March, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
mum of trouble and delay. A. C. Matthews, Central Palma; E. Miller, Compafiia Arucarera Atlantica del Golfo; F. Adair Monroe, Compafiia Cubana; H. J. Schreiber, Ingenio JatibonN O ; Percy A. Staples Hershey Corporation of Cuba; J. D. Stephenson, Central V)ioleta; N. N. Trinler, Central Preston; George T. Walker, Central Espana; and B. A. Sample, of Havana, provided facilities of all kinds and splendid hospitality. Special thanks are due F. A. Monroe, whose influence was of tremendous help in many difficult situations. The Coca Cola Corporation of Havana made available, without chargel mobile refrigerating facilities which made possible the collection of samples from all over the island. Leonard Wickenden, of New York, provided valuable connections in Louisiana and Cuba. LITERATURE CITED
(1) Andrews, J. S., Boyd, H. M., and Terry, D. E., Cereal Chem.,
19,55 (1942). (2) Baker, A. Z.,Wright, M. D., and Drummond, J. C., J. SOC. Chem. Id.,56, 191 (1937). (3) Connor, R. T., and Straub, G. J., IND. ENG.CEEM.,ANAL.ED., 13, 380 (1941). (4) Copping, A. M.,and Roscoe, M. H., ,Biochem. J., 31, 1879 (1937).
263
(6) Cowgill, 0.R., J. Am. Med. Assoc., 113, 2146 (1939). (6) Zbid., 122, 437 (1943). (7) Daniel, E. P., and Munsell, H. E., U. 8. Dept. Agr., iMisc. Pub. 275 (1937).
(8) Harris, L. J., and Leong, P. C.,J . SOC.C h m . Id.,56, 195 (1937). (9) Lapicque, L., and Chaussin, J., C m p t . rad., 166,300 (1917). (10) Moore, C. V.,and Brodie, J. L., Arch. Pediat., 42, 572 (1925). (11) Nelson, E. M.,and Jones, D. B., J . Agr. Research, 41,749(1930). (12) Pyke, M.,J . SOC.Chem. Id.,58, 338 (1939). (13) Sherwood, R. C., Nordgren, R., and Andrews, J. S., Cered Chem., 18,811 (1941). (14) Snell, E. E.,and Strong, F. M., IND. ENG.CHEM.,ANAL. ED., 11, 346 (1939). (16) Snell, E. E., and Wright, L. D., J. Biol. Chem., 139,676 (1941). (16) Spencer, G. L., and Meade, G. P., Handbook for Cane Sugar Manufacturers and Their Chemists, 7th ed., p. 234,New York, John Wiley t Sons, 1929. (17) Strong, F. M., Feeney, R. E., and Esrle, A., IND. ENG.CHE~M., ANAL.ED., 13,566 (1941). (18) Thomas, J. M.,Bina, A. F., and Brown, E. B., Cereal Chem., 19, 173 (1942). (19) Williams, R.R.,IND. ENG.CREM.,33, 718 (1941). PRSSS~NTBIII before the Division of Sugar Chemistry and Technology a t the 106th Meeting of the AMERICAN CHEMICAL SOCIIITY, Pittsburgh, Pa.
Corrosivity of Lubricating Oils EXISTENT AND POTENTIAL GEORGE W. WATERS‘ AND HUGH D. BURNHAM* Shell Oil Company, Inc., Wood River, Ill.
T
HE fundamental Bearing corrosion is analyzed into two concepts: ‘‘exOn the other hand requ&tes for coristent corrosivity” which occurs by virtue of the instanevery lubricant can cause rosion are a corroditaneous chemical state of a lubricant, and “potential corimmediate corrosion to: a ble and vulnerable metal rosivity” which occurs under conditions, representative degree, varying from zero surface, and the presence of those of service, which lead to the simultaneous oxidato relatively great magnition of the oil. The effects upon both types of corrosivity tude. This “existent corroof corrosive bodies, generally acidic. of temperature, time, nature of oil,concentrationof reactsivity” (EC) is defined as Corrosion of the bearants, and physical factors of tests are described. The imthe corrosion caused by portant functioh of protective lacquer films in preventing a lubricant under conings of an internal cornbustion engine by lubricatcorrosion and the interference with their action by deditions which lead to tergents are demonstrated. ing oil occurs in service no, or insignificant, change under conditions which efeither chemically or physifect simultaneously other cally, other than that chemical changes in the lubricant. In general, an unused oil condirectly associated with the occurrence of corrosion. Thus, tains no corrosive bodies; however, when an oil-metal system is an undoped, fresh oil would be expected to have very low aged under conditions representative of service, corrosion will EC, whereas the same oil after a period of service may have occur t o a degree dependent upon the oil and the severity of the developed an appreciable EC. Existent corrosivity is expressed fundamental factors. This tendency of an oil to become corroin the same units as potential corrosivity; however, the apparasive, called “potential corrosivity” (PC), is defined the extus and conditions of memurement necessarily differ markedly. tent of corrosion which occurs during the service life of the oil. Although the potential corrosivity of a lubricant is influenced Some liberty is taken in this definition since the corrosion which by the initial characteristics of the oil, it win depend to a major occurs in service is a function of the conditions, and varies not degree upon the conditions of aging. I n service, corrosion is only with type of engine but Over different units of the same inseparable from and dependent upon the oxidation of the oil. type. However, standard engine tests have been devised and Hence, in the laboratory, conditions of test for potential corrosivlimits assigned to permissible corrosion; potential corrosivity, ity should promote the oxidation of the lubricant. Herein lies therefore, is not an imaginary concept. Potential corrosivity a fundamental difference between potential corrosivity and pertains to fresh oils and is most accurately measured in the enexistent corrosivity; for the latter the maximum separation of &e. Laboratory tests, however, which age an oil under the variables is sought. Although existent corrosivity is useful in a conditions representative of service, predict with reasonable fundamental study, potential corrosivity remains more imaccuracy its potential COrrOSiVity. Potential COrrOSiVity is exportant practically since the quality of a lubricant is determined pressed as the milligram weight loss per square centimeter BUSby its performance in service, tained by the metal under the selected conditions.
* Present
address, Shell Oil Company, Inc., 50 West 50th Street, New
York 20, N. Y. 3 Present address, Lieutenant, Ordnance Department, The Proving Center, Aberdeen Proving Ground, Md.
APPARATUS FOR MEASURING CORROSIVITY
The corrosion and stability (C and 5) apparatus and test used to evaluate potential corrosivity were described elsewhere (8);
264
'
INDUSTRIAL AND ENGINEERING CHEMISTRY standard C and S tests were correlated wit,h engine performance and applied to the preliminary evaluation of lubricants. Later in this paper are discussed the effects of variables which influence potential corrosivity in the C and S apparatus.
Important requisites for the evaluation of EC are (a)rigid control of variables and ( b ) flexibility in conditions under which studies can easily be made. The EC apparatua, designed t,oinclude simplicity and durability, is represented in Figure I . Fabricated from Pyrex, it employs a vapor bat,h for the control of heat which is supplied electrically. U The volume of the reaction cell is sufficient to hold 20 cc. of oil at the test temperature and to allow aeration at HEATER moderate rates without loss through foaming. Although immersion of the test specimen is not complete when an ingot is used as illustrated in Figure 1, EC is calculated on d&!O VOLT A.C. t,he basis of area exposed to the oil. The capillary venting tube, the U-bend of which Figure 1. Reaction rests on the bottom of Vessel for RSeasuring the react,ion cell, flares into Existent Corrosivity a sintered-glass outlet about 15 mm. (0.59 inch) in diameter and is equipped along the shaft with glass rings to center and support the metal specimen normal to the sintered plate. The test specimen is customarily an ingot about 10 cm. (3.94 inches) in length and 2-2.5 mm. (0.079-0.098 inch) in diameter, cut either from wire or rod or cast from molten metal. Any metal or alloy can be studied; sections from engine bearings are frequently utilized with but slight modification in the procedure. Through the capillary tube is passed a controlled and metered flow of gas which ascends through the oil around the metal specimen. Any gas can be used; however, carbon-dioxide free, dry, and filtered air is generally employed. Auxiliary equipment, not shown in Figure 1, includes facilities for purifying and metering the air and a transformer control on the power input to the electrical heater. These are, in fact, identical with the equipment used for similar purposes with the C and S apparatus (8). During measurements, a thermometer is immersed in the oil sample with the bubbler-metal specimen assembly which facilitates recording the temperature throughout the run. I n conducting a test, the specimen is polished with steel wool, wiped with cotton soaked in 60-80" F. naphtha, and weighed to 0.1 mg. The oil sample is accurately weighed into the clean cell and brought to temperature with bubbler and thermometer in place. The air flow is then started, being measured by a manometric flowmeter. The specimen is introduced and allowed to remain in the oil for the test period. Upon removal, the specimen is washed in naphtha, then in acetone, and weighed. The loss in weight is expressed as mg. per sq. cm. of area which has been in contact with the oil. The specimen may be re-used after repolishing. MEASUREMENT OF EXISTENT CORROSIVITY
The selection of conditions for the standard EC test was governed by certain objectives. Since existent corrosivity precludes oxidation or deterioration during test, conditions must be such that minimal changes occur, other than those directly associated with corrosion. However, conditions must be sufficiently severe to differentiate oils and give results reasonably representative of practical bearing corrosion. The study of the effect of variables has led to the selection of conditions for a
Vol. 36, No. 3
standard test which do not depart unreasonably from the conditions existing in practice. A short test period was indicated to avoid significant changes in the oil through volatilization, oxidation, etc. Conditions selected for the standard EC test follow: Temperature of oil O F. Size of oil test samble, grams Effective area of metal specimen, sq. om. Aeration ml. dry air/min. Duratiod of test, min. Test specimen
313 20 5 6.3 20 P b or Cd
Observation of the effect upon EC of varying test conditions has been suggestive of the mechanism of corrosion. Except when stated otherwise, EC was measured under the standard conditions. EFFECTOF TEMPERATVRE. Figure 2 presents data illustrating the complex manner in which corrosivity can vary with temperature. Corrosivity at various temperatures is expressed as the percentage of that occurring at 313" F. All data of Figure 2 were obtained on lead ingots; curves 1 and 2 are based on test periods of one hour, curve 3, on the standard 20 minutes. SAE-20 mineral oil was used, unaged but rendered artificially corrosive by the addition of undecylic acid to a neutralization number of 2.0 (mg. of potassium hydroxide reacting per gram of oil). Corrosivity under these conditions (curve 1) shows a maximum in the neighborhood of 313" F. The lead specimens after test were covered 1%-ithlacquer films which increased in size with increasing temperature. The observed maximum was postulated, therefore, to be t,he result of competing mechanisms: (a) solution of metal which, on the basis of kinetics, might be expected to have a relatively low temperature coefficient, and
,
TEMPERATURE,
Figure 2.
OF.
Effect of Temperature on Weight Loss of Lead in EC Apparatus
( b ) deposition of products of oil oxidation which should undergo marked acceleration wit,h increasing temperature. This hypothesis could be tested if the deposition of lacquer could be eliminated. Examination v a s made in two ways. The first (illustrated by curve 2) involved the single modification that 2% of DI, a calcium soap-type detergent, was incorporated in the oil. The detergent should prevent the deposition of lacquer and thus maintain the metal surface vulnerable throughout the run. It vias essential that the added detergent itself altered neither the corrosive properties of the oil nor its oxidation stability. This was satisfactorily est,ablished through independent
March, 1944
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
experiments. Curve 2 shows that prevention of lacquer deposition not only eliminates the maximum in corrosivity with temperature, but results in a greater than linear increase over the entire range studied. A second method of curtailing the protection by lacquer films consists in reducing the test period (curve 3). The data were obtained from 20-minute rather than 1-hour runs; no detergent was present. Again the maximum disappears although the downward concavity of curve 3 indicates that lacquer deposition has not been completely eliminated. Since EC, by definition, should be independent of lacquer deposition, conditions of test must be such that interference from lacquer is minimized. Test periods have, therefore, been restricted to 20 minutes. Another factor which influences the temperature susceptibility of EO is the solubility of the metal soaps. This is discussed under the consideration of relative corrosivities of various organic acids. Talley, Larsen, and Webb ( 7 ) described the effect of temperature on corrosivity of oils as measured in the thrust bearing corrosion machine. They reported a sharp dependence of corrosion upon the detergent activity of the oil; our experiments are therefore consistent with their experience. EFFECTOF TIME. E C has been observed consistently to vary either linearly or slightly less than linearly with time of test up to 2-3 hours for oils which are free from the complicating effects of lacquer or sludge deposition. Although the E C of the lubricant is fixed initially and does not depend upon the development of any chemical specie, depletion of reactants might be expected to result in a reduced rate of reaction. This should be the case particularly for extended periods. However, as exposure of an oil to these conditions is increased, interfering reactions lend complication and analysis of the mechanism becomes difficult. Exposure of even an unaged oil for prolonged periods to aeration a t 313" F., as discussed below, would be expected to lead to some oxidation. With an already aged oil, this factor becomes more critical; in addition, the loss of volatile products of oxidation may alter the chemical nature of the oil. For example, venting an aged oil in the absence of metal at 313' F. for 2 hours reduced the neutralization number from 1.5 to 1.1. On the other hand, venting a fresh oil containing nonvolatile stearic acid under the same conditions for a similar period resulted in an increase in neutralization number from 4.5 to 5.0 and in saponification number from 5.0 to 6.7. From the shape of curve 3 i t may be judged that, with a test period of 20 minutes a t higher temperatures, all such interference is not eliminated. However, at 313' F., 20 minutes represents as short a period as can usefully be employed and be reasonably consistent with the aims of the test. EFFECT OF AERATIONRATE. Figure 3 indicates that E C varies almost linearly with rate of air flow over the range studied.
AIR
Figure 3.
FLOW'COMk
Effect of Air Flow Rate on Weight Loss of Lead in EC Apparatus
265
These data were obtained with a n unaged SAE-20 mineral oil containing undecylic acid to a neutralization number of 3.0, Lead was used, and conditions other than aeration were standard. Aeration provides agitation and supplies oxygen. The latter may influence corrosion either through oxidation of the metal, perhaps thus facilitating corrosion, or as a depolarizing agent removing hydrogen from the metal surface and allowing continued contact between metal and oil. Although Figure 3 does not disclose a sharp dependency of corrosion upon air flow, venting with nitrogen has been observed to decrease corrosion markedly. For example, in 60-minute runs at 313" F. the weight loss sustained by cadmium in an artificially corrosive oil was reduced from 6.4 to 2.4 mg. per sq. cm. upon substituting nitrogen for air (6.3 ml. per minute). Thus, i t is indicated that, although oxygen under these conditions enhances corrosion, its effect may not be critical.
+
LEAD, S A E - 2 0 MOTOR OIL CONTAINING UNDECYLIC ACID SAE-SO WHITE OIL CONTAINING UNDECYLK; ACID
0 CADMIUM,
NEUTRALIZATION NUMBER,
MG. KOH/GM.
Figure 4. Variation of EC with Total Acidity as Measured by Neutralization Number
TOTALACIDITY. Neutralization number is ah unreliable criterion of the corrosiveness of an oil in service (8) due principally to factors which are influential in determining corrosion, such as lacquer formation, but which are not directly related to neutralization number. EC, which is reasonably independent of such interfering factors, should correlate more closely with neutralization number. The data of Figure 4 confirm this to a degree. For a single oil-acid-metal system a smooth relation is obtained between E C and neutralization number. However, alteration of oil and metal distinctly differentiates the observed EC of blends of identical concentrations of the same acid. The separation of the curves of Figure 4 may be attributed principally to the higher corrodibility of lead over cadmium which, in turn, is due to the higher equivalent weight of lead. Moreover, EC, although measured with the same metal phase (lead), can vary with the nature of the oil media. Figure 5 presents EC data obtained upon the B-oils of the SAE Committee (6) which were aged initially under the respective standard C and S tests (8) for SAE-10 and -30 viscosity grade lubricants. For oils of similar viscosity a correlation of EC with neutralization number is indicated; however, magnitudes of EC are discrete for the two viscosity grades. This effect can be ascribed in part to the variation in corrosivity of acids of different molecular weighti.e., produced through the oxidation of oils of different distillation ranges. Additional factors probably include the variation in solvency for the products of corrosion and possibly
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
266
I
5.0
Vol. 36, No. 3
removal of lead from the surface. Polishing has a greater effect upon copper-lead than upon either cadmium-silver or cadmiumnickel probably because, in polishing, the softer metal (lead) is removed and a copper “shield” is abraded over the surface. This hypothesis finds support in the fact that etching the polished copper-lead with copper etchant restores it to its original corrodibility. A similar explanation can probably account for the decrease in corrodibility resulting upon the abrasion of cadmium-nickel. Cadmium is preferentially removed and the surface is enriched with the phase NiCd,, which is conceivably less susceptible to attack.
TABLE 111. REL.4TIVE CORRODIBILITIES IN E C TESTOF COMMON BEARING ALLOYS
1.0
2.0 NEUTRALIZATION
3.0
NUMBER,
4.0 5.0 HG. KOHIGM.
Figure 5. Existent Corrosivity toward Lead of SAE Correlation Oils Aged in C and S Apparatus
......
a solvent effect akin to those recognized in media of higher dielectric constant as influencing the thermodynamic activity of the involved specie. Figure 5 indicates that acids produced by the oxidation of the lighter grade oils are more corrosive per unit concentration. This correlates with the observation, discussed later, that corrosivity of organic acids increases with decreasing molecular weight. RATIOOF OIL VOLUME TO AREA OF ~ ~ E T ASURFACE. L As would be expected, increasing the area of metal exposed to a corrosive oil results in an increased loss in weight, other factors remaining constant. The data of Table I demonstrate this; weight loss persq. cm., however, tends to remain constant.
ON EC (LEAD)O F VARL4TION TABLE I. EFFECT RATIO
IN
(SAE-10 motor oil containing undecylic acid t o a neutralization number of 2.0; standard conditions except 2.4 sq. om. of metal surface exposed t o oil) Weight Loss, Mg./Sq. Cm. Polishedb Bearing Material Unused Usedo Polishedb & etched0 Copper-lead 1.4 i 0 . 2 0 . 8 6 f 0 . 0 6 0.56rt0.06 1.55-CO.05 Cadmium-silver 1.8 i o . 0 1.7 i o . 0 ...... Cadmium-nickel 1.6520.05 . . . . . . 1.25*0.05 Tin-base Babbitt 0.06 ...... ...... ...... Indium-plated Cu-Pb 1 . 0 & 0 . 1 ...... ...... ...... a Initially aged in thrust bearing corrosion machine ( 7 ) . b Polished on emery paper successively on grit Nos. 1/0, 2 / 0 , 3 / 0 , and 4/0. 0 Etched with copper etching reagent (1 part 30% HzOz, 1 part concd.
......
”
