US 3360452 A
Description (OCR text may contain errors)
Dec. 26, 1967 v F. E. M NULTY CATHODIC PROTECTION SYSTEM Filed Feb. 24, 1964 I I L l FRANK E. M NULTY INVENTQR.
United States Patent 3,360,452 CATHODIC PROTECTION SYSTEM Frank E. McNulty, Tulsa, Okla, assignor to Nee & McNulty, Inc., a corporation of Oklahoma Filed Feb. 24, 1964, Ser. No. 346,945 Claims. (Cl. 204-197) ABSTRACT OF THE DISCLOSURE transistor. Where the voltage on the base-emitter junction increases, the current flow increases through the transistor.
This invention relates to cathodic protection systems. More particularly it is concerned with a novel means by which the potential on the metal structure (cathode) which it is desired to protect from corrosion can be automatically held at a predetermined level.
Structures can be cathodically protected in two ways:
(1) By the use of sacrificial anodes, and
2) By means of impressed currents.
The most widely known sacrificial anode system is galvanized steel wherein the zinc coating is the anode which corrodes preferentially thereby protecting the steel. The important point is to have direct current entering the entire surface of the structure to be protected. Impressed currents involve passing a direct current from one external source through an anode, through the corrosive environment, and then to the structure. Usually a very corrosive resistant or so called permanent anode is needed. Carbon, platinum and silicon-iron alloys are typical examples of impressed current anodes. Underground pipelines, radar towers, seagoing vessels, olfshore wells and piers are examples of structures requiring cathodic protection for prolonging their usefulness.
The sacrificial anode and impressed current methods have not met with unqualified success. For example, in the case of the sacrificial anode method, the current flowing to the cathode is desirably just enough to prevent galvanic action at the local anodes and cathodes. As a practical matter, however, a much higher potential has to be maintained particularly in the cathodic protection of large areas because to maintain a protective potential at distant areas, areas near the anode(s) are over-protected. Thus, in the case of cathodic protection of ship hulls by magnesium anodes, areas next to the anodes receive excessive protection which frequently causes the paint on the hull to blister and peel off. Also, undesirable calcareous deposits build up on the surface in such overprotected areas. Further, the maximum current output is determined by the surface area of the anode and the inherent resistance in the system, such as the resistances of the lead connections. In order to provide cathodic protection over an extended period of time, the consumable anodes must be large enough to provide adequate electron flow or current flow up until their normal time of replacement. This dis advantage has been offset to a certain extent by inserting a relatively large resistance in the electrical circuit connecting the electrodes and over a period of time reducing this resistance as required. However, a lack of flexibility of current adjustment is nevertheless present. The cost of the use of an anode metal low in that series and impressing a direct current on the structure to be protected to provide the necessary protective current. A comparatively inert metal is used as the anode so that it will not take part in the galvanic process as would a more active metal. In this way the life of the anode is greatly extended. However, the current input to the cathode or protected metal must be high enough to protect areas remote from the anode, and this also results in undesirable coatings of calcareous materials on the cathode in the region near the anode. Likewise, on the ships hull, for example, the high current input causes the paint to strip away in the vicinity of the anode. This extensive protection in the anode area is particularly bad in the impressed current systems because the area of the anode is generally not great, as where the anode is made of platinum, and in order to achieve the proper level of current density, a relatively high voltage must be applied in the circuit to overcome the anode-to-sea resistance. This resistance in the case of a ship in fresh water, for example, is high for the small anode area available. Also, the relatively inert nature of the anode requires that a considerable voltage be applied in the circuit before the voltage can even overcome the difference in potential between the anode and the material to be protected. It will be apparent that all of these factors dictate the use of relatively high impressed voltages and in practice the voltages have proved to be much higher in the cathode areas immediately next to the anode than presently known paint films can withstand. Not only is the paint in these areas undesirably stripped away thus increasing the amount of cathodic protection which must be provided, but also it will be apparent that a great deal of power is wasted in overprotecting areas near the anode in order to adequately protect remote cathode areas. Like Wise, an inadvertent power failure will completely suspend cathodic protection until power is again obtained, and the former cathodic areas will become undesirably anodic with respect to the formed anodes.
Accordingly, it is an object of my invention to provide an automatic anti-corrosive system whereby protection can be applied to the surface of a metal structure which can be held at a predetermined potential effective to inhibit corrosion. It is another object of my invention to provide a cathodic protection system empolying sacrificial anodes, a self-contained voltage regulation device, and means for controlling the voltage through said regulating device whereby the current flow between the anodes and cathode is controlled. A still further object of my invention is to eliminate the need for an external power source for operation of the aforesaid voltage regulating device. It is another object of my invention is to improve the eificiency of a sacrificial anode with respect to the recovery of its electrical current capacity.
I have found that power transistors have the ability to regulate current flow across the emitter and collector of the transistor by varying the voltage input on the base of the transistor. When-the voltage on the base-emitter junction increases, the current flow increases through the transistor. Also, a relatively small amount of applied current on the base of the transistor results in a relatively large current flow between the emitter and the collector of the transistor. This current gain can be in the order of thirty to fifty times or more.
In practical application of my invention, a power transistor is included in the conductor between the sacrificial anode and the cathode, e.g., carbon steel, with the emitexample, be sea water, earth, etc., a separate anode is used to provide a current and voltage supply to the base of the transistor. If the reference anode to the transistor base is zinc, the base-emitter potential difference before any current has moved through the main conductor of the transistor would be the difference between 686 millivolts forcarbon steel and 1,106 millivolts for zinc, using a copper-copper sulfate reference electrode. This leaves a net voltage between the base and the emitter of the transistor of 420 millivolts. With the same reference electrode, magnesium alloy produces a potential of 1,550 millivolts. Actually the conductor and junction resistances to and in the transistor will probably cause a loss of approximately 100 to 150 millivolts. It has conveniently, therefore, worked out that the base-emitter voltage in this system increases the potential of the steel by approximately 270 millivolts using a type of transistor capable of operating at extremely low voltages. This is the difference between the theoretical potential of 420 millivolts mentioned heretofore and 150 millivolts lost in resistances in the transistor and conductor. Adding the 270 millivolts to the basic 686 millivolts of the carbon steel, it indicates'that the potential to steel would be raised to 956 millivolts. This is a desirable potential for steel because it is below the hydrogen disbonding or paint peeling level and above the potential required to prevent steel from corroding. In this illustration, when the steel has reached a potential of 956 millivolts, there is no further base to emitter voltage. This suppresses the current flow between the emitter and collector of the transistor. Should a change occur in which the resistance of the electrolyte is decreased, a base-emitted junction voltage immediately develops which in turn opens the transistor permitting increased current flow between the sacrificial anode and the cathode via the emitter and the collector of the transistor.
Because transistors have relatively low current capacities, used individually, it is possible to use a group of transistors in parallel in order to obtain high current outputs. For example, on a large ship it may be necessary to provide as much as 1,000 amperes of current at a given moment. This would be at top speed and in areas of maximum salinity and possibly in rough whether where the wetted area of the hull could be increasing and decreasing quickly. Experience actually shows that the amount of cathodicprotection required to protect an active ship can vary by a ratio of one to five. In this application a ship will use about one-fifth as much current at dockside as it will under maximum operating conditions.
In many offshore drilling structures located in sea water and on ships, the resistance of the electrolyte in the cathodic system varies continuously throughout the day due to changes in the salinity of the water, temperature of the water, and other factors. It is, therefore, desirable to have a controlled potential system that will respond instantly to these variations. In using a power transistor for this purpose, there is current flow-through the transistor at all times owing to the continuous emission of electrons from the cathode. The process is constantly taking place so that the transistor in effect is acting like a valve which automatically partially opens and closes according to the demand placed on the transistor by the cathode. The transistor senses this demand when the potential on the cathode drops which sets up a base to emitter potential at the transistor.
In the present invention, a standard EIA type transistor No. 2N2728-a PNP germanium type-was used. With this particular transistor, the geometry of its construction is such that it can be used with reversed polarity. The transistor can be connected to the conductor so that the collector is connected to the cathode and the emitter to the anode with good results. There are numerous transistors on the market today that will work in this application with either polarity. A transistor should be selected that will operate on a stable basis within the current capacities to which it is subjected. It is also necessary to use a transistor that is susceptible to low voltage operation with particular reference to emitter-collector voltage and base-emitter voltage.
My invention will be further illustrated by reference to the accompanying drawing in which a ship 2 having a carbon steel hull 4 floats in sea water (electrolyte) 6. From the cathode is a conductor 8 leading to sacrificial anode 10 which may be magnesium or any other metal higher in the electrochemical series than the metal of the ships hull. Likewise certain alloys of such metals can be used as sacrificial anodes. In conductor 8 is placed a selfcontained voltage regulating device such as, for example, a power transistor 12, and between the latter and cathode 4 is an ammeter 14 to measure current flow through conductor 8. Transistor 12 consists essentially of three main parts, base 16, emitter 18, and collector 20. Anode 22 is connected to base 16 by means of conductor 24. Variable resistance 26 enclosed by dashed lines is used where an anode material is employed capable of increasing the cathode potential above the desired level. The use of a resistor is unnecessary where anode 22 is a metal or alloy that is capable, without curtailing the current in any way, of producing a potential on cathode 4 high enough to inhibit appreciable galvanic action but low enough to prevent the stripping of paint off the cathode surface and to inhibit formation of calcareous deposits.
To check the operation of this cathodic protection system, it is desirable to have a standardized reference electrode 28 of silver-silver chloride or copper-copper sulfate joined to cathode 4 by means of conductor 30, having therein a volt meter 32 by which the potential between cathode 4 and refernce electrode 28 can be accurately monitored. In place of the half-cells mentioned above, any suitable metal higher in the electrochemical series than that of cathode 4 may be substituted such as magnesium, zinc or alloys of these metals with the voltmeter calibrated to read corresponding potential changes.
In operation, cathode 4 represents a ships hull to which sacrificial anode 10 is connected through conductor 8. A positive ion current flow travels from anode10 to cathode 4 via sea Water electrolyte 6. The normal potential between cathode 4 and reference electrode 28 which, for example, is a copper-copper sulfate half-cell, is about 686 millivolts. To inhibit corrosion of cathode 4 under such conditions, the potential thereof should be raised to about 800 millivolts and preferably to about 850 millivolts between cathode 4 and reference electrode 28. With the fiow of current from anode 10 to cathode 4, the potential or impressed voltage built up thereon tends to exceed a value of about 1,050 to about 1,100 millivolts, the maximum level at which protection of the cathode from corrosion is obtained without causing the painted surface to blister. Accordingly, the current flow from anode 10 to cathode 4 can be decreased by allowing cur rent to flow from anode 22 to base 16 of transistor 12. This flow of current to transistor 12 is controlled by variable resistance 26 when anode 22 is of magnesium or a metal of similar voltage producing capacity. Thus by setting resistor 26 at a level such that the base-emitter junction voltage of transistor 12 will not exceed a maximum of, say, 314 millivolts, the potential on cathode 4 cannot exceed 1,000 millivolts. As previously mentioned, the con ductor and junction resistances in transistor 12 generally cause a loss in voltage of to millivolts. Thus, if resistor 26 were set at a level such that a voltage of 1,100 millivolts on cathode 4 were theoretically possible, the maximum practical voltage produced thereon would be only of the order of 950 to 1,000 millivolts. By setting resistor 26 at a given value, this means that a certain current can flow from base anode 22 to the base transistor 12. To prevent an abrupt drop in potential on cathode 4,
if a sudden change in electrolyte resistance occurs, a capacitor (not shown) could be used. As the positive ion current flow from anode It} continues, the potential on cathode 4 increases, thereby narrowing the potential difference between the cathode and anode 10. This action continues until the base-emitter junction voltage approaches zero. When this condition exists, only equilibrium current flows through transistor 12. When the potential on cathode 4 decreases, a base-emitter junction voltage develops immediately causing transistor 12 to open and permit additional current to again flow and re-initiate the process of increasing the potential on cathode 4 once more to the desired level.
As has been previously mentioned, resistor 26 enclosed in dashed lines in the accompanying drawing may be dis pensed with where a metal is used for base anode 22 which, as a practical matter does not produce a potential on cathode 4 substantially in excess of about 1,000 but above about 850 millivolts. Zince is typical of such metals that may be used as a substitute for magnesium in base anode 22 in which case resistor 26 is omitted from the circuit. With resistor 26 removed, an auxiliary conductor segment 36, shown in solid lines, can be employed. Zinc, for example, in a system using a saturated copper-copper sulfate reference electrode, produces a potential of about 1,100 millivolts. Owing to conductor and transistor junction resistances, however, the ultimate potential on the steel cathode is only about 950 to 1,000 millivolts which is an ideal range for maximum protection of the cathode without causing damage thereto characteristic of that resulting from higher voltages.
Magnesium anodes however, may be substituted for zinc anode(s), 10 and there are several economic advantages in doing so. For example, the ampere hours per pound of zinc is in the order of 350 while magnesium has a theoretical ampere hour per pound in the order of 1,000. For practical purposes, it has been recognized that onethird as many magnesium anodes as zinc anodes are required for the same life expectancy in protecting a given cathode under specific operating conditions. In my invention, I am able to substitute magnesium for zinc and am able to control the potential on the cathode so that the magnesium actually functions as zinc in regard to the potential it develops on the cathodes.
It will be recognized that cathode 4 may be any metal structure which it is desired to protect from corrosion such as, for example, an underground or submarine pipeline, ofi-shore radar towers, drilling platforms, etc., and, of course, the cathode may be of any metal lower in the electrochemical series than the anode employed.
Because an NPN transistor is comparable to an ordinary vacuum tube, the bias control can be obtained by connfcting the transistor base to the cathode using a resistance in the conductor between the base and the cathode to control the potential of the latter. Thus, it will be apparent that a vacuum tube, providing it is operable at sufiiciently low voltages, is considered an equivalent of the NPN transistor in the system herein described. In the present case the PNP transistor bias can be obtained by a conductor between the anode 10 and transistor base 16 with a resistance placed in the conductor to furnish the desired base to emitter voltage to control the cathode potential. In order to improve the response of the transistor to varying load, a capacitor can be installed across this resistance whereby an adequate voltage level is assured.
While I have described my invention as being applicable to cathodic protection systems employing sacrificial anodes, it will be apparent that direct current impressed cathodic protection systems can be similarly controlled by the use of transistors as taught herein. In general it may be said that my invention is intended to cover any cathodic protection system wherein the potential on the cathode is automatically held at a substantially predetermined level in an environment subject to change in electrical conductivity, by means of a transistor or its equivalent whereby the use of an external power source, such as has been formerly employed, can be avoided.
1. A cathodic protection apparatus comprising, a cathode, a first anode, a first conductor joining said cathode and first anode, a power transistor in said first conductor having its collector connected to said first anode and its emitter connected to said cathode, a second anode, and a second conductor joining the second anode to the base of said transistor, said first and second anodes being of metal higher in the electrochemical series than said cathode.
2. The apparatus of claim 1 in which the cathode is steel, the first anode is magnesium and the second anode is zinc.
3. The apparatus in claim 1 wherein a variable resistance is place in the second conductor.
4. The apparatus of claim 3 wherein a current sensing means is placed in the first conductor between said cathode and said transistor.
5. The apparatus of claim 3 wherein the first and second anodes are higher in the electrochemical series than zinc.
6. In an electrolytic system for cathodically protecting a metal structure immersed in an electrolyte, a first anode, a first conductor joining said structure and first anode, a power transistor in the first conductor having its collector connected to said first anode and its emitter connected to said structure, a second anode, and a second conductor joining the second anode to the base of said transistor, the first and second anodes being of metal higher in the electro-chemical series than said structure and being immersed in said electrolyte.
7. The electrical system of claim 6 wherein a variable resistance is placed in the second conductor whereby the current flow between the first anode and said structure can be held at a predetermined potential to inhibit dissolution of said structure.
8. The electrical system of claim 6 in which the second anode is zinc.
9. A cathodic protection system in which the flow of current to the cathode is regulated by a power transistor wherein the collector and emitter ofsaid transistor are connected to a first sacrificial anode and a cathode, respectively, the base of said transistor being connected to a second sacrificial anode.
10. In a cathodic protection system in which current flows from a sacrificial anode to the cathode, the improvement which comprises controlling said current flow by the use of a power transistor wherein the collector and emitter of said transistor are connected to a first sacrificial anode and a cathode, respectively, the base of said transistor being connected to a second sacrificial anode.
References Cited UNITED STATES PATENTS 2,912,635 11/1959 Moore 30788.5 3,061,773 10/1962 Ellison et al 204l96 3,108,055 10/1963 Grant 204196 3,135,677 6/1964 Fischer 204196 3,143,670 8/1964 Husock 307 3,146,182 8/1964 Sabins 204l97 3,242,064 3/1966 Byrne 204196 0 ROBERT K. MIHALEK, Primary Examiner.
JOHN H. MACK, Examiner.
T. TUNG, Assistant Examiner.