INDUSTRIAL AND ENGINEERING CHEMISTRY
332
Vol. 19, No. 3
Controllable Variables in the Quantitative Study of the Submerged Corrosion of Metals' By 0. B. J. Fraser, D. E. Ackerman, and J . W. Sands INTERNATIONAL NICKEL Co., BAYONNE, N. J.
HE development of laboratory methods for determining
exerted over the test conditions, particularly these three : the probable life of corrosion-resistant metals and al- (a) composition of the medium, (b) velocity of relative moveloys has received much attention in recent years. ment between metal surface and medium, and ( c ) temperature. The ancient "beaker" test is no longer accepted as anything I n the absence of serious interference from compact films of more than an indication of the manner in which corrosion insoluble products of corrosion accumulating on the metal may take place. As such it is a valuable means of exhibiting surface, the relationships between rate of corrosion and these tendencies toward pitting or deep corrosion a t or near the variables are continuous and fairly simple. They have been solution surface, but quantitative determinations by this determined for the action of aqueous solutions of sulfuric method of exposing pieces of metal in quiet, unaerated solu- acid upon monel metal. tions are not in favor with engineers because they represent I n this .case the corrosion rate, a t constant temperature, conditions rarely met in practice. varies continuously with the concentrations of hydrogen ion Much confusion has arisen in a search for an accelerated (acid) and dissolved oxygen in solutions of from 0 to 80 Der test which would give recent sulfuric acid, by weight. liable information in a few With higher acid concentrahours. Concentrations of tions than this, the rate is The great influence of controllable variables on the corrosive components have also a continuous, but differresults of laboratory corrosion tests of the total-immerb e e n changed arbitrarily, ent, function of the consion type is discussed and illustrated by numerous or, even worse, rates of corcentration. The change in data obtained in a study of the action of sulfuric acid rosion obtained in solutions function is consequent upon solutions on monel metal. Three types of apparatus of one reagent have been an abrupt change in the are described, which provide for control of the variables, used to interpret the resistnature of the corrosion reand with which satisfactorily reproducible results have ance of metals to other quite action. been obtained. I n dilute sulfuric acid the different media. The imThe concentrations of dissolved oxygen and of the corrosion rate, at constant position of an external curactive ions of the solution, the velocity of relative temperature, is directly prorent on the metal to be motion between test pieces and solution, and the temportional to the oxygen constudied as anode, with an inperature must be controlled closely, because the vafiacentration, reaching a maxis o l u b l e cathode, has been tion in rate of corrosion with changes in any of these mum, therefore, when the suggested as a suitable variables is of considerable magnitude. Moreover, solution is saturated with it. means of acceleration, but such changes in rate of corrosion are not always in the C o n s e q u e n t 1y , the most the reliability and utility of same direction because of the concurrent differences effective means for controlthe method have so far not in oxygen solubility with changing ionic concentrations l i n g t h i s v a r i a b l e is to been demonstrated. It is and temperature. m a i n t a i n t h e solution at indicative of what may take saturation with respect to place when two metali of air. d i f f e r e n t potential are in The corrosion rate increases continuously with the velocity contact in the same solution, but not of the normal corroof relative movement between the test piece and the solution. sion of a single metal. That there is a general need for acceleration is questionable. The rate of change of corrosion rate diminishe3 as the velocity I n a majority of corrosive media metals, if they corrode a t all, increases, and approaches constancy a t a velocity of about are usually dissolved in sufficient quantity in 24 hours, or 200 feet per minute. Obviously, fairly high test velocities less, to permit accurate determination of rates of corrosion. are desirable so that the effect of variations in velocity during The quantity and physical structure of solid products of the time of test will be minimized. The corrosion rate, in the manner of reaction velocities corrosion very frequently govern the ultimate course of corrosion, so that 24 hours may not always be a sufficient generally, increases logarithmically with the temperature, time, but it has been found to be more than sufficient for doubling itself within about, 30" C. This relationship is most metals and alloys which have been studied by the au- maintained only to a maximum a t 70" C., beyond which the rate decreases because of rapid decrease in the solubility of thors. The maintenance of a maximum supply of air, and of move- oxygen a t the higher temperatures. The low temperature ment between test pieces and solution, are not, strictly coefficient, 1.3 for a rise of 10" C., is typical of diffusion procspeaking, steps to provide an accelerated method of testing, esses, and is characteristic of reactions in heterogeneous syssince these are very frequent conditions of service which any tems. Finish of test pieces is unimportant in most cases, so long test should simulate. as they are clean and reasonably smooth. Conclusions Development of Methods The principal conclusion to be drawn from the experimental Calcott and Wheteel2 apparently were the first to present a data recorded herein is that, if accurate information is to be obtained from laboratory corrosion tests on metals and al- critical discussion of general methods for laboratory study. loys totally submerged in liquid media, close control must be Following their contribution numerous other articles and
T
1
Received November 6, 1926.
2
Trans. A m . Insl. Chem. Eng., 16, I, 10923).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1927
papers have been presented.J Formerly, the great importance of providing for relative movement between test pieces and solution, and of maintaining constant aeration, was not realized. More recently, Thompson and McKay showed that rate of movement and aeration must be controlled if concordant and useful results are to be obtained. Alternate immersion methods were suggested by Rawdon and Krynitsky as being a means of controlling aeration. These have proved useful in studying the value of metallic coatings on iron and steel, but their general utility has not yet been established. Great differences in results may arise from such things as uncontrolled drafts, changing humidity, and hygroscopicity of salts on the drying surface, so that the results must be interpreted with caution. Influence of Controllable Variables
The principal variables over which control must be exerted in order to obtain concordant results are: (a) Composition of the solution with respect to all participants in the corrosion reactions; which implies constancy in concentration of the active components, including oxygen. ( b ) Velocity of relative movement between metal and solution. (c) Temperature.
333
the main volume of solution from a small volume within the enclosed space, which, because of the corrosion reactions, is soon of a composition very different from that of the main body. Through the operation of active concentration cells in such an environment, very rapid and deep localized corrosion can, and often does, take place beneath masses of products in solutions which corrode the metal initially a t a low rate. I n the case of monel metal in sulfuric acid solutions, there is ample evidence that the structure of the small amount of solid products formed is such as to offer only slight resistance to diffusion. Hence the rate determined from a 20hour test is not a t variance with one from a longer test period, or with one developed by measuring the weight loss over a final portion of a longer period, as recommended for general practice by Calcott and Whetzel. Shape and finish of test pieces are often of minor importance, particularly where the rate of corrosion is appreciable. The composition of the pieces should be truly representative of the material to be used in service, but no great precaution in surfacing is necessary beyond insuring that they are clean and reasonably smooth. There are exceptions, such as the action of strong alkaline hypochlorite solutions on nickel-copper alloys, where a polished surface resists for a considerable time the initial attack of solutions of much higher concentration than the limit for safe commercial use of such alloys. Another instance is the formation of rust on chromium-iron alloys, which is initiated more readily on rough than on smooth surfaces. Test pieces, of course, should be of proper size so that weighing errors for small changes in weight will be minimized.
It is essential that the composition of the solution, including oxygen, be maintained uniform throughout the time of test, particularly in the immediate vicinity of the test pieces. This requires movement between metal and solution, which insures continuous renewal of reactants and removal of soluble nroducts of corrosion. Absolute control of the rate of movementis not possible, but it may be approached closely. These Composition of the Solution points are of particular importance if results are to be applied CONCENTRATION OF PRINCIPAL CORROSIVE CONSTITUEFFto the design of equipment to be exposed to continuously renewed solutions. The temperature influence is of the usual Figure 1 and Table I show the variation of corrosion rate nature for chemical reactions, up to the point where decreasing oxygen solubility causes a sharp depression of the curve representing the relationship. The rate of corrosion within this range, as has been stated, bears a logarithmic relationship to the temperature. The duration of a test is .L important in many instances. 5 C a l c o t t a n d Whetze12 and Pratt and Parsons3 have drawn attention to the fact that the $ initial rate of c o r r o s i o n i s u s u a l l y considerably greater e than the later and more uniform rate. Cases are known 6 where the opposite is true; the 30 later rate is much greater t.han $ the initial. I n the one case tD s o l i d p r o d u c t s of corrosion 2 are d i s t r i b u t e d evenly, and 8 have a p h y s i c a l s t r u c t u r e w h i c h i n t e r f e r e s uniformly IO with the diffusion of reactants Figure 1 and resultants to and from O-Corrosion rate curve. O-Hydrogen-ion concentration. A-Oxygen solubility. the corroding surface. I n the other the solid products are of a different nature and dis- with acid concentration, a t room temperature. It is evident tribution, and frequently serve as diaphragms separating that a corrosion rate determined a t one concentration is not to service at a different concentration* 8 Thompson and McKay, THISJOURNAL, 16, 1114 (J923); Rawdon The break in the curve in the vicinity of 80 per cent and Krynitsky, Trans. A m . Electrochem. SOL, 46, 359 (1924); Rawdon, Krynitsky, and Finkeldey, Proc. A m . Soc. Testing Material.;, 24, 11, 717 sulfuric acid marks a change in the corrosion reaction. I n (1924); Farnsworth and Hocker, Trans. A m . Electrochem. Soc., 45, 281 lower concentrations it may be represented as the (1924); Wernlund, I b i d . , 45, 294 (1924); Pratt and Parsons, THISJOURNAL,
5
2 'i: 3
17, 376 (1925); Whitman and Russell, I b i d . , 17, 348 (1925).
M
+ H2SOd +
'/202
=
MSO4
4-HzO
(1)
INDUSTRIAL A N D ENGINEERING CHEMISTRY
334
where M represents both copper and nickel. I n the higher concentrations a second reaction intervenes, which is probably analogous to that stated by Piekering’ for copper in concentrated sulfuric acid, namely: 5 c U f 4HzS04 = c U & 3cUso1$- 4Hz0 (2) A corresponding reaction for nickel would be 4H2S04 = NiS f 3Nis04 f 4H20 4Ni (3) This explanation f& the break in the curve is supported by the fact that irregular patches of sulfides were formed on the test pieces in the 93.5 per cent solution. It is noteworthy that the two test pieces in 75 per cent solution were the only ones throughout the study which did not check closely.
+
+
Vol. 19, No. 3
reason that the maximum in the corrosion rate curve occurs at an acid concentration lower than that of maximum hydrogen-ion concentration. Table 11-Influence of Rate of Aeration on t h e Corrosion Rate of Monel Metal i n 5 Per Cent Sulfuric Acid Solutions, a t Room Temperature AERATION
RATE
No. OF CORROSION RATE TEST PIECES Max. Min. Av.
Cc./min./l.
soh.
0 25 75 100 150 260 450
Mp./sg. dm./day 2 2 2 4 2 2 4
0 129 141 170 159.5 164 182
0 114 140 160 157 161 165
0 121.5 140.5 165 158 162.5 171
Av. PEKETRATION
In./yeau 0.0000 0.0197 0.0228 0.0267 0.0256 0.0263 0.0277
Av. D E W ATION FROM
MEAN
Per cent to.0 t6.2 10.4
*2.6 11.6 *0.9 *3.1
Period of test, 20 hours. Test pieces in constant motion at 12.6 feet per minute.
Figure 2
The values for hydrogen-ion concentration, up t o 38.03 per cent sulfuric acid (10 N ) , were calculated from the data of Jones.5 I n 95 per cent solution sulfuric acid is dissociated to the extent of 1 per cent16which furnishes the last point on the hydrogen-ion curve. This point has been joined arbitrarily by a straight line to the remainder of the curve, in the absence of data for intervening concentrations. The oxygen solubility curve, which is for pure oxygen under atmospheric pressure, was constructed from Seidell’s data.’
I n the range of concentrations from 15 to 75 per cent sulfuric acid the corrosion rate curve is a straight line. I n this range i t has a slope differing only slightly from that of the assumed curve for hydrogen-ion concentration. The oxygen solubility curve becomes much less steep in this region, leaving the hydrogen-ion concentration as the principal variable. AERATION: OXYGEN-The influence of the rate of aeration is shown by Figure 2 and Table 11. For the run with zero air input, the solution was made up with freshly boiled water, cooled out of contact with air, and subjected to the passage of a stream of nitrogen throughout the test. The complete resistance of the alloy to corrosion in sulfuric acid in the absence of oxygen is of special note. It is evident that it is not enough just to add “some” air. There must be sufficient to maintain constantly the saturation concentration in the main body of solution, and hence a uniform rate of diffusion through the liquid layer a t the metal surface. An air supply of a t least 100 cc. per minute per liter of solution is necessary in the case under discussion. Of course, only a very small proportion of the total oxygen in this amount of air is actually consumed in the corrosion re-
Table I-Influence of Acid Concentration on Corrosion Rate of Monel Metal in Aerated Solutions of Sulfuric Acid a t R o o m Temperature Has04
No.
OF
TEST
CORROSION RATE
PIECES Max.
Per cent 0.1 1 2 4 5 10 15 25
50 75 83 85 93.5 ._ .
Min.
Av.
M g . / s g . dm./day 4 4 4 12 4 4 4 4 4 4 2 2 4 ~
95 144 212 250 227 221 202 144 61 31 32 65 727
87 130 188 157 216 206 193 135 55 18 31 67 585
93 139 202 219 221 215 196 139 56.5 23.5 31.5 66 668
Av. PEN&
Av.DevrATION F R O M
TRATION
MEAN
In./year
Per cent
0.0151 0.0226 0.0326 0.0356 0.0358 0.0349 0.0318 0.0226 0.0092 0.0038 0.0051 0.0107 0,1085
t3.2 k3.8 14.3 15.6 *1.6 t2.0 t1.7 *2.3 14.0 -21.3 *1.6 *1.5 *6.1
Period of test 20 hours. Test pieces in’ constant movement in circular path at rate of 16.75 feet per minute.
If the oxygen availability did not decrease as the acid concentration increased, it would be expected that there would be little change in the slope of the curve until the maximum hydrogen-ion concentration was reached. However, the oxygen availability, which is governed by its diffusion rate, does decrease as the acid concentration increases, not only because of the decreasing solubility of oxygen but also because of the increasing viscosity of the solution. It is probably for this J. Chem. Soc. (London), 33, 112 (1878). Z . p h r s i k . Chem., 66, 385 (1906). 6 Smith, “General Chemistry for Colleges,” 1 s t ed., p. 228. 7 “Solubilities of Inorganic and Organic Substances,” 2nd ed., 1919. 4
5
action. The excess passes off in the bubbles which escape from the solution. The maintenance of complete aeration is warranted be-
I S D USTRIA 11 A N D ESGINEERIYG CHE-MISTRY
March, 1927
335
cause, as has been stated already, this is a very frequent, and almost invariably the most severe, condition of service for metals and alloys in all kinds of liquid media. Moreover, it is much easier to maintain a solution completely saturated with air than a t some lower concentration. Figure 3 and Table I11 show the effect of varying the oxygen-nitrogen ratio in the saturating atmosphere, other conditions remaining constant. All atmospheres except that containing 21 per cent oxygen, which was the normal air supply of the laboratory, were meter-controlled mixtures of oxygen and nitrogen led into a mixing chamber, from which the gas mixture flowed into the corrosion vessels at the rate of 100 cc. per minute per liter of solution. Table 111-Influence of Concentration of Oxygen in t h e Saturating Atmosphere on Corrosion R a t e of Monel Metal in 5 Per C e n t Sulfuric Acid Solutions, at R o o m Temperature
C ~ ~ ~ ; ~ & G No. G A OF S CORROSION RATE TEST PIECES Max. &fin. Av. 0
R b y vol. 0 in ._
21a 50
100
70by vol. 100
sn ..
79 50 0
2 4
4 2
2
Mg./sq. dm./day 0 0 0 87 90 9:3.6 .. . 170 160 165 295 289 292 561 548 554
Av. PENE-
Av. DEVIATION
TRATION
FROM MEAN
In./year
Per cent 10.0
0.0000 0.0146
0.0267
0.0473 0.0898
/
/
I
INrlUfME OF TEI-IPERA TURE ON
--
E
13.2 12.6 Al.0 12.3
7H€ ORROS/ON OF /'?O#€L
&TAL
INAERATED S%H,SO,Sorurim
,
,
,
,
,
~
Normal air supply of the laboratory. Period of tests, 20 hours. Test pieces in constant motion at 12.6 feet per minute. Rate of input of gas mixture, 100 cc. per minute per liter of solution. a
The rate of corrosion is a linear function of the concentration of oxygen, and hence of its partial pressure, in the satnrating gas mixture, except for concentrations approaching zero. -4s the rate of diffusion of a gas through a fluid mediurr is proportional t o its concentration or partial-pressure gradient. the corrosion rate-oxygen concentration curve of Figure 3 is indicative as well of the influence of oxygen concentration in the saturating atmosphere on its rate of diffusion to the metal surface. Evidently this rate of diffusion is the limiting factor in the corrosion of monel metal in sulfuric acid solutions, as it is in so many other cases of corrosion. The curves of Figures 2 and 3 are quite different in character. The form in Figure 2 is undoubtedly due to variation in absorption efficiency with increasing air input, and not directly t o the effect of changing concentration, which determines the forni of the curve in Figure 3. I n Table I11 the ratio of corrosion rate with pure oxygen to that with air is 3.4. Speller8 states that the concentration of
Figure 4
oxygen in gases dissolved by water in equilibrium with air is 35 per cent, the enrichment from the normal 21 per cent in air being due to the greater solubility of oxygen. The data of ' "Corrosion: Causes and Prevention," p. 1 4 1 ( 1 9 2 6 )
Figure 5
Solid products of corrosion may or may not have an important influence on the rate of diffusion of oxygen, depending upon their quantity and physical structure. If they had. in this case, the relationship between corrosion rate and oxygen concentration would not be linear, because as the amount of corrosion increased with increasing oxygen the thickness of the layer of products would increase, and rate. of diffusion would be retarded to an increasing extent. h continuous, tightly adherent, nonporous envelope of products would be expected t o produce such an effect. I n the case of monel metal in sulfuric acid such an envelope is not formed. This statement is based upon other tests not reported herein, in which the influence of time was studied. There is a very small amount of brown product, probably hydrated oxideb. which increases in quantity with increasing weight loss as the concentration of oxygen in the saturating atmosphere is increased. The layer of product apparently is of an open structure, very easily rubbed off, and does not offer serious resistance to the diffusion of oxygen. Such a condition is far from general in the corrosion of metals, nor is it true for monel metal in all media. I n acetic acid solutions monel metal acquires a coating of products of quite different physical characteristics, which interferes to a marked degree with diffusion. Although no study of the point has been made in acetic acid, it is believed that the relationship between oxygen partial pressure and rate of corrosion would not he linear. The rates of corrosion with a sufficient supply of normal air are lower in Tables I1 and I11 than the corresponding d u e for the 5 per cent sulfuric acid solution in Table I. This is 0
472
Seidell, "Solubilities of Inorganic and Organic Compounds " p 458,
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336
due to the influence of temperature, and will be discussed as such in a later paragraph. VOLUMEOF SoLuTIox-Calcott and Whetze12 concluded, for quiet, unaerated tests, that a volume of 200 cc. per test piece, measuring 2 X 1 X 0.01 inches (0.508 X 0.254 X 0.00254 dm.), was sufficient. The exposed surface of such a piece is approximately 0.26 sq. dm. The test pieces for this study were disks 1 inch (0.254 dm.)in diameter and 0.25 inch (0.063 dm.) thick, having a total surface area per test piece of approximately 0.15 sq. dm. These were corroded in pairs in baths of either 2000 or 4000 cc. volumevery large in comparison with those suggested by Calcott and Whetzel. Such an excess provides for a large reserve of the corrosive components and avoids the building up of concentrations of soluble products of corrosion to dangerous levels. The acid contents of the baths remained constant, within the limits of analytical error, during the tests. The maximum amount of metal dissolved in any of these tests, had it all remained as soluble products, would have given a final metal content in the solution of only 0.04 gram per liter. This was in one of the 73" C. tests of Table V.
VOl. 19, No. 3
easily displaced than those closer t o the metal surface, where the electrical and surface tension forces exert a maximum influence, preventing complete destruction of the stationary layer. Sometimes, particularly a t high velocities, there would be a tendency also to throw off interfering solid products of corrosion, or to avoid their formation on the metal surface by separating the metal from soluble products before their precipitation in an adherent form could take place. Where hydrogen evolution accompanies corrosion the influence of velocity upon the overvoltage of hydrogen is important. This has been discussed thoroughly by Whitman, Russell, Welling, and Cochrane.lo The curve of Figure 4 is very similar in shape t o those of Whitman and Russell3 for the influence of velocity on the corrosion of copper in aerated sulfuric acid solutions. Their curves for copper in aerated acetic and hydrochloric acid solutions were of quite different shape, however, for which they advanced an extended explanation.
Velocity of Movement between Metal and Solution
The influence of velocity on corrosion rates has been studied by others.I0 There is a general tendency toward increasing rates of corrosion with increasing velocity, and Figure 4 and Table IV illustrate the tendency in the case of monel metal in 5 per cent sulfuric acid solution, a t room temperature. Table IV-Influence of Velocity of Solution Flow o n t h e Corrosion Rate of Monel Metal i n 5 Per C e n t Sulfuric Acid Solutions, a t R o o m Temuerature
VELOCITY
NO, OF
TEST
PIECES
Fl./min. 0
11 13 82.5 131 142.5 240
4 4
4 2 2 2 2
CORROSION RATE
Max. Min.
AP. PENE-
AV.
TRATION
Mg./sq. d m . / d a y 116 103 108 141 130 134 134 119 128 296 294 295 325 331 319 329 317 323 388 370 379
In./year
Period of tests, 20 hours. Rate of aeration, 100 cc.
minute
O.Oli5 0.021i 0 0207 0 04i8 0 0526 0 0523 0 0614
xv.
\r
Figure 6
DEVIATION FROM h l E A N
Per cent 13.5 *2.6 *3.7 h0.3 +l 8 *l D -2 4
liter of solution
Velocity changes are generally considered to influence rates of diffusion of reactants and soluble products of corrosion to or from the metal surface. I n every case of submerged corrosion t'here is held to be a stationary layer of liquid immediately adjacent to the metal surface. If corrosion is to proceed, dissolved metal ions must diffuse outward through this layer to the main volume of olution and corroding ions and oxygen must diffuse inward. The net rate of reaction will be governed by the slowest rate of diffusion. Diffusion rates are proportional to the concentration gradients through, and inversely proportional t o the thickness of, the stationary liquid layer. As the solubility of oxygen in sulfuric acid solutions is small, the rate of diffusion of oxygen through the stationary liquid layer is the controlling factor. The effect' of increasing the oxygen concentration gradient, as shown in Figures 2 and 3, is ample supporting evidence. Any decrease in the thickness of the fixed liquid layer should result in a proportional increase in the rate of diffusion of oxygen. A rapid movement of solution past the metal surface evidently decreases the thickness materially, for the rate of corrosion increases progressively, though a t a diminishing rate, as the velocity is increased. The diminishing rate may be explained by considering that the outer portions of this layer are more 10 hlcKay, Trans. A m . Elecfvochern. Soc., 41, 201 (1922); Friend and Dennett, J . C h e m . Soc. (London), 121, 41 (1922); Calcott and Whetzel, loc. c d . ; Whitman and Russell, loc. c i f . ; also, with Welling and Cochrane, THISJ O U R N A L , 15, 672 (1923); Speller and Kendall, [ b i d . , 15, 134 (1923).
The tests of which results are given in Table IV were run in a different type of apparatus from those of the preceding tables, to which is due their generally lower rates of corrosion. This apparatus is of a continuous-flow type with stationary test pieces, while that used in the other tests provides for test-piece movement. The apparatus is described in a later section. The lower rates in the tests of Table IV are believed to be due actually to differences in absolute velocities between solution and metal, which, in the moving test piece type of apparatus, are probably not the same as the known rate of movement of the test piece. Temperature
Figure 5 and Table V show the influence of temperature upon the corrosion rate. A logarithmic relationship exists over a major part of the total range, or up to 70" C. The equation connecting the two is of the type Log R
=
a
+ bT
(4)
where R is the corrosion rate, a and h are constants, and T is the temperature in degrees Centigrade. I n this case a = 2.097 and b = 0.0103. Calcott and Whetzel noted that the influence of teniperature in many corrosion reactions could be represented by the type equation for velocity of homogeneous reactions : LogR
= a
+ -b
ra
(5)
where T, is the absolute temperature. The curve in this case may be evaluated in terms of such an equation. TVhitman and Russell3 found values for the corrosion of steel and aluminum in 50 and 96.5 per cent sulfuric acid, within the range of 20" to 50" C., which comply with similar laws. I n 6 per cent acid neither metal complied with such a law, nor did steel in 20 per cent, nor copper in 6, 20, or 96.5 per cent sulfuric acid. Above 70" C. the oxygen solubility decreases to such a
-mall ~ a l u tliat e the I:LW
is not obeyed. ,411idea of the miinner i n which the solubility of oxygen decreases as tlie temperature increases mny he had from examination of Fiwre 0, which gives the relationship for water. A similar relationship probably holds for a 5 per cent solution of snlfnric acid. Speller," from whose book tlie curve of Fignre 6 was talien, found that the corrosim rate oi iron in natural water reached a lnaximum a t 80" (!. and tllen decreased at higller teinpcrnt,urcr, 1)cc:tiise of decreasing solubility of oxygen. 'roble \.. -Influence of Tern erafure on Corrowion Rate of Munel Mofnl In 5 Per &mt Sulfuric Acid Soluiion A". I'ilNt.
A". DI:YIITLOS
..I word of esplniiation may now be offered for tile difforent corro?ion rates in 5 per cent acid solutions ill Tables I to 111. The tempcratincs Twre not controlled thennostatically for tlie determinations recorded in these t.ahIes. Those of Table I were obtained in the summer, when the temperature ran8e over the 20-tiour test periods was roughly 18" to 30" C. Those of Tables I1 a.rid 111 u-ere found during the winter, when the laboratory t.emperature was as low as 10" C . during tlie night, and about 25" C. during the day. On inserting in equat,ion (4) the logarit.1ims of the corrosion rates for 5 per cent sulfuric acid from Tables I to 111, respectively, it is found that the valne of 221 lug. per sq. dm. per day, from Table I, corre,spondsto a temperature of 21"C., while tliat of 165 mg. per sq. dm. per day from Tables I1 and 111 corresponds to a temperature of 12" C. There is, therefore, no conflict among tlie data.
initially inucli more resistant to attack than the same met.al with a ground surface. The rcsults of the group of tests referred to in this paragraph are recorded in Table VI. These were also included in Table V for the rate of corrosion at 30"C. Apparatus
Tlrrcc differentsets of apparahs were employed, with all of a disk I inch (0.254 dm.) diameter by 0.25 ineh (0.063 dm.), is usually iised. l.'ignre 7 shows what is called the circular path apparatus, with which the data of Tables I, V, and VI were obtained. It is a development from an earlier apparatus of The type of motion oi the test. Tliampson and k 1 ~ K a y . ~ pieces has !ieen (:hanged from a vertical, reciprocating movemciit in t,lic earlier apparatus to a uniform mot-ement on a circular pntli, in a vertical plane. The test pieces are supported on glass stirrups, and w e set a t an angle bo tlie plane of tlroir orhits. This insures positive flow of solution past every point oil the metal surface at some part of each revolution. l'he absolute velocity of movement between solution and metal is greader than the rate of linear travel of the test piece because of the angular orientation described above, and because of vertical currents r e s u l h g from tlie manner of aeration. Except for bearings and gears, the apparatus is h i l t of monel metal rod and shapes. Two crank shafts, driven by lielical gears from a common drive shaft are supported on 5 frame of angle sections. Tlie cranks have a tlirow of 1.5 inches, and are connected by a cross arm, from four points in wliicti fire hung yokes, adjustable vertically, ior a.itac1imeiit of the glass stirrups on which t,lie test pieces rest. TIx soiirce oi power is a '/,-horsepower motor, connected t.lirougli reduction gears and cliain and sprockets to the comnioii drive sliaft. T l i e reduction gearing is adjustable for speed to a, rnaxirninn of NO r, p. i n . , which gives a niaximum w l i i c l ~a siandard size of test piece, in the form of
Table VI- -Influence of liinieh of Teat Piecea o n Corrosion Rare of Monel M e t r l in 5 Per Cent Sulfuric Acid Solution, B f 30° C.
NO.
