28
G. H. CARTLEDGE
Vol. 60
THE MECHANISM OF THE INHIBITION OF CORROSION BY THE PERTECHNETATE ION.’ 11. THE REVERSIBILITY OF THE INHIBITING MECHANISM BY G. H. CARTLEDGE ConttibutMnfrvm the C h i d r y Division of the Oak Ridge National Laboratory, Oak Ridge, Tenm8ee Recsiwd May 81, 1966
An attempt has been made to determine the extent to which the process responsible for the inhibition of corrosion by the pertechnetate ion ia reversible. It has been found that the disturbance of inhibition by added electrolytes involves a specific action and not merely an increase in electrical conductivity of the solution phase. The electrode potential of electrolytic d both in potsasium pertechnetate and in mixtures of this with other electrolytes. The potential waa found iron wan m to respond quickly to changes in the compositionof the solution in a manner that clearly re resented kinetic influences at the interface. It is concluded that the potential and the inhibition alike depend upon a l a b e atate at the interface that is quickly responsive to changes in the composition of the solubon.
I n the theories of inhibitor action that a m m e an adsorption of the inhibitorZ+ there is considerable uncertainty concerning the degree of reversibility of the adsorption. The most recent work has made w e of radioactive C+, and the difticulty in interpreting the results arises from the fact that no distinction has been made between h l y hound, unreduced chromate and insoluble or strongly held reaction products. Chromates stimulate the corrosion of carbon steel in acidic solution,14 and some reduction inevitably occurs under .such conditions. It is also likely that precipitated chromium(II1) oxide carries with it some chromium(VI). Thus,Uhligand Geary’O found 3 X 10l6 atoms of chromium per cm.z on Armco iron, half of which was removed by long soaking in distilled water. Hackerman and Powers6 were unable to remove the C P adsorbed on chromium, and found no activity at pH 11. The interpretation of the results at high pH’s, however, is complicated by the fact that corrosion is inhibited in sufjiciently alkaline aerated solutions even without the chromate.” If the inhibition were due to a film-repair mechanism’* then the deleterious effect of hydrogen ions and chloride ions would require ad hoc assumptions t o account for their action, such as dissolution, penetration or peptization of the protective film by the antagonistic agent. In such a case, the inhibiting mechanism itself would be completely irreversible. The experiments presented in the preceding paperIs made it apparent that no extensive reduction of the inhibitor necessarily occurs when the inhibition is (1) Thia work wan done for the U.8. Atomic Energy Commission. (2) Norman Hackerman and A. A. Makridea, Ind. Bng. C h m . , 46, 523 (1954). (3) E. Cook and Norman Hackerman. THISJOURNAL, 111, 549 (195 1). (4) H. R. Wi.“Metal Interfaces” A Symposium. Am. Sac. Metals. Cleveland. Ohio, 1951. p. 312. (5) R. A. Powers and N. Rackerman, J . Electrochem. Soc., 100, 314 (1953). (6) N. Hackerman and R. A. Powers. THISJOURNAL. 117,139 (1953). (7) T. P. Hoar and U. R. Evan& J . Chem. Soc.. 2476 (1932). (8) U. R. Evans. Tran-8. Blsdrochan. Soc.. 69, 213 (1986). (9) E. Chyseasld and U. R. Evans, ibU.. 76, 215 (1939). (10) H. H. Uhlig and A. Geary, J . Blectrochem. Soc., 101, 215 (1954). (11) J. E. 0. Mayne. J. W. Menter and M. J. Pryor, J . Chem. Sm., 3229 (1950). (12) T. P. Hoar and U. R. Evans. J . Eleclrochem. Soc.. S9, 212 (1952). (13) G. H. Cartledge, Trns JOURNAL. 6S, 979 (1955).
due t o the pertechnetate ion. The present experiments were therefore directed t o an attempt to determine, by some means not dependent upon the activity deposited on the metal, whether the inhibitory process in this case is reversible or irreversible. Two types of experiments were conducted, involving the effect of added electrolytes upon (a) the inhibiting property of the pertechnetate ion and (b) the electrode potential of electrolytic iron in a pertechnetate solution. As shown p r e ~ i o u s l y , ~ ~ inhibition by the pertechnetate was lost in the presence of sufficient concentrations of hydrogen ions or other electrolytes. For the present study, the sulfate ion was selected, since it is an excellent flocculating agent for hydrous iron oxide and could hardly be suspected of peptizing a protective a m . For a second non-inhibiting ion of different charge type the perrhenate ion was chosen because of its close similarity to the pertechnetate ion in most respects. Inhibition in the Presence of Added Electrolytes.-The inhibiting concentrations of potassium pertechnetate are so small (from 5 X 10-6 f up) that the solutions have a low electrical conductivity. Even 10 p.p.m. of added chloride ion is only 2.8 X 10-4f. Since added electrolytes were shown to weaken or destroy the inhibiting power of the pertechnetate, this effect might be ascribed to the increase in conductivity of the solution as well as to any specific action on the metal-solution interface, such as the competitive displacement of an adsorbed inhibitor. Because of the minute amounts of reaction film formed in the presence of the pertechnetate ion it would not be likely that the effect of added electrolytes could be ascribed to a decrease in film resistance. Parallel inhibition tests were therefore conducted in two solutions of essentially equal conductivities but of different ionic types. One solution was 0.01 f potassium perrhenate, KReO4, and the other was 0.005 f sodium sulfate. At. 25”) the equivalent conductance of 0.010284 f =eo4 is 119.15 mho cm.2 and that of 0.005 f Nai30, is 117.28 mho cm.2 Neither solution by itself inhibits corrosion.17 Each solution was made 3.6 X lo-‘ f in techne(14) C. H. Cartledge, Corrosion, 11, 335t (1955). (15) J. H. Jones, J . Am. Chem. Soc., 68, 240 (1946). (16) “International Critical Tables.” Vol. VI, McGraw-Hill Book Co., New York. N. Y., 1929, p. 236. (17) Negative resulte with the perrhenate ion will be preaented in the
next paper in this series.
&
.
Jan., 1956
mIBILl!l'Y
tium and 1 X 2 cm.emeried spi?cimens of carbon steel (sample no. 102) were exposed at 100" (23" overnight). The metal in the d a t e solution became heavily smudged almost immediately, the solution became turbid, and rusting soon followed. The specimen in the perrhenate mixture remained bright during the firsthour at 100" except for a few small blue-grey spots. After four dayt3 at 23" there were a few small areas of localised rusting in the rhenium specimen, whereas the d a t e specimen showed much rust. After three weeks, the specimen in the TcOa--R,eO,- mixture had much bright metal, some interference film, and a few very a l l pits. The specimen in the TcO4-4404- mixture was dark all over, except for a large bare area that was deeply corroded and numerous other areas less deeply corroded. There was much more loose rust in suspension in this tube than in the one containing the perrhenate mixture. It waa evident that whereas the presence of the perrhenate interfered to some extent with the pertechnetate inhibition, such effect was far less prominent than that observed in the sulfate solution. It appears, then, that the disturbance caused by the presence of electrolytes is not due merely to the increase in conductivity, but also involves specific effects such as competition between dserent ionic species in adsorption at the interface. Thisresult is in agreement with the data of Hackerman and Powers,' who showed that there is a corresponding competition between chromate and d a t e ions adsorbed on chromium. Electrode Potentials.-In another series of experiments the electrode potential of iron was m d in dilute solutions of potassium pertechnetate, potassium perrhenate, sodium d a t e and mixtures of these." Measurements were made in aerated solutions and also in solutions through which nitrogen was bubbled. The temperature was 23-24". Strips of very pure electrolytic iron 10 mil sheet about 5 111111. wide were emeried and covered with an insulating lacquer except at the ends. After the lacquer had been dried overnight at 110" the exposed end was again lightly emeried just before use. The cell consisted of the experimental M-cell bridged to a saturated calomel half-celi (S.C.E.). The measurements were made by means of a vibrating-reed electrometer and Brown recorder. Table I shows the results of a preliminary experiment. (In this table and in Figs. 1-4 the potential corresponds to the polarity of the iron electrade, Ce., the European Convention for signs is followed.) TAEILE I ELECCEODE POTENTI~LS OF ELECTROLYTIC I a o ~(hmmu SOLUTIONS) Potential
Electrolyte.
f
Jrreo* lo-' KTCOI
w.
s.cJ3.
(mv.)
+ 10
+ N d 3 0 ~lo-' + 0 to 8
K T c O . + N ~ l O - *-!BO N&Oh 5 X 10-4
-520. drifthg
negative
Otmermtions No cormmion No d o n No c o d o n : pH 6.37after 3 :rh Corroding: pH 6.60 after 2 br.
Slircht corrosion ovemkht. (18)The sothor is indebted to Mr. F. A. Pmey for bin of them meanvementa
ION ~NHIBITINGM -
OF THE PEB"EC"A!L'E
'
a in Cer-
29
In potassium pertechnetate alone (100 p.p.m. Tc) the potential became essentially constant at +lo mv. S.C.E. After 2.6 hours from the time of immersion, d c i e n t 0.05 f sodium d a t e was added to bring its concentration to 1 X lO-'f. The addition caused an immediate negative shift of the potential which did not exceed 5 mv.; the potential drifted slowly back to +8 mv. S.C.E. during the next 24 minutes. At this time additional sodium sulfate was added to raise its concentration to f. hmediately the potential debased rapidly and attained a nearly steady value of -290 mv. within about 20 minutes. The following day this value had debased further to -380 mv., pH had changed to 7.40,and slight corrosion had occurred. A second specimen was measured in 5 X lo-' f sodium d a t e alone. Corrosion set in at once; the potential went to -400 mv. almost at once and drifted steadily to lower values. After 2 hours it was -520 mv. and the following day it measured -570 mv.; corrosion was very extensive. A aimilar experiment was done in which the potentials were recorded over a twoday period and the electrode was carried through two complete cycles of being ennobled in pertechnetate alone and debased by addition of d a t e . Figure 1 shows the
\
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-
; -;
v) rn
-1000-
a.!r10-3 TcOl
A :(
'..
-I
9 ; -200IW
-
a I-
-400 _1
W
t
-
..
5~1O-~ 2.5~10-~
e.
-.
2.5~qO-~ 5rr0-3
f.
-300-
n 0
c. d.
b.
n O
SO; 0
p.
1x10-j
AERATED; pH 6.5; 24. ELECTRODE CONDITIONED IN KTc04 BEFORE USE
I
-1
0
I
2
3
4
5
6 7'24 TIME (hr).
I
1
1
1
25 26 27 2 8 . 2 9
Fig. l.-Effect of added d a t e ion upon the electrode potential of electrolytic iron in aerated potassium pertechnetate.
complete history of the experiment. Although this specimen became somewhat more noble in the pertechnetate solution than the electrode used in the first experiment and actually continued ennobling somewhat in the presence of 5 X lO-'f N&O+ the addition of sodium sulfate up to 2.5 X lo-' f induced immediate and extensive debasing. It is especially to be noted that the specimen returned to a noble potential rapidly when the sulfa-rtechiietate mixture was replaced by pertechnetate alone. When sulfate was next added to only 1 X 10-3 f, the electrode became unsteady and debased slightly, but the potential dropped extensively only when the d a t e concentration was again increased to 2.5 X 10-Jfand finally to 5 X lo-'$ The debasing and rwnnobling cycle was confirmed in still a third experiment (Fig. 2). The measurements leave no doubt that the potentialdetermining interface is in a labile state which involves an antagonis
30
,"-:
I
c W ;-2001,' 4 0
w -300 n
2
g
2
3
f
-400
o
w
0
1
4 5 6 TIME (hr.),
D, pH 6.6,2 4 -
2 2 2 3 2 4 2 5
Fig. 2.-Effect of added sulfate upon the electrode potential of electrolytic iron in aerated potassium pertechnetate.
tic action between the sulfate and pertechnetate ions. The variation in potential corresponds precisely to the observed behavior with respect to corrosion. These changes in potential were shown not to be associated with corresponding changes in the beta activity of the thoroughly washed electrode. Thus, i n the experiment summarized in Fig. 2, the electrode was removed and counted after it had debased by 125 mv. (point e). It was then returned to the aerated pertechnetate without sulfate and left overnight. A recount was made a t point g, the electrode being held as nearly in the same position in the counter as possible. The total activities found were 49 and 58 counts per minute a t points e and g, respectively. Because of the uncertain geometry, absolute values of the technetium content can be only approximately estimated as in the range of 1013-1014atoms per cm.2, but the magnitudes are seen to be very similar under both conditions. This observation again demonstrates that the counted activity is of only secondary importance for the inhibitory process itself. Had the debasing been allowed t o proceed t o still lower potentials, corrosion would have ensued in spite of the fact that the beta activity would then have increased greatly. In a similar series of measurements, the debasing effect of the perrhenate ion was determined. Figure 3 shows the results of one experiment, which indif cate that the noble potential reached in l
