INDUSTRIAL AND ENeINEERlNG CHEMISTRY PUBLISHED B Y
C H E M I C A L SOCIETY
WALTER J. M U R ? H Y , EDITOR
Laboratory Tests to Predict the Performance of Metals under Service Conditions
D. W. SAWYER AND R. B. MEARS, Chemical Mebllurgy Division, Aluminum Research Laboratories, New Kensington, PI.
To predict the performance of metals under service conditions, a laboratory test must give results that correlate directly with service results. The most certain method of designing a suitable taboratory tat i s to simulate the conditions of service as closely as possible. Attempts to accelerate the laboratory test by intensifying one of the facton encountered in service often load to misleading results. Several examples of laboratory tests illustrating these points are described.
HERE are several different reasons why engineers often T desire to predict, on the h i s of laboratory tests, the behavior of some specific metal or alloy under definite conditions of serv- ice (7). Such predictions are particularly desirable when equipment is required for some new chemical or process about which no service background has been built up. /Laboratory tests are also helpful in determining whether some untried metal or alloy offers promise of being more suitable than the material previously used. Laboratory tests are often useful in determin- ing the cause of attack which has been encountered in service and in developing methods to prevent or alleviate this attack.
These u~e9 of laboratory corrosion tests are fairly well known. They are concernec with the effect of the product under consid- eration en various metals or alloys under the conditions of serv- ice. However, often it is of importance to determine the effect of the metal or alloy on the properties of the product being proc- essed. &ch teats have not been described so frequently. In the present paper, tests of both types are described.
In the past, much emphasis has been placed on accelerated corrosion tests. All engineers would like to be able to evaluate the relative behavior of various metals or alloys in a matter of minutes instead of weeks or months. However, technical people are now beginning to realize that results from tests accelerated by altering certain factors encountered in service will generally lead to false conclusions. I t is becoming axiomatic that the more closely the laboratory test conditions approach the conditions of service, the more dependable the results will be. The teats de- scribed below illustrate the validity of this axiom.
GASOLINE STORAGE TESTS
Airplane gas tanks made of aluminum alloys have been widely used, and in general, have given very eatisfactory service. How- ever, in tanks of a particular design, corrosion was encountered after about one years service. In order to determine whether the newer fuels being used in these tanks were responsible for this attack, a series of tests was conducted (9) in which strips of several aluminum alloys were exposed in glasa bottles to the vari- ous fuels in both the presence and absence of liquid distilled water.
In no case were the specimens of any of the aluminum alloys appreciably attacked after one years exposure. Evidently the
fuels themGlves, even in the presence of water, were inert to aluminum. Some other factor or factors must have caused the attack under service conditions.
Open boxes were constructed of aluminum alloys. These boxes included torch-welded, spob welded, and riveted joints to simulate the construction of actual gasoline tanks and were equipped with cast fittings like those used in the actual tanks.
Since the previous test had indicated that the various fuels, even in the presence of distilled water, had no action it was de- cided to use a typical leaded aircraft fuel plus a 3.5% sodium chloride solution in distilled water. The gasoline layer waa changed every week and the sodium chloride solution was changed every month.
After one year it was apparent that corrosion had develo ed, but the type of attack obtained was not similar to that wkch had occurred in service. Therefore, the test was not considered to be dependable.
I n the meantime, with the cooperation of the aircraft operators, it was found that an aqueous phase collected in the bottom of the particular tanks in servioe and that these tanks were designed so that it waa impoesible to remove this entrapped liquid com- pletely. An attempt wag made to collect samples of thiB watery layer.
Draining8 from individual tanks after each flight gave from a few dro s to a tables onful of this liquid. These drainings were accumurated until a R u t 5 gallons were obtained. A new &Et was started using small enclosed aluminum alloy boxes provided with all the features of the service tanks. The boxes wem equipped with cast fittings and breather vents just as are the large tanks. A small portion of the tank drainings was laced in each small tank and then these tanks mere partially B e d with leaded aircraft fuel.
The tanks were installed in a delivery truck so that they could be agitated in a manner somewhat similar to that under service conditions. In addition, the gasoline layer was changed every week and the aqueous layer every month.
When all these precautions were followed, test remlts simulat- ing those of service were obtained. Once a dependable test waa available, it was possible to evaluate various alloys, types of con- struction, and other factors and thus to find a solution to the problem.
This is probably an extreme case, but it will illustrate the necessity for duplicating the conditions of service if a dependable laboratory test is to be obtained.
Therefore, a new test was run.
MUTUAL EFFECT OF GASOLINE AND METALS
In selecting a material for the construction of storage tanks or shipping containers for fuels, it is important to select one which has no deleterious effect on the fuel. There are several published references to the effect of metals on the rate of gum formation in fuels contacting them, but most of them tests were conducted a t elevated temperatures (11, 16). It is by no means certain that
2 ' " 3 U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 17, No. 1 - Ti lines
GaS0li"e Initial Blanks' 2S-I/.Hb Steel Copper steel i D . / l O O
mi. Mp. pm 100 m1. Straight run 0 . 2 Cracked 3 . 2 Blended 2 . 3 Sfriiighf run. leaded 0.6 Cracked leaded 2 . 5 Blended: leaded 1.8
7 . 6
0.8 5 . 6 3 . 2
0 . 4 7 .2 5 . 2 0 . 6 4 . 6 3 . 2
0 . 2 0 . 2 0 . 2 6 . 2 1525.8 9 . 2 2 . 2 3 1 . 2 1 . 4 1.0 2 . 0 ... 5 . 2 1724.0 6 . 4 2 . 4 144 .0 3 . 4
* Blanks stored in contact with glass. * Commereislly pYre aluminum. Table II. Induction Period of Garoliner before and after Storage for
Five Months F i n d
A1"mi""m S l l O Y Stainleas
Gasoline Initial Blanks" 2S-'/,Ib Steel Copper steel Iloliis Holrrs Hours Hours Hour Hours
Cracked g.0 2 .75 3.0 3 . 5 0 .25 3 . 2 5 Blended 4 . 0 4 . 2 5 4 . 5 0 . 5 4 . 5 Cracked.1eaded 3.6 4:O 4 . 0 4 . 0 0 . 5 3 . 5 Blended,leaded 4 .75 5 .25 5 . 5 6 . 5 0 . 5 5 .25
Blanks stored in contact with g1a.s. I , Commerclrlly pure alumiaum.
the relative behavior of the different metals would he the same a t high temperatures as a t room temperatures, yet storage oontainers are used a t room temperatures. For this reason, a lest WBS run under conditions simulating those of storage.
Some of the results are given in Tables I, 11, and 111. These results indicate that cracked or blended fuels are definitely af- fected by contact with copper. It can he Seen in Table I that the effect of copper on gum formation is most pronounced. The ef- fect of copper on the oxidation induction period is also definite, sa shown in Table 11. Although the effect of copper on 'the oc- tane numhers of the fuels was appreciable (Table III), the mag- nitude WBS le= pronounced than in the ease of the gum formation or oxidation induction periods of the fuels. No definite changes in the other measured properties were detected, so these are not reported here.
The gasoline stored in contact with the other metals WBS not affected, as judged by these property measurements. However, the gasoline stared in contact with low-carbon steel became 6lled with finely divided rust particles.
Figure 1 shows the appearance of the steel specimen a t the conclusion of the test. The heavy rust layer which formed he- tween the coil turns is e l w l y shown.
Figure 2 illustrates the aluminum and copper specimens at the
he discoloration of the gasoline in con- :aused by heavy gum formation.
aluminum specimens showed no evi- ed no more alteration in properties of with glass alone,
MUTUAL EFFECT OF OILS AND METALS
A somewhat similar study is being made of the mutual effect of lubricating oils and metals, employing B method of testing similar to the proposed A.S.T.M. method for studying the oxidation characteristies of steam-turbine oils (5).
Instead of the wire eoii used in the A.S.T.M. test, sheet specimens (10 X 15 em. in size) of several different metals bent in the farm of square tubes have been substituted. The surface area of these tubes was similar to that of the wire samples or- dinarily employed-that is, 1 sq. em. of metal per ml. of oil. Sheet specimens were used instead of wires, since wires of small diameter are affected to B much greater extent in certain environ- ments than are flat surfaces (le). Moisture -'as supplied to the oils, since in service moisture will generally be present. In tests being run st 90" C., 20 ml. of distilled water were added to each s a p : ? of 100 ml. of oil. In tests run a t 120' C . the oxygen, which was bubbled through the oil samples, was saturated with distilled water.
The tests were run in glass tubes 600 nun. long and 45 mm. in diameter, each equipped with an individual condenser. The volume of ail used in each tube was 300 ml. Oxygen was bubbled
Figure 1. Steel Specimen after Five Months' Exposure to Gasoline and
Distilled Water N o k hewv tun iwer which h s d brlnen the
.Oil t Y r n l
Tablolll. Octane Numbomof Garoliner beforeand after Storage for Five Months with Metals
Final Alum$- ""nl stam- alloy less
Gaaaline Initla1 Blanks- 2S-V.Hb Steel Copper ateel Straight r u 57.0 60.6 51 .1 50 .2 61 .1 Craoked 76.0 7312 72 .2 73.1 67 .6 7 3 . 0 Blended 68.3 63.8 6 4 . 3 63 .2 64 .8 Straightrun.1eade.d 7 5 . 3 73:4 72 .9 72 .7 7 1 . 9 C,+e$.+$ed 85.; 83.3 83.3. 83.1. 15.i !a:$ I)le"(lea.LeS(le(l 01.0 SU.3 I J . , OL.' 1 1 . 1 0 Y . O
a Blanks stored in oonlact with glass. b Commercially pure aluminum.
A N A L Y T I C A L E D I T I O N 3
slight and rhe oil exposed in contact with aluminum alloy 5 2 s l/&l was aEected no more than the oil exposed in the absence of any metal.
The tests being run at 90' C. are giving qualitatively similar results. The oil in contact with copper was definitely affected after 8 days, and marked sludging occurred (Figure 3). No sludging developed in the case of oil exposed to the other metal specimens. After degreasing, the appearance of some of these other specimens at the conclusion of the test is shown in Figure 4. The steel was definitely rusted and the tinplate was attacked near one edge. The zinc showed a few shallow corroded grooves and the other two specimens (aluminum alloy 14ST and stainless steel) showed only a mild surface staining.
The interpretation of these test results in terms of service iS not known. However, qualitatively similar results were ohtnined by Hunter and eo-workers (10) under conditions more nearly simulating those of service. It would he expected that where oils are used a t elevated temperature, in the presence of air and moisture, these test results would correlate qualitativdy with service performance.
CATHODIC PROTECTION TESTS
Fisure 2. SDocinrns at Conclusion of Test
Table IV. Oxidation of a Solvent-Refined Pennsylvania Lubri- cating Oil at 120" C. in Presence of Moistare and Various Metals
Hours Neutralization Metal Oxidized Number
Unoddised oil NO metal Aluminum alloy 14s-TO Aluminum alloy 2S-Ob Aluminum alloy 52S-1/zHc Copper Zi,X Low-carbon steel stain1es. steei Tinolate
1544 1344 1344 1344 117
0.05 0.78 1.35 1.08 0.80 2.70 1.05 2.13 1.23 1.48
a Aluminum slloy. nomind oompoaition: 4.4% Cu. 0.8% Si, 0.8% Mn. b Commercisily pwe al?mioum.
Aluminum aiioy. nommal cbmpoaition: 2.5% Mg. 0.26% Cr.
the conclusion of the test, other properties of the oil samples. such as interfacial tension (du Noiiy method), steam emulsion number (e), A.S.T.M. color number. (l), ,and viscosity (S) were also determined and the ehanees in weiaht or ameaxance
~ - .. of the metal samples were noted.
At the present time, one series of tests bas been completed, in which a solvent-refined Pennsylvania lubricating oil was used and the tests were run at 120' C. (Table IV).
Figure 3. Sludge
Another series of tests at 90' C. has hien started.
In the test run a t 120' C., it was found that copper definitely increased the neutralization number of the oil after 117 hours' exposure, so this sample was removed from test (see Table IV). The oil in contact with the low-carbon steel sample r$n for 472 hours, after which time ita nentralin;ation number was markedly increased; it was therefore removed from the test. The other samples were continued in test for 1344 hours. A t the end of this extended period of exposure, i t was found that the neutralization numbers of the oils had been aEected somewhat more in the casea of the samples exposed in con- tact with tinplate, aluminum alloy 1&T, or stain- less steel than for the remaining samples. The &ect of zinc and aluminum alloy 2SO was very
The use of solution potential measure- menta to predict whether cathodic protec- tion (8, 14) can he successfully applied furnishes an example of another. type of laboratory test. If two dissimilar metals are coupled together and immersed in an electrolyte, in general, au electric current will flow between them. The metal from which positive current leaves to enter the electrolyte is the anode and the other metal is the cathode. Normally attack of the anodic metal is stimulated by such a contsct while attack of the cathodic metal is reduced. This reduction in attack of the cathodic metal caused by current flowing to i t from the solution is termed cathodic protection. The mechanism of this protection has been discussed in pre- vious papers (8,13).
In the classical electromotive series, aluminum is listed as having a more anodic solution potential than zinc. However, in many natural waters, including sea water, zinc is auodic to aluminum. There- fore, in such waters zinc attachments CBO be used to protect aluminum chemical equipment csthodically. Furthermore, it should be possible to determine under what conditions cathodic Drotection of