EP0018124A1

EP0018124A1 - Anodically passivated vessel and method of passivating it

Info

Publication number: EP0018124A1
Application number: EP80301017A
Authority: EP
Inventors: Lawrence Louis Masters; Richard King Teague
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1979-04-02
Filing date: 1980-04-01
Publication date: 1980-10-29
Also published as: AU5702880A; JPS55134179A

Abstract

An easily adjustable method for anodically passivating a vessel (10) containing a corrosive liquid is provided. A first electrode (26) and a reference electrode (32) are secured within the vessel (10) and a second electrode (34) is secured within a nozzle. Each electrode communicates with a corrosive liquid and each extends to the exterior of the vessel. A control means (50) has a first (52) and second input (54), connected to the vessel (10) and the reference electrode (32), a direct current output (56), connected to the first and second electrodes, and an internal setpoint. If the potential measured between the first and second inputs varies from that of the internal setpoint, the control means (50) varies the direct current output (56) supplied to the first (26) and second electrodes (34). A variable resistance means (70) is provided between the output of the control means (50) and the second electrode (34). The variable resistance means (70) may be adjusted to vary the output from the control means to the second electrode (34) so that approximately the same vessel potential may be obtained at the first (26) and second electrodes (34), that is, so that protection will be uniform throughout the vessel being passivated. Efficient passivated vessels are provided.

Description

Background Of The Invention

This invention relates generally to an improvement in anodic passivation to minimize corrosion of metallic vessels containing corrosive liquids such as sulfuric acid, phosphoric acid, or solutions of acids.
Corrosion of a metallic object may be minimized by making it the anode in an electrical circuit. If the potential difference between the metallic object and a relatively inert cathode is maintained within a certain range, the metal becomes passive and corrosion is controlled.
The prior art discloses numerous sytems for anodically passivating a vessel containing a corrosive solution to control corrosion of the vessel. U. S. Patent 3,442,779 discloses the use of a pulsed direct current between the vessel wall and an inert electrode within the vessel to maintain the metal in a passive state. The pulsed direct current is controlled in response to the measurement of potential between the vessel wall and a reference electrode. The use pf pulses, or on-off controls, requires the use of switches which often fail after prolonged usage. Another system, shown in U. S. Patent 3,127,337, controls the direct current for anodic passivation of a vessel by regulating an alternating current supply prior to rectification.
It has been common to install the electrodes near the center of the vessel; however, since nozzles extend outwardly from the vessel wall, nozzles received less protection than the remainder of the vessel. Thus second electrodes have been installed within the nozzles. Installation of the second electrode created hot spots, area of accelerated corrosion caused by too great a potential between the vessel and the electrode. Elimination of the hot spots until now has been accomplished by changing the physical position of the second electrode. The vessel would be placed in operation and measurements of potentials made; then the vessel would be removed from operation so that the second electrode could either be inserted farther or partijtally removed from the vessel. This procedure could be repeated several times upon start up of the vessel or whenever a major change in operating conditions occurred.

Summary Of The Invention

An object of this invention is to provide an efficient passivated vessel for corrosive liquids by anodic passivation.
Another object of this invention is to provide a simplified method for adjusting the anodic current through the corrosive liquid between the walls of the vessel and the electrodes supported in the liquid while the vessel is in operation.
To achieve these as well as other objects we provide a vessel, having an interior and exterior surface and a longitudinal axis, which contains a corrosive liquid, for example sulfuric acid, phosphoric acid, or a mixture of acids. The vessel has a plurality of apertures or nozzles which serve as entryways for electrodes and for the flow of fluid into and out of the vessel. A first electrode is secured along the longitudinal axis within the vessel, in communication with the corrosive liquid, and extends to the exterior of the vessel through one of the plurality of nozzles. A reference electrode extends into the vessel through a coupling and it, like the first electrode, is in communication with the corrosive liquid. A second electrode is secured within a second nozzle and extends to the exterior of the nozzle. The second electrode communicates with the corrosive liquid which flows through the nozzle. A variable resistance means is connected to the second electrode at the exterior of the second nozzle. Control means having a first and second input and a direct current output is utilized to compare a measured potential with an internal setpoint potential. The reference electrode is connected to the first input and the vessel is connected to the second input. The direct current output is connected to the first electrode and, through the variable resistance means, to the second electrode. A variation in the measured potential between the first and second inputs, that is a variation in the measured potential between the reference electrode and the vessel, from the internal setpoint potential results in a corresponding change by the control means in the direct current output to the first and second electrodes. The variable resistance means may be adjusted to further vary the direct current to the second electrode to thereby provide a uniform potential throughout the interior of the vessel and the nozzle. Use of the variable resistance means provides a simplified method for adjusting the anodic current supplied to the second electrode and eliminates the necessity of removing the vessel from operation, as required in the past, so that the physical position of the second electrode could be varied.

Brief Description Of The Drawings

Attention is now directed to the drawings, in which:

FIGURE 1 is an elevation view of a typical vessel showing the location of the electrodes of this invention;
FIGURE 2 is a plan view of the second electrode installed within a spool piece.
FIGURE 3 is a plan view of an alternate embodiment of the second electrode;
FIGURE 4 is a perspective view of another alternate embodiment of the second electrode; and
FIGURE 5 shows the electrical circuit of this invention in cooperation with the vessel of FIGURE 1.

Detailed Description Of The Preferred Embodiment

Referring now to the drawings, wherein like referenced characters designate like or corresponding parts throughout the several views, a vessel is shown in FIGURE 1. The vessel 10 in FIGURE I is shown as an acid cooler, that is, as a heat exchanger for corrosive liquids, for example sulfuric acid, phosphoric acid, or solutions of acids. Nozzles, or apertures, 12 and 14 are provided for water flow into and out of the vessel 10. Nozzles,or apertures, 16 and 18 are provided for the flow of the corrosive liquids into and out of the vessel 10. Tubes within the vessel 10 which separate the water and the corrosive liquid are not shown but the vessel 10 is shown as containing a corrosive liquid 11. Each of the nozzles 12, 14, 16 and 18 are connected to a piping system; however, only the ends of the pipe attached to the nozzles are shown in FIGURE 1.
Couplings, or apertures, 20 and 22 are provided on vessel 10 for insertion of a reference electrode 32, shown at coupling 20, into the vessel 10. Nozzle 24 is provided so that a first electrode 26 may be inserted within the vessel 10. First electrode 26 is inserted through nozzle 24 and secured within the interior of the vessel. Electrode 26 extends parallel to the longitudinal axis of the vessel 10, that is, it is secured parallel to the length of the vessel 10. A spool piece 28 and 30 is shown connected to each of the nozzles 16 and 18 between the nozzles and the piping system to provide a housing for a second electrode 34. Though a spool piece 30 is used here to house the second electrode 34, the spool piece 30 is not essential as the electrode may easily be mounted within the nozzle 16. Similarly spool piece 28 would not be required if another second electrode 34 were installed on vessel 10 as the electrode could be mounted within nozzle 18.
Reference electrode 32 may be of any suitable type, such as a calomel cell, a silver-silver chloride cell, a hydrogen cell, or others known in the art. The reference electrode 32 is connected to the corrosive liquid 11 by an internal electrolytic bridge. The reference electrode 32 is inserted into the vessel 10 through coupling 20 and is positioned near the inner surface of the vessel. The vessel 10 is grounded, thus the reference electrode 32 must be electrically isolated from the vessel. A second reference electrode may be inserted through coupling 22. If more than one reference electrode is used,one may be used for measurement and control while the others are used to monitor the condition of the vessel.
The first electrode 26 which extends into the vessel 10 in the axial direction near the center of the vessel is also of a standard configuration, that is, it is formed of a long cylindrical metallic rod of a metal which is relatively inert to the corrosive liquid 11. The length of the first electrode 26 is sized to enable the electrode to extend from the exterior of vessel 10 through substantially the length of the portion of vessel 10 which contains corrosive liquid 11. Multiple parallel electrodes may be used rather than a single electrode. In this event each electrode would be inserted into the vessel 10 from the same end of the vessel. A nozzle 24 would be required for each electrode, each nozzle being offset radially from the center of the vessel such that each electrode would be on the same diametrical chord of the vessel as each of the other electrodes. The first electrode 26 extends to the exterior of vessel 10 where it is secured and supported by an assembly 31. The assembly 31 has an internal configuration (not shown) which provides physical support to the first electrode 26, provides a fluid tight seal to prevent leakage of corrosive liquid 11 through the nozzle 24 to the exterior of vessel 10, and provides electrical isolation between the first electrode 26 and the vessel 10. The assembly 31 additionally provides a location for an electrical connection to the first electrode 26.
The second electrode 34 is usually installed within a nozzle near the end of the vessel opposite the entry nozzle 24 of the first electrode 26; however, two second electrodes 34, one at each end of vessel 10, may be used. The D.C. electrical potential supplied to first electrode 26 is reduced by resistance losses along the length of the electrode. Thus the potential at the end of first electrode 26 farthest into vessel 10 will be less than the potential at the end nearest the entry nozzle 24. Placement of the second electrode 34 near the area of reduced potential and proper adjustment of the D.C. potential supplied to the second electrode 34 will provide a substantially uniform potential throughout the vessel 10.
Turning now to FIGURE 2, a second electrode 34 is shown installed in spool piece 30. The second electrode 34 is a metallic rod formed of a metal relatively inert to the corrosive liquid 11 which extends substantially across the diameter of spool piece 30 and extends to the exterior of the spool piece. The inner end 35 of second electrode 34, extending across the diameter of the spool piece 30, is spaced about 7.6 centimeters from the wall of spool piece 30 to prevent the formation of a hot spot, that is, an excessive amount of anodic current flowing between the second electrode 34 and a particular point on spool piece 30. The outer end 36 of second electrode 34 extends to the exterior of spool piece 30 where it is secured within assembly 37. A sheath 38 of electrically non-conductive material surrounds the outer end 36 of second electrode 34 and extends about 7.6 centimeters into spool piece 30 to prevent the formation of hot spots along the outer end 36. Assembly 37 provides physical support for the second electrode 34, provides a fluid tight seal to prevent leakage of the corrosive liquid 11 to the exterior of spool piece 30, and provides electrical isolation between the second electrode 34 and spool piece 30. The internal components of assembly 37 are not shown but many configurations are known which will provide the functions required of assembly 37.
Turning now to FIGURE 3, an alternate embodiment of the second electrode 34 is shown. The inner end 35' of second electrode 34' is shown in the shape of an annular ring. The ring shaped inner end 35' of second electrode 34' is sized such that when it is inserted within spool piece 30 the annular ring is spaced about 7.6 centimeters from the inner surface of the spool piece. An advantage of this embodiment is that the annular ring of second electrode 34' has a greater surface area than the straight..rod shape of the second electrode 34 shown in FIGURE 2. With the electrode having a greater surface area, a lower D.C. potential is required at the second electrode 34' to provide the same current density between the annular ring shaped second electrode 34' and the vessel 10 as between the straight rod shaped second electrode 34 and the vessel 10.
In FIGURE 4 another alternate embodiment of the second electrode 34 is shown. The second electrode 34" is shown with the inner end 35" having a plurality of fins 39. The inner end 35" of second electrode 34" is approximately 1.6 centimeters in diameter and the fins 39 are approximately 3.5 centimeters in diameter and 0.64 centimeters in thickness. The fins 39, like the annular ring 35' of second electrode 34', provide a greater surface area on the inner end 35" of second electrode 34" than the surface area of the inner end 35 of second electrode 34. The greater surface area allows the use of a lower D.C. potential to provide approximately the same current density between the second electrode 34" and the vessel 10. This alternate embodiment may eliminate the possible need for more than one second electrode within a nozzle.
The electrical circuitry of the invention is shown in FIGURE 5. A control means 50 which has a first input 52, a second input 54, and a direct current output 56 is shown. Vessel 10 is shown with first electrode 26 installed within the vessel through nozzle 24, reference electrode 32 in- talled through coupling 20 and second electrode 34 installed within spool piece 30. Vessel 10, which is grounded, is connected to the second input 54 of control means 50 by electrical cable 60. The reference electrode 32 is connected to the first input 52 of control means 50 by electrical cable 62. The direct current output 56 of control means 50 is connected to the first electrode 26 by electrical cable 64. The direct current output 56 of control means 50 is also connected to a first terminal 71 of a variable resistance means 70. A second terminal 72 of a variable resistance means 70 is connected to the second electrode 34 by electrical cable 66. When two second electrodes 34 are used, one in each of the spool pieces 30 and 28, a second variable resistance means 70 is connected between the direct current output 56 of the control means 50 and the second of the second electrodes 34. In operation, the control means 50 measures the difference in the electrical potential between its first and second inputs, that is, it measures the potential difference between the reference electrode 32 and the vessel 10 which is grounded. This potential difference is a measurement of the passivity of the vessel 10. Control means 50 compares the measured potential difference with an internal setpoint. If the measured potential difference varies from the internal setpoint, the control means 50 will change the D.C. current and potential supplied at the output 56, thus changing the D.C. current and potential supplied to first electrode 26 and second electrode 34.
When the vessel 10 is placed in service, it is important that the potential throughout the vessel from the first electrode 26 and the second electrode 34 be substantially uniform to provide uniform protection throughout the vessel. To make the potential uniform, the anodic current between the second electrode 34 and the vessel is adjusted. In the past, this adjustment has been made by physically moving the second electrode 34 so that greater or lesser lengths of the second electrode 34 are exposed to the corrosive liquid 11 flowing through the spool piece 30 into the vessel 10. Therefore in the past, the vessel 10 has been removed from service and the second electrode has been inserted farther or partially removed from the spool piece 30. The adjustment of the localized anodic current has been greatly simplified by the use of the electrical circuit, control means 50 and variable resistance means 70 of this invention, because the anodic current from the second electrode 34 may be adjusted without removing the vessel 10 from service. A second reference electrode 32' is inserted within the spool piece 30 through coupling 22 for use as a monitor when the vessel 10 is placed in service or when large changes occur in the process conditions. The potential measured by one of the reference electrodes 32 or 32' may vary from the potential measured by the other reference electrode which indicates a non-uniform vessel protection, that is a non-uniform potential at the vessel near the first electrode 26 and the second electrode 34. The anodic current supplied by the second electrode 34 must then be varied until the potential measured by the two reference electrodes 32 and 32' is in balance, that is, until the potential measured by one of the reference electrodes is substantially equal to the potential measured by the other of the reference electrodes. The anodic current supplied by the second electrode 34 of this invention may easily be varied by changing the resistance of the variable resistance means 70. Thus this anodic passivation system may be adjusted to provide a uniform potential throughout the vessel 10, and its nozzles, by comparing the potential measured by the reference electrodes 32 and 32' and adjusting the variable resistance means 70 until the potential measured by each reference electrode is substantially equal. It is not required that the vessel 10 be removed from service, nor is physical movement of the second electrode 34 required. A uniform potential throughout the vessel 10 and its nozzle is required to provide equal protection from corrosion for the entire vessel. Therefore, through the use of the anodic passivation system of this invention, an easily adjustable anodic passivation system is provided which assures uniform protection throughout the vessel being passivated.
Changes may be made in the combination and arrangement of parts or elements without departing from the spirit and scope of this invention. It is therefore to be understood that the present embodiment is to be considered as illustrative only and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description.

Claims

1. An anodically passivated vessel for a corrosive liquid characterized by:

a vessel having an interior and an exterior surface and a longitudinal axis and having a plurality of apertures, said apertures serving as entryways for electrodes and for fluid flow into and out of said vessel;

a first electrode secured in axial alignment within said vessel for communication with corrosive liquid, said first electrode extending to the exterior of said vessel through one of said apertures;

a reference electrode extending into said vessel through another of said apertures for communication with corrosive liquid;

a second electrode secured within a third of said apertures and extending to the exterior thereof for communicating with corrosive liquid flowing through said aperture;

variable resistance means connected to said second electrode at the exterior of said aperture; and

control means having a first input connected to said reference electrode, a second input connected to said vessel, means for connecting a direct current output to said first electrode and, through said variable resistance means, to said second electrode, and an internal setpoint whereby a variation in the potential between said first and second inputs from said internal setpoint results in a corresponding change in the direct current output from said control means to said first and second electrodes and said variable resistance means adjustable to further vary the direct current output to said second electrode to provide uniform protection by passivation throughout said interior of said vessel and said apertures.

2. The anodically passivated vessel of Claim 1 wherein said first and second electrodes comprise cylindrical metallic rods of a metal which is relatively inert to corrosive liquid.

3. The anodically passivated vessel of Claim 1 wherein said second electrode has an inner end and an outer end, said inner end being formed in the shape of an annular ring.

4. The anodically passivated vessel of Claim 1 wherein said second electrode has an inner end and an outer end, said inner end having the shape of a cylindrical rod with a plurality of fins.

5. In the method of anodically passivating a vessel having an interior surface containing a corrosive liquid, a first electrode immersed in said corrosive liquid within said vessel, a second electrode within a nozzle on said vessel and in contact with the corrosive liquid flowing through said nozzle, a reference electrode immersed in said corrosive liquid within said vessel, and a control means having first input connected to said reference electrode and second input connected to said vessel and a direct current output connected to said vessel and a direct current output connected to said first and second electrodes to supply a D.C. potential and current and an internal setpoint, by

measuring the potential between said reference electrode and said vessel,

comparing said measured potential to said internal setpoint of said control means,

varying the D.C. potential and current from said direct current output of said control means to said first and second electrodes in response to the difference between said measured potential and said internal setpoint, the improvement characterized by:

varying the D.C. potential and current supplied to said second electrode from said direct current output of said control means by varying the resistance of a variable resistance means connected between said direct current output and said second electrode to provide a uniform potential between said first and second electrodes and said interior surface of said vessel.