US3649492A

US3649492A - Method for determining the completeness of cathodic protection of corrodible metal structure

Info

Publication number: US3649492A
Application number: US73931A
Authority: US
Inventors: Glenn A Marsh; Edward Schaschl
Original assignee: Union Oil Company of California
Current assignee: Union Oil Company of California
Priority date: 1966-06-14
Filing date: 1970-09-21
Publication date: 1972-03-14
Anticipated expiration: 1989-03-14

Abstract

A METHOD FOR EVALUATING THE EFFECTIVENESS OF A CATHODIC PROTECTION SYSTEM IN WHICH THE COMPLETENESS OF CATHODIC PROTECTION OF A CORRODIBLE METAL STRUCTURE EXPOSED TO A CORROSIVE ENVIRONMENT IS ASCERTAINED BY MEASURING THE POLARITY OF THE STRUCTURE WITH RESPECT TO A SPECIMEN OF THE SAME METAL PLACED IN THE CORROSIVE ENVIRONMENT AND ELECTRICALLY CONNECTED TO THE STRUCTURE AND MAINTAINED IN ELECTROLYTIC CONTACT WITH THE CORROSIVE ENVIRONMENT AND OUT OF CONTACT WITH OXYGEN. ALL AREAS OF THE STRUCTURE ARE COMPLETELY CATHODICALLY PROTECTED WHEN THE STRUCTURE EXHIBITS THE SAME POTENTIAL AS THE TEST SPECIMEN, OR IS NEGATIVE WITH RESPECT TO THE SPECIMEN.

Description

March 1972 G. .A. MARSH ETA!- METHOD FOR DETERMINING THE COMPLETENESS OF CATHODIC PROTECTION OF CORRODIBLE METAL STRUCTURE Filed Sept. 21, 1970 INVENTOR5 64 51v ,4, MARS/7 [QM/4R0 SCH/45661 BY m M ATTORNEY United States Patent @flice- METHOD FOR DETERMINING THE COMPLETE- NESS F CATHODIC PROTECTION OF CORROD- IBLE METAL STRUCTURE Glenn A. Marsh and Edward Schaschl, Fullerton, Califl, assignors to Union Oil Company of California, Los Angeles, Calif.

Continuation-in-part of application Ser. No. 557,492, June 14, 1966, which is a continuation-in-part of application Ser. No. 213,171, July 30, 1962. This application Sept. 21, 1970, Ser. No. 73,931

Int. Cl. C23f 13/00; G01n 27/46 US. Cl. 204-148 8 Claims ABSTRACT OF THE DISCLOSURE A method for evaluating the effectiveness of a cathodic protection system in which the completeness of cathodic protection of a corrodible metal structure exposed to a corrosive environment is ascertained by measuring the polarity of the structure with respect to a specimen of the same metal placed in the corrosive environment and electrically connected to the structure and maintained in electrolytic contact with the corrosive environment and out of contact with oxygen. All areas of the structure are completely cathodically protected when the structure exhibits the same potential as the test specimen, or is negative with respect to the specimen.

This is a continuation-in-part of application Ser. No. 557,492, filed June 14, 1966, now U.S. Pat. No. 3,549,993, which in turn is a continuation-in-part of application Ser. No. 213,171, filed July 30, 1962, and now abandoned.

This invention relates to the cathodic protection of corrodible metal structures, and more particularly concerns a method for determining the completeness of cathodic protection of such structures.

Almost any metallic surface exposed to a corrosive electrolytic environment, such as those disposed in damp soil or water, can be cathodically protected from corrosion. In conventional cathodic protection systems for mitigating the corrosion of submarine or subterranean metallic structures, cathodic protection is attained by connecting sacrificial anodes of a metal higher in the electromotive series than the structure, such as magnesium or zinc in the case of ferrous structures, to the structure and disposing them within the electrolytic environment. Also, direct current electricity can be supplied to the structure to provide all or part of the current required for cathodic protection. Less expensive anodic metals, such as scrap iron, can also be utilized as anodes where an auxiliary current source is used to drive current from the anode to the structure being protected. Cathodic protection of the structure is achieved when cathodic areas of the structure receive all electrons utilized in the cathodic process from the auxiliary anode, and not from the local anodes of the structure itself. Generally, for carbon steel, protection is achieved when the potential of the structure has been depressed to O.85 volt with reference to a saturated copper-copper sulfate half cell. Under the condition of complete cathodic protection, local cathodes in the structure are assumed to be polarized to the open circuit potential of the anodes.

Our prior applications Ser. Nos. 557,492 and 213,171 disclose a method for measuring the maximum corrosion rate of a corrodible metal structure exposed to a corrosive environment in which the maximum corrosion rate of the most anodic portion of the structure is determined by measuring the corrosion rate of a corrosion test probe placed in the corrosive environment, electrically connected to the structure and maintained in electrolytic con- 3,649,492 Patented Mar. 14, 1972 tact with the corrosive environment and out of contact with oxygen. Oxygen is excluded from contact with the corrosion probe by placing a special barrier of an electrolytically conductive, oxygen-impermeable material around the probe. While this technique has proved advantageous in determining the maximum corrosion rate that might be expected in a cathodically protected system and, hence, can also be used to evaluate the effectiveness of the cathodic protection system, an expensive and complicated Wheatstone bridge type of instrument is required to measure the current flow through the corrosion probe in determining corrosion rate by the resistance method. Thus, need exists for a simple technique for evaluating the eifectiveness of a cathodic protection system that employs simple, inexpensive, readily available measuring instruments.

Accordingly, a primary object of the invention is to provide a method for evaluating the effectiveness of a cathodic protection system.

Another object of the invention is to provide a method for ascertaining and continuously monitoring, if desired, the elfectiveness of a cathodic protection system based on a single, readily obtainable measurement.

Still another object of the invention is to provide a method for determining the effectiveness of a cathodic protection system employing a simple, inexpensive, readily available instrument.

A yet further object of the invention is to provide a method for controlling the amount of current supplied to a cathodically protected corrodible metal structure to attain complete cathodic protection.

These and other objects of the invention will be apparent from the following description and the appended drawing which schematically illustrates the practice of the invention.

Briefly, the method of this invention is practiced by placing a specimen of the corrodible metal in the corrosive electrolytic environment in which a cathodically protected metallic structure is buried or submerged. The corrodible metal specimen and the structure being cathodically protected are electrically connected by an electric conductor, and a special barrier of electrolytically conductive, oxygen-impermeable material is placed around the specimen to maintain the specimen in electrolytic contact with the corrosive environment and out of contact with oxygen. The polarity of the structure with respect to the test specimen is then determined. All areas of the structure are completely cathodically protected when the structure exhibits the same potential as the test specimen, or is negative with respect to the specimen.

In the typical case, the corrosion rate of buried or submerged steel structures is determined by the availability of dissolved oxygen and by the conductivity of the electrolyte. Steel corrodes in an aqueous oxygen-containing electrolyte by local cell action, by long cell action, and by a combination of local and long cell action. Local cell action results in rather uniform loss of metal from the entire exposed surface. Long cell corrosion causes severe pitting of the metal surface in localized areas. In local cell action, available oxygen is reduced at microscopic cathodic sites and iron is oxidized at adjacent microscopic anodic sites. The pH of the environment is not a rate determining factor in the range from 5 to 9. As the conductivity of the electrolyte increases, long cell action becomes more and more possible. In long cell action, the cathodic and anodic sites or zones are separated by a macroscopic distance which may be from inch or less to more than feet, depending on the conductivity of the electrolyte. The driving force for the long cell action is derived solely from the difference in availability of oxygen between the two zones and is independent of the means by which this difference is achieved. Furthermore, long cell corrosion is a self perpetuating mechanism. Once cor-- rosion starts, corrosion products are formed at the anode which tend to occlude oxygen from this area, thus causing a greater diiferential concentration of oxygen, which further accelerates the corrosion reaction.

Buried or submerged structures in damp, aerated environments are subject to rapid localized corrosion due to differential aeration cell attack. It has been found that the oxygen which sustains galvanic corrosion can find its way to the cathodic area of submerged structures by dissolving in the body of water in which the structure is submerged. Similarly, air is carried to buried structures by dissolving in rain water which drains downward from the surface of the earth to the buried structure. Thus, in any case, the oxygen necessary to sustain the corrosion reaction is dissolved in the electrolyte solution.

Referring now to the drawing, the numeral represents a corrodible metal structure such as a pipeline buried in earth formation 12. While the drawing illustrates a pipe line buried in an earth formation, it is to be understood that the shape or utility of the corrodible metal structure is not part of the invention and that the method of the invention can be practiced on all manner of structures. Further, it is to be recognized that structure 10 can be submerged in a body of water rather than the illustrated earth formation. The cathodic protection system for metallic structure 10 is comprised of an external D.C. power source such as battery 14 connected to the structure through variable resistor 16 by conductor 18. Ammeter 20 measures the current flowing from battery 14 to structure 10. The positive terminal of battery 14 is connected to anode 22 by conductor 24. Alternatively, anode 22 can be a sacrificial anode, in which case battery 14 is not used. Sacrificial anode 22 is constructed of magnesium or other metal higher in the electromotive series than ferrous pipeline 10. Variable resistor 16 is adjusted to control the flow of electrical current to pipeline 10 and the amount of electrical current flowing to the pipeline is measured by ammeter 20.

Specimen 33 is a small piece or coupon of a corrodible metal of the same or similar type as corrodible structure 10. For example, where corrodible structure 10 is constructed of a carbon steel, specimen 33 is formed from the same or similar type of carbon steel. Specimen 33 is buried or submerged in the corrosive environment, such as subterranean formation 12, and is electrically connected to the corrodible structure by means of

electrical conductors

34 and 36 which are conveniently connected in junction box 38 located above the surface of earth formation 12. Normally,

conductors

34 and 36 are connected in the junction box so that specimen 33 is connected to structure 10. However, when a reading is to be made, this connection is broken and polarity indicating device 40 is connected between

conductors

34 and 36. Polarity indicating device 40 can be a microammeter, a galvanometer, a zero-resistance ammeter or the like. In one preferred embodiment, polarity indicating device 40 is a zero-center galvanometer having a red zone in one polarity and a green zone in the opposite polarity corresponding to insufiicient cathodic protection and complete cathodic protection, respectively. Polarity indicating device 40 may be connected each time it is desired to determine the adequacy of the cathodic protection, or the device can be permanently installed as a field monitor.

The barrier material used to ensheath the test specimen desirably exhibits the properties of high electrolytic conductance and impermeability to non-ionic dissolved gases and liquid fluids generally, and particularly to dissolved oxygen. Since the electrolytic conductance of the material is achieved by the diffusion of ions through the barrier material, materials having high permeability for ions are preferred. A number of materials having both a high degree of electrolytic conductance and which are substantially impervious to oxygen and non-ionic fluids are known in the electrolysis and electrodialysis arts. While an ideal suitable for the electrolytically conductive barrier is substantially imperivous to oxygen, materials having limited permeabilities are satisfactory in many applications. Increased oxygen permeability has the effect of reducing the observed corrosion rate by rendering the test specimen less anodic. However, errors introduced lay-using materials having low permeabilities to oxygen are generally small. Thus, in many applications, satisfactory test results are obtained by ensheathiug the probe in an electrolytically conductive, substantially oxygen-impermeable material.

Suitable electrolytically-conductive, substantially oxygen-impermeable materials include animal tissue, various gels such as silica gel and alumina gel, sponge saturated with agar gel, and and paper dipped in collodion or agar gel, porous paper such as laboratory filter paper, and multiple layers of cloth. Where a relatively thick barrier of a moisture absorbing material such as animal tissue, agar-agar, silica gel, etc., is used, it is preferred to incorporate therein a small amount of'a highly ionizing salt, such as sodium chloride or sodium sulfate, to increase the electrolytic conductivity of the barrier.

Also suitable are numerous resins which are commonly used in making semi-permeable barriers. Where the material utilized is of itself too soft to provide suflicient rigidity, such as agar-agar, it may be coated on a plastic screen, or an insulation-coated metallic screen. The barrier will absorb or adsorb sufficient water or electrolyte from the surrounding earth or body of water to maintain a high electrolytic conductivity. Thus, while most barrier materials achieve an equilibrium water content, the material is substantially impermeable to water and other nonionized fluids once the equilibrium is established.

Another barrier for isolating the apparatus of this invention from the oxygen-containing electrolytic environment while maintaining it in electrolytic communication with the anode of the cathodic protection system, is a mixture of sandy soil with about 1% to 10% by weight of bentonite. The bentonite is capable of absorbing and retaining a small amount of electrolyte, thereby main taining a high electrolytic conductivity. Upon absorbing water the bentonite swells and forms with the sandy soil a barrier capable of resisting the influx of additional ground water. Thus, the barrier material Will in itself be damp or even wet, but will be impermeable to the flow of additional quantities of the electrolytic environment. Such a barrier, which, although damp, is capable of resisting the flow of additional quantities of water meets the definition of a fluid-impermeable barrier, as used in this specification and in the appended claims. As in the case of some of the other barriers, it is preferred to incorporate in the bentonite a small amount of a highly ionizing salt to enhance the electrolytic conductivity thereof.

Ferrous metal in an oxygen-free environment assumes the open circuit potential of the anodic areas since there is no cathodic activity. In order to achieve cathodic protection, it is necessary to depress the potential of the structure to the open circuit potential of the anodic areas, i.e., 0.8 5 volt with reference to a saturated coppercopper sulfate half cell. Hence, the effectiveness of the cathodic protection of a corrodible metal structure in a corrosive environment can be determined by measuring the polarity of the structure and adjusting the current supplied to the structure to maintain its potenial more negative than the open circuit potential of the anodes.

In accordance with the method of this invention, the adequacy or completeness of the cathodic protection provided a corrodible metal structure exposed to a corrosive electrolytic environment is determined by placing a specimen of the same or a substantially similar metal as the structure in the corrosive environment. The specimen is surrounded by or encased in a sheath of an electrolytically conductive, oxygen-impermeable material of the type hereinabove described so that the specimen is maintained in electrolytic contact with the corrosive environment, but

out of contact with oxygen in the environment, i.e., oxygen in the corrosive environment is excluded from contact with the specimen. Also, the specimen is electrically connected to the structure in the above-described manner.

The adequacy or completeness of the cathodic protection provided the structure is determined by connecting a microammeter, galvanometer, zero-resistance ammeter, or other current measuring device between the specimen and the cathodically protected structure and measuring the polarity of the structure with respect to the specimen. Since the specimen is at its open-circuit, anodic potential, e.g., at or close to 0.85 volt for carbon steel, the cathodic protection of the structure is complete if the structure exhibits the same potential as the specimen, in which case no current will flow between them, or is negative with respect to the specimen. On the contrary, the cathodic protection is incomplete if the structure is positive with respect to the specimen.

Where it is ascertained that the structure has a positive potential with respect to the specimen, i.e., there is a flow of electrons from the specimen to the structure, complete cathodic protection can be provided by increasing the flow of current to the structure sufiiciently to render the potential of the structure equal to or more negative than that of the open circuit anodic potential of the specimen. Also, where a cathodically protected structure is provided with excess cathodic protection current, the current supplied can be reduced so long as the structure is not rendered positive with respect to the specimen.

The invention is further described by the following examples which are illustrative of specific modes of practicing the invention as defined by the appended claims.

EXAMPLE 1 The adequacy of the cathodic protection applied to a buried pipeline is determined by the method of this invention. The pipeline is constructed of type API 5L carbon steel, wrapped and buried and cathodically protected in conventional manner.

A small coupon of type 1020 cold rolled steel and a resistance type, temperature compensated corrosion probe are ensheathed in a mixture of bentonite, soil and water and buried approximately 12 inches below the surface and approximately 12 inches from the pipeline. An electrical conductor attached to the coupon and the leads from the corrosion probe extend above ground and terminate in a junction box. Also, an electrical lead attached to the pipeline extends to the junction box. Both the coupon and the corrosion test probe are maintained electrically connected to the pipeline except when readings are made.

A milliammeter is connected between the structure and the coupon and it is observed that the structure has a negative polarity with respect to the coupon, i.e., an electronic current of 0.3 milliamp is observed flowing from the structure to the coupon.

EXAMPLE 2 A corrodible carbon steel buried pipeline is cathodically protected substantially as illustrated in the drawings. Also, a carbon steel coupon is encased in an electrolytically conductive, oxygen impermeable sheath and buried near the pipeline. The coupon is electrically connected to the pipeline through an above ground junction box.

The electrical leads from the pipeline and from the coupon are disconnected and attached to a zero-center galvanometer. The galvanometer deflection indicates that the pipeline is positive with respect to the coupon.

The cathodic protection current supplied to the pipeline is then increased and a second polarity measurement made. The pipeline is now negative with respect to the coupon indicating that the potential of the pipeline is less than the anodic open circuit potential of the coupon and, thus, that the cathodic protection is adequate to substantially mitigate corrosion of the pipeline.

While particular embodiments of the invention have been described, it will be understood, of course that the invention is not limited thereto since many modifications can be made and it is intended to include within the invention such modifications as are within the scope of the claims.

The invention having thus been described, we claim: 1. A method for determining the completeness of the cathodic protection provided a cathodically protected corrodible metal structure exposed to a corrosive environment which comprises:

placing in said corrosive environment a specimen of a corrodible metal similar in composition to the corrodible metal structure under cathodic protection;

electrically connecting said test specimen to said metal structure under cathodic protection;

maintaining said specimen in electrolytic contact with said corrosive environment;

excluding any substantial quantity of oxygen from contact with said test specimen; and

determining the polarity of the structure with respect to said specimen.

2. The method defined in claim 1 wherein said test specimen is maintained in electrolytic contact with said corrosive environment and any substantial quantity of oxygen is excluded from contact with said specimen by encasing the specimen in a sheath of electrolytically conductive, substantially oxygen impermeable material.

3. The method defined in claim 2 wherein said electrolytically conductive, substantially oxygen-impermeable material is selected from the group consisting of animal tissue, agar-agar, silica gel, alumina gel, a mixture of sandy soil and about 1 to 10 percent by weight bentonite, a plurality of layers of cloth, and a plurality of layers of porous paper.

4. The method defined in claim 3 wherein a small amount of a strongly ionizing salt is added to the material forming said sheath.

5. The method defined in claim 1 wherein the structure is cathodically protected by supplying electrical current to said structure and wherein the flow of electrical current to said structure is adjusted so that the structure exhibits a potential equal to or more negative than the potential of the specimen.

6. The method defined in claim 1 wherein the polarity of the structure with respect to the specimen is determined by connecting a microammeter, a galvanometer, or a zero-resistance ammeter between the structure and the specimen.

7. A method for determining the completeness of the cathodic protection provided a cathodically protected corrodible metal structure exposed to a corrosive environment which comprises:

placing in said corrosive environment a specimen of a corrodible metal similar in composition to said corrodible metal structure;

encasing said specimen in a sheath of electrolytically conductive, oxygen-impermeable material selected from the group consisting of animal tissue, agar-agar, silica gel, alumina gel, a mixture of sandy soil and about 1 to 10 percent by weight bentonite, a plurality of layers of cloth, and a plurality of layers of porous paper whereby said specimen is maintained in electrolytic contact with said corrosive environment and out of contact with any substantial quantity of oxygen under cathodic protection;

electrically connecting said specimen to said corrodible metal structure;

periodically connecting a microammeter, a galvanometer, or a zero-resistance ammeter between the structure and the specimen; and

determining the polarity of the structure with respect to said specimen.

8. A method for cathodically protecting a corrodible metal structure exposed to a corrosive environment which compnses:

placing in said corrosive environment .a specimen of a corrodible metal similar in composition to the corrodible metal structure;

electrically connecting, said test specimen to said metal structure;

encasing said specimen in a sheath of electrically conductive, oxygen-impermeable material so that said specimen is maintained in electrolytic contact with said corrosive environment and out of contact with any substantial quantity of oxygen;

determining the polarity of the structure with respect to said specimen;

supplying electrical current to said metal structure to cathodically protect the metal structure;

determining the polarity of said structure with respect to said specimen; and

adjusting the flow of current to said structure so that it exhibits a potential equal to or more negative than the potential of the specimen.

References Cited UNITED STATES PATENTS Cowles 204-195" TA-HSUNG TUNG, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF QORRECTIQN Patent No. 3 6 49 +92 D d March 1 1972 Inventor(s) Glenn A. Marsh and Edward Schaschl It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, claim 7, line 65:

"under cathodic protection" should be deleted.

Column 6, claim 7, line 67:

"under cathodic protection" should be added before semi-colon.

Signed and sealed this 18th day of July 1 972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents