Corrosion of Metals bv Organic Acids in Hvdrocarbon Solvents J
J
C. F. Prutton, D. R. Frey, D. Turnbull and G. Dlouhy CASE SCHOOL OF APPLIED SCIENCE, CLEVELAND, OHIO
The presence of oxygen or peroxides is a necessary wndition for the corrosion of lead and cadmium by fatty acid solutions in hydrocarbon solvents. Although corrosion in these media will not take place to any large extent in the absence of acids, the rate of corrosion is mainly controlled by the rate of diffusion of oxidizing agent into the metal surface and by the chemical rate of oxidation of the metal rather than by acid strength or concentration. When an insoluble soap B m is. formed on the metal surface, the rate of corrosion is controlled by the rate of diffusion of the oxidizing agent through the a m . Accumulation of lead soaps in solution markedly decreases the rate of corrosion of lead. When dissolved oxygen is used as the oxidizing agent in the corrosion of lead, the limiting rate of corrosion varies directly with oxygen
pressure and may be represented as a function of temperature by the Arrhenius equation. Corrosion of lead becomes rapid around 4Q'C. and corrosion of cadmium around 80OC. Corrosion of lead proceeds at about the same rate in benzene as in xylene solutions, but much more slowly in white oil solutions due partly to the relatively high viscosity of white oil. Corrosion of bearing metals by lubricating oils containing fatty acids may be caused by dissolved oxygen and by organic peroxides. A t temperatures above 100' C., corrosion of lead in white oil due to dissolved oxygen becomes much smaller than at 100' C. or lower. A t higher temperatures corrosion of bearing metals by'acidic oils is probably caused principally by organic peroxides, as has been suggested by Denison.
T
portant factors involved in bearing corrosion and to discuss the mechanism of the reaction. Factors t o be considered are oxygen pressure, nature of the metal and of the solvent, nature and concentration of acid and of peroxide, temperature, time. Various concentlatiom of pure organic acids were made up in mineral white oil, xylene, or benzene, and permitted to react with pure metals for given periods of time under known oxygen preasure or in the presence of known amounts of added peroxide at some constant temperature.
HE corrosion of metals by lubricating oils has been attributed to the accumulation of fatty acids in the oils as a result
of oxidation. Howevcr, i t has often been observed that there is little correlation between the corrosivity of oils and the concentration and strength of acids present as a result of oxidation (7). It has long been known that weak organic acids will not attack metals in the absence of oxygen. I n 1883Fox (5)found that lead strips lost no weight over a period of days when hung in pure oleic acid at 100' C. in the absence of oxygen. Donath (E!) found that pure oleic acid did not attack such metals as zinc, brass, copper, or iron in the absence of oxygen at 75' C. However, the important role of oxygen in the corrosion of bearing metals by lubricating oils has largely been overlooked until recently. Denison ( I ) recently showed that corrosion of bearing metals by lubricsting oils containing acids will not occur to any appreciable extent unless peroxides are present in the oil. His results further indicate that the rate of corrosion ia determined by the concentration of peroxides in the oil, and is virtually independent within wide limits of the concentration or strength of acids present in the oil. He concludes that in the corrosion of bearing metals the presence of both acid and peroxide is necessary and that the corrosive action probably takes place in two steps: the reaction of peroxide with the metal to form a metal oxide, and subsequent reaction of the oxide with acid to form a salt. At the time Denison's results were published, this laboratory was attacking the problem of bearing corrosion from a somewhat different viewpoint, and results largely substantiated his conclusions. Denison followed the corrosion rate over long periods of time beginning with the complete unoxidized oil. In the present investigation acid and peroxide were not accumulated by oxidation of the solvent while corrosion was taking place, but were added in knownhmounts at the beginning of the reaction. This procedure i s advantageous from t a o standpoints: (a)The kinetics of acid formation in the oil are eliminated from consideration, and the mechanism of bearing corrosion by the action of acid and oxygen can be studied more directly; (b) when the acid is added directly, the corrosion takes place rapidly and sufficient information about the course of the reaction can be obtained in 3- 4 hours. The purpose of the present paper is to evaluate the most im-
EXPERIMENTAL PROCEDURE
PREPARATION OF TEST PIECES. Lead and cadmium were chosen for investigating the mechanism of corrosion since they are common components of bearing metals. They also corrode with much greater rapidity than other metale used in bearings such as tin, copper, or silver. Test strips were cut from chemically pure lead or cadmium sheet, and were about 5.08 X 1.91 cm. with a total surface area of about 20 s cm. Just prior to each determination, the stri s were cleaned& rubbing with an etheralcohol mixture appliecfwith a soft cloth. Rubbing waa continued until a clean portion of the cloth waa not discolored when rubbed across the surface. REACTION VESSEL. Pyrex tubes about 250 mm. long and 20 mm. in diameter, with a capacity of about 100 cc., were employed as reactors. The upper end of the tube waa closed by a ground glass joint sealed to a 2-mm. sto cock. High-vacuum stopcock grease waa used to seal this grounfjoint and the stopcock. AWPATION OF SAMPLE DURING REACTION.Agitation was provided during each test by rocking the reactor at the rate of 100 cycles per minute. The lowest point of the reactor traveled through a 2-inch arc. This rocking motion also served t o agitate the electrically heated diethylene glycol baths in which the reactors were immersed. DETERMINATION OF CORROSIVE ACTION. A metal test piece and sufficient solution (about 35 cc.) to cover i t were placed in the reactor. The reactor was closed with the ground-glass joint and attached to the vacuum-pump and manometer. All of the air, including that dissoked in the t a t solution, was pum ed out at room tem erature. After the test solution had attaineitern rature equiibrium oxygen WM admitted to a definite preKbrmined pressure, the stopcock was closed, and the agitator started. The amount of oxygen absorbed was determined by measuring the ressure at the end of the run. When the corrosion loss waa of t i e order of 0.6 milliequivalent or greater, the gas phase wm water-saturated, and correction for water vapor pressure WM made in calculating the milliequivalents of oxygen absorbed, 90
Jenaary, 1945
INDUSTRIAL A N D ENGINEERING CHEMISTRY
When the corrosion loss ww 1- than 0.6 milliequivalent, no correction for water ressure waa made and the oxygen absorption calculated from tfeae results thus tended to be low. After the test piece was removed from the solution, it was rinsed with benzene and cleaned with alcohol and ether by the procedure used to pre are the test piece ori 'nally. Several mechanical factors €!EPRODUCIBILITY OF %ESULTS. which mi ht affect the reproducibility of the corrosion tests were examined! Only two were found to be of major im ortance: (a) Position of test piece in the reactor; the test piece s%ould lie in a plane perpendicular t o the direction of vibration, and it waa held in osition by indentations in the side of the @ass reactor tube. Vibrations in the rocker frame; these variations were noted in preliminary work, and a plication of a spring to o p q e the thrust of the eccentric which g i v e s the rocker eliminate: this difficulty. In a series of seven testa on a 0.1 N solution of lauric acid in xylene the mean deviation was found to be *4% with a maximum deviation of 12%. D ~ R M I N A T I OOFN PEROXIDES. The method of Kokatnur s a d Jelling (6)was used. Lead salta of the fatty acids cause some interference due to the formation of yellow lead iodide. However with a little experience the proper end point can be ascertained. Whenever lead salta were present, the end point was sharpened b allowin the samples t o stand for 48 hours befqre titrating. &ith a bfank. correctiop this procedure gave satisfactory results when a phed to various known mixturea of laur071 peroxide and &tt&tyl hydroperoxide and lead stearate in 011s. DETSIWINATIONOF WATER. Water contents of the various solutions before and after reaction were determined by titration with Karl Fischer r e a p t , $repared accordi to the method of Smith, Bryant, and itche (6). This proc3ure is satisfactory for nonviscous solutions. I n ex riments where white oil was used, it waa found necessary t o a d g few cubic centimeters of abso: lute methanol to keep the reagent dspersed. Better resulta are obtained If the solution is warmed until all of the salts are dissolved. Excessiye heating should be avoided. This procedure was checked against a standard water solution made up in rtbsolute methanol and found to be satisfactory.
91
TABLE I. EFFECTOF OXYQEN ON CORROSION OF LEADBY LAURIC ACIDSOLUTION Water in Os Pressure wt. Loas, Solvent Xylene Xylene Xylene X lene v?hite oil White oii
Solvent
Mm. of Hi
ME.
Sstd.
0 0 166
2.3 112.5
None
Satd.
None
166
None
0 166
None
3.7 134.9 1.7 67.1
(1)
*
PRELIMINARY OBSERVATIONS
NECESSITY FOR OXYQENOR PEROXIDES. A number of preliminary observations were made to determine the conditions necessary for the attack of lead by solutions of fatty acids in nonpolar solvents. Tests were run for 6 hours a t 100" C. using 0.1 N solutions of lauric acid in xylene or white oil. A number of reactors were evacuated to a pressure of less than 0.01 mm. of mercury and sealed off while others were closed off in a n atmosphere of air before being subjected t o the test. Results of these tests (Table I) showed that the extent of reaction of the metal with fatty acid in either white oil or xylene is negligible in the absence of oxygen. In accord with Denison's results i t was found that acid test solutions which had accumulated peroxide on standing corroded lead rapidly in the absence of oxygen. Solutions of fatty acids in xylene accumulate rather high concentrations of peroxides when permitted to stand for several days. When a fatty acid solution in xylene which had been standing at room temperature for several days was permitted to react with lead in the absence of oxygen, the loss in weight of the test piece was approximately equivalent to the amount of peroxide which had accumulated (0.01 equivalent per liter). In all tests involving xylene, it was necessary t o use newly prepared solutions made from xylene freshly distilled from sodium in order t o eliminate peroxides. E F F EOF ~ WATER. It WBB further observed that no appreciable reaction took place at 100" C. in the absence of oxygen when the test solution of laukic acid in xylene was saturated with water. Results of this test did not preclude the possibility that the rapid reaction which occurred when oxygen waa not excluded was catalyzed by the presence of small amounts of water in the medium. To test this possibility, the amount of reaction of lead with acid solutions in xylene saturated with water was compared withhhe amount of reaction with dry solutions a t 80' and 100" C. at a definite oxygen pressure. Results at 100" are shown in Table I. At 80' under an oxygen pressure of 267 mm. in an interval of 20
minutes, five tests using dry xylene as the solvent gave a mean lead loss of 2.8 * 0.2 X lo-* milliequivalent per sq. cm. Three tests made in water-saturated xylene gave a mean loss of 2.1 * 0.2 x IO-* milliequivalent per sq. cm. Thus, water appears t o retard the action of the fatty acids on the metal. To verify further the fact that the presence of water wm not required to produce corrosion, a test was run in which butyric anhydride was added to a 0.1 N solution of lauric acid in xylene. An amount of butyric anhydride was added equivalent to the sum of the traces of water present in the original solution and the water which would be formed by complete reaction of the lRuric acid according to the following equation: ~
PbO
+ 2CiiHnaCOOH
+
H20
+ Pb(CiLHz&OO)o
After 2 hours at 86" C. under an oxygen pressure of 400 mm. the loss in weight of the lead test piece was nearly (85%) equivalent to the sum of the lauric acid and butyric anhydride. Thus, it can be demonstrated that the presence of water is a negligible factor in the corrosion of metals by solutions of fatty acids in hydrocarbon solvents. ACTION OF OXYQEN AND PEROXIDES I N ABSENCE OF ACIDS. When lead is placed in an acid-free medium of white oil or xylene containing added peroxide or dissolved oxygen a t 80" C., the test piece becomes coated with oxide. However, the amount of lead oxidized under these conditions is negligible in comparison with the amount of lead lost when acid is present. Furthermore, when a lead piece oxidized by heating in air for a long period a t 200-300" C. was placed in an acid solution in xylene at 80" C. containing no peroxide or dissolved oxygen, the oxide coating readily dissolved. This supports Denison's conclusion that the metai oxide is actually an intermediate in the corrosion process. RELATION BETWEEN OXYQBN ABSORBED h ~ Loss ) IN WEIQHT OF TEST PIECE.With the metal oxide as an intermediate in the corrosion reaction, assuming no other oxygen-consuming reactions, the amount of oxygen consumed during the test period should be equivalent to the metal removed from the test piece. In many cases the oxygen absorbed and the metal lost were found t o be equivalent. Hornever, in many instances, particularly in xylene solutions a t the higher temperatures or under relatively high preesures of oxygen (300-400 mm.), the equivalents of oxygen absorbed far exceeded the equivalents of metal lost. This was traced t o the fact that xylene will absorb oxygen in considerable amounts a t 100" C. or above even in the absence of catalyst. In the presence of the lead or cadmium salts of fatty acids, much greater amounts of oxygen are absorbed. Thus in one experiment a t 100" C. a solution of 0.1 N lauric acid In zylene absorbed 12% of the oxygen present initially; a 0.08 M solution of lead stearate absorbed 77% of the initial oxygen over the same period (6hours) at the same initial pressure of oxygen (400 mm.). Similar results were obtained with unoxidized white oil as the solvent although the amount of oxygen absorbed by white oil solutions is considerably less than that absorbed by xylene solutions under the test aonditions employed. Although dylene solutions of fatty acids accumulate more peroxides than does pure xylene when stored a t room tempersr ture and atmospheric pressure, the presence of fatty acids seems to have little effect on the amount of oxygen absorbed during a
92
test. Thus, at comparatively low oxygen pressures and a t low temperature (SO-SO0 C.) lead is probably oxidized in preference to the xylene when acid is present; however, to obtain unequivocal proof that oxygen is absorbed as a result of the corrosion reaction, it was necessary to resort to solvents which would not oxidize to any appreciablc extent under the test conditions. It waa found that solutions of lead stearate and lauric acid in h e p tane, diphenyl, or diphenyl oxide did not absorb oxygen in any appreciable amount a t 85' C. over a period of several hours.
s+?.Or FIGURE I
EFFECT OF OXYGEN PRESSURE ON CORROSION OF LEAD BY 0.1N LAURIC ACID IN WHITE OIL AT 100' C.
-0 tm180min.
@*I ts.0
/9
/O
RELATION BETWEEN METALDISSOLVED AND WATERFORMED. Both the oxygen absorption and t.he formation of a normal salt indicate that a normal metal oxide may be intermediate in the corrosion reaction. A further proof would be a determination of the amount of water formed when an inert solvent was used. For this purpose benzene was chosen, and data from tests made a t 70" C. a t several oxygen pressures are presented in Table IV. M E T A L S OTHER THAN LEADAND CADMIUM. Qualitative Observations on tin, aluminum, magnesium, copper, and d v e r indi-
0
9
3
FIGURE 2- CORROSION OF LEAD AS A FUNCTION OF TIME USING 0.1N LAURIC ACID IN WHITE -~ OIL AT IOO'C. AT CONSTANT OXYGEld PRESSURES
P 6.0
t=gOmin.
a 5.0c
n
rf
"!
t 30 min.
w
0 P=200 mm.
d cn
3z
5 2.0
0
2 .o
v)
I .o
0
a a I
I
I
200
300
400
-
OXYOEN PRESSURE [Mn.of Hg]
8.0 FIGURE 3 - EFFECT OF OXYGEN
'c
3
/
0
t = 60 min.
100
/0p=400mm
\
3
E
Vol. 37, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
I 500 180 min.
t
PRESSURE ON CORROSION OF 7.0 LEAD BY 0.25 N LAURIC ACID IN WHITE OIL AT 100.C.
8
-8
50
FIGURE 4
X
6.0
0t
I
9
105 min.
HY DROPLROXIDL
I
100
I
I
I
200
300
400
OXYGEN PRESSURE
- [Mmo f Hg]
1
200
TIME IN MINUTES
- EFFECT OF OXYGEN PRESSURE ON CORROSlEN OF LEAD BY 0.04 N LAURIC ACID IN WHITE OIL AT IOO'C.
p"c
9 t = 180 min.
2 5.0
O ," 0
I SO
IO0
g 2.0 0
I
-
---
t = 30 min.
I
500
Solutions of lauric acid in each of these solvents were permitted to react with lead a t 85' C.for 2 hours, and the oxygen absorbed and lead weight loss were determined. The results summarized in Table I1 show that there is rough agreement between the equivalents of lead lost and the amount of oxygen absorbed in all three solvents. I n addition to showing that the metal oxide is intermediate in the reaction, these results indicate that the reaction proceeds in a solvent which resists oxidation a t a rate comparable to that in a solvent which is readily oxidized. Although the lead test pieces always exhibited a metallic luster when removed from test solutions containing acid, in some instances a considerable amount of a black substance was removed in the cleaning operation. This may have been an accumulation of the black suboxide of lead. FORMATION OF A NORMAL SALT. In a number of experiments the lead salt formed by the action of lauric acid in xylene on lead was separated, dried, and weighed. The amount of lead in the salt was calculated on the assumption that the normal salt, Pb(CIIHuOa)*,was formed. Weights of lead thus calculated are compared with the weight losses of the corresponding test pieces as shown in Table 111; the comparisons appear to justify the assumption that the normal lead salt is formed in the reaction.
cate that these metals do not react with lauric acid solutions in white oil or xylene to any marked extent in the temperature range 80-100" C. in the presence of oxygen. This is probably due to the insolubility of the oxides in the test solution. Under the same conditions, however, zinc will react rapidly. CORROSION OF LEAD
Corrosion of lead by lauric acid solutions was studied in the three solvents-white oil, xylene, and benzene. It is of interest to compare rates of corrosion in xylene and white oil because of their marked difference in viscosity. However, since both of these solvents are oxidized with comparative ease, particularly in the presence of lead soaps, pome measurements of corrosion were made in benzene solutions which are comparatively inert to oxidation in the temperature range employed. Variation with Oxggan CORROSION IN WHITEOIL SOLUTIONS, Pressure and Time. Corrosion tests were run at constant time intervals a t 100" C. under oxygen pressures ranging from 50 to 450 mm. The extent of corrosion a t the various test times, expressed a.s milliequivalents of lead per square centimeter, is plotted against pressure of oxygen in Figure 1 for 0.1 N lauric acid. Oxygen pressures were measured a t the beginning and end of the
January, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
reaction, and Figure 1 gives mean values. Actually a t 100' C. the total extent of oxidation of both lead and white oil was never large enough to deplete the oxygen initially available by any more than 15%. It is apparent from Figure 1 that the amourit of lead lost in a given time varies almost linearly with oxygen pressure and that no reaction takes place at aero oxygen pressure. Over the shorter intervals of time, a t which the total reaction is mall, the curves are more nearly linear than over the longer periods of reaction. I n Figure 2 the amount of corrosion is plotted against time at even oxygen pressures of 400, 200, and 100 mm. for 0.1 N lauric acid. The points for Figure 2 were read from Figure 1 a t the indicated pressures. These curves exhibit a slight induction period which might be attributable to a building-up period of metal oxide on the metal. Following the induction period there is a n interval in which the reaction rate is practically linear; finally at the longer times, particularly at the higher oxygen pressures, there is a marked decrease in the rate. Corrosion by tert-Butyl Hydroperozicle and Lauric A d . . Figure 2 also shows the corrosion loss of lead in 0.1 N lauric acid solution which was initally 0.036N with respect' to tertbutyl hydroperoxide. I n these and subsequent tests with added peroxide, atmospheric oxygen was removed by evacuation at room temperature. This solution exhibits a reactivity in the initial stages which would be shown by lauric acid and molecular oxygen a t a partial pressure of 800-900 mm. From the limited data available, the solubility of oxygen in white oil was estimated to be of the order of 0.02 equivalent per liter a t 900 mm. oxygen pressure, which is somewhat lower than the concentration of hydroperoxide used. Thus, the high reactivity of the hydroperoxide solution appears to be due primarily to its high concentration of active oxygen. Effect of Acid Concentration. T o determine the effect of acid concentration upon the corrosion of lead in white oil, the tests were repeated with 0.04 and 0.25 N lauric acid solutions. Dependence of the reaction rate upon oxygen pressure was similar to that in the case of the 0.1 N acid (Figures 3 and 4). The extent of reaction of lead with the different concentrations of acids as a function of time is shown in Figure 5 at the even pressures of 400, 200,and 100 mm. It is noteworthy that the extent of corrosion of
TABLE 11. COMPARISON OF OXYGENABSORPTION WITH LEAD Loss IN INERT SOLVENTS (Teata run at 85' C. and initial oxygen pressure of 400 mm.; solutions 0.1 N with respeot to lauric acid) Lead Loas Ox gen Absorbed, Solvent ,\lilheqmvalcLts dlliequivalenta Heptane 3.9 3.4 Diphenyl oxide 2.4 2.1 3.4 2.0 Diphenyl
TABLE111. COMPARISON OB LEAD Loss FORMATION
AND
NORMALSALT
(Teat8 run for 6 hours on 0.1 N aolution of lauric acid in xylene) Tyz., Or Pressure Milliequivalenta of Lead Mm. of H i Loot Salt Formed 0.70 0.60 100 129 1.70 1.50 323 80 1.87 1.72 80 416 1.49 1.66 80' 0
lead may be represented by a single curve a t any given oxygen pressure over a period of 1-1.5 hours, irrespective of the initial lauric acid concentration over the range 0.04 to 0.25. At lower pressures the independence of the rate with respect to acid concentration extends to longer periods, and for an oxygen pressure of 100 mrri. the rate is independent of acid concentration over a period of at least 3 hours. It is apparent that at the higher oxygen pressures the rate is independent of acid concentration for a t least 3 hours when the concentration is 0.1 N or greater. From these results it is concluded that, although corrosion of lead will not take place in white oil to any appreciable extent in the absence of acid, the rate of corrosion is independent of the acid concentration within wide limits. I n the case of lauric acid, this range is from about 0.02 N to a t least 0.25 N, and the corrosion rate is determined mainly by the concentration of oxygen or peroxide in solution. The conclusion that the corrosion rate is large!y independent of acid concentration is in agreement with Denison's results. To test further the dependence of rate on acid concentration, the corrosion of lead in 0.04 N and 0.10 N lauric acid solutions containing 0.036 equivalent of tertbutyl hydroperoxide per liter waa compared. Figure 6 shows that the reaction rates a t the two concentrations of acid are nearly identical over the first half-hour period. A slight dropping off in rate is observed after the acid concentration has dropped to about 0.02 N. This further substantiates the conclusion that the rate of corrosion is nearly independent of acid concentration. DEPENDENCE OF CORROSION RATEON AREA AND AQITATION. Measurements using test pieces of different area showed that the amount of corrosion waa directly proportional to area in the early stages of the reaction. This, coupled with the fact that the rate depends upon the extent of agitation, indicates that a diffusion process partly determines the reaction rate. LIMITINGRATESOF CORROSION.Initial or limiting slopes (dz/dt) of the curves in Figure 5 were measured a t different pressures, and their dependence upon oxygen pressure was found. These slopes are summarized for an oxygen pressure of one atmosphere in Table V; they are dependent upon the extent of agitation of the test solution but are virtually independent of acid strength. Inhibitim of Corrosion by Lead Soaps. Addition of lead soaps to the lauric acid solutions before the test greatly diminished the extent of corrosion. Thus, a 0.1 N lauric acid solution in white oil, containing initially a lead stearate concentration of 0.15 mole per liter, caused a lead loss of 6 mg. under conditions which caused a 40-mg. loss in the absence of lead stearate. This effect depends markedly upon the concentration of the lead soap, and a low concentration (0.002 mole per liter of lead stearate) had no appreciable effect upon the amount of corrosion. From these results it appears that the decrease in corrosion rates at the longer periods observed in Figures 2 and 5 is due to inhibition of the reaction as a result of the accumulation of lead laurate in the solution. Esect of Temperature on Corrosion Rate. Figure 7 shows the extent of corrosion of lead in a 0.1 N solution of lauric acid in white oil a t 125' C. as a function of pressure a t various time inter-
Test piece precoated with PbO.
TABLE IV. COMPARISON OF LEADLoss AND WATERFORMATION Tim$. 70 70 70 70
(Tests run on 0.1 N solution of lauric acid in bensene) Time Os Pressure Milliequivalents Min: M m . of Hi Pb lost H i 0 Formed 1b 216 0.47 0.38 90 216 1.33 1.27 lb 400 0.68 0.80 00 400 1.07 1.42
93
TABLE V. SUMMARY OF LIMITINGRATECONSTANTS FOR CORROSION OF LEAD [do/&
-
Solvent Benaene Bensene Xylene Benzene Xylene Xylene Xylene Xylene White oil White oil
-
kP (where P oxygen reasure in atmospheres) and k = milliequivalenta lea$/sq. om.-hr.-atm.] Try., Acid k X 10' Laurio 45 0.7 00 18.6 Laurio 70 20 _. Lauric 66 12 Laurio 80 29 Lauric 100 50 Laurio 80 27 Steario 80 21 Phenol 100 6.0 Laurio 126 8.0 Lauric
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
94
FlQURE 7
FIGURE 5 OF ACID CONCENTRATION UPON CORROSION BY LAURIC ACID SOLUTIONS IN WHITE OIL AT CONSTANT OXYQEN PRESSURES cI
t
3.0-
-
Vol. 37, No. 1
EFFECT OF OXYGEN PRESSURE ON CORROSION OF LEAD BY 0.1 N LAURIC ACID IN WHITE OIL AT 125.C.
0
2.0 -
t = 180 min.
0
7.0
100
A'-
4.01
200
300
-
400
500
OXYGEN PRESSURE [Mm.of Hgd u)
0
0 P 200 mm. 0
0
2.o
X
5a
/
m n
YCY,
-A-
-0-
I."-
0.25 N 0.10 N
50
6-
r
I
I
I
I
8 -
FIGURE 9
$'"-
-
100
150
200
TIME IN MINUTES EFFECT OF TEMPERATURE ON CORROSION OF LEAD BY 0.1 N LAURIC ACID IN WHITE OIL UNDER AN OXYQEN PRESSURE 0t OF 400 MU.
l0O0C.
-I
-
A Gi
0
Q
5.0r
8 . 4.0 1..
/
0.10N ACID
P A 0.04N / ACID
-
FIGURE 6 EFFECT OF ACID CONCENTRATION ON CORROSION OF LEAD BY LAURIC ACID AND 0.036 N t - B U T Y L HYDROPEROXIDE IN WHITE OIL AT 100'C.
2.o I.o
50
IO0
150
TIME IN MINUTES
vale. Figure 8 is a cross plot of the same data, showing the dependence of corrosion upon time a t various even prewures. Although the dependence of corrosion upon oxygen pressure is similar to the type of dependence shown e t lW', the dependence upon time is quite different. This is shown plainly in Figure 9; while the initial rate of corrosion is greater at 125' than a t 100' C., i t falls off rapidly to an approximately constant value after 30 minutes which is much less than the rate at 100' at corresponding times. Thus, over a period of 1 to 3 hours, the total corrosion at 125' is much less than a t 100' C . At 140' the total corrosion after 30-90 minutes is still less than a t 125'. It is probable that the initial rate is greater at 140" than at 125' C., but initial readings cannot be taken a t 140" with sufficient accuracy to establish this definitely. At SO0 C. the mechanism of the reaction was complicated by the formation on the surface of the test piece of a film of lead laurate, virtually insoluble in white oil a t this temperature. The limiting rate was not measured but the total corrosion over a onehour period was only half as large as at 100' C. over the same period. At 100' or above, no lead soap film was encountered, CORROSION IN XYLENE SOLUTIONS. Variation with Pressure
s
t4.0
0 t :125OC.
8 2.0 Gi p 1.0 5_ 0 _
1
100 ~-
I50
200
TIME IN MINUTES
and Time. Corrosion of lead by 0.1 N lauric acid in xylene was measured at 55', 80', and 100' C. in a manner similar to that described for white oil. Corrosion took place with much greater rapidityin xylene than inwhite oil, but the shapes of the corrosion curves were similar for the two solvents. Efect of Temperature. Figure 10 shows the total corrosion at 55' C. at various oxygen pressures as a function of time; for comparison the total corrosion which resulted wheB 0.036 N terkbutyl hydroperoxide was used as oxidizing agent in place of molecular oxygen. At 55' a solid film of lead laurate is present on the test piece from the earliest stages of the reaction a t all pressures, and this apparently causes the marked falling off in reaction rate with increasing time. For oxygen pressures up to 200 mm. the reaction rate varies linearly with pressure; beyond 200 mm. the rate appears to be practically independent of pressure. At 80" and, 100" C . the reaction mechanism is no longer complicated by the presence of a solid film on the surface of the test piece. The amount of corrosion at 80' is shown at four pressures in Figure 11. I n the case of the nonvolatile white oil, the extent of reaction was entirely independent of total pressure at a con-
INDUSTRIAL AND ENGINEERINQ CHEMISTRY
January, 1945 FIGURE K)
- CORROSION OF LEAD AS A FUNCTION OF TIYE USING 0.1 N LAURIC ACID IN XYLENE AT SS'C.AND
FIGURE I3
-
CONSTANT OXYGEN PRESSURES
6.0r
95 EFFECT OF ADDED PEROXIOES ON CORROSION OF LEAD BY 0.1 N LAURIC ACID IN XYLENE
KEY2
5.0
8- 0.036 N
5.0
LAUROYL PEROXIDE
0-0.036N t-BUTYL HYDROPEROXIDE
4 .O
55OC.
3.O
0 P =100mm.
2 .o
55oc.
I .o loo
FIGURE II 0 0
I
200
300
TIME IN MINUTES
400
- TIME CORROSION OF LEAD AS A FUNCTION OF USING 0.1 N LAURIC ACID IN XYLENE
0
I50 - _
TIME IN MINUTES
FICURE 14
AT 8O'C.AND CONSTANT OXYGEN PRESSURES
-I
100
SO
";
-
CORROSION OF LEAD AS A FUNCTION OF TIME USlNC 0.1 N STEARIC ACID IN XYLENE AT SO'C. AND CONSTANT OXYOEN PRESSURES
a
n
0
10.0
P = 215 mm.
I
cn
3
U'
---o
'-:
2
0
ia
P = 125 mm.
-0
B0: 0
P= 65 mm.
0
J
100
TIME IN MINUTES
50
100
150
1
200
TIME IN MINUTES
0 P=335mm.
6 .O
,o[yO
P
215 mm.
4 5 .O -01
~0
25 __
FIGURE 12- CORROSION OF LEAD AS A FUNCTION OF TIME USING 0.1 N LAURIC ACID IN XYLENE AT 100'C.AND CONSTANT
50
7s
TIME IN MINUTES
stant pressure of oxygen. This was not true for xylene and the other volatile solvents used unless the total pressure was higher than the vapor pressure of the solvent. Apparently at pressures below the vapor pressure of the solvent, oxygen was swept out of solution by refluxing of the solvent. I n all measurements involving xylene and benzene, sufficient nitrogen was introduced into the oxygen to make the total pressure substantially greater than the vapor pressure of the solvent. Results obtained a t 100" are shown in Figure 12. Here, as in white oil at 125" C., the initial reaction rate is hi$h and then falls off rapidly with increasing time. Thus, although the initial reaction rate is higher at 100" than at SO' C., the total corrosion over a one-hour period will be much less than a t 80" because of the more rapid decrease in rate with time. The marked decrease in corrosion rate with increasing time observed in xylene a t 100"and in white oil at 125' may, perhaps, be attributed to the consumption in oxidation of the solvent of part of the oxygen ordinarily available at the surface for oxidation of
1
-
FIGURE I5 OF LEAD AS A FUNCTION
W TIME USING 0.1 N PHENOL IN XYLENE AT 80%. AND CONSTANT OXYGEN PRESSURES
1 7
V
I
50
I
100
I 150
TIME IN MINUTES
the lead. At 100' C. the oxidation of xylene proceeds rapidly in the presence of lead soaps, and the amount of oxygen absorbed by the system when the metal is present is considerably larger over the longer time intervals than can be accounted for by oxidation of the metal alone. At 80", however, there is usually much closer correspondence between the amounts of oxygen absorbed and the amount of lead corroded in xylene solutions (Table VI). From these typical data it appears that at SO" the lead is, in general, oxidized in preference to the xylene; at 100' the xylene is oxidized in preference to the lead. The same considerations apply in explaining the rapid dropping off of the corrosion rate in white oil a t 125" C. At this temperature the e q u i d e n t s of oxygen absorbed were larger over the longer time intervals than the equivalents of lead corroded; it may be expected that the concentration of oxygen at the lead surface will be much Iower as a result of the more rapid side reaction with the solvent which is catalyzed by lead soaps formed in the process of corrosion.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
96
FIGURE 16CORROSION OF LEAD AS A FUNCTION OF TIME USlNG 0.1 N LAURIC ACID IN BENZENE AT 70.C. AND CONSTANT OXYGEN PRESSURES
8 - FIGURE 17 - EFFECT Of
A 7.0-
5
TEMPERATURE ON CORROSION OF L E I D BY 0.1N LAURIC ACID IN BENZENE UNDER OXYGEN PRESSURE OF 2 7 0 MM.
$6.0n
\
4.0
-
a I .O
a
0
TIME IN MINUTES
100
Vol. 37, No. 1
25
Effect of Peroxides on Corrosion Rates. The corrosivity of oils, which may be due to dissolved atmospheric oxygen or peroxides acting in conjunction with organic acids, will probably be markedly affected, as Denison suggests, by the reactivity of any peroxides present in the oil. T o test the effect of reaction of lead with 0.1 N lauric acid solutions in xylene containing comparable concentrations of peroxides exhibiting two different types of linkage corrosion was measured at 55" and 80' C. a t various times. tertButyl hydroperoxide and lauroyl peroxide were chosen (Figure 13). These curves show that the corrosion rate in terl-butyl hydroperoxide is much greater than that in lauroyl peroxide at either temperature. At 80' the limiting rate for 0.036 N tertand for 0.036 N laubutyl hydroperoxide is roughly 50 X royl peroxide, about 12 X lo-* milliequivalent per sq. om.-hr. Dissolved oxygen would exhibit the same reactivity under oxygen pressures of ahnut 1200 and 300 mm., respectively. The corrosion rates of the two peroxides in benzene a t 60" C. indicated about the same ratio as was observed in xylene. Corrosion by Stearic Acid and Phenol. Although the rate of reaction is largely independent of lauric acid concentration, this does not rule out the possibility that specific effects due to the acid may be observed when acids other than lauric are used. Limiting rates were measured for the reaction of 0.1 N stearic acid in xylene on lead a t 80" C. The results are plotted in Figure 14, and the limiting rate of reaction (Table V) is little different from that found for lauric acid a t 80". Measurements were also made using 0.1 N phenol in xylene a t 80" (Figure 15); an acid such as phenol, which is much weaker than the carboxylic acids, nevertheless gives a limiting rate in xylene which is of the same order of magnitude as that obtained for the carboxylic acids. CORROSION IN BEKZENE. Effect of Oxygen Pressure and Time. I t is probable that the limiting rates of reaction found in xylene and white oil are independent of the oxidation of the solvent, particularly at lower temperatures. However, i t was of interest to examine the reaction mechanism in a solvent which would not oxidize to any appreciable extent under the conditions of the reaction. Benzene Containing lauric acid and large concentrations of lead soap did not absorb oxygen in any detectable amounts a t temperatures up to 85" over 6-hour periods. Figure 16 shows the extent of corrosion of lead hy 0.1 N lauric acid in benzene under various conditions a t 70" C. From Table V it is apparent that the limiting rate of corrosion of lead in lauric acid solutions in benzene is the same order of magnitude as for lauric acid solutions in xylene. This indicates that, for a given oxidizing agent, the corrosion mechanism in xylene and white oil which are oxidizable under the reaction conditions is similar to the corrosion mechanism in benzene which is not oxidizable under these conditions. Effect of Temperature. Corrosion rates of lead in 0.1 N lauric acid solutions in benzene under an oxygen pressure of 270 mm. were measured a t 60" and 45" C. Figure 17 shows these results
compared with those at 70" a t a corresponding pressure. At both of the lower temperaturea solid films of lead laurate were present on the test pieces from the earliest stages of the reaction. The shape of the curve a t 60" shows that the presence of the solid film on the test piece operates to cut down the reaction rate I I I as time increases; at 50 75 too 45' no such effect is TIME IN MINUTES observed and the rate is constant. Thus, at 60" the reaction rate is apparently determined primarily by the rate of diffusion of reactant through the solid film on the sixface; at 45" the rate is determined b y the rate of the chemical reaction on the surface of the test piece, which is nearly independent of film thickness. At 30" the amount of corrosion is not appreciable over 6-hour periods. Thus, at temperatures around 40" the rate of chemical attack of lead becomes appreciable; between 45" and 60" the rate of diffusion of reactant into the surface partly controls the rate of reaction. CORROSION OF CADMIUM
VARIATIONS WITH OXYGEN PRESSURE AND TIME IN WHITEOIL. Measurements on the corrosion of cadmium in 0.1 N lauric acid solutions in white oil were made only at 100" C. (Figures 18 and 19). An unusual feature of these results is that the rate and extent of corrosion of cadmium passes through a pronounced maximum at an oxygen pressure of about 250 mm. This rather unexpected result was also observed at about the same oxygen pressure using 0.1 N lauric acid in xylene a t 80". I t was fully confirmed by four independent determinations, made at different times and with different test pieces at each of two oxygen pressures, 200 mm. (in the vicinity of the maximum) and 400 mm. (well beyond the maximum). Although the rate of corrosion falls off a t high oxygen pressures, the rate using 0.036 N tert-butyl hydroperoxide having a higher concentration of available oxygen is larger than the maximum rate obtainable with oxygen itself (Figure 19). The interpretation of the results on cadmium is probably closely connected with the composition and physical character of the intermediate oxide film which is formed in the reaction. Thus the falling off in corrosion rate at high pressure might be due to the accumulation of a n oxide film which reacts less rapidly than the film formed a t lower pressures. The acceleration in corrosion rate with increasing time cannot be attributed to the accumulation of peroxides since analysis showed that no detectable concentration of peroxide was present during any stage of the reaction. It is possible that the initial reaction rate is slowed, due to an in-
TABLE VI. Temp.. O
c.
100
COMPARISON OF OXYGEN ABSORPTION WITH Loss AT 80' A N D 100"C.
(Lauric acid solutions in xylene) 01 Absorbed, Time, Min. h-lilliequivalents
15 48
90
LEAD
Lead Loss, Milllequivalents
0.33 0.67 1.04 1.04 1.90
0.40 0.80
0.96 1.60 2.2
0.55
1.40 1.69 1.85
0.72 0.98
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1945
-
FlQURf 90
FIGURE 18 EFFECT OF OXYQEN PRESSURE ON CORROSION OF CADMIUM BY 0.1 N LAURIC ACID IN WHITE
-8 4 .O
5.0,-
-
97
COMPARISON OF THE CORROSION OF LEAD AND CADMIUM BY 0.1 N LAURIC ACID IN WHITE OIL UNDER AN OXYGEN PRESSURE OF 200 MM. AT IOO*C.
n
' 0t = 270 mln. t = 150min. f 1.0
3
200
100
300
OXYGEN PRESSURE
i
0
8
400
500
6.0-
FIGURE 19 CORROSION OF CADMIUM AS A FUNCTION OF TIME USING 0.1 N LAURIC ACID IN WHITE OIL AT IOO°C.AND CONSTANT OXYQLN PRESSURES
0 P=250 mm.
5.0 -
TIME IN MINUTES
(Mmof Hg]
-
3
P
-
FlQURE PI
2 IS*Or
- CORROSI6N OF CADMIUM AS A FUNCTION OF TIME USiNO 0.1 N LAURIC ACID IN XYLENE AT
L.ot w6
4.0 P =400 mm. P = 100mm.
3.O
'-
, 100
200
0.036 N t-BUTYL HYDRqPEROXlM
300
A
3z
0 t * lOO*C.,P= 416 All).
5.0
2 u)
8 e 8
TIME IN MINUTES
about one tenth as great m in xylene when the comparison is made at 100" C. These results, in conjunction with the evidence that the limiting rates are nearly independent of acid strength and concentration within wide limits, suggest that the corrosion rate is controlled by the rate of diffusion of molecular oxygen or an oxygen carrier into the surface of the metal and by the chemical rate of oxidation of the metal. Wben lead is corroded b y molecular oxygen in conjunction with fatty acids, the possibility exists that the lead is oxidized b y a peroxide intermediate which could be formed by the reaction of oxygen with the acid or solvent. Analysis for peroxides during the course of the corrosion showed that no appreciable amounts of peroxide accumulated except in white oil at 125' and 140" C. Furthermore, when the pure solvent or test solution was subjected to the action of oxygen in the absence of metal, the accumulation of peroxides was much smaller than the amount of lead corrosion over the same period (Table VII). However, the possibility that a highly reactive peroxide may be formed and discharged so rapidly that it escapes detection must be examined.
TABLE VII. ACCUMULATION OF PEROXIDES I N TESTSOLUTIONS OF XYLENE AND WHITEOIL
NATURE OF OXYGEN CARRIER IN CORROSION f
Examination of the limiting rates of corrosion of lead by fatty acids and molecular oxygen summarized in Table V indicates that all may be closely represented by an equation of the type:
kP
t * 100*C.,P- 215 m i .
t 88*C, 0.036 N t-BUTYL HYDROPEROXIDE
duction period required for nucleation of the intermediate oxide. Cadmium test pieces exhibited badly pitted surfaces when the extent of corrosion was high. COMPARISON OF CORROSION RATESOF LEADAND CADMIUM. Figure 20 compares the extent of corrosion of cadmium and lead in a 0.1 N lauric acid solution in white oil under a n oxygen pressure of 200 mm. The initial rate of corrosion is much larger for lead than for cadmium but becomes less after 2.5 hours. EFFECT OF TEMPERATURE ON CORROSION RAW OF CADMIUM. Measurements on the corrosion of cadmium at lower temperatures were made in 0.1 N lauric acid solutions in xylene and benzene. In no w e was the corrosion appreciable over a &hour period at temperatures below 80" C. Results obtained in xylene above 80" are shown graphically in Figure 21. The curves are similar in shape t o those for cadmium in white oil. The reaction rate is greatly accelerated with increasing temperature; the much greater rate at an oxygen pressure of 215 mm., compared with that at the higher pressure of 415 mm., is striking. A curve showing the extent of corrosion when 0.036N tert-butyl hydroperoxide is used in place of oxygen is included in Figure 21 for comparison.
dxldt =
p
r\
u)
TIME IN MINUTES
L
--"
CONSTANT OXYQEN PRESSURES
'b' I
(1)
Comparison of the limiting rates for lauric acid in the three solvents show that in xylene and benzene, whose viscosities differ little, they are nearly the same; in white oil, whose visaosity is about eighteen times that of xylene, the corrosion rate is only
(No metal present; solutions 0 , l N with respect to lauric acid) Expected T f m z , Ot Pressure Time, Peroxides Lead Loss Solution Mm. of Hd Min. Milliequi;. Milliequiv: Xylenea 0.15 X lens 0.0 d i t e oil Xylene *.. 0.0 Xylene X lene hite oil0 k h i t e oil a Pure solvents containing no acid.
d
0.0 0.18 0.47 0.0
0.30
1.0
2.2
... ...
2.1
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
98
I n view of the fact that the corrosion rate is largely independent of acid concentration, such a peroxide could hardly be a peracid in solution. This does not exclude the possibility that adsorbed peracid on the metal test piece may be the oxidizing agent. Thus, if the fatty acid were strongly adsorbed, its concentration and that of the peracid on the metal surface might be nearly independent of the acid concentration in solution, and the reaction rate might be controlled by the rate a t which oxygen diffused into the surface layer to form peracid.
0-2.5
where C: = concentration of acid at surface of test piece concentration of acid in solution CS diffusion coefficient of acid in solvent Dt depth of diffusion layer y,
--
5
REACTION OF ACID AND METALOXIDE PIECE TO FORMMETALSALT. dxldt =
(MO)(Cd
TEST (5)
-,q
BENZENE
From the experimental evidence, steps represented by Equations 4 and 5 may be assumed to be very rapid for the reactions under consideration and will not be rate controlling. If this is the case, the concentration of “active metal centers” will be essentially constant and Equation 3 becomes:
I n a steady state the rate of oxide formation given by Equation 6 should equal the rate of salt formation given by 5, and &/dt
3.0 I/T
kq
AT SURFACE OF
where z = concentration of metal salt kn = chemical rate constant for reaotion of metal oxide with acid
-
FIGURE 22 RELATION BETWEEN LIMITING RATE CONSTANTS AND TEMPERATURE FOR THE CORROSiON OF LEAD ey 0.1 N
Vol. 37, No. 1
x 103
The similarity in magnitude and oxygen preasure dependence of the corrosion of lead in xylene and benzene solutions indicates that an intermediate peroxide formed by reaction between oxygen and the solvent is unlikely. As further evidence that a transitory peroxide in solution is not the oxygen carrier a t low temperatures, the lack of any appreciable induction period and the comparatively rapid rate of corrosion of lead at temperatures as low as 45” C. may be cited. There is no reason to believe that lead will not oxidize rapidly at the temperatures employed‘if some reagent is present to remove continually the protective oxide layer which is ordinarily formed. KINETIC FORMULATION AND PROPOSED CORROSION MECHANISM
Leaving out of consideration adsorption ‘and desorption processes on the surface of the test piece, and assuming that molecular oxygen itself is the oxidizing agent, the corrosion process may be visualized as taking place in the following steps at speeds given by the accompanying rate expressions:
DIFFUSION OF OXYGENINTO SURFACE OF TESTPIECE FROM
BODYOF SOLUTION.
where C,O = concentration of oxygen in solution CI = concentration of oxygen at surface of test piece Dl = diffusion coefficient of oxygen in solvent yl = depth of diffusion layer
P
kl CI
(7)
Also, in a steady state the rate of diffusion of oxygen into the surface must equal its rate of reaction at the surface; equating 2 and 6,it follows that:
Since by Henry’s law, C:
where P
kr
a
Plki
= partial rewure of oxygen 5
Henry E w constant
then the limiting rate may be represented as a function of oxygen pressure as follows:
Thus the assumption that the corrosion rate is controlled by the rate of diffusion of oxygen into the metal surface and by the chemical rate of oxidation of the metal leads to the functional form of the rate expression actually observed (Table V). When the diffusion of oxygen is very slow in comparison with the chemical oxidation, klyl>>D1 and Equation 10 become:
When the diffusion of oxygen is very rapid relative to the rate of chemical oxidation, Dl>>kly,, and
REACTION OF OXYGENWITH METALAT SURFACE.
‘9 k: (CI) ( M ) =
where (MO) = concentration of metal oxide on surface ( M ) = concentration of “active metal centers” k; = chedical rate constant for oxidation
DIFFUSION OF ACIDINTO SURFACE OF TEST PIECE.
dz=k_!p dt
ks
Thus a similar functional dependence upon oxygen pressure, but with different constants, is obtained whether the corrosion rate is controlled mainly by the rate of diffusion of oxygen into the metal or by the rate of oxidation of the metal. The fact that the corrosion rate varies with the degree of agitation of the solution shows that a diffusion process is important in controlling the rate but gives no indication of the relative importance of chemical oxidation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1945
To evaluate the role of the rate of oxidation of the metal in determining the rate, it is of interest to examine the temperature coefficients of the limiting rate constants. Since the solubility of oxygen in liquids does not change markedly with temperature in the range under consideration as a f i s t approximation, the change in the Henry law constant with temperature may be neglected. Both the diffusion coefficient D, in Equation 2 and the chemical rate constant kl in Equation 6 should vary exponentially with temperature. Actually it was observed that the three experimental rate constants obtained for xylene and benzene (Figure 22) followed closely an equation of the expected type: logto k
- --
2.3gRT 4-
The constants E (energy of activation) and C which satisfy Equation 13 for the three solvents are summarized as follows: Solvent Beneene X lane hite oil
wy
E , Calories 9800 7900 3600
C 5.65 1.85 0.79
For diffusion processes in liquids the energies of activation to be expected are of the order of 3000-5000 calories (J), and the energy of activation found in white oil is of this order of magnitude. For xylene and benzene the observed activation energies are considerably higher than would be expected for a diffusion process, and it is probable th,at the chemical rate of oxidation is partially rate determining in the corrosion of lead by lauric acid contained in these solvents. Because of the high viscosity of white oil relative to xylene, the rate of diffusion of oxygen will be much slower in white oil than in xylene, and the diffusion step is thus more likely to be rate determining in white oil. The fact that cadmium has a lower initial reaction rate than lead may be due to the slow rate of formation of the oxide intermediate or to the slow rate of solution of the oxide in fatty acid. Since the initial rate varies almost linearly with oxygen pressure, it appears that chemical oxidation of the metal may be the controlling factor. INHIBITION BY LEADSOAPS.Since only the limiting rates of corrosion have been dealt with, the effect of salt accumulation upon the rate of corrosion has not been considered. As previously pointed out, lead salts inhibit the rate of corrosion of lead in these solutions. I n addition to promoting oxidation of the solvent (operative only for white oil and xylene), lead salts might decrease the rate of corrosion by (a) adsorption on the surface of the test piece, thus cutting down the concentration of "active metal centers", or (a) increasing the viscosity of the diffusion layer surrounding the test piece. COMPARISON WITH DENISON'S RESULTS
Denison's conclusions that a metal oxide is a necessary intermediate in the corrosion of metals by oils and that the rate of corrosion is largely independent of fatty acid length and concentration as long as acid is present are fully substantiated by the results of this investigation. Denison's results showed that the rate of corrosion of bearing alloys is largely controlled by peroxide concentration; the results of this investigation indicate that in the presence of fatty acid the corrosion rate may still be very rapid, aa a result of molecular oxygen, without any appreciable accumulation of peroxide, particularly at lower temperatures. In this connection, it may be remarked t h a t Denison's measurements were made at approximately 140"C., at which temperature the extent of oxidation of the metal by molecular oxygen was found in this investigation t o be comparatively small. Thus, the oxidation of metal a t these temperatures may be due primarily t o the presence of peroxides which have accumulated in the oil.
99
At lower temperatures, however, particularly around 100" C., it appears that the corrosion of bearing metals in the presence of fatty acids is caused by dissolved molecular oxygen as well as by any accumulated peroxides. Thus, while the formation of fatty acids in oil and the corrosion of metals by oil at high temperatures (above 125") may be catalyzed primarily by peroxide intermediates, 'bold corrosion" by oils containing fatty acids may be effectively catalyzed by dissolved molecular oxygen in the absence of detectable quantities of peroxides. CONCLUSIONS
1. The corrosion of lead or cadmium by fatty acids does not take place at an appreciable rate in nonpolar media, such as white oil, benzene, or xylene, in the absence of molecular oxygen and peroxides. 2. When lead is corroded by fatty acids in solvents which themselves do not rapidly oxidze in the presence of oxy en or peroxides, an amount of oxy en is used up which is equivaknt t o the amount of metal corrode8 3. I n the absence of fatty &ids, molecular oxygen or peroxides attack lead very slowly as a result of the formation of a protective oxide 6lm. When fatty acida are present, the oxide film doas not accumulate and the metal corrodes rapidly. 4. Corrosion of lead by fatty acids in nonpolar media is not greatly influenced by the presence or absence of water. When oxygen and p d e s are absent, no appreciable corrosion takes place when t e media are water-saturated, and in the presence of oxygen the rate of corrosion is somewhat leas for water-saturated than for dry media. 5. When the effect of molecular oxygen, tert-butyl hydroperoxide, and lauroyl peroxide u on the rate of corrosion of lead is compared in the presence of g t t y acid, it is observed that their effectiveness decreases in the order named. These differences ma be due in part to different diffusion coefficients and in part to gfferent inherent reactivities. 0. The limiting rate of corrosion of lead by lauric acid in the solvents studied in the presence of oxygen (dzjdt) may be represented within wide limits of acid concentration by an equation of the type:
dx/dt = kP 7. Limiting corrosion rate constants k may be expressed as a function of temperature by an equation of the type:
logzo k
-A/T
+C
8. At, temperatures below 50-60" C. in benzene and xylene, the rate of corrosion of lead by fatty acids in conjunction with molecular oxygen a pears to be controlled principally by the chemical !ate of oxiJation of the lead. Between 50" and 70" C. the chemical reaction is comparatively rapid and the rate 1s largely controlled b diffusion of molecular oxygen through the insoluble film of 1eaJsoap; above 70" the rate is largely controlled by diffusion through a liquid diffusion layer of constant thickness. 9. The kinetic evidence suggests that molecular oxygen itself directly oxidizes the lead when roxides have not accumulated but does not exclude the possibiEy that adsorbed peracid on the metal surface may be the oxy en carrier. 10. Corrosion of lead by f a t t y acids and oxygen takes place a t rates which are of the same order of magnitude in xylene and benzene, two solvents having similar viscosities. In white oil the corrosion rate is about ten times slower than in xylene, which is robably due in part a t least t o the higher viscosity of the white Oil!
11. Chan es in lauric acid concentration above about 0.02 N have little e#ect upon the corrosion rate of lead in white oil at 100" C. From the measurements in xylene at 80" (using stearic acid, lauric acid, and phenol), it appears that acid strength has little influence on the corrosion rate within broad limits. 12. Corrosion of cadmium by fatty acids in conjunction with 'molecular oxygen or peroxides proceeds ra idly only a t temperatures in excers of 80' C. The initial rate orcorrosion of cadmium in white oil at 100' is much leas than that for lead, but in the later sta es of the reaction the rate is higher. 13. temperatufa around 100' or lower it a pears that the corrosion of metals in oils containing fatty acids is &e to dissolved molecular oxygen as well as to peroxides which have accumulated in the oil. 14. At higher temperatures with peroxide-formin solvents containing acids, the corrosion of lead and cadmium &e to dis-
It
INDUSTRIAL AND ENGINEERING
100
solved oxygen is much less important than at lower temperatures, and the corrosion rate under these conditions is probably controlled principally by the concentration of peroxides in the oil as Denison suggested. ACKNOWLEDGMENT
The authors extend their appreciation to the Lubri-Zol Corporation which sponsored this work.
CHEMISTRY
Vol. 37, No. 1
LITERATURE CITED
i;;
:r~~~~e~”~~:,3~~,42783((1~~~
(3) Fox, Analyst, 8, 116 (1883).
(4) Glasstone, Laidler, and Eyring, “Theory of Rate Processes”, p. 401, New York, MoGraw-Hill Pub. Co., 1941. (5) Kokatnur and Jelling, J . Am. Chem. SOC.,63, 1432 (1941). (6) Smith, B ~and Mitchell, ~ ~ bid.,~61, 2407 ~ (1939). , (7) Staeger, Petroleum, 33, No. 7 , 1 (1937).
Useful Life of a Molecularly Dehydrated Phosphate in Sulfuric Acid ROBERT W. ATTEBERRY AND DONALD S. HERR The Resinous Products & Chemical Company, Philadelphia, Pa.
M
ORGEN and Swoope (2) measured the useful life of various molecularly dehydrated phosphates under varying conditions. The useful life of Calgon in 1 N sulfuric acid was not included among these studies, and so was investigated. It was 6rst necessary to set up an analytical method. The five following solutions were made up: SOLUTION I. 270 grams of 95.5% sulfuric acid were poured into 840 ml. of distilled water. After cooling, the volume was adjusted to 1 liter with distilled water. SOLUTION 11. 20 grams of C.P. ammonium molybdate were dissolved in distilled water and diluted to 1 liter with distilled water. SOLUTION 111. 0.5 ml. of concentrated stannous chloride solution (Betz and Betz Code No. 239) was added t o 20.0 ml. of distilled water, made up fresh daily. SOLUTION IV. 1.5073 grams of disodium orthophosphate (IYa2HP04)were dissolved in 1 liter of 1.0265 N sulfuric acid. This solution was found to be 0.0106 3f by the gravimetric method. SOLUTION V. Solution I V was diluted tenfold with 1.0265 N sulfuric acid. To 5.0 ml. of solution V were added 10.0 ml. each of solutions I and 11. Then 2.5 ml. of solution I11 was added, and after standing 30 minutes, n spectral transmission curve was run on the Coleman Universal spectrophotometer model 11. This curve showed a transmission maximum in the range 520-540 mw. [The general analytical method was described by Dunajew ( I ) . The point of maximum transmission was chosen since the spectral absorption TABLE I. TRANSMISSION DENSITIESOF ORTHOPHOSPHATE STANDARDS 1.0265
NO.
N
H*SO,,
M1.
Soh. Type
1.0 n n
v V
Ortho-
phosphate
Volume, 311.
Standard PO4---,
P.P.M.
Trans-
mission Density
4.0
TABLE 11. RATEOF CALGON HYDRATION ~ No.
i Hr.
~ ~ , Density Transmission Standard Calgon
P.P.M. PO* --in Cslgon
Hydration, yo
properties of reduced phosphomolybdic acid are more dependent upon p H than upon orthophosphate ion concentration. A low p H was used to recjxce the color intensity and wave length of maximum transmission was employed to obtain the greatest sensitivity of orthophosphate ion concentration.] The KlettSummerson photoelectric photometer, model 2071, which has a long logarithmic scale calibrated to read in relative terms of transmission density, was therefore abridged with the green No. 54 filter for use in this work. Standards of orthophosphate ion were set up as described in Table I , to which 10.0 ml. each of solutions I and I1 were added, and then 2.5 ml. of solution 111 were added. The transmission density vas measured in exactly 30 minutes after solution I11 was added in the tube model cuvette. Since the transmission density is a linear function of the concentration of orthophosphate ion, this method can be used to check the rate of hydration of Calgon. A 300-mg. portion of Calgon was dissolved in 1 liter of 1.0265 N sulfuric acid, and this solution was placed in a water bath maintained a t 25” * 2’ C. After measured intervals of time, 5.0 ml. of the solution were pipetted into a flask to which 10.0 ml. each of solutions I and I1 were added. Then 2.5 ml. of solution I11 were added, and the transmission density was measured in exactly 30 minutes in the tube model cuvette. Each time such an analysis for orthophosphate was made, a standard similar to No. 8 of Table I was analyzed, and the result served as a reference point for the linear relation between transmission density and the concentration of orthophosphate ion. Table I1 shows the results of the rate of the hydration. The per cent hydration is calculated by multiplying the ratio of orthophosphate ion concentration at the specified time to the final constant concentration of orthophosphate ion by 100. Thus, when per cent hydration is plotted against time, it is found that 50% ‘hydration occurred in 28 hours. Hence, the half life, or useful life in this case, of Calgon in normal sulfuric acid a t 25” C. is 28 hours. The use of sulfuric acid containing relatively small amounts of Calgon has been found more beneficial for the regeneratim of a cation exchange material than sulfuric acid alone. The effect of the metaphosphate even in the presence of acid prevents the deposition of calcium sulfate, etc., upon the exchanger bed, directly improves theefficiency of such a regeneration, and decreases the subsequent rinse water requirements. LITERATURE CITED
(1) Dunajew, A,, 2. anal. Chem., 80,252 (1930). (2) Morgen and Swoope, IND.ENO.CHEM.,35,821 (1943).
