0
0
orrosion
The corrosion of steel in dilute solutions of oxygen and carbon dioxide was observed. Definite concentrations of these gases were maintained over a period of 21 days in an apparatus in which water continually flowed over the specimens ; the prevailing concentrations of the dissolved gases were checked by regular periodic analyses. In these experiments the water temperature was 60" C. =t5" and the velocity of flow-was 2.5 cm. per minute. During the progress of an experiment the water became saturated with ferric hydroxide. The corrosion rate was found to be a function of the carbon dioxide and oxygen concentrations. This relation is established in a series of curves. One p. p. m. of dissolved oxygen is approximately as corrosive as 20 p. p. m. of dissolved carbon dioxide, and a t zero concentration of either gas the corrosion rate is not zero but a value dependent upon the concentration of the other gas.
ISCE Whitney proposed the electrochemical theory of corrosion in 1903 (as),the mechanism of the process has been studied almost continuously. Because of the effect of carbon diox-
S
ide on the concentration of the hydrogen ion in the purer forms of industrial water and because of the effect of oxygen on the displacement of the equilibrium, the effect of these two dissolved gases, both individually and together, has received well-merited attention. The great importance of the effect of dissolved oxygen and carbon dioxide on the corrosion of iron by the purer forms of industrial water and of steam condensate justifies a study of the relative effects of water solutions containing different quantities of these gases. Many investigators of this phase of the corrosion problem have defined the corroding water as being aerated, as containing carbon dioxide, as saturated with oxygen, etc. The quantitative significance of the amount of oxygen present has been recognized by many investigators; Wilson (24) suggested a formula for the calculation of corrosion rates in which oxygen was considered. The effect of the amount of oxygen as a controlling factor in the rate of diffusion of the dissolved gas to the metal has been brought forward by Whitman and Russell (21) and by Bengough, Stuart, and Lee @)* Corrosion rates Tvere found to be proportional t o the oxygen content over a large part of the range by Denman_and Bartow (6); by Cos __.. . _and Roetheli
Quantitativ Effect of
Dissolved Oxygen and Carbon Dioxide THOMAS J. FINNEGAN, RICHARD C. COREY New York Steam Corporation, New York, h'. Y. AND DAVID D. JACOBUS
Stevens Institute of Technology, Hoboken, N, J.
iron specimens, and found that, when corrosion rate was plotted against oxygen concentration, a t high pH values the corrosion rate passed through a maximum a t certain definite oxygen concentrations, while a t low pH values the corrosion rate increased progressively with increasing oxygen concentrations. The quantitative significance of the amount of carbon dioxide has not been given the same attention as has the significance of oxygen. Carbon dioxide was used to obtain pK values on the acid side by Groesbeck and Waldron ( 9 ) al-
bon dioxide are always present when corrosion takes place. Hall and Mumford ( I O ) showed that, in accordance with the laws of Henry and Dalton, with the usual quantities of oxygen and carbon dioxide found in steam only a very small concentration of dissolved gases could be expected to be present in the condensate if the steam were the only source of the gases. Frequently higher contents of dissolved gases are found, however, and these are believed to be caused by inleakage from the atmosphere or in special cases by rapid condensation of steam in such apparatus as water heaters. As part of the program of study of corrosion in heating systems, it was decided to perform some experiments in which water containing varying amounts of carbon dioxide and oxygen would flow over weighed iron specimens. The loss of weight of the specimens after prolonged exposure in such a n experiment would give a n indication of the relative importance of these two gases in the corrosion of iron. Walker (20) has reported the results of an extensive field investigation of this phase of the corrosion problem, but in practical heating systems it was found impossible to control the gas content of the condensate, especially the carbon dioxide. Walker’s results indicated that oxygen was decidedly the controlling factor and that for a given oxygen content the quantity of carbon dioxide was not of great importance. On the other hand, Hayes, Henderson, and Staneart (11) conducted a limited number of laboratory experiments in a glass apparatus in which condensates containing varying amounts of carbon dioxide and oxygen were made to flow over steel test specimens; they found that reduction of the carbon dioxide content of the condensate to very low values caused a considerable reduction in the corrosion rate. Hall and Mumford ( I O ) , while they did not minimize the importance of carbon dioxide, laid considerably more stress on oxygen which they called the “quantity factor” of corrosion, carbon dioxide being termed the “intensity factor.” The investigation reported here, therefore, seemed to be justified, and, although i t was started with return-line corrosion of a heating system in mind, the results are believed to be applicable to the general subject of corrosion because of the fact that the experiments have been conducted in glass in a laboratory under conditions which have been closely controlled.
Apparatus I n order to simulate conditions found in practice, a continuous flow type of apparatus was considered necessary to the conduct of this investigation. I n the past the continuous flow types of apparatus (8, 9,19) utilized a pump to effect
in this Lvestigation was warm water, and by selecting the point a t which FIGURE2. METHODOF heat was applied in the cycle INTERCONNECTIXG a design was developed which TEST TJNITS resulted in thermal circulation and eliminated the necessity for a mechanical pump. Figure 1 illustrates the circulating apparatus which was used : A 5-gallon reservoir, a heating flask, and a glass specimen chamber were connected together with glass tubing as shown. When heat was applied to the flask, water rose in the vertical tube, flowed through the 250-cc. (8.4-ounce) sampling bottle, through the specimen chamber, thence to the reservoir, and back to the flask. The sampling bottle was incorporated in the circuit immediately before the specimen chamber to make certain that the water which was analyzed represented that which flowed over the specimens. A by-pass enabled the sampling bottle to be removed for analysis without disturbing the continuity of operation. The heating flask was surrounded with asbestos paper, and the vertical tube leading upward from the flask was lagged with insulating cement. This kept the water in the specimen chamber a t a temperature not greatly lower than that in the flask and enabled the water in contact with the specimens to be maintained a t the proper temperature without the necessity of heating the flask so high that there would be danger of venting the dissolved gas,. The burner was protected from draughts by means of a cylindrical metal shield. A thermometer was inserted in the specimen chamber, and the rubber stoppers were covered with tin foil after having been previously treated with sodium hydroxide solution. Ten such assemblies were set up and operated simultaneously throughout the 3-week time period selected for each experiment. I n this way several groups of experiments were obtained. In any one group each apparatus had the same oxygen content within the limits of control, but the carbon dioxide was different for each apparatus in the group. By running several groups of experiments, data were obtained for several different values of oxygen in the range 0 t o 2 parts per million and for values of carbon dioxide in the range 0 to 40 p. p. m. It was found after several trials that it was possible to maintain the proper gas concentrations and eliminate contamination due to air inleakage by maintaining a space filled with nitrogen a t a slight positive pressure above the circulating water. There was no such space in the specimen chamber, which was kept completely filled with water. The vapor spaces of all ten units were interconnected. The nitrogen entered at one end of the system
116
"OLd. 21, NO. 7
IKDUSTRIAL AND ENGIKEERING CHEMISTRY
and escaped through a small opening in the tube at the other end. There was considerable loss of oxygen when experiments using the higher oxygen concent'rations were being run. To prevent this loss, a small aspirator operated by the nitrogen flopused t o dra-v a requisite amount of dry air, free of carbon dioxide, into the system. This scheme is indicated in Figure 2, hut for the sake of simplicity only two units are shown.
Procedure The procedure for starting a series of experiments -,yas as folloTTs: The specimen chambers were removed from the ten units, and lengths of glass tubing were substituted. The bottles and connecting tubes 'iyere filled with distilled water to a predetermined mark. Xitrogen Fas then bubbled through this water for about 48 hours or until the oxygen and carbon dioxide content had been reduced to a very low figure as shown by analysis. The specimen chambers containing the specimens were then put in place, and to each bottle a proper volume of oxygen-saturated lvater and one of carbon-dioxide-saturated water were added until the Kater in the B-gallon bottle was made up to the proper concentrations of dissolved gases. A. supply of such 15-ateru-as kept at hand by having in a 12-liter (12 7-quart) balloon flask a fairly large quantity of water through nhich oxygen or carbon dioxide continuallv bubbled. A 1-liter (1.06-quart) buret was connected to all of thk &gallon reservoir bottles as shown in Figure 2. The buret was filled with gas-saturated Iyater, and the proper amount was allowed to flox- to each of the reservoirs. The term "oxygen- and carbon-dioxide-saturated v, ater" is used in a qiialitative sense. Actually this mater was analyzed before berng added t o the test units, and the analpsis .ic.hlchwas found governed the amount of mater added t o the reservoirs. The buret with connecting tubes which was used to add the gas-saturated water at the beginning of a test ~ 3 - also a ~ used t o make up the water removed as samples and to correct the concentrations when analysis showed it t o be necessary.
inserted in the apparatus, having heen preserved in a desi+ cator until needed for test purposes. At the completion of an experiment the corrosion products viere removed with sodium hydroxide and zinc after the method of Cournot and Chaussain (4). The loss in weight ~ y a sascertained by reweighing the cleaned specimens. The corrosion rate can be calculated by means of i h e formula published by the National District Heating Association (17) which takes into consideration the progressive decrease In the diameter of the wire as corrosion proceeds, The formula is:
xhere R 5"
= =
D, = W1 =
W
=
corrosion rate as av. penetration, inches per year time, years initial diam. of \Tire, inches initial weight of wire final weight of wire
Jvhen the corrosion loss is of the magnitude found in these TI1 .'Wis practically equal to W, and for a 21experiments, d day exposure of jyire 0.05 inch in diameter the formula can be reduced to ~
R
=
0.217
w1 - w _ I I _ _
WI
This expression was used for calculating the corrosion rates in this investigation because the difference between the results obtained by i t and those derived from the more exact expression is small enough to be negligible when conlpared x i t h the effect of the variation in the experimental determination of the corrosion rates for each of the four specimens of a single test unit considered separately. Because of the small The corrosion specimens were of the type which has been values usually found for R, the custom of expressing the corused by the Corrosion Committee of the Sational District Heating Association in the corrosion tester which this cornrosion rate in the unit R X 1000 was adopted. The values of R X 1000 which were obtained were plotted mittee has devised for the purpose of studying corrosion in against oxygen with carbon dioxide as a parameter and against buildings. This tester and the manner of its use have been described at length in reports of the Corrosion Committee carbon dioxide with oxygen as a parameter. Five groups of experiments were run. I n the first the (15, 16, 17') and also by Walker (20). The essential features o x y g e n c o n t e n t of of t h i s t e s t e r a r e t h e w a t e r w a s apthree specimen coils mounted in tandem proximately 0.1 p. p. m. and the total in a small supporting frame. The connecc a r b o n dioxide was i n c r e a s e d b y small tions t o the frame and i n c r e m e n t s in each from one specimen to unit from 1.5 p. p. n?. the next are made by means of insulators of i n t h e first to 82 p. p. m. in the last. Micarta. The indiThe second group had vidual specimen is a 0.5 p. p. m . o x y g e n helix of ten turns of with carbon dioxide Bessemer steel wire from 0.0 to 22 p. p. m. made from a special T h e t h i r d h a d 1.0 ingot which had been p. p. m. oxygen and heavily cropped to in2.0 to 28 p. p. ni. carsure against cavities or inclusions. The bon d i o x i d e . The 4%" O F A P P h R 4 T L S wire was d r a w n t o FIGURE 3. PHOTOGR fourth had 1.5p. pani. oxygen and 2 to 34 0.05 inch diameter =t p. p. m. carbon dioxide. The fifth had 2 p. p. m. oxygen and 0.0005 inch before it was fully annealed, to eliminate the effect 1 to 39 p. p. m. carbon dioxide, of strain, and pickled to give a clean iron surface. Each coil During the progress of each group of experiments analyses is approximately ll/lb inch (0.79 em.) outside diameter, xeighs of the water were made as frequently as the time of two assistabout 2.5 grams (0.088 ounce) and has about 1.5 square inches ants who were employed full time in that worlr would permit. (24.6 sq. em.) of surface. The oxygen was determined by the JGnkler method as deI n the present investigation four such coils were mounted in tandem in each chamber, each coil being electrically insuscribed by the American Public Health Association (1). The carbon dioxide was determined by the evolution method lated between Micarta connectors as is done in the N.D. H. A. originally described by Schroeder and Fellows (18). tester. With the progress of any experiment, considerable quantiEach coil was weighed on an analytical balance before being
JULY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
ties of ferric hydroxide appeared in the water, indicating that the water was saturated with respect to ferric hydroxide and therefore contained a small amount of ferric ion. As this ion is supposed to cause an error in the Winkler oxygen method, some tests of the water were made by the method described by Yoder and Dresher (25). No error was found, indicating that the ferric-ion concentration was low enough to be disregarded for the purposes of these analyses. The p H values were determined colorimetrically with regular periodic checks by the quinhydrone electrode. When the p H value of the water was higher than 6.0, the final determinations were made by means of indicator solution which had been adjusted to p H 7.0. When the p H value of the water was between 5.0 and 6.0, indicator adjusted to p H 6.0 was used. By regulating the heat supply by means of burner adjustments, it was found that the velocity of flow and the temperature of the water in the corrosion chamber could be maintained fairly constant. The temperature was 60" C. plus or minus 5", and the velocity was 2.5 cm. per minute which is in the region of viscous flow. Figure 3 is a photograph of the apparatus.
Results of Corrosion Experiments All of the experiments were run for a period of 21 days a t a velocity of 2.5 em. (1 inch) per minute. The water in the corrosion chambers was a t a temperature of about 60" C. (140' F.), which would be the order of the cooler condensates encountered in practice. During the progress of the tests the solutions became saturated with respect to ferric hydroxide. The suspended material varied in color from a yellow to a brick red; those solutions in which the oxygen content was relatively high were redder in color than where low oxygen concentrations prevailed. The test specimens themselves were, for the most part, coated with a deposit of black oxide overlaid with red ferric oxide. The black oxide was somewhat loose in texture, and it was possible to remove the major portion of the deposit by rubbing the specimen with a damp cloth. Portions of some specimens were bright a t the conclusion of the tests because the oxides fell off of their own weight. The results obtained are listed in Table I. The second column shows the average amount of oxygen in solution during the course of each run. The third column shows the prevailing amount of total carbonate in solution, expressed as p. p. m. of carbon dioxide and determined by the evolution method. The fourth column gives the average p H value during each run. The B t h column, free carbon dioxide, is calculated from the tabulated values of total carbonate and p H by the method of McKinney (fd). The sixth column shows the average corrosion rate of each set of four test coils, and the seventh, the deviation of the corrosion rates of the individual coils from the average value. The concentration of oxygen and total carbonate and the pH value listed for each individual test represent the average of all of the individual observations taken during the 21-day run. The deviations of the individual observations from the average values given in Table I are rarely larger than 20 per cent, which indicates a very close control of the experimental conditions when the difficulties of maintaining a continuous yet very small gas content over a 21-day period are considered. Inspection of the amount by which the corrosion rates of a set of individual coils deviate from the average value of R x 1000 will reveal few cases where the deviation is as large as 10 per cent, a fact which indicates that the errors contingent upon the determination of the corrosion loss are well within the experimental accuracy of the apparatus. All of the tests conducted to date have been in solutions
771
TABLEI. RESULTSOF TESTS Unit NO.
Total
1
0.22 0.09 0.08 0.12 0.05 0.08 0.08 0.05 0.06 0.06
1.5 1.8 3.5 4.8 10.3 20.2 28.6 46.9 62.0 82.3
2 3 4 5 6 7 8 9 10
1
0.55 0.50 0.33 0.38 0.38 0.35 0.39 0.41 0.42 0.44
0.0 0.6 1.3 2.1 4.4 8.2 15.8 13.4 19.0 22.2
1 la 2 3 4 4a 5 6 7 8 9 10
1.23 1.05 1.05 0.94 0.93 1.18 1.00 1.11 1.00 0.96 0.89 0.98
2.1 1.0 2.3 3 0 5.8 4.9 7.2 10.3 14.6 19.6 23.6 28.3
1 2 3 4 5 6 7 8 9 10
1.49 1.44 1.48 1.37 1.48 1.49 1.41 1.39 1.53 1.59
2.2 2.3 2.5 4.7 7.3 10.0 14.4 20.0 24.1 34.0
1 2 3 4 5 6 7 8 9 10 10a
2.00 2.07 2.14 1.90 2.17 2.08 1.86 1.98 2.04 2.11 1.89
0.9 1.3 2.8 5.4 8.3 11.7 14.6 21.9 25.6 35.0 39.2
2 3 4 5 6 7 8 9 10
Free
Oxygen Carbonate pH P . p. m. P. p . m. COS
Corrosion Deviation of Test
COz
Rate Pieces P . p . m. COa Inches per year X 1000 G r o w No. 1 7.6 0.10 1.15 0.08 7.3 0.22 0.62 0.08 6.5 1.7 1.14 0.04 2.3 6.5 1.78 0.05 6.2 6.6 1.84 0.12 6.0 15.0 4.93 0.11 6.0 21.2 5.11 0.40 6.0 34.8 5.30 0.34 5.8 50.8 8.17 0.71 5.8 67.3 10.47 0.61 Group No. 2 7.6 0.00 2.50 0.17 7.8 0.02 1.57 0.09 7.7 0.07 1.46 0.10 6,s 0.65 1.50 0.11 6.3 2.6 2.37 0.16 6.3 4.8 3.61 0.21 6.2 10.2 3.90 0.42 6.1 9.3 4.06 0.42 6.0 14.1 4.75 0.20 5.9 17.4 4.65 0.30 Group KO.3 7.9 0.07 5.31 0.30 6.7 0.36 5.61 0.29 7.5 0.19 4.75 0.45 7.3 0.38 3.72 0.28 6.4 3.1 4.40 0.28 6.2 3.1 6.77 0.32 6.2 4.6 6.18 0.28 6.1 7.1 6.36 0.32 6.0 7.03 10.5 0.69 6.0 14.4 8.45 0.40 18.5 5.9 9.17 0.35 22.2 5.9 12.16 0.28 Grouu No. 4 7.2 0.34 6.25 0.40 6.9 0.61 7.38 0.39 7.0 0.55 6.92 0.41 6.4 2.5 7.13 0.59 6.3 4.3 8.49 0.54 6.2 6.4 10.43 1.14 6.0 10.7 10.90 1.28 6.9 15.6 13.35 0.68 5.8 19.7 13.42 0.57 5.7 29.0 15.19 0.58 Group No. 5 6.9 0.24 8.59 0.79 6.7 0.47 10.37 0.62 6.3 1.65 12.04 0.52 6.2 3.5 10.23 0.70 6.2 5.3 14.08 0.24 6.0 8.7 12.96 0.45 5.8 12.0 13.85 0.91 5.8 17.9 18.34 0.94 5.5 23.0 17.33 1.07 5.6 30.7 18.69 0.58 5.5 35.2 20.96 0.44
having sufficiently high hydrogen-ion concentration so that all of the carbonate which is present is in the form of free carbon dioxide and bicarbonate ion. I n no single test did an equilibrium study show as much as 0.01 p. p. m. of divalent carbonate ion present. On the other hand, none of the solutions have a p H value as low as would be the case if all of the carbonate present were in the form of free carbon dioxide. We may therefore state that the following equation represents the equilibrium which prevailed:
+ M + OH-
M+HCOj-
+ H,O
(1)
The symbol M represents any metallic ion with which the carbonic acid can react. Because each solution was made up of pure distilled water and purified carbon dioxide gas, the bicarbonates which were found to be present in small amounts must have been due to a reaction of the carbonic acid with either the glass walls of the apparatus or with iron derived from the submerged test pieces, since there were no other solids in contact with the solution. The equilibrium represented by Equation 1 can be expressed as
778
IKDUSTRIAL A S D ENGINEERING CHEMISTRY o r & - = - (HCOI-) (H’) (HzC03)
Because K , the equilibrium constant of carbonic acid, is known, Equation 2 makes it possible to calculate the amounts of HC03- and H2C03 in solution, where the pH value of the solution and the concentration of total carbonate have been determined by analyses. As before mentioned, this calculation has been performed by the use of McKinney’s tables (1.4). The term “free carbon dioxide” as used in this paper is synonymous with the symbol HzC08, where both the term
I8
I6
I4
12
g 10 9 b
4
2 ‘0
S
IO
IS 20 2.5 30 3s TOTAL CARBONATE- RRM. C O L
40
43
FIGURE4. RATEOF CORROSION AS FUNCTION OF TOTAL CARBONATE
and the synibol may be defined as meaning the amount of carbon dioxide which is in solution and which is not present as bicarbonate or carbonate ions. It is possible to postulate that a portion of the “free carbon dioxide” is dissolved as an anhydrous gas and that another portion is present as undissociated carbonic acid, but no analytical method has yet been devised which will permit making a quantitative distinction of this nature (3, I S ) . The authors adhere to the conventional use of the term “free carbon dioxide” as meaning the amount of anhydrous gas plus the amount of undissociated carbonic acid in solution, and in equations have designated this quantity by the symbol H2C03. The uniformity with which the solutions were prepared has caused the free carbon dioxide t o be in all cases roughly proportional to the total carbonate in solution. Hence the corrosion rates observed in this particular group of experiments can be related either to the amount of total carbonate or to the amount of free carbon dioxide in solution. The results of these tests will be discussed largely in terms of dissolved oxygen and free carbon dioxide, since in all of the experiments the carbonate was largely present as free carbon dioxide. The fact that in this particular investigation the corrosion rate can be related to the concentration of total carbonate is shown in Figure 4. This chart is strictly limited in its application to solutions in which the carbonate is present very largely as carbonic acid. The data of Table I have been treated graphically in Eigures 4, 5 , and 6. It is the opinion of the authors that these curves, derived as described in the remaining paragraphs of this section, present a satisfactory interpretation of these data. It has been found that the observed corrosion rates can be directly related to the oxygen and carbon dioxide concentrations prevailing during each run. This correlation has been accomplished by constructing a three-dimensional surface, one axis being corrosion rate, another axis being oxygen concentration, and the third axis being free carbon dioxide. The best representative surface was drawn through the plotted points, where each point represented a separate 21-day run, After constructing such a surface, i t was found that the indi-
VOL. 27, YO. 7
vidual observations deviated by an average of only 11 per cent from the plotted surface. This surface is pictured in Figure 5 by plotting the intersections of planes of constant oxygen content with the surface. The same procedure has been followed in Figure 6, where planes of constant carbon dioxide content have been allowed to intersect the surface. I n Figure 5 , observed corrosion rate is the ordinate with free carbon dioxide the abscissa, and each line represents a series of observations made a t constant oxygen concent)ration. Because each series of tests was made with all of the units a t about the same oxygen concentration, it is possible to plot each individual observation on this chart without violence to the data. It will be observed that the plotted data and the lines of constant oxygen content are in good agreement. I n Figure 6, observed corrosion rate is the ordinate with oxygen the abscissa. The lines on this chart represent the intercepts of the planes of constant free carbon dioxide with the surface which is descriptive of the complete relationship. The data have been gathered into several groups, each group representing a series of tests in which the free carbon dioxide was about equal t o the concentration represented by one of the plotted lines. These groups of data are represented by characteristic symbols. It will be observed that there is good agreement between the plotted data and the curves. Kone of the data has been smoothed to bring it in line with a certain concentration; the procedure followed was to plot directly each run which had a carbon dioxide concentration about equal to that of one of the plotted lines. Inspection of either Figure 5 or Figure 6 s h o w that oxygen and carbon dioxide mutually influence the corrosion rate. Let us take as an example a condensate containing 1 p. p. m. (0.7 cc. per liter) of oxygen and 10 p. p. in. of free carbon dioxide. According to the charts, such a condensate would produce corrosion a t the rate of 0.007 inch per year. If the oxygen could be reduced to aero, 10 p. p. m. of free carbon dioxide would still cause corrosion to take place at the rate of 0.002 inch per year. Similarly, if the carbon dioxide were reduced to zero, 1 p. p. m. of oxygen would cause corrosion a t the rate of 0.005 inch per year. Figures 5 and 6 are truly applicable only to the particular set of conditions under which
ia lb li)
I2
g- IO ;;a b 4
2
0 F R l E CARBON DIOXIDE
.PPM COL
.4s FLJNCTIOY FIGURE5. RATEOF CORROSIOX OF FREECARBON DIOXIDE
these tests were conducted, but they show in a graphical manner that the quantity of either gas will influence the rate a t which corrosion occurs, the total effect being due to the combined influence of both gases. A very rough generalization of the relative effects of carbon dioxide and oxygen can be made by saying that 20 p. p. m. of free carbon dioxide have about the same influence on the corrosion rate as 1 p. p. m. of oxygen. The concentrations at which these two gases occur in actual steam-return lines depend on the steam supply and on the operating character-
JULY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
istics of the line in question. If, in a given installation, knowledge of the concentrations of the two gases were available, it should be possible to evaluate the relative effects. I n this connection, however, the authors do not believe that the data secured in this investigation can be used to secure a direct estimate of the actual life of a piping installation.
Application to Heating-System Corrosion When steam is produced in a boiler, it will ordinarily be mixed with both oxygen and carbon dioxide. The oxygen is some part of that which is dissolved in the raw water. Carbon dioxide is produced by the breakdown of the carbonates contained in the raw water when heated to boiler temperatures. The dry steam leaving the boilers is not corrosive, even when it contains these two gases. This statement is in accordance with the accepted corrosion theories and is borne out in practice by the fact that corrosion of the steam piping of a power plant or distribution system is not noticeable after fifty years of continuous service. When the steam condenses in a piece of equipment, such as a radiator, the amount of these two gases which dissolves in the condensate depends on the partial pressures of the gases in the system in accordance with the laws of Henry and Dalton. The actual amounts of the gases which dissolve are negligibly small under usual operating conditions. I n investigations of corrosion in heating systems, condensates are often found which do not have the expected low concentrations of dissolved gas but instead have concentrations equal to the higher magnitudes studied in these experiments. Inleakage of air from the atmosphere can be responsible for high oxygen concentration, but carbon dioxide is not present in the atmosphere in sufficient concentration to be a source of serious contamination. A high concentration of both gases, however, can result from the operation of certain pieces of equipment in which the steam is rapidly condensed out of contact with a vapor space. Some types of kitchen equipment can produce high concentrations of dissolved gas in the outflowing condensate. A type which often gives trouble consists of a steam coil of small dimensions, immersed in a relatively large volume of water. The outward flow of condensate from the steam coil may be controlled by a thermostatic valve, actuated either by the temperature of the water surrounding the steam coil or by the temperature of the outflowing condensate itself. The operation of the thermostatic valve on this type of equipment often causes condensate to be impounded ahead of the trap in such a manner as to seal the system with respect to the escapement of the noncondensable gases entrained with the steam. As a specific example, one might consider a heating coil 5 feet long, of 3/8 inch diameter copper tubing, supplied with steam a t 5 pounds per square inch gage pressure, which is heating water a t a rate that will produce a condensate flow of 40 pounds per hour. At this rate of condensation approximately 800 volumes of steam will condense in the copper coil during each hour of operation. If it be assumed that the steam supply contains 1 p. p. m. of oxygen and 10 p. p. m. of carbon dioxide, the respective partial pressures of these gases in the steam supply will be 0.75 X 10-6 and 5.5 x 10-6 atmosphere, and their solubility in condensate formed in contact with freshly condensed steam will be only 0.000018 and 0.0026 p. p. m. After the unit has been in operation for 1 hour, the concentration of the gases trapped in the copper coil will be increased 800 fold because of their limited solubility in the outflowing condensate. Since the solubilities of the gases are directly proportional to their partial pressures, at dilute concentrations, 0.014 p. p. m. of oxygen and 2.1 p. p. m. of carbon dioxide are soluble in the outflowing condensate. The above description considers the solution of
779
gases from an equilibrium standpoint, a condition which is rarely approached in practice. It is more probable that in restricted spaces where condensation is taking place rapidly, noncondensable gases are trapped as bubbles with the condensate, subsequent solution taking place when the condensate flows toward the trap. I n its passage toward the trap an interface between the liquid and the steam is formed, and the condensate which flows away from this interface is cooled below the steam temperature. The gases which are entrapped as bubbles with the liquid go into solution because of the fact that they are subject to the operating pressure of the system. I n a number of field studies rapid corrosion has been noted where this type of equipment is in use, the corrosion occurring a t the point where the outflowing condensate first comes into contact with an iron pipe or fitting.
18 16 14 I2
010
8* 8 6 4
2
0 OXYGEN
- RRM.
FIGURE 6. RATEOF CORROSION AS FUNCTION OF OXYGEN
Corrosion difficulties which may be encountered with this type of equipment cannot be remedied by any ordinary treatment of the steam supply. The correct approach to the problem is the proper design of the equipment to avoid conditions that favor the appreciable solution of oxygen and carbon dioxide in the condensate. At the present time the development of such equipment is under consideration by leading manufacturers. Although the quantities of gases in the steam do not ordinarily make a corrosive condensate, the district steam producer has the responsibility of delivering his product with as little of these gases as is industrially practicable. Oxygen should be kept as low as is obtainable with the best heater practice, and carbon dioxide should be a t the minimum value, depending on the natural carbonate content of the raw water. The use of carbonates for feed-water treating agents is to be avoided. The user of the steam has the responsibility of maintaining his system tight,. particularly against inleakage of gases, and of taking all possible precautions to avoid inleakage of air into his returns, I n the design and operation of the system, locations a t which very rapid condensation occurs should be regularly vented or designed to provide space for a vapor phase.
Conclusions These experiments on the effect of oxygen and carbon dioxide on the corrosion rate of mild steel have indicated the following conclusions : 1. Oxygen and carbon dioxide mutually influence the corrosion rate, 20 parts of carbon dioxide having approximately the same influence as 1 part of oxygen. 2. At constant oxygen content the corrosion rate varied
INDUSTRIAL AND EZGISEERISG CHEMISTRY
780
linearly with the amount of free carbon dioxide in solution. At constant carbon dioxide content the corrosion rate varied linearly with the oxygen concentration except when the oxj-gen nas below some point in the region 0.0 to 0.5 p. p. m. At these low values the curve appears to flatten out. 3. At zero concentration of either of these gases, the corrosion rate mas not zero but a value dependent upon the concentration in solution of the other gas.
Literature Cited Am. Pub. Health Assoc., Standard Methods for Examination of K a t e r and Sewage, 6th ed., pp. 59-61 (1925). Bengough, G. D., Stuart, J. M., and Lee, A. R . , Proc. Roy. SOC. (London), A116, 426-67 (1927). Clark, W. M.,“Determination of Hydrogen Ions,” 3rd ed., p. 561, Baltimore, Villiams and Wilkins Co., 1928. Cournot, J,,and Chaussain, M., Compt. rend., 194, 1823 (1932). Cox, G. L., and Roetheli, B. E., ISD. ENG.CHEM.,23, 1012-16
It has been mentioned that the flow of the water in the apparatus was in the viscous region. As the mechanism of corrosion may be affected by the nature of the flow, these results are strictly applicable only to the condition in which the flow of the water is not turbulent. I n most reported cases of corrosion of steam returns, the corrosion is localized a t sections adjacent to the point of condensation, such as trap connections, where the condensate flow is slow and probably viscous. S o attempt has been iiiade to relate the results of these experiments to the rate of corrosion of steel pipe in actual practice where other factors than those considered here may have an influence. The experiments show the relative influences of oxygen and carbon dioxide on the corrosion rate, but it is the opinion of the authors t h a t it is inadvisable to relate these measurements directly to the life of a given piping installation because so many factors would have to be assumed or neglected.
(1931). Denman, W~L., and Bartow, E., I h i d . , 22, 36-9 (1930). Evans, E. R., and Hoar, T. P., Proc. Roy. Soc. (London), -4137, 343-65 (1932) Fraser, 0. B. J., Ackerman, D . E., and Sands, J . W., ISD.EKG. C H E M .19, , 332 (1927). Groesbeok, E. C., and Waldron, J. L., Pmc. Am. Sor. Testing X a t e r i a l s , 31, 279-91 (1931). Hall, R . E., and Mumford, -4. R . , Heating, P i p i n g Ai? Conditioning, 3, 943-59, 1041-9 (1931). Hayes, Henderson, and Staneart, Engineering Expt. Sta. Iowa St’ate Coil., Bull. 84 (1927). Lee, A. R., Trans. Faraday Soc., 28, 707-15 (1932). Lewis, G. N., and Randall, M., “Thermodynamics and Free Energy of Chemical Substances,” 1st ed.. p. 297, S e w T o r k , LTcGraw Hill Book Co., 1923. McKinney, D . S.,XND. EKG.CHEJI.,Anal. Ed., 3, 192-7 (1931). S a t l . District Heating Assoc., Proceedings, 23, 273-81 (1932). Tbid.,24, 195-204 (1933). Natl. District Heating Assoc., preprint 1933 meeting. . SOC.M e e h . Schroeder, TT. C., and Fellows, C. H., T ~ u n sA.m. Engrs., 54, R. P. 54-13, 213 (1932). Speller, F. N., “Corrosion Causes and Prevention,” 1st ed., p. 228, S e w York, McGraw Hill Book Co., 1926. Walker, J. H., Heating and Ventilating, 30, No. 5 , 28-32 (1933). Whitman, W.G., and Russell, R . J., J . Soc. Chem. I n d . , 43, 193 T,197-9 T (1924). Whitman, W,G., Russell, R . P., and Altieri, V. J., IXD,EKG. CHEST., 16, 665-70 (1924). Whitney, W. R.. J . Am. Chem. Soc., 25, 394 (1903). Wilson, R. E., ISD.EKG.OHEX, 15, 127-33 (1923). Yoder, J. D., and Dresher, J. D., Combustion, 3, No 10, 18- 22 (1934).
Acknowledgment Although these experiments were carried out entirely in the laboratory of the Xew York Steam Corporation, the authors wish to acknowledge the part played by the Sub-committee on Corrosion of the Real Estate Board of New York, the meinbers of which initiated the work and encouraged its progress. The authors wish to acknowledge also the assistance furnished by the research staff of the Kern York Steam Corporation, members of which have read this report and have made many valuable suggestions.
RECEIVED &larch 29, 1935. Presented before the Division of Industrial and Engineering Chemistry at the 89th Meeting of t h e American Chemical Society, Kew York, N. Y . , April 22 t o 26, 1935.
oasting o olubilitv of Alunite
Effect of J
LUKITE is a hydrous, basic sulfate of potassium and aluminum of the formula Kz0~3-41~034SO3 6H20, on which basis its theoretical coniposition is: potassium oxide, 11.37 per cent; alumina, 36.92; sulfur trioxide 38.66; and water, 13.05 per cent. It is of interest, therefore, as a source of potash and alumina, and is so utilized in certain foreign countries. TTithin the United States, however, although there are important deposit. of this mineral in Utah and rariouq western states (S),it has riot been utilized except to a limited extent in the raw or roasted state as a low-grade fertilizer since the war-time period of intense potash activity. Interest in utilization is being revived from the broader viewpoint of industrial planning for the western states (a); this interest ifi further indicated by pilot-plant experimentation now in progress t o test processes, the nature of which has not been published. The Utah deposits hare been the inost accurately surveyed and are conrervatively estimated (Z) to contain 3,000,000 tons of higli-grade mineral containing 10 per cent
VOL. 27, NO. 7
J . RICHkRD iDA3lS Fertilizers Investigations, Bureau of Chemistry and Soils, Washington. D. C,
potash (K2O) and little or no silica. In the same region are additional reserves whose tonnages are estimated in large figures ( 7 ) ~
Chemical Properties Aside from the patent literature, of doubtful value as a source of physical constants, little information has been published to describe the chemical properties of alunite. The usefulness of the available data often suffer: througli lack of information as to the source or character of the material under examination, since it is obvious from what has been publiihed that alunite from different deposits sonietinies differs TI idely in physical and possibly, therefore, chemical properties. It is assumed (unless otherwise stated) that, in the reqearciies reported froin American laboratories, alunite from the be5tknown domeqtic deposit-the Marysvale region of iouthern Utah-has been employed; accordingly, only reference< to
