US3280020A

US3280020A - Anodic polarization system

Info

Publication number: US3280020A
Application number: US232323A
Authority: US
Inventors: Norman L Conger
Original assignee: Continental Oil Co
Current assignee: ConocoPhillips Co
Priority date: 1962-10-01
Filing date: 1962-10-01
Publication date: 1966-10-18
Anticipated expiration: 1983-10-18

Description

5 Sheets-Sheet 1 Filed Oct. 1, 1962 'ELEJ.

TllTrr-Z iNVENTOR. A/OPMAN L. GONG-5Q dill ,4 OQA/EY Oct. 18, 1966 N. L. CQNGER 3,280,020

ANOD'IC POLARIZATION SYSTEM Filed Oct. 1, 1962' 5 Sheets-Sheet 2 1 71 E-JA INVENTOR. IVOQMAN L GOA/65E Oct. 18, 1966 N. L. CONGER 3,280,020

ANODIC POLARIZATION SYSTEM Filed Oct. 1. 1962 5 Sheets-Sheet 3 t SYSTE'M 56 28a CONT. 3 I 24 /6 SYSTEM 56 70 co/vr. 5/ 24 "fisr-r INVENTOR. Nap/WAN Lo Co/vcii-e A GPA/E) United States Patent tinental @il Company, Ponca City, Okla, a corporation of Oklahoma Filed Oct. 1, 1962, Ser. No. 232,323 5 Claims. (Cl. 204196) This is a continuation-in-part of applicants co-pending application, Serial No. 858,094, filed December 8, 1959,

now abandoned, and entitled Saturable Reactor Switch for an Anodic Polarization System.

This invention relates to an improved apparatus for the anodic protection of metals against corrosion.

The present invention is an improvement in the apparatus disclosed and claimed in co-pending application, Serial No. 1,136, filed January 7, 1960, now US. Patent No. 3,208,925, and entitled Anodic Protection Against Corrosion, which co-p'ending application is assigned to the same assignee. Application, Serial No. 1,136, discloses a novel system which provides a periodic application or" a control current to a vessel in such a manner that the corrosive eifects of a caustic or other corrosive solution contained in the vessel will be substantially eliminated. This prior system utilize an on and oii type of controller operating a switch in the control current circuit for interrupting the anodic polarization current each time the corrosion of the vessel reaches a predetermined minimum, that is, each time the vessel reaches a predetermined degree of passivity. Each time the corrosion of the vessel increases to a predetermined maximum, the switch is closed and anodic polarization current is again passed through the corrosive solution to re-establish the passivity of the vessel. This prior system is highly useful in minimizing corrosion in a storage tank or other vessel where there is little change in the corrosive solution or in the storage conditions, and particularly in systems where the required anodic current is low, such as to amps at 15 to 25 volts. However, under changing conditions, such as when the vessel i a process vesssel and the corrosive solution is frequently changed, the corrosion rate changes rapidly. Under these conditions, the on and off controller provides a rapid making and breaking of the switch in the anodic current circuit, and the switch is worn out and rendered ineffective in short order.

An important object of this invention is to effectively control corrosion of a vessel containing a corrosive solution, even though frequent changes occur in the corrosive solution or in the storage conditions.

Another object of this invention is to rapidly passivate a vessel containing a corrosive solution at any time the passivity is partially destroyed, without damage to the anodic protection system.

A further object of this invention is to provide a systern for protecting a vessel against corrosion by means of an anodic current wherein high currents can be passed from the vessel through a corrosive solution contained in the vessel during the times that passivation is being established.

A further object of this invention is to delay the opening and closing of the anodic current circuit of an anodic protection system to provide rapid passivation of a vessel being controlled by the system each time passivation of the vessel is interrupted.

A still further object of this invention is to provide an anodic protection system which is simple in construction, has a long service life and which requires a minimum of repair.

The present invention contemplates a novel system for anodically passivating a vessel containing an electrolytic corrosive solution wherein the vessel is subject to frequent changes in corrosion rate, which system features a saturable core reactor having its A.C. winding interposed in the anodic current circuit and having its control winding controlled by a suitable switch means connected in series with a DC. source having a much smaller power output than the power supplied through the anodic control circuit. The switch means is operated by a simple on and oil controller which monitors the corrosion rate of the vessel to close and open the switch means in response to predetermined maximum and minimum corrosion rates. With this arrangement, the anodic current is not interrupted until after the vessel has become passivated, even though the controller rapidly opens and closes the switch means in the DC. control circuit in response to rapid changes in the potential measured or monitored by the controller. In this connection, it may be pointed out that the controller operates rapidly during the time that a high anodic current is being utilized, even though the vessel has not been passivated. The present system is particularly useful in systems requiring high anodic currents, such as amps at 25 volts.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with the accompanying drawings which illustrate the invention.

In the drawings:

FIGURE 1 is a schematic illustration of a previous anodic protection system, such as is disclosed in application, Serial No. 1,136, mentioned above.

FIGURE 1A is a schematic illustration of the anodic current provided by the system shown in FIG. 1 during a time in which the vessel is being passivated.

FIGURE 2 is a schematic illustration of the present anodic protection system.

FIGURE 2A is a schematic illustration of the anodic current provided by the system shown in FIG. 2 when the vessel is being passivated.

FIGURE 3 is a schematic illustration of another em bodiment of the present invention.

FIGURE 4 is a schematic illustration of still another embodiment of the present system.

Referring to the drawing in detail, and particularly FIG. 1, reference character 10 designates a vessel, such as a tank, containing an electrolytic corrosive solution 12 and which is to be protected against the corrosive action of the solution 12. The vessel 10 is constructed of metal normally mild or stainless steel in view of the required service. The solution 12 may be, for example, sulfuric, phosphoric, or nitric acid.

An inert electrode 14 is suspended or otherwise supported in the solution 12 and is connected to the nega tive terminal of a rectifier 16 by a conductor 18. The electrode 14 may be any material which is inert to the solution 12 and resists changes in potential as current is passed therethrough, that is, a material which preferably does not polarize. Preferable electrode materials are platinum or carbon. The positive output terminal of the rectifier 16 is connected by a conductor 20 to the vessel 10. It will thus be seen that the vessel funcions as an anode and the electrode 14 functions as a cathode.

A reference electrode 26 is placed in electrical communication with the solution 12, as by being suspended in the solution 12. The reference electrode 26 is made of a material which is located in the table at a lower or more noble position than the material of the vessel 10, and may be, for example, a calomel electrode when the vessel 19 is stainless steel. An on and off controller 24 is connected across the vessel 1% and the reference electrode 26

conductors

21 and 19 to monitor the potential between the vessel 10 and electrode 26, for pur poses to be described. The input impedance of the controller 24 should be at least ten times as great as the internal impedance of the reference electrode 26 in order to prevent an erroneous indication of the vessel potential, since the controller 24 draws current from the reference electrode 26. -In an anodic protection system, the reference electrode has a high internal impedance, and in order for the controller 24 to accurately measure the potential between the vessel It) and the reference electrode 26, the controller 24 must have a correspondingly high input impedance, preferably ten to one hundred times the internal impedance of the reference electrode 26. The controller 24 is mechanically connected to a switch 28 interposed in the power supply circuit extending from an A.C. power supply 36 to the input of the rectifier 16.

As is known in the art, the rate of corrosion of the type being considered varies with the potential of the vessel being corroded, and the corrosion may be prevented, or at least minimized, by anodic polarization. The potential of the vessel is determined with respect to the reference electrode 26. When the vessel is made the anode of an electrochemical cell, as shown in FIG. 1, the potential of the vessel shifts in a more noble direction. When this shift is of suflicent magnitude, the corrosion stops and it is said that the vessel has become passive.

In operation of the system shown in FIG. 1, the controller 24 monitors the potential between the vessel It and reference electrode '26 to open and close the switch 28. The switch 2d is closed when the corrosion rate of the vessel 10 reaches a predetermined maximum (as indicated by the potential between the vessel 10 and electrode 26) to establish the anodic current from the power supply 36 and rectifier 16. The flow of the anodic current from the vessel It) to the electrode 14 pacifies the vessel It) and minimizes the corrosion rate. When the corrosion rate reaches a predetermined minimum (as de termined by the potential between the vessel 10 and the electrode 26), the controller 2 opens the switch 28 and interrupts the anodic current. The passivity of the vessel 10 gradually decreases until the corrosion rate again reaches the predetermined maximum; whereupon the anodic current is again turned on to maintain the passivity of the vessel 10 between predetermined maximum and minimum levels.

As previously indicated, the controlled 24, in combination with the switch 28, works very eiTectively when the vessel 10 has once been pacified and when the solution 12 and other storage conditions do not vary to any appreciable degree. However, if the passivity of the vessel 10 is interrupted (as by a change in the solution 12), the potential between the vessel lit) and reference electrode 26 varies rapidly while passivity is again being established. As a result, the controller 24 rapidly opens and closes the switch 28 to apply and interrupt the anodic current. Under these conditions, the anodic current will appear as illustrated in FIG. 1A.

As shown in FIG. 1A, the anodic current is in the form of a square wave varying from zero to a predetermined level (such as 70 amps) depending upon the power output of the AC. power supply 36. When passivity is being established, as illustrated by the left-hand portion of the wave shape in FIG. 1A, the anodic current varies rapidly by the rapid opening and closing of the switch 23. For example, the switch 28 may be opened and closed ten times per second. As passivity is established, which may require several hours (as long as five or six hours with a mild steel vessel containing 99 percent sulfuric acid), the anodic current is turned on and off at a decreasing frequency. The rapid opening and closing of the switch 28 during the establishment of passivity quickly wears out the switch and eventually renders the system ineffective to prevent corrosion of the vessel 16.

In accordance with the present invention, and as shown in FIG. 2, the anodic current is controlled directly by a saturable core reactor 30 controlling the AC. supplied to the rectifier 15, rather than the use of a mechanical switch as described above. The reactor 3% has a core 4%) on which are wound a pair of AC.

windings

31 and 32. The

windings

31 and 32 are connected in series with one another and in series with the AC. power supply 36 and one input of the rectifier 16 by a conductor 34. A mechanical switch 28a is interposed in the conductor 39 and is connected to the controller 24 for operation in the same manner as the switch 28 previously described in connection with FIG. :1. The remaining portion of the system shown in FIG. 2 is the same as that shown in P16. 1.

In operation of the system shown in FIG. 2, the controller 24 closes the switch 28a when the potential between the vessel it) and the reference electrode 26 reaches a predetermined maximum, indicating a predetermined maximum rate of corrosion. Closure of the switch 28a causes current to flow from the DC. power supply 41 through the control winding 37 of the reactor 30 to saturate the core 40; whereupon the AC. from the power supply 36 flows through the conductor 34 and the AC.

windings

31 and 32 pass the anodic current from the vessel it) to the electrode 14. When the predetermined degree of passivity is established, as indicated by the potential between the vessel 10 and the reference electrode 26, the controller 24 opens the switch 28a and interrupts the DC. supplied to the control winding 37 of the reactor 30; whereupon the A.C. power supply is isolated from the rectifier 16 and the anodic current is turned 0E.

As previously indicated, the system shown in FIG. 2 is particularly useful when passivity of the vessel 10 is frequently interrupted, such as by a change in the solution 12. Under these circumstances, and as described above, the controller 24 operates rapidly until passivity of the vessel is established. The anodic current produced by the system shown in FIG. 2 is illustrated in FIG. 2A as it occurs during the establishment of passivity. As will be observed, the anodic current increases rapidly when the switch 28a is first closed, but not instantaneously as shown in FIG. 1A. The inherent time delay required to saturate the core itl upon imposition of the DC. from the power supply 4-1 provides a short time delay in the buildup of the anodic current. Likewise, an inherent time delay is required in the unsaturation of the core 49 upon opening of the switch 28a. As a result, the anodic current will fluctuate between a predetermined maximum and a predetermined minimum which is above zero upon rapid opening and closure of the switch 28a, as when passivity is being established for the vessel 19. As passivity of the vessel is established, the anodic current more nearly approaches a square wave, with the troughs of the wave being at zero.

It will thus be seen that the fluctuation of the anodic current above zero (provided by the system shown in FIG. 2) will provide a higher average amplitude for the anodic current during the initial phases of the establishment of passivity to more quickly pacify the vessel, compared with the use of a mechanical switch in the anodic current network as shown in FIG. 1. Furthermore, the saturable reactor 36 has no moving parts to become worn or cause arcing. Thus, the system shown in FIG. 2 is particularly useful for preventing corrosion of the vessel when the vessel is subjected to varying storage conditions, such as frequent changing of the solution 12. Each time passivity of the vessel 10 is interrupted, it will be quickly re-established to minimize the overall corrosion of the vessel.

The embodiment shown in FIG. 3 is the same as that shown in FIG. 2 except that the A.C. winding 31 of the saturable reactor 30 is connected directly across conductors 34 and 35, and the A.C. winding 32 is connected directly to the input of the rectifier 16. In this embodiment, when switch 28a is closed, the core 40 will likewise saturate, however, since the energy from winding 31 must be transferred to winding 32 through core 40', as core 40 is saturated, little energy will be transferred thereto. When switch 28a is opened, core 40 provides an effective transfer medium for energy between the Wind ings 3-1 and 32. It is to be understood that

windings

31 and 32 must have sufficient impedance so they will not be damaged by the voltage from the power supply 36 when core 40 is saturated.

The embodiment shown in FIG. 4 is also the same as that shown in FIG. 2, with the exception of the switch interposed in the conductor 39 extending from the D.C. power supply 41 to the control winding 37 of the reactor. In the embodiment shown in FIG. 4, the switching means comprises a transistor 50 having its emitter and collector in series with the conductor 39 and having its base 51 connected to the controller 24. It will be understood that the controller 24 must develop suitable bias for the base 51 of the transistor 50, such that the transistor will be driven between saturation and cut off to provide a switching action. It is, of course, understood by those skilled in the art that operating conditions for transistor 50 other than those previously stated may be employed if the efliciency of transistor 50 may be sacrificed.

From the foregoing, it will be apparent that the present invention provides a novel anodic protection system which is particularly useful for controlling the corrosion of a vessel or the like which is subject to frequent changes in corrosion rate. Each time passivity of the vessel is interrupted, a high anodic current is utilized to quickly reestablish passivity in a minimum of time and with a minimum of corrosion of the vessel. The only mechanical switching element which may be utilized in this system is utilized to control a negligible D.C. current rather than the higher anodic current which is actually imposed to control corrosion. It will also be apparent that the present system is simple in construction, may be economically manufactured and will have a long and trouble-free service life.

Changes may be made in the combination and arrangement of parts or elements as heretofore set forth in the specification and shown in the drawings, it being understood that changes may be made in the embodiments disclosed without departing from the spirit and scope of the invention as defined in the following claims.

What is claimed is:

1. A system for anodically passivating a vessel containing an electrolytic corrosive solution and subject to frequent changes in corrosion rate, comprising:

an electrode immersed in the solution;

a rectifier having its negative output terminal connected to the electrode and its positive output terminal connected to the vessel to make the electrode a cathode and the vessel an anode;

an A.C. source;

a saturable core reactor having an A.C. winding connected in series with the A.C. source and the rectifier and having a control winding;

a reference electrode electrically communicating with the solution;

an on and off type controller connected across the vessel and the reference electrode to monitor the potential between the reference electrode and the vessel, said controller having an input impedance substantially greater than the internal impedance of the reference electrode;

a D.C. source connected in series with the control winding of the reactor; and

switch means operably connected to the controller and interposed in the connection of the D.C. source to the control winding of the reactor to alternately energize and de-energize the reactor in response to low and high potentials between the reference electrode and the vessel.

2. A system as defined in claim 1 wherein the switch means comprises a switch mechanically connected and operated by the controller.

3. A system as defined in claim 1 wherein the switch means comprises a transistor having its emitter and collector in series with the D.C. source and the control winding of the reactor and having its base electrically connected to and operated by the controller.

4. A system for anodically passivating a vessel containing an electrolytic corrosive solution and subject to frequent changes in corrosion rate, comprising:

an electrode immersed in the solution;

an A.C. source;

a saturable core reactor having a pair of A.C. windings connected in series with each other and with the A.C. source and the rectifier, said reactor also having a control winding;

a reference electrode electrically communicating with the solution;

an on and 011? type controller connected across the vessel and the reference electrode to monitor the potential between the reference electrode and the vessel, said controller having an input impedance substantially greater than the internal impedance of the reference electrode;

a D.C. source connected in series with the control winding of the reactor; and

5. A system for anodically passivating a vessel containing an electrolytic corrosive solution and subject to frequent changes in corrosion rate, comprising:

an electrode immersed in the solution;

an A.C. source;

a saturable core reactor having a pair of A.C. windings, one of said windings connected in series with the A.C. source and the other connected in series with the rectifier, said reactor also having a control winding;

a reference electrode electrically communicating with the solution;

a D.C. source connected in series with the control winding of the reactor; and

switch means operably connected to the controller and interposed in the connection of the D.C. source to the control Winding of the reactor to alternately en- OTHER REFERENCES ergize and de-energize the reactor in response to 10W Power Engineering, vol. 1 1 January 1957 and high potentials between the reference electrode page 65 and the vessel. 5 Crow: saturating Core Devices, 1949, pp. 61 and 273. References Cited by the Examiner UNITED STATES PATENTS JOHN H. MACK, Przmary Exammer.

04,905 10 1961 Sabins 204--196 MURRAY TILLMAN Exammer- 3,009,865 11/1961 Mueller et a1. 204147 T. TUNG, Assistant Examiner.

Claims

1. A SYSTEM FOR ANODICALLY PASSIVATING A VESSEL CONTAINING AN ELECTROLYTIC CORROSIVE SOLUTION AND SUBJECT TO FREQUENT CHANGES IN CORROSION RATE, COMPRISING: AN ELECTRODE IMMERSED IN THE SOLUTION; A RECTIFIER HAVING ITS NEGATIVE OUTPUT TERMINAL CONNECTED TO THE ELECTRODE AND ITS POSITIVE OUTPUT TERMINAL CONNECTED TO THE VESSEL TO AMKE THE ELECTRODE A CATHODE AND THE VESSEL AN ANODE; AN A.C. SOURCE; A SATURABLE CORE REACTOR HAVING AN A.C. WINDING CONNECTED IN SERIES WITH THE A.C. SOURCE AND THE RECTIFIER AND HAVING A CONTROL WINDING; A REFERENCE ELECTRODE ELECTRICALLY COMMUNICATING WITH THE SOLUTION; AN "ON" AND "OFF" TYPE CONTROLLER ACROSS THE VESSEL AND THE REFERENCE ELECTRODE TO MONITOR THE POTENTIAL BETWEEN THE REFERENCE ELECTRODE AND THE VESSEL, SAID CONTROLLER HAVING AN INPUT IMPENDANCE SUBSTANTIALLY GREATER THAN THE INTERNAL IMPEDANCE OF THE REFERENCE ELECTRODE; A D.C. SOURCE CONNECTED IN SERIES WITH THE CONTROL WINDING OF THE REACTOR; AND