US 3055813 A
Description (OCR text may contain errors)
Sept. 25, 1962 E. scHAscHL ETAL 3,055,813
CURRENT CONTROLLER FOR USE IN CATHODIC PROTECTION OF STEEL STRUCTURES Filed Nov. 17, 1958 FIG. I
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INVENTORS 22 23 GLENN A. MARSH BY EDWARD SCHASCHL ATTORNEY Patented Sept. 25, 1962 3,4l55 813 QtIiNTidtipLLliii lFflR USE IN CATHODEC i i'lfiTE CTiiiN Ufa STRUCTURES Edward lichaschi and Gienn A. Marsh, (Crystal Latte, iii
assignors to The Pure Uil Qompany, Qhicago, 1533., a
corporation of @hio l iied Nov. 17, 1958, Ser. No. 774,366 6 (Ilaims. (til. Eddid?) This invention relates to a method and apparatus for use in cathodic protection of metal structures whereby means are provided to detect conditions of current fluctuation and to automatically adjust the current supplied to the system in accordance therewith.
Metallic structures, including steel pipe, oil tanks, engine mountings, oil-rig base-members, etc., develop a flow of current from anodic and cathodic portions when placed in an electrolytic atmosphere. The proximity of grounded power lines or electric railways may contribute to the current flow along the surfaces of the metallic structures. At those points or areas where the current leaves the structure, small quantities of metal from the structure pass into solution at the anodic portions. This results in pitting and corrosion of the metallic structure. One of the most etficient methods of counteracting electrolytic corrosion is to make the metal structure, which is usually buried in soil, cathodic instead of anodic by connecting it to a sacrificial metal anode composed of a more active metal, such as aluminum or magnesium, or any other metal more active in the electrochemical series, than the metal being protected. Another method is to connect an anode to a source of external DC. current (generator).
In order to protect buried or submerged metal structures against corrosion, the usual practice is to wrap or coat same with an impermeable material before installation. Such coatings offer less than complete protection because of unavoidable imperfections in the surface which expose isolated areas of the metal to contact with the corrosive environment. Cathodic protection in some measure compensates for the inadequacies of the coating method.
However, numerous problems arise with cathodic protection systems. It is difficult to determine the amount of cathodic protection required in a given (installation in order to avoid both underand over-protection. Another problem is that of determining the corrosion rate at the exposed areas so that reasonable, inexpensive and accurate cathodic protection systems can be installed. Methods and apparatus have been developed to determine the current density of an underground metal structure located in an electrolyte, such as wet soil or salt water, and con nected to a sacrificial anode buried adjacent thereto. Also, methods have been developed for checking cathodic protection of buried structures to obtain direct data without regard to the type of electrode being employed and without the necessity of interpolating the data in light of standard electrodes. However, these systems do not take into account the changes in the electrolytic system which influence the amount of current or current density needed for adequate protection of metal structures in contact with the electrolytic system. The instant invention is directed to a method and means for determining the changes in current density demand for adequate cathodic protection due to changes in the electrolytic system.
Thus, in a cathodic protection system, a supply of direct current is provided to make a steel structure cathodic to its environment. The current usually is maintained constant in the range of 15 milliamperes per square foot of exposed structural steel surface. However, the actual amount necessary to completely protect the structure varies from time to time, depending primarily on the amount of oxygen in the vicinity of the structural surface. For example, during a rain-storm, air-saturated water is carried to the structure, and there is made available a large supply of oxygen at the metal surface. The current demand can rise to as high as 30 Ina/sq. it. under such conditions.
The present invention is directed to a method and apparatus for detecting these temporary increases in current requirements, and for automatically increasing the current supply to the cathodic protection system for as long as it is needed to offset the increased corrosion rate.
Accordingly, it becomes a primary object of this invention to provide an apparatus for detecting temporary or permanent increases or decreases in current requirements for a cathodic protection system, and to vary the current supply to the system in accordance therewith for as long as necessary to oiiset the unbalance in the system.
Another object of the invention is to provide an apparatus including means for detecting increases in electrolytic action within an environment to actuate a relay and increase or decrease automatically the current density supplied to the system.
Another object of the invention is to provide such an apparatus including a metal couple, a relay, a source of current supply and means for varying said current supply in accordance with the signal received from said metal couple.
Other objects of the invention will become apparent as the description of the invention proceeds.
In the accompanying drawings there is clearly and fully illustrated one embodiment of this invention, in which:
FIGURE 1 is a schematic wiring diagram of an apparatus for establishing cathodic protection and controlling the current density of same to counteract changes in the electrolytic properties of the environment, and
FIGURE 2 is a modified circuit therefor.
Broadly, this invention is based on the discovery of an improved apparatus and method for controlling the current density on a metal structure located in an electrolytic environment, such as wet soil and salt water. The apparatus is connected through an external source of DC. current to an anode, also in contact with the environment, to provide cathodic protection that is proportional or adjusted to the requirements of the system. In this apparains and method 'a metallic couple comprising spaced coupons of dissimilar metals is positioned in the environment in close proximately to the surface of the metallic structure, along with the inert anode of the external current source. The inert anode is connected in the usual manner through a source of electrical power to the metal structure. The couple is connected to a relay operating a switch within a bypass electrical line from said power source. The metallic couple detects changes in electrolytic conditions in said environment and actuates the relay, thus cutting in more power to the anodic system.
Referring to the drawing, one application of the method and apparatus is shown as applied to controlling the current density at area J. of pipe 2 with the earth 3 acting as the corrosive electrolytic environment. Pipe 2 is connected at area 1 by means of lead wire 4 to coil 5 which forms the secondary winding of power-supply transformer 6. Core 7 and primary coil 8, connected through leads 9 and 10 to AC. generator 11, complete the elements of the transformer 6 and its electrical power source. Primary coil 5 has intermediate tap 12 connected to switch contact 13. Lead 14 connects the other end of primary coil 5 to switch contact 15. Switch arm 16, pivotally mounted at junction 17, is connected by lead 18, through half-wave rectifier 19 to anode 20.
Also, buried within the earth 3 is couple 21 comprising spaced, opposed plates 22 and 23 consisting of dissimilar metal coupons. Couple 21 is located in the proximity of pipe 2 and anode 20, and is preferably located within the area immediately surrounding pipe 2 or in an area between or substantially between pipe 2 and anode 20. Plate 22 is connected to lead wire 24 to coil 25 of relay 26. Lead wire 27 connects the other end of coil 25 to plate 23. The magnetic core of relay 26 is represented at 28.
The normal position of switch arm 16 is, as shown, connected to switch contact 13 and intermediate tap 12. The voltage between area 1 of pipe 2 and anode 20 is dependent on the resistance of earth 3 surrounding these elements. Transformer 6 is adapted to produce the required amount of current for cathodic protection of the system under these normal conditions. Couple 21, being of dissimilar metals, is sensitive to changes in electrolytic activity of the earth 3 between its plates 22 and 23. Thus, increases in moisture content, oxygen content, salt content, or pH are sensed by couple 21 and a polarity in plates 22 and 23 is established. This causes a current to flow through coil 25, actuating core 28 which moves switch arm l6 to the dotted-line position to connect with switch contact 15. As a result, the cutout portion 5' of coil 5 is placed in the secondary circuit and the curent density supplied to area 1 is increased to give the necessary cathodic protection under the new condition. F or example, an iron-aluminum couple, that is, one wherein plate 22 is made of iron and plate 23 is made of aluminum, is quite sensitive to the oxygen content of an electrolytic environment such as soil 3. When the dissolved oxygen content of the soil is substantially zero, the potential between the plates 22 and 23 is also zero and the potential across coil 25 is similar. As the oxygen content of the soil increases due to rain falling thereon and carrying dissoved'oxygen to plate 22, the potential increases to about 0.25 volt under static conditions. Higher voltages may be detected by such a couple Where dynamic conditions are present, as where there is agitation or trickling of water containing dissolved oxygen or salts between the plates of the couple. Relay 26 is adjusted to remain closed under dry, low-oxygen conditions in soil 3 so that switch-arm '16 is connected to contact 13, providing a low potential of protective current from power source 11. The system is adapted to detect any changes in electrolytic activity between plates 22 and 23 which causes current 13 to flow through relay 26, thereby moving lever-arm 16 from contact 13, putting the 'by-passed portion 5' of coil 5 in the circuit, and increasing the amount of cathodic protection current applied to pipe 2.
As thus far described, the apparatus employs a sensitive relay adapted to function on fractional voltages. Another expedient would be to use a sensitive relay to operate a power relay having its own switch and power source. Such an arrangement is shown in FIGURE 2 wherein couple 21 is connected as before to sensitive relay 26 adapted to operate switch-arm 30 to move same into electrical connection with contact 31 (dotted line position) to connect coil 32, through leads 33, 34 and 35, to battery 36 and actuate core 37, thereby moving switcharm 16 as before. Instead of using battery 36, leads 34 and 35 maybe connected to a portion of coil 5.
Sensitive relay 26 may be any of the many types of general-purpose, current-actuated, or voltage actuated, sensitive relays adapted for power-supply control. Sensitive low-energy relays, having a pull-in current of 1.0 ma., may be used. The rectifier 19 may be a generalpurpose, silicon diode, having a volt VMI 25 C. The power source 6 may be a transformer, having an output voltage of 6 volts. Inert anode 20 may be of carbon.
What is claimed is:
1. A method for controlling the current density on a metal structure located in an electrolytically corrosive environment and connected to an anode located in said electrolytically corrosive environment to provide cathodic protection comprising establishing a bi-metallic couple composed of spaced, dissimilar plates of metal in a circuit separate from said anode circuit, and exposing said couple to the electrolytically corrosive environment in close proximity to said metal structure and said anode, continuously detecting changes in electrolytic activity in said environment by means of said couple while said structure remains under cathodic protection, and applying said detected change in the form of a signal to means for increasing the cathodic current in response to increases in said electrolytic activity.
2. In combination with an underground metal structure, located in an electrolytically active environment and connected to an anode buried in said environment to provide cathodic protection therefor, an apparatus for controlling the current density on the metal structure comprising a metallic couple composed of spaced dissimilar plates of metal, a relay electrically connected between said plates, a power souce connected to said anode and said metal structure, and a by-pass in said power source opened and closed by said relay whereby changes in electrolytic activity between said couple controls the amount of current supplied from said power source.
3. In acathodic protection system the combination comprising a metal structure exposed to an electrolytically active environment, an anode within said environment and connected through a source of current to said metal structure, a switch means connected between said power source and said anode, a metallic couple in contact with said environment and adapted to detect changes in the electrolytic activity of said environment in the form of a low-voltage impulse, relay means connected to said couple and actuatable by said low-voltage impulse, said relay means operating said switch means to cut in more cathodic current from said power source as said electrolytic activity increases and less cathodic current as said electrolytic activity decreases.
4. A cathodic protective system in accordance with claim 3 in which said relay means comprises a sensing relay and a power relay.
5. In combination with an underground metal structure, located in an electrolytically active environment and connected to an anode buried in said environment to provide cathodic protection therefor, an apparatus for controlling the current density on the metal structure comprising a metallic couple composed of spaced dissimilar plates of metal, an electric power source connected to said anode and said metallic structure, and means responsive to the flow of current between said plates electrically connected therebetween for increasing the output of said power source in response to increase in current flow between said plates, whereby changes in electrolytic activity between said couple control the amount of current supplied from said power source.
6. In a cathodic protection system the combination comprising a metal structure exposed to an electrolytically active environment, an anode within said environment and connected through a source of current to said metal structure, a couple including two dissimilar-metal electrodes in contact with said environment and adapted to detect changes in the electrolytic activity of said environment in the form of a low-voltage potential between said 2,483,397 Bonner Oct. 4, 1949 electrodes, and means responsive to the magnitude of said 2,759,887 Miles Aug. 21, 1956 potential for increasing the output of said source of cur- 2,3 3,797 COWIeS Oct- 20, 1957 rent in proportion to increases in said potential. FOREIGN PATENTS 503,946 Great Britain July 9, 1937 References Cited In the file of this patent 669,675 Great Britain APR 9 1952 UNITED STATES PATENTS OTHER REFERENCES 1,910,021 Legg May 23, 1933 10 Logan: The Petroleum Engineer, Reference Annual, 2,221,997 Polin Nov. 19, 1940 1943, 168-180.