The corrosion of copper by atmospheric sulphurous gases

Corrosion Science, Vol. 23, No. II, pp. 1141-1152, 1983 Printed in Great Britain.

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THE CORROSION OF COPPER BY ATMOSPHERIC SULPHUROUS GASES T. E. GRAEDEL, J. P. FRANEY and G. W. KAMMLOTT Bell Laboratories, Murray Hill, NJ 07974, U.S.A. Abstract-The sulfurization of copper by atmospheric gases is widely recognized, but the importance of the potential causative agents of sulfurization and the mechanisms involved have remained unresolved. In this work, polycrystalline copper has been exposed to the atmospheric gases hydrogen sulfide (H,S), carbonyl sulfide (OCS), carbon disulfide (CS,), and sulfur dioxide (SO,) in humidified air under carefully controlled laboratory conditions. At room temperature, the rates of sulfurization by H,S and OCS are comparable, and are some two orders of magnitude greater than those by CS, and SO,. Given the atmospheric concentrations of these gases, it is clear that OCS is the principal cause of atmospheric sulfurization of copper except near sources of the gases where high concentrations may render H,S (and possibly SO,) important. At constant absolute humidity, the sulfurization rate of copper by OCS is found to be inversely proportional to temperature over the range 21-80"C, a property attributed to reduced quantities of surface water at high temperatures and the subsequent decrease in the rate of hydrolytic transformation of OCS into a reactive form. In a final series of experiments, the initial sulfurization of copper by 2.2 ± 0.2 ppm H,S in humidified air at 22°C has been studied in detail. The first stages of sulfurization involve rapid attack by H.S at surface defect sites. As these corrosive mounds spread and merge, diffusion of copper to the surface is impeded and the fraction of H 2S molecules striking the surface that become incorporated into the corrosion film drops sharply from ~ 5 x 10- 6 (at t = 5 s) to ~ 8 X 10-7 (at t = 72 h). INTRODUCTION

COPPER is among the metals particularly sensitive to atmospheric corrosion, a property that limits its use in certain applications (e.g. electrical connectors) and promotes its use in others (e.g. architectural decoration). Its corrosion films are responsible for the light green patina which copper acquires after prolonged outdoor exposure; although these films contain oxides, hydroxides, carbonates, and chlorides, their primary constituent is copper sulfate. l .2 The mechanism by which the copper sulfate is formed (and, indeed the atmospheric corrodant responsible for its formation) has never been satisfactorily determined. This uncertainty has arisen in part from the difficulty of performing definitive laboratory experiments and in part from a paucity of atmospheric measurements. Of initial concern is the identification of the active sulfur species. Sulfur dioxide (S02) has long been considered the most likely, since it is a product of coal combustion and hence present in the atmosphere, since copper sulfate seemed a reasonable product for a reaction chain involving copper and S02' and since experiments involving exposure of copper to extremely high (10-10,000 ppm) S02 concentrations resulted in the formation of copper sulfate. 3 • 4 However, more recent work indicates that the sulfate formation is seen only at very high humidities and S02 concentrations,l and that atmospheric S02 concentrations are generally in the range of only 1-100 ppb. 5 Alternate candidates for the title of principal sulfur corrodant are hydrogen sulfide (H 2S) and carbonyl sulfide (OCS). The reaction of copper with H 2 S is known to be Manuscript received 29 November 1982; in amended form 15 March 1983. 1141

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T. E. GRAEDEL, J. P. FRANEY and G. W.

KAMMLOTT

rapid. 6, 7 A similar effect has recently been observed when copper was exposed to humidified air containing OCS.8 Sulfur-containing gases other than S02' H 2S, and OCS are rare in the atmosphere, although very low concentrations « I ppb) of carbon disulfide (CS 2), methyl mercaptan (CHaSH), dimethyl sulfide (CHaSCHa), and dimethyl disulfide are sometimes present. 8-lO Free sulfur is not known to exist in the atmosphere. 9 The rate of atmospheric sulfurization of copper is difficult to establish, in part because the corrosivity of field sites covers a very broad range and because the corrosive processes may involve synergistic effects with other pollutants. Limited laboratory results have been presented for H 2 S6.7,1l-la and for S02,l1 and indoor corrosion rates have been shown to be best correlated 14 with the atmospheric concentration of "reduced sulfur", a quantity determined by a measurement technique sensitive to H 2S, OCS, and perhaps other atmospheric reduced sulfur gases. 8 Sulfurization of copper by H 2S produces Cu2S as the first stable compound. 6, 15 It has been suggested that the initial step is dissolution of H2S in a water film at the surface. The overall activation energy for sulfurization is consistent with a free radical process, perhaps involving HS or HS-.7 Regardless of the manner in which the Cu2S is formed, it gradually oxidizes to form the sulfate. 16 Sulfurization of copper by S02 cannot proceed as above, since the first stable compound is the sulfate. It has been suggested that S02 dissolves in a water film to form H 2SOa which is subsequently oxidized.! No experiments designed to investigate the chemical process in a definitive manner have been performed. Thus, although the sulfurization of copper is a process of substantial interest, there is uncertainty concerning the corrodant species, the rates of sulfurization have not been well defined, and the physical processes involved in the sulfurization remain to be satisfactorily described. In this paper we present experimental data and analyses that permit us to make progress toward the resolution of these concerns. EXPERIMENTAL METHOD Samples preparation The experiments described in this paper were performed on OFHC Copper (99.99% pure), using sample disks 1 cm in diameter and 2 mm thick. The samples received an initial wet polish with 600 grit paper on a rotating wheel. Further polishing was performed with a slurry of aluminum oxide powder and shaving cream'7 on a rotating wheel covered with Buehler "MicrocIoth". Al,O. powder of average diameter 1.0 fLm was used, followed by Al,O. powder of average diameter 0.3 fLm. The samples were rinsed with de-ionized water and ethanol, and dried in a stream of nitrogen. They were then inserted into the exposure chamber. This process of mechanical polishing of copper introduces a relatively uniform plastic deformation of the surface, but retains the crystalline character of the surface layer.'· Instead of a sequential polish, some samples were prepared by etching. These samples received the same initial polish, but were then dipped into a 50% HNO., 50% H.O solution for 30 s, followed by a 30 s dip in de-ionized water. They were then rinsed with ethanol and dried in a stream of nitrogen. Generation, calibration and measurement of corrosive atmospheres Exposures were carried out in a system consisting of three modules: a gas generation system, an exposure facility, and a monitoring and data reduction system. The first two of these modules have been described in detail by Franey;'" we summarize the description here for completeness. For H,S, OCS, and SO., all of which are gases at room temperature and are available in pressurized form, the corrosive gas mixture is generated by filling a length of teflon tubing with the gas and allowing it to permeate through the tubing into a controlled flow of carrier gas. The mixed gas flows into the exposure chamber, where temperature and relative humidity are monitored continuously.

Corrosion of copper by atmospheric sulfurous gases

1143

As the mixed gas leaves the exposure chamber, a portion of it is fed to a detection system which continuously monitors the sulfurous gas concentration by ultraviolet irradiation of SO" followed by detection of the SO, resonance fluorescence. To monitor H,S or OCS concentrations, sample flow through a high temperature converter is mandatory to oxidize the reduced sulfur molecules to SO,. The system is calibrated for H,S and SO, by the permeation tube technique (e.g. [20]). The calibration procedure for carbonyl sulfide was detailed, since it was not known whether the detection system could measure that gas. First the detector response to a sample of 11.0 ppm OCS in N, prepared by gas dilution techniques (Matheson Gas Products, Inc.) and diluted with 0, and N. to provide ~ 4 ppm OCS in 79 % N., 21 % O. was measured. The result showed that the detector responded to OCS approximately as it did to H.S. A permeation wafer device was then used (Metronics Inc.) to prepare OCS at various concentrations. When the catalyst was heated to ~ 500oC, conversion of OCS to SO, was ~ 99 %. The wafer calibration was confirmed by measuring the OCS content of a tank of 4.0 ppm OCS in N. (Scientific Gas Products, Inc.). Finally, a mixture of 103 ppb SO, and 482 ppb OCS in N, was used, which had been cross-certified by Scott Environmental Services, Inc. and by Dr. Alan Bandy of Drexel University. Our measurement of the sulfurous gases gave 100 ppb SO, and 473 ppb OCS. The fourth atmospheric sulfur-containing species included in the experiments, carbon disulfide (CS,), is a liquid at room temperature. Gas dilution techniques are therefore not suitable for generating a mixture of CS, in humidified air. A simple exposure experiment was performed as follows: a mixture of water and CS, was prepared in a beaker. The odor of CS, at the beaker rim was detectable but not overpowering. Since the odor threshold for CS, is 0.9 ppm", the CS, concentration was estimated within the beaker as ~ 10 ppm. Copper samples prepared for exposure were then placed within a small Petri dish which was floated on the mixed liquid, and the beaker was capped. After the designated exposure period, the samples were removed for study. This procedure for CS" while crude, proved satisfactory since the effects of CS, on copper were shown to be very slight (see below). Had they been severe, the calibration of the CS, concentration would have had to have been made with a precision similar to that employed for the other sulfurous gases. Corrosion film analysis

The morphology of the copper corrosion films was studied with a Kent-Cambridge 2A SEM equipped with a solid state X-ray detector and a multichannel analyzer. The thickness of the corrosion films was determined by energy-dispersive X-ray analysis (EDXA). The preparation of thickness standards necessary for this method" is such that these films were not pure Cu,S, but include some cuprous oxide and perhaps some adsorbed water, as do the films formed in our experiments and in the atmosphere. The calibration thus applies to the total sulfur signal from sulfidized copper (a mixed film composed predominantly of the sulfide 6), rather than to that from copper sulfide. If sulfur concentration gradients in the bulk copper are disregarded and the sulfur is assumed to be present as CU,S,6.15 the intensity ratios may be plotted as a function of equivalent average corrosive film thickness. The result is a calibration curve of X-ray intensity ratios to film thickness accurate to ± 20% or ± 8 nm, whichever is smaller. The thickness measurements are average values of a sampling area of about 1 mm'. EXPERIMENTAL RESULTS

Su/furization as a function of sample preparation It is often remarked that reproducible results can be obtained in corrosion studies only if samples are polished and cleaned in an effective and consistent manner (e.g. ref. 23, 24). To evaluate the reproducibility of our technique, three samples that had been prepared by the sequential polishing method were simultaneously exposed, together with three samples prepared by the nitric acid etching process. (Both methods are described in the previous section.) The resulting determinations of film thickness are summarized in Table I. It should be noted that samples prepared with care but by different techniques do not give identical results. This is very likely due to differences in surface roughness, since the principal areas of initial corrosive attack are at grain boundaries, defect structure, etc.,25 structures which tend to be exposed preferentially by etchants (e.g. ref. 26). Another characteristic of Table 1 is that the standard deviation of the polished samples was far less than that of the etched samples. Indeed,

T. E. GRAEDEL, J. P. FRANEY and G. W. KAMMLOTT

1144

TABLE 1. CORROSION FILM THICKNESS (nm) ON MATCHED COPPER SAMPLES PREPARED BY DIFFERENT TECHNIQUES· Sequential polish preparation Sample 1 Sample 2 Sample 3

105.6 106.5 104.9

Mean (S.D.)

105.7 (1.1)

Acid etch preparation Sample 1 Sample 2 Sample 3

390.0 287.6 433.1

Mean (S.D.)

370.2 (105.8)

·AII exposures were performed under the following conditions [H.S] = 1.97 ppm; T = 22°C; r.h. = 90%. Total exposure = 7.9 ppm-h.

similar agreement among results obtained on copper samples prepared and exposed by the techniques described here over a period of some two years was seen. Consequently, the sequential polishing technique was adopted as a standard surface preparation technique for all of the corrosion testing. Relative sulJurization rates of different gases The rate of sulfurization for different atmospheric sulfurous gases is shown in Fig. I, where the combined previous results for H 2 S7 and OCS8 with additional data Su lf idotion of copper

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Corrosion of copper by atmospheric sulfurous gases

1145

for S02 and eS 2 generated in this work are shown. The sulfurization domains for H 2S and oes represent the areas bounded by the 90 %confidence limits for the leastsquares fits to the data.27 Within these confidence limits there is little difference between the sulfurization rates of the two gases, though that for oes is slightly higher. For the eS 2 exposure, no sulfur was detected on the sample: that is therefore a single upper limit for eS 2 sulfurization. The eS 2 data point is given a large probable error for exposure, reflecting the necessity of estimating its concentration during the experiment. For the four gases, quantitative comparisons may be made by utilizing the experimental fact',s that the growth of the first 150 nm or so of sulfide film on copper by H 2S and oes is an approximately linear function of the atmospheric sulfur gas concentration, that is, the rates are pseudo-first-order in [H2S] or rOeS]. For S02 and eS 2 the rates are much slower. This implies a different reaction mechanism, which must involve a reduction in valance for the sulfur atom in S02 and extensive bond breaking in es 2. For H 2S and oes R Cu ,;

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oes suljurization as a function of temperature In principle, it is possible to derive activation energies for copper sulfurization by performing experiments at different temperatures; the resulting activation energies can then provide insight into the controlling physical process. Accordingly, polished copper samples were exposed to oes at different temperatures. The results are shown in Fig. 2 for each of the experimental temperature groups. The comparison shows that the sulfurization rate is highest at the lowest temperature (i.e. less exposure is required to form a film of a prescribed thickness). At least at the higher exposures, there is an inverse relationship between the sulfurization rate and the temperature. The implications of this result are discussed below.

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Initial sulfurous film growth on copper Since it is known 7 • 15 that H 2S is a significant factor in the atmospheric sulfurization of copper, short term exposures of copper to H 2S were performed in order to examine the initial sulfurization process in more detail. The rate of corrosive film growth on copper in atmospheres containing H 2S involves different physical processes at different stages. It has been previously suggested 7 that the film growth from ~ '" 30 nm to ~ '" 200 nm is controlled by the supply of gaseous H 2S. For thicker films, the diffusion of copper ions and/or H 2S molecules controls the rate of growth. In the present work, the capability of rapid sample insertion and removal in the multiport chamber19 was used to study film growth in the 0-80 nm region. The results are shown in Fig. 3. The initial film is seen to form at a rapid rate. During this initial period changes in the surface morphology can be seen (Fig.4), the most obvious of which is the

Corrosion of copper by atmospheric sulfurous gases

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appearance of bright "mounds" of growth near the defects in the surface. The subsequent changes in surface morphology have been previously illustrated. 7 DISCUSSION

The data from our experiments have implications for the preparation of samples for corrosion tests, the surface finishing of metal components, the susceptibility of copper to different atmospheric environments, and the rates and mechanisms of copper sulfurization. Each of these areas is discussed below. Materials and components placed in field service receive a variety of surface preparations; the surface finishes thus have significant differences. This can affect the susceptibility to corrosion and also the degree to which material performance in the laboratory simulates that in the field. These points are clearly indicated by the results of Table 1, in which samples prepared by a specific etching process are shown to sulfurize at much higher and less uniform rates than those prepared by sequential polishing, i.e. sulfurization is minimized on surfaces that are relatively smooth rather than those with raised etch patterns. Mechanically polished surfaces thus seem more appropriate for experiments emphasizing reproducibility, and perhaps for components subjected to highly corrosive field environments. The rate constant data in Table 2 provide, to the best of our knowledge, the only directly comparable information on the relative susceptibility of copper to sulfurization by different sulfur-containing atmospheric gases. By combining this information with

T. E. GRAEDEL, J. P. FRANEY and G. W. KAMMLOTT

1148

typical atmospheric concentration of the gases, sulfurization rates can be calculated from equation (2). The results are shown in Table 3, where the assumed conditions arc T = 20o e, r.h. ". 90 %, and only one of the sulfurous gases present. Near the sources of the sulfurous gases, the gas concentration is relatively high. H 2S or oes (or, under high concentrations, perhaps S02) might rapidly sulfurize copper in such environments. In the ambient air away from sources, however, only the relatively longlived oes has a significant sulfurization rate. The possible synergistic effects on sulfurization of the combined presence of two or more of the sulfurous gases or of other atmospheric trace gases have not been investigated. Such studies will be necessary if a complete picture of the atmospheric sulfurization of copper is to be assured. TABLE 3. Corrosive gas H.S OCS CS. SO.

CALCULATED SULFURIZATION RATES FOR COPPER IN DIFFERENT AMBIENT ENVIRONMENTS

(nrn ppb-h-l)

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Figure 2 shows that the rate of sulfurization of copper by oes is inversely proportional to temperature over the range 21-80o e. Since most reactions show an increasing rate with temperature, this behavior suggests that something other than a simple bimolecular reaction of copper and oes is involved. It is known that oes requires water to be corrosive,28 and the data in Fig. 2 suggest that the amount of surface water is also significant. Sharma29 has measured the amount of water adsorbed on eu 20 at different relative humidities over a temperature range of 18-45°e. If those results are assumed to be valid also at 80 e (since no differences with temperature are seen over the range of the experiments), the amount of surface water on the copper samples in our experiments was ~ 0.5 monolayers (80°C), ~ 3 monolayers (38°C), and ~ 13 monolayers (21°C). Since the first layer of water on oxide surfaces is relatively immobile,30 it is clear that in the 80 e experiments the H 20 molecules were incapable of oes solution. Much the same situation exists at 38°e, since a solvated molecule or ion is associated with 5-15 water molecules 31 ,32 and clusters of such size would have significantly retarded velocities and thus reduced reactivies in a water layer of similar dimensions. At 21°e, the amount of water on the surface is sufficiently representative of bulk water that normal oes hydrolysis can occur. The inverse temperature dependence of copper sulfurization by oes is therefore ascribed to a decrease in the amount of water available to catalyze the primary sulfurization reaction. Experiments at constant temperature but varying humidity are in progress and will be presented separately. What characterizes the first stages of the sulfurization process? From Fig. 3 it can be seen that very short exposures of copper are sufficient to generate detectable films. The electron micrographs of Fig. 4 indicate that the films are not uniform, however. From an initially clean surface, showing scratches characteristic of a surface 0

0

Fro. 4.

Electron micrographs of the initial stages of film growth on copper during exposure to H,S. (a) E, 3 ppb-h; (b), 25 ppb-h; (c), 111 ppb-h.

Corrosion of copper by atmospheric sulfurous gases

1151

roughness of '" 0.3 [Lm, small bright "mounds" appear (Fig. 4a), principally along surface scratches and irregularities. These mounds grow and spread with increasing exposure (Fig. 4b). Figure 4(c) pictures the surface after an exposure of III ppb-h. The equivalent Cu 2 S thickness for this sample is '" 10 monolayers. The spatial resolution of X-ray mapping is insufficient to locate the sulfur on the surface, but we infer that it is concentrated in the mounds (the principal new features), since the growth of the oxide film during this time period is known to be negligible. 5 An alternative way of studying the initial sulfurization process is to compute the H 2S "incorporation coefficient", i.e. the fraction of H 2S molecules striking the surface from which the sulfur atom becomes incorporated into the surface film. (Note that the detection technique does not specifically determine the molecular form of the sulfur, but rather its presence.) The incorporation coefficient, y, is given by 7 (2) where ~ is the equivalent Cu 2S film thickness (nm), [H 2S] is the average concentration of the corrosive gas, and t is the exposure time in s. The resulting values and a polynomial curve fitted to them are shown in Fig. 5. From the first determination of y '" 5 X 10-5 , at t = 5 min, the coefficient decreases to a value of y '" 8 X 10- 7 at t = 72 h. The figure also indicates the exposures at which 1, 3 and 10 equivalent monolayers of Cu 2 S were formed. y is quite high initially, levels off after the formation of the first one or two equivalent monolayers and declines after the formation of a few tens of monolayers. The interpretation of this behavior is that the initial sulfurous molecules are incorporated readily into the surface at the defect sites. Once these favored sites are occupied, the rate of incorporation becomes relatively constant as

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Exposure (ppb-hl FIG. 5. H.S incorporation coefficient as a function of total exposure during the initial stages of film growth. The curve is a polynomial fit to the data points. The total exposures at which 1, 3, and 10 equivalent monolayers of Cu.S have formed are indicated.

1152

T. E. GRAEDEL, J. P. FRANEY and G. W. KAMMLOTT

the "mounds" of corrosion products grow and coalesce. When a relatively cohesive and thick corrosion film is formed, diffusion of copper atoms is impeded and the incorporation coefficient becomes negligible. Acknowledgement-We thank Dr. Alan Bandy of Drexel University for his cooperation in the interlaboratory calibration of our OCS detection system. REFERENCES 1. H. LEIDHEISER, Jr., The Corrosion of Copper, Tin, and Their Alloys. John Wiley, New York (1971). 2. E. MATTSSON and R. HOLM, in Atmospheric Corrosion (ed. W. H. AILOR), pp. 365-381. John Wiley, New York (1982). 3. W. H. J. VERNON, Trans. Faraday Soc. 27, 255 (1931). 4. R. ERICSSON and T. SYDBERGER, Werkstoffe Korros. 28, 755 (1977). 5. T. E. GRAEDEL, in Handbook of Environmental Chemistry, Vol. 2A, p. 107. Springer, Heidelberg (1980). 6. S. P. SHARMA, J. electrochem. Soc. 127, 21 (1980). 7. J. P. FRANEY, T. E. GRAEDEL and G. W. KAMMLOTT, in Atmospheric Corrosion (ed. W. H. AILOR), pp. 383-392. John Wiley, New York (1982). 8. T. E. GRAEDEL, G. W. KAMMLOTT and J. P. FRANEY, Science, N. Y. 212, 663 (1981). 9. T. E. GRAEDEL, Chemical Compounds in the Atmosphere. Academic Press, New York (1978). 10. P. J. MAROULIS and A. R. BANDY, Geophys. Res. Lett. 7, 681 (1980). 11. D. W. RICE, P. PETERSON, E. B. RIGBY, P. B. P. PHIPPS, R. J. CAPPELL and R. TREMOUREUX, J. electrochem. Soc. 128,275 (1981). 12. W. E. CAMPBELL and U. B. THOMAS, Electrical Contacts-1968, lIT Res. lnst., Chicago, p. 233 (1968). 13. R. V. CHIARENZELLI, Proc. 3rd Int. Conf. Elec. Contact Phenom., University of Maine, Orono, p. 83 (1966). 14. D. W. RICE, R. J. CAPPELL, W. KINSOLVING and J. J. LASKOWSKI, J. electrochem. Soc. 127, 891 (1980). 15. W. H. ABBOTT, IEEE Trans. Parts, Hybrids, and Packaging PHP-I0, 24 (1974). 16. R. F. ROBERTS, N. D. HOBBINS, T. E. GRAEDEL, J. P. FRANEY and G. W. KAMMLOTT, unpublished work (1980). 17. M. E. DAVIS and T. J. LOUZON, Metallography 13,185 (1980). 18. D. M. TURLEY and L. E. SAMUELS, Metallography 14, 275 (1981). 19. J. P. FRANEY, Corros. Sci. 23, 1 (1983). 20. G. O. NELSON, Controlled Test Atmospheres. Ann Arbor Science Publishers, Ann Arbor (1971). 21. W. H. STAHL (ed.), Compilation of Odor and Taste Threshold Values Data, ASTM DS-48. Amer. Soc. for Testing and Materials, Philadelphia, PA (1973). 22. G. W. KAMMLOTT, Appl. Spectroscopy 35,324 (1981). 23. W. E. CAMPBELL and U. B. THOMAS, Trans. Electrochem. Soc. 76, 303 (1939). 24. G. OELSNER, Galvanotechnik 64, 460 (1973). 25. J. H. BROPHY, R. M. ROSE andJ. WULFF, The Structure and Properties of Materia Is, Vol. 2, p. 155. John Wiley, New York (1964). 26. G. PETZOW, Metallographic Etching, Amer. Soc. for Metals, Metals Park, OH (1978). 27. M. KENDALL and A. STUART, Advanced Theory of Statistics, Vol. 1, 4th ed. Macmillan, New York (1977). 28. W. BRAKER and A. L. MOSSMAN, Matheson Gas Data Book, 5th edn, p. 115. Matheson Gas Products, East Rutherford, N.J. (1971). 29. S. P. SHARMA, J. Vac. Sci. Technol. 16, 1557 (1979). 30. C. F. CEROFOLINI and C. ROVERE, Thin Solid Films 47,83 (1977). 31. A. W. CASTLEMAN, Jr., B. D. KAY, V. HERMANN, P. M. HOLLAND and T. D. MARK, Surface Sci. 106,179 (1981). 12. M. ARMBRUSTER, H. HABERLAND and H.-G. SCHINDLER, Phys. Rev. Lett. 47, 323 (1981).