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T h e conditions of the experiments were identical in every respect except t h a t the charcoals were different. Charcoal F shows a maximum hydrogen chloride production of 34 per cent, while Charcoal E reaches 26 per cent. The curves fall off at t h e end of 800 min. t o 1 8 per cent in the case of F, and 7 per cent in the case of E. Other charcoals are known t o show even greater differences a t low temperatures. Accurate comparisons are not available a t higher temperatures. It is to be noted in Figs. I , 2 , and 3 t h a t the delay in the appearance of hydrogen chloride is due t o the 6 absorptive action of the charcoal on this gas and not t o any induction period in the reaction.
1
[:I 8qo 0
K, 50
541
PI/'' %!
SUMMARY
FIG.Z-CURVES S H O W I N G THE PRODUCTION O F HYDROGEN CHLORIDE (MARKED A N D AMOUNT OF CHLORINE(MARKED0) ESCAPING ACTION WHEN VARYING AMOUNTS OF WATER ARE PRESENT CONSTANTS:Charcoal E; depth of layer, 10 cm.; cross section, 10.75 cm2 ; weight of charcoal, 67 g.; mesh, 8 to 10 per in.; temperature, 25"; velocity of gas, 500 cm. per minute: air-chlorine ratio, 500 to 1; pressure of gas above charcoal, 1 atmosphere VARIABLES: Water content of air, El, 0; Ez, 20; Ea, 40; and E4,80per cent saturation The niolar ratios of water to chlorine are, 0:l: 3.1: 1: 6.2:l; 12 4.1
+)
the reaction itself. The really significant parts of these curves are the ones indicating t h a t the production of hydrogen chloride is in each case reaching a constant rate dependent upon the amount of water present. Later experiments conducted without air and with the water-vapor greatly in excess of t h e requirements of t h e reaction indicate t h a t large-scale production of hydrochloric acid is possible by this method. C A T A L Y T I C VALUE O F D I F F E R E N T CHARCOALS-It is well known t h a t different charcoals, and charcoals treated by different processes, have greatly varying powers of absorbing gases, and other substances such as dyes, colors, and impurities from solutions. I t is also evident from the work of Bohart and Adamsl t h a t different charcoals show great differences in the catalysis of the reaction between chlorine and water.
%t-
,
i
,
, ,
, , ,
, ,
1
FIG.STCURVES S H O W I N G THE P R O D U C T I O N O F HYDROGEN CHLORIDB (MARKED +) A N D AMOUNT OF CHLORINE (MARKED 0) CONSTANTS:Charcoal F: depth of layer, 10 .em.; cross section, 10.75 cm2.; weight of charcoal, 65 g.; mesh, 8 to 10 per in.; temperature, 25'; velocity of gas, 500 cm. per min.; air-chlorine ratio, 500 t o 1; pressure of gas above charcoal, 1 atmosphere; pressure of gas below charcoal, 1 atmosphere minus 0.97 cm. of water; molar ratio of water t o chlorine 7.7: 1
As an illustration of this fact the curve Ez, Fig. I, may be compared with the hydrogen chloride curve of Fig. 3. 1
1 . 0 ~cil. .
I-It is shown t h a t the reaction between chlorine, water, and charcoal between o o and about 130' C. produces hydrogen chloride and carbon dioxide, and t h a t the most important factors influencing the speed of the reaction are the character of t h e charcoal, the temperature, and the relation between the concentrations of the water and the chlorine. a-A reversal of Deacon's process is discussed and i t is interesting t o note t h a t changing industrial conditions have made i t of possible commercial application. THE CORROSION OF BRASS IN DILUTE ELECTROLYTES1 By J. H. Reedy and Bertram Feuer DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS,AND WATERSURVEY DIVISION,STATE OF ILLINOIS, URBANA, ILLINOIS Received January 21, 1920
The corrosion of metals and alloys is conditioned b y a number of influences, such as the composition of t h e metal, the dissolved substances present in the liquid in contact with i t , temperature, stirring, dissolved oxygen, homogeneity, contact with metals and catalytic agents, the previous mechanical and thermal treatment of the metal, the nature of its surface, and so forth. I n view of this large number of influences which are operative in the process of corrosion, and especially inasmuch as some of them are decidedly indefinite and hard t o control, i t is not surprising t h a t the results and theories of corrosion are as diverse and conflicting as the literature shows them to be. The principle of studying each influence-individually seems the most promising method of attack for this problem, and h a s been used in the present work. A large amount of valuable work has been done on this problem, and the authors have freely availed themselves of the information already in the literature. Very noteworthy in this respect are t h e elaborate 1 This article is published as a more or less preliminary report. The writers are fully aware that certain phases of the problem have been only superficially investigated, and others have been omitted altogether. It is planned t o continue this study at a later time. The Fourth Report of the Corrosion Committee of the Institute of Metals, written by Bengough and Hudson ( J . Inst. Metals, 2 1 (1919), 37). It was received after the experimental work of this paper was finished appears t h a t part of the work of these investigators has been duplicated in our work and the conclusions t h a t have been reached on certain points are identical. However, i t has been deemed best to publish our paper in its. original form, not only as a substantiation ofthe results of the above investigators, but particularly t o present a unified account of our work which contains a largefamount of experiment and inference not covered in thc British paper.
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
542
researches of Bengough and his collaborators1 on t h e corrosion of brass condenser tubes by sea water. These investigators, however, have considered t h e influence of t h e components of t h e solution erz masse, and have given no attention t o t h e individual behavior of each of t h e various ingredients. I n t h e work of the Illinois State Water Survey, i t has been found t h a t certain waters are very corrosive t o t h e brass working parts of water meters and pumps. I t was this fact which suggested t h e present investigation’ and led t h e writers t o limit their experimental work t o such ions as ordinarily occur in natural waters. Bengough has defined two types of corrosion,2 “complete” and “selective.” I n “complete” corrosion t h e components of t h e brass are equally corroded-that is, in t h e proportion in which they occur in t h e alloy, This form of corrosion is due t o direct oxidation b y dissolved oxygen. I n “selective” corrosion only one ingredient (the zinc) is corroded. The action is electrochemical and depends on differences of potential, either within the brass itself, or between t h e brass and some metal in contact with it. S E L E C T I V E OR E L E C T R O L Y T I C C O R R O S I O N
It is generally accepted t h a t a n y set of conditions t h a t will bring about a difference of electric potential between two metals in metallic connection, or between t h e components of an alloy, will result in electric currents, and corrosion will take place a t t h e expense of t h e metal of highest solution pressure. I n t h e case of brass, t h e corroded element is usually t h e zinc, and its removal is called “dezincification.” The characteristic of this selective form of corrosion is pitting, or in t h e more serious cases, perforation. The zinc is removed, leaving t h e copper matrix in a porous and weak form. The action is localized around t h a t p a r t of t h e couple which functions as t h e cathode, and in general appearance i t differs conspicuously from corrosion due t o oxidation. I n order t o study t h e part t h a t t h e various ions of a solution may play in electrolytic corrosion, t h e writers have carried out the following experimental work. M A G N I T U D E O F P O T E N T I A L DIFFERENCES-The potential differences between pieces of bright copper and bright zinc were measured a t room temperature (21’ t o 2 2 O C.)in sever81 solutions, obtaining initial values of 0.9-1.0 volt (Table I). Assuming t h a t there are discrete particles of copper and zinc present on t h e surface TABLEI-POTENTIALDIFFERENCES BETWEEN COPPER AND ZINC Volts Cu 0.1 M HCl : Zn................. 0 92 Cu:O.lMKCI :Zn 0.92 Cu : 0 1 M KNOa : Zn 0.92 Cu : 0.1 M KNOz : Zn 0.98 Cu.OIMNH4Cl :Zn 0.95 Cu : 0 1 M NaHC03 : Z n . . 1.04
.
................. ................ ................ ................. ...............
of t h e brass, this gives an approximate measure of t h e intensity factor operating a t t h e start in t h e electrolytic corrosion of brass. This difference becomes smaller as corrosion proceeds, due t o polarization effects, but since in practice t h e volume of t h e solution is exceedingly large and since t h e liquid is frequently in constant motion, the working value of this potential 1
J . Inst. Metals, 10 (1913), 13; 16 (1916), 37.
* Ibid., 10 (1913), 21.
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may remain high. This was shown i n t h e following manner: The electrodes just referred t o were shortcircuited for 24 hrs. in a 0.1 M potassium chloride solution, washed as thoroughly as possible in t h a t solution, and upon remeasuring gave a potential of 0.76 volt. This indicates t h a t , with a n electrolyte of good conductivity, electrolytic corrosion of brass m a y be very considerable. . ROLE O F THE IoNs-The corrosive influence of t h e common ions of natural waters was investigated b y means of current-potential curves. An electrolytic cell was set up, using as a n anode a piece of 80 : 2 0 brass, z X 2.5 cm., and as a cathode a piece of bright sheet platinum. Care was taken t h a t t h e relative positions of t h e electrodes should be unchanged in t h e various experiments, and t h a t t h e stirring should be as uniform as possible. Fig. I shows a diagrammatic arrangement of t h e apparatus. Increasing potentials were applied by means of t h e sliding resistance R, and were read directly from t h e voltmeter V, which was placed on a shunt around t h e cell and t h e milliammeter M.
/CATHODE
FIG. DIAGRAM O F APPARATUS
The curves for 0.1 molar solutions of potassium nitrate, chloride, and bicarbonate are shown in Fig. 2. The curves for 0.05 M KzS04 and 0.1 M KNOa are almost superposable with t h a t for 0.1 M KNOa, and for t h a t reason were not included in t h e figure. On account of t h e double flexure in t h e potassium chloride curve i t is not easy t o compare t h e influences of these anions on corrosion. Assuming t h a t t h e operative potential lies within t h e range 0 . 7 t o 1.0volt, i t would appear t h a t there is very little difference in t h e corrosive effects of C1, NOa, NOa, and SO4 ions. The action of HC03 ion is, for some unexplained cause, much less t h a n t h e others. The sinuous form of t h e chloride curves requires an explanation. All the chloride solutions used showed decided double flexures, with t h e exceptions of hydrochloric acid a n d ammonium chloride. The double flexure in current-potential curves is generally interpreted t o mean a change in reaction a t one of t h e electrodes. I n t h e case of solutions containing C1 ions, t h e anode
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1920
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reaction with brass electrodes a t low potentials seems t o be t h e discharge of C1 ions, forming cuprous chloride as a primary product. Owing t o t h e insolubility of cuprous chloride, polarization effects are very slight , and t h e current mounts rapidly with t h e first increase in potential. At higher potentials a second reaction begins--the direct ionization of the copper of t h e brass--.and this gives a second stimulus t o t h e current.
543
chlorides of equivalent concentration, the difference being due t o the hydrolytic formation of free hydrochloric acid in the first two solutions, Fig. 4 shows the effect of concentration on electrolytic corrosion. I t is significant t h a t * t h e current increases with the conductance of the solution, indicating t h a t in electrolytic corrosion high concentrations, by lowering t h e resistance of the liquid part of t h e circuit, would strongly stimulate corrosion.
It has been suggested by some writers1 t h a t a solution containing two or more salts might be more corrosive t h a n the combined effect of the component salts when taken separately. Current-potential curves were taken for several mixtures, and the total current was found t o be very nearly equal t o the sum of the currents for separate solutions of single salts of equivalent concentrations t o those in which they occurred in t h e mixture. I t is inferred t h a t the influence of t h e various ions on electrolytic corrosion is an additive property. CORROSION PRODUCTS-very naturally corrosion products vary with the electrolyte. The products for low potentials in neutral solutions are basic salts, only loosely adherent t o the anode. A t higher potentials the solution becomes turbid and its color shows a n increasing amount of dissolved copper. Basic copper chloride was found t o be very stable, a n d constituted the principal part of t h e corrosion product with neutral chlorides. With nitrates and sulfates, basic zinc salts seemed t o predominate. Solutions containing HC03 ions appeared rather non-corrosive from the electrolytic point of view, and no appreciable precipitate formed unless high potentials were used. I N F L U E N C E O F DISSOLVED O X Y G E N I N E L E C T R O L Y T I C VOLT3
Fw.2-INFLUENCE
OF
ANIONSIN ELECTROLYTIC CORROSION
T h e second flexure is in the neighborhood of a potential of 1.0 volt, its position being displaced in different solutions b y different cathode potentials. I n some cases, as for instance hydrochloric acid a n d ammonium chloride, curves of apparently different types were obtained, but t h e abnormality is more apparent than real. In hydrochloric acid the second flexure is displaced t o the left by an elevation of the cathode potential, and in ammonium chloride t h e first p a r t of t h e curve is modified b y a side reaction-the formation of NH3 complexes. Current-potential curves were taken for sets of solutions having a common anion, so as t o get comparative values for t h e influence of several cations (Fig. 3). With hydrochloric acid solution marked corrosion is indicated, as would be expected from the high mobility of the hydrogen ion. With solutions of metallic chlorides, the corrosion is much less. This is due t o t h e fact t h a t t h e cathode reaction is t h e discharge of hydrogen ions, and the amount of current passing a t t h e cathode depends directly upon t h e concentration of these ions. From this i t follows t h a t solutions which are acid, either by reason of a free acid or owing t o hydrolysis, are favorable mediums for corrosion. Ammonium and calcium chloride solutions appear more corrosive t h a n potassium and sodium
coRRosIox-Some have assumed t h a t dissolved oxygen would stimulate electrolytic corrosion by acting as a depolarizer a t the cathode. I n order t o verify this assumption, current-potential curves were taken for 0.1 M KNO9, using a brass anode and a copper cathode. I n one experiment, t h e solution (previously boiled t o expel air) was stirred by a jet of nitrogen bubbled into the solution close to the cathode. I n a second run, the solution was stirred with filtered COz-free air instead of nitrogen, The curves were identical within t h e limits of experimental error. A solution of 0.1 M KCl gave similar results. It is therefore concluded t h a t dissolved oxygen is without significant effect in electrolytic corrosion. COMPOSITION OF T H E BRASS-The measurements cited above were made with a n anode of commercial brass containing 80 per cent copper and 2 0 per cent zinc. Other brasses ranging from 90: I O t o 60: 40 gave potential measurements of the same order as those with the 80 : 2 0 form. The results given here are merely representative. The specific effects of variation in composition of t h e brass will be studied further. I N F L U E N C E O F TEXPERATURE-At elevated ternperatures the effects of electrolytic corrosion are greater2, and t h e life of brasses exposed t o agrated 1 Gaines, Eng. News, 19 (1908). 578; Heyn and Bauer, Mitt. kgl. MaterialDrilfungsamt, 88 (1910). 62. f Bengough and Jones,J . Inst. Metals, 10 (1913), 50.
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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
natural waters is much shortened, owing t o pitting and possible perforation. High temperatures favor corrosion in two ways, viz.: I-The mobility of t h e ions is increased, t h u s greatly reducing t h e resistance of t h e liquid phase of t h e circuit. a-There is an acceleration in t h e speed of t h e reactions involved, such as t h e ionization of t h e zinc, and so forth. The above experiments suggest very strongly t h a t , so f a r as electrolytic corrosion is concerned, t h e role of most ions is immaterial, except as they maintain t h e conductivity of t h e solution. Certain ions, on t h e other hand, show a specific corrosive action. I n t h e case of chlorine ions, this is connected with t h e removal of t h e ions of t h e anode metal b y converting them into insoluble compounds or into complexes of low ionization. I n other words, they act as depolarizers.
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product containing copper oxide. It is clear t h a t such reactions are not electrolytic, since in t h e first two cases t h e metals are quite homogeneous and t h e corrosion is general, not local. I n t h e case of brass, electrolytic corrosion would hardly result in t h e solution of t h e nobler component. It appears t h a t this form of corrosion is due t o oxidation b y dissolved oxygen, known as “complete” or “general” corrosion. O X Y G E N E S S E N T I A L T O C O M P L E T E CORROSION-In corrosion hydrogen must be liberated, or oxygen (or some chemically analogous substance) must be taken up from t h e air or other oxidizing agent. I n order t o learn whether hydrogen was liberated, pieces of soft brass, I . j X 7 cm., were placed in 0.1 M solutions of HC1, KC1, NH4C1 and NHINO,, each of which had been boiled t o expel absorbed oxygen. The apparatus consisted of 2 0 cc. glass-stoppered weighing bottles, each of which, after introduction of t h e brass, was completely filled with t h e solution so as t o exclude all bubbles. The bottle was placed in a cylinder a n d covered with an excess of t h e same solution t o seal i t from access of air b y leakage. After standing for varying lengths of time (Table 11),t h e brass remained TABLE11-EFFECT os ABSENCEOF DISSOLVED OXYGEN O N COMPLETE CORROSION Loss in Length of Test Weight SOLUTION Dissolved Air Days Grams Saturated by frequent shaking 37 0.0125 0.1 M KCI 0.1 M KCI None 37 0.0010 0.1 M HC1 None 26 0.0012 0.1 M NHaCl None 26 0.0003 24 0.0006 0 1 M NHdN03 None 6 MHCl hTone 17 0.0010
VOLTS
FIG.3-INFLUENCE
OF CATIONS IN ELECTROLYTIC CORROSION
As another example, waters containing hydrogen sulfide are corrosive t o brasses, not so much on account of their acidity as t h e formation of insoluble zinc sulfide and copper sulfide. Solutions of ammonium salts act very much in t h e same way, in t h a t t h e solution becomes alkaline and free ammonia is formed, which unites with Zn and Cu ions t o form NH3 complexes of low ionization. This explains t o some extent t h e large current with ammonium chloride solution. COMPLETE CORROSION
It is easily demonstrated t h a t fine copper is tarnished in t h e presence of moist air. Pure zinc is similarly coated with a layer of zinc oxide. Under similar contditions brass corrodes, but much more slowly, t o a
bright a n d no bubbles of gas appeared in a n y of t h e tubes, indicating t h a t no hydrogen was evolved. With 6 M HC1 (sp. gr. 1.12) brass was likewise inactive a t ordinary temperatures, b u t upon heating evolution of hydrogen began at temperatures slightly below 50’. Table I1 shows t h a t , while there was no visible liberation of gas, there was a certain amount of corrosion in all cases, t h e amount, however, being small in comparison with t h a t occurring when air was present. Hence i t is concluded t h a t oxygen is necessary t o extensive corrosion of brass in dilute solutions a t ordinary temperatures. Similar results were obtained b y Adiel in his studies of iron, and have since been fully confirmed b y the very careful investigations of Friende2 N E A S U R E O F coRRosIoN-The usual procedure in determining t h e amount of corrosion has been t o remove t h e corrosion product by some mechanical means, such as carefully polishing with an abrasive, or rubbing with a brush or rubber-tipped rod, a n d determining t h e corrosion by loss in weight. I n t h e present work it has been found far quicker and more accurate t o remove t h e deposit chemically. The corroded test piece is immersed in cold hydrochloric acid (sp. gr. 1.12) for one minute, rinsed thoroughly with distilled water which has been boiled t o expel dissolved air, and dried between filter papers. I n a series of four blank tests, using brass pieces 2.5 X 6. j cm., t h e average loss on this treatment was 0.0003j gram. Ammoniacal and alkaline cyanide solutions 1
Min. Proc. I n s f . C k i t Eng., 4 (1845), 323.
2
“Corrosion of Iron and Steel,” 1911, 142
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
were tried as solvents for t h e corrosion products, b u t discarded because of poor efficiency and a marked tendency toward oxide formation in the subsequent rinsing. A very promising means of determining corrodibility of brass depends upon considering t h e oxygen consumed an index of corrosion. Pieces of brass of convenient size are placed in small tubes connected with mercury manometers of t h e open-arm type. The solution t o be used is shaken with air a t room temperature until saturated, and a definite volume introduced. T h e tube is best closed b y sealing off t h e capillary end of a vent tube, so as not t o disturb t h e level of the mercury in t h e manometer. This method affords a very convenient means of learning not only the relative corrodibility of two or more specimens of
VOLTS
Fro 4-INFLUENCE
OF
CONCENTRATION ON ELECTROLYTIC CORROSION
brass, but gives t h e necessary information about t h e speeds of corrosion. The use of rubber stoppers in this work is not recommended, since in most of t h e experiments where they were used t h e results were vitiated by leakage around t h e tubes or diffusion through t h e rubber. Ground glass joints seem t o be re qurr e d. I N F L U E N C E O F I O S S O S C O M P L E T E CORROSION-&?any observers have reported t h a t there are wide differences in t h e corrosive action of various solutions. T o determine t h e specific influence of various solutions, pieces of soft brass, 2.j X 6.5 cm., were placed in I 50 cc. glass-stoppered bottles and covered with I O O cc. of the solution t o be investigated. The bottles were thoroughly shaken so as t o saturate the solutions with air, and t h e shaking was repeated every 24 hrs. These tests were carried out in groups of 18, and every care was taken t o make the conditions as uniform as possible. After standing 1 5 days, t h e pieces were cleaned in hydrochloric acid of 1 . 1 2 specific gravity as described above, and t h e loss in weight determined. Table I11 gives t h e results obtained. Comparative values of t h e influence of various anions may be obtained from a consideration of t h e
545
TABLE 111-COMPARISONOF COXPLETE CORROSION IN VARIOUS SOLUTIONS Distilled Hz0 0.1 M HCl 0.1 i M KC1
WT. Grams REMARKS 0.0016 Brownish red tarnish 0.0744 No stain. Cu-plate on edge 0.0105 Lozal Led stains. Slight precipitate containing
0.1 M NaCl
0.0033
0.1 M NHaCl
0.0721
0.05 M CaClz
0.0151
SOLUTION
LOSS IN
,m,
0.05 M MgClz
0.0166
0.1 M KNOi
0.0032
0.05 M KzSOd
0.0044
0.1 M KNOz 0.1 M NHaNOa
0.0017 0.1120
0.1 0.1 1.0 0.01
M NaHCOs M KHCOa 24 KC1
M KCl
1.0 MNHdCl
0.3306
0.01 M NHiCl
0.0294
LU
Local Led stains. Slight precipitate containing zn cu No tbmish. Colorless solution. Blue precipitate containing Cu only Slight reddish stain. Precipitate containing Zn Cu Slighi reddish stain. Precipitate containing Zn, Cu Local black splotches. Traces of NHI and NOz ions in solution General tarnish. Local black stains. Trace of sulfide in corrosion product Local black stains General black tarnish. Colorless solution. Precipitate containing Cu, Zn General darkening, with local red stains General darkening, with local red stains Bright. Slight precipitate containing Cu Covered with red-brown stain Bright. Blue solution. Green precipitate containing Cu Red-brown stain. Blue solution. Greenish precipitate containing Cu, Zn
0.1 M potassium salts in t h e above table. They fall into t h e following order (the most corrosive first): H C 0 3 , C1, Sod, NOS, NOz, OH. T h e order of t h e cations is: H , "4, Mg, Ca, K, Na. The H C 0 3 ions were t h e first t o show visible action. I n from 5 t o 6 hrs. t h e test pieces were almost wholly covered with a black deposit of what seemed t o be copper oxide containing carbonates. The deposit seemed t o exert little protection for t h e uncorroded brass, as is indicated by the high corrosion. Marked corrosion has always been noted in solutions containing chlorides. Their specific action seems t o be directly related to: ( I ) The insolubility of the basic chlorides of zinc and copper. ( 2 ) Their marked power for dissolving oxygen. A characteristic of the corrosion by neutral chlorides is t h e formation of brownish splotches which appear t o be regions of pure copper, and not of cuprous oxide, as some workers have reported. This conclusion is based upon t h e insolubility of these stains in dilute hydrochloric acid and potassium cyanide solutions. Another interesting observation, made only in chloride solutions, is t h e apparent copper plating of the brass. T h e deposit has exactly t h e same apbearance as t h a t formed upon placing brass in a solution of cupric chloride. I t is suggested t h a t the copper first goes into solution by general corrosion, and is later precipitated by the zinc of t h e brass. Sulfates are characterized by a general darkening of the brass, and later by t h e formation of dark splotches. On the fourth day a slight precipitate of what was taken t o be basic zinc sulfate appeared. A trace of CuS was formed during t h e corrosion, though the amount seemed too small t o account for t h e total corrosion loss. It is thought that t h e main action is direct oxidation, rather t h a n t h e reduction of sulfate t o sulfide. I n nitrate solutions brass might be expected t o behave as a zinc-copper couple, reducing the nitrate readily t o ammonia. Here, as in other respects, there is a sharp contrast between brass and its constituents in t h e form of a couple. I n t h e 0.1 M K N 0 3 solution t h e brass piece seemed almost inert. With t h e exception of dark stains localized on some scorings formed i n
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546
rolling the brass, the piece remained bright. At t h e end of the I 5-day period of corrosion, Nessler's reagent and a-naphthylamine-sulfanilic acid reagent showed t h e presence of small amounts of ammonium and nitrite ions, respectively. Nitrites are even less corrosive t h a n nitrates. The corrosive influence of the cations has already been forecasted. The action of free acids-that is, of solutions containing the hydrogen ion-consists in dissolving oxides as fast as they form. Again, if t h e free acid has a specific oxidizing influence, like nitric acid, its corrosive effect will be accordingly intensified. Formation of free acids hydrolytically, as hydrochloric acid from magnesium chloride or nitric acid from ammonium nitrate, explains the marked increase of the corrosive action of these solutions over t h a t of potassium chloride or nitrate. The ammonium group has an exceedingly corrosive effect by reason of t h e formation of NHI complexes. Tests with litmus paper showed a development of alkalinity in all t h e neutral salt solutions used. INFLUENCE
OF
DISSOLVED
OXYGEN
ON
COMPLETE
coRRosIoN-Undoubtedly a contributory influence in general corrosion is t h e solubility of oxygen in t h e various solutions.' Oxygen is more soluble in an acid t h a n in one of its salts of t h e same concentration, and more soluble in a salt t h a n in t h e corresponding base of t h e same concentration.2 Furthermore, solubility diminishes with increase in concentration. For this reason (see 1.0M KC1, 0.1 M KCI and 0.01 M KCI in Table 111) corrosion falls off a t higher concentrations, and a high enough concentration may completely inhibit it. Such limiting values of concentration have been defined in t h e corrosion of iron.3 The divergence of acids,and ammonium salts from this behavior may be inferred from what was stated above. MECHANISM OF COMPLETE CORROSION-ASa working hypothesis t h e following are assumed t o be t h e fundamental reactions in complete corrosion by dissolved oxygen: 2Cu 2Hf 0 2Cu+ H20 (I) Zn 2Hf 0 _% Znff HzO (2) The equilibrium may be shifted in t h e forward direction in three ways: I---Increase in t h e concentration of H ions, t h a t is, t h e acidity. 2-Increase in t h e concentration of t h e dissolved oxygen, which varies with different solutions. 3-Removal of t h e Cu and Zn ions by (a) precipitation as oxide or other insoluble substance, ( b ) complex ion formation, as Cu(NH3)2+ and Zn(NH3)4++, and ( c ) oxidation of Cu+ t o CU++. This conception of an equilibrium gives a ready explanation of several of t h e outstanding features of brass corrosion. The well-known fact t h a t alkalies of moderate concentration inhibit corrosion follows from t h e repression of t h e H ion. The so-called catalysis by means of dissolved carbon dioxide is explained not
+ +
+ +
+
+
1 Trans. Far. SOC., 6 (1909), 68; J . Am. C h e m . Sac., 33 (1911), 362; American Public Health Association, "Standard Methods for Water Analysis Analysis," 1917, 68. a Giffcken, 2. physik. Chem., 49 (19041, 257. 8 Friend, LOC.cit , p. 131.
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12, NO.
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so much by t h e slight increment of H ions as by t h e removal of Cu and Zn ions by the HCO, ion. Those solutions which are t h e best solvents for oxygen are generally t h e most corrosive. O T H E R S I C R I F I C A N T INFLUENCES-The effect of stirring on the speed of corrosion is well known. Contact action is one of t h e most important factors in corrosion, but the writers have not been able t o include i t in t h e present investigation. An illustration came up in one of t h e manometer experiments. I n one of t h e tests a piece of soft brass stood in a 0.1 M KN03 solution for several weeks with no sign of corrosion; b u t when once a splotch of black CuO appeared a t one point, corrosion proceeded rapidly and the stain spread over t h e whole test piece. This behavior has its analog in the rusting of iron and steel, where the hydrated oxide catalyzes rust formation t o a marked degree.' Contact with metals nobler t h a n copper strongly catalyzes the corrosion of brass. The nature of t h e surface seems of significance. Pieces of brass which have been etched chemically, corrode more rapidly t h a n polished pieces.2 This may be partly due t o increase in surface. Corrosion usually begins a t the edge of t h e test piece, indicating t h a t t h e strained condition caused b y t h e cutting leaves t h e edges in a more reactive form. In a few cases corrosion started on t h e scorings left in rolling t h e sheet brass a t t h e mill. This behavior is comparable t o t h a t of steel^,^ where strains in t h e metal affect its corrodibility t o a noticeable extent. SUMMARY
The experiments described above point very decidedly to the conclusion t h a t there are two kinds of brass corrosion, v i z . , electrolytic and complete. The specific influences of various factors in corrosion may be summarized as follows: I-vigh concentration of electrolytes favors electrolytic corrosion in an additive way, and retards, o r even inhibits, complete corrosion. 2-Elevated temperatures favor electrolytic corrosion, b u t diminish complete corrosion by diminution of dissolved oxygen. 3-Contact with metals nobler t h a n brass favors electrolytic corrosion, and is without influence on complete corrosion. 4-Dissolved oxygen has no appreciable effect on electrolytic corrosion, b u t is essential t o complete corrosion. j-The presence of ions or groups t h a t combine with zinc and copper ions t o form compounds of low ionization favors both electrolytic and complete corrosion. 6-There is no specific influence of ions in electrolytic corrosion except in t h e case of ions t h a t form complexes of low ionization and in t h e case of hydrogen ion; on the other hand, there is marked specificity in complete corrosion, believed t o be largely, determined by oxygen solubility. Friend, Lac. c i t . , p. 96. Eden, Rose and Cunningham, Proc. Brit. Inst. Mech. Eng., [3 and 41 1911, 839; Moore, Mech. Eng., 41 (19191, 731. 8 Hambuechen, Univ. of Wisconsin Bulletin, Engineering series 8. (1900); Heyn and Bauer, J . Iron Steel Inst.* 1 (1909), 109. 1
2
June, r g z o
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
7-Homogeneity reduces electrolytic corrosion, and has no effect on complete corrosion. %-Other influences-catalysis, nature of surface, strained areas in t h e alloy, etc.-are operative, b u t have not been included in this investigation. It is doubtful whether in practice either form of corrosion ever occurs t o t h e exclusion of t h e other. I n complete corrosion, dezincification occurs as a secondary process b y t h e displacement of copper from t h e solution, and since electrolytic corrosion usually takes place in contact with the air, primary oxidation of the alloy is possible. GASOLINE FROM NATURAL GAS. I-METHOD REMOVAL By R. P. Anderson
OF
UNITSD NATURAL GAS CO., OIL CITY, PENNSYLVANIA Received December 31, 1919
The hydrocarbon content of natural gas consists essentially of a mixture of such hydrocarbons of t h e parafin series as exert an appreciable vapor pressure at t h e temperature of gas production. Similarly natural gas gasoline consists essentially of a mixture of those hydrocarbons of natural gas t h a t may exist in liquid form a t atmospheric temperatures and pressures, As natural gas hydrocarbons, there may be included t h e normal and various isomeric compounds from methane t o undecane, inclusive; and as constituents of natural gas gasoline, all of t h e natural gas hydrocarbons except methane, ethane, propane, a n d butane. For convenience of reference, various d a t a concerning t h e normal paraffin hydrocarbons from methane t o undecane, inclusive, are given i n Table I. The constituents of natural gas gasoline may be removed from natural gas and collected in liquid form by either t h e compression, t h e refrigeration, or t h e absorptiogz method. I n t h e general discussion which follows no attempt will be made t o consider details of apparatus or operation; on t h e contrary, t h e present purpose is merely t o present briefly t h e principles and applications of the different methods and t o bring out t h e relationships t h a t exist between them.’ COMPRESSION METHOD
The compression method for t h e production of gasoline operates on the well-understood principle t h a t t h e vapor pressure of a substance is independent of t h e pressure on t h e gas of which t h e vapor forms a part. Accordingly, when t h e pressure on the gas is increased, t h e partial pressure of the vapor increases only until its saturation pressure is reached, and from this point, increase in pressure results in condensation of increasing amounts of t h e vapor as liquid. The amount of t h e substance remaining in vapor form is 1 For details, consult the following publications of the Bureau of Mines: “The Condensation of Gasoline from Natural Gas,” by Burrell, Seibert and Oberfell, Bulletin 88 (1915). “12xtraction of Gasoline from Natural Gas by Absorption Methods,” by Burrell, Biddison and Oberfell, Bulletin 120 (1917). “Recovery of Gasoline from Natural Gas by Compression and Refrigeration,” by Dykema, BuZZetzn 151 (1918). “llecent Developments in the Absorption Process for Recovering Gasoline from Natural Gas,” by Dykema, Bulletin 176 (1919).
54 7
inversely proportional t o the number of compressions employed, using t h e pressure a t which t h e partial pressure becomes equal t o saturation pressure as a basis. For example, if a gas is saturated with pentane vapor a t atmospheric pressure, compression t o z atmospheres will condense one-half t h e pentane; if t h e gas is onetenth saturated, compression t o I O atmospheres will be necessary t o increase t h e partial pressure t o t h e saturation point and this last pressure must be doubled t o cause condensation of one-half t h e pentane vapor. Such difficulties as are encountered in t h e efficient operation of a compression plant result from t h e simultaneous condensation of t h e several mutually soluble hydrocarbons t h a t constitute natural gas gasoline. What is t r u e when pentane is t h e only condensable substance no longer holds when other condensable substances are present. The saturation pressure for each gasoline hydrocarbon is influenced by t h e other condensable hydrocarbons and varies as condensation becomes more and more nearly complete. The result is t h a t t h e most efficient method of operating a compression plant for t h e production of gasoline can be determined, a t the present time, only by experiment.
FIG.1
I n actual practice t h e degree of compression varies from a few pounds t o about 300 lbs., depending upon the kind of gas being treated. I n some cases a high vacuum is maintained on old wells in oil-bearing sands from which t h e original gas has almost entirely been withdrawn. The more volatile portions of t h e oil are gradually vaporized and only a slight pressure is required t o condense these as gasoline after they reach t h e surface of t h e earth. On t h e other hand, some natural gases give no condensation of gasoline a t 300 lbs. pressure. Gases t h a t yield much less t h a n 0.5 gal. per M cu. ft. of gas a t 300 lbs. pressure are not usually compressed for their gasoline content. Additional condensate may usually be obtained b y using pressures higher t h a n 300 lbs., but butane and propane are its principal constituents and these of course cannot be retained by themselves in liquid form a t atmospheric pressure and room temperature. There is no question b u t what butane may remain in solution in
