , is ~ ~ , ~rent ~ , should ~ ~ be the same for all thicknesses, since the film an escape mechanism for an electron trapped in a potential thickness is incorporated into the field. As can be seen well. The photoexcitation process in amorphous materials from Figure 9 this holds true for the thicker films of 19involves localized states, which act as traps for the phoand 29-nm thickness, but the quantum efficiency is less toexcited electron. The photocurrent is dependent upon for the 11-nm film for all photon energies. This may be the escape of the electron from the localized state, and this due to a reverse-tunneling process, as described earlier, process is strongly affected by the electric field, which which tends to lower the net photocurrent to such an exlowers the escape barrier. The equation for the field detent that a t very low fields cathodic photocurrents are pendence of the photocurrent based on the Poole-Frenkel observed. This can be seen in Figure 9 by the rise of 171 effect is the following:
iph = const exp(@/2/k!l')
(2)
where p = ( e 3 / ~ c ~ o and ) 1 / 2E and eo are the dielectric con(23) DiQuarto, F.; Russo, G.; Sunseri, C.; DiPaola, A. J . Chem. SOC., Faraday Trans. 1 1982 78, 3433. (24) Danzfuss, B.; Schultze, J. W.; Stimming, U. Mat. Sci. Eng. 1985, 69, 213.
(25) Hill, R. M. Philos. Magn. 1971, 23, 59. (26) Vincent, G.; Chantre, A.; Bois, D. J. Appl. Phys. 1979,50,5484. (27) Martin, P. A.; Streetman, B. G.; Hess, K. J. Appl. Phys. 1981,52, 7409. (28) Danzfuss, B.; Schultze, J. W.; Stimming, U. In Principles of Electrode Reactions; Schultze, J. W., Ed.; Dechema Monographs Vol. 102, Frankfurt, 1986; p 465. (29) Kung, H. H.; Jarrett, H. S.; Sleight, A. W.; Ferretti, A. J. Appl. Phys. 1977,48, 2463.
Langmuir 1987, 3, 910-916
910
Fe,
themselves, as has been assumed for the passive films on titanium under certain condition^.^^^^ This can possibly explain why the Poole-Frenkel behavior still holds at photon energies well above the band gap energy (4.8eV), as found from the intercept of Figure 7. This again brings up the question of the meaning of the band-gap energy as determined from plots according to eq 1for noncrystalline materials. Poole-Frenkel behavior has also been observed on the film formed on FemZrmas shown in Figure 10 for photon energies of 3.4 and 4.4 eV, arbitrarily assuming a flat-band potential of 0 V. These results together with the spectral dependence of these films suggest that they too are highly disordered and most probably amorphous, given that their substrate is amorphous.
in 1M Na,SO,
Zr,
hu /eV m
4.4
/-
/ I
0.5
1.o
(u-u,b)I/* /v
1.5 'I2
Figure 10. Poole-Frenkel plot for the air formed oxide film on FemZrsoat hv = 4.4 and 3.4 eV. at low potentials. Another possible explanation is that in thin films all of the light may not be absorbed by the passive film, but some may be reflected at the metal-oxide interface. The Poole-Frenkel effect as applied to passive films is dependent upon the involvement of localized states. From the spectral behavior of the passive films formed on zirconium, it was thought that localized states were involved in the photoexcitation process within these films. By studying the potential dependence and observing the Poole-Frenkel behavior, it seems that localized states do exist, implying that these films are highly disordered. Localized states exist most often within the so-called band gap but can also arise from the localization of the bands
Conclusions By use of photoelectrochemical techniques, the electronic and structural properties of the passive fiis formed on zirconium and on iron-zirconium amorphous alloys have been investigated. From their spectral behavior, together with their potential dependence, in particular their Poole-Frenkel behavior, it can be suggested that these passive films are a t least highly disordered or amorphous. The observance of cathodic photocurrents on films of 11-nm thickness on zirconium suggests that a reverse-tunneling process takes place and illustrates the importance of localized states in the photoexcitation process. By further exploration of the electron-transfer processes, for example, the possibility of reverse tunneling as a competing process to the Poole-Frenkel and forward tunneling processes for sub-band-gap photoresponses, the understanding of absorption in noncrystalline passive fiis can be expanded. Acknowledgment. The amorphous Fe-Zr alloy samples were prepared by Dr. L. McCormick, National Bureau of Mines, Avondale. The discussions with N. Wheeler and Dr. L. McCormick about the properties of the Fe-Zr alloys are greatly appreciated. Financial support by the National Science Foundation and Columbia University is gratefully acknowledged. Registry No. Zr, 7440-67-7; FeS3Zr6,,70162-67-3;FemZrm, 77506-55-9; Na2S04,7757-82-6;NaClO,, 7601-89-0.
Effect of Metalloid Elements on Passivity of Glassy Metalst M. Janik-Czachor Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland Received June 25, 1986. I n Final Form: September 30, 1986 In the group of glassy alloys consisting of late transition metals (Fe, Co, Ni) and metalloids (C, B, Si, P) the latter play the role of glass stabilizers. Their required content is about 20 atom % , and their effect on corrosion behavior of the glasses is distinct. In this paper the influence of the metalloids on the anodic behavior of glasses is reviewed. The following aspects are discussed: ability to passivate; dissolution within the passive region; composition of passivating films; stability of the passive state.
Introduction Most metals used in practice and suffering from corrosion problems have a well-defined structure; i.e., the relative disposition of atoms is ordered and regular over a long 'Presented a t the Symposium on "Corrosion", 191st National Meeting of T h e American Chemical Society, New York, NY, April 13-18, 1986.
0743-7463/87/2403-0910$01.50/0
range. Some metals, however, can be solidified in such a way that the disordered atomic structure characteristic of the liquid state is preserved. Such a rigid, but disordered, material is called a glass. In principle any material should produce a glass if the gap between a material's freezing point and its glass transition temperature is bridged faster than the time required for crystallization to occur. In the case of metals the quench rates required for glass formation 0 1987 American Chemical Society
