PRINCIPLES AND DEFINITIONS

A. ELECTROLYSIS IN GENERAL

1. Electrolysis is the process whereby an electric current pass ing from an electrode to an electrolyte or vice versa causes chemical changes to take place in the electrolyte. Electrolysis also includes any chemical changes at the surface of an electrode resulting from the chemical changes in the electrolyte. Electrolysis is independent of the heating effect of the electric current.

NOTE. These changes usually occur in a water solution of an acid, alkali, or salt. By the passage of an electric current through it, water (containing a trace of acid) is decomposed into hydrogen and oxygen, copper is deposited from a solution of copper sulphate, silver from solutions of silver salts. Electroplating, electrotyping, and refining of metals by electrodeposition are useful applications of electrolysis in the arts. Electrolysis is involved in the charge and discharge of storage batteries, and in the operation of primary batteries.

In order that electrolysis may occur, the following conditions must be present:

(a) There must be a flow of electric current through a conducting liquid from one terminal to another;

(b) The conducting liquid must be a chemical compound or solution which can be altered by the action of the electric current.

2. Electrolyte, Electrode, Anode, Cathode. The electrolyte is the solution (or fused salt) through which the electric current flows; the conducting terminals are the electrodes; the terminal by which the current enters the solution is the anode; the terminal by which it leaves is the cathode.

NOTE. The chemical changes caused by the current may affect both the electrolyte and the electrodes. In the case of a solution of copper sulphate with copper plates as electrodes, copper is removed from the anode by the current and carried into solution; an equal amount of copper is deposited upon the cathode. In general the metal travels with the current toward the cathode.

3.

Amount of Chemical Action. (Faraday's Law.) The amount of chemical action taking place at the anode and also at the cathode (as expressed by Faraday's Law) is proportional to (1) the strength of current flowing, (2) the duration of the current, and (3) the chemical equivalent weights of the substances.

NOTE. Otherwise expressed, the quantity of metal or other substance separated is proportional to the total quantity of electricity passing and the electro-chemical equivalent of the substance or substances concerned. The electro-chemical equivalent of a metal is proportional to its atomic weight divided by its valence. Faraday's Law is so exactly realized in practice under favorable conditions that it is used as the basis for the definition of the international ampere, one of the fundamental electrical units. (See Passivity, Paragraph 15.)

4. Cause of Current Flow. The current flowing through the electrolyte may be due (1) to an external electromotive force or (2) to the difference of potential due to the use of electrodes of different materials or to solutions of different concentrations.

NOTE. The first case is illustrated by electrolysis of dilute sulphuric acid using two lead plates and an external battery; the second by the electrolysis of the same solution using a zinc and a copper plate, which touch each other inside or outside the solution. The first occurs in charging a storage battery; the second in the discharging of a primary battery or a storage battery.

5. Electrolysis by Local Action. Instead of two plates of different metals the same result may follow with one plate if it is chemically impure or otherwise heterogeneous, when immersed in an electrolyte.

NOTE. Such a plate excites local currents and a loss of metal occurs at all the anode areas. This local action causes impure zinc to dissolve rapidly in a solution which has no action on pure zinc.

6. Anodic Corrosion is the term applied to the loss of metal by electrolysis at the anode.

NOTE. When iron is anode the iron is carried into solution by the current, the first product being a salt of iron, the nature of which depends upon the character of the electrolyte. In dilute sulphuric acid, ferrous sulphate is formed; in hydrochloric acid, ferrous chloride, etc. These first products of electrolysis are frequently modified by secondary reactions.

7. Secondary Reactions are the chemical changes which occur at or near the electrodes, by which the primary products of electrolysis are converted into other chemical substances, and are sometimes followed by other reactions,

NOTE. Ferrous hydroxide formed by the union of iron with hydroxyl ions set free at the anode, is subsequently converted into an iron oxide due to the reactions with oxygen dissolved in the electrolyte. When lead is cathode in an alkali soil or solution, the alkali metal (such as sodium or potassium) reacts with water at the cathode and forms alkali hydroxide, setting hydrogen free. This hydroxide may react with the lead chemically and form lead hydroxide (especially after the current ceases), which in turn may combine with carbon dioxide, forming lead carbonate.

8. Cathodic Corrosion is the term applied to the corrosion due to the secondary reactions of the cathodic products of electrolysis, as described in the preceding paragraph. The metal of the cathode is not removed directly by the electric current but may be dissolved by a secondary action of alkali produced by the current.

NOTE. The anodic corrosion is more common and more serious; cathodic corrosion, however, sometimes occurs on lead and other metals that are soluble in alkali. Cathodic corrosion never occurs in the case of iron.

B. ELECTROLYSIS OF UNDERGROUND STRUCTURES

9. General. As used in this report, the term "electrolysis" embraces the entire process of accelerated corrosion of underground metallic structures due to stray current. In the electrolysis of gas and water pipes, cable sheaths, and other underground metallic structures, and the rails of electric railways, the moisture of the soil with its dissolved acids, salts, and alkalis is the electrolyte, and the metal pipes, cable sheaths and rails are the electrodes.

NOTE. Wherever the current flows away from the pipes they serve as anodes and the metal is corroded. Metal or gas or alkali, according to the nature of the soil, will be set free at the cathode.

10. Self Corrosion is the term applied when a pipe or other mass of impure or heterogeneous metal buried in the soil is corroded due to electrolysis by local action.

NOTE. This is called "self corrosion" because the electric current originates on the metal itself, without any external agency to cause the current to flow. Self corrosion may also be due to direct chemical action.

11. Acceleration of Local or Self Corrosion. Self corrosion is accelerated by the presence in the soil water of acid or salts which lower its resistance as an electrolyte, and also by cinders, coke or some other conducting particles of different electric potential which augment the local electric currents. In the latter case the metal need not be heterogeneous.

NOTE.-A pipe may be destroyed in a relatively short time by self corrosion or local action if buried in wet cinders or in certain soils.

12. Stray Current is that current which has leaked from the return circuit of an electric railway system and flows through the earth and metallic structures embedded therein.

13. Anodic and Self Corrosion. Anodic corrosion due to stray currents and self corrosion due to local action may occur simultaneously, and the former may accelerate the latter.

NOTE. Hence the corrosion due to a given current plus the increased self corrosion induced by that current may give a greater total corrosion than called for by Faraday's Law. This explains how the coefficient of corrosion may exceed unity.

14. Coefficient of Corrosion. The coefficient of electrolytic corrosion (sometimes called corrosion efficiency) is the quotient of the total loss of metal due to anodic corrosion (after deducting the amount of self corrosion if any) divided by the theoretical loss of metal, as calculated by Faraday's Law, on the assumption that the corrosion of the anode is the only reaction involved.

NOTE. In practice it is found that the coefficient of corrosion varies widely from unity, being sometimes as low as 0.2 and sometimes even above 1.5, but commonly between 0.5 and 1.1.

15. Passivity is the name given to the phenomenon in which a current flows through an electrolyte without producing the full amount of anodic corrosion which would occur under normal conditions.

NOTE. This restricted definition of passivity has regard only to its effect in electrolysis. Many conditions affect the degree of passivity attained, an initial large current density being favorable to it. Plunging iron into fuming nitric acid renders it temporarily passive. A satisfactory explanation of passivity has not been given.

16. Polarization Voltage (sometimes called polarization potential) is the temporary change in the difference of potential between an electrode and the electrolyte in contact with it due to the passage of a current to or from the electrode. This change in potential difference is due to the change in the conditions of the surface of the electrode or change in the concentration of the electrolyte (or both), and under some conditions is approximately proportional to the current flowing, but in many cases is not so proportional. The magnitude of the polarization voltage also depends on the material of the electrode, the nature of the electrolyte, and the direction of the current.

17. Alternating or Frequently Reversing Direct Currents. If alternating currents (or frequently reversing direct currents) flow through the soil between pipes or other underground metallic structures, the metal removed during the half cycles when a pipe is anode may be in part replaced when it is cathode. Hence, the total loss of metal on a given pipe may be less than is indicated by computing the loss on the basis of the positive part of the cycle only, and in the case of alternating current at commercial frequency may be less than 1 per cent. of such computed values.

NOTE. In slow reversals of current the recovery effect is less, but the loss will be less than with direct current continuously in the same direction (excepting possibly where the phenomenon of passivity may affect the result).

18. Action on Underground Metallic Structures. Faraday's Law applies to electrolysis of metallic structures in soil as elsewhere, the total chemical action being proportional to the average current strength and the time the current flows and to the electrochemical equivalent of the metal or other substances concerned. Although local action and passivity affect the loss of metal and so apparently modify Faraday's Law, it is still true that the total chemical action resulting from the current flow is proportional to the total current when local currents are included.

NOTE. Sometimes this chemical action is concerned only with corroding the anode; sometimes it is concerned with breaking up the electrolyte, as when the anode is a noble metal or in the passive state (as iron and lead sometimes are); sometimes both these effects occur.

The theoretical loss of iron per year per ampere is about twenty pounds and of lead is 3.7 times this amount, or about seventy-four pounds. The loss in volume of lead is 2.4 to 2.6 times that of iron. The greater loss in lead is due to the higher electrochemical equivalent of that metal.

19. Electrolysis Mitigation. The two primary features of electrolysis mitigation are (1) the reduction of the flow of current through the earth and the metallic structures buried in the earth, (2) the reduction of the anode areas of such structures to a minimum, where the current is not substantially eliminated in order to reduce the area of destructive corrosion as far as possible.

NOTE. The current in the underground metallic structures will be decreased, other conditions remaining the same, by (1) increasing the conductance of the return circuit, (2) increasing the resistance of the leakage path to earth, (3) increasing the resistance between the earth and the underground metallic structures, (4) increasing the resistance of the underground metallic structures.

The anode areas of the underground metallic structures will be. decreased, other conditions remaining the same, by providing suitably placed metallic conductors for leading the current out of the underground structures so that the flow of the current directly to the earth shall be minimized. This will change a portion of the anode area to cathode.

20. Electrolysis Survey. An electrolysis survey is the operation of determining by means of proper measurements all relevant facts pertaining to electrolysis conditions, such as the voltage drop in the grounded railway return; the location and extent of the areas in which the metallic structures are in danger from stray currents; the condition of the structures and adjacent soil in the danger areas, and the extent of any damage that may have occurred; the seriousness of electrolytic action in progress and the source of the stray current producing the damage, its course and magnitude and the conditions in neighboring structures tending to produce electrolysis. It will generally be found desirable to make some preliminary tests for the purpose of indicating the lines along which the complete survey should be made.

21. Overall Potential Measurements. Overall potential measurements are measurements which are made to determine the difference in electric potential between points in the tracks at the feed limits of the station and the point in the tracks which is lowest in potential, and are obtained by means of pressure wires and indicating or recording voltmeters. This is most commonly applied to measurements of voltage between the point of lowest potential in the grounded portion of a railway return system and the points of approximately highest potential on its various branches.

22. Potential Gradient. A potential gradient is the voltage drop per unit of length between two points on a single conductor or in the earth, and is usually expressed in volts per thousand feet.

23. Potential Difference. In electrolysis work the term "potential difference" usually means the difference in potential which exists between nearby points on separate systems of conductors, or between conductors and the earth, e. g., between pipes and rails, lead sheaths and rails, lead sheaths and earth, etc.