Applied electrochemistry and welding

The principles of Electrochemistry are almost as old as the science of electricity itself. The phenomenon of electrolysis was discovered in 1800, and its laws were experimentally determined by Faraday in 1833; again the electrolytic cell, with its simple electrodes and conducting liquid, was very early used to accomplish the dissociation of chemical compounds in the same manner as it is now used in chemical industries; the electric furnace was really discovered almost simultaneously with the arc lamp and in its essentials is identical with it.

The cheapening of electrical power and the increased use of the products involved have been largely responsible for the progress along these lines and, today, the preparation of electrolytic copper is a great industry; hydrogen and oxygen gases are now obtained by the electrolytic decomposition of water; and the method of electrolyzing fused aluminum oxide has brought the price of aluminum to a practical basis. Again, by means of the electric furnace, several highly resisting chemical reductions have been accomplished and methods have been perfected for the manufacture of calcium carbide, silicon products, carborundum, graphite, and steel.

Welding, one is rather inclined to think, is an unimportant process applied exclusively to the repairing of broken down machinery, but one glance at this section in the volume shows what a commanding position the electric arc, butt, and spot welders are taking in the manufacturing world, and gives a clear idea of the applications of gas and thermit welding to all sorts of processes which are usually supposed to be purely machine operations..

Finally, when by the aid of intense electrical discharges in air, even the nitrogen of the atmosphere is made available for our use, the results seem to approach the miraculous. To think of the world's supply of nitrates being augmented from the very atmosphere itself seems more like a dream of a Jules Verne or a Wells, than an actual twentieth century accomplishment.

All of these scientific marvels are intensely interesting and the treatment has been made exceedingly practical by the authors. The material is written in a clear readable style and is designed to appeal to both the trained engineer and the la3nnian. It is the hope of the publishers that a study of this volume may widen the acquaintance of many readers with this branch of industrial electricity and stimulate their interest in the general scientific development of the world.

Chemical Reactions. Chemical reactions are frequently, if not in fact almost universally, associated with changes in electrical energy. The science and art of electrochemistry deal with the relationship of electrical and chemical forms of energy. It is well known that many chemical reactions take place with the liberation of energy in the form of heat; thus when coal bums, combining with the oxygen of the air, there is a liberation of heat energy. Other chemical changes involve an absorption of heat energy, that is, heat must be applied to materials in order to cause certain reactions to take place. The formation of calcium carbide is an example of the production of a useful compound by heating lime and carbon to a high temperature. All chemical reactions may be classified as endothermic (heat-absorbing) or ezothermic (heat-liberating).

Numerous chemical transformations also occur with a liberation of electrical energy, a fact upon which are dependent the various types of primary cells or electric batteries. Electrical energy may, on the other hand, be made to produce chemical changes by the passage of electric current through an electrolyte, and this finds practical application in various forms of electrolytic cells.

Storage batteries constitute an important class of electrical apparatus, consisting of a certain combination of metals and electrolyte in which the electrochemical action is reversible, that is, in passing current through the battery in one direction certain chemical changes take place, or the battery is charged, and these chemical reactions take place in a reverse direction when the current is allowed to flow in a reverse direction, as when the battery is discharged.

Range of the Subject. The fundamental units and principles of electricity, the elementary principles of electrochemical action, the primary cell, and the secondary, or storage, cell have already been considered in previous articles, and consequently this article will be confined to a consideration of the wider field of applied electro-chemistry, dealing with the more important practical uses of electrical energy in producing useful chemical transformations.

Electrical energy may be applied to materials by various methods, the more important of which are the following:

(1) Electrolysis, or the electrolytic change brought about by the passage of a direct current through an electrolyte.

(2) Electrothermics, or the production of chemical change through the heat effect produced by electrical means.

(3) Electrical discharge in gases.


ELECTROLYSIS AND ITS APPLICATIONS

ELECTROCHEMICAL THEORY

KINDS OF CONDUCTORS

All materials may be divided, first, into two classes, depending upon whether or not they conduct electrical current. If they conduct, they are called "conductors" and if they do not, they are designated as "insulators". In turn, materials which conduct may again be subdivided into two more classes commonly designated: metallic conductors, or conductors of the first class; and electrolytic conductors, or conductors of the second class.

It is important that as a basis for the study of electrolysis a clear idea be acquired as to the distinctive differences between metallic and electrolytic conductors.

Metallic Conductors. As implied by the name, metallic conductors, the metals belong to this class; and in addition to the metals and metallic alloys, there are a few other elements and various compounds which conduct in a similar manner and are therefore designated as metallic conductors. In this class of conductors the flow of current produces only a heating effect without producing chemical change.

Non-Metallic Elements. Of the few non-metallic elements which conduct, the most important is carbon, or graphite. Silicon, boron, and selenium are other elements possessing metallic conductivity to some degree.

Chemical compounds do not as a rule conduct metallically. The important exceptions include peroxide of lead, which is a constituent of one of the electrodes in a storage battery; magnetic oxide of iron, which is used as an anode material for electrolytic purposes; sulphides of lead and of silver, various metallic carbides, silicides, borides, etc.

Specific Resistance, The specific resistances of the metals and metal alloys cover a comparatively limited range, a high resistance metal such as mercury having a specific resistance about one hundred times that of the best conductive metals, copper and silver.

It is a characteristic of the metals that the resistance varies in a minor degree with variations in temperature. The resistance usually increases with increase in temperature, or, in other words, the metals have a positive temperature coefficient. With pure metals the temperature coefficient is a constant, i.e., the resistance is approximately proportional to the absolute temperature and, if the resistance be plotted for various temperatures, the line points toward absolute zero, suggesting that if the metals could be cooled to that point they would possess no resistance and thus become perfect conductors.

With the conductive metalloids or non-metals, and with the compounds which conduct metallically, a higher order of specific resistance is encountered as well as a greater variation in temperature coefficient. To the electrochemist, carbon is the most important of these conductors. Its specific resistance varies through a wide range from that of the diamond, which, is practically an insulator, to graphite, and then to "metalized" carbon which has a con-
ductility comparable to that of mercury.

Carbon, such as is used for electrode purposes, may consist of plates made up of finely ground carbon mixed with a binding material, molded into shape and subjected to a high temperature baking. The resistance is dependent upon the quality of the carbon flour, the purity, the nature of the binding material, the pressure of forming, and the baking temperature. The higher the firing temperature used, the lower is the resistance and, by carrying the temperature to the highest attainable value, the material is transformed into a more conductive form known as graphite. The discovery of this method of graphitization by electric heating constitute one of the most important of the electrochemical discoveries, furnishing not only the basis of a large artificial graphite industry but also sup- plying an electrode material of inestimable value to the electrochemist.

Electrolytic conductors invariably consist of definite chemical compounds. It should be borne in mind that, as pointed out under metallic conductors, not all chemical compounds which conduct are electrolytic conductors.

Classification. Electrolytic conductors may be either fused materials or certain solutions of materials in water or other solvents. Some evidence of electrolytic conductivity has been detected in a few solid compounds, but this phenomenon is of little importance from the practical standpoint. It is with liquid conductors that electrolysis commercially applied has to deal and for practical purposes these liquid conductors may be placed in three divisions:

(1) Electrolytes consisting of substances dissolved in water.

(2) Electrolytes consisting of substances dissolved in solvents other than water.

(3) Electrolytes consisting of chemical compounds in a state of fusion.

The first group is the one of the greatest importance, since water is a great universal solvent, which, on account of its abundance and low cost and great solvent properties, furnishes an essential material in most industrial electrolytic processes. Non aqueous solutions, while attracting much interest from the theoretical and scientific points of view, have as yet few technical applications. A more extensive use of these solvents, however, may safely be anticipated as a result of future development.

Electrolytes consisting of fused materials have important technical applications in industries such as the manufacture of aluminum, sodium, magnesium, and calcium.


THE ELECTROCHEMICAL CELL

Definitions. An electrochemical cell is a form of apparatus in which all industrial electrolytic processes are carried out. It may be defined as a combination of two metallic conductors, constituting the electrodes, and an electrolytic conductor, constituting an electrolyte which joins the electrodes. A suitable containing vessel is also an essential part.

The anode is the electrode at which the current enters the electrolyte, and the cathode is the electrode at which the current leaves the electrolyte.

The cell is inactive if no current flows, and it becomes active when the current passes, which in turn means that to be active it must be connected to an external source of electrical energy. This external energy may be obtained from any generator of direct current, such as a primary battery, a storage battery, or a dynamo.


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