I N D U S T R I A L A N D ENGINEERING CHEMISTRY
November, 1930
20
1197
of temperature change occurs with increase in sugar concentration. Moreover, the rate of heat transfer is not a p preciably affected by the viscosity of the liquid. Acknowledgment
.T e m p e r d u n ~n ( e n t m oftrr .T e n ~ e r o l u Rjn i e n k r d t c r
3. Temprrotvn
a
IO
20
30
40
So
60
70
/ n centeroite.
80
90
h o u o tbw8np 8 hours thoring I8 bourstkarinq
’4
100
Figure 12-Effect of Sugar Concentration on Rate of Temperature Change d u r i n g Thawing
$ W
The writers gratefully acknowledge the aid of A. W. Luhrs in securing funds for these investigations from the Paperboard Industries Association. Thanks are due to the American Can Company, especially Mr. Culver, for kindly furnishing the cans used. Literature Cited (1) Birdseye, IND.E N G .CHEM.,21, 414, 573 (1929). (2) Diehl, U. S. Dept. Agr., Tech. Bull. 148 (1930); Glass Packer, 3, 179, 190 (1930). (3) Evans, Physico-Chemical Tables, p. 204, London, 1902. (4) International Critical Tables, Vol. 11, p. 346 (1927). (5) Nelson and Lang, Food Industries, 2, 184 (1930).
The Initial Corrosion Rate of Steels’ H. 0.Forrest, B. E. Roetheli, and R. H. Brown DEPARTMENT OY CHEMICAL ENGINEERING, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
HE rapid development
Recent publications on the corrosion-resistant propsteels should be of the same and increased producerties of chromium steels have laid considerable stress order of magnitude as for ortion of comosion-rc+ on t h e oxide films a s a preventive for corrosion. Studdinary steels, decreasing very &ant steels by alloying iron ies of t h e initial period, during the first few minutes, rapidly to a negligible value. with chromium and nickel in If one of the additional ashave shown that the corrosion rates of an 18 per cent varying proportions have led sumptions is granted-that chromium-8 per cent nickel steel and of a 14 per cent to numerous studies of the in neutral distilled water the chromium steel are of the same order of magnitude as single potentials of the steel those of an ordinary steel. The actual values of oxygen reasons for their b e h a v i o r . Many theories have been adused in t h e above order are 0.0095, 0.0145, a n d 0.0192 cc. alloys and steel are of the vanced, that which seems to per liter per square decimeter per cubic centimeter of same order of magnitude, or that the rate of diffusion of offer the most plausible exoxygen present per minute. After a 10-minute period p l a n a t i o n being the wellthe rate for ordinary steel decreases only slightly, while oxygen to the metal surface k n o w n “oxide film” theory those for the high-chromium steels become essentially controls the reaction-t h en zero. These results give further evidence of t h e exthe initial corrosion rates of which, in essence, postulates the building up of an intreme importance of films i n corrosion processes. r e s i s t a n c e s t e e l s should s o l u b l e o x i d e film on the be not only of t h e s a m e surface of the metal. This film serves to prevent diffusion order of magnitude but essentially the same as for low-carbon and, therefore, further attack. The same mechanism may steel. The present research was undertaken for the primary be said to take place in a greater or lesser degree on all iron surfaces which are attacked in media in which an purpose of developing a method of determining initial coroxide is insoluble. I n the case of the resistant steel the film rosion rates of steels in order to ascertain the effect of film formed is supposedly dense, adherent, and not penetrated by formation. As may be obvious, the design of such an experithe corroding medium. mental apparatus entailed many difficulties, but it was finally Many facts have been presented in the literature in recent developed to a point where consistent check values could be years which seem to substantiate this theory. Among the regularly obtained, and where the possibility of results being most important of these are: due to any mechanism other than oxidation of the metal sample was eliminated. Studies were then made of the initial (1) The inability of “air-oxidized” iron to Plate out copper corrosion rates of oxide-free metal surfaces of annealed lowfrom a copper sulfate solution ( 6 ) . (2) The resistance to corrosion imparted to iron by immersion carbon, 14 per cent chromium, and 18 per cent chromium-8 in solutions of oxidizing salts such as chromates (1,2). per cent nickel steels in distilled water. (3) The resistance to the flow of current through iron, nickel, and cobalt electrodes after anodic oxidation (7). Experimental Method (4) The inactivity of metals having basic oxides in neutral or alkaline media and their subsequent activity in acids in which The apparatus used in this investigation was designed to the solubilities of the oxides are greater (3). permit the measurement of the corrosion rates in water of (5) The actual presence of tough, thin, transparent oxide films on “air-passive~~ materials as determined by their isolation active metal surfaces by means of the decrease in the oxygen from carbon and stainless steels by selective dissolution of the content of the water for definite periods of immersion. For underlying metal (4, 5 ) . successful operation the apparatus was required to permit the following processes: (i)treatment of metal wit6 acid, If the assumption is made that an oxide is necessary to (2) washing free of acid with oxygen-free water, (3) introducprevent corrosion, then the initial rate of corrosion of stainless tion of oxygenated water, and (4) determination of the decrease of oxygen concentration of corroding medium. 1 Received August 9, 1930.
T
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1198
7
FIGURE I
f
J
*-TESTING JAR 0-BOTTLES FOR 1G*b UCL JOLUTlW C-BOTTLE FOR OXYGENATE0 01STILLED WATER . 0-COOLING COIL E-STEEL PIPE r i u m wrrn IRON
r-
FILINGS COPPER TANK r o R HEATING
DISTILLED WATER
C-SUCTIW
DUCT
H-BOTTLE roR NITROGEN STORED OVEII SATURATED 2.M- SOLUTIOH I- BOTTLE 1OR SATUWITEO ZmSOq SOLWlON J- WASH BOTTLES FOR POTASSIUM
PVRDCALUTE
1.13 -STOPCOCKS
M-MOTOR
SCHEMATIC DIAGRAM O f INITIAL CORROSION RATE APPARATUS
Figure 1 shows schematically the details of the apparatus used. The acid treatment is accomplished by placing a cylindrical specimen of stainless iron in a Bakelite or glass jar, A (shown in detail in Figure 2), and pickling with a 10 per cent hydrochloric acid solution. The pickling solution is stored in bottle B and run into the jar by gravity when stopcocks 1 and 2 are opened, all other stopcocks except 4 and 5 being closed to allow the air to escape from the jar. During the pickling process the sample is rotated on a spindle (Figure 2b shows the jar and spindle used) driven by motor M . After pickling for 15 or 20 minutes the sample is washed free of acid by means of oxygen-free water. This water is prepared by removing part of the oxygen by boiling in copper tank, F , equipped with a steam coil for this purpose. The hot water is then run by gravity through two pieces of 3-inch (7.6-cm.) steel pipe 30 inches (76 cm.) long, E , filled with iron filings. From these the hot oxygen-free water is run into a copper cooling coil, D, and thence into a gas separator, C. An appreciable quantity of hydrogen is evolved when the hot water passes over the iron filings. The gas rises to the top of the jar and is allowed to escape by means of stopcock 9. From the gas separator the water is run into the Bakelite jar and drained through stopcock 7. After the specimen has been washed until a pH of 7 is obtained in the effluent, the residual water is expelled by nitrogen from the bottle H. The nitrogen is purified by passing the gas from a high-pressure nitrogen cylinder through potassium pyrogallate, and is stored over a saturated solution of zinc sulfate. The jar A is filled in a period of approximately 8 seconds with water of known oxygen content through stopcock 8. The specimen is then rotated a t 230 r. p. m. and the oxygen concentration is determined before and after each run by the Winkler method. The samples are forced out with nitrogen when stopcock 5 is opened. At 230 r. p. m. considerable turbulence is maintained and the concentration of oxygen throughout the liquid is substantially uniform. Because of the low corrosion rate of stainless steel compared with that of ordinary steel, it was found necessary to use a smaller ratio of water volume to area of specimen than in the case of ordinary steels. To achieve this end, a hard-rubber spindle shown in place in the jar (Figure 2B) was used. The volume of the jar for stainless steels was 620 cc. The spindle
Vol. 22, s o . 11
used for ordinary steels ia shown in Figure 2.4, and the volume of the jar with this spindle was 1000 cc. Some tests made with the hard-rubber spindle using low-carbon steel showed that there was essentially no difference in the results. The actual drop in a 3-minute run using this method was 0.061 cc. per square decimeter per cubic centimeter of oxygen per liter as compared with 0.057 for the same conditions using the other spindle. I n addition several runs were made with the smaller spindle using resistant steels and, although the accuracy was poor due to the low oxygen drop, the results were quite comparable with those with the larger spindle. In the case of chromium-nickel-steel it was found necessary to wash the specimens with hot oxygenfree water to remove adsorbed hydrogen. The hot washing was continued only as long as the exit water was acidic. Washing was then finished with cold water.
Method of Calculation The corrosion rates were reported as oxygen drop, since no absolute conversion factor to inches per year of penetration is yet available. The rates are expressed as cubic centimeter per liter per square decimeter. I n order to correct for small differences in average oxygen concentration, it was assumed that the rate was proportional to the concentration. Whether or not this was true would have only a slight effect on the results, since an effort was made to keep the drop as nearly constant as possible by varying the ratio of water to metal. DETAIL OF CORROSION
JAR FIGURE 2
M-MERCURY S E A L S- SPECIMEN C-SPINDLE FOR STAINLESS STEEL 0- H A R D RUBBER BEARING W I T H DRAINAGE VENTS E- SPINDLE F O R OADINARY STEEL F - CORROSION J A R
INDUSTRIAL A N D ENGINEERING CHEMISTRY
November, 1930
TIME-MNUTES
Results
Figure 3 shows graphically the rates of osypen consumption by annealed low-carbon steel. 14 per cent chromium steel, and 18 per cent chromium-8 per cent nickel s t r e l during the first few minutes of the corrosion proccsc. Figure 1 shows the same curves plotted for a longer period 2 of time.
1199
The slopes of the curves determine the corrosion rate, and these decrease with increased time (Figures 3 and 4) in the case of each metal, the change for lowcarbon steel being quite gradual. This indicates building up of a resistant film even in this case, although the degree of protection increases much more slowly than for the resistant steels. A visual inspection of the low-carbon steel during the progress of the corrosion reaction reveals a gradual graying of the metal and the color of the film becomes decper as time goes 011, but the corrosion rate continues to be finite although progressively smaller. In the case of t h r resistant steels no visible films were observed. although their presence waq indicated as previously discussed. The work of Evans ( 5 ) shows that films on stainless steels are transparent, extremely thin. and tough. The decreaye in rorrosion rate of the lowcarbon steel is inore marked than would normally be expected from prei ious data. This indicates more rapid buildIng iip of a resiqtant film than under normal conditions. The explanation of this facat is reserved for a later publication, but it may he stated that the effective water velocity was high, resultiiig in a relatively uniform corrosion product.
Discussion of Results
The curves in Figure 3 show that each of the metals used corrodes at a finite rate during the initial period, and the slopes of the lines a9 drawn indicate values of 0.0192, 0.014.5, and 0.0095 cc. per liter per square decimeter per minute for the 0” o 2 low carbon, 14 per cent chromium, and 18 per cent chromium-8 per cent nickel steels, respectively. 3 S i n c e the c u r v e s for the resistant steels be- $- come horizontal very rapidly, it is possible that the initial rate, a t zero time, is essentially the same for each metal. After a very brief period no further oxygen is used up, and if this is due to film formation the time required t o build a protective film is approximately 7 minutes in the case of 14 per cent chromium steel and 3 minutes in the case of 18 per cent chromium-8 per cent nickel steels. By making a series of assumptions as to the composition, density, and molecular weight of the film, its tliirkness may be calculated approximately as only a few molecules. A question might well be raised as to whether this initial drop were not due to experimental error, hydrogen adsorbed on the metal surface, or loss of oxygen into the nitrogen. A numher of blank runs were made, which showed that the maximum error, including oxygen loss in the nitrogen, was less than 0.010 cc. and was usually zero. while the minimum drop measured during a run with the resistant steels mas approximately 0.5 cc. in the 1-minute sample. The hydrogen adsorbed mas actually measured in several cases, and found to be negligible as compared with the oxygen used. It may therefore be concluded that the oxygen drop was a direct measure of the amount of corrosion.
e
New Cane Yields Extra Ton of Sugar per Acre-Producing nearly a ton of sugar more t o the acre than its nearest competitor, the sugar-cane variety known as C. P . 807 has raised high expectations among the federal scientists who watched and developed the plant through three years of experimental planting and released a stock of seed cane for commercial planting this year. C. P. 807 is a seedling produced at the Sugar Plant Field Station .operated a t Canal Point, Fla., by the Bureau of Plant Industry,
5
10
15
20
25
30
35
40
TIME MINUTES
Conclusions
(1) Oxide-free surfaces of chromium and chromium nickel steels have finite and high initial rates of corrosion which are nearly equal to that of ordinary steel. (2) Chromium and chromium-nickel steels resist corrosion in water because of the formation of resistant oxide or hydroxide films on tJhesurface of the metal. (3) Differences in corrosion resistance of steels and steel alloys are due to differences in the physical and mechanical properties of the films formed. Literature Cited Byers. J . Am. Chem. Soc., SO, 1718 (1908). Cushman, ZranJ. A m . Eleclrochem. Sor., 12,403 (1907). E v a n s . Tranr. Faraday Soc.. 19, 789 (1923). Evans. J. Chem. Sor., 1927, 1020. Evans, Ib,d., 1929, 651. Krir, Trans. Roy. Sot. London, 80, 359 (1790). Mulier and Spitzer, 2. anorg. Chem., SO, 321 (1906).
U. S. Department of Agriculture. The station was founded in 1920. There the specialists in sugar-cane culture have been breeding new varieties and watching the performances of the seedlings. This variety is the first they have considered worthy of general introduction. The scientists sought a sugar cane which would produce a heavy yield of sugar. They also were breeding for a high degree of resistance t o sugar-cane diseases.
