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I N D U S T R I A L A N D ENGINEERISG CHEMISTRY
In the presence of dissolved oxygen, a film of ferrous rust is maintained against the metal and is suficiently soluble to keep the thin liquid film on the metal surface somewhat alkaline thus repressing the tendency to evolve hydrogen gas. In the absence of oxygen, however, this alkalinity of the liquid film is not necessarily maintained because the corrosion is much less rapid and the alkali which is produced has a chance to diffuse away. Under these conditions, therefore, the tendency for hydrogen gas evolution is increased and hydrogen can be evolved in appreciable amounts.
As would be expected, with the low hydrogen-ion concentration, evolution is slow but is apparently proportional to the p H value. Figure 4 shows an experimental determination of p H versus rate of hydrogen evolved in the absence of dissolved oxygen.23 This point is well illustrated by the corrosion of cast iron where the ferrite corrodes along the line of contact with flakes of graphite without the aid of oxygen. Although the reaction is slow in the absence of oxygen, the solution of a small amount of metal, as in the case of cast iron, often has a serious weakening effect on the structure of the metal. With non-scale-forming water at high temperatures and high velocities, as found in steam boilers, the corros-on taking place in the absence of oxygen may in time become noticeable unless the alkalinity of the water is maintained a t a safe point by proper treatment. CoAclusion
During the past twenty years the electrochemical theory has been considerably broadened in detail and now may be said to be generally accepted in explanation of the fundamental reactions and factors of corrosion. It will be seen later in this work that the application of this theory to the practical solution of the corrosion problem has been of great assistance. The number of factors involved and their effect on one another is often bewildering. Some of the %econdary” factors, such as film protection, often exert a predominating influence and may, in fact, stop the primary reactions entirely. It is not necessary for progress in study of this problem that the effect of all the secondary factors mentioned should be explained by one theory, as they are often independent of the initial corrosion reactions. Supplementary theories may account for them, but they do not explain the fundamental factors on which the rate of corrosion is based, as does the electrochemical theory. The important consideration is to determine the effect of these factors and their relationship to each other. Carbonic acid is the basis for certain reactions of importance in certain cases; colloids and emulsoids may aid as carriers of oxygen and influence film formation; film interference must always be given due consideration; but no matter how the reader may choose to interpret the effect of the many influences bearing on this problem, it is encouraging to feel that the underlying fundamental reactions, without which corrosion cannot occur, have a satisfactory explanation in the electrochemical theory which has to far stood the test of time. 23
Shipley and McHaffie, Can. Chcm. Met., 8, 121 (19241.
* The Corrosive Action of Various Soils-Panoramic pictures have been made of the specimens of pipes removed from the ground in 1924 in the course of the Bureau of Standards’ soil corrosion investigation. These pictures, which are 6 inches wide and as long as the circumference of the pipe, show quite clearly the size and distribution of the pits and, where supplemented by the available data on the individual corrosion losses, are very helpful in the study of the effects of the soil on pipe materials. Individual pictures or complete sets of prints may be purchased by those desiring them. Copies of the progress report on the soil corrosion investigation may be purchased from the American Foundrymens Association, 140 S. Dearborn St., Chicago, Ill., a t 30 cents per copy.
Vol. 17, No. 4
Accelerated Corrosion Tests of Copper-Zinc Alloys b y Salt Spray By U’. H. Bassett a n d H. A. Bedworth .4YERICAN
BRASSC O . ,
u’ATBRBURY,
CONI\‘.
Corrosion tests of a series of commercial copper-zinc alloys by an accelerated salt spray t e s t a n d immersion tests in sea water, extending over a period of t e n years, have s h o w n that alloys c o n t a i n i n g between 70 a n d 85 p e r c e n t copper a r e best a d a p t e d t o resist salt water corrosion. Results obtained by accelerated spray tests were in excellent general a g r e e m e n t w i t h effects produced by longt i m e immersion t e s t s in sea water. T h e procedure a n d results of the spray t e s t s are described in s o m e detail, a n d the relation of d u r a t i o n of t e s t s to results obtained is discussed.
T
HE subject of corrosion of metals is one of great im-
portance and in recent years notable advances have been made by a number of investigators in the study of the principles underlying corrosion. Viewing the situation from a practical standpoint, the need for accelerated tests, not only to determine the alloys best suited to withstand certain corrosive conditions, but also to maintain the present rate of progress in this field, is quite apparent and well appreciated. Tests requiring comparatively long periods of time may give reliable results, but in practice the time consumed is prohibitive. I n order that an accelerated test may be adequate it is of prime importance that it shall give results which will closely parallel those obtained in service. It is the purpose of this paper to show a comparison of the effect upon a copper-zinc series of alloys of long-time immersion tests in sea water with salt spray corrosion tests, and discuss some points in the technic of testing by salt spray. Apparatus a n d Description of Tests
The apparatus used for the salt spray tests was similar t o that described by Finn.’ It was an inclined Alberene stone box covered with a glass plate and fitted with an atomizer operated by compressed air, the samples being supported by glass rods. The solution used contained 20 per cent of common salt by weight, and was renewed from time to time by adding more solution. No attempt was made to control the concentration of the solution, as all specimens were tested under the same conditions, so that results for the various alloys were strictly comparable. The test was intermittent in character; that is, the spray was not operated overnight and over week-ends. The tests were conducted a t room temperature, which varied from 15’ to 27” C. The apparatus and procedure for the long-time tests have been fully described by Bassett and Davis.2 I n these tests the specimens were suspended on glass rods, the ends of which were k e d in a wooden frame. This entire arrangement was totally immersed in a reservoir containing sea water obtained a t regular intervals from the East River a t New York Harbor. Analyses of the water, which represents more or less polluted sea water, are given in the article by Bassett and Davis. These tests were carried out a t room temperature, 18’ to 23’ C., and the water was circulated and aerated by means of a centrifugal pump. Proc. A m . S O L .Tesling Materials, 18, Pt. I, 237 (1918). “Corrosion of Copper Alloys in Sea Water,“ presented at a meeting of the American Institute of Mining and Metallurgical Engineers, February, 1
9
1925.
mercial copper-zinc alloys, ranging in composition from copper down to l l u n t z metal, or 60:40 brass, in the form of soft annealed sheet. All alloys were cold-rolled, with the exception of 60:40 brass, which was hot-rolled. The metal had received the customary finish in the mill-that is, had been pickled in dilute sulfuric acid. The specimens were cleaned, before testing, by rubbing with fine flour emery, which cleaned but did not scratch or polish. T a b l e I-Analvses of M e t a l Used (Per cent)
Nominal mixture 97:3 95:s 94:6 90: 10 87.5: 12.5 85315 80 : 20 7.5325 70: 30 66.7:33.3 63 : 37 60: 40
Copper 96.80
Zinc 3.17
Lead 0.01 0.01 0.02 0.02
Iron
Tin
0.02
. . ....... . . . ....
.............. ...... ... .. . . . . . . .. .
Copper.. Silver.. .4rsenic. . Antimony.. Lead,.
........ . . ..
of P e n e t r a t i o n - E n d of O n e Y e a r Calcd. from Depth of deepest pit loss in weight Actual measurement Nm. hfm. ALLOY Copper 0.0183 0.13 0.15 97:3 0.0160 95:s 0.15 0.0160 0.15 0.0173 94:6 0.18 90: 10 0.0145 0.18 8 7 . 5 :1 2 . 5 0,0146 0.1: 8:: 1: 0.0130 80:2u 0.0127 0.10 7. 5 . 2 6 n nim 0 0,08 io dezincified dezincified 70: 30 0,0155 66.7 :33.3 0.0163 Not pitted-dezincifieda 63 : 37 0.0254 Not pitted-dezincified" 60:40 0.0297 Not pitted-dezincified" a The depth of dezincification was not measured as it varied too greatly a t different points of the specimen, being especially deep a t edges and corners. The average depth, however, was as great or greater than the depth of pitting in the alloys of higher copper content. T a b l e 111-Depth
,0.02
0.05 0.06 0.04 0.10 0.12 0.15 0.05 N. E. C. ELECTROLYTIC COPPER 99.964 Iron.. , , ,, , 0.0004 Nickel.. , . ,, 0,0008 Selenium ... . , .. 0.0002 T e l l u r i u m , . . , . .. , Sulfur.. . . , . . . . . . . , 0.0011 Oxygen 0.037 Total. . . ,100.0098
T a b l e 11-Total Loss in W e i g h t (Mg. per' sq. cm.) ALLOY 4 Weeks 8 Weeks 1 2 Weeks 24 Weeks 28 Weeks 52 Weeks 16.28 Copper 1.81 7.43 9.95 4.05 5.46 97:s 1.97 3.29 14.09 8.59 10.52 4.79 95:s 2.25 3.50 14.03 8.37 10.03 4.87 94:6 2.34 4.05 5.61 15.50 8.87 10.67 90: 10 1.92 6.94 8.96 3.25 12.71 4.51 67.5:12,5 2.06 3.55 12.66 6.57 8.87 4.65 11.18 83:15 1.97 3.27 5.68 :.OO 4.05 11.00 s0:20 1.49 3.19 5.66 ,.90 4.05 ia:25 1.38 11.36 5.06 6.85 2.76 3.71 13.23 5.92 7.66 70 : 30 1,52 3.27 4.34 66.7:33.3 1.39 8.20 7.98 4.11 13.84 3.21 63:37 2.14 4.36 7.16 10.35 13.05 21 40 60:iO 2 00 4.51 24.80 13.08 16.00 8.64
0.15 0.0019 0.0022 0,0005 0,0004 0.0013
The copper-zinc alloys used for the sea water tests covered The rates of corrosion as calculated from the results obthe same range as for the spray test. There were 21 specimens of each alloy, 4 in the annealed condition, 4 each t'ained from tests of different durations are set forth in Table rolled 1, 2 , 4,and 6 B. & S. numbers, and 1 rolled 8 B. & S. IT,and shown graphically by Figure 3. numbers hard. A complete description of the materials is T a b l e I\--Rates of Corrosion for Different D u r a t i o n s of given in the art'icle by Bassett and Davis.? Continuous Test Salt Spray Tests
The specimens were exposed to the action of salt spraj. for various periods. At the end of each period the specimens were removed and cleaned, first by dipping in dilute sulfuric acid t o remove corrosion products, and finally by rubbing lightly with flour emery. After weighing they were replaced in the spray box for subsequent tests. The losses in weight are shown by Table I1 and graphically by Figure 2 . The depth of penetration as calculated from the loss
IO0
95 F i g u r e 1-Corrosion
SO
85
80
.
75
(Loss in mg. per s q . cm. per month) ALLOY 1 Month 3 Months 6 Copper 2.00 0.65 97:3 1.67 1.27 95:5 1.63 1.16 94:6 1.84 1.09 90: 10 1.63 0.81 8 7 . 5 : 12.5 1.74 0.64 85:15 1.60 0.54 80 :20 1,57 0.54 75:25 1.38 0.45 70:30 1.52 0.48 66.7: 3 3 . 3 1.47 0.70 63:37 2.46 1.07 60:40 2.85 1.49
70
C O P P E R -PER CENT of C o p p e r Alloys in S e a W a t e r . R a t e s of Loss of B r a s s e s C o m p a r e d .
65
Months 1.05 0.60 0.67 0.81 0.62 0.64 0.53 0.53 0.76
0.93
0.98 1.39 1.46
60
T o t a l T i m e , 98 M o n t h s
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
348
Tests by Immersion in Sea Water
I n these tests the specimens were removed at the end of various intervals, cleaned with a stiff bristle brush, and weighed. They were then replaced for further test. Figure 1, taken from Bassett and Davis,2shows graphically in a relative way the cumulative rates of corrosion at the end of various intervals. Discussion
The results by both the salt spray tests and by the longtime immersion test showed a decided minimum loss in weight for the alloys between the limits 85:15 and 70:30. Observation of the specimens after corrosion in both tests showed that a t 75 per cent copper and below there was dezincification and above the composition 85: 15 there was pitting. T h e effects increased, in a general way, towards the extremities of the series. Depth of either dezincification or pitting is a most vital factor in determining the life of copper-zinc alloys. It will be noted in Table I11 that the actual depth of pits was from six to twelve times the depth of calculated average penetration by solution and the depth of dezincification was apparently as great or greater than the depth of pits. I n the process of corrosion by either salt spray or sea water, the copper-zinc alloys become covered with a coating of corrosion products, which appears to have a protective influence on the most resistant alloys, whereas the attack on those alloys whose tendency is to become pitted or dezincified is accelerated by these deposits. The mechanism of these phenomena is doubtless electrochemical in nature.
100
a0 COPPER
Fiaure 3-Salt
70
- PLR LCNT
60
Spray Corrosion Tests. Copper-Zinc Alloys. of Corrosion
Rates
Figure 3 shows some very interesting relations between the rate of corrosion by salt spray and the continuous duration of the test. The rates for tests of 3 and 6 months’ duration showed remarkably good agreement, whereas the rates ob-
Vol. 17, No. 4
served a t the end of 1 month were about twice as great, and might have led to erroneous conclusions in comparing the different alloys. The effects of coatings of corrosion products are clearly brought out, in slowing down the general rate of loss in weight as time advances, and in accelerating pitting and dezincification in those alloys which are subject to these effects. A minimum duration of continuous test of a t least 3 months has seemed essential in these tests in order to obtain reliable results and to allow sufficient development of pits and dezincification. Summary
1-Tests of a series of commercial copper-zinc alloys by salt spray and long-time immersion in sea water have shown that those alloys containing between 70 and 85 per cent of copper offered the greatest resistance to corrosion by salt water. Alloys richer in copper showed a tendency toward pitting and those richer in zinc a tendency toward dezincification, and these factors really determine the life of alloys in the copper-zinc series. 2-Results obtained by the accelerated salt spray tests were found to be in very satisfactory general agreement with those obtained by a long-time immersion test. It has been possible to reach conclusions in a few months by means of the accelerated tests, whereas several years were required by the immersion tests. It is gratifying that these results are also quite in accord with accumulated data representing the performance of a number of these alloys under conditions of actual service, not only in sea water, but in natural waters of corrosive nature.
The Acid Corrosion of Metals Effect of Oxygen and Velocity ’
By W. G. Whitman and R. P. Russell
MASSACHUSETTS INSTITUTE
OF
TECRNOLOCY, CAMBRIDGE,
MASS.
Part I presents an experimental survey of the effect of dissolved oxygen in the corrosion of steel, aluminium, lead, copper, nickel, tin, and several alloys by sulfuric, hydrochloric, nitric, and acetic acids. The method used was to compare the corrosion rates in two solutions, one of which was saturated with oxygen and the other with hydrogen. Part I1 deals with the effects of velocity on the corrosion of copper by sulfuric, acetic, and hydrochloric acids, and on the corrosion of steel by concentrated sulfuric acids. An apparatus in which the samples are suspended in the acid from a horizontal rotating wheel was used, and provision was made for air saturation or for total exclusion of oxygen. The results emphasize the importance of oxygen in corrosion by dilute nonoxidizing acids. They also show that dissolved oxygen may act as a passivifying agent in some cases, thereby actually reducing corrosion. Velocity increases corrosion when oxygen is a vital factor, and also markedly accelerates corrosion where protective films may beremoved.
HE importance of dissolved oxygen in the corrosion of metals by neutral waters has been emphasized by many investigators, and the prevention of corrosion by removal of oxygen is widely practiced in hot water systems and boiler installations. The role of oxygen in acid corrosion, on the contrary, has received little attention, despite such well-known facts as the resistance of copper to corrosion by nonoxidizing acids when air is excluded. Similarly, the effect
T
