densities ranging from 1.7 milliamperes per sq. ft. (18.3 milliamperes per sq. m.) to 54 milliamperes per sq.. ft. (581 milliamperes per sq. m.), the loss of weight of iron by electrolysis is at least equal to that calculated by Faraday's law, and is in general greater than the theoretical loss. In all cases electrolysis tends to cause localized corrosion and decided pitting. Surface scale appears to accelerate corrosion from electrolysis with all irons except cast iron; this was especially pronounced in the case of the steel pipes tested. When the surface scale was removed there was practically no difference in the amount of corrosion produced by a given current leaving iron for damp soil between commercial steel, commercial wrought iron, ingot iron and cast iron.

It should be pointed out that the electrical resistivity of cast iron is about ten times as great as that of wrought iron, steel, or ingot iron, and the usual lead joints in cast iron pipes also have a resistance which is many times greater than the screw coupling joints usual with wrought iron and steel pipes. For these reasons a given voltage drop through ground will cause a much smaller current to flow on a cast iron pipe than on a wrought iron or a steel pipe, thus practically making cast iron pipes much less subject to electrolysis than wrought iron or steel pipes. It must also be noted that when a cast iron pipe is corroded by electrolysis, the iron is oxidized but remains in place as a graphitic mass having little mechanical strength, but possessing the ability to maintain the pipe gas-tight and sometimes even water-tight for considerable periods, while with wrought iron or steel pipes this does not occur, so that holes and consequent leaks are more quickly produced. Frequently where cast iron pipes appear to be immune from electrolysis because no evidences of leakage have developed, an examination of the pipes would reveal that a great deal of corrosion has actually taken place and that the pipes have been very greatly weakened.

The tests described in this paper are by no means considered complete. There are in fact so many possible variables, such as different kinds of soil, different degrees of wetting the soil, different kinds of iron, different voltages, different current densities, etc., that it would be extremely laborious to make a complete set of tests. The writer expects, however, to continue the experiments along the line outlined in this paper and hopes that the discussion will bring out suggestions which will serve to make the next series of tests more valuable.

44

DISCUSSION ON ELECTROLYTIC CORROSION OF IRON BY DIRECT CURRENT IN STREET SOIL" (GANZ), BOSTON, MASS., JUNE 25, 1912.

Carl Hering: It seems to me that the tests made by Professor Ganz are very valuable, and we are fortunate to have had the benefit of his experience. But I think that to speak of voltage as Professor Ganz does in this paper, is somewhat misleading. The voltage for the electrolytic corrosion of iron is negative. Therefore it should be possible that with no external voltage at all there might be some corrosion. There are always two voltages, one at each electrode, and we generally refer to their sum, as it is difficult to separate them. The result described was perhaps due largely to over-voltage " at the copper electrode which he used. In my opinion there is also an important mechanical effect in underground electrolysis in the form of the rate of diffusion of the liquid which does the electrolyzing. If that liquid cannot circulate rapidly there will be much less electrolysis than if it can, and it therefore seems to me that this effect of the circulation of the liquid through the soil is an important factor in determining whether the corrosion will be bad or not; it must also have a very decided effect on the voltage. Professor Ganz says nothing about the inside of the pipe and whether the results there are the same or not.

Edward B. Rosa: Regarding the excessive loss of weight by corrosion, it does not seem to me that it is necessary to assume that it is due to the removal of metal by mechanical means. At the Bureau of Standards we have made some similar experiments, and under some conditions the excess above the calculated value is considerable. It is well known that if iron pipes are embedded in cinders or in certain soils, the corrosion may be very greatly accelerated. I have known of one case where a line was laid through cinders and the pipe was destroyed within a year without the application of any outside current whatever. If the current puts the surface in a different condition from the surface of the pipe exposed as a blank experiment, the local action of self-corrosion may be thereby accelerated. These experiments are of great importance, and I think they emphasize the need of making laboratory experiments under as nearly as possible practical conditions.

Irving Langmuir: The corrosion of the pipe and the consequent loss in weight, in excess of that calculated by Faraday's law, must be due to oxidation. The iron in the ferric condition reacts with the iron itself to produce iron in the ferrous condition, thus causing a greater corrosion of the pipe. Some experiments made at Stevens Institute several years ago threw some light on the pitting. We placed two iron plates in the soil and passed a current between them for several days. At the end of that time we opened the circuit and found that there was a potential difference between the plates, in the same direction as the original current, thus tending to maintain the current.

Therefore, if the current leaves a pipe at one place with a little higher current density than at another, there will be a voltage set up at that place which will make the current concentrate at that point. The explanation of this is difficult to find. There are one or two other cases where a similar phenomenon is noticed. For example, the same effect may be observed with hydrochloric acid alone. If a current of very low density be passed between two platinum electrodes in a dilute solution of hydrochloric acid through which hydrogen is bubbling, it is found, upon opening the circuit, that there is a difference of potential between the electrodes in the same direction as that which originally produced the current. This effect, however, persists only a short time.

C. H. Sharp: The idea that I desire to express about the excess of corrosion over and above the theoretical amount, is that the action of the current accelerates the normal or so-called "chemical" oxidation of the iron. Consequently the ordinary oxidation goes on more rapidly when the current is flowing than when it is not. One interesting thing is Professor Ganz's suggestion as to the reason for the greater durability of cast iron pipes than wrought iron and steel pipes, explaining the fact on a rational basis. He did not say anything about scale or about surface conditions or anything like that, but he showed that the higher resistivity of cast iron alone would result in greater durability.

Albert F. Ganz: Dr. Hering has said that the voltage which I have measured in my experiments is somewhat misleading, because there are always two voltages, one at each electrode. This is, of course, true, but I was measuring the total applied voltage between the electrodes as we do when we make the usual potential survey. Regarding the insides of the pipes, I wish to say that in my experiments only the outside surfaces of the pipes were in contact with the soil, so that the inside surfaces were not affected by corrosion. Referring to Dr. Rosa's remarks, I wish to say that the large excess of corrosion over that calculated from Faraday's law, amounting to several hundred per cent in some cases, was undoubtedly due to the effects of scale on the surface of the pipes, because in the last series of experiments where this scale was removed by turning down the pipes before the test was begun, the excess of corrosion over that computed from Faraday's law was only from 3 to 23 per cent, and this excess of corrosion is probably due to the accelerated natural corrosion set up by the electrolytic action.

There has been so much written about the electric furnace since it entered into regular commercial use about 20 years ago that the presentation of a paper on the subject treating it in a general way is not apt to be interesting. But a promise to attempt the preparation of such a paper having been given, it was thought that there might be some interest attaching to a review of the development of some furnaces during the past 30

Moreover, it is often useful to look back over ground that has been covered in order to obtain suggestions as to the best direction to be taken in attempting further advances.

It is now 30 years since Sir William Siemens melted about 20 pounds of steel, as well as platinum, in notable quantities in an electric furnace with which he had been experimenting since 1878, and since then the electric furnace has so far developed that there are great numbers, both in Europe and this country, regularly engaged in the commercial manufacture of steel. While it is true that others had made some use of electrothermic methods at a much earlier day, for example Despretz, whose source of current was 600 Bunsen cells, yet Siemens's furnace must be considered the first really practical one, coming as it did after the invention of a cheap source of energy-the electric generator. Siemens's work is of particular interest because he saw the possibility of using the electric furnace for steel manufacture, and, so far as the principles are concerned, they are the same as those in actual commercial use to-day. One of his furnaces was made of a graphite or other refractory crucible enclosed in

a jacket of heat-insulating material. Inserted in the bottom of the crucible was an electrode of iron, platinum or carbon, while passing through the cover of the crucible was another electrode. The latter was connected to an automatic regulating device consisting of a solenoid, this serving to vary the length of the arc and thus keep the rate of generation of energy constant. In working the furnace the steel or other metal was introduced into the crucible and made contact with the lower electrode, while an arc was drawn between the upper electrode and the charge. In such a furnace Siemens melted steel in quantities of more than 20 pounds.

In another form of electric furnace devised by Siemens, the electrodes entered the crucible in a horizontal direction near the top and opposite one another so that an arc was formed between them and heated the charge below by radiation.

Siemens's work must be considered as the forerunner of at least two well-known kinds of electric steel furnaces which are in existence to-day in actual commercial use for the manufacture of steel, although he never was able to do commercial work with his apparatus because electrical engineering was not sufficiently advanced.

The growth of the Siemens electric furnace for steel making was at first slow, for numerous practical difficulties in its working had to be overcome, but so many of these have at last been met successfully by men like Heroult, Girod, Stassano and others who have modified the apparatus in various ways, that now we have furnaces like those of the Steel Corporation in Chicago and Worcester working on charges of 15 tons of steel.

Siemens in his furnace used direct current and laid particular stress on the point that the charge should be connected to the positive side of the circuit, since it is well known that in the electric arc it is at the positive electrode the main generation of heat occurs. In the modern furnaces, however, alternating currents are used for obvious reasons, and the surface of the molten bath is covered with a layer of slag which becomes intensely heated, not only by the arc but by the current which it carries. In this way ideal conditions are obtained for refining the metal, as the steel and the molten slag between which chemical reaction is desired are intensely heated at their surfaces of contact. Moreover, the slag effectually prevents the introduction of carbon from the electrodes into the metal. The problem of regulating the electrodes automatically has also been