I N D U S T R I A L A N D ENGINEERING CHEMISTRY
April, 1930
mislead should be read favorably to the accomplishment of the purpose of the act. The statute applies to food and the ingredients and substances contained therein. I t was enacted to enable purchasers to buy food for what it really is. Literature Cited (1) American 1929
Medical Association, “ S e w and Non-official Remedies,”
335
(2) Mix and Sale, J . A m . M e d . ~ s s o c n . ,86, 1963 (1925). (3) Skinner and Sale, J. IND. ENG.CHBM.,14, 949 (1922). (4) U. S. Dept. Agr., Notices of Judgment 3869 (19151, 4310 (1916). (5) U. S. Dept. Agr., Notice of Judgment 5906 (1918). (6) U. S. Dept. .4gr., Notice of Judgment 12,367 (1924). (7) U. S. Dept. Agr., S. R. A , , F. D. No. 1 (1927). (8) U. S. P u b . Health Service, Pub. Health R c p f s . , 40, N o . 15 (1925). (9) Warren, U. S. Dept. Agr., Farmers’ Bull. 1448 (1925).
Galvanic Corrosion on Cast-Iron Pipes’ R. J. Kuhn N E W ORLEAKSPUBLIC S E R V I C E , I N C . , N E W ORLEANS,LA.
Cast-iron pipes t h a t have been buried in soils in t h e vicinity of New Orleans are susceptible to a type of corrosion due to electrolytic currents. A study has been made to determine the source of these currents. The current densities of t h e discharge a t t h e surface of t h e pits have been determined both by t h e method used a t t h e Bureau of Standards and by t h e potential tangent method. The latter is believed to be more accurate as t h e current does not flow in radial paths from t h e pits, but in all directions. Bf this method t h e current density a t t h e surface of the pits is found to be 54.8 milliamperes per square foot. A study of t h e cycle of t h e potential changes of t h e pipes shows t h a t t h e rate of corrosion is proportional to . ....
HE soils encountered in X’ew Orleans are very different
T
from those found in most other sections of the country. There are three main soil areas-the sandy loam section near the Mississippi River, the Sharky clay section in the old swamp area two or three miles from the river bank, and the muck section near the shores of Lake Pontchartrain. The loamy soil is fairly uniform and contains very little decayed vegetation. It is situated on ground about 15 feet above sea level, comparatively high for New Orleans. This section is considered the least corrosive of any in the city. The Sharky clay section is a t sea level and contains ft great deal of stumps and decayed vegetation. This area was formerly subject to tidal flow. The clay area is considered mediumly corrosive. The muck section has an underlying strata of clay and white sand, but the top soil consists of about 1 to 3 feet of nearly pure muck or decayed vegetation. This area was also subject to tidal flow. The muck area is one of the most corrosive soils in the United States, according to the U. S. Bureau of Standards. All three soils are extremely damp or even wet, the first least of all and the others in the order named. The electrical resistivities of the soils are generally very low and uniform for any one location. An average value for the clay and loam would probably be about 1000 ohm-centimeters, but values as low as 260 ohm-centimeters have been found and 500 ohm-centimeter soils are common. The low electricbl resistivity of the soil facilitates the flow of current from underground metallic structures to the earth with its resulting corrosion, regardless of whether due to straycurrent or to galvanic-current electrolysis. The texture and uniformity of the soil facilitates the measurement of these currents. For this reason N e f Orleans provides an ideal location for the study of this type of corrosion. Appearance of Corroded Pipes in New Orleans In many places in New Orleans cast-iron pipes, when corroded by stray-current electrolysis or by soil corrosion, 1
Received May 2, 1920.
Revised paper received March 8, 1930.
t h e difference in potential of pure iron in t h e pipe as indicated by its place in t h e electromotive series and t h e actual potential of t h e pipe in t h e earth. Various theories have been advanced to explain this difference of potential, but t h e writer is inclined to believe t h a t it is due t o a great extent to the presence of oxide foundry films acting similarly to mill scale on steel. An experiment with a piece of cast-iron pipe immersed in ferroxyl jelly which bears out this theory is described. This corrosion has been overcome to some extent in New Orleans by a system of electrolysis drainage, which causes a collection of current to counteract t h e discharging galvanic currents.
present a very characteristic appearance. These pipes look like new, the coal-tar dip glistening, and there is usually very little or no red rust visible. Upon pecking with a hammer or knife, deep holes or pits may be cut in the pipe, and these pits often extend through the pipe wall and several square inches in area. The material removed from them is the black graphitic residue ordinarily found on pipes corroded by electrolysis. This residue is a conductor of electricity, is porous, and contains soil moisture. On corroded cast-iron pipes in New Orleans the pitted area of the surface is usually but a small percentage of the total area. On tracing the outline of pits on paper a’nd measuring the area with a planimeter, the pitted area was shown to be 8 per cent of the total exposed surface area of the pipe. The pipe in question was very badly damaged by stray-current electrolysis and the area of pits was apparently large compared with that ordinarily encountered on pipes. Figure 1 shows a corroded cast-iron pipe after being washed of all loose dirt, and Figure 2 shows the same pipe after being thoroughly sand-blasted. I n the course of stray-current electrolysis investigations many cast-iron pipes were found which were very badly corroded in a manner similar to corrosion due to stray currents. In many of these cases there could not be linked any evidence of stray currents having influenced the corrosion. Conclusions as to stray-current electrolysis were drawn only after very careful investigation and analysis by some of the most up-to-date methods, using special electrical instruments. Measurement of Stray-Current Electrolysis With stray-current electrolysis current discharges from the pipes, mainly from the pits, and does not return t o the pipes except by some indirect circuit through substations, trolleys, rails, earth, and distant parts of the pipe system. The relatively heavy discharge of current from the pitted areas causes the rapid corrosion of the metal in the pits.
I N D l i X T R I A L A N D ENGYNEERING C H E M I S T R Y
April, 1930
Calculation of Current Densities
Determination of Current Density a t Surface of Pits
Assuming in Figure 5 that the current was discharging and collecting radially from the pipe in the particular plane measured the current densities a t the pipe surface may be calculated from the Bureau of Standards formula ( 1 ) : 2 E,‘ rDlog,(l
+ -)2 L
i=-
D+2a where i
= current density on pipe surface, in amperes per
square foot voltage drop between electrodes, in volts (voltage drop between earth a t positions A and C in Figure 5) r = resistivity of earth, in ohm-feet D = external diameter of pipe, in feet L = distance between electrodes a t positions A and C, in feet a = distance from pipe surface to nearest dectrode a t A , in feet
E,’
=
In Figure 5, E,’ r D
L a
=
variable for different positions 0 to 48
= 16.4 ohm-feet (soil a t location of test) = 4.33feet =
0.5foot
= 0.25foot
Solving for the current density, O D 1 Y f i e
z = -~
16.4 X 4.33 log,
(1
+
0,5 4.33
)
337
= 0.148 E,’
+ 2 x 0.25
Substituting the value of E,‘ in position 0, Ee,’ = (0 050 - 0.009) = 0 041 volt z = 0.148 X 0 041 = 0 006068 ampere or 6 068 milliamperes per square foot
The current density a t any position between 0 and 48 may now be calculated by substituting in the above equation the difference of potential between the rows in the earth A and C in Figure 5 . For example, a t position 4 the potential a t C is + 5 millivolts or SO.005 volt, while a t A the potential is +13 millivolts or $0.013 volt. The difference between A and C or E,‘ is 0.008 volt with the current discharging, as the earth near the pipe is a t a higher potential than the earth away from the pipe. The current density a t the pipe surface a t position 4 is 0.148 X 0.008 = 0.001183 ampere or 1.184 milliamperes per square foot. The current densities a t every position along the pipe have been calculated and plotted as shown in Figure 6, the length of the lines indicating the density a t the various positions and the arrowheads the direction of current flow. The maximum current density thus calculated is 6.068 milliamperes per square foot, discharging a t position 0. Smaller discharges and collections are shown a t other parts of the surface. These densities, as previously stated, were calculated on the assumption that the current was flowing in radial paths. It has been demonstrated many times that galvanic currents and even stray railway currents, in discharging from the small, isolated pitted areas of a pipe, do not follow radial paths, but flow in all directions in the surrounding earth away from the pits. Therefore the potential gradients produced by the current several inches from the pipe are less than those that would be produced by a radial flow in which the current was confined to narrower paths. The potential gradient measurements taken in the earth several inches from the pipe therefore give but little indication of the gradients and current densities existing at the surface of the pit.
Several methods have been devised to determine accurately the current density of discharge a t the surface of pits, one of which is the potential tangent method. This method is based on the theory that a tangent drawn to a point on a potential gradient curve will represent the potential gradient a t that point on the curve. The potential gradient and the soil resistivity being known, the current density may be calculated. The curve reprwenting the potential gradient in the earth in a line radial from the surface of a pit is obtained by measuring the potential a t various points in the earth on this line, referred to a distant point in the earth, by tlie use of two copper sulfate electrodes and the RIcCollum earth-current meter. The potential a t the surface of tlie pit is obtained by cleaning the earth entirely away from the pit and measuring the potential difference between olle electrode placed on a small lump of earth on the surface of the pit and another electrode in distant earth. Curve -4 in Figure 7 represents the potentials of the earth in a line 111 position 0 radial from the pipe shown in Figure 5 with reference to the potential of remote earth taken as zero. The curve shows a decided gradient away from the pipe. Taking the potential 1 alues for one point 3 inches and the other 9 inches from the pipe and assuming that these values were produced by a radial flow of current, hypothetical potentials have been calculated for various distances from the pipe and plotted as curve B , Figure 7. This curve follows the law P D = K , where K is a constant, P is the potential a t any point above the potential of remote earth, and D is the distance from the point in question to the center of the pipe. From this hypothetical curve the potential of remote earth is shown on Figure 7 as being 174 millivolts below the actual remote-earth potential. For this to he possible there mould have to be a steep gradient in the earth away from the pipe on all sides, and this would indicate a general discharge of current from the pipe which might be furnished by a source of current such as a battery. This was not the case, however. Other tests made in the nearby earth demonstrated that the potential of the earth 3 or more feet from the pipe represented the general potential of the earth. CURRENT TIFRMINALS FOR MAKING SOIL RESlSTlVl
M€ASUREM€NTS NOTSHO
€COLLUM EARTH CURRENT
METER
OUS POSITIONS OF CTROOES IN FACE OF Figure 3-Method
TRENCH of Measuring Earth C u r r e n t s a r o u n d a Pipe
Actually, there was not so much total current flowing in the earth several feet from the pipe as there was between the positions 9 - 0 and C-0 shown in Figure 5 . Part of the current which was flowing between A-0 and C-0 producing the values shown on curve A (Figure 7) spreads out sideways, causing a decrease of potential drop in the remainder of the radial path along position 0 under what it would have been had it all flowed radially and produced the values shown on curve B , thereby accounting for the 174-millivolt difference between the actual and the remote-earth potentials calculated. Assume for the present that the current does discharge radially from the pipe and produces a gradient such as shown
338
INDUSTRIAL AND ENGINEERING CHEMISTRY
on curve B, although actually the current fans out in all directions. The current density a t the pipe surface is obtained by dividing the potential a t the pipe surface by the resistivity of the soil. The gradient a t the pipe surface is obtained graphically by drawing a tangent to the curve B where it intersects the pipe surface a t point c and extending it until it intersects t o the base line a t b. The ordinate ,-MOVABLE ELECTRODE
5 POSITIONS OF LE ELECTRODE
Figure 4-Method of Measuring Galvanic Potential Differences in Earth Surrounding a Pipe i n a V-Shaped Trench
Vol. 22, No. 4
densities which may be several times lower than the actual values. Rate of Corrosion of Cast-Iron Pipe
One ampere discharging continuously from cast iron to an electrolyte will corrode 20 pounds of cast iron in one year, assuming 100 per cent corrosion efficiency. Cast iron weighs approximately 450 pounds per cubic foot. A slab of cast iron 1 foot square and weighing 20 pounds will be approximately 0.5 inch thick. Should 1 ampere discharge uniformly from the square foot surface a t 100 per cent corrosion efficiency, the slab would be eaten entirely away in one year. With 1 ampere discharging from 1 square foot the current density is 1000 milliamperes per square foot; 54.8 milliamperes per square foot will therefore corrode uniformly to a depth of 0.5 inch on cast iron in approximately 17 years. Current densities even greater than this have been encountered. The highest found was 146 milliamperes per square foot, and this density will eat entirely through cast-iron pipe in about 6 years. Failures from soil corrosion in New Orleans in 6 years have been encountered. Relation between Corrosion and Electrical Potential
A study of the causes of this current from an electrical a-c, o r 9 3 millivolts, divided by the abscissa a-b, or 10 inches, gives the potential gradient in the earth a t the pipe surface viewpoint indicates that cast-iron pipes when buried in New of 9.3 millivolts per inch or 111.6 millivolts per foot. Di- Orleans go through a definite cycle of potential changes, viding this quotient by the soil resistivity expressed in the seventy-two specimens having behaved in the same manner. same units, or 16.4 ohm-feet, gives the hypothetical current When newly buried, the pipes have a high electrical potential density in the earth a t the surface of the pipe, a value which in the electromotive series, much above that of pure iron, should check with the value obtained by the earth-current and large galvanic currents flow. The potential lowers in meter formula. From this graphic method the current a few days until it assumes a position close to that of pure density obtained is 6.8 milliamperes per square foot, while iron, and practically no galvanic currents flow. After the density obtained from the formula was 6.07 milliamperes several months the potential gradually returns to its former high position and the galvanic currents resume their flow per square foot, which is a rather close check. It was shown above that the Bureau of Standards formula and continue. A study of the effects of stray current on ferrous structures is based on extending the gradient curve to the pipe surface from two known points in the earth, assuming the current has shown that where structures such as electric railway to be flowing radially. Since the current does not flow rails, tie rods, spikes, pipes, etc., are maintained at a potential radially, but flows in such a manner as to produce a gradient above the natural potential of pure iron with respect to the curve which is much steeper a t the pipe surface than the surrounding earth, these structures will rapidly deteriorate hypothetical gradient curve, it is more accurate to use the actual curve made from a series of tests in a radial line rather than from the two-point method. By drawing a tangent to the actual potential curve A in Figure 8 a t a point where the curve intersects the pipe surface, a gradient of 900 millivolts per foot is obt a i n e d , which represents a current density of 54.8 milliamperes per square foot. This is q u i t e a c o n t r a s t to 6.0 7 milliamperes per s q u a r e foot as calcuFigure 5-Galvanic Potential Differences in Earth Surrounding a n Experimental Isolated Length Of lated by the Bureau of 48-Inch Cast-Iron Pipe in V-Shaped Trench in New Orleans Standards formula, in Values in millivolts above or below reference point, lines connect points of equipotential. which it was assumed the Eurrent which produced the potential values a t points from the action of ordinary corrosion and electrolysis. A-O-and C-0 in Figure 5 flowed radially from the surface of Where the same types of structures are maintained a t a the pipe. Owing to the fanning out action of the current potential below that of pure iron, as when cathodically discharging from the pits, the formula gives values for current protected in electrically drained electrolysis areas, these
April, 1930
ISDCSTRIAL A X D ESGIiYEERING CHEMISTRY
339
structures are protected not only from electrolysis but also from self-corrosion. I t appears reasonable t o conclude that the type of corrosion on cast-iron pipe with which we are dealing, whether from electrolysis or soil corrosion, is due directly to the natural or artificial potential a t which the pipe is maintained with respect to the potential which the pure iron in the pipe
8.0
7.0
6.0 5.0
4.0 3.0
-
2.0:
1.0:
-
03
ninth the area of discharge. In other words, the cathodic current densities would be nine times the anodic current densities. For a cast-iron pipe wall '/2 inch thick to fail from electrolytic attack in 6 years an anodic current density of approximately 146 milliamperes per square foot is required. Assuming the corrosion to be due to galvanic action in microscopic circuits between the iron and carbon particles, then the current density of collection on the c a t h odic carbon surfaces would be 9 times 146, or 1314 milliamperes per square foot. Under such a heavy SHADED DENOTES DISCHARGING current density polarization would occur in a UNSHADEDDENOTES C O L L E C T I O D ~ v e r y s h o r t period to such an extent as to stop the flow of current due to the p o t e n t i a l difference between iron and carbon. Ferroxyl Jelly Experiment
An i n t e r e s t i n g exp e r i m e n t was made with a piece of castiron pipe and a large Figure 6-Current Densities i n Milliamperes per Square Foot in Earth around 48-Inch Cast-Iron Pipe. These Values Obtained from Equipotential Values Shown i n Figure 5 vat containing several gallons of f e r r o x y l would have were it in equilibrium with the surrounding soil, jelly, a corrosion indicator made o f agar-agar and small assuming that all other conditions, such as chemistry of the amounts of phenolphthalein and potassium ferricyanide. soil, are the same and are of such a nature as to cause corro- The phenolphthalein turns pink by the production of alkali sion. In other words, the rate of corrosion is directly pro- a t a cathodic surface, while the potassium ferricyanide turns portional to the difference between the potential of pure iron blue by the dissolving of iron a t an anodic ferrous surface. in the pipe as indicated from its place in the electromotive Two plugs 2 inches in diameter were cut from a piece of 6series and the actual potential of the pipe in the earth. A inch coal-tar-dipped cast-iron pipe about 2 feet long, the non-polarizing copper sulfate electrode is used as a reference surfaces brightened down to bare cast iron, and reset in the holes from which they were cut but in such a manner as to be electrode. insulated from the pipe. The spaces between the plugs Theories as to Cause of Differences in Potential and pipe were filled with insulating compound both for insulation and to make the interior of the pipe waterproof. There is quite a variance of opinion as t o what causes this difference of potential which makes the iron in the pipe Small insulated wires were welded to the inner surface of anodic to the surrounding soil. Soil variations, differences each plug before it was reset in the pipe and one t o the inner in oxygen content of soil and soil waters, variation in composi- surface of the pipe itself, these wires to serve as test leads. tion of material, and presence of oxide foundry films acting The ends of the pipe were plugged with coal tar to make them waterproof and the pipe was placed in a large wooden similarly t o mill scale on steel have been mentioned. The writer is inclined to believe the oxide-foundry film vat of ferroxyl jelly. Tests were made of potential of iron plugs on open circuit to be the chief cause of this difference of potential on coalto a copper sulfate reference electrode in the jelly, of potential tar-dipped cast-iron pipe and that on undipped pipe other factors probably dominate, particularly variation in oxygen of pipe on open circuit to the electrode, and of potential of content of soil and soil waters and degree of surface rusting. plugs and pipe short-circuited to the electrode. PotentialIn both cases, however, it seems to be a matter of surface difference measurements by a null method between plugs and pipe were also made. The current flowing between oxidation. One theory advanced is that the deterioration of cast iron plugs and pipes on short circuit was also determined by a by self-corrosion is due to the local action between the iron null method. The plugs and pipe were short-circuited, a particles and the impurities in cast iron. This seems ex- condition existing in practice when scarred surfaces of casttremely doubtful, especially when the large polarizing iron pipe are in contact with the soil, and after several hours effects of such galvanic currents on the impurities are con- the discolorations in the ferroxyl jelly were noted. Later potential-difference measurements in the jelly were sidered. The carbon content of cast iron is about, 3 per cent by weight, or about 10 per cent by volume. The area of made in a manner similar to that illustrated in Figure 4, exposed carbon on the surface of a cast-iron object is there- so that equipotential and current stream lines in the jelly fore approximately 10 per cent of the total surface area, around the pipe could be plotted. The results showed an initial difference of potential of leaving about 90 per cent of the area iron. With the iron anodic to the carbon, the iron would discharge current to the -0.087 and -0.093 volt between the two plugs and the pipe surrounding electrolyte in microscopic galvanic circuits on open circuit, and that this difference was due to the and the carbon would collect this same current on one different positions in the electromotive series occupied by the
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Val. 22, No. 4
ASSUMING A RADIAL FLOW PRODUCES POINTS P2265 0 2 ~ 2 9 AND ?3z24P3a35 THEN PnDa:C:Pm 0, AND Pm+(P2-P,):P,t(65-24)=Pmf41
WHERE & 4NOPm ARE P O T E Y T J A L S OF Pz AND?, MEA5URED FROM P O l E N T l A L
BASE LINE OF THEORETICAL R A D I A L C U R V E THROUGH POINTS p2 D, AND p3 D3 D, OR(Pnt41) D, 2 pZ :&D3 SOLVIIJG ( P m t 4 ~ ) z 9 -pm x 3 s Pm =I98( A B O V E NEW BASE LINE)
40
35
5 D,=b
TANG
CALCULATE D
P O I NT 5
P x D 198 x 3 5
= 6930
c
100 X 69.3 = 6930 150 X 46.2 = 6930 2 5 0 X 27.7 z 6930 300 X 23.1 = 6930 350 X 19.8 = 6930 400 X 17.3 = 6930
SURFACE
CALCU
FLOW CORV
OF P
30
=
?
: ? -c
J
POTENTIAL OF REMOTE EARTH FROM KTUAL MEASUREMENTS
12
I
-
-
.
'
.
.
I
.
.
FOR R A D I A L F L O W C U R V E a T o b = IOINCHES TO c = 9 3 ~ . v .
POTENTIAL GRADIENT IN EARTH AT SURFACE OF PIPE (CALCULATED CURVE)
.
.
,
\,CALCULATED
-
93 =9.3M.V/INCH. =Jll.6M.V/FT. RESISTIVITY OF EARTH :\6.4OHM-FT. CURRENT DENSITY. g = ' 3 4 = 6 . 8 MA/SQ.FT.
FOR
,
"
,
2 4 27 30 33 36 39\42 4548 51 54 57 6 0 63 6669 72 DISTANCE FROM CEN PIPE I N I N C H E S
*\..
ACTUAL CURVE
= 2 INCHES a T o e = 150 M.V.
'
.
75 78 81 84
POTENTlAL
-a..-.
BY USING FOORMUCA FOR RADIAL FLOW
aT0d
ACTUAL POTENTIAL GRADIENT l~ EARTH AT
'3
CURRENT DENSlTY AT P I P E S U R F A C E = 6 . 0 7 M I L L ) - A M PER EJ/SQUARE FOOT
SURFACE OF PIT ON PIPE = = 75M.V/INCH i 9 0 0 M . V Fr. RCSISTIJITY OF EARTH =I6.4OHM-FT. ACTUAL CURRENT DENSITY AT SURFACE OF PIT=%+= 54.8 M.A/SQ.FT.
NOTE C L O S E CHECK B E T W E E N GRKPHIC AN0 CALCULATED V A L U E S Figure 7-Determination
POTENTIAL OF REMOTE EARTH FROM HYPOTHETlCqL VALUES
J
of Current Density a t Surface of Pits by Potential Tangent Method
brightened and the oxidized cast iron as it comes from the foundry. The potentials of the brightened cast-iron plugs were -0.710 and -0.730 volt, referred to a copper sulfate electrode in the jelly, while the potential of the pipe was -0.620 volt or an average of 0.100 volt positive to the iron. On short-circuiting, the small exposed iron surfaces had little effect in changing the potential of the relatively huge pipe, and the potential assumed by the combination of brightened iron and oxidized iron pipe on short circuit was very close to the open-circuit potential of oxidized iron pipe. On short circuit the measured potential to a copper sulfate
electrode reference was -0.632 volt, only 0.012 volt negative to the open-circuit potential of the cast-iron pipe, but 0.088 volt positive to the open-circuit potential of the brightened cast iron. In other words, the brightened iron in the pipe was made 0.088 volt positive to the ferroxyl jelly, thereby forcing current to discharge into the solution and causing the brightened iron plugs to corrode. The current flowing between the two brightened cast-iron plugs and the pipe on short circuit was initially 0.520 and 0.775 milliampere, which represented anodic current densities of 62.6 and 77.5 milliamperes per square foot on the submerged areas of the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
April, 1930
plugs. These are extremely heavy current densities and are inch in about sufficient to corrode cast iron to a depth of 12 years, assuming 100 per cent corrosion efficiency and also assuming that these currents continue. On specimens buried in the earth current densities twice as great as the above have been found. Tests made about 12 hours later showed that the current had decreased and that it was due directly to a decreased difference of potential between the plugs and the pipe, from 0.087 and 0.093 volt to 0.038 volt for each plug. Potential measurements of the brightened cast iron and the oxidized pipe to the copper sulfate reference electrode showed that the potentials of the brightened iron plugs had remained stationary in one case a t -0.730 volt and in the other case had showed very little change, from -0.720 volt to -0.730 volt. The potential of the oxidized pipe had changed materially, from -0.620 volt to -0.690 volt, and it was this change that caused the reduction of the difference between the brightened cast iron and the pipe. This change mas probably due to polarization, which in turn was probably due to the collection on the surface of the pipe of galranic current, not only from the plugs, but from innumerable other small galvanic cells on the surface, as evidenced by the fact that cast-iron pipes not having artificially brightened surfaces go through the same cycle. Potential-difference measurements were now made in the jelly surrounding the pipe with the aid of two copper sulfate electrodes and the earth-current meter. The current discharging electrolytically from the plugs had decreased to such a low value, however, that the equipotential and current stream lines drawn from these particular results did not give a good demonstration. Distinct equipotential and current stream lines were to be seen, but they were radiating from other anodic points on the pipe. Examination of the discolorations in the jelly showed that the brightened castiron plugs were entirely covered with blue, indicating the dissolving of iron and the passage of electricity to the solution. The remaining areas of the pipe were surrounded, generally, by a pink discoloration, indicating an alkaline condition of
341
the solution caused by the collection of current. A few other small blue spots appeared, indicating other anodic areas. The fact that the currents died down in the above ferroxyl test may be taken as a good indication that these galvanic currents are transient and of no serious consequence. The decrease of current was due to the change of potential in the negative direction due to polarization. Long-time tests have shown, as previously stated, that after several months a depolarization sets in and the galvanic difference of potential is again restored. When this takes place the extent of corrosion is probably a matter of the formation of protective films over the surfaces of the anodes. Should proper films form, corrosion will probably cease, while should it not form, we see no reason why the iron should not continue to go into solution from ordinary electrolysis caused by galvanic forces. Remedies for Galvanic Corrosion
This general quest:m of corrosion is important to utilities, as in many communities the railway, water, gas, and power systems are owned by different interests and the matter of differentiating between soil corrosion and electrolysis comes up very frequently. Remedies for stray-current electrolysis conditions, while in many cases not simple, are not, however, very difficult, but to overcome the galvanic action on a system of uncoated gas or water mains which have already been laid is rather difficult. Electrolysis drainage, or the connection of underground metallic structures to the negative busses of railway substations, a measure which was applied to the water mains in New Orleans primarily to protect them from stray currents, is not only accomplishing its purpose, but is also protecting a large part of the system from the ravages of destructive galvanic currents. The drainage causes a collection of current which counteracts the discharging galvanic currents. Literature Cited (1) McCollum and Logan, Bur. Standards, Tech. Paper 551 (1927).
Autoxidation of Corn Oil as Related to Its Unsaponifiable Constituents’ H. A. Mattill and Blanche Crawford D E P A R T M E N T OF CHEMISTRY, S T A T E UNIVERSITY OF
HE spontaneous oxida-
IOWA, IOWAC I T Y ,
IOWA
and the production of a great The keeping qualities of fats and oils are primarily tion or autoxidation of dependent upon the relative proportions of “prooxivariety of partial oxidation fats and oils is a corndants” and “antioxidants” which they contain. Heatproducts, some of which may plex and puzzling process of treated oils often show a much shorter induction period in turn be or become perinterest both in thetechnology than untreated oils. Observations on corn oil in various oxides. The reaction is thus autocatalytic, and when the of,drying o i l s a n d in t h e stages of preparation relate the antioxidizing sterols p r e p a r a t i o n o f industrial and the products of heat treatment with susceptibility values for oxygen consumed are plotted against time the and edible fats. It is deto oxidation. usual form of parabolic curve p e n d e n t in t h e f i r s t i n stance upon the presence of one or more dvuble bonded car- for such reaction is obtained. lions. According to the conceptions developed by Engler Prooxidants and Antioxidants and Weissberg (b), Powick ( I d ) , Kerr and Sorber (9), Eibner and Pallauf Tschirch (16); and Holm, Greenbank, and But the mere presence of unsaturated carbon is of less Deysher (8),peroxide-like substances (ozonides, “moloxides”) moment for the process of oxidation than is that of subare formed by the addition of molecular oxygen a t the double stances which initially either accelerate or retard the reaction. bond. The further oxidation of these more easily oxidizable The compounds which favor the reaction are presumably compounds causes the rupture of the chain a t the double bond similar to the initial products just mentioned, substances of potentially high oxidizing ability. Contact of such a peroxide 1 Received February 14, 1930.
T
a),
