374
A . L. MCCLELLAN AND NORMAN HACKERMAN
(24) ROBINSON, R. A., WILSON,J. M.,
AND
AYLINO,H. 8.: J. Am. Chem. 800. 64,1469-71
(1942). (25) SACHS, A.: Z.Kryst. Mineral. 38, 497 (1904). (26) SCHREINEbfAKERS, F. A. H.: physik. Chern. 11, 81 (1893). (27) SXITH,D. M., BRYANT, W. M. D., A N D MITCHELL, J., JR.:J . Am. Chem. SOC.61,2407-
z.
12 (1939).
(28) SPATH,E.: Monatsh. 33, 853 (1912). (29) STOKES, R. H., A N D ROBINSON, R. A.: J. Am. Chem. SOC.70, 187C-8 (1948). (30) v. UNRUH,A. : Dissertation, Rostock, 1909 (see Gnielin’a Handbook, System-Number 55, p. 117). (31) WARREN,G. G.: Can. Chem. Process Inds. 29, 370 (1945); Chem. Abstracts 39, 3221 (1945). (32) WYROUBOFF, G.: Bull. soc. franc. mineral 32, 349 (1909).
(33) ZERBAN,F. W.,
AND
SATTLER,I,.: Ind. Eng. Chem., Anal. E d . 18. 138-9 (1946).
T H E SORPTION OF GMES ON METALS AT ROOM TEMPERATURE’ A. L. McCLELLAW A N D NORMAN HACKERMAN
Department of Cheniastry, University of Texas, Austin, Texas Received March 17, 1950
The adsorption of gases on metals has been investigated by many workers (2,5,6,9, 10, 12, 14). These investigators most frequently reported data obtained at very low temperatures (ca. - 190’C.) or above 300°C.,and most of the work was concerned with oxygen, hydrogen, nitrogen, or carbon dioxide as adsorbates. The work reported here was carried out a t room temperature and includes gasmetal systems used in the references cited, plus others. The research stemmed from an interest in the passivity of metals and especially the occasional reports of the effect of supposedly physically adsorbed gases on this phenomenon (4, 8). More complete sorption information specific to such systems is necessary in order to evaluate the results of such studies of metal passivity. This paper reports the investigation of the sorption of oxygen, chlorine, nitrogen monoxide, and nitrogen dioxide on a passive metal, chromium, a passive alloy, 18-8 stainless steel, and an “active” metal, steel at about 30°C. EXPERIMENTAL
Apparatus The measurements were made using a quartz-spiral spring balance of the McBain-Bakr type. The sensitivity of each spring was determined to 1 per cent and all showed about 1 mm. extension per milligram of load. The rated capacity 1 Presented before the Division of Colloid Chemistry a t the 117th National Meeting of the American Chemical Society, Houston, Texas, March, 1950. 9 Present address: Department of Chemistry, University of California, Berkeley, California.
SORPTION OF QASES ON METALS
375
of the springs was 200 mg., and the calibration was linear up to this value. The average weight of all samples used was 140 mg. A filar microscope was used to measure the extension, and the optical magnification was such that the overall sensitivity of the measurements was 1.7 f 0.1 X g. of sorbate. The system contained the usual pumps, traps, and gauges; and mercury seal stopcocks were used throughout. Gas pressures were determined with an oil manometer (Octoil S) and a cathetometer. All of the experiments were carried out a t room temperature, 25°C. =k 5", with the variation always less than 2°C. for any particular isotherm. Corrections were made to compensate for thermal expansion of the spring and spring tube during individual determinations. The reproducibility obtained in these experiments is indicated on the curves for oxygen in figure 6, where the experimental points are from three different runs. Two sets are on the same metal sample, with the pretreatment indicated in the caption given between determinations, and the third is on a separate sample. The maximum variation in any of the isotherms is about 10 per cent. Materials The hydrogen used for reduction was Magnolia Airco tank hydrogen, purified by catalytic reaction of the oxygen present and removal of condensable matter. Tank oxygen was used after the condensable matter had been removed in a cold trap. Chlorine, nitrogen monoxide, and nitrogen dioxide were obtained from the Mathieson Company and used without further treatment. The metals mere used in the form of powders, two of which were obtained commercially. The stainless-steel powder, nominally 18 per cent chromium and 8 per cent nickel,a had been made by high-temperature sensitization followed by intergranular corrosion in cupric sulfate-sulfuric acid solution. The resultant powder was then washed, dried, and annealed. SAE 1020 steel powder' (hereafter referred to as steel) had been prepared by atomization of the metal at 1620°C. It was then annealed at 1000°C. in a hydrogen atmosphere containing some propane to prevent excessive oxidation and decarburization. Powdered chromium was prepared here by grinding sheets of electrodeposited chromium in a glass mortar and pestle. The electroplate, which was stripped from its copper base by concentrated nitric acid and then rinsed thoroughly with distilled water, was spectroscopically pure. All the powders were degreased with three rinses of hot acetone and dried in air at room temperature. Photomicrographs of the metals showed the steel particles to be fairly smooth and generally spherical in shape, while the particles of the other metals were more irregular in shape. The chromium contained a few glass particles broken from the mortar and pestle in the grinding operation. The areas of the powders were determined by adsorption of argon at liquidnitrogen temperatures. Figure 1 shows the isotherms and table 1 gives the values of the areas. a 4
Supplied by Micro-Metallic Corporation, 193 Bradford Street, Brooklyn, New York. Supplied by Esomet Corporation, Conneaut, Ohio. It contains 0.20 per cent carbon.
376
A. L. MCCLELLAN AND NORMAN HACKERMAN
The other methods consisted in determinations of average particle radius, e.g., microscopically or by sedimentation; calculation of the areas, assuming spherical shapes; and the application of an arbitrary roughness factor of 10. The estimations were made before the areas were obtained by the B.E.T. method. The only sample of the specially prepared chromium powder was accidentally destroyed by heating above its sintering temperature, so that its area by adsorption is not available. The other procedures indicated an area for this metal close to 0.60 m.’/g., and the absence of a more precise value made it necessary 0
AI
I
I
I
I
I
/I
I
A
1
-
I
I
I
I
I
PRESSURE, MM. HG
FIQ.1. Adsorption of argon at -193°C.: (A) on stainless steel; (B) on steel TABLE 1 Areas of the powders METAL
Steel ................................. Stainless steel. ....................... Chromium.. ..........................
1
AREA
B.E.T.
I
Othermethda
W#.V#.
m.’/g.
0.11 0.46
0.097 0.55 0.60
to use this value. However, table 1 indicates that the approximate methods agree with the area determined by the B.E.T. method to about 25 per cent, so that the use of this figure for chromium powder is not wholly unjustified. Before each run the metal surface was treated in one of two ways, each in situ: (I) Continuous evacuation during overnight baking a t 450°C.These surfaces are designated “baked.” @) Reduction at 450°C.by hydrogen, which passed first through a Deoxo catalyst tubes and then through a cold trap, followed by evacuation a t 450°C.The cycle was repeated until the sample weight was constant. This treatment and the surfaces so produced are referred to as “reduced.’) 1 Fisher
Scientific Company, Pittsburgh, Penneylvanis.
377
SORPTION OF G A S E S ON M E T A L S ISCTHERMS FOR T H E QAS-METAL
SYSTEMS
Each point of every isotherm was measured until both pressure and spring extension were constant. These constant values are plotted in the isotherms
0.8
2W z
Q VI
$0.4
c6-
z
0
0.4 P R E S S U R E , CM. HG
0.8
FIG.2. Sorption on baked steel a t room temperature: 0 , oxygen; 0 ,chlorine; A , nitrogen dioxide; V , nitric oxide.
30 J
a I-
W
g2 c ul W
W
I O
0
0.4 P R E S S U R E , CM
0.8 HG
FIG. 3. Sorption on baked stainless steel at room temperature: 0 , oxygen; 0 , chlorine; A , nitrogen dioxide; V , nitric oxide.
shown in figures 2-7. It is interesting to note that the isotherms gave a good fit when plotted according to the linear form of the Langmuir equation, even though more than a monolayer equivalent of gas was taken up in each case. The equivalent number of layers of gas acquired by the metals was calculated from the maximum sorption, the measured surface areas, and the molecular
378
A. L . MCCLELLAN AND NORMAN H A C K E R l l A S
dimensions of the gases, assuming hexagonal close-packing of the adhering phase. The calculated values listed in table 2 are for comparison only and are not intended to represent the actual number of gas layers.
I
V
1
I
I
I
I
I
0.4
0
PRESSURE,
CM. HG
FIG.4. Sorption on baked chromium at room temperature: 0, oxygen; 0, chlorine; A , nitrogen dioxide; V, nitric oxide. I
I
I
I
1
FIG.5 . Sorption on reduced steel a t room temperature: 0, oxygen; 0 , chlorine
Some of the metal-oxygen systems reported here have been studied by others and the available data are compared in table 3. Although agreement is good for the passive alloy, considerable differences are seen for steel, especially in the unreduced form. The lack of agreement may be attributed to differences in samples and in methods of preparation. It is clear that the steel powder uaed in this work is more reactive, possibly by virtue of a thinner or more porous oxide layer originally. This explanation is supported by the much smaller discrepancy
379
SORFTIOS OF GASES O S METALS
3 -1
2 W
=2
e in
z0 . 0
21
0
0.4
0.8
P R E S S U R E , CM. HG
,n ,oxygen (there
FIG.6. Sorption on reduced stainless steel a t room temperature: 0, separate runs); 0 , chlorine.
0
Y
I
I
I
I
i
0.4 PRESSURE, CM. HG
0.8
i . Sorption on reduced chromium a t room temperature: 0 , oxygen; 0 ,chlorine
- .
STXEL
STAINLL8S S I X E L
CEPOXlQM
GAS
0 2 . ..
.. ... . .. ..
Clr., , . . . . . . . . ,
NO;.. . . . . . . . . ,
Baked
Reduced
21 6.1' 20' 14
44
66'
Baked
,
Reduced 1 7
1.4
14
2.5 2.1
12
2.3
xo... , , . , . , . 2.9 * Estimated from isotherm, which still had not ,
Reduced
1
levelled off.
5.6 1.9
380
A. L. MCCLELLAN A N D NORMAN H A C K E R M A S
noted for the reduced steel samples. Not only is the agreement for the passive alloy good between the two investigations cited, but also the reproducibility was better than that for steel in each case. This leads to the conclusion that the surfaces of such metals are more reproducible, and possibly more nearly uniform, than are those of steel. Continued evacuation at the temperature of the experiment was ineffective in removing any measurable amount of sorbed material for any of the systems. Thus physical adsorption, in detectable extent, was not involved in any of the equilibrium systems. This, plus the relatively small effect of the reduction step TABLE 3 Comparison of available data o n some metal-ozygen systems YETAL ~
PRETEEAIYEN+
'
OXYGEN
PEP 0.1 ti,'APEA
'C.
Steel wire
APPARENTLAYERS OFOXYGEN SOBBED
SORBED
Degreasec 20
I
mg.
0.036
Steel powder
Baked
0.71
Steel wire
Reduced
0.420
Steel powder
Reduced
1.55
Iron powder
Reduced
Number
1
0.7
I
1
1 1
'
I
1 1
21 9.2 40
'
'
Calculated from
Reference
Sitrogen adsorpPa) tion a t -183°C. Argon adsorption This work a t -195% Nitrogen adsorp@a) tion a t -183°C. Argon adsorption r h i s work a t -195°C. (3)
(1) ,
Stainless-steel powder.
IReduced
monoxide adsorption a t
- 183'C. 30
1 0.13
3.2
Argon adsorption rhis work a t -195°C.
on the sorption capacity of the passive metals, affords evidence that physically adsorbed gases are not likely to play a major role in passivity. DISCUSSION
The data show that steel takes up each of these gases to a greater extent than do the other metals and that reduced surfaces sorb more than do unreduced surfaces. Stainless steel and chromium each acquire about the same amount of oxygen. The unexpected correspondence between the amount of oxygen sorbed on stainless steel and that sorbed on chromium suggests that the surfaces of these two metals may be similar, perhaps implying a concentration of chromium or chromium(II1) oxide in the upper portion of the stainless-steel surface. This is in conflict with the work of some authors ( l l ) ,who found that the iron content in the uppermost surfaces of various chromium-iron alloys exceeded that of the I d k phase. In those experiments the metals mere maintained at very high
SOR€TION OF QASES ON METAL8
38 1
temperatures for long periods of time, so that some migration of the components was possible. It is interesting to note the generally larger uptake of chlorine than of oxygen, particularly on the passive metals. Pretreatment of the surface was not a major factor in chlorine sorption on the passive metals, the reduced and unreduced samples of each picking up roughly the same amount of the gas (see table 2). The lack of effect of the reduction step on chlorine uptake indicates an oxide layer not reducible by the treatment used. Other possibilities suggest themselves. For example, the chlorine may be attached to material already on the surface and not removable by the reduction treatment, e.g., chemisorbed oxygen. The absence of this effect with steel would then be due to the greater ease with which the relatively more massive surface oxides can be reduced. Also, nonuniform surfaces could allow simultaneous uptake of two or more gases; specifically, with fewer sites available for chlorine on the passive metals even after reduction. Both the heterogeneity of metal oxide surfaces (13) and the incompleteness of oxygen films on iron (7) have been discussed in recent publications. Thicknesses of the surface reaction products were calculated for the steeloxygen and steel-chlorine systems. I t was assumed that (1) the products were simple compounds of iron, (t?)the bulk densities were applicable, and (3) the layers were uniform. For the formation of ferrous oxide on the unreduced steel it was found that a layer approximateLy 50 A. thick was formed, while on the reduced steel the thickness was 110 A. The oxide thickness on the original powder was determined by reducing a laLge sample with hydrogen at 800"C., and it was found to be of the order of 100 A. This means that simple outgassing and baking at 450°C. permitted the oxide layer to increase by about 50 per cent on subsequent exposure to oxygen. Assuming that the en situ reduction was effective in reducing the surface oxide almost completely, the interesting observation obtains that just about the same thickness formed following this pretreatment as was there originally. This is the only evidence that evacuation and heating might cause additional and relatively rapid reaction of the iron. I t should be noted that the oxide thicbness, assumingoformation of Fe203only, would be as follows: unreduced, 40 A . ; reduced, 88 A. This is not markedly different even for this extreme case and for Fe301the values would fall somewhere in between. For the steel-chlorine system the hydrogen treatment causedoa considerable increase in reactivity. In the case of the unreduced metal a 15-18 A. iayer formed and for the reduced metal the thickness was between 160 and 195 A. The lower value was obtained considering ferric chloride as the product and the higher value for ferrous chloride. Since the system was free of moisture, there are no complications because of hydrolysis or hydration. Similar calculations are less meaningful for the nitrogen oxides, since the probable products cannot he as firmly deduced. This holds also for the passive metal systems, largely for another reason. In all but on? of the latter, i.e., chlorine-stainless steel, the calculated thicknesses are 10 .\. or less. Detection of surface layers as thin as these is not easily possihle, c.g , by techniques such as
382
.+. L.
Mc'CLELLAN AND S O R M A N HACKERMAN
light interference or surface reflection electron diffraction, and the significance of stoichiometric compounds in surface layers so thin is debatable. SUMMARY
Isotherms for the sorption of oxygen, chlorine, nitrogen monoxide, and nitrogen dioxide at 30°C. on unreduced and reduced surfaces of SAE 1020 steel, 18 chromium-8 nickel stainless steel, and pure chromium powders have been determined by the use of a quartz-spiral spring balance. The results are compared with those of other investigators on some of the same systems. In general, each of the gases was picked up by the metals in the following decreasing order: steel, stainless steel, chromium. Reduction by hydrogen treatment increased the uptake of both oxygen and chlorine on steel but made very little difference with the passive metals. There was no firm evidence of physical adsorption. Not more than six equivalent layers of gas were !orbed on the passive metals, except in one case. Calculations showed that a 50 A. laytr as ferrous oxide formed on unreduced steel exposed to oxygen and that a 110 A . layer formed on reduced steel. This work was carried out under contract NSori-136 T.O. I1 with the Office of Naval Research. This support is gratefully acknowledged. In addition, we wish to thank Dr. George Jura for his help in determining the area of the powders by gaseous adsorption and Mr. H. L. Wang for constructive criticism of the manuscript. REFERENCES (1) ARMBRUSTER, M. H.: J. Am. Chem. SOC. 70, 1734 (1948). (2) . , ARMBRUSTER. M. H.. A N D AUSTIN. J. B.: (a) . . J. Am. Chem. SOC. 88. 1347 (1946): . . , (b) . . 66, 159 (1944). ' (3) RURSHTEIN, R.,SHUMILOVA, N.,A m GOLBERT, K . : Acta Physicochim. U. R. S. S. 11. 785 (1946). (4) CONE,W. H., A N D ANDERSON, D . H.: J. Am. Chem. Soc. 67, 1640 (1945). CONE,W.H., A N D TARTAR,H. V . : J. Am. Chem. SOC. 69, 937 (1937). P.H . : J. Phys. & Colloid Chem. 61, 1232 (5) DAVIS,R . T.,DEWITT,T. W., A N D EMMETT, (1947). (6) EMMETT, P. H., A N D CINES, M.: J. Am. Chem. SOC.61, 1329 (1947). (7) ERSHLER,B.:Discussions of the Faraday Society 1, 277 (1947). (8) FONTANA, M. G., A N D BECK,F. H.: Metal Progress 61, 939 (1947). W. G.:J. Am. Chem. SOC.66, 1827 (1944). (9) FRANKENBURG, (10) GULBRANSEN, E. A,, AND WYSONE,W. S.: Am. Inst. Mining Met. Engrs., Inst. Metals Div., Metals Technol. 14, (6), Tech. Pub. Nos. 2224 and 2226,September, 1947. E. A,, PHELPS,R. J., A N D HICKMAN, J. W.: Ind. Eng. Chem., Anal. Ed. (11) GULBRANSEN, 18, 640 (1946). (12) LANGMUIR, I.: J. Am. Chem. SOC. 40, 1380 (1918);S8, 2273 (1916). H . S.,A N D LIANG,S. C.: J. Am. Chem. Soc. 69,1306, 2989 (1947). (13) TAYLOR, (14) WILKISS,F. J.: Proc. Roy. SOC.(London) A M , 496 (1938). ,
