1816
CECILV. KINGAND SHELDON EVANS
Vol. 63
RADIOTRACER STUDIES OF ZINC-ZINC ION EXCHANGE BY CECILV. KINGAND SHELDON EVANS Department of chemistry, New York Universitg, New York, N . Y . Received February 17,1068
Measurements have been made of the rate and extent of acquisition of radioactivity by zinc coupons when immersed in solutions of zinc perchlorate containing Znasas tracer. Air-saturated and deaerated solutions were used, as well as neutral and acid solutions containin dichromate as corrosion inhibitor. Activity pickup is much greater in the presence of air, due to the formation of soli8 corrosion products. Exchange is ver small in the presence of dichromate, and even retreatment with dichromate or chromic acid inhibits exchange. I n &aerated solutions of Zn(C104)zcorrosion is very s & ~ , and the continuin activity pickup is ascribed to self-diffusion within the metal. Calculated diffusion coefficients are too large, which may 6e the result of applying the theory of homogeneous diffusion to polycrystalline specimens.
Measurements of zinc-zinc ion exchange reported in the literature have been, for the most part, qualitative in nature, and have not solved the problems presented by dissolution and corrosion, crystal nature of the metal or possible adsorption of ions a t the interface. Not much is known about the mechanism of exchange, or the relation of this mechanism to values of the exchange current. The present work is only semi-quantitative in nature and does not answer all these questions. However, it does show some of the factors which must be taken into account in future work if measurements are to be interpreted in terms of rates and mechanism. It was first shown by Rollinl that zinc does exchange, by shaking zinc powder with ZnClz solution containing Zna6as tracer. The metal became radioactive; little more than this fact was shown, The experiments were done in the presence of air, and the metal corroded somewhat. Haenny and Mivelaz2rotated zinc coupons a t 900 r.p.m. in radioactive ZnSOl solutions, in the presence of air, and found that the activity acquired by the metal reached an apparent maximum in 2 or 3 hr. On reimmersing in inactive ZnSOl little or no activity was lost. The authors concluded that the activity was sealed in an oxide layer, which does not exchange with the solution. Dissolving a layer less than 25 p thick in HC1 removed all the activity. The experiments do not really tell whether exchange is involved, or only precipitation of zinc ion from solution, due to corrosion. Matsuura3 shook powdered zinc with active ZnSOd solution and showed that the activity of the solution decreased to a minimum in about 50 hr. In similar experiments with zinc sheet, the activity 0 7 5 sand apparent of the solution decreased 4 equilibrium was reached in 200 hours; a plot of log (1 - F ) was linear with time, where F is the ratio of activity lost from the solution at time t, to activity lost during 200 hr. (However, the zinc metal had not come to true equilibrium with the solution, i.e., the ratio of active to inactive zinc was not the same in the two phases. The experiments apparently were conducted with air present, so we do not know the relative importance of corrosion.) Matsuura points out that self-diffusion within the metal is an important factor in the exchange. (1) B. V . Rollin, J . Am. Chem. Soc., 62, 86 (1940).
(2) C. Haenny and P. Mivelaz, Helu. Chim. Acta, 31, 633 (1948). (3) N. Matsuura, Sci. Papers, C o l l . Gen. Educ., Univ. T o k y o , 6, 97 (1955).
He does not give details in the paper cited3 but states that from experiments with electropolished zinc specimens the value of D, the self-diffusion coefficient, was estimated to be about 10-l2 cm.2/ sec. The author says that such a large value, a t room temperature, is only possible in a solid of extremely disordered crystal lattice. A few experiments on Zn-Zn++ exchange with single crystals have been carried After immersion in 0.03 M Zn*S04, it was found that the prism facets had acquired some 10% more activity than the base facets. There is a definite difference in electrical potential, the prism facets being more anodic. The present work with zinc was undertaken to learn more about the metal-solution interface in the case of an active metal; the effect of corrosion with air present and the much slower corrosion in the absence of air; the effect of corrosion inhibitors and cathodic protection; the role of internal diffusion and possible adsorption on the metal. The possible mechanisms by which a metal can acquire activity in a labeled solution of its own ions have been discussed elsewhere6; other factors than simple exchange must be considered. Experimental Zinc disks 3.74 cm. in diameter were cut from 99.99% pure sheet, 0.05 cm. thick. A small hole in the center served to mount the disk on a motor shaft or to suspend it on a $lass hook. The disks were usually polished with No. 600 sihcon carbide paper, washed and dried with alcohol and ether. Some were annealed at 110' and etched as described later. Zn" was obtained as ZnClz in HC1. Since preliminary ex eriments showed that Zn(ClO4)z is less corrosive than the chroride, sulfate or acetate, HC1 was ex elled from the active solution by repeated evaporation. &all amounts of tracer were added to solutions of inactive Cd(ClO4)z which were then diluted to final concentration. Measurements were made with a thin mica window GM tube, with conventional scaler and shielding. The activity was high enough t o make counting errors small except as described below. Counts were converted t o gram atoms of pickup per cm.* by comparison with evaporated samples of the solutions. Since appreciable radiation penetrated the disks (Zn" !i a positron- and yemitter), both sides of the calibration disks were counted and the sum taken, or solution was evaporated on both sides and the average count taken. Since both sides of the experimental disks were exposed to the solution, both sides were counted and the average taken. The disks were immersed in 100 ml. of 0.01 M Zn(ClO4)z at room temperature (about 23'). To deaerate, nitrogen was passed over hot copper, then through water and through the solution cell for 1 hr. before introducing a coupon. (4) B. N. Bushmanov and G . S. Vozdvizhenskii, Dokladu Akad. Nauk S S S R , 114, 1046 (1957); in English translation, Proc. Acad. Sei. U.S.S.R., Sect. Phys. Chem., 114, 397 (1957). (5) C.V. King and N. E . MoKinney, Can. J . Chem., SI, 205 (1959).
Y
Nov., 1959
RADIOTRACER STUDIES OF ZINC-ZINC ION EXCHANGE
Air-saturated vs. Deaerated Solutions.-Figure 1 shows the activity ickup on single immersions up to 1 hr. in the two cases. $wo to four disks were used for each point, and the vertical lines indicate the reproducibility. Multiple immersions gave similar results, L e . , the activity acquired in air-saturated solution was 5 or 6 times as great as in deaerated solution. Long immersion in the presence of air (up to 280 hr.) led to weight gains of as much as 10 mg. per coupon (of 22 cm.2 area). The disks became dull gray with many white spots, which proved to be corrosion products covering deep pits. The pickup in deaerated solution was somewhat larger for repeated short immersions, with 2-minute cold water washing, than for a single immersion of the same total time. The behavior is much like that of silver.6 Complete exchange in one atomic layer corresponds to about 2.7 X 10-0 gram atoms/cm.2, and the exchange in a single short immersion corresponds to about 3 apparent layers. This may indicate a roughness factor of 3, with rapid exchange in one layer. The exchange current evidently is large. In deaerated solutions the coupons remained bright up to 48 hr. On immersion up to 250 hr. they became slightly dull, but the brightness was restored by dipping in 0.01 M K&rzOp, which removed a few per cent. of the activity. In the range of 4-24 hours the measured activity became erratic (.t 50%). In the range 48-250 hr. the acquired activity varied as much in amount, but this was a smaller per cent. of the total. If continuing pickup is due entirely to homogeneous diffusion within the metal, the activity should be linear with the square root of time. The actual plots show continuous to 9.5 X upward curvature with slopes from 2 X gram atoms cm.-’J sec.-’/a. This is analogous to the corresponding curves for silver.’ Re-exchange.-On immersion of active disks in inactive solution some activity is lost, the rate evidently depending on the previous depth of penetration within the metal. Given sufficient time, the active atoms obviously must distribute themselves in the same ratio (to inactive atoms) throughout the metal and the solution. Effect of Dichromate.-Chromates and dichromates can inhibit the corrosion of zinc; for example, a zinc cylinder has been rotated for 48 hr. without weight loss, in 0.5 M acetic acid containing 0.06 M KNO, and 0.01 M KzCrzOp.* The metal dissolves rapidly in the same solution of acid and nitrate, without dichromate. Zinc perchlorate does not destroy the protection, but other anions do, as shown in Table I. The potentials found are all polarized, since the reversible potential is about 1.03 volt; the values remained about the same in the least corrosive solutions but rose with time to about 1 volt in corrosive solutions. The solutions were not deaerated.
1817
100
5
80
X u4-
e 60 3 4 40 !I is
20
10 20 30 40 Time of immersion, min.
50
60
Fig. 1.-Activity acquired by zinc disks on single immersions in 0.01 M Zn(ClO&: upper curve, air-saturated solution; lower curve, deaerated solution.
TABLE I1 PICEUP FROM 0.01 M Zn(ClO&, 0.01 M K2CraO7, GRAM ATOMS
x
~OD/CM.P
HAo,
KNOa,
...
Time, hr.
...
... ...
0.5 .5 .5 .5
... ...
...
1 12 24 1 26
0.06 0.06
3 20
M
...
M
Activity
0.86 0.90 1.02 0.90
.90 -90 .90
A number of zinc disks were etched in a chromic acid solution used to show grain structiir:.D Some of these disks had been annealed in vucuo a t 110 , which resulted in the formation of large crystals around the edges and center holes. The etched disks were washed, dried and then immersed in active Zn(C104)zsolution. The activity picked up in 10-60 minutes corresponded to about 0.45 X 10-9 gram atoms/cm.a, independent of the exact treatment or time. A brief dip in dilute HC104-KNOs solution restored the TABLE I ability to pick up the normal 70 X 10-8 gram atoms/cm.2 in CORROSION OF ZINC DISKS ROTATED IN SOLUTIONS CON- 30 minutes (in air-saturated solution). Similar inhibition to exchange was conferred by 30TAINING 0.5 M ACETICACID, 0.01 M K~Cr207. E vs. SATD. minute immersion in 0.01 M K2CrzOl alone, or in 0.5 M KCL CALOMEL CELL acetic acid containing 0.01 M K2Cr207, followed by washing. Wt. Other Experiments with Dichromate.-Coupons which KNOa, E, loss, Time, Zn salt had been made active by short immersion lost much of their V. mg. hr. 0.01 M 1cf activit on immersion in KzCrzO, solution, as shown in Table ... ... 0.65 0 48 111. *he gases ( 0 2 , Hz, Nz) were bubbled through the ZnAcp 0.06 .70 0.4 5 solutions for 1 hr. in advance. It is of interest that coupons ZnClz 0.06 .74 31 3 activated in air or oxygen lost so much of their activity; while those treated in hydrogen acquired twice as much activity as ZnSO4 ... .88 45 7 min. those treated in nitrogen and also retained more. The point Zn( Clod2 ... -72 0 48 of main interest is that dichromate always dissolves some ... .60 0 24 Zn( Clod2 zinc, releasing it to the solution, and does this within a few Acetate. seconds. When activated coupons were immersed in 0.01 M potassium chromate (which has a higher pH than the dichroZinc disks, polished with No. 600 silicon carbide aper, mate), much less activity was removed, even on standing were immersed in solutions containing radioactive Zn&104)2 several hours. and KzCrz07, as shown in Table 11. The disks remained Cathodic Polarization.-To simulate conditions of cabright and there was no weight 108s in any of the runs. The thodic protection, zinc disks were rotated in active 0.01 M radioactive counts were only 10-20 c.p.m. above back- Zn( ClO& and polarized cathodically. A silver anode in KC1 ground, corresponding to about */, atomic layer. It is solution was separated from the cell by an agar bridge. evident that the film which prevents corrosion, whatever its Currents of even a few microamperes/cm.2 led to weight nature, also inhibits exchange. gains and to greatly increased pickup. The weight gains were 3 to 5 times greater than the weight of zinc correspond(6) C. V. King and A. Simonsen, J . EEectrochem. SOC.,104, 194 ing to the activity count; evidently the solution at the inter(1957).
(7) H. Gerischer and R. P. Tischer, 2. Elektrochem., 68, 819 (1954). (8) C. V. King and E. Hillner, J . Electrochem. Sac., 101, 79 (1954).
(9) C. J. Smithells, “Metals Reference Book,” Interscience Publishers, Inc., New York, N. Y., 1949, p. 267.
CECILV. KINGAND SHELDON EVANS
1818
Vol. 63
TABLE 111 fusion coefficient and a. is the surface activity in COUPONSIMMERSED 30 MIN. I N 0.01 M Zn(ClO& IN gram atoms/cm.a. (Of course, total gram atoms VARIOUS ATMOSPHERES, THENIN 0.01 ' i k K&rgO,, GRAM are used, being proportional to the a's.) This ATOMS
x
109
Atm.
Activity
Min. in I
