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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 test and immersion tests in sea water, extending over a period of t e n years, have shown that alloys containing between 70 and 85 per cent copper are best adapted t o resist salt water corrosion. Results obtained by accelerated spray tests were in excellent general agreement with effects produced by longt i m e immersion tests in sea water. The procedure and results of the spray tests are described in some detail, and the relation of duration of tests 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 and 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.
