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
Passivity of Glassy Metals
Langmuir, Vol. 3, No. 6, 1987 911
0.1 N H2S04 (Hashimoto e t a1 )
c L
X
E ' "SCE
Figure 2. Potentidynamic polarizationw e s for Fe70Crl$13X7, X = B, Si, C, P, alloy. d E l d t = 8.5 VIh.4
Figure 1. Effect of substituting 7 atom % of C, Si, or P for B in a Fe70Crl,,Bmalloy on the average corrosion rate.4
are very high 1010-106 K/s.'s2 Certain metallic alloys have rather lower critical cooling rates; especially the late transition metal (Fe, Ni, Co)metalloid (B, P, Si, C) alloys.2 In these alloys the metalloids play the role of glass stabilizers. Their required content is large, about 20 atom %. This requirement causes a certain difficulty; namely, studying this group of metallic glasses we are able to change the degree of substitution a t a constant total content of the metalloids in the glass but not to investigate the effect of one specific metalloid as an alloying element. This is presumably why there is no concensus concerning the effect of various metalloids on corrosion properties of metallic glasses, although the existing experimental data suggest that this effect is large and related to both the anodic and cathodic Hashimoto4was probably the first to recognize the effect of metalloids, his measurements of an average corrosion rate of Fe70Cr,J313X7glasses, with X = C, P, Si, or B have shown that P is most beneficial, reducing the corrosion rate up to 20X (Figure 1). The effect was then studied by several author^.^*"^ An excellent review of work performance up to 1983 has been published by Diegle et al.3 The scope of the present paper is limited; only an influence of metalloids on the anodic behavior of glassy metals is examined. The following aspects of the problem are discussed ability to passivate; dissolution rate in the passive region; composition of passivating films; stability of the passive state.
Characteristics of the Materials Most of the corrosion data available and reviewed here concern glassy ribbons produced by melt spinning. A common factor in all modifications of these rapid quenching techniques is that the product is inevitably very thin (typically 50 pm), a prerequisite if the cooling rates of -lo6 K / s required for glass formation are to be achieved. (1)Davies, H. A. In Rapidly Quenched Metals ZZ& Cantor, B., Ed.; The Metal Society: London, 1978; Vols. 1-21. (2)Roper, G.W. Surf. Eng. 1985,1, 289. (3)Diegle, R. B.;Sorensen, N. R.; Tsuru, T.; Latanision, R. M. In Treatice on Materials Science and Technology; Scully, J. C., Ed.; Academic: New York, 1983;p 80. (4)Hashimoto, K.;Naka, M.; Noguchi, J.; Asami, K.; Masumoto, T. In Passtuity of Metals; Frankethal, R. P., Kruger, J., Eds.; The Electrochemical Society: London, 1978;p 156. (5) Janik-Czachor, M. h o c . 9th ZCMC, Toronto; 1984; p 234. (6)Janik-Czachor, M. Werkst. Korros. 1985,36,441. (7)Kovacs, P.;Farkas, J.; Takacs, L.; Awad, M. Z.;Vertes, A.; Kiss, L.; Lovas, A. J. Electrochem. SOC. 1982,129,695. (8)Moffat, T. P.; Flanagan, W. F.; Lichter, B. D.; Proc. 9th ZCMC, Toronto; 1984, p 454.
Figure 3. Quasi-stationary polarization curves for Fe70Si5X16, X = B or P.l8
It should be recognized, however, that these cooling rates still do not provide an ideally homogeneous material. The liquid-quenched glassy ribbons may exhibit an inhomogeneity due to a cooling rate profile within the sample. Some chemical seggregation is possible and microstructural order may vary within the sample. Differences in compositiong and consequently in the corrosion behaviorgt1 have sometimes been observed between the shiny and dull side of the liquid-quenched ribbons. Higher quenching rates, e.g., those attainable during vapor quenching, 10" K/s, eliminate the diffusion-related inhomogeneities. However, corrosion rate data concerning these materials are ~ c a n t y . ~ @ J ~ The liquid-quenched samples have a fairly rough surface.12J3 The roughness coefficient carefully measured with BET is typically 10-30.14J5 In contrast, during vapor quenching a very smooth, uniform surface can be obtained.17
-
Ability To Passivate Cr-containing glasses exhibit an excellent ability to passivate even under very severe condition^,"^ but it may (9)Devine, T.M. J. Electrochem. Soc. 1977,124,38. (10)Kapusta, 5.; Heusler, K. E. Z. Metallk. 1981,72,785. (11)Kiss, L.; Kovacs, N.; Farkas, J.; Lovas, A. Zashch. Met. 1982,28,
_--. 1Q?.
(12)Plieth, W.; Linker, U. Werkst. Korros. 1983,34,391. (13) Janik-Czachor, M.; Mazurkiewicz, B. Corrosion 1978,43, 194. (14)Yokoyama, A.; Komiyama, H.; Inoue, H.; Masumoto, T.; Kimura, H. M. J. Catal. 1981,68,355;Scr. Metall. 1981, 15,365. (15) Schay, Z.,private communication. (16)Diegle, R. B.;Merz, M. D. J. Electrochem. SOC. 1980,127,2030. (17)Williams, R. M.; Thakoor, A. P.; Khoma, S. K.; Johnson, W. L. J . Electrochem. SOC. 1984,131, 2791.
Janik- Czachor
912 Langmuir, Vol. 3, No. 6, 1987 Borate buffer oFe B
3 00
pH:U -400.
75 25-xsix F@79B16s'5
?
250
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borate butter pH.8.4
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9
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,
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Figure 6. Effect of substituting Si or P for B in Fe base glassy alloys on critical passivation potential, Ecr.
X Yo
Figure 4. Effect of substituting Si or P for B in Fe base glassy alloys on the critical cd for passivation, Icr. still be altered by the presence of various metalloids. Figure 2 shows some examples of potentiodynamic polarization curves for Fe,0Crl,,B13X7measured by Hashimoto et aL4 One should note that the apparent critical current density (cd) of passivation is by an order of magnitude lower for the glass with X = P than for the other alloys. Also, the cd within the active region is the lowest for X = P, and E , is shifted in a noble direction. These results suggest that the effect of partial substitution of P for B is, in fact, to decrease the active dissolution rate (although the authors interpretation is different) and to increase the ability of the glass to passivate. The conclusion is in agreement with the results by Moffat et al.? Janik-Czachor and Wishska,'* Codet e t al.,19 and Kr61ikowski.20 When discussing potentiodynamic measurements for a multicomponent alloy, we have to bear in mind that the results may be affected by a transient selective dissolution of the most electronegative component,l0P2lresulting in a dealloying of the electrode surface layer. In the steady state the glassy alloys dissolve according to their bulk composition~~'3~z1although their surface composition may differ from the bulk. Therefore, it was of interest to compare the effect of metalloids in quasistationary conditions. An example is given in Figure 3 for two simple Fe base alloys which generally exhibit poor corrosion resistance. The results for Fe75X16Si5with X = B or P confirm the above-mentioned conclusion concerning the effect of P. The slopes and positions of the cathodic and anodic polarization curves indicate in particular that corrosion cd is distinctly lower for P-substituted alloy. Figure 4 shows the critical cd of passivation, I,,, for the same two alloys as compared with data for a series of Fe-B-Si glasses, at various degree of substitution of Si for B. The results indicate that substituting P or Si for B reduces I,,, but, the effect of P is substantially larger. Figure 5 summarizes the effect of substituting Si or P for B in Fe-BSi glasses on critical potential of passivation, E,. Substitution of Si lowers E,. Substitution of P makes E,, more noble, but, as seen from Figure 3, the dissolution (18) Janik-Czachor, M.; Wislawska, M.; manuscript in preparation. (19) Cadet, P.; Keddam, M.; Takenouti, H. In Passivity of Metals and Semiconductors; Froment, M., Ed.; Elsevier: Amsterdam, 1983; p 311. (20) Kr6likowski, A. 36th Meeting of ISE, Salamanca; 1985. (21) Heusler, K. E.; Huerta, D. R o c . 9th ICMC, Toronto; 1984; p 222.
1
5
6
7 PH
8
9
Figure 6. Effect of solution pH and substitution of Si or P for B in Fe-Ni base glassy alloys on critical passivation potential, Ecr. ( 0 )The data points evaluated from ref 12. 0.5 M 50; IHeusier et aI.1
N
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y
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-
2
l
5 yl
0 a
-
1 10
0
10
0
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3
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Figure 7. Effect of solution pH on cd within the passive region, I,,,, for FeBoBzo and Fe40Ni40B20.'0
rate is still much lower than that of B-containing alloy. The above effects for simple Fe base alloys could only be studied at pH 28. In neutral and acidic solutions these alloys do not undergo a stable passivationzz and a severe active dissolution destroys the sample in the course of an experiment. (The validity of this statement has been checked also with very small samples when the total anodic current passing was far below the maximum output current of the potentiostate.) On the other hand, Fe-Ni base (22) Janik-Czachor, M. Werkst. Korros. 1983, 34, 451.
Passivity of Glassy Metals
Langmuir, Vol. 3, No. 6, 1987 913
borate buffer pHz8.4
k7?25-XSiX 5.
A
FenBl6SiS
A
knP&
4-
high anodic potentials, the cd for FeBOB2, in sulfate solutions does not depend on pH, the surface being covered with a sulfate layer (Figure 7). On the other hand, Fe40Ni40B2, exhibits a true oxide-type passivity, with log I,,, decreasing linearly with pH, the slope being d log I,,,/dpH = 0.4. Such a relationship is characteristic of passivity of crystalline Fe and its alloys in aqueous solut i o n and ~ ~ is ~ strongly related to the mechanism of metal dissolution through the film. It is of importance to check the generality of the considered behavior for other glassy metals. IPm is quite large for glassy metals without Cr (even if one considers the large roughness coefficient for the glassy ribbons14J5),as compared with the corresponding figures for crystalline Fe or Ni6JzS The existing data suggest that P reduces An example is given in Figure 8. For Fe-B-Si alloys I,,, practically does not depend on the degree of substituting Si for B. However, substituting P for B results in a dramatic drop of I thus suggesting a distinct reduction of the ionic con$%ivity of the film there. On the other hand, for Fe-Cr-Mo-B-Si alloys24a 10-fold decrease of IWhas been observed, when increasing Si content from 5% up to 15% (Figure 9). Ipass.4J89m
"
6
9
12
15
18
x% Figure 8. Effect of substituting Si or P for B in Fe base glassy alloys on cd within the passive region, I,,.
1o2
ESCE
Figure 9. Effect of Si content in Fe-Cr-Mo-B-Si
quasi-stationary polarization curves.24
alloys on
glasses passivate easily in acidic solutions. An effect of substituting Si or P for B is similar to that observed in Fe base glasses. Figure 6 shows Ecritfor Fe-Ni base glasses a t various solution pH's. P-substituted alloy exhibits the most noble Ecrit.The effect of pH on Ecrit is quite large here: AEcfit/ApH c 90 mV, suggesting a change in composition of the passive film with pH, whereas for crystalline Fe or Ni a slope of -60 mV is common.23 Surprisingly, for Cr-containing glasses the beneficial effect of Si seems to be confied to the passive region only. E.g., for Fe-Cr-Mo-B-Si alloys, both I,, and E,, are the same a t various degrees of substitution with Si (see the next paragraph and Figure 9). There is a large body of data suggesting that B is detrimental; substitution of any other metalloid for B in a glass enhances its ability to passivate and improves corrosion resistance.3-6J8~22 Certain authors believe, however, that the effect of B may be neglegib1e.l9s2lThere is only one unconfirmed report that B may reduce the corrosion cd.I
Dissolution within the Passive Region As already mentioned, Fe base glasses without Cr exhibit very poor ability to passivate. In some cases they may, however, produce a low protective salt film which affects alloy dissolution a t high anodic potentials. An interesting example has been reported by Kapusta and Heusler.lo At (23) Vetter, H. J. Elektrochemische Kinetik; Springer Verlag: Berlin, 1961; p 605. (24) Hashimoto, K.; Asami, K.; Kawashima, A. Proc. 9th ICMC, Toronto; 1984; p 208.
Composition of Passivating Films There is large surface analytical evidence that metalloid-oxygen compounds are constituents of the passivating films formed a t glassy alloys. Boron-oxygen compounds are present within the passive film formed a t B-containing glasses.3-6~10~12~24~21-2g These compounds are probably responsible for degradation of the protective properties of the films, at large B content in the substrate. Silicon-oxygen compounds are the main constituents of films formed a t Si-bearing a110ys.5*6~24,28-30 The corresponding XPS data for two groups of alloys, FeB-Si and Fe-Cr-Mo-Bi-Si, are plotted into the Wagner diagram (Figure 10). The diagram demonstrates that the magnitude of 0 1s and Si 2p binding energies for various silicon-oxygen compounds lies in a narrow band corresponding to a line difference of 429.0-429.6 eV.31 The line energies for silicates are lower than those for both silicon oxides and hydroxides, thus offering a possible diagnostic test to distinguish between the two groups of compounds. This diagram appears to be useful for identifying siliconoxygen surface compounds as well.32 From XPS and with the aid of Wagner diagram it has been found for Fe-B-Si alloys that silicon oxides at 312% Si and silicates at 3 X teresting to note here the results by Kolotyrkin et al.& who has found a substantial improvement in the corrosion behavior of rapidly quenched FelmSi, alloys in HC1, when x was increased up to 35%. These alloys are not glassy but microcrystalline ribbons and evidently exhibit a much better resistance to chloride attack than Fe-B-Si glasses. This bolsters a suggestion already made that B is detrimental and Si is beneficial. More research with rapidly quenched, microcrystalline binary alloys may help to understand the role of individual metalloids, since no requirements concerning their total amount in the alloy is necessary in this case. Results obtained with Fe-X-Si alloys (X = B or P) confirm the suggestion that P is beneficial. The P-bearing glass is much more resistant to pitting than the other one. Figure 12 shows the breakdown potential, E,,, vs. CCL- for (40)Vasiliev, V. Yu.; Klochko, A. N.; Pustov, Yu. A. Zashch. Met. 1985,21, 199.
(41) Diegle, R. Corrosion 1980, 36, 362. (42) Janik-Czachor, M. J.Electrochem.SOC.1981, 128, 513C. (43) Engell, H. J.; Stolica, N. D. 2.Phys. Chem. N.F. 1959,20, 115. (44) Heusler, K. E.; Fischer, L. Werkst. Korros. 1976, 27, 555. (45) Vetter, K. J.; Strehblow, H. H. Localized Corrosion, NACE-3, 1974; p 240. (46) Kolotyrkin, Ya. M.; Kniazheva, V. M.; Sokolov, S. A.; Novikov, S. V. Proc. 8th Europ. Corr. Congress, Nice;1985; p 81-1.
I
Figure 12. Effect of Cl- concentration on breakdown potential, Enp,for Fe72Si5X16, X = B or P.18 Fe75B25-XS'X
A Fe79*16s'5 14
A
% 1' 6
"5
x%
Figure 13. Effect of substituting Si or P for B in Fe base glassy alloys on the width of passive region, LIE.
the two alloys. E,, for X = P is always more noble. There are two regions in the curves. For X = P, the first region, a t low Ccl- and relatively noble E,,, exhibits a high slope of -970 mvldecade, thus showing a strong aggressive effect of chloride here. At Ccl- > 5 X M, the slope is only -120 mvldecade, and E,, becomes low and close to Ecrit(compare Figure 3). It is quite possible that such behavior is similar to that reported by Okamoto for stainless ~teels.4~ He suggests that at low anodic potentials the passive film may contain bound water thus being able to defend itself from the aggressive action of chloride. Consequently an increase in Ccl- will not shift E,, much in the negative direction since the bound water will "buffer" the chloride action. In contrast, at high noble potentials where no bound water is present within the film the aggressive action of chloride becomes irreversible. Therefore, any change in CCI-,increasing a probability of localized chloride built up, will strongly change E,,. It should be pointed out here that the stability of the passive state of Fe base glasses is so low that they may undergo pitting even in absence of any aggressive ions. E.g., in borate buffer, the stable passivity range AE for Fe-B glasses is only AE 300 mV.39 Substituting Si for B may improve it, thus making AE wider (Figure 13). Moreover, a full substitution of P for B in Fe-B-Si glasses results in AE = 1100 mV already at a low Si level (Figure 13), suggesting again that Si and P are beneficial, whereas (47) Okamoto, G. Corr. Sci. 1973, 13, 471.
Janik-Czachor
916 Langmuir, Vol. 3, No. 6, 1987
10’1
II
I
borate b u f f e r p H = 8.4
.
I
erative when the glass contains a “film former”. Taking the above into account we may conclude that a chemical rather than a structural inhomogeneity in passivating films on glassy metals is probably responsible for localized corrosion of these materials. E.g., a suggestion was m a d e previously that an inhomogeneous distribution of boron-oxygen compounds may exist within the passivating film formed on B containing glasses.16 A localized agglomeration of these compounds may provide sufficient weak spots to facilitate passivity breakdown. Careful surface analytical measurements with a high lateral resolution SAM may prove the validity of this assumption.
Concluding Remarks I 6
9
12
IS
1 18
x ,% Figure 14. Effect of substituting Si or P for B in Fe base glassy alloys on the time interval, r,, pquired to break the passive film down during an open-circuit potential decay.
B is detrimental. A similar tendency is observed when measuring open-circuit potential decay of prepassivated glasses. Time to breakdown (activation) 7, becomes larger the higher the degree of substitution of Si for B in Fe-B-Si alloys is. Moreover, a t lower Si level T , becomes 2 orders of magnitude higher when P is substituted for B (Figure 14). A question arises as to what kind of an inhomogeneity may be responsible for the passivity breakdown in metallic glasses. Although these materials do not contain grain boundaries and other defects which are inherent in crystalline alloys, the glassy ribbons still may contain some structural inhomogeneities, as already discussed. These may provide “weak spots” in the film, which altogether is not well protective in the absence of Cr. Also, the high surface roughness may make the surface more reactive. One may argde, however, that basically the surface spots characterized by the lowest activation energy for dissolution26exhibit also the lowest activation energy for passivation,* hence they may repassivate the easiest. Moreover, the possible inhomogeneities a t the surface become inop(48) Frankenthal, R. P. J. Electrochem. SOC. 1967,114, 542.
Glassy metals do not contain any defects characteristic of the crystalline materials and, therefore, were expected to he particularly resistant to localized breakdown of passivity. However, glasses without Cr or other “filmforming” element appeared to be very susceptible to localize corrosion. Chemical composition of a glass and thus of the passive film, rather than the amorphous structure, s e e m to be a decisive factor in determining the ability of the alloy to passivate and the stability of the passive state. Metalloid elements, playing the role of glass stabilizers in metal-metalloid glasses, affect also corrosion properties. Si or P, when substituted for B, are beneficial. XPS and AES surface analysis suggests that silicon-oxygen and phosphorus-oxygen compounds are the main constituents of passivating films formed on Si- and P-containing glasses. Silicon oxides (or hydroxides) improve the protective ability of the film, whereas silicates do not. Phosphates [P(O)] and some, not yet identified, lower valency phosphorus-oxygen compounds may considerably reduce ionic conductivity of the film and improve its stability. On the other hand, boron-oxygen compounds introduce some kind of instability which leads to localized corrosion even in the absence of any aggressive anions.
Acknowledgment. I am grateful to Professors R. P. Frankenthal and K. E. Heusler for helpful discussions concerning the electrochemistry of glassy metals and to Dr. P. Kedzierzawski for critical reading of the manuscript. This work has been done within the research project CPBR 2.4.
