ELECTRODE POTENTIALS Relation to Corrosion of Alloys and Metals in Lubricating Oils FRANK HOVORKA Western Reserve University,
trodes in contact with oil. Each metallic part has a definite potential against its surroundings, which ordinarily is oil. This potential may be large or it may be so small that i t is immeasurable by the best potential measuring instruments. However, whichever it may be, the final result is the presence of a large number of cells, each contributing its electromotive force caused by various chemical reactions which may prove destructive to the surfaces of the various metallic parts. For example, let us take a bearing made of metal X and a crankshaft composed of metal Y . These two are contacted by some oil, and the whole setup acte as a galvanic cell. The following results may be expected as the cell works: (1) X or Y may, because of its electrolytic solution pressure, dissolve and form a conducting solution. (2) X or Y may serve as electrodes a t which some of the components of the oil are oxidized, and then these oxidation products may chemically attack the metals in contact. (3) The oxidation products may cause further chemical reactions in the oil which may result in corrosive substances. Thus it may be possible that the electrode potential is either directly or indirectly responsible for some of the corrosion in the case illustrated.
Experimental Procedure
JOHN K. ANTHONY . The Cleveland Graphite Bronze Company, Cleveland, Ohio AND
TEMPERATURE CONTROL.A regular 4-liter Dewar vessel fitted with a stirrer and knife-edge type of heater was filled with mineral oil. This served as an outside temperature controlling bath. It was possible when desired to hold the temperature even at 170' C. to *0.2' C. THE CELL. A Pyrex glass beaker served as a container for the cell. It was held in place by suspending it from a Bakelite board that was securely fastened to the top of the wooden frame holding the Dewar vessel (Figure 1). In order to avoid any stray electromotive forces, the oil in the cell was heated entirely
Data are presented on the electrode potentials of various bearing metals and alloys in several lubricating oils. Potentials were found to start at about 110" C. in unstirred cells and at about 70' C. in stirred cells. Voltage as high as 1.1 volts at 170" C. was found. Potentials varied considerably with temperature. A reversal of polarity was found in several instances. The possibilities of correlating these potentials and their variations with the problem of bearing corrosion are discussed. It is pointed out that much work is yet to be done before any definite conclusions may be drawn.
ING
7 DEWAR c
FLASK
TEST
C
ELEMENT
ORROSION of metals dates from the time when man first prepared and used them. It is not the purpose
OIL
BEAKER
of this paper to discuss the various prevailing theories of corrosion but rather to point out a possible factor which appears to have been thus far overlooked by workers in this field-namely, the electrode potential of metals and alloys in contact with oils and the resulting electromotive force of the various connected parts.' The purpose of this work is to determine (a) whether a measurable electromotive force exists, and ( b ) if it does exist, to measure it, using several metals and alloys in different oils a t various temperatures. For example, in the automobile engine much work has been done to explain and eliminate corrosion. However, work thus far has not taken into consideration that each part is made up of various alloys or metals, and all of these parts are connected in such a way as to form a network of elec-
FIGURE 1. DIAGRAM OF CELLASSEMBLY
by conduction of heat from the outside temperature bath. A stirrer in the cell assured uniform temperature throughout. The electrodes were approximately5 cm. long, 0.9 cm. wide, and 1mm. thick, and were held tightly about 1 cm. apart. At first it was thought desirable to hold the electrodes quite close together in order t o approximate the actual working conditions in an automobile engine. After numerous runs when the electrodes were held 0.007 to 0.015 cm. apart, it was found almost impossible to obtain any reproducible and therefore trustworthv results owing to poor oil circulation. In future work a cell will be constructed with moving electrodes, making it possible to reproduce the actual working conditions of an engine. Only
I A suggestion of this possibility was first made about two years ago during a conference by Zay Jeffries, General Electric Company, Cleveland, Ohio.
959
INDUSTRIAL AND ENGINEERING CHEMISTRY
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VOL. 29, NO. 8
ON RELATIVE CONDUCTIVITY OF OILS IN VARIOUS CELLS TABLEI. EFFECTOF TEMPERATURE
Cell
,--T
Al--used oil-Cd-Ag + annealed AI--oil B-Cd-Arr * annealed Ai--oii A-Cd-Ai + annealed Steel +-oil B-Cd-Ag - annealed Steel +-oil B-Cd-Ag- annealed Steel +-used oil-Cd-Ag wnannealed Steel +-used oil-babbitt metalCu +-used oil-Cd-Ag - unannealed Steel +-used oil-Al-
140 1.3 1.3 1.1 1.1 1.2 2.0 1.5 2.5 1.4
-T
"C. (First Cycle)165 155 170 3.6 1.6 3.0 2.7 2.3 1.6 2.4 3.9 2.4 2 . 9 ~ 1.9 1.2 2.8 2.4 3.8 6.1 3.9 4.1 2.1 3.0 1.5 6.0 5.0 3.3 3.9 2.8 2.1
about half of the electrodes were allowed to dip into the oil, in order to eliminate any electromotive force that might be due to connecting metals. Heavy copper leads, attached to the electrodes,,passed through a cork which was held in position by fitting it tightly in the Bakelite board. Since oils exhibit very high resistance, THE POTENTIOMETER. it was necessary to use a sensitive galvanometer in order to cope with the low microamperage of the cell. A type H. S. Leeds & Northrup reflecting galvanometer was available, and, although not the most adaptable one Leeds & Northrup make, it gave sufficiently qualitative results to justify its use. In future work a galvanometer will be used which will have specifications to meet the requirements of the new cell in which the metals serving as electrodes will be as close as they are in their natural working conditions. ?he galvanometer was mounted on a solid wall and was used with a lamp and scale at 1 meter distance. The potentiometer used was the Leeds & Northrup type Kz. CONDUCTIVITY. The conductivity was determined at various times in order to obtain a few comparative values. Two methods were employed: (a) A direct current of 120 volts was passed through a conductivity cell, and the resulting amperage was determined. (b) A regular Leeds & Northrup Kohlrausch bridge using two-stage amplification was used. However, since only qualitative results were obtained at this time, it was thought desirable to report only in relative terms the effect of temperature on conductivity. This was done separately for each cell by using the term I / E as determined from the total deflection of the galvanometer and the electromotive force. For simplicity the conductivity at 120' C. was made unity (Table I).
140 1.1 1,6 1.6 1.2 2.0 3.0 1.7 5.0 2.2
140 1.3 1.2 1.1 1.3 2.2 3.3 1.1 5.8 2.1
OC. (Second Cycle)-155 170 155 2.1 3.4 3.1 1.6 2.1 1.7 3'. 1 4.1 3.9 2.8 2.3 2.2 3.4 4.0 2.8 6.7 4.7 8.5 3.4 1.7 3.6 8.5 11.3 8.9 5.0 3.3 3.1
140 2.7 1.1 1.6 1.4 2.2 5.2 1.9 7.2 2.3
MATERIALS.The oils used in this investigation consisted chiefly of new oils A and B and used oil. The used oil was obtained by ipetting the oil out of 40 V-8 Fords in which the oil was used a t least 500 miles, but less than 1000 miles. All of the forty fractions were mixed in order to give a better average used oil. No attempts were made to re-refine or to dry the oils. For a great many of the experiments the A and B oils were treated
8r
TABLE 11. OIL CONSTANTS Gravity ftt 60° F., OA. P. I. Flash point, O F. Fire point F Saybolt UAiver'sal viscosity,
Oil. A 0il.B Used Oil A (Oxidized) Oil B (Oxidized) Oil 27.7 27.3 29.4 28.1 26.8 440 446 435 440 240. 505 500 495 500 360
with oxygen. This was accomplished b bubbling oxygen A definite darkthrough the oil for 24 hours at'about 170' ening of the oil occurred in each case due to the oxidation. The constants of these oils are given in Table 11. For some of the work, new oils C and D were also used. Three types of regular production bearing alloys were furnished by The Cleveland Graphite BronzB Company. These
8.
.3
.z .I 0
.3
D€G
FIGURES 2
TO
c.
5. DATA ON ALUMINUM-OIL-CADMIUMSILVER CELLS
FIGURES 6
TO
D€G. C. 8. DATA ON CADMIUM-SILVER-OILSTEEL
CELLS
INDUSTRIAL AND ENGINEERING CHEMISTRY
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96 1
sidered as annealed during the second cycle. The conductivity during the second cycle was from 20 to 40 per cent greater than during the first cycle. This was the experience with all the following cells. During both of the cycles the steel-oil-babbitt metal cell (Figure 10) exhibited a decrease in the electromotive force on. cooling. When no stirring was employed, the opposite effect was found (Figure 9). This cell also had a strong tendency to reverse itself in polarity. As would be expected, a relatively flat curve was obtained for the cadmium-silver-oil-cadmium-silver annealed cell (Figures 11 to 14) for the two-cycle run; very irregular curves with frequent changes in polarity were obtained for the eingle runs. Figures 15 and 16 illustrate cells with other metals.
DEG. C.
FIGURES 9 AND 10. DATAON STEEL-OIL-BABBITT META CELLS
alloys were babbitt metal, cadmium-silver, and copper-lead. The annealing of some of the cadmium alloys was carried on at about 175" C. The steel and aluminum electrodes were made of steel and aluminum automobile parts. Pure copper served as a copper electrode. PROCEDURE. The speed of the stirrer and the amperage of the heaters were controlled manually by rheostats. No readings of the electromotive force were taken until the temperature of the cells became constant. At first each cell was heated without regular stirring from room temperature to 170" C. or above and then cooled down to room temperature. Readings of the electromotive force were taken every 10" C., both when increasing and decreasing the temperature. Later, to learn the effect of efficient and regular stirring and also the effect of heating and cooling on the recovery of the system, it was decided t o run continuously through two complete cycles- that is, the oil was (a) heated from room temperature to 170' C. or above, ( b ) cooled to room temperature, (c) again heated to 170' C . , and (d) cooled to room temperature. As before, the recordings were taken for every 10" C . change in temperature, and check readings were taken at the highest temperatures. Efficient stirring was used for both of the cycles. A complete run required 12 to 20 hours.
Results To simplify the discussion, the more important data were graphed using only the electromotive force and temperature. I n each single-cycle run the electromotive force obtained on heating up to 180" C. appears on the left of the voltage axis; the electromotive force obtained on cooling appears on the right of the axis. The graphs of the two-cycle runs are plotted as follows: The first cycle is identical with a single-cycle run, but the second cycle is designated by a broken line; its electromotive force on heating appears on the right and that of the cooling appears on the left of the voltage axis. I n each graph the original polarity of the electrodes is indicated. Data for one of the two-cycle runs appear in Table 111. The results of this work may be expressed in several ways. For convenience, various electrode systems will be taken individually a t 170" C. Equilibrium conditions are probably most easily attained, a t this temperature, partly because of low viscosity and partly because of higher conductivity conditions prevailing. The cell aluminum-oil-cadmium-silver (Figures 2 to 5) gave about 0.2 volt. The electromotive force of the twocycle runs reproduced itself almost exactly. Figure 2 shows the irregularities when stirring is not employed. In the cell cadmium-silver-oil-steel (Figures 6 to 8) the cadmium-silver was negative; in the cell used for Figures 2 to 5 i t was positive. Since the unannealed electrodes were heated to 170' 0. during the first cycle, they should be con-
TABLE 111. ELECTROMOTIVE FORCE OF UNANNEALED CADALLOYSAGAINST STEELIX USEDOILAT MIUM-SILVER VARIous TEMPERATURES Time Temp., ' C. E. M. F. Time Temp., ' C.a E. M. F. 7:40 A. M. 7:50 8:02
20 60 75 90
... ...
0.30 0.359 0.356 0.357 0.337 0.336 0.385 0.505 0.526 0.547 0.567 0.566 0.556 0.566 0.554 0.468 0.381 0.342 0.331 0.322 0.313 0.300 0.249 0.22 0.23
8:13 8:19 ' 100 8:24 109 8:29 120 8:35 129 8:43 139 8:50 150 8:58 159 9:05 169 9:lO 170 9:11 170 9:18 165 9:23 161 9:29 153 9:40 143 9:53 132 123 10:04 10:17 113 10:34 103 10:51 93 11:10 83 11:21 73 11:31 64 11:40 58 11:50 53 11:58 49 a Allowed t o cool slowly to 29' C.
2:oo P. M. 2:08 2:13 2:17 2:24 2:29 2:35 2:40 2:47 2:55 3:07 3:15 3:19 3:22 3:35 3:60 4:OO 4:11 4:22 4:37 4:54 5:lO 5:24 5:34 5:50 6:04 6:14
29 59 76 89 99 109 120 129 139 150 160 170 170 170 159 161 141 131 122 112 102 91 80 71 61 55 51
...
0 : 3i4
0.372 0.366 0.347 0.347 0.366 0.364 0.507 0.408 0.407 0.408 0.407 0.314 0.304 0.289 0.232 0.217 0,191 0.181 0.170 0.175 0.18 0.18
When various new untreated oils, including A, B, C, and D, were tried with different electrode systems, they gave electromotive forces. Usually, the voltage did not appear until a temperature above 100" C. was reached, and it was of small extent. However, the conductivity of all the oils was very different. For instance, the voltage in C and D oils was less than 0.1 volt a t 170' C., but the conductivity in C oil was about ten times greater than that in D oil. I n A oil, on the other hand, the conductivity was about twice as great as in C oil.
Discussion of Results Inasmuch as this work is preliminary in this field, one cannot offer definite conclusions because it is impossible as yet to reproduce every step in the various experiments. This is due in part to the uncertainty of reproducing the oils, particularly the used oil. The following general observations, however, seem to be warranted by the experimental data : All of the various combinations of metals and alloys gave rise to an electromotive force in the oils thus far used. In almost all of the single runs where there was a change of temperature but no stirring, the electromotive force was greater at lower than a t higher temperatures. At correspondingly high temperatures the electromotive force of a single-cycle run agreed usually well with that of a twocycle run. This is probably due to high diffusion when the viscosity is low a t high temperature.
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FIGURES 15 AND 16. DATAON CILLS WITH OTHERMBTALS
O€G.
FIGURES 11
TO
c.
14. DATAON CADMIUM-SILVER-OILCADMIUM-SILVER CELLS
I n almost all of the single-cycle runs the electromotive force started and stopped approximately a t 115” to 120°C. However, in the case of two-cycle runs the electromotive force appeared a t a much lower temperature and usually persisted on cooling to a n even lower temperature. The presence of water in all of the oils used may be a factor in explaining this behavior. Steel in different combinations with annealed and unannealed cadmium alloys exhibited a positive potential. However, steel against aluminum showed a normal behavior as in water. Steel in combination with babbitt metal exhibited a greater electrolytic solution pressure under certain conditions. This may explain why in some cases a steel crankshaft becomes pitted when a motor has been idle for a long time. I n order to relate corrosion and electromotive force, it is necessary to know the conductivity of the oils used. It was found that the conductivity of the oils increased considerably with temperature, This is due to many factors. Some of the more important are decrease in viscosity, presence of water, increase in the oxidation products which may yield ionized substances, etc. Since the amount of chemical reaction is proportional to the amperage it is poesible to have (a) a high electromotive force and low amperage and ( b ) low electromotive force and much higher amperage than in case a. Both of these cases have been demonstrated during this work. In used oil the amperage was always much higher than for the same voltage with A and B oils. When twocycle runs were made, the amperage was always greater during the Recond cycle by 20 to as much as 100 per cent in spite of the fact that the electromotive forces were nearly equal. This means a decrease in resistance which results in higher conductivity (Table I). This property ought to
be of importance to the oil chemist because with each heating the oil may become more and more conductive, which may result in greater and greater corrosion, regardless of whether the corrosion is due to galvanic action or to straight chemical reaction or to both. To inquire into the cause of what contributes to the conductivity of oil is to inquire into the physical and chemical make-up of the oil itself. (In the case of used oil the colloidal particles no doubt contribute a great deal to the high conductivity through cataphoresis, and this effect would probably be more evident during the second cycle.) No doubt many substances added to oils for various reasons may prove detrimental if they are conductive by themselves, or if they are easily converted into good conductors. This fact is shown by the work on entirely new and untreated oils. For example, if such a n oil as C starts with ten times the conductivity of oil D, there is apt to be great corrosion immediately, and such a n oil may prove to be almost useless in a short time. However, it is also quite possible to have a new oil with a very high resistance, which will contain substances that are easily converted into conductive solutions; the final result will be the same as the example just given. Much work needs to be done to correlate corrosion in oils (with and without the various agents commonly added to them) with such factors as conductivity and electrode potentials of metals and alloys in contact with them, and further to correlate these factors with actual service conditions. Undoubtedly some of these properties will be used eventually to characterize oils just as much as viscosity is today. For the present these experimental data do not permit the discussion of the effects of such factors as polarization and oxygen gas electrodes. The presence of either one of them or both would help to explain some of the results. A. P. Anderson of the Shell Petroleum Corporation called the writers’ attention to the fact that Wawrziniok [ Automobiltech. Z., 35,428-30, 600-1 (1932); Chem. Abstracts, 27,407, 1153-4 (1933)] published some work on “Corrosion of Automobile Engine Parts Caused by Lubricating Oils,” in which it is claimed that “corrosion of motor parts may be due to galvanic currents generated between different metal parts, the oil serving as electrolyte.” The summary of Wawrziniok’s data in Chemical Abstracts reveals that his results are of the same order as the present ones. RECEIVED November 18, 1936.
