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On the mechanism of silver and copper sulfidation by atmospheric H2S and OCS
CorrosionScience,Vol. 25, No. 12, pp. 1163-1180, 1985 Printed in Great Britain
0010-938X/85 $3.00+ 0.00 (~) 1985PergamonPressLtd.
ON THE MECHANISM OF SILVER SULFIDATION BY ATMOSPHERIC T. E.
AND COPPER H2S AND OCS
GRAEDEL,J. P. FRANEY,G . J. GUALTIERI,G . W . KAMMLOTY a n d D. L. MALM AT&T Bell Laboratories, Murray Hill, NJ 07974, U.S.A.
Abstract--Hydrogen sulfide (H2S) and carbonyl sulfide (OCS) are the principal atmospheric corrodents for silver and copper. To investigate the mechanisms by which the sulfidation reactions occur, the two metals have been exposed to each of the sulfurous gases under a wide range of relative humidities. The sulfidized samples were analyzed by scanning electron microscopy, X-ray fluorescence spectroscopy, low energy ion scattering spectrometry, spark source mass spectroscopy, Auger spectroscopy and X-ray photoemission spectroscopy. The results demonstrate that sulfidation of both metals by both gases is strongly dependent on the relative humidity, that negligible carbon or oxygen is incorporated into the sulfide layer during OCS sulfidation and that the products of sulfidation by either H2S or OCS are Ag2S and Cu2S. These results are used to develop a general mechanism for atmospheric sulfidation in which the absorption of the gas into the surface water layer is followed directly by dissociative coordination with metal atoms. Experiments capable of confirming some of the steps in this mechanism are proposed. INTRODUCTION THE SULFIDATION o f m e t a l s u p o n e x p o s u r e to the a t m o s p h e r e is a c o m m o n p h e n o m e n o n , a n d s t u d i e s o f the r a t e s o f sulfidation h a v e b e e n c o n d u c t e d for half a c e n t u r y . D e s p i t e this h i s t o r y , t h e r e is c o n s i d e r a b l e u n c e r t a i n t y c o n c e r n i n g the s p e c i e s a n d m e c h a n i s m s i n v o l v e d in t h e sulfidation p r o c e s s . T h e s i t u a t i o n has g r o w n still m o r e c o m p l e x with t h e d i s c o v e r y t h a t c a r b o n y l sulfide, d e t e c t e d in the a t m o s p h e r e r e l a t i v e l y r e c e n t l y , is c a p a b l e of sulfidizing c o p p e r , 1 c o p p e r alloys 2 a n d silver. 3 C o p p e r sulfidation b y h y d r o g e n sulfide (H2S) is k n o w n to be a sensitive function of r e l a t i v e h u m i d i t y ( R H ) . B a c k l u n d et al. 4 s h o w e d t h a t the sulfidation r a t e c o u l d b e i n c r e a s e d b y m o r e t h a n an o r d e r o f m a g n i t u d e as the R H i n c r e a s e d f r o m 0 to 100%. S h a r m a , 5 w o r k i n g at m u c h l o w e r a n d m o r e a t m o s p h e r i c a l l y realistic H2S c o n c e n t r a tions, c o n f i r m e d this finding. A similar result was o b t a i n e d by R i c e et al. ,6 using a test involving s e v e r a l c o r r o d e n t gases. N o s t u d i e s o f the r e l a t i v e h u m i d i t y d e p e n d e n c e o f c o p p e r sulfidation b y O C S h a v e b e e n r e p o r t e d . Less a g r e e m e n t exists a m o n g l i t e r a t u r e r e p o r t s on the r e l a t i v e h u m i d i t y d e p e n d e n c e of the H2S sulfidation o f silver. T h r e e sets of e x p e r i m e n t s , two using H2S as the o n l y c o r r o d e n t 4'7 a n d o n e using m i x e d c o r r o d e n t s i n c l u d i n g H2S, s r e p o r t a positive d e p e n d e n c e , with t h e sulfidation r a t e i n c r e a s i n g s h a r p l y a b o v e 70% R H . In c o n t r a s t , the m o s t r e c e n t w o r k utilizing m i x e d c o r r o d e n t s including H2S finds no r e l a t i v e h u m i d i t y d e p e n d e n c e . 6 NO studies involving O C S have b e e n p e r f o r m e d . T h e r e a c t i o n m e c h a n i s m s of c o p p e r a n d silver suifidation a r e o f significant scientific a n d t e c h n i c a l i n t e r e s t , b u t r e m a i n u n c e r t a i n . It has b e e n s u g g e s t e d 5 t h a t , in Manuscript received 22 March 1985. 1163
1164
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIERI, G. W. KAMMLOTrand D. L. MALM
the absence of water (a condition virtually never attained in the earth's lower atmosphere), H2S can be dissociatively adsorbed on copper by the reaction H2S ~ H2 + S2- + 2@,
(1)
where O represents a positive hole in the sulfide. The S2- is then available to react with metal ions. Alternatively, it is possible 9 that on cuprous oxide the conversion does not involve an intermediate ion: Cu20 + H2S --~ Cu2S + H20.
(2)
The evidence to support either of these mechanisms is not yet compelling. In the presence of water saturated with O2, the formation of free sulfur has been proposed: lO H2S + ½02--o S + H20.
(3)
Although the presence of free sulfur in some reaction vessels I1 may offer support for this inherently multistep mechanism, it may merely indicate that free sulfur is produced at high solution concentrations by a variety of mechanisms. Similar reactions have been proposed for silver,I° although without substantial evidence for their validity. The purpose of this work is to ameliorate this paucity of information on the degradation processes of widely used metals. To this end, we have conducted careful exposure experiments and analyzed the corrosion films by a variety of analytical techniques capable of determining the thickness of the corrosive films and their elemental and molecular content. The results of our work are presented below.
EXPERIMENTAL
METHOD
Sample preparation and exposure The copper samples used for these experiments were 'oxygen-free, high conductivity' copper (99.99% pure), prepared as disks 1 cm in diameter and 2 mm thick. The silver samples were 99.9999% pure and were prepared by machining 1 cm z coupons from sheet stock. Following initial wet smoothing with 600 grit AI203 paper, the copper and silver samples were sequentially polished with finer AI203 powders, culminating in polishing with powder of average diameter 0.3 p,m. The technique is described in more detail by Davis and Louzon. ~z The process yields a surface with mean roughness <0.3/~m. Replicate samples prepared by this technique show variations in sulfidation rate of < 2 % , much less than that of replicate samples prepared by etching techniques.13 The corrosive environment to which the samples were exposed consisted of approx. 3.5 parts per million (ppm) of hydrogen sulfide or carbonyl sulfide gas in humidified air at room temperature (21 + 2°C). The stability of the sulfurous gas concentrations during the experiments was of order +5%. 14H2S or OCS concentrations higher than a small fraction of a ppm are rare in field environments I~ but our finding that the sulfidation of copper and silver is a function of total exposure ~6permits us to use higher concentrations to shorten the exposure time. The samples were readied for exposure in the multiport chamber ~4by attaching them to flexible inserts sized to fit the chamber ports: this permitted samples to be inserted and withdrawn rapidly and without noticeably altering the corrosive environment. Control of the HzS or OCS concentration in the chamber was achieved by using polymer permeation techniques) 7 The water content of the air was varied by adjusting a portion of the carrier air supply that passes through a humidifier. A dew point hygrometer and hot wire anemometer monitored the dew point and flow rate continuously. Each sensor was sampled periodically throughout the exposure period by a dedicated desktop computer system.
Silver and copper sulfidation by H2S and OCS
1165
Sulfide film thickness analysis The thicknesses of the sulfide films grown on the copper and silver were determined by energydispersive X-ray analysis, using a Kent-Cambridge 2A SEM equipped with a solid state X-ray detector and a multichannel analyzer. The preparation of thickness standards necessary for this method is discussed elsewhere, ts The calibration curves that were used relate the ratio of intensities of metal and sulfur X-rays to the thickness of the sulfidation film. The results are accurate to _+8--10 nm or _+20%, whichever is smaller.
Auger and X-ray photoemission spectroscopy Auger and X-ray photoemission (XPS) spectra were taken on a commercial ESCA/Auger spectrometer (PHI, model 548) employing a double pass cylindrical mirror analyzer with a concentric electron gun. Auger spectra were collected in the standard differential scanning mode with AE/E = 0.65% at an excitation energy of 2 kV with approximately 20 p~A of beam current. The modulation voltage for all spectra was 3 V peak-to-peak. Photoemission spectra were collected using AI K a radiation. The instrumental bandpass contribution to the resolution was fixed at 0.32 eV for the Cu 2p3/2 core levels. The intrinsic linewidth of the A1 K a source contributes an additional 1.1 eV to the overall linewidth. Binding energies of unsputtered surfaces were referenced to the carbon Is core level at 285.0 eV. The Cu 2p3/2 and Ag 3d5/2 photopeaks were employed for alignment after sputtering and were not found to be affected by sputtering within an experimental error of +0.5 eV.
Low energy ion scattering spectrometry Low energy ion scattering spectrometry (LEISS) was used to study the elemental composition of the corrosion films, LEISS is described in detail by Smith 19and by Honig and Harrison 2, The method makes use of elastic binary collisions of <10 keV rare gas ions (He, Ne or Ar) with surface atoms to provide energy spectra characteristic of the atomic mass of the scattering centers. This information can be used to determine the composition of the outermost atomic layer of a solid regardless of its conductivity, with a detection limit of approx. 0.1 at.%. By the use of controlled ion-bombardment etching (sputtering), profiles of the distribution of the elemental constituents as a function of depth can be determined. The sample spot size was approx. 1 mm in diameter; the maximum penetration depth studied in these experiments was approx. 300 nm. The instrumental response was not quantitative, but profiles for the respective samples could be compared on a relative basis.
Spark source mass spectrometry This method, referred to hereafter as SSMS, provides for the vaporization, ionization and mass analysis of positive ions originating from the elemental constituents of a solid. This type of ion excitation is relatively non-selective in that the respective ion fractions generated in the arc are proportional to the atomic composition of the sample to within a factor of five. In the surface analysis mode, sampling is carefully confined to the outer 5-10/~m of surface thus providing chemical composition information for this region that is independent of that at greater depth. The principal advantages of this technique for monitoring relative compositional changes in thin films are the speed and sensitivity with which the presence of surface species can be determined. Results can be made quantitative on a relative basis for impurities which are present at the 1 at.% level or lower, provided they do not differ significantly in ionization potential or volatility. Accurate, absolute determinations of impurity concentrations at this level require the use of standards. The sampling procedure used for surface analysis was to record spectra from at least 10 distinct areas of the corrosion film. The arithmetic means and standard deviations of the ion intensities of interest were recorded. The results are expressed as ion percent by first computing the total ion exposure and then taking the percentage of the total for each ion species. Additional details concerning SSMS are available in the literature.2~ ._,2 'EXPERIMENTAL
RESULTS
Sulfidation by hydrogen sulfide Relative humidity dependence. I n F i g . 1 w e p r e s e n t
data on the thickness
of
1166
T . E . GRAEDEL, J. P. FRANEY, G. J, GUAI,T1ERI, G. W. KAMMLOTTand D. L. MALM 102 -
LEGEND [3 : 9 5 %
d
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TOTAL EXPOSURE, PPM- HR
FIG. 1. The sulfidation of silver by hydrogen sulfide at different relative humidities. All experiments were performed with [H2S ] ~ 3.5 ppm and T = 21 _+ 2°C. The lines are linear least squares fits to the data points for RH = 5, 31 and 95%.
sulfur-containing corrosion films formed on silver by H2S exposure at different relative humidities. The data indicate clearly that the sulfidation is strongly dependent on the relative humidity, in agreement with previous work. 4,7 Similar data for copper are shown in Fig. 2. The sulfide film growth is quite similar at 5 and 39% RH, but reaches significantly higher levels at 95% RH. Again the results agree with previous data. 4~ It is clear that water is involved in the sulfidation process, particularly at high humidities. TABLE 1.
RELATIVE
ION
PERCENT
OF TARNISH
CONSTITUENTS
AS
DETERMINED
BY S P A R K
SOURCE
MASS
SPECTROMETRY
Sample number
Sample
Metal
Sulfur
Oxygen
Carbon
96.9+3.1 86.3_+7.9 76.8± 16.3 81.9_+ 14.4 88.6-+ 12.1 80.6+7.6 85.2 -+ 9.0 76.8± 13.[)
n.d.* 9.4_+5.8 18.3_+ 13.3 13.3+ 10.4 n.d. 10.2+4.3 10.3 -+ 4.4 16.7± 12.4
1.8+2.1 2.9+2.8 3.6+6.2 2.2-+3.2 5.5+5.7 5.6-+3.5 2.9 + 3.4 1.9±2.0
1.4+ 1.9 1.4_+1.4 1.3± 1.9 2.5-+2.7 5.9-+6.8 3.6-+3.2 1.6 -+ 2.3 1.6_+ 1.6
1.
Unexposed silver
2.
Ag, H~S,5%RH
3.
Ag, H2S,95% RH
4.
Ag, OCS,95%RH
5. 6. 7. 8.
Unexposed copper C u , O C S , 5 % RH Cu, H2S, 95% RH Cu, OCS,95% RH
* n.d. = not detected.
Silver and copper sulfidation by H2S and OCS
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Fro. 2. The sulfidation of copper by hydrogen sulfide at different relative humidities. All experiments were performed with [H2S ] = 3.5 ppm and T = 21 _+ 2°C. The lines are linear least squares fits to the data points for RH = 5 and 95°/,,.
Elements in the corrosion films. Samples of silver and copper that were prepared but unexposed were examined with SSMS, as were samples exposed to H2S under low and high RH conditions. The results are presented in Table 1. Since SSMS examines approximately the top 5-10/xm of the sample and since the corrosion films are 0.01q3.5/zm thick, the entire film as well as the bulk material is being sampled. The unexposed samples 1 and 5 have no detectable sulfur, and carbon and oxygen signals that have very large error bars but may reflect a minor degree of surface deposition of unincorporated material. For silver exposed to H2S (samples 2 and 3), the sulfur signal is well above the background level and is higher at the higher humidity. No significant carbon or oxygen signature is detectable. The situation is similar for copper exposed to H2S at high humidity (sample 7). The depth profiles of elements in the surface films are shown by the LEISS analyses displayed in Fig. 3. For copper at low humidity [display (a)], the oxygen signal is confined to the outer skin of the sample, indicating that oxygen is not continuously incorporated into the growing corrosion film. The sulfur is present at relatively high levels for a depth of 100-200 nm. At high humidity [display (b)], the oxygen behavior is similar but the film containing sulfur is much thicker. In the case of silver [display (c)], no oxygen is detected (recall that silver does not form an oxide at these temperatures) and the sulfur-containing layer is much thinner than that of copper.
1168
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIERI, G. W. KAMMLOTTand D. L. MALM 0.5 0.4
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DEPTH (ARBITRARYUNITS)
FIG. 3. Chalcogen to metal (c/m) ratios as functions of depth determined by low energy ion scattering spectrometry for surfaces exposed to hydrogen sulfide. The solid lines are for sulfur, dashed lines for oxygen. Where no dashed line is plotted, the oxygen was undetectable. (a) Copper, T = 21 + I°C, RH < 5%, (b) copper, T = 21 _+ I°C, R H = 90 + 2%, (c) silver, T = 20 + I°C, R H = 91 + 2%.
Compounds in the corrosion films. Silver and copper samples given total exposures of 30-80 ppm hr -~ H2S were examined by both Auger and XPS prior to any argon ion sputtering. Minor amounts of both carbon and oxygen were detected, and it was noted that the carbon/oxygen ratio was somewhat higher on samples exposed to H2S. The carbon and oxygen signals detected on the surfaces proved to be from thin contaminant overlayers, however, and were not observed to persist into the bulk film in any significant concentrations. Typically, 5 rain of Ar + sputtering at 2 kV (2 × 10 -~ torr), with the ion beam rastered over an area of approx. 1 cm 2, was sufficient to reduce these signals to the noise level. X-ray photoemission spectra of silver and copper samples exposed to H2S at high humidity were obtained for core levels corresponding to Cu 2p312, Ag 3dr2 and S 2p. Note that the effective sampling depth corresponding to the photoelectron energies is approx. 5 nm. The results are presented in Table 2, together with certain reference values given in the literature. The absolute values of the reference spectra binding
Silver and copper sulfidation by H2S and OCS TABLE 2.
1169
MEASURED AND REFERENCEBINDING ENERGIES ( e V )
Sample
Cu 2p3/2
Cu/H2S Cu/OCS Cu CuS Cu2S CuSO~ Ag/H2S Ag/OCS Ag Ag2S Ag2SO4
931.7 931.9 932.4 932.0 932.3 934.5
Ag d~p_
S 2p
Ref.
162.5 162.4
367.8 368.0 368.0 367.8 367.7
Thiswork Thiswork This work, 23 161.9" 24.25 161.7* 26 168.5t 24, 27 160.6 Thiswork 161.0 This work This work, 23 160.8" Thiswork 168.3:[: Thiswork, 28
*S 2p3/2--unresolved S 2p spectra shift the binding energy up by --~0.3-4).5eV. +Cu 2p3/2--S2p from Ref. 24; alignment of absolute Cu 2p.v2from comparison of data of Refs 24 and 27. $Ag 3d~/2shift from Ref. 28, Ag 3d5/2-S2p from this work.
energies are often scattered over a range of 5 V between authors for a given core level in a fixed compound. It was necessary to examine chemical shifts relative to metallic Cu or Ag as well as Cu 2p3/2-S 2p and A g 3d5/2-S 2p splittings and then to reference all data to metallic Cu 2p3/2 and Ag 3ds/2 core levels at 932.4 and 368.0 eV respectively. In this manner a set of reference data is derived which should be self consistent within +0.5 eV. Examination of Table 2 shows that the films observed on either Cu or A g suggest the formation of sulfides. The formation of sulfates or sulfites can be excluded based both on the core level binding energies and the virtual absence of oxygen within the bulk films. A u g e r spectra of silver and copper samples are shown in Figs 4 and 5. The spectra are indicative of conditions in approx, the upper 3 nm of the films. The small residual carbon signals seen in the spectra were reduced after a 5 min sputter by approx, a factor of 50-70, indicative of an adsorbed surface contaminant layer due to air exposure during handling. On the lightly sputtered samples the Auger lineshapes are relatively constant, but with prolonged sputtering the Cu/S peak-to-peak ratio was observed to increase. This indicates that some selective sputtering of sulfur occurs. The situation is similar for the silver samples. As seen from Table 2, XPS core level shifts would not serve to distinguish between the phases Cu2S and CuS or their intermediates. Data in the literature 5 for sulfide films grown using ppm concentrations of H2S clearly indicate, however, that Cu2S is the favored product. Similar considerations apply to the sulfidation of Ag using ppm concentrations of H2S; 29'3° once again Ag2S is the indicated film component following exposure to H2S.
Sulfidation by carbonyl sulfide Relative humidity dependence. Data on the thickness of sulfur-containing corrosion films formed on silver by OCS exposure at different relative humidities are
1170
T . E . GRAEDEL, J. P. FRANEY, G. J. OUALTIERI, G. W. KAMMLOTTand D. L. MALM
Ag I
I
I
S
Ag/H2S
Ar
C
UJ "o
uJ Z LU
Ag/OCS Ag
r Ar
i
0
I
100
i
I
C
i
I
i
I
200 500 400 ELECTRON KINETIC ENERGY(eV)
i
I
500
F~G. 4. Derivative Auger spectra of lightly sputtered films grown on Ag in an atmosphere of purified air at 95% RH containing approx. 3.5 ppm H2S (top trace) or 3.5 ppm OCS (bottom trace).
shown in Fig. 6. It is apparent that sulfidation due to OCS is a strong function of relative humidity. Similar data for copper are presented in Fig. 7. As with H2S, the thickness of the film grown at relative humidities of approx. 50% or lower is virtually the same, that for 95% humidity is much thicker. These are the first data to be acquired on the sulfidation of metals by OCS as a function of relative humidity. Elements in the corrosion films. The relative ion percents of elements in corrosion films produced by OCS exposure were examined by SSMS; the results are included in Table 1. For the silver sample exposed at high humidity (sample 4), the sulfur signal is strong, while those of oxygen and carbon are very weak and similar to those of the unexposed sample. The situation is similar for copper samples exposed at low or high humidity (samples 6 and 8), although there is some suggestion of minor oxygen incorporation on the former sample. These conclusions are reinforced by the
Silver and copper sulfidation by H2S and OCS
1171
SS
Cu/H2S
I 0
LLI
t.d Z LIJ
Cu/OCS
U: I
I
0
I
100
J
I
200
I
i
I
300
o
I
I
400
i
I
500
ELECTRON KINETIC ENERGY (eV)
FIG. 5. Derivative Auger spectra of lightly sputtered films grown on Cu in an atmosphere of purified air at 95% RH containing approx. 3.5 ppm of H2S (top trace) or 3.5 ppm OCS (bottom trace).
LEISS depth profiles shown in Fig. 8. The corrosion film on silver is thin and no oxygen is seen. The film on copper exposed at low R H shows some oxygen incorporation. At high humidity the sulfide film that grows on copper is thick and does not contain any detectable oxygen.
Compounds in the corrosion film. The results of XPS analyses of the corrosion films formed on silver and copper at high humidity by OCS are presented in Table 2. The Cu/S peak ratios for H2S- and OCS-exposed samples sputtered for identical times are not equal because the contaminant overlayers were not initially equivalent in thickness. A similar statement is true for the Ag samples. One can clearly see, however, that the bulk films consist essentially of Cu/S and Ag/S, respectively and that the compounds which form under HzS or OCS exposure have identical Auger lineshapes.
1172
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIER1, G. W. KAMMLOTT and D. L. MALM 10 2 -LEGEND u : 9 5 % RH
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TOTAL EXPOSURE, P P M - H R
FIG. 6. The sulfidation of silver by carbonyl sulfide at different relative humidities. All experiments were performed with [OCS] = 3.5 ppm and T = 21 + 2°C. The lines are linear least squares fits to the data points at RH = 4, 70 and 95% R H .
We are unable on the basis of XPS data and literature information to distinguish between Cu2S, CuS or their intermediates. However, the similarity in the Auger lineshapes for films grown by either OCS or H2S (Figs 4 and 5) supports the identical assignment of Cu2S to the compound. In the case of silver, Ag2S is the only stable compound. For either metal, a two phase mixture of the sulfides and products containing carbon and/or oxygen can be ruled out based on the absence of the latter constituents in the bulk films.
Atmospheric sulfidation at low relative humidity With the information developed in the experimental work just described, together with information contained in the scientific literature, we can describe a number of aspects of the dry sulfidation process.* The simplest case is that for silver, which does not form an oxide film at temperatures of the order of 21°C, 3t and H2S. The initial step is dissociative adsorption of H2S onto the metal lattice. 32'33 The favored site is at a surface defect 34 where the normal bonding in the silver crystal is unsatisfied. The adsorption may involve an intermediate stage in which an HS radical * 'Dry', in the sense of this work, refers to air with a dew point of - 2 0 ° C . This is equivalent to a relative humidity of ~ 5 % at 2(I°C, or a water content of ~ 1RI0 ppm, and will result in much less than a monolayer of adsorbed water on the metal or metal oxide surface. Although it represents an extreme lower limit for an atmospheric regime, the H20 concentration is more than a thousand times greater than that of any of the corrosive atmospheric gases under field conditions.
Silver and copper sulfidation by H2S and OCS
1173
LEGEND
o : 95*/. RH o : 52% RH
&
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,10a Z
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TOTAL EXPOSURE, PPM-HR
FIG. 7. The sulfidation of copper by carbonyl sulfide at different relative humidities. All experiments were performed with [OCS] ~ 3.5 ppm and T = 21 + 2°C. The lines are linear least squares fits to the data points at 52 and 95% RH.
occupies a surface site, with an H atom on an adjacent site. 35 In any case, hydrogen is readily released. 35The product of the reaction process is Ag2S. 29"3°Once nucleation of the Ag2S occurs, the islands of sulfide grow laterally across the silver surface until they coalesce into a continuous film. At this point diffusion through the Ag2S layer becomes necessary if further growth is to occur. This diffusion process provides the limit to sulfidation by H2S at long exposure times. In the case of copper, the sulfidation process begins on a layer of cuprous oxide about 4 nm thick.5"36 The growth of the sulfide layer thus requires diffusion through the oxide layer or a breakdown of the oxide layer throughout the entire process. H2S dissociatively adsorbs on copper metal to form Cu2S, 37 although there appear to be no comparable studies on the oxide, we envision a similar process. Once nucleation of Cu2S occurs, the subsequent growth of the film will be qualitatively similar to the case for silver. The data of Figs 1 and 2 indicate that at very low relative humidity (note that relative humidity, not absolute humidity appears to be the critical water p a r a m e t e r 38) the sulfide films formed on copper are substantially thicker than those on silver. Dry sulfidation by OCS involves the initial chemisorption of the OCS molecule to the metal surface. We know that the metal surfaces are receptive to the sulfur atom in HeS. It is therefore reasonable to assume that the sulfur atom in OCS behaves similarly. In support of this supposition that such bonding occurs, we note from Table 1 that the oxygen and carbon content of the surface layers of sample 6 is negligibly different from the unexposed sample 5, suggesting that OCS is cleaved at the C = S
1174
o4F
T.E. GRAEDEL,J. P. FRANEY,G. J. GUALTIERI,G. W. KAMMLOTTand D. L. MALM
(a)
0.3 E 0.2 0
0.t 0.0
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=
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L
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I
500
DEPTH ( ARBITRARYUNITS) FIG. 8. Chalcogen to metal (C/M) ratios as functions of depth determined by low energy ion scattering spectrometry for exposures involving carbonyl sulfide. The solid lines are for sulfur, dashed lines for oxygen. Where no dashed line is plotted, the oxygen was undetectable. (a) Copper, T = 21 _+ I°C, RH < 5%, (b) copper, T = 21 + I°C, RH = 90 _+ 2%, (c) silver, T= 21 _+ I°C, RH = 91 + 2%.
b o n d (a w e a k e r b o n d t h a n the O - - - C bond39). This c l e a v a g e will p r o d u c e C O , which is o n l y m i n i m a l l y a b s o r b e d o n t o silver, 4° c o p p e r 4~ a n d o x i d e 42 surfaces, a n d thus escapes. T h e final p r o d u c t , at least at high t e m p e r a t u r e s , is t h e sulfide .43 T h e d e t a i l e d p r o c e s s thus differs f r o m t h a t involving H2S in the d e t a i l s o f the initial a d s o r p t i o n of the g a s e o u s c o r r o d e n t m o l e c u l e .
Atmospheric sulfidation at high relative humidity A s the r e l a t i v e h u m i d i t y is i n c r e a s e d , s e v e r a l m o n o l a y e r s o f w a t e r b e g i n to a d s o r b o n t o the A g a n d C u 2 0 surfaces. In the initial stages, the w a t e r f o r m s clusters on the surface, 44"45 p r o b a b l y at t h e grain b o u n d a r i e s a l o n g which diffusion o f m e t a l ions is f a v o r e d . F o r h u m i d i t i e s > 9 0 % , the w a t e r l a y e r is s e v e r a l m o n o l a y e r s thick on silver 38'46'47 and c u p r o u s o x i d e , 48 so that the p r o p e r p i c t u r e o f the sulfidation p r o c e s s at high h u m i d i t y involves an a q u e o u s q u a s i - s o l u t i o n p h a s e a t o p the solid lattice. 49
Silver and copper sulfidation by H,S and OCS 50 -
(a)
50
SILVER
1175
(b)
SILVER
z
20
/
-iI.-
f .,,~___d,
OCS 0 0
I 20
500-
z
f
/
I
I
40 60 RH,%
"7 I
I
80
100
////
2-.o, /
//
iO
u_
(c)
ram" 20
HzS
laJ z
ocs
COPPER
0
500
4OO -
p
/
d
1
I
I
5 O(H20)
t0
0
z=
(d)
COPPER
400
~
//
l
/'
w 500 m z
~
"r 200 F-
-~ too 0
I
0
Fro. 9.
o..~___~..o/ 20
I
200
U_I00 °"~"~'~cf/ I
40 60 RH, %
I
1
80
t00
0
0
//
.
/ocs
"
I
I
I
S
10
15
0 (H20)
The dependence of H2S and OCS sulfidation as functions of atmospheric and
s u r f a c e water. All ordinate values are derived from linear fits to the data of Figs 1,2, 6 and 7 and are for total s u l f u r o u s g a s e x p o s u r e s of 100 ppm hr ~. (a) Sulfidation of silver as a
function of relative humidity; (b) sulfidation of silver as a function of the monolayer coverage of adsorbed water, where O is the number of monolayers; (c) sutfidation of copper as a function of relative humidity; (d) sulfidation of copper as a function of the monolayer coverage of adsorbed water.
The presence of the aqueous phase enhances sulfidation to a very large degree. To demonstrate this, we use the data from Figs 1,2, 6 and 7 to plot (at the left of Fig. 9) the sulfide film thickness at a total sulfurous gas exposure of 100 ppm hr Las a function of relative humidity. On the right side of Fig. 9 we plot the same film thickness data as functions of the number of water monolayers on the surface, using the surface coverage data from the sources cited in the previous paragraph. Of particular note is the rapid increase in copper sulfidation once three or more monolayers of water are present on the surface. Silver sulfidation shows a more gradual increase as the surface water layer thickens. C o n s i d e r first the case of sulfidation by H~S. The gaseous molecule dissolves readily in water 5" and dissociates. The p K for H,S is approx. 7.5. 5~ The water layer on silver is p r o b a b l y weakly acidic because of dissolved atmospheric CO2, so the H2S
1176
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIERI, G. W. KAMMLOVrand D. L. MALM
D (,/') (/3 ILl Z
~~7~i-:
(,3 m
I-._1 Ix. ttl O
I.O >
tlz TOTAL EXPOSURE
FI6. 10. A schematic representation of a typical sulfidation curve. G is the region where the supply of corrosive gas is the rate-limiting step in the process, D the region where the flux of diffusing metal ions is rate-limiting. Initially (1) the film grows atop pure metal (e.g. silver) or oxide (e.g. Cu_,O). Once a sulfide layer has formed (2), the subsequent growth occurs atop a sulfide surface.
will be predominantly in molecular form. In contrast, Cu20 is weakly hydrolyzed in water, 52 with the resulting solution being slightly basic. In at least the initial stages of copper sulfidation by H2S, therefore, H S - ions will be present. It is interesting to note that the overall activation energy for the sulfidation of copper by H2S is consistent with a free radical process. ~6 The overall stoichiometry for the H2S sulfidation reactions is 2Ag + H2S---+ Ag2S + H2 2Cu + H2S---+ C u 2 S + H 2
(4) (5)
where the reactions and products have all been measured. The reactions are likely to be multistep processes, however, with the prospect of rate-limiting control by different mechanisms at different times. Consider the schematic diagram of Fig. 10, which is representative of the sulfidation of either silver or copper by either H2S or OCS. 3"~3 In the initial stage (1), sulfidation occurs on a silver or Cu20 surface. As soon as a continuous sulfide layer is formed, the surface upon which growth takes place is the sulfide. Over an extended range of total exposure the suifidation is limited by the rate at which sulfurous gas is supplied to the surface, as demonstrated by the fact that short, high concentration exposures produce sulfidation results identical with those for long, low concentration exposures. 3'j3 For very long exposures the sulfidation becomes independent of the sulfurous gas supply, since the thickness and restricted defect structure of the sulfide layer render ion diffusion the controlling factor. We cannot say with certainty what role is played by water in the sulfidation process. Since H,S is very soluble in water, however, it seems likely that the function of the water is to absorb the H2S and make it more readily available to the metal. Absorption can only occur when the water is several monolayers thick, a circumstance that appears to offer a reasonable explanation for the increase in sulfidation
Silver and copper sulfidation by H2S and OCS
1177
that occurs when the adsorbed water on the substrate reaches that average thickness. Since the water tends to be present in clusters or drops, 44'45 the actual volume into which molecules are absorbed will be greater than is suggested by the average monolayer thickness. For the case of carbonyl sulfide, the chemistry becomes somewhat more complex. As with HzS, suifidation by OCS produces Ag2S or CuzS, in a process that is limited by the sulfurous gas supply at low and moderate exposures and by metal ion diffusion at high exposures. The sulfidation rates of OCS and H2S are the same to within the precision of our experiments; hence the presence of sulfur as OCS rather than H2S is not rate-limiting. The hydrolysis of OCS is known to give H2S as a product, 11'53 although the presence of dissolved 02 reduces the H2S yield per OCS molecule to <50%.tl The hydrolysis is base-catalyzed, t1"54"55 and Phillip and Dautzenberg 54 have suggested that the reaction sequence is
/ -0
S II c
+ o.-
N OH
----- cso~- + .ao.
(6)
The support for this mechanism comes from the evidence for an H2S product, the existence of base-catalyzed hydrolysis, and Phillip and Dautzenberg's detection of CSO~-, which they suggest is formed in basic solution by
OCS
-4- OHI I t
,,, -0
/
S II C
\ OH
-.;
0 II ; /C\
-0
SH
0 H']-H2~ ~-.~C / X HO
P H2S + CO 2 .
SH
This constitutes evidence, though indirect, for the intermediate in the base-catalyzed hydrolysis sequence. A problem with attempting to apply this hydrolysis sequence to our results is that it does not explain the fate of the hydrolyzed OCS not detected as H2S. Derdall and Hyne 1~suggest that rapid H2S oxidation by H2S + ½02 ~ S + H 2 0
(7)
is the cause, since the measured H2S yield is decreased by the presence of dissolved 02. Such a reaction is unlikely to proceed in a single step and it seems more likely that unspecified reactive intermediates are involved. Another problem with applying Phillip and Dautzenberg's mechanism to our work is the O H - catalysis step, since most of the sulfidation in our experiments occurs in weakly acidic solution. Basic conditions can be anticipated only in the early stages of sulfidation on water-covered Cu20, since the oxide is slightly soluble in water, C u 2 0 + H 2 0 ~ 2Cu + + 2 O H -
(8)
1178
T . E . GRAEDEL,J. P. FRANEY, G. J. GUALTIERI,G. W. KAMMLOTTand D. L. MALM
....
[M
tl
H20 ~ ; .
.
.
H20
.
IM
I M
ti
L.. . . . . . . .
J
IM
Fl~. 11. A schematic representation of OCS and H2S sulfidation of metals. The wavy lines indicate absorption from the gas phase while the broken lines indicate possible chemical reaction paths. M = metal; I = unspecified intermediate. The reaction paths are discussed in the text.
and will produce a weakly basic solution until the evolving sulfide layer isolates the oxide from the water. OCS hydrolysis in neutral or acidic solution is quite slow, lt,53 despite the fact that the compound is an acid anhydride. (Note that thiocarbonic acid, H O C S O H , does not exist in the free state.56) However, OCS coordinates readily to metal centers with cleavage of the C - - S bond, 39 so that the sulfidation process may not be hydrolysis at all, but may proceed dissociatively by HzS or OCS coordination to the solid surface• In this case, the relative humidity dependence of the sulfidation would reflect either the ability of the water to absorb the sulfurous gases and make them conveniently available or for the water to be directly involved in aiding the coordination reaction• These considerations lead us to the schematic reaction sequence for sulfidation shown in Fig. 11. In the initial step, OCS or H2S is absorbed from the gas phase into the surface water layer. If direct dissociative coordination with the metal occurs (step 1), the result will be an intermediate (I) which will form the sulfide by reaction with a second metal atom (step 4). Alternate pathways are possible• OCS may hydrolyze to form H2S (step 2) and proceed as does the H2S molecule• H2S may dissociate (step 3), with the thiyl ion being the reactive coordination species• It is interesting to note the possibility of an experiment in which NH3, the only basic gas commonly present in the atmosphere, is added to a gaseous H2S mixture• If the H2S sulfidation proceeds through HS (step 3), the pH alteration produced by NH 3 adsorption should alter the sulfidation rate. A second experiment would consist of adding NH 3 to a gaseous OCS mixture• If the OCS sulfidation proceeds by hydrolysis to H2S (step 2), the O H - dependence of the hydrolysis should be demonstrated, although the fact that both steps 2 and 3 are O H - dependent may make the results difficult to interpret• Such modifications may occur in the atmosphere as a result of varying levels of NH3.15
Silver and copper sulfidation by H~S and OCS
1179
CONCLUSIONS T h e e x p e r i m e n t a l w o r k d e s c r i b e d h e r e i n has r e s o l v e d s e v e r a l a s p e c t s o f the a t m o s p h e r i c sulfidation p r o c e s s e s involving H2S a n d O C S . T h e sulfidation rates for silver a n d c o p p e r are s h o w n to be s i m i l a r for H2S a n d O C S . W e h a v e d e m o n s t r a t e d t h a t t h e r e l a t i v e h u m i d i t y d e p e n d e n c e of the sulfidation, while s o m e w h a t d i f f e r e n t for silver a n d c o p p e r , is s i m i l a r no m a t t e r which of the t w o gases is used. W e find t h a t no excess c a r b o n o r o x y g e n is d e t e c t e d on the m e t a l surfaces f o l l o w i n g O C S sulfidation, i n d i c a t i n g that c l e a v a g e of t h e C - - S b o n d is facile u n d e r t h e s e c o n d i t i o n s . F i n a l l y , we h a v e d e t e r m i n e d that t h e sulfidation p r o d u c t s in all cases are the m e t a l sulfides, suggesting t h a t the m e c h a n i s m s o f H2S a n d O C S sulfidation ( a n d p e r h a p s the r e a c t i v e i n t e r m e d i a t e ) a r e s i m i l a r o r i d e n t i c a l . A l t h o u g h the initial r e a c t a n t s a n d final p r o d u c t s a r e k n o w n in e a c h case, the linking processes and intermediate species remain uncertain. We tentatively regard d i r e c t c o o r d i n a t i o n with the m e t a l of the s u l f u r o u s gas m o l e c u l e d i s s o l v e d in a surface w a t e r l a y e r as the m o s t likely p r o c e s s u n d e r typical a t m o s p h e r i c c o n d i t i o n s . T o w h a t e x t e n t can the results be g e n e r a l i z e d ? In a d d i t i o n to silver a n d c o p p e r , H2S is k n o w n to sulfidize s e v e r a l o t h e r m e t a l s . F o r O C S , b e s i d e s the silver a n d c o p p e r e x p e r i m e n t s , only l i m i t e d e x p o s u r e s o f b r o n z e s 2 h a v e b e e n p e r f o r m e d . In e a c h o f t h e cases d i s c u s s e d in this p a p e r , we h a v e d e a l t with r e a c t i o n s b e t w e e n r e d u c e d sulfur gases a n d t r a n s i t i o n m e t a l s . It s e e m s p o s s i b l e that the c o n s i d e r a t i o n s that a p p l y to silver a n d c o p p e r will a p p l y as well to nickel, t u n g s t e n a n d zinc, t h r e e o t h e r t r a n s i t i o n m e t a l s k n o w n to sulfidize 3s57"ss u p o n e x p o s u r e to c o r r o s i v e a t m o s pheres.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20.
REFERENCES T. E. GRAEDEL,J. P. FRANEYand G. W, KAMMLOTT,Science212, 663 (1981). L. SOTO,J. P. FRANEY,T. E. GRAEDELand G. W. KAMMEOYr,Corros. Sci. 23,241 (1983). J. P. FRANEY,G. W. KAMMEOTTand T. E. GRAEDEL,Corros. Sci. 25, 133 (1985). P. BACKEUND,B. FJELLSTROM,S, HAMMARBACKand B. MAIJGREN,Ark. Kemi 26,267 (1966). S. P. SHARMA,J. electrochem. Soc. 127, 21 (1980). D. W. RICE, P. PETERSON, E. B. RIGBY,P. B. P. PH1PPS, R. J. CAPPELLand R. TREMOUREUX,J. electrochem. Soc. 128,275 (1981). J. DRo'rr, Ark. Kemi 15,181 (1959). J. A. LORENZEN,Proc. hist. Environ. Sci. 110 (1971). J. L. SANSREGRET,J. electroehem. Soc. 127, 2083 (1980). S. LIEIANFIEEDand C. E. WHITE,J. Am. chem. Soc. 52,895 (1930). G. DERDALLandJ. B. HVNE,Can. J. chem. Engng57, 112 (1979). M. E. DAVISand T. J. LouzoN, Metallography 13, 185 (1981)). T. E. GRAEDEL,J. P. FRANEYand G. W. KAMMLOTI'Corros. , Sci. 23, 1141 (1983). J. P. FRANEV,Corros. Sci. 23, l (1983). T. t . GRAEDEL, in Handbook of Environmental Chemistry (O. HUTZINGER,ed), Vol. 2A, pp. 107-143. Springer, Heidelberg (1980). J. P. FRANEY,T. E. GRAEDEEand G. W. KAMMEOTT,in Atmospheric" Corrosion (W. H. AILOR,ed), pp. 383-392. John Wiley, New York (1982). J. P. FRANEY,J. electrochem. Soc. 126, 2159 (1979). G. W. KAMMLOYr,Appl. Spectrosc. 35,324 0981). D. P. SMITH,Bull. Am. phys. Soc. 14,788 (1968). R. E. HONIGand W. L. HARmNGTON,Thin Solid Films 19, 43 (1973).
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T . E . GRAEDEL,J. P. FRANEY,G. J. GUALTIERI,G. W. KAMMLOTTand D. L. MALM
21. A.J. AHEARN,Spark Source Mass Spectrometric Analysis of Solids, pp. 347-376. National Bureau of Standards, Monograph 100, 28 April (1967). 22. D. L. MALM,in Physical Measurement and Analysis of Thin Films (E. M. MURTand W. G. GULDNER, eds), pp. 148-167. Plenum Press, New York (1969). 23. C. D. WAGNER,Faraday Discuss. chem. Soc. 60,291 (1975). 24. M. ROMAND,M. ROUBINand P. DELOUME,J. Electron. Spectrosc. Relat. Phenom. 13,229 (1978). 25. H. GONSKA, H. J. FREUND and G. HOHLNEICHER,J. Electron. Spectrosc. Relat. Phenom. 12, 435 (1977). 26. P. E. LARSOH,J. Electron. Spectrosc. Relat. Phenom. 4,213 (1974). 27. D. C. FROST,A. ISHITANIand C. A. McDOWELL, Molec. Phys. 24, 861 (1972). 28. S. W. GARENSTROOMand N. WINOGRAD,J. chem. Phys. 67, 3500 (1977). 29. H. FtSCHMElSTERand J. DRorr, Acta Metall. 7,777 (1959). 30. W. A. CROSSLANDand E. KNmHT, Proc. 5th Int. Symp. Electric Contact Phenom., pp. 324-328. Berlin, VDE Verlang Gmblt. (1970). 31. W. E. CAMPBELLand U. B. THOMAS, Trans. electrochem. Soc. 76,303 (1939). 32. J. PERDEREAOand G. E. RHEAD, Surf. Sci. 7, 175 (1967). 33. J. BERNARD,J. OUDARand F. CABAN~-BROUTV,Surf. Sci. 3,359 (1965). 34. V. PHILLIPS,J. appl. Phys. 33,712 (1962). 35. J. M. SALEH,C. KEMBALLand M. W. ROBERTS, Trans. Faraday Soc. 57, 1771 (1961). 36. E. MATrSSONand R. HOLM, in Atmospheric Corrosion (W. H. AILOR, ed), pp. 365-381. John Wiley, New York (1982). 37. J. L. DOMANGEand J. OUDAR,Surf. Sci. 11,124 (1968). 38. D.W. RICE, R. J. CAPPELL,P. B. P. PHIPPSand P. PETERSON,in Atmospheric Corrosion (W. H. AILOR, ed), pp. 651-666. John Wiley, New York (1982). 39. J. A. IBERS, Chem. Soc. Rev. 11, 57 (1982). 40. T. E. WHVTE,JR., Catal. Rev. 8, 117 (1973). 41. I. TOYOSmMAand G. A. SOMORJAI,Catal. Rev. 19, 105 (1979). 42. R. S. STONE,Adv. Catal. 13, 11 (1962). 43. P. HADJ1SAVAS,M. CAILLET,m. GALERIEand J. BESSON,Rev. Chim. Minerale 14,572 (1977). 44. K. KLIER,J. H. SHEN and A. C. ZETrLEMOVER,J. phys. Chem. 77, 1458 (1973). 45. P. G. HALLand F. C. TOMPK~NS,Trans. Faraday Soc. 58, 1734 (1962). 46. F. P. BOWDENand W. R. THROSSELL, Proc. R. Soc. A 209,297 (1951). 47. H. E. BENNETT, R. L. PECK,D. K. BURGEand J. M. BENNETt,J. appl. Phys. 40, 3351 (1969). 48. S. P. SHARMA,J. Vac. Sci. Technol. 16, 1557 (1979). 49. P. B. P. PH~PPSand D. W. RICE, in Corrosion Chemistry (G. R. BaUBAKERand P. B. P. PHIPPS,eds), Series 89, pp. 235-261. ACS Symp. (1979). 50. H. STEPHENand T. STEPHEN,Solubilities oflnorganic and Organic Compounds I, Part I. Macmillan, New York (1963). 51. M. EmEN, W. KRUSE,G. MAAHSand L. DEMAEVER,in Progress in Reaction Kinetics 2 (G. PORTER, ed), Chap. 6. Macmillan, New York (1964). 52. W.M. LAT1MERand J. H. HILDEBRAND,Reference Book on Inorganic Chemistry, 3rd edn. Macmillan, New York (1951). 53. H. W. THOMPSON,C. F. KEARTONand S. A. LAMa,J. chem. Soc. 1033 (1935). 54. B. PHILIPPand H. DAUTZENBERG,Z. Phys. Chem. 229,210 (1965). 55. M. M. SHARMA,Trans. Faraday Soc. 61,681 (1965). 56. Dictionary of Organic Compounds, 4th rev. edn, Vol. 5 (1965). 57. W.Z. FRIENn, Corrosion of Nickeland Nickel-Base Alloys, pp. 103ff. John Wiley, New York (1980). 58. D.J. SPEDDING,Corros. Sci. 17, 173 (1977).
0010-938X/85 $3.00+ 0.00 (~) 1985PergamonPressLtd.
ON THE MECHANISM OF SILVER SULFIDATION BY ATMOSPHERIC T. E.
AND COPPER H2S AND OCS
GRAEDEL,J. P. FRANEY,G . J. GUALTIERI,G . W . KAMMLOTY a n d D. L. MALM AT&T Bell Laboratories, Murray Hill, NJ 07974, U.S.A.
Abstract--Hydrogen sulfide (H2S) and carbonyl sulfide (OCS) are the principal atmospheric corrodents for silver and copper. To investigate the mechanisms by which the sulfidation reactions occur, the two metals have been exposed to each of the sulfurous gases under a wide range of relative humidities. The sulfidized samples were analyzed by scanning electron microscopy, X-ray fluorescence spectroscopy, low energy ion scattering spectrometry, spark source mass spectroscopy, Auger spectroscopy and X-ray photoemission spectroscopy. The results demonstrate that sulfidation of both metals by both gases is strongly dependent on the relative humidity, that negligible carbon or oxygen is incorporated into the sulfide layer during OCS sulfidation and that the products of sulfidation by either H2S or OCS are Ag2S and Cu2S. These results are used to develop a general mechanism for atmospheric sulfidation in which the absorption of the gas into the surface water layer is followed directly by dissociative coordination with metal atoms. Experiments capable of confirming some of the steps in this mechanism are proposed. INTRODUCTION THE SULFIDATION o f m e t a l s u p o n e x p o s u r e to the a t m o s p h e r e is a c o m m o n p h e n o m e n o n , a n d s t u d i e s o f the r a t e s o f sulfidation h a v e b e e n c o n d u c t e d for half a c e n t u r y . D e s p i t e this h i s t o r y , t h e r e is c o n s i d e r a b l e u n c e r t a i n t y c o n c e r n i n g the s p e c i e s a n d m e c h a n i s m s i n v o l v e d in t h e sulfidation p r o c e s s . T h e s i t u a t i o n has g r o w n still m o r e c o m p l e x with t h e d i s c o v e r y t h a t c a r b o n y l sulfide, d e t e c t e d in the a t m o s p h e r e r e l a t i v e l y r e c e n t l y , is c a p a b l e of sulfidizing c o p p e r , 1 c o p p e r alloys 2 a n d silver. 3 C o p p e r sulfidation b y h y d r o g e n sulfide (H2S) is k n o w n to be a sensitive function of r e l a t i v e h u m i d i t y ( R H ) . B a c k l u n d et al. 4 s h o w e d t h a t the sulfidation r a t e c o u l d b e i n c r e a s e d b y m o r e t h a n an o r d e r o f m a g n i t u d e as the R H i n c r e a s e d f r o m 0 to 100%. S h a r m a , 5 w o r k i n g at m u c h l o w e r a n d m o r e a t m o s p h e r i c a l l y realistic H2S c o n c e n t r a tions, c o n f i r m e d this finding. A similar result was o b t a i n e d by R i c e et al. ,6 using a test involving s e v e r a l c o r r o d e n t gases. N o s t u d i e s o f the r e l a t i v e h u m i d i t y d e p e n d e n c e o f c o p p e r sulfidation b y O C S h a v e b e e n r e p o r t e d . Less a g r e e m e n t exists a m o n g l i t e r a t u r e r e p o r t s on the r e l a t i v e h u m i d i t y d e p e n d e n c e of the H2S sulfidation o f silver. T h r e e sets of e x p e r i m e n t s , two using H2S as the o n l y c o r r o d e n t 4'7 a n d o n e using m i x e d c o r r o d e n t s i n c l u d i n g H2S, s r e p o r t a positive d e p e n d e n c e , with t h e sulfidation r a t e i n c r e a s i n g s h a r p l y a b o v e 70% R H . In c o n t r a s t , the m o s t r e c e n t w o r k utilizing m i x e d c o r r o d e n t s including H2S finds no r e l a t i v e h u m i d i t y d e p e n d e n c e . 6 NO studies involving O C S have b e e n p e r f o r m e d . T h e r e a c t i o n m e c h a n i s m s of c o p p e r a n d silver suifidation a r e o f significant scientific a n d t e c h n i c a l i n t e r e s t , b u t r e m a i n u n c e r t a i n . It has b e e n s u g g e s t e d 5 t h a t , in Manuscript received 22 March 1985. 1163
1164
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIERI, G. W. KAMMLOTrand D. L. MALM
the absence of water (a condition virtually never attained in the earth's lower atmosphere), H2S can be dissociatively adsorbed on copper by the reaction H2S ~ H2 + S2- + 2@,
(1)
where O represents a positive hole in the sulfide. The S2- is then available to react with metal ions. Alternatively, it is possible 9 that on cuprous oxide the conversion does not involve an intermediate ion: Cu20 + H2S --~ Cu2S + H20.
(2)
The evidence to support either of these mechanisms is not yet compelling. In the presence of water saturated with O2, the formation of free sulfur has been proposed: lO H2S + ½02--o S + H20.
(3)
Although the presence of free sulfur in some reaction vessels I1 may offer support for this inherently multistep mechanism, it may merely indicate that free sulfur is produced at high solution concentrations by a variety of mechanisms. Similar reactions have been proposed for silver,I° although without substantial evidence for their validity. The purpose of this work is to ameliorate this paucity of information on the degradation processes of widely used metals. To this end, we have conducted careful exposure experiments and analyzed the corrosion films by a variety of analytical techniques capable of determining the thickness of the corrosive films and their elemental and molecular content. The results of our work are presented below.
EXPERIMENTAL
METHOD
Sample preparation and exposure The copper samples used for these experiments were 'oxygen-free, high conductivity' copper (99.99% pure), prepared as disks 1 cm in diameter and 2 mm thick. The silver samples were 99.9999% pure and were prepared by machining 1 cm z coupons from sheet stock. Following initial wet smoothing with 600 grit AI203 paper, the copper and silver samples were sequentially polished with finer AI203 powders, culminating in polishing with powder of average diameter 0.3 p,m. The technique is described in more detail by Davis and Louzon. ~z The process yields a surface with mean roughness <0.3/~m. Replicate samples prepared by this technique show variations in sulfidation rate of < 2 % , much less than that of replicate samples prepared by etching techniques.13 The corrosive environment to which the samples were exposed consisted of approx. 3.5 parts per million (ppm) of hydrogen sulfide or carbonyl sulfide gas in humidified air at room temperature (21 + 2°C). The stability of the sulfurous gas concentrations during the experiments was of order +5%. 14H2S or OCS concentrations higher than a small fraction of a ppm are rare in field environments I~ but our finding that the sulfidation of copper and silver is a function of total exposure ~6permits us to use higher concentrations to shorten the exposure time. The samples were readied for exposure in the multiport chamber ~4by attaching them to flexible inserts sized to fit the chamber ports: this permitted samples to be inserted and withdrawn rapidly and without noticeably altering the corrosive environment. Control of the HzS or OCS concentration in the chamber was achieved by using polymer permeation techniques) 7 The water content of the air was varied by adjusting a portion of the carrier air supply that passes through a humidifier. A dew point hygrometer and hot wire anemometer monitored the dew point and flow rate continuously. Each sensor was sampled periodically throughout the exposure period by a dedicated desktop computer system.
Silver and copper sulfidation by H2S and OCS
1165
Sulfide film thickness analysis The thicknesses of the sulfide films grown on the copper and silver were determined by energydispersive X-ray analysis, using a Kent-Cambridge 2A SEM equipped with a solid state X-ray detector and a multichannel analyzer. The preparation of thickness standards necessary for this method is discussed elsewhere, ts The calibration curves that were used relate the ratio of intensities of metal and sulfur X-rays to the thickness of the sulfidation film. The results are accurate to _+8--10 nm or _+20%, whichever is smaller.
Auger and X-ray photoemission spectroscopy Auger and X-ray photoemission (XPS) spectra were taken on a commercial ESCA/Auger spectrometer (PHI, model 548) employing a double pass cylindrical mirror analyzer with a concentric electron gun. Auger spectra were collected in the standard differential scanning mode with AE/E = 0.65% at an excitation energy of 2 kV with approximately 20 p~A of beam current. The modulation voltage for all spectra was 3 V peak-to-peak. Photoemission spectra were collected using AI K a radiation. The instrumental bandpass contribution to the resolution was fixed at 0.32 eV for the Cu 2p3/2 core levels. The intrinsic linewidth of the A1 K a source contributes an additional 1.1 eV to the overall linewidth. Binding energies of unsputtered surfaces were referenced to the carbon Is core level at 285.0 eV. The Cu 2p3/2 and Ag 3d5/2 photopeaks were employed for alignment after sputtering and were not found to be affected by sputtering within an experimental error of +0.5 eV.
Low energy ion scattering spectrometry Low energy ion scattering spectrometry (LEISS) was used to study the elemental composition of the corrosion films, LEISS is described in detail by Smith 19and by Honig and Harrison 2, The method makes use of elastic binary collisions of <10 keV rare gas ions (He, Ne or Ar) with surface atoms to provide energy spectra characteristic of the atomic mass of the scattering centers. This information can be used to determine the composition of the outermost atomic layer of a solid regardless of its conductivity, with a detection limit of approx. 0.1 at.%. By the use of controlled ion-bombardment etching (sputtering), profiles of the distribution of the elemental constituents as a function of depth can be determined. The sample spot size was approx. 1 mm in diameter; the maximum penetration depth studied in these experiments was approx. 300 nm. The instrumental response was not quantitative, but profiles for the respective samples could be compared on a relative basis.
Spark source mass spectrometry This method, referred to hereafter as SSMS, provides for the vaporization, ionization and mass analysis of positive ions originating from the elemental constituents of a solid. This type of ion excitation is relatively non-selective in that the respective ion fractions generated in the arc are proportional to the atomic composition of the sample to within a factor of five. In the surface analysis mode, sampling is carefully confined to the outer 5-10/~m of surface thus providing chemical composition information for this region that is independent of that at greater depth. The principal advantages of this technique for monitoring relative compositional changes in thin films are the speed and sensitivity with which the presence of surface species can be determined. Results can be made quantitative on a relative basis for impurities which are present at the 1 at.% level or lower, provided they do not differ significantly in ionization potential or volatility. Accurate, absolute determinations of impurity concentrations at this level require the use of standards. The sampling procedure used for surface analysis was to record spectra from at least 10 distinct areas of the corrosion film. The arithmetic means and standard deviations of the ion intensities of interest were recorded. The results are expressed as ion percent by first computing the total ion exposure and then taking the percentage of the total for each ion species. Additional details concerning SSMS are available in the literature.2~ ._,2 'EXPERIMENTAL
RESULTS
Sulfidation by hydrogen sulfide Relative humidity dependence. I n F i g . 1 w e p r e s e n t
data on the thickness
of
1166
T . E . GRAEDEL, J. P. FRANEY, G. J, GUAI,T1ERI, G. W. KAMMLOTTand D. L. MALM 102 -
LEGEND [3 : 9 5 %
d
58% RH
o=
,8' o RH
,
-y-
:E
z
RH
o =
j
101
w Z v 0 -r I..J
/
,.i 10°
1
'10 .4
I
I
104
i
I
I Ill[
I
I
I
I
I Ill]
402
i
I
105
TOTAL EXPOSURE, PPM- HR
FIG. 1. The sulfidation of silver by hydrogen sulfide at different relative humidities. All experiments were performed with [H2S ] ~ 3.5 ppm and T = 21 _+ 2°C. The lines are linear least squares fits to the data points for RH = 5, 31 and 95%.
sulfur-containing corrosion films formed on silver by H2S exposure at different relative humidities. The data indicate clearly that the sulfidation is strongly dependent on the relative humidity, in agreement with previous work. 4,7 Similar data for copper are shown in Fig. 2. The sulfide film growth is quite similar at 5 and 39% RH, but reaches significantly higher levels at 95% RH. Again the results agree with previous data. 4~ It is clear that water is involved in the sulfidation process, particularly at high humidities. TABLE 1.
RELATIVE
ION
PERCENT
OF TARNISH
CONSTITUENTS
AS
DETERMINED
BY S P A R K
SOURCE
MASS
SPECTROMETRY
Sample number
Sample
Metal
Sulfur
Oxygen
Carbon
96.9+3.1 86.3_+7.9 76.8± 16.3 81.9_+ 14.4 88.6-+ 12.1 80.6+7.6 85.2 -+ 9.0 76.8± 13.[)
n.d.* 9.4_+5.8 18.3_+ 13.3 13.3+ 10.4 n.d. 10.2+4.3 10.3 -+ 4.4 16.7± 12.4
1.8+2.1 2.9+2.8 3.6+6.2 2.2-+3.2 5.5+5.7 5.6-+3.5 2.9 + 3.4 1.9±2.0
1.4+ 1.9 1.4_+1.4 1.3± 1.9 2.5-+2.7 5.9-+6.8 3.6-+3.2 1.6 -+ 2.3 1.6_+ 1.6
1.
Unexposed silver
2.
Ag, H~S,5%RH
3.
Ag, H2S,95% RH
4.
Ag, OCS,95%RH
5. 6. 7. 8.
Unexposed copper C u , O C S , 5 % RH Cu, H2S, 95% RH Cu, OCS,95% RH
* n.d. = not detected.
Silver and copper sulfidation by H2S and OCS
LEGEND
.-m-
[3 = 95*/. RH o = 30% RH '~ = 5% RH
z
~
~ ~
-e~-/
L
"r~-v
1167
+~///~
~/ /~// ,
/////TI ,#
.
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i / / /
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/
I I
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I
I lllllll
I
I I IIIIII
IO 0 t01 IO 2 TOTAL EXPOSURE, PPM-HR
I
I I IIIIII
10 3
Fro. 2. The sulfidation of copper by hydrogen sulfide at different relative humidities. All experiments were performed with [H2S ] = 3.5 ppm and T = 21 _+ 2°C. The lines are linear least squares fits to the data points for RH = 5 and 95°/,,.
Elements in the corrosion films. Samples of silver and copper that were prepared but unexposed were examined with SSMS, as were samples exposed to H2S under low and high RH conditions. The results are presented in Table 1. Since SSMS examines approximately the top 5-10/xm of the sample and since the corrosion films are 0.01q3.5/zm thick, the entire film as well as the bulk material is being sampled. The unexposed samples 1 and 5 have no detectable sulfur, and carbon and oxygen signals that have very large error bars but may reflect a minor degree of surface deposition of unincorporated material. For silver exposed to H2S (samples 2 and 3), the sulfur signal is well above the background level and is higher at the higher humidity. No significant carbon or oxygen signature is detectable. The situation is similar for copper exposed to H2S at high humidity (sample 7). The depth profiles of elements in the surface films are shown by the LEISS analyses displayed in Fig. 3. For copper at low humidity [display (a)], the oxygen signal is confined to the outer skin of the sample, indicating that oxygen is not continuously incorporated into the growing corrosion film. The sulfur is present at relatively high levels for a depth of 100-200 nm. At high humidity [display (b)], the oxygen behavior is similar but the film containing sulfur is much thicker. In the case of silver [display (c)], no oxygen is detected (recall that silver does not form an oxide at these temperatures) and the sulfur-containing layer is much thinner than that of copper.
1168
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIERI, G. W. KAMMLOTTand D. L. MALM 0.5 0.4
-
(o)
E 0.3 u 0.2 0.'1
\
0.0
~
I
i
I
)
I
i
I
i
i\
I
i
I
I
I
i
I
I
0.5 0.4 E 0.3
"~
o.z
-\
O. 1 0.0
\
0.4
I
(c)
E 0.3 o
0,2 0.t
0.0
i
O
I
I
100
I
200
I
I
I
300
I
400
t
I
500
DEPTH (ARBITRARYUNITS)
FIG. 3. Chalcogen to metal (c/m) ratios as functions of depth determined by low energy ion scattering spectrometry for surfaces exposed to hydrogen sulfide. The solid lines are for sulfur, dashed lines for oxygen. Where no dashed line is plotted, the oxygen was undetectable. (a) Copper, T = 21 + I°C, RH < 5%, (b) copper, T = 21 _+ I°C, R H = 90 + 2%, (c) silver, T = 20 + I°C, R H = 91 + 2%.
Compounds in the corrosion films. Silver and copper samples given total exposures of 30-80 ppm hr -~ H2S were examined by both Auger and XPS prior to any argon ion sputtering. Minor amounts of both carbon and oxygen were detected, and it was noted that the carbon/oxygen ratio was somewhat higher on samples exposed to H2S. The carbon and oxygen signals detected on the surfaces proved to be from thin contaminant overlayers, however, and were not observed to persist into the bulk film in any significant concentrations. Typically, 5 rain of Ar + sputtering at 2 kV (2 × 10 -~ torr), with the ion beam rastered over an area of approx. 1 cm 2, was sufficient to reduce these signals to the noise level. X-ray photoemission spectra of silver and copper samples exposed to H2S at high humidity were obtained for core levels corresponding to Cu 2p312, Ag 3dr2 and S 2p. Note that the effective sampling depth corresponding to the photoelectron energies is approx. 5 nm. The results are presented in Table 2, together with certain reference values given in the literature. The absolute values of the reference spectra binding
Silver and copper sulfidation by H2S and OCS TABLE 2.
1169
MEASURED AND REFERENCEBINDING ENERGIES ( e V )
Sample
Cu 2p3/2
Cu/H2S Cu/OCS Cu CuS Cu2S CuSO~ Ag/H2S Ag/OCS Ag Ag2S Ag2SO4
931.7 931.9 932.4 932.0 932.3 934.5
Ag d~p_
S 2p
Ref.
162.5 162.4
367.8 368.0 368.0 367.8 367.7
Thiswork Thiswork This work, 23 161.9" 24.25 161.7* 26 168.5t 24, 27 160.6 Thiswork 161.0 This work This work, 23 160.8" Thiswork 168.3:[: Thiswork, 28
*S 2p3/2--unresolved S 2p spectra shift the binding energy up by --~0.3-4).5eV. +Cu 2p3/2--S2p from Ref. 24; alignment of absolute Cu 2p.v2from comparison of data of Refs 24 and 27. $Ag 3d~/2shift from Ref. 28, Ag 3d5/2-S2p from this work.
energies are often scattered over a range of 5 V between authors for a given core level in a fixed compound. It was necessary to examine chemical shifts relative to metallic Cu or Ag as well as Cu 2p3/2-S 2p and A g 3d5/2-S 2p splittings and then to reference all data to metallic Cu 2p3/2 and Ag 3ds/2 core levels at 932.4 and 368.0 eV respectively. In this manner a set of reference data is derived which should be self consistent within +0.5 eV. Examination of Table 2 shows that the films observed on either Cu or A g suggest the formation of sulfides. The formation of sulfates or sulfites can be excluded based both on the core level binding energies and the virtual absence of oxygen within the bulk films. A u g e r spectra of silver and copper samples are shown in Figs 4 and 5. The spectra are indicative of conditions in approx, the upper 3 nm of the films. The small residual carbon signals seen in the spectra were reduced after a 5 min sputter by approx, a factor of 50-70, indicative of an adsorbed surface contaminant layer due to air exposure during handling. On the lightly sputtered samples the Auger lineshapes are relatively constant, but with prolonged sputtering the Cu/S peak-to-peak ratio was observed to increase. This indicates that some selective sputtering of sulfur occurs. The situation is similar for the silver samples. As seen from Table 2, XPS core level shifts would not serve to distinguish between the phases Cu2S and CuS or their intermediates. Data in the literature 5 for sulfide films grown using ppm concentrations of H2S clearly indicate, however, that Cu2S is the favored product. Similar considerations apply to the sulfidation of Ag using ppm concentrations of H2S; 29'3° once again Ag2S is the indicated film component following exposure to H2S.
Sulfidation by carbonyl sulfide Relative humidity dependence. Data on the thickness of sulfur-containing corrosion films formed on silver by OCS exposure at different relative humidities are
1170
T . E . GRAEDEL, J. P. FRANEY, G. J. OUALTIERI, G. W. KAMMLOTTand D. L. MALM
Ag I
I
I
S
Ag/H2S
Ar
C
UJ "o
uJ Z LU
Ag/OCS Ag
r Ar
i
0
I
100
i
I
C
i
I
i
I
200 500 400 ELECTRON KINETIC ENERGY(eV)
i
I
500
F~G. 4. Derivative Auger spectra of lightly sputtered films grown on Ag in an atmosphere of purified air at 95% RH containing approx. 3.5 ppm H2S (top trace) or 3.5 ppm OCS (bottom trace).
shown in Fig. 6. It is apparent that sulfidation due to OCS is a strong function of relative humidity. Similar data for copper are presented in Fig. 7. As with H2S, the thickness of the film grown at relative humidities of approx. 50% or lower is virtually the same, that for 95% humidity is much thicker. These are the first data to be acquired on the sulfidation of metals by OCS as a function of relative humidity. Elements in the corrosion films. The relative ion percents of elements in corrosion films produced by OCS exposure were examined by SSMS; the results are included in Table 1. For the silver sample exposed at high humidity (sample 4), the sulfur signal is strong, while those of oxygen and carbon are very weak and similar to those of the unexposed sample. The situation is similar for copper samples exposed at low or high humidity (samples 6 and 8), although there is some suggestion of minor oxygen incorporation on the former sample. These conclusions are reinforced by the
Silver and copper sulfidation by H2S and OCS
1171
SS
Cu/H2S
I 0
LLI
t.d Z LIJ
Cu/OCS
U: I
I
0
I
100
J
I
200
I
i
I
300
o
I
I
400
i
I
500
ELECTRON KINETIC ENERGY (eV)
FIG. 5. Derivative Auger spectra of lightly sputtered films grown on Cu in an atmosphere of purified air at 95% RH containing approx. 3.5 ppm of H2S (top trace) or 3.5 ppm OCS (bottom trace).
LEISS depth profiles shown in Fig. 8. The corrosion film on silver is thin and no oxygen is seen. The film on copper exposed at low R H shows some oxygen incorporation. At high humidity the sulfide film that grows on copper is thick and does not contain any detectable oxygen.
Compounds in the corrosion film. The results of XPS analyses of the corrosion films formed on silver and copper at high humidity by OCS are presented in Table 2. The Cu/S peak ratios for H2S- and OCS-exposed samples sputtered for identical times are not equal because the contaminant overlayers were not initially equivalent in thickness. A similar statement is true for the Ag samples. One can clearly see, however, that the bulk films consist essentially of Cu/S and Ag/S, respectively and that the compounds which form under HzS or OCS exposure have identical Auger lineshapes.
1172
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIER1, G. W. KAMMLOTT and D. L. MALM 10 2 -LEGEND u : 9 5 % RH
o: o:s:gH
i+
z if) Z
T I'--
101
:E d LL
uJ
10 0
10 t
I
I
I
I
I
Illl
I
I
1
I
t0 2
I
I II
I
10 3
TOTAL EXPOSURE, P P M - H R
FIG. 6. The sulfidation of silver by carbonyl sulfide at different relative humidities. All experiments were performed with [OCS] = 3.5 ppm and T = 21 + 2°C. The lines are linear least squares fits to the data points at RH = 4, 70 and 95% R H .
We are unable on the basis of XPS data and literature information to distinguish between Cu2S, CuS or their intermediates. However, the similarity in the Auger lineshapes for films grown by either OCS or H2S (Figs 4 and 5) supports the identical assignment of Cu2S to the compound. In the case of silver, Ag2S is the only stable compound. For either metal, a two phase mixture of the sulfides and products containing carbon and/or oxygen can be ruled out based on the absence of the latter constituents in the bulk films.
Atmospheric sulfidation at low relative humidity With the information developed in the experimental work just described, together with information contained in the scientific literature, we can describe a number of aspects of the dry sulfidation process.* The simplest case is that for silver, which does not form an oxide film at temperatures of the order of 21°C, 3t and H2S. The initial step is dissociative adsorption of H2S onto the metal lattice. 32'33 The favored site is at a surface defect 34 where the normal bonding in the silver crystal is unsatisfied. The adsorption may involve an intermediate stage in which an HS radical * 'Dry', in the sense of this work, refers to air with a dew point of - 2 0 ° C . This is equivalent to a relative humidity of ~ 5 % at 2(I°C, or a water content of ~ 1RI0 ppm, and will result in much less than a monolayer of adsorbed water on the metal or metal oxide surface. Although it represents an extreme lower limit for an atmospheric regime, the H20 concentration is more than a thousand times greater than that of any of the corrosive atmospheric gases under field conditions.
Silver and copper sulfidation by H2S and OCS
1173
LEGEND
o : 95*/. RH o : 52% RH
&
:~
~ : 4Y° RH
,10a Z
i,i Z
-r t"
101~ hl >
I0
I
10°
I
I
I IIII1
I
t0 t
I
I
I III1[
I
t0 2
I
I
I Ill|[
10 3
TOTAL EXPOSURE, PPM-HR
FIG. 7. The sulfidation of copper by carbonyl sulfide at different relative humidities. All experiments were performed with [OCS] ~ 3.5 ppm and T = 21 + 2°C. The lines are linear least squares fits to the data points at 52 and 95% RH.
occupies a surface site, with an H atom on an adjacent site. 35 In any case, hydrogen is readily released. 35The product of the reaction process is Ag2S. 29"3°Once nucleation of the Ag2S occurs, the islands of sulfide grow laterally across the silver surface until they coalesce into a continuous film. At this point diffusion through the Ag2S layer becomes necessary if further growth is to occur. This diffusion process provides the limit to sulfidation by H2S at long exposure times. In the case of copper, the sulfidation process begins on a layer of cuprous oxide about 4 nm thick.5"36 The growth of the sulfide layer thus requires diffusion through the oxide layer or a breakdown of the oxide layer throughout the entire process. H2S dissociatively adsorbs on copper metal to form Cu2S, 37 although there appear to be no comparable studies on the oxide, we envision a similar process. Once nucleation of Cu2S occurs, the subsequent growth of the film will be qualitatively similar to the case for silver. The data of Figs 1 and 2 indicate that at very low relative humidity (note that relative humidity, not absolute humidity appears to be the critical water p a r a m e t e r 38) the sulfide films formed on copper are substantially thicker than those on silver. Dry sulfidation by OCS involves the initial chemisorption of the OCS molecule to the metal surface. We know that the metal surfaces are receptive to the sulfur atom in HeS. It is therefore reasonable to assume that the sulfur atom in OCS behaves similarly. In support of this supposition that such bonding occurs, we note from Table 1 that the oxygen and carbon content of the surface layers of sample 6 is negligibly different from the unexposed sample 5, suggesting that OCS is cleaved at the C = S
1174
o4F
T.E. GRAEDEL,J. P. FRANEY,G. J. GUALTIERI,G. W. KAMMLOTTand D. L. MALM
(a)
0.3 E 0.2 0
0.t 0.0
I
=
u
E U
0.7 0.6 0.5 0.4 03 0.2 0.t 0.0
I
(b)
0.4
051 ~ 0.3 0.2 0.t 0.0 J
L
i
I
I
i
I
(c)
1
I
100
I
200
I
300
I
400
J
I
500
DEPTH ( ARBITRARYUNITS) FIG. 8. Chalcogen to metal (C/M) ratios as functions of depth determined by low energy ion scattering spectrometry for exposures involving carbonyl sulfide. The solid lines are for sulfur, dashed lines for oxygen. Where no dashed line is plotted, the oxygen was undetectable. (a) Copper, T = 21 _+ I°C, RH < 5%, (b) copper, T = 21 + I°C, RH = 90 _+ 2%, (c) silver, T= 21 _+ I°C, RH = 91 + 2%.
b o n d (a w e a k e r b o n d t h a n the O - - - C bond39). This c l e a v a g e will p r o d u c e C O , which is o n l y m i n i m a l l y a b s o r b e d o n t o silver, 4° c o p p e r 4~ a n d o x i d e 42 surfaces, a n d thus escapes. T h e final p r o d u c t , at least at high t e m p e r a t u r e s , is t h e sulfide .43 T h e d e t a i l e d p r o c e s s thus differs f r o m t h a t involving H2S in the d e t a i l s o f the initial a d s o r p t i o n of the g a s e o u s c o r r o d e n t m o l e c u l e .
Atmospheric sulfidation at high relative humidity A s the r e l a t i v e h u m i d i t y is i n c r e a s e d , s e v e r a l m o n o l a y e r s o f w a t e r b e g i n to a d s o r b o n t o the A g a n d C u 2 0 surfaces. In the initial stages, the w a t e r f o r m s clusters on the surface, 44"45 p r o b a b l y at t h e grain b o u n d a r i e s a l o n g which diffusion o f m e t a l ions is f a v o r e d . F o r h u m i d i t i e s > 9 0 % , the w a t e r l a y e r is s e v e r a l m o n o l a y e r s thick on silver 38'46'47 and c u p r o u s o x i d e , 48 so that the p r o p e r p i c t u r e o f the sulfidation p r o c e s s at high h u m i d i t y involves an a q u e o u s q u a s i - s o l u t i o n p h a s e a t o p the solid lattice. 49
Silver and copper sulfidation by H,S and OCS 50 -
(a)
50
SILVER
1175
(b)
SILVER
z
20
/
-iI.-
f .,,~___d,
OCS 0 0
I 20
500-
z
f
/
I
I
40 60 RH,%
"7 I
I
80
100
////
2-.o, /
//
iO
u_
(c)
ram" 20
HzS
laJ z
ocs
COPPER
0
500
4OO -
p
/
d
1
I
I
5 O(H20)
t0
0
z=
(d)
COPPER
400
~
//
l
/'
w 500 m z
~
"r 200 F-
-~ too 0
I
0
Fro. 9.
o..~___~..o/ 20
I
200
U_I00 °"~"~'~cf/ I
40 60 RH, %
I
1
80
t00
0
0
//
.
/ocs
"
I
I
I
S
10
15
0 (H20)
The dependence of H2S and OCS sulfidation as functions of atmospheric and
s u r f a c e water. All ordinate values are derived from linear fits to the data of Figs 1,2, 6 and 7 and are for total s u l f u r o u s g a s e x p o s u r e s of 100 ppm hr ~. (a) Sulfidation of silver as a
function of relative humidity; (b) sulfidation of silver as a function of the monolayer coverage of adsorbed water, where O is the number of monolayers; (c) sutfidation of copper as a function of relative humidity; (d) sulfidation of copper as a function of the monolayer coverage of adsorbed water.
The presence of the aqueous phase enhances sulfidation to a very large degree. To demonstrate this, we use the data from Figs 1,2, 6 and 7 to plot (at the left of Fig. 9) the sulfide film thickness at a total sulfurous gas exposure of 100 ppm hr Las a function of relative humidity. On the right side of Fig. 9 we plot the same film thickness data as functions of the number of water monolayers on the surface, using the surface coverage data from the sources cited in the previous paragraph. Of particular note is the rapid increase in copper sulfidation once three or more monolayers of water are present on the surface. Silver sulfidation shows a more gradual increase as the surface water layer thickens. C o n s i d e r first the case of sulfidation by H~S. The gaseous molecule dissolves readily in water 5" and dissociates. The p K for H,S is approx. 7.5. 5~ The water layer on silver is p r o b a b l y weakly acidic because of dissolved atmospheric CO2, so the H2S
1176
T . E . GRAEDEL, J. P. FRANEY, G. J. GUALTIERI, G. W. KAMMLOVrand D. L. MALM
D (,/') (/3 ILl Z
~~7~i-:
(,3 m
I-._1 Ix. ttl O
I.O >
tlz TOTAL EXPOSURE
FI6. 10. A schematic representation of a typical sulfidation curve. G is the region where the supply of corrosive gas is the rate-limiting step in the process, D the region where the flux of diffusing metal ions is rate-limiting. Initially (1) the film grows atop pure metal (e.g. silver) or oxide (e.g. Cu_,O). Once a sulfide layer has formed (2), the subsequent growth occurs atop a sulfide surface.
will be predominantly in molecular form. In contrast, Cu20 is weakly hydrolyzed in water, 52 with the resulting solution being slightly basic. In at least the initial stages of copper sulfidation by H2S, therefore, H S - ions will be present. It is interesting to note that the overall activation energy for the sulfidation of copper by H2S is consistent with a free radical process. ~6 The overall stoichiometry for the H2S sulfidation reactions is 2Ag + H2S---+ Ag2S + H2 2Cu + H2S---+ C u 2 S + H 2
(4) (5)
where the reactions and products have all been measured. The reactions are likely to be multistep processes, however, with the prospect of rate-limiting control by different mechanisms at different times. Consider the schematic diagram of Fig. 10, which is representative of the sulfidation of either silver or copper by either H2S or OCS. 3"~3 In the initial stage (1), sulfidation occurs on a silver or Cu20 surface. As soon as a continuous sulfide layer is formed, the surface upon which growth takes place is the sulfide. Over an extended range of total exposure the suifidation is limited by the rate at which sulfurous gas is supplied to the surface, as demonstrated by the fact that short, high concentration exposures produce sulfidation results identical with those for long, low concentration exposures. 3'j3 For very long exposures the sulfidation becomes independent of the sulfurous gas supply, since the thickness and restricted defect structure of the sulfide layer render ion diffusion the controlling factor. We cannot say with certainty what role is played by water in the sulfidation process. Since H,S is very soluble in water, however, it seems likely that the function of the water is to absorb the H2S and make it more readily available to the metal. Absorption can only occur when the water is several monolayers thick, a circumstance that appears to offer a reasonable explanation for the increase in sulfidation
Silver and copper sulfidation by H2S and OCS
1177
that occurs when the adsorbed water on the substrate reaches that average thickness. Since the water tends to be present in clusters or drops, 44'45 the actual volume into which molecules are absorbed will be greater than is suggested by the average monolayer thickness. For the case of carbonyl sulfide, the chemistry becomes somewhat more complex. As with HzS, suifidation by OCS produces Ag2S or CuzS, in a process that is limited by the sulfurous gas supply at low and moderate exposures and by metal ion diffusion at high exposures. The sulfidation rates of OCS and H2S are the same to within the precision of our experiments; hence the presence of sulfur as OCS rather than H2S is not rate-limiting. The hydrolysis of OCS is known to give H2S as a product, 11'53 although the presence of dissolved 02 reduces the H2S yield per OCS molecule to <50%.tl The hydrolysis is base-catalyzed, t1"54"55 and Phillip and Dautzenberg 54 have suggested that the reaction sequence is
/ -0
S II c
+ o.-
N OH
----- cso~- + .ao.
(6)
The support for this mechanism comes from the evidence for an H2S product, the existence of base-catalyzed hydrolysis, and Phillip and Dautzenberg's detection of CSO~-, which they suggest is formed in basic solution by
OCS
-4- OHI I t
,,, -0
/
S II C
\ OH
-.;
0 II ; /C\
-0
SH
0 H']-H2~ ~-.~C / X HO
P H2S + CO 2 .
SH
This constitutes evidence, though indirect, for the intermediate in the base-catalyzed hydrolysis sequence. A problem with attempting to apply this hydrolysis sequence to our results is that it does not explain the fate of the hydrolyzed OCS not detected as H2S. Derdall and Hyne 1~suggest that rapid H2S oxidation by H2S + ½02 ~ S + H 2 0
(7)
is the cause, since the measured H2S yield is decreased by the presence of dissolved 02. Such a reaction is unlikely to proceed in a single step and it seems more likely that unspecified reactive intermediates are involved. Another problem with applying Phillip and Dautzenberg's mechanism to our work is the O H - catalysis step, since most of the sulfidation in our experiments occurs in weakly acidic solution. Basic conditions can be anticipated only in the early stages of sulfidation on water-covered Cu20, since the oxide is slightly soluble in water, C u 2 0 + H 2 0 ~ 2Cu + + 2 O H -
(8)
1178
T . E . GRAEDEL,J. P. FRANEY, G. J. GUALTIERI,G. W. KAMMLOTTand D. L. MALM
....
[M
tl
H20 ~ ; .
.
.
H20
.
IM
I M
ti
L.. . . . . . . .
J
IM
Fl~. 11. A schematic representation of OCS and H2S sulfidation of metals. The wavy lines indicate absorption from the gas phase while the broken lines indicate possible chemical reaction paths. M = metal; I = unspecified intermediate. The reaction paths are discussed in the text.
and will produce a weakly basic solution until the evolving sulfide layer isolates the oxide from the water. OCS hydrolysis in neutral or acidic solution is quite slow, lt,53 despite the fact that the compound is an acid anhydride. (Note that thiocarbonic acid, H O C S O H , does not exist in the free state.56) However, OCS coordinates readily to metal centers with cleavage of the C - - S bond, 39 so that the sulfidation process may not be hydrolysis at all, but may proceed dissociatively by HzS or OCS coordination to the solid surface• In this case, the relative humidity dependence of the sulfidation would reflect either the ability of the water to absorb the sulfurous gases and make them conveniently available or for the water to be directly involved in aiding the coordination reaction• These considerations lead us to the schematic reaction sequence for sulfidation shown in Fig. 11. In the initial step, OCS or H2S is absorbed from the gas phase into the surface water layer. If direct dissociative coordination with the metal occurs (step 1), the result will be an intermediate (I) which will form the sulfide by reaction with a second metal atom (step 4). Alternate pathways are possible• OCS may hydrolyze to form H2S (step 2) and proceed as does the H2S molecule• H2S may dissociate (step 3), with the thiyl ion being the reactive coordination species• It is interesting to note the possibility of an experiment in which NH3, the only basic gas commonly present in the atmosphere, is added to a gaseous H2S mixture• If the H2S sulfidation proceeds through HS (step 3), the pH alteration produced by NH 3 adsorption should alter the sulfidation rate. A second experiment would consist of adding NH 3 to a gaseous OCS mixture• If the OCS sulfidation proceeds by hydrolysis to H2S (step 2), the O H - dependence of the hydrolysis should be demonstrated, although the fact that both steps 2 and 3 are O H - dependent may make the results difficult to interpret• Such modifications may occur in the atmosphere as a result of varying levels of NH3.15
Silver and copper sulfidation by H~S and OCS
1179
CONCLUSIONS T h e e x p e r i m e n t a l w o r k d e s c r i b e d h e r e i n has r e s o l v e d s e v e r a l a s p e c t s o f the a t m o s p h e r i c sulfidation p r o c e s s e s involving H2S a n d O C S . T h e sulfidation rates for silver a n d c o p p e r are s h o w n to be s i m i l a r for H2S a n d O C S . W e h a v e d e m o n s t r a t e d t h a t t h e r e l a t i v e h u m i d i t y d e p e n d e n c e of the sulfidation, while s o m e w h a t d i f f e r e n t for silver a n d c o p p e r , is s i m i l a r no m a t t e r which of the t w o gases is used. W e find t h a t no excess c a r b o n o r o x y g e n is d e t e c t e d on the m e t a l surfaces f o l l o w i n g O C S sulfidation, i n d i c a t i n g that c l e a v a g e of t h e C - - S b o n d is facile u n d e r t h e s e c o n d i t i o n s . F i n a l l y , we h a v e d e t e r m i n e d that t h e sulfidation p r o d u c t s in all cases are the m e t a l sulfides, suggesting t h a t the m e c h a n i s m s o f H2S a n d O C S sulfidation ( a n d p e r h a p s the r e a c t i v e i n t e r m e d i a t e ) a r e s i m i l a r o r i d e n t i c a l . A l t h o u g h the initial r e a c t a n t s a n d final p r o d u c t s a r e k n o w n in e a c h case, the linking processes and intermediate species remain uncertain. We tentatively regard d i r e c t c o o r d i n a t i o n with the m e t a l of the s u l f u r o u s gas m o l e c u l e d i s s o l v e d in a surface w a t e r l a y e r as the m o s t likely p r o c e s s u n d e r typical a t m o s p h e r i c c o n d i t i o n s . T o w h a t e x t e n t can the results be g e n e r a l i z e d ? In a d d i t i o n to silver a n d c o p p e r , H2S is k n o w n to sulfidize s e v e r a l o t h e r m e t a l s . F o r O C S , b e s i d e s the silver a n d c o p p e r e x p e r i m e n t s , only l i m i t e d e x p o s u r e s o f b r o n z e s 2 h a v e b e e n p e r f o r m e d . In e a c h o f t h e cases d i s c u s s e d in this p a p e r , we h a v e d e a l t with r e a c t i o n s b e t w e e n r e d u c e d sulfur gases a n d t r a n s i t i o n m e t a l s . It s e e m s p o s s i b l e that the c o n s i d e r a t i o n s that a p p l y to silver a n d c o p p e r will a p p l y as well to nickel, t u n g s t e n a n d zinc, t h r e e o t h e r t r a n s i t i o n m e t a l s k n o w n to sulfidize 3s57"ss u p o n e x p o s u r e to c o r r o s i v e a t m o s pheres.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20.
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