US3207678A - Process for determining cathodically protecting current densities - Google Patents

Description

Sept.

cpRRoslolv RA TE G. A. MARSH ETAL PROCESS FOR DETERMINING CATHODICALLY PROTECTING CURRENT DENSITIES Filed June 1, 1960 I I2 I I 8 [0m 0 IO'M "*CA THODIC ANODIC APPUED CURRENT/mafia F I 6. I

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INVENTORS GLENN A. MARSH Y EDWARD SCHASCHL A TTORNE Y United States Patent 3,207,678 PROCESS FOR DETERMINING CATHODICALLY PROTECTING CURRENT DENSITIES Glenn A. Marsh and Edward Schaschl, Crystal Lake, 11].,

assignors to The Pure Oil Company, Chicago, 111., a

corporation of Ohio Filed June 1, 1960, Ser. No. 33,161 9 Claims. (Cl. 204-1) This invention relates to mitigating the corrosion of submerged or subterranean metal structures by means of cathodic protection. More specifically, it relates to a method for determining the minimum cathodic current density necessary to alTord protection in a cathodic protection system.

Almost any metal surface exposed to soil or water can be cathodically protected from corrosion. In this corrosion-prevention method a voltage, great enough to cause a certain amount of current to flow to the metal structure to be protected and render all parts of the structure cathodic, is applied from an external source to the structure. To evaluate the economics of the cathodic protection system, it is necessary to determine the amount of current necessary to protect the structure. The current flow should be no greater than the minimum required to provide substantially complete protection, not only from the standpoint of useless waste of power, but also to avoid excessive destruction of the anodic member of the electrical protection system.

The prior art describes various methods of determining the current density requirements to achieve cathodic protection of a metal, especially steel, in an electrolytic environment. Several arbitrary criteria for deciding the adequacy of the applied current have been devised. One of the most common theories is that the applied current density should be the electrochemical equivalent of the rate of free corrosion of the structure in the corrosive environment. This theory, which is in reality an application of Faradays law, has been found to provide a fair approximation of the actual minimum current which will provide complete protection in selected systems. In other electrolyte-metal systems, it has been found that the minimum current is in fact at variance with that predictable from Faradays law, usually being slightly less than that which would be predicted, especially in the case of steel in an aqueous salt solution. The use of various correction factors for estimating minimum current density has been proposed for specific corrosion systems. Other authorities have taken the position that minimum current density can be determined for each corrosion system only by trial and error experiments.

One method for determining the minimum current required to achieve substantially complete cathodic protec tion of a steel structure disposed in an electrolyte is described in US. Patent 2,869,003, of the instant inventors. The method of the present invention is in part an improvement over the method described and claimed in this patent in that it provides a simplified technique by which minimum current density can be estimated from data obtained in a single experiment.

Accordingly, it is a primary object of this invention to provide a method for rapidly and conveniently determining the minimum current density which must be applied to a metal structure, susceptible to electrolytic corrosion, to achieve substantially complete protection of said structure. It is another object of this invention to provide a method for rapidly evaluating and controlling cathodic protection applied to a corrodible, submerged structure.

In accordance with this invention, the minimum current density required to achieve substantially complete cathodic protection of a corrodible metal in an electrolyte can be 3,207,678 Patented Sept. 21, 1965 determined by disposing two metal specimens in the electrolyte, causing a current of known density to flow between the specimens for a suitable period of time, and measuring the rate of corrosion of each specimen during the period of current flow. From the data thus determined a linear equation expressing corrosion rate as a function of current density can be obtained, and this equation can be solved to determine the minimum current density required to protect the selected metal in the selected electrolytic environment.

This invention will be described with reference to the drawings, of which:

FIGURE 1 is a graph showing corrosion rate as a function of applied current.

FIGURE 2 is a perspective view of a corrosion probe which may be used in the method of this invention.

FIGURES 3 and 4 are schematic drawings of circuits which may be used with the corrosion probe of FIG- URE 2.

When .a corrodible metal specimen in a corrosive electrolytic environment is made positive by applying a current from an external source, the corrosion rate of the specimen may increase by an amount proportional to the applied current, as is predictable from Faradays law. Then,

Where I is the new corrosion rate expressed as the equivalent corrosion current, 1 is the rate of free corrosion of the selected specimen in the selected environment, and I, is the applied current. It will be understood that every corrosion rate has an equivalent galvanic current, in accordance with Faradays law. Accordingly, in performing mathematical calculations or in constructing graphs, corrosion rates can be converted to their equivalent currents and all calculations made in terms of currents, or, alternatively, all currents and corrosion rates can be expressed in terms of corrosion rates, and calculations similarly performed. It will be understood that in this specification and the appended claims, the two methods are considered to be equivalents. Experimenters of the prior art have found that the measured corrosion rate, I, for certain metals under certain conditions was less than the sum of I and 1,. Under other more commonly occurring conditions, it has been found that the measured corrosion rate, I, is greater than the sum of I and 1,.

From an inspection of the foregoing equation, it is evident that When I is negative, and equal in magnitude to I the rate of free corrosion, the actual corrosion rate I must be zero. Some experimenters of the prior art concluded from this equation that the current necessary to achieve complete cathodic protection of a corroding metallic structure in an electrolytic environment is the current equivalent of the rate of free corrosion of the structure. Since, as was explained above, in a few instances I is in fact less than the sum of I and I and in the more usual circumstances I is greater than the sum of I and 1,, the calculation of the current required to achieved complete cathodic protection of a structure using this method was only an approximation, and subject to serious error. The workers of the prior art, being aware of this error of method, proposed various correction fac tors which may be applied, but none of which were altogether satisfactory in that While the methods that they proposed might be suitable for a specific metal in a specific environment, the method still could not be applied generally without the introduction of serious error.

Referring to FIGURE 1, the graph is seen to depict variations of corrosion rate as a function of galvanic current for a corrosion system, which may be steel in an aqueous sodium chloride solution. Point 2 represents the free corrosion rate of the specimen in. the electrolyte.

Through this point pass two curves, 4 and 6. Curve 4 represents the corrosion rate which would be predicted from Faradays law, and this curve passes through the point 2 since it is apparent that the specimen will corrode at its free corrosion rate when not under the influence of galvanic current. Assuming that galvanic currents and corrosion rate are both represented in terms of current, the slope of curve 4 will equal 1, in accordance with Faradays law. When a galvanic current is caused to fiow to or from the corroding specimen, curve 6 is found in fact to define the relationship between corrosion rate and galvanic current. The slope of this line, for systems in which the corroding specimen is not subject to anodic polarization, is greater than one.v The workers of the prior art were aware that a curve corresponding to 6 existed, but only in the range of point 2 to point 8, that is, where a cathodic current is applied to the corroding specimen. It has now been discovered that curve 6 extends through the free corrosion rate point, 2, and can be plotted for anodic currents. It has further been found that curve 6 is linear for applied galvanic currents, both cathodic and anodic, where these currents have values within the range of zero to slightly less than the current equivalent of the free corrosion rate. Thus curve 6 of FIGURE 1 is linear in the range of point 8 to a point somewhat beyond point 110. Points 10 and 12 are obtained for 10 milliamperes per square foot current densities, cathodic and anodic, respectively.

The foregoing discoveries have led to the method of this invention, and are set out to provide a basis of understanding for the application of the method of this invention. It will be seen from FIGURE 1 that the current required to achieve complete protection of a corroding specimen in an electrolytic environment is equivalent to the free corrosion rate only when the slope of line 6 becomes equal to 1, that is, where the actual corrosion rate varies with applied cathodic current in accordance with Faradays law. In the majority of cases in which it is desired to cathodically protect a corrodible structure in practice, the system is, such that corrosion rate varies in accordance with applied currents as defined by a curve such as curve 6. It is evident that in such cases the slope of the curve is greater than 1, and the current density which must be applied to achieve complete cathodic protection of the structure is sub stantially less than the current equivalent of the free corrosion rate of the specimen in the environment under study. Some specimen-environment combinations may exist wherein the corrosion rate will be controlled by anodic polarization. In such instances, curve 6 may have a slope of less than unity, and the curve will not be linear. The method of this invention cannot be expected to predict minimum current required to achieve substantially complete cathodic protection with the desired degree of accuracy in such cases. Accordingly, it is intended that the method of this invention be applied only to the usual case where the free corrosion rate is not controlled by anodic polarization.

As has been stated, point 2 represents the free corrosion rate of a corrodible test specimen, such as steel, in a corrosive electrolyte, such as sodium chloride in an aqueous solution. When a cathodic current of 10 milliamperes per square foot is applied to the specimen, the rate of corrosion drops more rapidly than would be predicted from Faradays law, and point 12 is found to represent the actual corrosion rate observed at an applied cathodic current density of 10 milliamperes per square foot. An equivalent anodic current is then applied to the specimen, and point It) is determined. It is found that points 2, I2, and 10 all he along the same straight line. This method has been extended using other current densities, such as 5 milliarnperes per square foot or milliamperes per square foot. The points were found to fall along straight line 6 as long as the applied currents, anodic or cathodic, did not exceed about 85% of the current equivalent of the free corrosion rate of the specimen-electrolyte combination under study. It now becomes apparent that if two corrodible specimens are placed in an electrolyte, and a current of known density of suitable magnitude is passed between the specimens for a suitable length of time, the corrosion rates of the two specimens can be measured, and two points such as 10 and 12 will be thereby determined. These points may be plotted on a graph and a straight line joining the points may be extended to its intersection with the horizontal axis representing zero corrosion rate. The cathodic current indicated by this intercept is that current required to afford complete cathodic protection for the test specimen-electrolyte system.

Referring to FIGURE 2, a test probe which may be used to make the necessary experiments is depicted. Basically, the probe comprises a base 20, two parallel, exposed test specimens 22 and 24, and one coated specimen 26. The probe is used in combination with a current-supply means, as shown in FIGURE 3. A bridge circuit adapted for determining the relative corrosion rates of the specimens is shown in FIGURE 4.

In U.S. Patent application, Serial No. 528,032, filed August 12, 1955, by the instant inventors, now abandoned, there is described a temperature-compensated, corrosion-testing probe which determines corrosion loss during the corrosion of specimens by measurement of resistance change occuring in the test specimen. In the basic embodiments of the corrosion-testing probe, two test specimens of the metallic material of construction under consideration are disposed within the corrosive environment in a suitable specimen holder which permits the specimens to be serially interconnected. One of the specimens is left unprotected while the other specimen is ensheathed with a protective coating, such as a corrosion-resistant plastic to prevent its corrosion. These specimens are serially connected and form separate resistances in one branch of a conventional electrical bridge circuit. This combination of resistance elements constitutes a corrosion-testing unit, or probe, and functions as a sensing element for the complete apparatus. The remainder of the bridge network, which in its simplest form consists of a second resistance branch in parallel with the first resistance branch, a metering instrument such as a galvanorneter connected across said resistance branches, and a power source, is positioned outside of the corrosive environment at a point which will facilitate the making of observations in the corrosion study. In the second resistance branch, a variable resistance forms the second bridge arm opposed to the corrodible speci men exposed to the corrosive environment. Instrumentation which can be used in connection with this corrosion-testing unit includes electrical bridge circuits such as are described in US. Patent 2,824,283.

In the method of this invention a modification of the afore-described test probes is preferably used. In this modification, two bare corrodible specimens and one coated compensating specimen are preferably included in the probe, as depicted in FIGURE 2. The circuit depicted in FIGURE 3 provides the means for passing a current of known magnitude between the two bare specimens. In FIGURE 3, the compensating element 26 has been omitted for clarity. Elements 22 and 24 serve as anodic and cathodic electrodes and are connected through lead wires 30 and 32 to power source 34, which may be a battery, through variable resistance 36, and ammeter 38. Thus the current applied to the specimens may be varied over a wide range. It is apparent that the circuit is complete only when elements 22 and 24 are immersed in an electrolyte, such as wet soil, water, aqueous solutions, and so forth. It is especially preferred that elements 22 and 24 lie in parallel planes to avoid uneven corrosion of the specimens with resulting inaccuracies. FIGURE 4 shows the manner in which bare specimens 22 and 24, together with protected specimen 26, are connected in a bridge-type measuring circuit. Specimen 26 may be permanently connected in the bridge circuit, whereas specimens 24 and 22 are adapted for connection in sequence so that the change in resistance of each specimen may be measured individually. The second branch of the bridge is provided by potentiometer 40, and power source 42 together with galvanometer 44 comprise the remainder of the network. The method of operation of the corrosion-probe measuring circuits of the resistanceratio type is well known to the art, and accordingly will not be further described. The corrodible specimens used in the method of this invention preferably are foil-like, cold-rolled, steel sections about 3 inches long by /s inch Wide by 0.001 inch thick. Specimens haivng other dimensions may be used, but very thin ribbon-like specimens are preferred, because they permit the rapid obtaining of accurate data. By using suitable current densities, preferably in the range of 50% to 85% of the current equivalent of the estimated free corrosion rate of the test specimens, accurate data can be obtained by applying the current between the specimens for a period as short as one-half hour to six hours and then making the necessary corrosion rate measurements.

The resulting data are handled as shown in FIGURE 1 to determine the minimum current required to protect the specimen. The corrosion rates, expressed as current, of the anodic and cathodic specimens are plotted against applied current, as indicated by points 10 and 12, respectively. Line 6 is then drawn through the points and extended to intersect the applied current axis. Point 8, or the axis intercept, is equivalent to the minimum protective current. While point 2 need not be plotted using the method of this invention, it is evident that such a point exists, and that it can be determined, if desired, by observing the point at which line 6 crosses the zero ordinate of applied current. Since line 6 is linear, it becomes evident that the free corrosion rate of the metal specimens can be determined by averaging the two measured corrosion rates arithmetically.

In some cases, the free corrosion rate of the metal specimens in the electrolytic environment under study may be known. In such cases, in accordance with this invention, a second simplified method of determining the minimum current density required to achieve complete cathodic protection may be used. In this method, a corrosion probe using two bare corrodible specimens, as depicted in FIGURE 3, is utilized. No compensating specimen is necessary. The probe is inserted in the electrolyte and a current of suitable magnitude is passed between the two specimens. The two specimens are then connected in a corrosion-probe bridge circuit, as depicted in FIGURE 4, one specimen being placed in each arm of the bridge, as represented in FIGURE 4 by elements 24 and 26. Since no reference specimen is used, and since both of the bare specimens will corrode to some extent, the corrosion-meter circuit indicates the magnitude of the difference in the corrosion rates of the two bare specimens and does so in a single meter reading, rather than indicating the absolute corrosion rates of the two specimens separately. The difference in corrosion rates having been observed, the current required to achieve cathodicprotection can be calculated from the formula,

ZI L, T where,

I is the minimum current to be determined, d is the measured difference in corrosion rates, I is the free corrosion rate, and I is the current caused to flow between the specimens.

In instances where the free corrosion rate of the metal is not known, it can be readily determined by use of an electric resistance-change-type corrosion meter. A particular advantage of the method of this invention is that it is only necessary to cause current to flow once between two bare, corrodible metal specimens. The necessity for making a plurality of corrosion-rate determinations at a plurality of rates of current flow is avoided. Utilizing the method of this invention it is essential that the two corrodible metal specimens be fabricated of identical materials of construction, and it is preferred that these speci mens have equal surface area. It is desirable, but not essential, that the two specimens have the same ratio of surface area to volume, and that this ratio be high. These conditions can readily be met by using two identical ribbon-like specimens fabricated from the same material of construction. The compensating specimen, if one is used, may similarly be fabricated of the same material of construction and have the same dimensions as the test specimens. This is not necessary however, provided the compensating specimen has a temperature resistance characteristic practically identical with that of the two bare specimens, so that effective temperature compensation is obained. The current applied between the two bare specimens need not be of any particular value, provided that the current is less than that ultimately found to be necessary to achieve complete cathodic protection. It is preferred that the currents be maintained as high as practical, because greater accuracy is obtained using this method when the applied current approximates, but is somewhat less than, the current required to provide complete cathodic protection to the specimen. Accordingly, it is preferred that the applied current be in the range of 50% to 75% of the electrochemical equivalent of the free cor rosion rate of the test specimen. The free corrosion rate can usually be estimated by one skilled in this art with reasonable exactness. Where it is found that the applied current lies in the range of 50% to of the current determined to provide cathodic protection, it may be assumed that an accurate determination has been made. Where it is found that the applied current is less than 50 of the current determined to provide complete cathodic protection, it is desirable that the experiment be repeated using an applied current slightly less, say 10% less, than that which was indicated would provide cathodic protection. Where the current determined to provide cathodic protection is found to be less than the applied current, it is imperative that the experiment be repeated using a lower current density, since the accuracy of the first determination is doubtful.

As a specific example of the method of this invention, a probe (as depicted in FIGURE 2) is constructed having two bare specimens 3 inches long, /8 of an inch wide, and 0.001 inch in thickness. The probe is exposed to a cor rosive environment comprising a 1% aqueous sodium chloride solution containing 7 parts per million of dissolved oxygen and having a pH of 7. A current of 62.5 microarnperes is caused to flow between the two bare specimens for a period of 2 hours. The corrosion rates of the two bare specimens are then measured, using the coated specimen as a reference element, by means of a resistance-change corrosion-measuring instrument. The corrosion rate of the anodic specimen is determined to be 2.8 microinches penetration per hour which is equivalent to 50.5 milliamperes per sq. ft. The corrosion rate of the cathodic specimen is determined to be 1.1 microinche per hour, which is equivalent to .20 milliamperes per sq. ft. Using this information, and the known current applied between the two specimens, a graph corresponding to that shown in FIGURE 1 is constructed. The two points are plotted and the straight line joining them is extended to the zero corrosion-rate intercept. At this point, the minimum current required to achieve complete cathodic protection of the selected specimens in the corrosive electrolyte is read from the graph-in this case 28 milliamperes per sq. ft.

As another example of the method of this invention, two bare steel test specimens are inserted in a test probe in parallel relationship as depicted in FIGURE 3. The

probe is inserted in the electrolytic solution described in the prior example, and a current having a density of 12 milliamperes per sq. ft. is caused to flow between the two specimens. The time of current flow is 2 hours. At the expiration of this time, the two corroded specimens are connected to an electric resistance-change corrosion meter and the difference in the rates of corrosion of the two specimens is read directly, and found to be 30.5 milliamperes per sq. ft., or 1.72 microinches per hour. The free corrosion rate of the same material of construction in the same environment is determined by inserting a conventional electric resistance-change-type corrosion probe, having a bare corrodible element fabricated from the same material as the afore-described bare elements, and the rate of corrosion of this probe is determined to be 35.6 milliamperes per sq. ft. or 2.0 mircoinches per hour. The data thus obtained are then substituted in the formula,

and the current density required to achieve complete cathodic protection of the specimens is determined to be 28 milliamperes per square foot.

The embodiments of the invention for which a special property or privilege is claimed are defined as follows:

1. A method for determining the minimum current density required to achieve substantially complete cathodic protection of a corrodible metal in an electrolytic environment comprising disposing a cathode and an anode consisting of two bare specimens of said metal having a high ratio of surface area to volume in spaced relationship in said environment, applying between said specimens a potential from an external source to cause a direct current flow between the said specimens, determining the current density applied to each of said specimens, measuring the rate of corrosion of each of said specimens occurring during the period of the applied current flow, determining from said current density and from said measured corrosion rates a linear equation expressing the corrosion rate as a function of current, and solving said equation for current at zero corrosion rate to determine the minimum current capable of achieving substantially complete cathodic protection of said metal in said electrolytic environment.

2. A method for determining the minimum current required to achieve substantially complete cathodic protection of a corrodible metal in an electrolytic environment comprising determining the free corrosion rate of said metal in said environment, disposing a cathode and an anode consisting of two elongated bare specimens of said metal in spaced relationship in said environment, said specimens having equal surface areas and a high ratio of surface area to volume, applying between said specimens a potential from an external source to cause a direct current of known magnitude to flow between said specimens, determining the difference in the rates of corrosion of said specimens during the period of said direct current flow by measuring the change in the ratio of the resistances of said specimens occurring during said period, and determining the minimum current density required to 8 achieve substantially complete cathodic protection of said metal in said environment by substitution in the formula:

9 21:51., where,

I is the minimum current to be determined,

d is the measured difference in corrosion rate,

I is the free corrosion rate, and

I, is the current caused to flow between the specimens.

3. A method according to claim 8 in which the said applied current is in the range of 50 to of the current determined to provide complete cathodic protection to said metal.

4. A method according to claim 11 including the steps of disposing a third reference specimen in said environment, said reference specimen having a temperature-resistance characteristic similar to that of said two specimens, and being insulated from the corrosive effects of said environment, and determining the rates of corrosion of said two specimens by measuring the change in the ratio of the resistances of said two specimens with respect to said reference specimen, said corrosion occurring during the period of current flow.

5. A method according to claim 4 in which the said applied current is in the range of 50 to 75 percent of the current determined to provide complete cathodic protection to said metal.

6. A method according to claim 4 in which the said applied current is about of the current determined to provide complete cathodic protection to said metal.

7. A method according to claim 6 in which said specimens are ribbon-like and are disposed in parallel, laterally-displaced relationship.

8. A method according to claim 2 in which the free corrosion rate of said metal in said environment is determined by disposing two elongated specimens of said metal in said environment, one of said specimens being bare and the other being insulated from the corrosive effects of said environment, and determining the rate of change of the ratio of the resistances of said specimens as said bare specimen corrodes.

9. A method according to claim 3 in which the two bare specimens are ribbon-like and are disposed in parallel, laterally-displaced relationship in said environment.

References Cited by the Examiner UNITED STATES PATENTS 2,786,021 3/57 Marsh 204 2,796,583 6/57 Marsh et al. 32471.3 2,803,797 8/57 Cowles 3247l.3 2,834,858 5/58 Schaschl 32471.3 2,869,003 1/59 Marsh et al 324-713 OTHER REFERENCES Evans: Metallic Corrosion Passivity & Protection, 1948, pages 256, 257, 441 and 412.

Blum et al.: Transactions of the American Electrochemical Society, vol. 52, 1927, pages 403-429.

JOHN H. MACK, Primary Examiner.

SAMUEL BERNSTEIN, JOHN R. SPECK, MURRAY TILLMAN, WINSTON A. DOUGLAS, Examiners.

Claims (1)

1. A METHOD FOR DETERMINING THE MINIMUM CURRENT DENSITY REQUIRED TO ACHIEVE SUBSTANTIALLY COMPLETE CATHODIC PROTECTION OF A CORRODIBLE METAL IN AN ELECTROLYTIC ENVIRONMENT COMPRISING DISPOSING A CATHODE AND AN ANODE CONSISTING OF TWO BARE SPECIMENS OF SAID METAL HAVING A HIGH RATIO OF SURFACE AREA TO VOLUME IN SPACED RELATIONSHIP IN SAID ENVIRONMENT, APPLYING BETWEEN SAID SPECIMENS A POTENTIAL FROM AN EXTERNAL SOURCE TO CAUSE A DIRECT CURRENT FLOW BETWEEN THE SAID SPECIMENS, DETERMINING THE CURRENT DENSITY APPLIED TO EACH OF SAID SPECIMENS, MEASURING THE RATE OF CORROSION OF EACH OF SAID SPECIMENS OCCURRING DURING THE PERIOD OF THE APPLIED CURRENT FLOW, DETERMINING FROM SAID CURRENT DENSITY AND FROM SAID MEASURED CORROSION RATES A LINEAR EQUATION EXPRESSING THE CORROSION RATE AS A FUNCTION OF CURRENT, AND SOLVING SAID EQUATION FOR CURRENT AT ZERO CORROSION RATE TO DETERMINE THE MINIMUM CURRENT CAPABLE OF ACHIEVING SUBSTANTIALLY COMPLETE CATHODIC PROTECTION OF SAID METAL IN SAID ELECTROLYTIC ENVIRONMENT.

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