I N D U S T R I A L A N D ENGINEERIN# CHEMISTRY
February, 1923
of water. This generally gives figures of a convenient order of magnitude. I n cases where the rate of corrosion is being measured in a long pipe and the oxygen content is greatly different at the exit from that at the entrance, it can readily be shown that the effective average oxygen content is not the arithmetic mean of initial and final values, because it drops more rapidly a t first, and therefore the greater portion of the length is subject to a concentration lower than the arithmetic mean. For such cases the “logarithmic mean” is mathematically the correct one to use. This value may be calculated by means of the formula: Log mean concn. =
initial concn.
- final concn.
133
It is interesting to note that, other things being equal. the specific rate of corrosion is proportional to the logarithm of the ratio of the initial oxygen to the final. It is important to keep this relationship in mind because there is much loose thinking with respect to the effect of the rate of corrosion on the oxygen content. For example, under one set of conditions the oxygen content of water ppssing through a given deactivator or pipe might drop from an initial value of 8 cc. per liter to a final value of 1 cc. per liter, whereas under slightly different conditions it might drop from 8 to 0.3 cc. per liter. This is a tendency to say that the rate of corrosion in the second case is
- Om3>or 1.1 times that in the 8-1
first, whereas actually the specific rate of corrosion, as defined above, is more than 1.5 times as great, although the amount 1 of metal corroded is only 1.1times as great. or more readily from the chart in Fig. 3, which gives the The formula also brings out the fact that accurate results logarithmic (and to make the method general, the geometric) on rates of corrosion by the drop-in-oxygen method can only be obtained under conditions such that the log initial 03 final 02 is accurate, which is only true where the ratio is not too near unity or infinity. The method is most accurate where the amount of oxygen removed is between 25 and 80 per cent, and should not be used for rate determinations outside the limits of 10 and 96 per cent removed. It is thus possible t o correlate specific rates of corrosion obtained in a variety of ways. A few typical sets of results from the Iiterature have been calculated by this method and are presented in the following paper. The considerations pointed out in preceding sections show that, in order for corrosion data in natural waters to be of value, the composition of the water is not important, save fo; its oxygen content, and its tendency to form protective films, while the usually neglected factor of velocity should FIG. 3 be carefully measured and specified. The author appreciates fully the fact that reasoning as to mean in terms of per cent of the arithmetic mean, for various reaction rates is a rather dangerous proceeding, and that a ratios between the initial and final concentration. number of the deductions herein presented have not as yet To calculate k in the units above, from data on the depth been adequately verified by experiment; but the great need of corrosion of iron or steel, since 1 cm. depth equals about for such simplification and correlation of corrosion data, 7800 mg. per sq. cm., the formula becomes and the fact that the writer is now forced to terminate his present investigations along these lines, seem to justify a k = 7800 X measured depth in cm. time in years X cc. 02 per liter somewhat detailed presentation of what he believes to be I n cases where the rate of corrosion is studied by measuring the essential mechanism of underwater corrosion. the drop in oxygen in passing through a pipe or sheet-iron ACKNOWLEDQMENT deactivator, as in the following paper by Speller and Kendall, The writer desires to acknowledge his indebtedness to it is merely necessary to make use of the fact that the ratio of cubic centimeter SO2 to milligrams Fe in Fez031’is 1:3.32. William H. Walker and F. N. Speller for their helpful suggestions and criticisms in connection with the development We may then write: of the foregoing line of reasoning; and also to many members cc. 0 2 consumed per year X 3.32 of the staff of the Research Laboratory of Applied Chemistry area of iron surface in s q . cm. x av. concn. 01 and the Department of Chemical Engineering for the data 3.32 (initial conc. On - final conc. Oa) X liters water on which much of this paper is based. per min. X 1440 X 365 c 2.303 log
initial
final
-
-
area of iron surface X initial concn. Oz final conc. 01 2.303 initial concn. On final concn. On initial On 4,020,000 X liters water per min. X log
P
area of iron surface in sq cm. 16,400 X gal. water per min. X log
C I
initial
area of iron surface in sq. ft.
01
‘
Indeed, samples of 17 Iron does not always corrode to give FezOa. rust taken from the inside of pipes generally analyze around 1 / t FeO to ‘/a FeiOa. Most of the ferrous iron, however, is in the layer nearest the steel surface which has not yet had an opportunity to oxidize completely, and, when steady conditions are established, practically all the iron corroded eventually goes to form FezOs. At any rate, the corrwtion to the conversion factor above is not over 10 per cent.
Staff Correspondents Since our previous publication of the list of staff correspondents representing the various local sections, the following additional correspondents have been appointed for THISJOURNAL and the News Edition: Arkansas: J. W. READ Cincinnati: A. B. DAVIS Delaware: F. C. ZSISBERG Maryland: A. E. MARSHALL Midland: T. A. GANN Minnesota; J. J. WILLAMAN Omaha: W. M. BARR Rhode Island: LESLIS BAMBBRGER
In connection with Cancer Week, the State of New York and the City of Philadelphia each bought two grams of radium for the use of its citizens.
