INDUSTRIAL AND ENGINEERING CHEMISTRY
9Q2
ponent and the equilibrium relation was discontinuous for mixtures both rich and lean in the more volatile component. Binary mixtures having an envelope curve with a minimum exhibit equilibrium relations which are discontinuous in the middle ranges of concentration and, in addition, may be discontinuous in concentrations either rich or lean in the more volatile component. Such systems, together with those in which immiscibility occurs, are comparatively rare and are not considered further. The data of Kuenen (4) on unknown mixtures of ethane and butane indicate that mixtures of the normal paraffin hydrocarbons have a maximum pressure higher than the critical pressure of either component and that their equilibrium relations in the critical region probably present limitations similar to those of the carbon dioxide-sulfur dioxide system. The equilibrium relation a t 10 atmospheres in Figure 6 is that of Dodge (2, 3) and was determined by analyzing directly liquid and vapor in equilibrium. The constant-pressure equilibrium relations at higher pressures were obtained by the method outlined from three dew-point and boiling-
Vol. 23, No. 8
point curves determined by Kuenen and collaborators (6,6). The figure shows qualitatively that the direct and indirect methods are in agreement. The limitations imposed upon rectification by pressure are summarized as follows: Rectification a t constant total pressure may be effected with increasing difficulty a t pressures up to the critical pressure of the component of lowest critical pressures; to a limited degree between this pressure and the maximum pressure; and not at all above the maximum pressure. Literature Cited (1) Caubet, Compt. rend., 1S0, 828 (1900). (2) Dodge, Chem. M e t . Eng., 88, 622 (1928). (3) Dodge and Dunbar, J . A m . Chcm. Soc.. 49, 591 (1927). (4) Kuenen, Proc. Roy. SOC.Edinburgh, 21, 433 (1895-97). (5) Kuenen and Clark, I b i d . , No. 150 (1917). (6) Kuenen, Verschoyle, and Van Urk, Comm. Phys. Lab. Leiden, No. 161 (1922). (7) Roozeboom, “Heterogenen Gleichgewichte,” Vol. 11, Part 1 (1904).
Effect of Temperature on the Corrosion of Zinc’ G. L. Cox RESEARCH LABORATORY OF APPLIEDCHEMISTRY, DEPARTMENT OF CHEMICAL ENGINEERING, MASSACHWSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
The corrosion of zinc in distilled water continually water and changes its physisaturated with air is affected tremendously by temcal c h a r a c t e r i s t i c s , and metals in oxygenated perature. The most important factor influencing a t 125’ C. the hydrous oxide water a t varying temthese rates at the different temperatures is shown to is reported to have the comperatures are influenced, in be the physical nature of the corrosion-products film. position Zn(OH)*. g e n e r a l , by the manner in The experimental results are satisfactorily explained Although the above obserwhich t e m p e r a t u r e affects by a consideration of the physical properties of the v a t i o n s were made of the (1) the s p e c i f i c r e a c t i o n corrosion-products film, rate of transfer of dissolved precipitated hydroxide withrates of the various corrosion oxygen, and oxygen concentration of the corroding out reference to its formation processes, (2) oxygen solumedia. on a metal surface d u r i n g bility of the water, (3) the corrosion processes, it is rate of transfer of dissolved oxygen through the liquid, and (4) the nature of the corrosion highly probable that, if the character of the precipitated hyproducts. I n general, all other conditions being constant, droxide changes with temperature, the products of corrosion equal small increments of temperature cause approximately will likewise change, and thereby in some manner affect the the same multiplication of specific reaction rates of chemical ultimate corrosion rates. On the basis of these indications, processes, but it has been demonstrated that the corrosion tests have been made of the corrosion rate of zinc in aerated rate of iron in water of constant oxygen concentration in- distilled water a t different temperatures, and the physical concreases linearly with temperature, and that the corrosion re- dition of the corrosion-products film has been studied with the action is influenced chiefly by the rate of transfer of oxygen view of explaining some of the observed phenomena. to the metal surface ( 7 ) . The rate of transfer of oxygen will Experimental Method be deterinined largely by the nature of the corrosion products and the effect of temperature upon the rate of diffusion. The testing apparatus (Figure 1) consisted of a 4-gallon However, on account of the decreasing solubility of oxygen in Pyrex-glass jar containing distilled water continually satuwater with rising temperature, the corrosion rate of iron in rated with air, covered, and fitted with a condenser to prevent water in open systems is a balance between the rate of transfer loss of water by evaporation. No effort was made to reof oxygen to the metal surface and the oxygen solubility. move the carbon dioxide from the water or the air for Therefore, over a range of temperature, the net result is that aeration. The temperature of the water in the jar was the corrosion rate passes through a maximum. maintained by an oil bath heated with an electric immersion It is believed that this explanation will hold in general for heater and controlled by an external variable resistance. a number of active metals, provided the corrosion products When once adjusted for any particular temperature, the water of all are affected by temperature in the same manner. It is a in the jar remained substantially constant, the variation recognized fact that the hydroxide, or hydrous oxide, of zinc being not more than * 2 O C. The temperatures were noted has varying properties as the temperature is changed (2, daily, and the averages of the entire test period were used. 3 , 4 , 5 ) . At room temperature the hydrous oxide precipitates Specimens 8 x 12 x 0.076 cm. were cut from sheets of in the form of a gel with an indefinite amount of adsorbed zinc (99.9 per cent purity), and drilled with 6-mm. (‘/d-inch) water. When the precipitate is heated, it gradually loses holes near each end. The total area of the faces and edges of each specimen was approximately 190 sq. cm. The speci1 Received April 24, 1931.
HE corrosion rates of
T
IAVDUSTRIAI. ALVDENGINEERIXG CHEMISTRY
August, 1931
mens were suspended in the water by means of glass rods supported on a wooden disk 6 inches in diameter, rotated a t a speed of 56 r. p. m., and exposed to the aerated water for 15 days. At the end of this time the corrosion products on the specimens were examined, removed, and the loss in weight of the metal determined in order to calculate the rate of corrosion.
YO
IRON B a
Figure 1-Rotating
Tester
Discussion of Results
The corrosion rates of zinc in distilled water a t different temperatures, expressed as penetration in centimeters per year, are shown graphically in Figure 2 . Each point on the curves represents the average rate of corrosion of two check specimens, the deviation being not more than 5 per cent. Two curves are shown in the figure, one representing the actual rates of corrosion, and the second, the rates calculated per unit concentration of oxygen in the water.2 Table I shows a summary of the physical conditions of the corrosion products on the specimens a t the end of the tests. Table I-Conditions of Corrosion Products at Various Temperatures TEMPERAAPPEARANCE OF TENACITY OF TURE PRECIPITATE PRECIP~TATE
903
curve. If the corrosion products did not change during the process of corrosion over the temperature range, the corrosion rates, when calculated on a basis of constant oxygen concentration, should, from the previous discussion, follow very nearly a straight-line relationship with temperature. The dotted curve indicates clearly that the rates of corrosion are influenced tremendously by some factor other than the temperature coefficient of either the rate of Oxygen transfer or oxygen solubility. A visual examination of the corrosion products a t the various temperatures studied (Table I) showed that a t the lower temperatures (20-50" C.) the corrosion products exist as a gelatinous and adherent substance. This type of film has been found to offer high resistance to aqueous corrosion a t low temperatures (I). An increase in temperature to 55" C. is accompanied by a definite change in the corrosion products, the gelatinous form being transformed to a granular, nonadherent type. At 65' C. the corrosion filmis completely granular and somewhat non-adherent, although it appears to be more compact and should accordingly possess greater protective qualities. Increasing the temperature further is accompanied by a further rapid increase in density (compactness) and tenacity of the film. I n fact, a t 100" C. the corrosion products could not be readily removed, and the rate of corrosion was therefore determined by the increase in the weight of the specimen, assuming the product to be zinc hydroxide. Although this method is admittedly inaccurate, it serves the purpose of showing qualitatively the increased tenacity and compactness of the film as formed a t the higher temperatures. Therefore, it seems most probable that the sudden increase in the corrosion rate from 50" to 65" C. may be attributed chiefly to the abrupt change in the nature of the corrosion-products film from a protective to a non-protective state, and that the corrosion rate decreases rapidly from 65' C. owing largely to the increase in the protective qualities of the film. 0.S
028
2
I
1
I
I
I
I
.
I
I - q A T E S CALCULATCD PER ic.0, PER LlTiR
024
I
I
I
i
I
c.
20 50 55 65
75
95 100
Definitely gelatinous Slightly less gelatinous Mostly granular Decidedly granular, becoming flaky, and compact Decidedly granular. flaky. and compact Compact, dense, and flaky Varies from grayish white to black, very dense, resembling enamel
Very adherent Adherent Non-adherent Won-adherent Non-adherent Adherent, removed by bending specimen and fractunng scale Very adherent and difficult to remove by mechanical means
Figure 2 indicates clearly that up to 50" C. the rate of corrosion increases only slightly with rising temperature, changes abruptly a t approximately 53 " C., and increases very rapidly with further increase in temperature until it reaches a maximum a t approximately 65" C., above which it decreases rapidly. These facts are explained by a consideration of the effect of temperature on the solubility and rate of transference of oxygen and of the physical nature of the corrosion-products am a t the different temperatures. Although an increase in temperature causes a gradual diminishing of the viscosity of the liquid and of the amount of oxygen dissolved in the water, the decrease in these quantities is not sufficient to account for the peculiarities of the * T h e data of the temperature coefficient of oxygen solubility were obtained from Seidell (6).
TEMPEPANRGY.
Figure 2-Effect
of Temperature on t h e Corrosion Rate of Zinc in Distilled Water
Conclusion
These observations are believed to indicate definitely that the corrosion rate of zinc in oxygenated distilled water over a range of temperature is controlled largely by the nature of the corrosion-products film, rather than the temperature co-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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efficient of the specific reaction rate of the corrosion process, the rate of transfer of oxygen through the liquid, or the oxygen solubility. Acknowledgment The author wishes to acknowledge the assistance rendered by W. B. McCluer, of the Research Laboratory of Applied Chemistry, Massachusetts Institute of Technology, in the interpretation of phases of the investigation.
Vol. 23, No. 8
Literature Cited (1) (2) (3) (4) (5)
Brown, Roetheli, and Forrest, IND. END. CHEM.,23, 350 (1931). DeFarcrand, Comfit. rend., 177, 765 (1923). Fricke and Ahrndts, 2. anorg. Ckem., 134,344 (1924). Goudriaan, Rec. trav. ckim., 89, 505 (1920). Linder and Picton, J . Ckem. Soc., 61, 130 (1892). (6) Seidell, “Solubilities of Organic and Inorganic Compounds,” Van Nostrand, 1918. (7) Speller, “Corrosion-Causes and Prevention,” p. 144, McGraw-Hill, 1926.
Absorption of Ultra-Violet Light b y Lacquer Films’ D. C. Duncan,2 D. R. Wiggam,3 and Wheeler P. Davey4 PENNSYLVANIA STATECOLLEGE, STATECOLLEGE, PA.
I
T IS a matter of common experience that films of nitrocellulose lacquers are affected by ultra-violet light and that the effect is different in amount for films of different composition. The present work was undertaken in the hope of finding (1) whether the effect is due primarily to some one wave length or to the whole ultra-violet region, and (2) whether the effect is due primarily to some one or more of the ordinary ingredients of lacquers. Measurements were therefore made of the coefficient of absorption for various wave lengths of light of lacquer films of various compositions, and of the individual lacquer ingredients. The data indicate that, within the range of wave lengths found in sunlight, the effect of ultra-violet light is not due to any one wave length, but increases rapidly as the wave length is decreased beyond B threshold limit. Of all the ingredients of common lacquers, ester gum is the most affected by ultraviolet light.
Using the value of I
to
E,=? the absorption coefficient, a, was determined by means of the customary formula
I
io
=
A similar procedure was followed in the case of the solid films, except that no cells were used. Materials Tested and Final Results
The materials tested were chosen so as to bring out any significant effect of the degree of nitration, and the viscosity of the nitrocellulose as well as the individual effect of each of the film components (nitrocellulose, plasticizer, gum). All the films were made from solutions having the following composition:
Method
Preliminary tests showed that none of the materials to be studied exhibited marked selective absorption for any narrow wave-length region. It therefore seemed unnecessary to use one of the more troublesome and unsteady sources of continuous radiation, such as the under-water arc. A mercury arc in quartz was used because of its constancy of light emission under properly controlled conditions. I n the determination of the absorption coefficients of the liquid materials, alternate photographs were taken with a Hilger E2 spectrograph, placing in the path of the light first a blank absorption cell, A , then a similar cell, B, containing a definite thickness, d, of the material under test. The exposure time, 4, for cell A was kept constant while that for cell B was varied over a considerable range. This gave on the same plate a series of alternate spectrograms such that each of the spectra of B lay between two control spectra of A . Relative intensities of the mercury lines in the B spectra were obtained in terms of the time required to produce a blackness on the photographic plate equal to that of t h e corresponding line in the adjacent A spectrum. This assumes that constant blackness is obtained for the same product of intensity times time. This assumption is not necessarily correct, but it yields results which differ from the true results by altering only the steepness of the curves. Owing to the smooth, continuous nature of the curves, the interpretation of the final results is unaffected by the validity of this assumption. This work was done at the instance of the 1 Received April 25, 1931. Hercules Powder Co. and is published with their permission. 2 Professor of physics, Pennsylvania State College. Assistant director, Experimental Station, Hercules Powder Co. 4 Professor of physical chemistry, Pennsylvania State College.
PQYrS
Nitrocellulose Plasticizer Ester gum Toluene Butyl acetate
10 5
8
47 30
Series ( a ) . This series contained five films, alike in every respect except for the nitrogen content of the nitrocellulose, which varied as follows: 10.79, 11.41, 11.57, 11.97, and 12.68 per cent. The coefficients of absorption for various wave lengths are shown in graphical form in Figure 1. All five films gave identical results. The curve shows no evidence of selective absorption, but only a continuously increasing absorption for shorter wave lengths. As is shown later, any possible small effect of variation in the nitrogen content is masked by the large effect due to the presence of ester gum. Evidently, then, the effect of nitrogen content of the nitrocellulose on the absorption of ultra-violet light is only a matter of academic interest. Series (b). The films tested in this series contained three different plasticizers-tricresyl phosphate, dibutyl phthalate, and blown castor oil-and nitrocellulose whose viscosities, measured by the dropping-ball method in the manner customary in the trade, ranged in steps from four seconds to less than l/r-second. These viscosities may be more definitely described as follows: VISCOSITY IN STANDARD SOLVENT NOMINAL VISCOSITY RATING ACTUAL
Sec. 4
Sec.
3.2 2.8 4.0 4.8 0 (35%) 0 The standard solvent contained 25 per cent of a (95 per cent), benzene (0.5 per cent), and water (4.5 per of ethyl acetate, and 55 per cent of toluene. b Concentration of solvent.
’/* ’/:
%b
12.2 20.0 25.0 35.0 mixture of alcohol cent): 20 per cent
