I
J. R. THOMAS, 0.1. HARLE, W. L. RICHARDSON, and L. 0. BOWMAN California Research Corp., Richmond, Calif.
Copper-Lead Bearing Corrosion Inhibition in Diesel Service Following intensive study of factors involved in corrosion and corrosion inhibition, a class of inhibitors suitable for use with both copper-lead and silver bearings was developed as additives for oils
THE
corrosion of copper lead bearings by oxidized lubricating oil has been the sub,ject of extensive study for many years. The small number of service problems associated today with corrosion of this type is in part attributable to the excellent performance of the corrosion inhibitors currently used in crankcase lubricants. The use of silver bearings in certain heavy-duty internal combustion engines has raised a growing problem in connection with the use of these inhibitors, however. This difficulty arises because these inhibitors almost universally contain active sulfur, which causes silver corrosion. In general, there appears to be an inverse correlation between the effectiveness of lubricants containing these now classical inhibitors in controlling copper-lead corrosion and their suitability for use with silver bearings. This report describes research aimed at obtaining a better understanding of corrosion and corrosion inhibition and, thereby, the development of new inhibitors suitable for use with both copper-lead and silver bearings. Corrosion Mechanism Denison (7) proposed the first significant mechanism of copper-lead bearing corrosion, which he represented as follows: Pb
+ ROOR’ = PbO + ROR’
PbO
+ 2HA = PbAz + H2O
(1) (2)
where ROOR’ is a peroxide or hydroperoxide and HA is an organic acid. Both the peroxide and acid are formed by oxidation of the lubricant. The loss of copper is insignificant compared to that of lead. Prutton, Frey, Turnbull, and others (2, 4,5; 7) confirmed this general scheme and, in a series of publications, elaborated in detail upon the mechanism involved. One of their contributions
was the demonstration that oxygen and other oxidizing agents, as well as peroxides, could oxidize lead in the systems under consideration. Pb
+
‘/202
=
PbO
(la)
Furthermore, they demonstrated that the rate of corrosion was first-order with respect to oxidizing agent in the presence of high relative concentrations of acid and first-order with respect to acid a t high relative concentrations of oxidizing agent. In addition, they showed that the rate was first-order with respect to the surface of lead available. These facts can be simply expressed by the following rate equation for lead corrosion using oxygen as the oxidation agent.
where kl’ and kz refer to the rate constants for the oxidation and dissolution reactions, respectively. similar to Equations l a and 2, and S represents the total surface of lead available. This equation results from a reasonable assumption involving the rapid establishment of a steady state at the metallic surface. Corrosive Conditions in GM 3-71 Engine To establish the corrosive conditions actually existent in a typical heavy-duty engine, corrosion of copper-lead and of silver bearings was studied in a G M 3-71 engine operated under modified Navy propulsion load (MIL-P-17269) conditions. A typical copper-lead corrosion curve, obtained by plotting the accumulated weight loss against time of operation, is shown in Figure 1. The oil in this case was a 60 V.I. SAE 40-grade solvent refined base stock from California crudes, containing 20 mmoles per kg. of basic detergent additive but .no oxidation or corrosion inhibitor (the reference oil). Also shown in Figure 3 is a plot of the pH of the oil as a function of time. The used oils from a number of engine runs of this type, both with and without inhibitor-type additives, were examined for peroxide and acid content during the engine tests, including the periods of high corrosion rate. Special care was
taken to ensure that decomposition of peroxide was avoided following the withdrawal of the sample from the engine. The peroxide number of the oil (equivalent to the cubic centimeters of oxygen per 100 grams of oil) was less than 10 in all cases, while the upper limit of the corrosive acid concentration was on the order of 0.01 to 0 02N. The amount of peroxide found should be compared with the solubility of oxygen in oil, which was found to be 3 cc. per 100 grams of oil for oxygen a t 0.2-atm. partial pressure, relatively independent of temperature. T o determine whether oxygen or peroxide is more important as an oxidizing agent under these conditions, the relative rates of corrosion by oxygen and a peroxide were determined by comparing, with these separate oxidizing agents, the rates of weight loss from a lead strip suspended in a medicinal white oil containing oleic acid in some cases and capric acid in others. The acid concentration was high enough so that the rate of corrosion was controlled by the concentration of oxidizing agent. In the experiments with oxygen, the white oil was inhibited against oxidation with Nphenyl-1 -naphthylamine. Tetralin hydroperoxide and tert-butyl hydroperoxide were picked as typical peroxides. The rate constant a t 100’ C. for corrosion using oxygen as the oxidizing agent was roughly 10 times that found when using either hydroperoxide. Expressing the rate constants in the terms described below, for experiments made with excess capric acid in white oil at 100’ C., oxygen yields a kl’ value of 0.55, while Tetralin hydroperoxide and tert-butyl hydroperoxide each give a kl’ value of 0.06. This result differs from that reported by Prutton and others, (2, 4) who found the rate constants for oxygen and tertbutyl hydroperoxide to be about the same. This discrepancy may be due to the fact that diffusion processes were controlling their rate, whereas, in this case the corrosion processes appeared to be independent of diffusion as judged by the independence of corrosion rate upon rate of stirring. Consequently, and in view of the amounts of oxygen and peroxide present in crankcase oils, it apVOL. 49,
NO.
10
OCTOBER 1957
1703
pears likely that oxygen is more important than peroxides as an oxidizing agent in the engine operated under the conditions chosen and with the typical presentday oils used. In attempting to answer the important question as to whether acid or oxidizing agent is rate-controlling under engine conditions, the kinetics of corrosion of pure lead strips was studied as described below. The goal of these experiments was to find the rate-determining step by comparing laboratory kinetic data extrapolated to the concentration range of reactants found in used oils. For this purpose. the data obtained were used to evaluate rate constants, kl' and k?, for the two reactions of Equation 3. The constants refer to lead corrosion rates measured as the equivalent rate of oxygen consumption in cubic centimeters per hour a t atmospheric pressure and 25' C.; S is given in square centimeters and is established at a fixed value of 50 sq. cm.; oxygen pressure is given in atmospheres; and acid concentration is given in normality. In these terms, the rate constants in Equation 3 for oleic acid dissolved in a medicinal white oil at 121' C. are 5 for kl' and 35 for k?. The data also show a kl' value of 0.55 and a ks value of 7.4 for capric acid in white oil at 100' C. The data of Prutton and others (2) are not complete enough to yield a good value of kp, but a value was calculated for kl' of 0.31 at 100' C. for lauric acid in white oil. Under usual corrosive conditions in the engine, the maximum acid content of the lubricating oil was found to be of the order of 0.01 to 0.02N. TVith this acid concentration, and if the oil is assumed to be saturated with oxygen at its partial pressure in air, the rate of corrosion, according to these rate constants, should be about linearly dependent upon acid concentration with a relatively small or negligible dependence upon oxygen concentration during most of the engine test period. Mechanisms of Corrosion Inhibition
The mechanisms of corrosion inhibition by inhibitors have not received as much attention as corrosion itself. However, most of the methods whereby inhibitors might be effective in reducing corrosion have been suggested (3. 6 ) . They include : Destruction of oxidizing agents. Destruction of acidic agents. Inhibition of lubricant oxidation. Destruction of oxidative catalysts. Reduction in the yield of corrosive oxidation products. Formation of impervious protective films.
As an aid in the search for new corrosion inhibitors for use with both copperlead and silver bearings, it was felt that further information on the relative im-
1704
portance of these various modes of action with conventional sulfur-containing additives would be helpful. For the service and base oil considered here, the use oE additives to destroy peroxides and other oxidizing agents appears trivial, because oxygen is the principal oxidizing material present, as discussed above. Moreover, in so far as the corrosion rate is primarily controlled by the concentration of acid, as in the case of the systems in the GM 3-71, modest reduction in concentration of oxidizing agent would have little effect. The manner in which basic additives reduce corrosion appears to be straightforward. As long as the build-up of a significant concentration of corrosive acid is prevented, corrosion cannot take place. Base is primarily neutralized by acids arising from oil oxidation and by sulfuric acid arising from the fuel. The relationship of the basicity of the oil to corrosion, for an oil free OE corrosion inhibitors, is shown in Figure 1, where the corrosion of the copper-lead bearing and the pH of the oil are plotted against time. There, the corrosion rate does not become significant until the pH of the oil has reached about 5.5. The oil in this case was the previously mentioned 60 V.I. SAE 40 grade oil containing 20 mmoles per kg. of base in a basic detergent formulation without additional additives. Laboratory tests employing ox-
idized oils confirm this relationship (Figure 2). The data plotted there were obtained by oxidizing the reference oil in the laboratory and determining the pH and the corrosion of a copper-lead strip suspended in the oil as a function of time. The oxidation was carried out under conditions similar to those described later in Corrosion Test B. The use of initial pH of a given oil containing basic additives wa2 justified by the finding that the base concentration remaining after oxidation or engine service was linear with the pH of the oil over the pH range 3 to 10, as measured in the titration solvent described below. In all cases, complete titration curves were obtained. The amount of acid generated by oxidation or engine service is proportional to the loss in pH. The pH limit, 5.5, at which corrosivity becomes apparent, is the pH at which significant amounts of free acids, as strong as or stronger than simple carboxylic acids, first appear in the oil; above this pH value, these acids are present only as salts. The control of oxidation, either by inhibition or by catalyst deactivation. and the reduction in yield of corrosive oxidation products were studied together. In laboratory tests, the oxidation rate oi the oil was measured in an oxidator equipped with a high speed stirrer and a recording gasometer. The oils were oxidized at
Table I. Effect of Inhibitors on Catalyzed Rates of Oxidation and Base Depletion at 171" C. 0 xidation Rate, c c . 02/100 Grams Oil/Hr.
Oil Reference oil (contains 20 mmoles/kg. base in basic detergent formulation) Reference oil
970 41
+ 6 mmoles/kg. zinc organic dithiophosphate
+ 10 mmoles/kg. zinc organic dithiocarbamate Reference oil + 0.5% sulfurized diparaffin sulfide Reference oil + 15 mmoles/kg. Additive B Reference oil + 26 mmoles/kg. Additive A
Reference oil
+ 50 mmoles/kg. + 50 mmoles/kg.
Reference oil
Reference oil Additive A
INDUSTRIAL AND ENGINEERING CHEMISTRY
14.5
73
8.0
620
14.5
1000
17.1
210
4.7
Influence of Additive A on Oxidation and Base Depletion Rates
Oil Reference oil Additive A Reference oil Additive A
20
4.0 (uncatalyzed)
830
Reference oil
Table II.
Base Depletion Ratc, Mmoles/Kg./Hr.
+ 50 mmcles/kg.
Total 0 2 Uptake, C c . 02/100 Grams Oil
None
800
pH of Oxidized Oil 6.4
None
800
8.25
7.5
None 0.05% iron naphthenate
2400
6.25
20.0
800
6.7
5.0
O.O5y0iron naphthenate
800
8.4
6.0
Catalyst
Oxidation Time, Hr. 11.5
C O R R O S I O N OF COPPER-LEAD BEARINGS g 1000CORROSION
B
i'
d m 2
.P
800-
I 600L$
3
.
w Y
400-
s
rn
-
4 200E
.
8
0 -
n
~~
0
20
40
HOURS 60 80
100 120
PH
Figure 1. Bearing corrosion and pH of lubricant in GM 3-71 engine
HOURS
Figure 2. Corrosion of comer-lead strir, as a futkiion of initia'l pH of oxidizing oil
Figure 3. A.
B. C.
D.
171' C. with pure oxygen. Samples of oil were withdrawn at periodic intervals and titrated for amount of base remaining measured to a p H of 5.5. Soluble catalyst, composed of mixed metal naphthenates, was added to the oil before oxidation, The catalyst was that described in Corrosion Test B, and was used at 0.02%. The reference oil in these tests was the 60 V.I. SAE 40 grade base oil containing a basic detergent combination previously discussed. The results of these tests are given in Table I. In addition to conventional sulfur. containing additives, results are also given for two experimental additives. These data show two things: The rate of oxygen absorption can be reduced by an inhibitor with a subsequent lowering in the rate of base consumption, and the decrease in the rate of base consumption is not proportional to the decrease in the rate of oxidation. This latter point is clearly shown by comparing the effects of the dithiocarbamate with that of Additive A. The oxidation rate with the latter material is three times greater than with the former, yet the base consumption rate is only about one half as great. A similar lack of proportionality is shown by comparing the data from the catalyzed and uncatalyzed oxidation. The dithiocarbamate appears to be an example of a true oxidation inhibitor (or catalyst deactivator), while Additive A is an example of a material which appears to reduce the yield of corrosive acids, as well as to reduce the oxidation rate moderately. These conclusions, of course, are true on only a relative basis. Neither the dithiophosphate nor the sulfurized diparaffin sulfide shows a marked effect upon oxidation rate or upon base depletion rate. Further evidence of the unusual effect of Additive A upon base depletion in an engine oil during oxidation was obtained. The results are given in Table I1 for experiments with and without catalyst. The reference oil was the same as de-
Bearing corrosion in GM 3-71 engine
Reference oil (containing 20 mmoles l / k g . basic detergent additive Reference oil f 26 mmoles/kg. Additive A Reference oil f 15 mmoles/kg. Additive B Reference oil f 10 mmoles/kg. zinc organic dithiocarbamate -t- 6 mmoles/kg. zinc organic dithiophosphate
scribed previously; the oxidations were performed by the method used in the previous experiments. The pH of the oils before oxidation was 11.O. For a given oxygen uptake, the loss of basicity was much less severe in the presence of Additive A, as indicated in drop of pH. The rate of base depletion, furthermore, was considerably lowered by this inhibitor, even when, as in the uncatalyzed oxidations, the rate of oxidation was increased by its use. Effects parallel to these observed in the laboratory for the dithiocarbamate and Additive A wei-e observed in engine operation. Table 111 shows the hours of running in the GM 3-73 engine required for the oil to reach a p H of 5.5. The reference oil is the same as that described above,
Table 111. Time for Oil to Reach pH of 5.5 in GM 3-71 Navy Propulsion Load Test Oil
Reference oil Reference oil 26 mmoles/kg. Additive A Reference oil 5 10 mmoles/kg. zinc organic dithiocarbamate 6 mmoles/kg. zinc organic dithiophosphate
+
Hours t o pH 5.5 50 a 150
+
200
The extension of base reserve in the engine by both mechanisms, reduction in oxidation rate and reduction in yield of corrosive products, appears to correspond to laboratory evidence. The influence of these additives upon corrosion in the engi.ne is shown in Figure 3, where the copper-lead weight loss in the engine is plotted against time. T o study the importance of protective film formation, the corrosion of copper-
lead bearing strips was determined as a function of time. These strips were suspended in vigorously stirred oil held a t 171' C. for 20 hours (Experimental Section, Corrosion Test B). In one set of experiments, the same strip was maintained in the test oil for the entire period; in another set. the strip was replaced every 4 hours with a new strip. The oil in each case contained 0.02yo soluble metal catalyst. The reference oil was the same as that discussed previously. Separate experiments showed that the catalytic effect of the metal strips was minor compared to that of the soluble catalyst (Figure 4). The accumulated weight loss found at 20 hours in the replacement strip test is much greater than that observed for the single-strip test. This demonstrates that these additives do form protective films which are highly impervious to corrosive agents. Moreover, the films appear to be relatively stable in the absence of abrasive forces. The high corrosion rates obsrrved late in the replacement strip tests indicate that the useful life of the inhibitors is short compared to the length of the experiment. The difference in corrosion observed for the reference oil in the single and the replacement strip experiments is due to organic lacquers which also function to protect the surface of the metal. This effect is very pronounced in the absence of detergent additives, which are effective in preventing deposition of this type of lacquer. The effect of the inhibitors in reducing corrosion in the replacement strip test, in addition to demonstrating the film-forming properties of the inhibitors, appears to reflect the ability of the zinc organic dithiocarbamate to control oxidation which was discussed previously. The results obtained in the studies of oxidation inhibition (either by true inhibition or catalyst deactivation) and VOL. 49, NO. 10
OCTOBER 1957
1705
HOURS
A. E.
F.
G.
Reference oil Reference oil f 6 mmoles/kg. zinc organic dithiophosphate Reference oil 3. 0.5% sulfurized diparaffin sulfide Reference oil + l o mmoles/kg. zinc organic dithiocarbamate
Acid and oxygen constant Sulfur concentration, mmoles/kg.
0.
A.
None
0.75
0.1.5 the formation of protective films support the following conclusions: Dithiophosphates and sulfurized diparaffin sulfides reduce lead corrosion primarily by protective film formation, and dithiocarbamate functions both by the formation of protective film and by the inhibition of oxidation. An indication of the relative importance of these two mechanisms in GM 3-71 operation was obtained by determining the effectiveness of the sulfurized diparaffin sulfide as a corrosion inhibitor in the engine. The test was run with a slightly different type of basic detergent additive than that used in the tests previously discussed, so that a strictly quantitative comparison cannot be made. However, the corrosion at 100 hours of operation. with the same concentration of sulfurized diparaffin sulfide used throughout this series of tests, averaged in excess of 1000 mg. per whole bearing for two tests as compared to a value for the reference oil without in. hibitor of about 800 mg. per whole bearing. This clearly demonstrates that protective film formation alone, by an additive whose life is short compared to the length of the experiment, is not adequate. In the presence of a good oxidation inhibitor, however, the useful life of this type of film-forming additive can be greatly extended. Protective Film Formation
Of all the mechanisms by which inhibitors can function to reduce corrosion, it was decided that the most profitable for further study was the one involving the formation of protective films, because corrosion inhibition by protective films is a positive method capable of functioning, at least in principle, under the highly corrosive conditions frequently encountered in Diesel operation. Furthermore, relatively little work appeared to have been devoted to the
1706
study of protective films, whereas a great deal of effort has been devoted to the study of oxidation inhibition and catalyst deactivation. A number of factors must determine the effectiveness of protective film-forming inhibitors. Rate of formation of film. Resistance of film to removal by detergents. Imperviousness of film to corrosive agents. Resistance of film itself to chemical attack. Resistance of film to abrasive removal. Life of film-forming inhibitor in service. The importance of some of these factors was demonstrated by the experiments described later. From a consideration of the situation of a bearing in an operating engine, it is apparent that the bearing will quickly become coated with the protective film provided by the inhibitor and that metal will be lost from the bearing only after this surface is scratched to expose fresh metal. Once the surface is scratched, inhibitor present in the oil will immediately begin to repair the scratch, and, after a certain period, the length of which will be determined by the speed with which the inhibitor reacts to form the film, the scratch will be repaired. During this period of repair, corrosion of lead from the bearing surface will take place in accordance hith the corrosion mechanism outlined previously According to the reaction sequence of Equations l a and 2, the exposed lead surface of the bearing scratch will be composed of either elemental lead or lead oxide. Under steady-state conditions, which means that these reactions are fast compared to the inhibiting reaction, the ratio of lead surface to lead oxide surface is given by
INDUSTRIAL A N D ENGINEERING CHEMISTRY
A.
3.0
0. 6.0 (4)
An inhibitor which functions by forming a protective film can do so, then, by reacting with either lead or lead oxide. For the GM 3-71 engine operated under the conditions described here, the surface is largely lead oxide because the corrosion rate is determined by the acidity rather than the oxygen concentration, indicating that the most effective type of inhibitor would be one reacting with lead oxide. Expressing the inhibitory reaction as Pb (or PbO)
3. X
ks ---f
PbX
(5)
where X is the corrosion inhibitor, it is readily shown that the lead loss from a scratch of initial area, S,, as a function of time, is
for an inhibitor which reacts with oxide under conditions where the acid concentration controls the corrosion rate. When the inhibitor reacts with metallic lead
A similar set of equations can br derived for the other extreme condition of corrosion-namely, that where the concentration of oxidizing agent is rate-determining. Experiments were conducted to test these rela tionships and to observe the protective effectiveness of the films formed. These consisted of studying the rate of weight loss of a pure lead strip exposed to controlled conditions of acidity, oxygen content, and inhibitor concentration (see Experimental Section). Figure 5 shows lead loss curves us. time at a
CORROSION
*
single acid concentration, one oxygen pressure, and four different concentrations of elemental sulfur under conditions such that the rate of lead corrosion is controlled by the acidity. Sulfur was picked as inhibitor because it was expected that it would be a simple reagent capable of combining directly with lead to form a protective film of lead sulfide. The effectiveness of sulfur as an inhibitor is clear from the data given in Figure 5. Furthermore, the general shape of the weight loss curve is in agreement with the requirements of Equations 6 and 7. The plateau values of lead loss are in the ratio of 1 :2 :4.5 : 9.9 for reciprocal inhibitor ratios of 1 : 2 :4: 8. These results are in good agreement with the requirements of Equations 6 and 7. Figure 6 shows the lead loss curves at constant oxygen pressure, constant inhibitor concentration, and three concentrations of acid. Here, the plateau losses are in the ratio 1 :1.4: 2 for acid concentration ratios of 1 :2:4. The dependence of the plateau losses upon acid concentration shows, surprisingly, that the sulfur is functioning by reacting with lead oxide rather than lead in accordance with the difference between Equations 6 and 7. The dependency of the plateau loss upon the square root of acid concentration rather than directly upon the acid concentration indicates a complexity of the kinetics which has not been taken into account, however. Figure 7 shows the loss curves for a condition of constant inhibitor and acid concentrations but for three different oxygen pressures. Here, the lead loss is the same in all three experiments, showing complete agreement with Equation 6, which calls for the lead loss to be independent of oxygen pressure. This demonstrates again that the reaction of sulfur to form a protective film is with lead oxide rather' than lead. This un-
OF COPPER-LEAD B E A R I N G S
expected conclusion was shown to be reasonable by studying the reaction of elemental sulfur dissolved in medicinal white oil with lead oxide. The reaction a t 132' C. was found to be fast and to yield a product consisting of 70 f 10% lead sulfide with the remainder being lead sulfate. Identification and analysis of these products was made by x-ray diffraction. These results indicate that the inhibiting reaction is 4Pb0
+ 4 s = JPbS + PbSOa
(8)
Study of the inhibition of corrosion, as carried out above, was attempted under other conditions of acidity and oxygen pressure, frequently without success. With sulfur as the inhibitor, heavy films were sometimes formed which appeared pervious to corrosive agents. Frequently, these films were only poorly adherent and were easily wiped off. Under conditions of very high corrosion rate, films were extremely hard to establish. Undoubtedly, the' physical properties of the films are important in determining the degree of inhibition attained, and the conditions under which the films are formed apparently markedly influence their physical properties. The importance of the kinetics of film formation and the importance of selecting inhibitors in accordance with the requirements of equations, such as Equations 6 and 7, in light of the existing corrosive conditions is apparent.
Acid-Type Film-Forming Inhibitors
,
With the aid of the above research on mechanisms of corrosion and inhibition in the GM 3-71 engine, a class of inhibitors was developed which effectively inhibited corrosion of copper-lead bearings. At the same time, oils containing these materials effectively lubricate silver bearings and are not corrosive to them.
These materials were designed to be acidic and, consequently, are capable of reacting with lead oxide to give tough refractory films. Their acid strengths were selected so that their salts are basic to sulfuric acid and oxyacids in order not to impair the detergent and antiwear properties of basic additives. An example of this class of inhibitor is an oil-soluble arsenic compound, Additive B in Table I. In the standard reference oil, 0.5y0 of this material reduced the bearing weight loss in the GM 3-71 at 100 hours from about 800 mg. per whole bearing to 20 mg. per whole bearing. Its corrosion us. time is show-n in Figure 3. This material has no effect on the rate of oxidation of the oil or upon the rate of depletion of base, either in the engine or in laboratory tests as seen in Table I. Another type of material, Additive A, does function in these ways. The superior film-forming characteristics of the arsenic additive and its long effective life can be demonstrated in the replacement strip test. In this test a replacement strip weight loss of 10 mg. was observed, compared to a singlestrip weight loss of about 3 mg. Without inhibitor these values would be 725 and 250 mg., respectively. This superior film-forming characteristic is demonstrated by the data plotted in Figure 8. The data shown in Figure 8 were obtained by determining the rate of corrosion of lead strips suspended in samples of used engine oils. The oils were vigorously stirred in an atmosphere of oxygen under conditions such that the oxygen absorption was accounted for stoichiometrically by the lead loss. Under these conditions, no further oxidation of the oil takes place during the time of the experiment. In Figure 8, corrosion rates are plotted against p H of the oils irre. spective of the time of engine operation.
60
180
120
2.10
MINUTES
Figure 6. corrosion
Effect of
acid
concentration
Sulfur and oxygen constant Capric acid concentration, mmoles/kg.
0. 40
A. 20
0.10
on inhibited
Figure 7. corrosion
Effect of oxygen concentration on inhibited 0.
A.
.JI
Air
Oz/Nz = 1 Pure02
VOL. 49, NO. 10
OCTOBER 1957
1707
pH
5
OF USED EN1
Figure 8. Corrosivity of used oil samp!es withdrawn during engine tests
Iron
A. C.
Manganese
0.002 0.0005
Chromium
0.004
H.
Reference oil Reference oil $- 15 mmoles/kg. Additive B Reference oil 0.25% sulfurized diparaffin sulfide + l O kg. zinc organic dithiophosphate
+
These data are given for the reference oil containing basic detergent additive run in a G M 3-71 engine, reference oil plus 15 mmoles per kg. of Additive B run in the G M 3-71 engine, and an oil containing sulfurized diparaffin sulfide and zinc organic dithiophosphate run in another test engine. The excellent anticorrosive effect of the arsenic compound is apparent even after the oil has become highly acidic. X-ray fluorescence analysis of surface films of lead strips used in experiments similar to those cited for elemental sulfur demonstrates the presence of arsenic. The form in which the arsenic is deposited was not determined. Experimental
Engine Test Procedure. A General Motors Series 71, two-cycle, threecylinder Diesel engine was employed according to MIL-P-17269 (Ships), "Procedure for the Evaluation of Diesel Engine Lubricating Oils Under Severe Operating Conditions." Deviations from the standGard procedure were as follows : No synthetic sea water was added. (The addition of a 2% volume of sea water is specified at the beginning of the test.) Cycling conditions were not observed. For this particular research, the engine was run continuously, except for weekend shutdowns. pH Determinations. A 10.0-gram sample of the oil was dissolved in 100 ml. of a solvent mixture composed of isopropyl alcohol, ethyl ether, and water in the proportions 40: 40 : 20 by volume, respectively. The titration was performed with 0.100X sodium hydroxide and a Beckman pH meter. Peroxide Determination. Peroxide was determined by reaction with potassium iodide in a glacial acetic acidchloroform mixture under carbon dioxide, followed by titration with standard sodium thiosulfate in the presence
1 708
2
tion, so that diffusion of oxygen into the sample was never a rate-determining factor. Copper-Lead Strip Corrosion Test B. This test was carried out in an oil bath held by thermostat a t 171 C. The test oil (300 ml.) was placed in a lipless, 400-ml. Berzelius beaker in which a polished, weighed, copper-lead strip was suspended. The test oil was stirred vigorously. After two hours, a synthetic catalyst was added to provide the following catalytic metals (as naphthenates) : 70
"moles/
of water. End points were difficult to determine because of the intense color of the used oils. Disappearance of the starch color in the aqueous phase was used. Oxidation Rates and Corrosion Test A. The oxidator, or laboratory oxidation apparatus, used in the oxidation experiments consisted of a reaction cell and a recording gasometer. The reaction cell consisted of a cylindrical flat-bottomed reactor of 150-ml. capacity fitted with a standard-taper male joint at the upper edge; a lid with the corresponding female joint was fitted to the reactor to give a gas-tight joint. The lid contained an induction-driven armature to which was fitted a glass stirrer extending into the reactor. The stirring mechanism was driven at high speed. A gas inlet permitted filling the gas space of the reaction cell with oxygen at atmospheric pressure and, in addition, permitted connection to the recording gasometer. The recording gasometer fed oxygen at atmospheric pressure to the reaction cell as oxygen was consumed there, sirnultaneously and continuously plotting the volume of oxygen delivered against time. This provided a measure of Oxidation rate. I n corrosion experiments, a rack of nickel wire was placed in the reactor. Upon this could be suspended as many as eight lead strips, each with a total surface area of 6 sq. cm. By withdrawing strips serially for weighing, the rate of corrosion could be determined during the course of oxidation. A 150-ml. sample of the test oil was ordinarily charged to the reactor at the start of each experiment. This was sufficient to cover all the lead strips completely in experiments where corrosion was also measured. The stirring rate was determined to be always sufficient to maintain the concentration of oxygen in the test oil at satura-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Lead
Copper
0.008 0.004
The test was continued a total of 20 hours, after which the copper-lead strip was removed, washed with hexane, and reweighed. The weight loss served as a measure of the corrosivity of the test oil. This test was designed to correlate with bearing weight loss in the GM 3-71 engine test. Kinetics of Corrosion Inhibition by Sulfur. All kinetic data were taken in Squibb's mineral oil inhibited toward oxidation with 0.01 % Ar-phenyl-l-naphthylamine. Weighed amounts of sulfur were dissolved by heating the inhibited mineral oil briefly at around 150" C. Capric acid was weighed in tared beakers and rinsed into the test oil with portions of the oil. The reaction vessel containing 1.0 kg. of the test oil was held by thermostat at 100' C. The test atmosphere was introduced through a sintered glass disk in the bottom of the reactor at a flow rate of 730 cc. per minute at standard temperature and pressure, which served to stir the solution. Corrosion rates were independent of flow rate in this region of flow. 'IVeighed pure lead strips 3.5 by 0.5 by '/le inch were suspended in the oil by glass hooks. The strips were removed at intervals, washed in hexane. and reweighed to determine the weight loss due to corrosion. Literature Cited (1) Denison, G. H., IND.ENG.CHEM.36,
477 (1944).
( 2 ) Guttenplan,' J. D., Prutton, C. F., Lubrication Eng. 4, 125 (1948). ( 3 ) Losikov, B. V., Makasheva, 0. P., Aleksandrova, L. A,, Neftyanoi Khoziaisluo 32, 65 (1954). (4) Prutton, C. F., Day, J. H., Lubrication Eng.53, 1101 (1949). ( 5 ) Prutton, C. F., Frey, D. R., Turnbull,
D., Dlouhy, C., IND.END.CHEM. 37. 90 1194s). ( 6 ) Prutfoni 'C. F:, Turnbull, D., Frey, D. R.: Ibid., 37, 917 (1945). ( 7 ) Turnbull, D., Frey, D. R., J . Phys. €8 Colloid Chem. 51, 681 (1947).
RECEIVED for review November 21, 1956 ACCEPTED April 25, 1957 Division of Petroleum Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
