{I) The mean value for the faraday, 96,538 coulombs, given in column SI11 has a precision of O . o O j $ ~ ~ The average deviation of the individual results from the mean is O O I O ~and ~ the mdximum is only 0.012%. These deviation3 are caused Ily the silver coulometer, nut by the iodine coulometer ( 2 ) The value 96,538 agrees astonishingly well with the value, 96,535, obtained by Richards with his copper coulometer (bee Table I). ( 3 ) The results with the iodine coulometer confirm the conclusion, already reached, that the silver deposit obtained in the silver coulometer, even under the best conditions, is greater than that required by the reaction, Xg" - (-) = Ag. (4) The value 96,538 coulombs per equivalent corresponds to the value I. I I 74 milligrams per coulomb for the t r w electrochemical equivalent of silver This is o.o8y0 lower than the value obtained by Smith, Mather and Lowry and 0.0jcjO lower than the recent result of Rosa, Vinal and McDaniel (see Table I). ( 5 ) Although we have no reason to believe that our results are affected by any source of error amounting to 0.03%~they are, perhaps, too few in number to justify the conclusion that the value, 96,538 coulombs, is correct within this limit. We believe, however, that the evidence presented in the foregoing pages justifies the statement that the value of the faraday is certainly greater than 96,500 coulombs and that the t w e electrochemical of silver is certainly less thun 1 . 1 180milligrams per coulomb. We hope to be in a position to make a more definit statement during the coming year. iksn'\.%.i r r
Tune 2 , , 1912
PASSIVITY OF IRON "?DER BOILER COIVDITIONS. BY HOEWE CJ E Y E R S A N D FWYD T VORIS Received August 10, 1912.
Introduction. The use of substances which inhibit the rusting of iron in order to protect it :rom the action of corrosive agents was suggested by Cushman,l who says that '' small quantities of inhibitive substances in boilers should he highly efficacious in preventing pitting." Wyatt' suggests the use of sodium bichromate in boilers to precipitate the scale-formkg materials and states that the chromic acid formed does not attack the iron. On the other hand, Schoch and Randolph3 state that " the corrosive character of an aqueous solution cannot he extensively lessened by additions of certain electrolytes such as chromates, hydroxides, etc." Also it was shown by one of us4 that in many solutions, in which iron as an anode is OEce Public Roads, t? S.Pept .4gric, Bull. 30, 29 f-tip
' I '
Mqjt
l'iit~ic
, 60, 2 2 0 Ckem., 14,719.
Ii\-er% I'rrls
TOIxN.4L
30,
1718
PASSIVITY OF IRON UNDER ROILER CONDITIOKS.
I369
passive a t ordinary temperatures, the passive state is destroyed at temperatures ranging from 60' to So', and that i t is impossible to render iron passive in halogen salt solutions by the use of anodic current density alone. It therefore seemed to be of interest to determin which of these views, (Cushman zs. Schoch), neither of which is adequately supported, is verified experimentally. Since the work was begun two papers bearing on the problem have appeared. Dunstan and Hil1,I have shown that the presence of salt solutions destroys the passive condition of metals and that on the other hand iron could be obtained in a passive state in boiling potassium bichromate, sodium carbonate and potassium hydroxide. J. I,. R. Hayden2 has shown that iron plates remain passive in the presence of o.o?yo of ammonium chloride but corrode slightly in 0.047~ of the same salt. The present contr bution is an effort to go into the subject more thoroughly and a t temperatures approaching those prevailing under steam boiler conditions. After the details of the experimental conditions and results are presented, a summary will be offered. Experimental, Figs. I and 2 show the apparatus devised to determin a t what temperatures iron can be kept passive as an anode in bichromate solutions of various concentrations and the arrangement of the coulometer, ammeter,
n
Fig.
I.
voltmeter, etc., to measure the anode loss and current used. In Fig. 2 is shown a boiler which consists of a heavily capped piece of gas pipe, the upper cover of which is pierced with two holes, inch in diameter, to carry the anode and cathode. The boiler also carries a pressure gauge and safety valve and a broad base in order that it may set securely in a a
J . Chem. SOC., 99, 1835. J . Frank. Inst., 172,295.
13i0
HORACE G . RVERS AS!) FI,OI'D T. VORIS.
large paraffin bath. The cathode and anode consist of standard iron wire (99. Si%) of about 0 . 7 mm. diameter, sealed in glass tubes and passed through corks securely plugged into the cover of the boiler. The area of :the anodes mas calculated from the diameter of the wire and the length of the portion immersed, but because of the difficulty of exact determination of this latter quantity, the density per sq. cm. given in the tables is only approximate. The current used was determined approximately by means of a milliammeter graduated to read to tenths and the amount regulated by a slide wire resistance. To get exact values the current used was measured by means of a copper coulometer with a variable cathode. In conducting the experiments a voltmeter was connected with the electrodes and a key in order to determin when the anode was passive. The readings are Fig. 2. not recorded since the voltages have no other significance than to indicate when the iron goes into the passive condition. When active the potential fall between the electrodes was below 2 . 5 v. and when passive upwards of 4 v., varying of course with the concentration of the electrolytes used and with the current strength employed. Where temperatures of less th&n 100' are involved the boiler was replaced by a porcelain beaker (B. Fig. I ) and when this was used at or near boiling temperature the level of the electrolyte was maintained as accurately as possible by meaix of a hot water bottle. Temperatures were determined by a thermometer immersed in the bath close to the boiler and, as the rise of temperature in heating the bath was very gradual, represent very accurately the temperature of the electrolyte. The electrolytes used were prepared in the form of 5% stock solutions of the salts used and the various concentrations employed were obtained by placing the required volumes of each in a zoo cc. flask and diluting t o the mark with distilled water.
PASSIVITY OF IRON UNDER BOILER CONDITIONS.
TABLEI. Per cent of KzCrzO,.
NO.
. 2.5 . 4.0 3.... 2.0 4.. . . 1.0 j... . 0.50 6 . .. . 0.25 I.. .
2...
Weight of Time copper in min. deposited.
Temperature.
80 to 113 100to
39 0.03048 75 o.00810 91 0.00735 151 116 0.09390 147 82 0.07235 120 120 o.oj142 160 107 0.04831
135 I I Z to 146
103 to
106to
to 7 . . , . 0.125 122 to 111
Weight of iron lost.
MilliamCurrent in peres milliam- per sq. cm. Condiperes. anode. tion.
0.00048 o.ooooj ,..
... 0.00024
o.00010
0.00018
39.57 52.9 5.47 6.73 4.09 j.24 40.98 31 .o 44.67 40.6 22.20 32.5 22.86 31.48
Passive Passive Passive Passive Passive Passive Passive
Table I gives the results obtained for bichromate solutions of a concentration varying from 4% to 0 . 1 2 5 % . The temperatures used varied from 80' to 160' and in all cases the iron is wholly passive, the minute losses and gain of weight being due to rusting above the area immersed in the electrolyte. In each case the iron was presumably active when immersed, being cleaned thoroughly either by pickling in dilute suifuric acid or by scrubbing with fine emery paper. Temperatures above 160' were not reached because of softening of the cork and loss of steam. Having thus determined that the solutions keep the iron passive a t temperatures a t and above IOO', the next step was to determin what concentration of sodium chloride is sufficient to destroy the inhibiting influence of bichromate. The results obtained a t room temperature are shown in Table 11. At all ratios of concentration of less than 40% bichromate to I of salt the iron remained active, but the loss of weight of iron is not given because the pitting and rusting were so great as to render cleaning of the anode impracticable. In two cases, Nos. 11 and 12, the iron used was previously made passive by immersion in bichromate TABLE11. Per cent Ratio of of Per cent K,Cr207 KCrzOr. NaC1. to NaC1.
No.
I . . . . I . O
2.. .
.
3.. .. 4.
2/I
ZIO
0.25
4/1
18.5
2.0
0.25 0.25
8/1
19.0
16/1
0.10
40/1
0.10 0.025
40/1
0.0225
20/1
19.5 23 93 16 14.5
0.0125
17
I1
0.0125
30/1 40/1
I5
47
0.0125
25/1
19 16
5 . .. , 6.., .
4.0 4.0 j . . . . 0.3125 8....0.25 9 . . . .0.375 11.
0.50
. . . 0.3125
12...
40 31 40 40 40 32 8
0.5
1.0
. . . 4.0
IO. . . .
Temperature.
Milliamperes Condition per sq. of anode Time Weight of Current cm. of at close in min- copper in milliiron of experiutes. deposited. amperes. anode. ment.
12.5'1
. 0.3125 o.orz5 25/1
13. . . . 0.3125 0.0125 25/1 14. . . . 0.3125 0.0078 40/1 15.... 0.3125 0.0104 30/1
21 20.5
I9
11
12
40 40 45 30
Active Active 5 5 . 3 Active 64.9 Active 70.,o Passive 0.02032 32.Ij 48,j Passive 0.00413 '7.42 24.8 active ? Active 0.00575 26.46 0.00450 20.71 ? Active Passive 0.02398 25.83 2 8 . 2 Qual. .. Active (20.0) 0.01518 19.21 26.4 Active 0.01495 19.45 32.4 ,Active 0.02404 27.04 30.2 Passive 0.01 633 27.06 29.9 Active
0.02565 0,02053 0.02584 0.02840 0,02618
32.53 33.58 32.88 38.52 41.68
52.8 56.j
1
-* - I7
-
HORACE G. BYSRS AND FLOYD
T. VORIS.
1)eiore beginning the experiment in the mixed electrolyte. These reiults are in accord with those obtained by Dunstan and Hill1 and by Harden. The next step was to determin the ratio of b'chrornate required to keep iron passive in salt solution a t about boiling temperature. The temperature registered was 93 O as that was conveniently maintained withoiit great loss oi solution and without deposition of salts upon the wire outside the electrolyte. The fact previously shown (see introduction) that the general effect of elevation of temperature is to destroy passivity, naturally led us to expect that a larger ratio would be necessary. Quite the reverse was the case and the results given in Table I11 show that a ratio of 2 , j of bichromate to I of salt is sufficient to render the iron almost ii not quite wholly inactive In Kos. 12 and 13 a small degree of activity is observed but only a very small fraction of that demanded for accord {Viith Faraday's law. In Nos 4, 7 and 8, it is seen that even a ratio of 20 to I is practically effective. In No. I z the solution used in No. I I was poured into a flask, which was filled, allowed to cool and used with the same electrodes The anode was almost theoretically active, probably wholly so if the adhering oxide could be quantitatively removed. In KO. 13 a cold solution of the ratio 2 j I showed itself wholly active in spite of the anode being previously rendered passive in pure bichromate solution, and when used lint as in N o 14 was effective in producing the passive state. The next step, the determination of the ratio a t temperatures above 1oo3, was carried out with results as indicated in Table IV. Here again a somewhat surprising result is obtained. While a ratio of 40 : I (see 30s. j,,; S ) is absolutely effective in inhibiting the activity of the iron even where the concentration is so small as o 1 7to~0 . 0 2 5 % ~ there is appreciable rusting a t 3 j to I and yet the anode loss is not at all equal to that which takes place when iron is wholly active. The exact amount of loss is not indicated by the loss of weight recorded, since much oxide adheres even to the carefully cleaned anode, as in Sos. I and 2. The Variation in temperature noted in the table is as follows: The boiler was immersed in hot parafiin and when the temperature ceased to fall the current waS allowed to pass through the electrodes. The higher temperatures were then gradually attained by continued heating and the apparatus kept at tile highest mark for from fifteen minutes to half an hour, when leak usually deseloped Since in the natursl waters likely to be used for boiler purposes the only substances other than chlorides which would remain in solution after treatment with bichromates, are sulfates and carbonates of the alkalies, I o
1:
PASSIVITY OF IRON UNDER B O X E R CONDITIONS.
... .. .
.. .. .. ... . .. . ... . .
.
... ..
.. .
. . .. . . . . . . . . .. .. . . . ' O N L & G 4 A idr; &
. . . .. .. . . . . . . .
"2
1373
.
.
.
. . . ... .. . ... ... .. .. .. ..
i$i
HORACE I;. BYERS A N D FLOYD T. VORIS.
"
c
5.
0
3
m
PASSIVITY O F IRON UNDER BOILER CONDITIOSS.
I3 75
a series of tests was made to ,determin the ratio of bichromate to sulfate
which would keep iron passive. Some additional interest attaches to these experiments since Dunstan and Hi11,l whose paper appeared during the progress of the work, state that sulfates and nitrates as well as chlorides reader iron active when added to solutions in which it is passive. That nitrates do not necesdarily do so is shown sufficiently by the experiments of Haydenl when he shows that 1 % of potassium nitrate will be sufficient to keep iron passive in 0 . 0 2 % of ammonium chloride. He also shows that 0 . 5 % of bichromate will keep iron passive in the presence of 1 % of ammonium sulphate. Table V shows that with a ratio of I to I , bichromate and sodium sulfate, the iron is always practically passive, not only a t ordinary temperatures but at near the boiling point and a t about 1 5 0 ~ . When the concentration ratio is less than I : I the influence of current density in producing the passive state becomes evident as may be seen by reference to Nos. 6 and 7, and also to Nos. 18 and 19, where the only difference discernable is that of the density of the current employed. Table V I contains the results obtained, a t the temperatures indicated, with bichromate and various concentrations of sodium carbonate. It would not have been necessary t o have performed these experiments in view of the results previously obtained by one of us2 but for the work of Krassa, in which it was shown that iron is apparently active as an anode a t elevated temperatures in hot, strong solutions of sodium hydroxide if the current density is not too great. It will be seen from the results that at all the temperatures and concentrations and variations of current density used the iron could not be made to rust, this passivity is produced despite the fact that the anodes were carefully made active by dipping in dilute sulfuric acid until hydrogen began to be liberated and then were washed and dried. Connections were also made by immersion of the cathode and closing the circuit by means of the anode. No. 12 of the table shows that iron is passive, even with the slight current employed, in a sodium hydrogen carbonate solution a t 93'. A few experiments were made to determin if iron is normally passive in water containing free carbonic acid. The results were negative. With current densities varying from 0 . 4 to 75 milliamperes per sq. cm. of anode the iron remained active throughout at room temperature. The statement of Dunstan and Hill' that iron does not rust in the absence of air even when immersed in sodium chloride solution, led to some successful experiments to determin if the anodic passivity could be established in a salt solution in the absence of air. It has already been shown by Byers' L O C . cit. Ryers, LOC.cat. Z. Elektrochem., 15, 490.
I!,(JR.\CE 0. EYERS .4XD F1,OYD
2
0
T. VORIS.
O G C
0 0 0
PASSIVITY O F IRON UNDER BOILER CONDITIONS.
N
r-
* I”
%
c
0
W
f
m N
v, 3
M
0
% 9
0
*‘t v)
W
r. N
8 8
c
0
d 0
3
5. m
m
9
9
N
h
e
e
?
N
v; e
9
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h
v)
i
0
0
C
0
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3
4
P-
c
a2
* 01
m
m H
0 v)
?
0
s
Q\
F, Q\
LI
c
N
-2
CI
N
3
8
2S g.9,
0 -
+.r
v)
+
7;.
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v)
10
I
I
0
0
v)
N v)
d
I n
$ 2 ‘32 3
Ff
I
0
0
2
z
H
2
2
Per cent Uf
So. K2CrZO:.
0.12j
Per cent of Na.CO?.
Ratid 01 K2CrnO;
..
,
reniperato Sa2COa. ture.
peres , rime it'cipht of Current per sq. in niin- copper in milliam- cm. of utes. deposited, pews. anode.
Same solution 0.i2j
0.250 O.
