January, 1945
A N A L Y T I C A L EDITION
(9) Mr~nroe,C. E., and Ti5my. 3. E., U. 5. Bur. Mines, Bull. 346 (1931). (10) Snttlling, W.O., Prac. Ew.Soe. West. Pmm.. 28,673 (1912). (11) starm. C. G.,and Cope. W.C., U. 9. Bur. Mines. Tech. Papm 1 2 5 (1916). (12) Taylor. C. A., and Munroe, C. E., U. S. Bur. Mines, Rcpt. I n o c A k 7 a f h 2558 (1923). (13) Taylor. G.B., and Cope, W.C., U. 8.Bur. Mines, 2 ' 4 . Pnpsr 162 (1917). (14) U. S. Treasury Dept.. Procurement Div.. Erplosives and Blast-
Measurina the
If
19
ins Aooesaories. General Schedule of Supplies,Class 4, Suppl. 1 (Jan.1 toDeo. 31,1944). (15) Wehler, L.. 2. ges. Schim-Spwstoffw.. u), 145-50. 165-9 (1925): 21,1-5.35-8,55-7.97-9, 121-3 (1926). (16) Wahler. L.,Roth, J. F.. snd Ewald. K., I t i d , , 22, 95-9, 135-9 (1927). A s s r a a n i o from Bnmm of Minm Technical Paper 677 (in prcss): puh. M~W. fished by per-aian of the Director. u. s. ~~e~~
xistent Corrosivity" of IJsed
ine
Oils
R. G. LARSEN, F. A. ARMFIELD,
AND L. D. GRENlD T Shrll Development Company, Emeryville, ~
A test for determining the "existent conorivity" of used engine oils independently of previous history provides a means ior evaluating in simple lashion, by the use of test *ips coated with lead or other mebl in gredueted thicknesses, a property of used oils not heretofore setisfadorily rnearured by routine engine oil tnh. It also has pncticel application in determining the caun of bearing failures and indiceting necossety oil drain periods.
tives produce t tect against oorroslon. I t was the purpose Of the present wort to develop a simple teat which would give a reliable indication of the corrosivity of used oils, and thus t o guide the engineer in determining the cause of bearing failures; and to help the operator decide when an oil change is necessary. Most of the testa reported in the literature (4, 8, If, 1s) are used to predict what is termed by Waters and Burnham (M)as "potential corrosivity"-i.e.. the extent of corrosion wbich occum during the service life of the oil. Such methods are useful in the research necessary to provide oils qf improved performance, but they do not answer the needs outlined above. Engine testa give only the combined results of corrosivity of the oil modified by the action of any protecting surface films formed on the bearing. Since these two phenomena often represent a delicate balance. lack of corrosion-in one particular e-ngine may not ensure rhc m n e fortunate condiriom in a similar engine under slightly different condirions. Furthermore, the engine must be torn doun and bearings removed before any observation of corrosion can be made. What these teats do not measwe is the unmodified corrosivity of the oil at any given time, or what has been called "existent corrosivity" (18)). As already mentioned, many operators use the acidity of an oil as a criterion of existent corrosivity. Waters and Burnham (f3)
ECAUSE of efficient and compact design, internal combustion engines can today be COnStNCted which produce considershly more power for a given weight than was possible before. As part of this development, the bearings now employed are smaller and are required to carry considerably greater loads and owrate at hieher temperatures. These advances, however, have oitcu been arcomplished with little regard to chemical relationships. Thw metals now used in bearingj can withstand higher loads and temperatures but are more susceptible to chemical attack tlran those greviowly used. This led to bearing failures at first, but the situation has subsequently been largely corrected through the efforts of oil manufacturers to understand the nature of the failures involved. Lubricants now produced give improved performance in many ways and ~ v the e engine designer greater latitude. Unfortunately, occasional bearing failures still occur. TLese may be traceable to mechanical Step Type Wedse Type factors such a 8 poor alignment and fit, poor cooling due to insufficient oil flow, poor bonding of the hearing alloy t o the hacking, or imperfect structure of the allay, or ta weakened structure as a result of chemical attack. Normally, the only certain method of deciding which of these factors caused failure is microscopic examination of the cross-sectioned bearing. Improvement in oil performance has generally been accomplished by the use of additives. Often these additives are detergent in nature and prevent the formationof films which normally U K D M lA8ORAT6SY protect the bearing, thus leaving it susceptible USmLEm to corrosive attack. A second additive may then he used to prevent corrosion by reducing oxidation or by passivating the surface. When these additives are depleted, normal or even increased corrosion may ocsur; it is, therefore, , . . . ..UF..-. - . . . .. desirable to change oil before the critical stage bas been reached. I n practice, the oil is usually changed when acidity develops, since it is assumea that acids cause corrosion. Yet it is well 1F5T STRlP ATTAWEE TO MP mcX knowd that acid number alone is not a reliable indication of corrosivity. I n fact, certain addiFigure 1. h e n - S t e p and Wedge-Type Corrosion Tolt Strips ~~
.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
20 ~~
~~
Table I.
~
~~
Corrosion Rate of Lead Castings in Lauson Engine By-Pan Line (One hour at 140’ C.)
Oil Condition Corrcaive Mildly corrolive Noncorrosive
Corrosion Penetration (Calculated) Microna/hour 1 5-61 0.1-0.76 0-0 06
Corrosion Losl Mo./rp. cm./hour 1.7-6.7 0 114.88 0-0 067
have shown that a relation exists between this property and neutralization number for oils in which acidity has been developed under rigidly controlled laboratory conditions. However, even such factors as the viscosity of the oil determine the magnitude of existent corrosivity for a given neutralization number. In the case of acids added to fresh oils, existent corrosivity de: pends upon the chain length of added acid. In engine operation, matters are complicated not only by the factors mentioned above but also by the effect of acids obtained from partial combustion of the fuels.
’;I 3;:!
HEIGHT ADJUSTMENT AND
=)
AMMETER
BATTERY
LEAD
u PLATING BATH
Figure 9. Plating Citcuit for Making SevenStep Corrosion Test Strips
Two test machines designed to measure existent corrosivity in the laboratory are the thrust bearing corrosion machine of Talley, Larsen, and Webb (ZO),and the existent corrosivity apparatus of Waters and Burnham (18). The corrosion test described in the present communication is not intended to supplant these or similar tests, but merely to indicate the corrosivity of a used oil with the use of nothing more complicated than a hot plate, a test tube, and thermometer, or to be adapted to the crankcase of an operating engine by the simple expedient of using a modified dip stick. CORROSION TEST STRIPS
Early in the development of the test it was decided that the most practical embodiment would consist of metal deposited in such a manner that its removal by corrosion could be followed simply by visual observation, An acutely tapered wedge of corrodible metal, suitably mounted, would satisfy this requirement, for reduction in the length of the wedge would measure the corrosion penetration into the surface (9). Several samples of this type of test strip were made by evaporating alternate wedges of copper and lead and of lead alone onto glsss microscope slides. Subsequent laboratory tests using these strips in corrosive oils showed that only the lead WM attacked on slides containing both lead and copper, in agreement with microscopic studies of corroded copper-lead bearings wherein the lead is selectively removed. Glass-supported metal wedges have the advantage that
Vol. 17, No. 1
the end of the wedge is easily seen by transmitted light, but they are fragile and not readily produced. Copper wedges, if present, act as a convenient reference, although in some cases corrosion of this metal may also be expected. I n view of the results obtained with the wedge-shaped test strips, other strips were made by plating lead on copper in various thicknesses (9). Greenshields and Wilson (6) had earlier found that a lead casting placed in the pressure by-pass line of a Lauson engine incurred the losses recorded in Table I, column 3. These data were converted into rate of corrosion penetration (colunn 2) which was then used as the basis for thickness of the wedge and lated test strips. The first strips made of the latter design had Eve “steps” of lead plated on copper, the layers being 0.05, 0.18, 0.58,1.78, and 5.08 microns thick, as calculated from the quantity of electricity required for the deposition. Preliminary testin indicated that the difference in thickness between the second and third step was too great and that the last step would be of little use; hence, subsequent strips were prepared with seven 1.27-mm. (0.5-inch) steps of 0.08, 0.18, 0.36, 0.58, 1.19, 1.78, and 2.54 microns (3, 7, 14, 23,47, 70, and 100 X 10-6 inch). This is the type of strip used in the present investigation, but experience indicates a reduction in thickness of the fifth step to be desirable; further modification to thickness of 0.08, 0.18, 0.36, 0.61, 0.94, 1.52, and 2.54 microns is sug ested. (Inquiries regarding supplies of the strips should be adfiressed to Randall and Sons, 2512 Etna St., Berkeley 4, Calif. The 1st-named arrangement of steps is designated as Type C.) Figure 1 shows new and corroded test strips of the wedge and step types, with an exaggrated constpctional view of each, and also a corrosion test strip attached to an oil dip stick for use in the crankcase of an engine. Although most of the work has been done with test strips of lead plated on copper, the method of preparation and testing can be applied to other metal combinations as well, it being preferable to deposit the more easily corroded metal upon the less easily corroded. Figure 2 is a schematic view of the plating circuit used in making the step-type strips. Necessary equipment includes a hardrubber or lead bath to hold the plating solution, a lead anode, a source of direct current, an ammeter of suitable range, and a means for adjusting the current supplied to maintain constant current density as the strip or sheet being plated is lowered step wise into the plating bath. Details of the plating technique employed are beyond the scope of this paper. Table I1 summarizes the data for plating lead on both sides of a single copper strip, 100 x 6.3 mm. (4 X 0.25 inches). The plating solutions used was made up in the following proportions : Bssic lead carbonate Hydrofluoric acid (50% concentration) Boric acid Glue Water to make
120 grams 192 grama 84 grams 0 . 1 5 gram 800 cc.
The first and thinnest plating of 0.08-micron ( 3 X 10-8) inch thickness was ap lied with 8.9 cm. (3.5 inches) of the strip’s length immersed gelow the surface of the plating bath. After this plating the strip was removed from the bath, nnsed, and dried. Subsequent steps are applied by lowenng the strip into the bath in 1.27-cm. increments. For the current density used (1.68 x 10-1 ampere per square centimeter), 1.6 seconds were require‘d for each 0.025 micron (0.0OOOOlinch) of plating thickness. I t can be seen from the last four columna of the table that the amount of lead deposited, as determined by actually weighing the strip after each operation, agrees well with the values calculated from the amount of current passed. The method can easily be applied to plating sheets of metal which are later cut to the desired width, If the finished strips are to receive much handling, it may be advantageous to coat them with paraffin, which is readily removed by the hot test oil. USE IN CONNECTION WITH ENGINE TESTING
An advantage of the present test method is that it can be used without removing the oil from the crankcase of the engine. Thus, in engine testa in which it is not desirable to remove samples for
ANALYTICAL EDITION
January, 1945
testing, the development of corrosivity in the crankcase oil can he followed directly by inserting into the crankcase oil, at periodic intervals, test strips which have been attached to the engine dip stick. This method of testing has the further advantage that corrosivity is measured under engine environmental conditions and without an intermediate cooling period. However, it may not always be possible or convenient to test oils in this manner, and in such cmes the oil is tested outside the engine as discussed below.
6 t-
TaMe
II.
21
Plating Directions for 7-Step Corrosion Test Strips, Plated on Both Sides
(Current denaity. 1.68 X 10-4 ampere per sq. mm. R a t e of deposit, 15.86 X 10- mm. Der aecond. Time necessarv to deDosit 0.026 micron. 1.6 aeconda) Weight of Lead Deposited Area Thick- Time to Calculated Experimental Plat- to Be Current ne88 of Give This This Thy ing Plated Necessary Plating Thickness plating Z plating Z Sa. mm. Ampere Macron Sa. Ma. Ma. Ma. Ma. 1 1280 0.215 0.08 4.8 1.1 1.1 1.1 1.1 2 183 0.0307 0.76 48 1.6 2.7 1.4 2.5 3 366 0.0615 0.58 37 2.4 5.1 2.3 4.8 4 650 0.0925 0.61 38 3.8 8 . 9 4.0 8.8 6 733 0.123 0.23 15 1.9 10.8 1 . 9 10.7 0.18 6 916 0.154 11 1.85 12.6 1.7 12.4 7 1097 0.186 0.10 6.6 1.27 13.9 1.4 13.8
set. In the latter case, the bearing is modified by the protective action of any surface films formed before the oil has become corrosive, and thus is not subject to corrosive attack of the oil to the same degree as is the unprotected test strip.
5 / j
u? 4
2 0
Figure 3.
For example, in the Chevrolet heavy-duty test, oil B-2 is corrosive to copper-lead bearings. The plated test strip also indicated the used oil to be corrosive, and COhtinUOUS immersion of the strip in the crankcase of the engine for 2 hours from the start of the test resulted in complete removal of the lead plating. Oil B-4, on the other hand, is not corrosive to copper-lead bearings in the above engine test, but the plated strip indicated the oil to be very corrosive. Immersion of the strip for a period of one hour resulted in removal of lead equivalent to 6 to 7 steps of the 7-step strip in every test after the second hour. To obtain additional information, oil B-4 was subjected to the Chevrolet test for 2.25 hours and then changed to a 1aborat)ory corrosion tester using copper-lead bearings. This test showed the oil to be very corro-
c
* ,
-
