US3425921A

US3425921A - Methods and systems for protecting metal structures

Info

Publication number: US3425921A
Application number: US539961A
Authority: US
Inventors: Leon P Sudrabin
Original assignee: Wallace and Tiernan Inc
Current assignee: US Filter Wallace and Tiernan Inc
Priority date: 1966-04-04
Filing date: 1966-04-04
Publication date: 1969-02-04
Anticipated expiration: 1986-02-04
Also published as: GB1184484A

Description

Feb. 4, 1969 P. SUDRABIN METHODS AND SYSTEMS FOR PROTECTING METAL STRUCTURES Filed April 4. 1966 Sheet 0f 5 i E INVENTOR T v Zw/v Pfiwmaw RM 5. OW

Aime/v Feb. 4, 1969 L. P. SUDRABIN METHODS AND SYSTEMS FOR PROTECTING METAL STRUCTURES Filed April 4, 1966 Sheet 2 Of?) INVENTOR Z .EO/V P Sup/mam Feb. 4, 1969 L. P. SUDRABIN 3,425,921

METHODS AND SYSTEMS FOR PROTECTING METAL STRUCTURES 1 Filed April 4. 1966 sheet 3 or:

540E fb/A/T 55EC7E17 Para/n44} /eirwe 1 M E CONTROL O Cueesn/r DENS/ ry 7 INVENTOR [56W Sup/mew BY xmsom United States Patent 3,425,921 METHODS AND SYSTEMS FOR PROTECTING METAL STRUCTURES Leon P. Sudrabin, Berkeley Heights, NJ., assignor, by mesne assignments, to Wallace & Tierman Inc., East Orange, N.J., a corporation of Delaware Filed Apr. 4, 1966, Ser. No. 539,961 U.S. Cl. 204-147 12 Claims Int. Cl. B01k 3/00 This invention relates to corrosion-preventing systems and methods, for protecting metal structures immersed in corroding electrolytes, wherein a direct electric current is passed through the electrolyte between the metal structure to be protected and auxiliary eletcrode means, as for cathodic protection or anodic passivation of the structure. In a more particular sense, the invention concerns systems and methods of this type involving continuous regulation of the applied current in response to variations in the potential of the protected structure, for controlling the structure potential. Specifically, the invention is directed to such systems and methods wherein variations in structure potential are sensed in a manner independent of variations in applied current and/or electrolyte resistivity, affording special advantages for operation with structures immersed in high-resistivity electrolytes.

By way of specific illustration, particular reference will be made herein to the invention as applied to cathodic protection, for maintainng the polarzation potential of the protected structure substantially constant at a predetermined value.

It is understood that corrosion of a metal structure immersed in an electrolyte results from flow of local current through the electrolyte between localized anodic and cathodic portions of the structure surface, the corrosion occurring at the anodic surface portions. A familiar example is the corrosion of iron or iron alloy structures immersed in water. Prevention of such corrosion by cathodic protection involves passing direct current (supplied by a suitable current source) through the electrolyte from one or more anodes immersed therein to the metal structure to be protected, which is connected to the negative terminal of the current source to constitute the cathode of the system. The purpose of providing this applied current is to establish and maintain at the structure surface, including the localized anodic portions thereof, a negative polarization potential effective to prevent the corrosion-producing local current flow.

Since the effectiveness of a cathodic protect-ion system is dependent on maintenance of a sufficient eletcro-negative polarization potential at the structure to be protected, it is desirable to control the operation of the cahtodic protection system, as by adjustment of the applied current flow, in response to changes in the structure potential. Such control may be accomplished by measuring the structure potential and actuating appropriate control means in accordance with the potential measurement to vary the current supply from the direct current source so as to maintain the polarization potential at a desired value.

If the electrolyte in which the protected structure is immersed has a sufficiently low resistivity, the polarization potential of the structure may conveniently be determined by immersing in the electrolyte a suitable nonpolarized reference electrode of fixed potential and directly measuring the difference of potential between this ref- 3,425,921 Patented Feb. 4, 1969 erence electrode and the structure. In such circumstance, the potential drop through the electrolyte between the reference electrode and the structure is so small as to be negligible in its effect on control of the system, i.e. variations in the potential difference between the electrode and structure are due substantially only to variations in structure polarization potential and hence provide eifective system control.

However, when the resistivity of the electrolyte is high (e.g. greater than about 1,000 ohm-centimeters, as in the case of electrolytes such as potable waters and soils), the potential difference measured between the reference electrode and the structure includes a significant potential drop resulting from the flow of cathodic protection current through the resistive electrolyte between the electrode and structure. This potential drop varies with changes in the applied current and/or in the resistivity of the electrolyte; for example, in the case of a reference electrode and a plane surface spaced one foot apart and immersed in water, the relation between current. density, electrolyte resistivity and potential drop between the reference electrode and structure surface are indicated in the following table:

Potential drop Current density Electrolyte resistivity included in (ma/[17. (ohm-cm.) polarization potential measurement (volts) As a result, in operation in a high-resistivity electrolyte, utilizing measurement of potential difference between a reference electrode and the protected structure to control the operation of a cathodic protection system as heretofore known, the system responds to variations in electrolyte resistivity or applied current density as well as to variations in the polarization potential of the protected structure. This is undesirable, since the potential drop resulting from applied current flow through the electrolyte has little or no relation to the effectiveness of control of local current flow at the metal surface to be protected; accordingly, for proper regulation of a cathodic protection system to maintain a desired polarization potential at the structure surface, the determination or sensing of the polarization potential should be accomplished in a manner that is independent of the latter potential drop, i.e. that effectively eliminates variations in such potential drop as a control factor in the system.

While the undesired potential drop component of the polarization potential measurement can be very substantially reduced by placing the reference electrode on the protected surface, such arrangement presents difficulties in that the reference electrode then senses the potential of only a vary limited area of the structure surface, and in addition the electrode may partially shield the surface from the applied protective current. Other arrangements heretofore proposed for compensating for this potential drop component have been attended with the disadvantages that all the protective current must flow through the potential-measuring circuit, which is expensive and impractical especially for systems requiring protective currents of substantial magnitude; and that the measurin circuit must be adjusted upon change in electrolyte resistivit Difiiculties similar to those described above are encountered in controlling anodic protection systems operating for passivation of metal surfaces (e.g. such as stainless steel surfaces) immersed in highly resistive electrolytes. In an anodic passivation system, the structure to be protected is connected to the positive terminal of a direct current source, the negative terminal of which is connected to auxiliary electrode means immersed in the electrolyte. To effect and maintain passivation of the structure surface, it is necessary that the structure potential (determined by comparison with standard reference electrode means) be controlled within a limited range of values, by regulation of the current source; if the structure potential departs from this range, the anodic protection operation may actually enhance the rate of structure corrosion. As in the case of cathodic protection, it is desirable to eliminate, from the measurement of structure potential used for system control, variables due to electrolyte resistivity and current density, which may be introduced in the measurement if the electrolyte resistivity is sufficiently high to provide an appreciable potential drop between the measuring reference electrode means and the structure through the electrolyte.

An object of the present invention is to provide systems and methods for protecting metal structures immersed in corroding electrolytes by passage of direct current through the electrolyte between the structure to be protected and auxiliary electrode means, wherein the potential of the protected structure is controlled by regulation of the applied current in accordance with variations in structure potential, and wherein further the Sensing of structure potential is independent of variations in resistivity of the electrolyte and current density therethrough.

A further object is to provide new and improved cathodic protection systems of the type wherein the polarization potential of the protected structures is maintained substantially constant at a predetermined desired value, and suitable for use to protect metal structures immersed in corroding electrolytes of high resistivity, e.g. resistivity above 1,000 ohm-cm. Another object is to provide such a system including provision for sensing the level of structure polarization potential independently of variations in applied current or electrolyte resistivity thereby to eliminate from the polarization potential measurement potential drops due to applied current flow through the electrolyte, and wherein the applied cathodic protection current is regulated in accordance with the sensed level of polarization potential to maintain such level at a desired value. A further object is to provide such a system incorporating reference electrode means for sensing the polarization potential, wherein such reference electrode means may be spaced at a substantial distance from the protected structure in order to sense the potential of an extended area of the structure surface without introducing, into the structure polarization potential measurement, potential drops representing flow of applied current through the electrolyte between the reference electrode means and the structure. Another object is to provide such a system including a new and improved arrangement of reference electrode means affording superior freedom from reference electrode polarization over extended periods of use. Yet another object is to provide such a system affording polarization potential measurement without requiring that all or a major part of the applied current flow through the potential measuring circuit.

A still further object is to provide a method of cathodically protecting a structure immersed in a corroding electrolyte by applying a flow of unidirectional current to the structure through the electrolyte from an anode immersed therein, while controlling the current flow in accordance with the level of polarization potential of the structure to maintain such polarization potential at a desired value, and while sensing the polarization potential for such control, independently of potential drops due to flow of applied current through the electrolyte. A further object is to provide such method including establishing a circuit for sensing the structure polarization potential independently of variations in applied current or electrolyte resistivity.

Still another object is to provide anodic passivation systems and methods wherein the applied current is regulated in response to variations in potential of the protected structure, measured in a manner independent of variations in current density and electrolyte resistivity.

Further objects and advantages of the invention will be apparent from the detailed description hereinbelow set forth, together with the accompanying drawings, where- FIG. 1 is a schematic view of a cathodic protection system embodying the present invention in a particular form;

FIG. 2 is a circuit diagram representing the system of FIG. 1;

FIG. 3 is a schematic view of an arrangement of reference electrodes particularly suitable for use in the cathodic protection systems and methods of the present invention;

FIG. 4 is a simplified schematic view of an anodic passivation system embodying the invention; and

FIG. 5 is a graph wherein potential of the protected structure, in the anodic passivation system of FIG. 4, is plotted against anode current density.

Referring to the drawing, there is shown a steel or like ferrous-metal tank 10 which contains a body of a corroding electrolyte 11, such as potable water of high resistivity (e.g. above 1,000 ohm-centimeters), having immersed therein a plurality of anodes 12 disposed in appropriate spaced relation to each other and to the tank. T0 constitute the anode and tank as a cathodic protection system for prevention of corrosion of the tank, there is provided an alternating current power supply generally designated 14, and a rectifier 15 connected to the output of the A.C. power supply to produce a direct current output. The positive terminal 16 of the rectifier is connected by a lead wire 17 to the anode 12, while the negative terminal 18 of the rectifier is connected through lead 20 to the tank 10 (which thus constitutes the cathode of the system), for effecting a continuous flow of direct current through the electrolyte from the anodes to the tank.

In accordance with the invention, two matched standard reference electrodes respectively designated 22 and 23 are immersed in the electrolyte in spaced relation to each other and to the tank and anodes, being so positioned that the potential drop resulting from passage of current between reference electrode 22 and the tank through the electrolyte is greater than the potential drop resulting from passage of current between reference electrode 23 and the tank through the electrolyte. To provide this relation, reference electrode 22 is spaced further from the tank 10 than is reference electrode 23, so that the current path through the electrolyte from electrode 22 to the tank has a higher resistance (being longer) than that from electrode 23 to the tank. Conveniently, these reference electrodes are of the so-called salt bridge type, i.e. including a metal electrode member immersed in a saturated solution of an appropriate salt which is in electro chemical contact with the electrolyte body 11; certain preferred forms and arrangements of such reference electrodes having special advantages for use in the present system are hereinafter more particularly described. Stated generally, it is important that these two reference electrodes be of substantially constant potential and substantially equal in potential to each other and that they remain nonpolarized over very extended periods of use, the function of these electrodes being to sense the potential in the path of protective current flow so as to enable the reading, in effect, of the desired polarization potential at the metal tank surface without introducing any polarization potentials or the like at the locality of sensing.

The system of the invention in the form shown further includes a bridge potentiometer 25 having opposed

end terminals

26 and 27 and a variable intermediate contact or tap 29. The end terminal 26 of potentiometer 25 is connected to the reference electrode 22 (i.e. the reference electrode spaced furthest from the tank wall) by means shown for simplicity in FIG. 1 as a lead wire 31, To the opposite terminal 27 of potentiometer 25 there is connected one terminal of .an adjustable reference potential source 33, the other terminal of which is connected to tank 10.

As shown, potential source 33 may (by way of illustration) comprise a direct current supply such as a cell 36 and a variable resistor 37 connected across the terminals of cell 36; the voltage produced by this source is measured by a voltmeter 38. It will be appreciated that by adjustment of the tap of the variable resistor 37, the voltage produced by the source may be varied over a range of values and, once the tap is set, will be maintained at the selected value,

Reference electrode 23 (the reference electrode positioned closest to the tank wall) is selectively connectable to tap 29 of potentiometer 25 either through a galvanometer 40 or through a meter relay control 42, by means of a switch 43. Meter relay control 42, which constitutes a very highly sensitive DC signal responsive relay system, is connected to control means such as a motor 45 for regulating the output of the alternating current supply 14 of the cathodic protection system.

By way of specific example of the above-described system, four anodes 12 may be arranged so that each is four feet away from the structure to be protected, i.e. the tank (cathode) surface 10. The electrolyte may be potable water having a resistivity above 7,500 ohm-centimeters.

Reference cells

22 and 23 are spaced between the anodes 12 and tank in the electrolyte, reference electrode 22 being positioned three feet away from the tank surface and reference electrode 23 being positioned one foot away from the surface. In this example, the control 42 is a DC. directional sensitive 1 microampere0(-)1 microampere meter relay control including a very high sensitivity meter relay (operated directly by the DC. signal hereinafter further referred to) which actuates slave relays that operate a directional motor drive 45 of a variable transformer 48. The latter transformer regulates the AC. power voltage applied to an isolation transformer 49, the secondary power voltage being applied to bridge-type rectifier 15. The meter relay control, motor drive and associated instrumentalities may be of well-known types and accordingly need not be described in detail. In the lead wire from the negative terminal of the rectifier to the tank 10, there is connected a resistor 50 and a switch 53 arranged to by-pass approximately 10% of the resistor 50 when closed.

In this system the bridge potentiometer may conveniently have a resistance in a range of 10 ,000 to 2,000,000 ohms, depending on the sensitivity of the galvanometer 40 and meter relay control 42 used and the magnitude of continuous current flow that can be tolerated through the

reference electrodes

22, 23 without effecting their polarization. An exemplary value of resistance for the potentiometer is 100,000 ohms. It has been found that reduction in total ohmage of the potentiometer increases the magnitude of the signal to the control 42 but also results in increased current fiow through the reference electrode 22 and may cause it to become polarized.

It will be noted that in the system as shown, tank 10, reference potential source 33, potentiometer 25 and reference electrode 22 are connected as a series circuit with source 33 interposed between tank 10 and potentiometer 25 and reference electrode 22 connected to the potentiometer terminal remote from the potential source.

The practice of the present method with the abovedescribed system may now be readily understood, and in this connection reference may be made to FIG. 2, which is a circuit diagram of the system of FIG. 1 illustrating certain further operational features of the system. In FIG. 2, R represents the resistance of the portion of the circuit between tap 29 and the reference voltage source, including the resistance of that part of potentiometer 25 which is between tap 29 and terminal 27 and the resistance of the part of variable resistor 37 included in the circuit; R is the resistance of the current path through electrolyte 11 between reference electrode 23 and the tank 10; R represents the total value of all resistance in this portion of the circuit extending from tap 29 through reference electrode 22 to the electrolyte 11, including the resistance of that portion of the potentiometer resistor which is between tap 29 and terminal 26; and R is the resistance of the current path through the electrolyte between the two

reference electrodes

22 and 23. Further in FIG. 2, E represents the constant predetermined voltage provided by potential source 33 while =E is the polarization potential of the tank 10. The resistance of the current path through the electrolyte between anode 12 and reference electrode 22 is represented at 55.

As will be apparent from FIG. 2, the circuit constituted by reference electrode 22, potentiometer 25, potential source 33, tank 10, and reference electrode 23 connected to tap 29 of the potentiometer, including the current paths through the electrolyte between the tank and reference electrode 23 and between the two reference electrodes, is in effect a modified form of Wheatstone Bridge circuit, wherein the first leg includes potential E and resistance R the second leg includes potential E and resistance R the third leg includes resistance R and the fourth leg includes resistance R The bridge of the circuit is provided by the connection, through galvanometer 40 or meter relay control 42, between reference electrode 23 and tap 29.

In carrying forward the present method, switch 43 is closed to connect reference electrode 23 to tap 29 through galvanometer 40. The potential source 33 is set, by appropriate adjustment of the tap of variable resistor 37, to provide the desired predetermined reference voltage (i.e. equal to the desired tank polarization potential) as measured on voltmeter 38. From the AC power supply 14 through rectifier 15, a continuous flow of protective current I is applied from the anodes 12 through the electrolyte to the tank 10. Within a short time, this impressed current establishes a polarization potential E at the tank surface.

With the polarization potential established, the tap 29 of potentiometer 25 is moved to vary the relative magnitudes of resistances R and R While for each setting of tap 29 switch 53 is opened and closed to vary the magnitude of the impressed current I This procedure is followed until a setting of tap 29 is found at which successive opening and closing of switch 53 produces no deflection of the galvanometer 40. When this condition is attained, the Wheatstone Bridge circuit is balanced, i.e. the resistances have the proportional relation.

In explanation of this balancing operation, it may be noted that opening and closing of the switch 53 varies the magnitude of the impressed current by changing the resistance of element 50 through which the impressed current flows. With switch 53 connected as described to bypass approximately 10% of resistor 50, the impressed current when the switch is closed is larger than when the switch is open by a value (dependent on the total resistance of the impressed current path including the resistance of the electrolyte between the anodes 12 and tank 10) equal to somewhat less than 10% of the current with the switch closed; therefore, such opening and closing of the switch varies the magnitude of the current flowing through the Wheatstone Bridge circuit by a corresponding amount.

Assuming (as is normally the case) that the polarization potential E of the tank 10 is not equal to the desired fixed reference potential E there will be current flow through the galvanometer 40 whether or not the Wheatstone Bridge circuit is balanced, and this current flow will be indicated on the galvanometer by a deviation of the galvanometer indicator from zero position. However, if the described proportionality of resistance exists, the current flow through galvanometer 40 is unchanged by variations in I and thus, in the latter condition, the galvanometer indicator exhibits no movement or deflection upon successive opening and closing of switch 53. Accordingly, when the galvanometer reading (although not equal to zero) remains constant as switch 53 is opened and closed, tap 29 is positioned to provide the desired balanced bridge condition. Of course, if E should happen to equal E the galvanometer will show a zero reading when the bridge is in balanced condition.

In this connection it may further be explained that although the change in magnitude of 1 produced by opening and closing of switch 53 will necessarily tend to change the value of the polarization potential E over a period of time, such change occurs relatively slowly; e.g. it may take as much as five seconds or more for the potential E at the tank surface to change appreciably with change of current flow I through the electrolyte. Consequently, with reasonably rapid opening and closing of switch 53 the short interval variations in I thus effected do not appreciably alter the value of E for the purposes of the described bridge-balancing operation.

As an alternative way of affecting variation of I for balancing the bridge, resistor 50 and switch 53 may be eliminated from the cathodic protection circuit and, during the balancing operation, the output of rectifier may be manually adjusted to 10% of its output current capacity; the rectifier output may then rapidly be turned on and off, achieving the same effect in varying the magnitude of I as the above described operation using switch 53. Under these circumstances, the other voltages present, including particularly the fixed reference voltage E 3, Sufiiciently energize the bridge so that the galvanometer reading is possible for the condition when the rectifier is turned off and there is no flow of protective current 1 Whatever arrangement is used for varying I desirably the change in current flow should be small as compared to the magnitude of impressed current utilized to provide the desired cathodic protection, to enhance the aforementioned lag or delay in change of polarization potential with change in current and thereby to facilitate balancing of the bridge.

When the bridge has been adjusted to the described balanced condition, switch 43 is opened to disconnect reference electrode 23 from galvanometer 40 and closed to connect the latter reference electrode to tap 29 through meter relay control 42, while continued flow of the cathodic protection current I is maintained. Assuming that reference potential E is not equal to the tank polarization potential E there will be a flow of current between reference electrode 23 and tap 29 through the meter relay control 42 in a direction determined by whether potential E is larger or smaller than E This flow of current, constituting a D C control signal, operates the meter relay control to actuate motor drive 45 to adjust the AC. voltage applied from variable transformer 48 and thereby to adjust the impressed current I in a manner to raise or lower the tank potential E to substantial equality with the reference potential E Thus, if potential B is of smaller electronegative magnitude than the preselected reference potential, the meter relay control is actuated by the direction of current flow therethrough to operate directional motor drive 45 to raise the AC. voltage applied from transformer 48; the increase in power voltage, applied from transformer 49, increases the rectifier voltage, thereby increasing the current flow I between anodes 12 and tank 10. The increased current flow enhances the polarization effect at the tank 10, raising potential E to a larger electro negative value. Correspondingly, if tank potential E has a higher electro negative value than reference potential E the signal current flowing through meter relay control 42 is in such direction as to actuate the directional motor drive 45 to reduce the AC. voltage from variable transformer 48, decreasing the protective current l and thereby reducing the polarization effect and potential E In this way, potential E (whether initially at a larger or smaller negative value than E is made substantially equal to the fixed reference potential E at which time current ceases to flow through the meter relay control 42 between reference electrode 23 and tap 29. If thereafter the polarization potential of the tank changes, i.e. departs from the preselected value of B a signal current will again flow through the meter relay control and the presence and direction of such current will once more actuate the control to adjust the applied current as in the manner described above to bring the tank polarization potential back to the desired value. In this way, with continuous application of protective current I supplied by the rectifier 15 and passing from anode 12 through the electrolyte to tank 10, the polarization potential of the tank is maintained substantially constant at a predetermined desired value by automatic adjustment of the applied current in response to departures of the polarization potential from such value.

It will be understood that in this system, changes in magnitude of the applied current I do not affect the reading or sensing of tank polarization potential that is used to control the system, for the reason that since the bridge circuit is balanced, variations in impressed current 1,, do not change the magnitude or direction of the signal current (if any) flowing through meter relay control 42, except, of course, insofar as variations in I produce changes in the tank polarization potential E this is demonstrated by the fact that when the bridge is initially balanced the opening and closing of switch 53 (which varies the magnitude of I causes no deflection or change in reading of galvanometer 40. Variations in resistance of the electrolyte 11 likewise have no effect on the sensing of change of polarization potential. Such changes in electrolyte resistivity alter the magnitude of bridge resistances R and R,, by the same factor, since both of these resistances are functions of the electrolyte resistivity; consequently, the proportionality of resistances in the bridge is unchanged regardless of variations in electrolyte resistance. It will be appreciated that so long as this proportionality is maintained, the flow of signal current through meter relay control 42 is unaffected by changes in magnitude of the bridge resistances. In other words, then, utilization of the balanced bridge measuring circuit for sensing the polarization potential E enables the continuous reading of E for control of the cathodic protection system, which is entirely independent of potential drops occasioned by flow of protective current through electrolyte. Hence the system can be used with superior advantages for maintenance of a desired substantially constant tank polarization potential regardless of electrolyte resistivity, i.e. even if the resistivity of the electrolyte is as much as 10,000 ohm-centimeters or higher.

In effect, the bridge circuit including the reference potential source 33 and tank 10 is a comparison circuit for comparing the tank polarization potential E to a predetermined value E if a disparity exists between the tank potential and reference potential, an error signal is developed through the meter relay control 42., this error signal serving to operate the meter relay control and thereby to regulate the impressed current supply for adjustment of the impressed current to bring the tank potential to the desired value.

It will be appreciated that various modifications may be made in the described system. For example, signal sensing and control instrumentalities other than meter relay control 42 and directional motor drive 45 with variable transformer 48 may be employed. Thus, the error signal developed in the bridge and represented by current flow between tap 29 and reference electrode 23 may be amplified, in any suitable manner, to regulate the AC. voltage output of a device such as a saturable reactor or silicon controlled rectifier, which output is applied to the bridge-type rectifier 15.

As stated above, satisfactory performance of the described system requires that the

reference electrodes

22 and 23 must match each other in potential and must remain stable, without becoming polarized, over long periods of continuous or intermittent operation of the cathodic protection system. Problems of avoidance of polarization are especially serious with respect to the reference electrode 22. Although, as will be apparent, in the present system the desired control is effected without requiring that all the applied current flow through the measuring circuit, and indeed involves only a relatively very minor flow of current through the bridge circuit, nevertheless there is some current fiowthrough reference electrode 22. Owing to the fact that in operation the adjustment of the impressed current maintains the bridge through meter relay control 42 in essentially null con dition (i.e. a condition approaching zero current flow), the current flow through reference cell 23 is ordinarily limited to negligible amounts.

As stated above, it is convenient to use so-called salt bridge electrodes in the present system, i.e. reference half cells each constituted by a non-conductive vessel containing a saturated salt solution exposed to a metal electrode, and a semipermeable conductive membrane separating the salt solution from the electrolyte, the metal of the electrode and the salt of the solution being mutually selected to provide desired reference electrode properties.

Reference electrodes

22 and 23 should be of the same type, so as to be matched in potential. Examples of such reference cells are calornel electrodes, wherein the metal electrode member is of platinum surrounded by mercury and mercurous chloride in contact with a saturated KCl solution. Other examples are cells having a copper electrode member in a saturated solution of copper sulfate (CuSO or a silver-silver chloride electrode member in a saturated solution of potassium chloride (KCl). However, reference electrodes of these types as heretofore known tend to polarize upon current flow through them, and long stable service life is not reliably obtained owing to loss of the saturated copper sulfate solution or chloride ion solution respectively.

However, in accordance with the present invention it has been found, for example in the case of a copper-copper sulfate reference electrode, that the rate of copper sulfate solution loss can be reduced at least five-fold by inverting the reference cell from the position normally used, i.e. so that the semipermeable membrane separating the salt solution from the electrolyte is at the upper rather than the lower end of the reference cell. This and other features of reference electrode arrangements embraced by the present invention in particular aspects thereof are shown schematically in FIG. 3 which illustrates such an arrangement of copper-copper sulfate reference electrodes.

In this arrangement, reference electrode 23 includes a copper metal electrode member 58 supported in the base 57 of an electrically non-conductive, vertically oriented vessel 60. An aqueous saturated solution of copper sulfate is contained within the vessel 60, in contact with electrode member 58. To maintain the solution 61 in saturated condition, an excess of copper sulfate crystals 63 is provided within the vessel surrounding the electrode member 58. The top of the vessel 60 is closed by a semipermeable membrane 65 e.g. of wood, which is conductive, and which separates the copper sulfate solution 61 from the electrolyte 1 1, having opposite sides respectively in contact with the copper sulfate solution and with the electrolyte to provide electro-chemical communication between the interior of the reference electrode 23 and the electrolyte. An insulated copper lead wire 67 connects electrode member '58 to switch 43. It will be seen that in this reference electrode 23 the semi-permeable membrane is atthe upper end of the cell and, as stated, provides advantages in materially retarding the rate of loss of copper sulfate solution from the cell.

Also shown in FIG. 3 is an arrangement of reference electrode 22 for preventing polarization of the latter electrode, under conditions of long continued flow of the current needed for energizing the bridge potentiometer 25. In structure, the reference cell 22 is similar to the cell 23 just described, i.e. including a copper electrode member 58 positioned at the lower end of a non-conductive vessel 60 containing a copper sulfate solution 61' with an excess of copper sulfate crystal 63 provided to maintain the solution in saturated condition, and further including a semi-permeable conductive membrane 65' closing the upper end of the vessel 60 and arranged to separate the solution 61' from electrolyte 11 while being in contact on opposite sides with the solution and electrolyte to provide electrochemical communication therebetween. However, for purposes hereinafter further described cell 22 in the form shown also includes a second copper electrode member '68, likewise positioned in the base of the vessel 60 but in spaced relation to the member 58, both

members

58 and 68 being in contact with the copper sulfate solution 61'.

Electrode member 58' is connected as by an insulated copper lead wire 70 through a microammeter 71 to the terminal 26 of the bridge potentiometer 25. An auxiliary circuit generally designated 73 is connected to a point 74 intermediate electrode member .58 and microammeter 71, the other end of this circuit 73 being connected by a further insulated copper lead wire 75 to the supplemental electrode member 68. This circuit 73 includes, in series between point 74 and electrode member 68, a dry cell 77 connected to provide current flow in a direction opposite to the energizing signal current through lead 70; a variable resistor 78; and a microamrneter 79. The resistance of variable resistor 78 is preferably at least times as large as the resistance of the lead wire 70 between electrode member 58 and point 74. Variable resistor 78 is so adjusted as to provide a current flow through circuit 73 (from cell 77) equal and opposite to the current flow through trnicroammeter 71, this adjustment being made manually in the example shown with the setting of variable resistor 78 selected to make the reading of microam-meter 79 equal to that of microammeter 71.

A baflie 80, of suitable nonconductive (insulating) material, is mounted in the base of the reference electrode 22, interposed between and extending substantially above

electrode members

58, 68 so as to compartmentalize the copper sulfate solution around each electrode member. This baffie determines the direction of the current path between the electrode members through the solution in a manner that insures proper superimposition of the effect of the current from circuit 73 upon the current that flows through membrane 65 and thence through resistance R for prevention of reference electrode polarization.

It is found that with the abovexlescribed condition of equal and opposite current flow maintained, the reference electrode 22 remains non-polarized as desired. It will be appreciated that While manual means for control of the current fiow through microammeter 79 are shown, an automatic control of this current may be provided. It will further be appreciated that, while the described arrangement has been shown as incorporating copper-copper sulfate reference cells, it is likewise applicable (and with the same advantages) to other forms of salt bridge reference electrodes, such as reference electrodes having silver-silver chloride electrode members and saturated solutions of potassium chloride.

Referring now to FIG. 4, there is shown a modified embodiment of the system of the present invention arranged for so-called anodic protection operation. The system of FIG. 4 is essentially the same as that of FIG. 1 (the same reference numerals beingemployed to identify like elements of the system), except that the positive terminal 16 of rectifier 15 is connected to the tank through lead 82, and the negative terminal 18 of the rectifier is connected to the auxiliary electrodes 12, so that the tank is the anode and the electrodes 12 are cathodes of the system. The tank may, for example, be a steel or stainless steel vessel confining a body 11 of highly resistive water; and

reference electrodes

22 and 23 may be saturated KC1 calornel reference electrodes.

Protection of the immersed steel structure by anodic passivation with this system is effected by initially developing a passive state (i.e. passivating the steel surface) and thereafter maintaining the potential of the structure within a predetermined range of values at which the structure surface remains passive. In the operation of the system, the passive range of potential values for the immersed structure is determined by the potentiostatic technique. By way of illustration of this technique, after the bridge circuit including potentiometer 25 is balanced (in the same manner as described above for the system of FIG. 1), and with the power supply operating to provide continuous current flow through the electrolyte, the reference voltage source 33 is set at a first preselected potential value as read on voltmeter 38, for example a value of -0.5 v. After five minutes, the rectifier current output is measured and recorded; the voltage source 33 is then reset at a second, less electronegative value (e.g. 0.4 v.) and maintained at that value for five minutes, after which the rectifier current is again measured and recorded. This procedure is repeated, with successive adjustments of the reference potential into the more passive (less electronegative) range, until the passive range has been determined, as indicated by reduction in measured value of rectifier current output. Since each resetting of the reference voltage source involves a change in the value of that portion of resistor 37 which is included in bridge resistance R (FIG. 2), the bridge circuit should preferably be rebalanced after each adjustment of the reference potential; as a practical matter, however, when the resistance of potentiometer 25 is very large with respect to that of resistor 37, only a .minor imbalance of the bridge results from resetting of the latter resistor.

For a further understanding of this procedure, reference may be made to FIG. 5, wherein the structure potential (in volts, relative to the reference electrode) of a typical immersed steel structure, in a system as shown in FIG. 4, is plotted against current density. It will be seen in FIG. 5 that as the potential becomes less negative, the rectifier current requirements initially increase. However, When the so-called Flade point is reached, representing development of a passive surface on the protected structure, the rectifier current output requirement markedly decreases. Throughout the passive range of potential values (0.2 v. to 0.0 v. relative to the reference electrode, in the example illustrated) the current requirement is low. When the structure potential reaches the transpassive range (i.e. becomes more positive than 0.02, in the example shown), the rectifier current flow again increases. Corrosion of the tank metal increases when the potential is in the transpassive range.

The system of FIG. 4 functions to apply suificient current initially to develop the passive state at the protected structure surface, and then to maintain the strucure potential at a selected value within the passive range as determined by the above-described technique, by appropriate setting of the reference potential source. For

12 example, with a passive range as shown in FIG. 5, the reference potential may be set to maintain the structure at 0.1 v. relative to the reference electrode.

It will be understood that the system of FIG. 4 operates in the same manner as that of FIG. 1 to maintain the potential of the protected structure at a constant predetermined value, and affords like advantages with respect to sensing of structure potential independently of changes in electrolyte resistivity or applied current density, enabling effective passivation of metal surfaces immersed in high-resistivity electrolytes. Thus, in a specific example of operation, with the reference potential set at -0.1 v. relative to the reference electrode the control instrumentalities operate to increase the rectifier output voltage if the tank potential becomes more negative than O.1l0v. relative to the reference electrode, and to decrease the rectifier output voltage if the tank potential becomes less negative than 0.090 v. relative to the reference electrode, in each instance effecting restoration of the tank potential to the desired predetermined value.

The system of FIG. 4 is shown for purposes of illustration as arranged for control of structure potential within a passive range that is negative relative to the reference electrode. If the passive range of structure potential happens to be positive relative to the reference electrode the reference potential source (i.e. cell 36 in FIG. 4) should be reversed in polarity, and the direction of operation of motor 45 should likewise be reversed, to provide appropriate response of the system to departures of structure potential from the selected control value within the passive range.

An arrangement of reference electrodes as shown in FIG. 3 may be employed in the system of FIG. 4 for avoidance of reference electrode polarization, the polarity of cell 77 being reversed to provide the proper direction of super-imposed current flow through circuit 73 for the anodic passivation operation.

The particular salt bridge reference electrode structures herein disclosed are not claimed per se herein, but certain features of such reference electrode structures are disclosed and claimed in the copending application of Leon P. Sudrabin (applicant herein), Ser. No. 604,369, filed Dec. 23, 1966 for Salt Bridge Reference Electrode, which is a continuation-in-part of this application.

It is to be understood that the invention is not limited to the procedures and embodiments hereinabove specifically set forth but may be carried out in other Ways without departure from its spirit.

I claim:

1. In a cathodic protection system for maintaining at a predetermined electronegative value of polarization potential a metal structure immersed in an electrolyte, in combination, at least one anode immersed in said electrolyte in spaced relation to said structure; direct current supply means having a positive terminal connected to said anode and a negative terminal connected to said structure for passing unidirectional current through said electrolyte from said anode to said structure; first and second reference electrodes of substantially equal and constant potential, immersed in said electrolyte and interposed between said anode and said structure in spaced relation thereto and to each other, said reference electrodes being so disposed that the potential drop represented by passage of current through said electrolyte from said first reference electrode to said structure is greater than the potential drop represented by passage of cur rent through said electrolyte from said second reference electrode to said structure; a reference potential source providing a constant voltage equal to said predetermined value of polarization potential; resistor means providing first and second resistances in series so proportioned that the ratio of said first resistance to the resistance of the current path through said electrolyte between said reference electrodes equals the ratio of said second resistance to the resistance of the current path through said electrolyte between said second reference electrode and said structure; and control means for regulating said direct current supply means, to maintain the polarization potential of said structure substantially constant at said predetermined value, in response to presence and changes in direction of an electrical signal of unbalance, said control means including means for sensing the presence and direction of said signal; said structure, said reference potential source, said resistor means and said first reference electrode being connected to constitute a series circuit wherein said reference potential source is interposed between said structure and said second resistance, and said first resistance is interposed between said second resistance and said first reference electrode; and said signal sensing means being connected between said second reference electrode and a point on said series circuit intermediate said first and second resistances for sensing the presence and direction of an electrical signal of unbalance between said second reference electrode and said point resulting from differences in relative magnitude of said constant voltage and the polarization potential of said structure.

2. A system as defined in claim 1, wherein each of said reference electrodes comprises a vertically oriented nonconductive vessel having a closed lower end and an open upper end, containing a solution of a salt and immersed in said electrolyte; an electrode member of metal disposed at the lower end of said vessel in contact with said salt solution, said salt and said metal being mutually selected to provide a reference half cell of substantially constant potential; means for making an electrical connection to said electrode member; and an electrically conducti-ve semipermeable membrane extending across the upper end of said vessel to separate said salt solution from said electrolyte, opposite sides of said membrane being in contact with said salt solution and said electrolyte respectively to provide electrochemical communication therebetween.

3. A system as defined in claim 2, wherein said first reference electrode includes first and second metal electrode members disposed at the lower end of the vessel of said first electrode in spaced relation to each other and in contact with the salt solution contained in said first electrode vessel, and wherein said first electrode member is connected to said resistor means, and further including circuit means having a first connection to said first electrode member and a second connection to said second electrode member, said circuit means comprising means for producing a continuous flow of current equal and opposite to the flow of current between said first electrode member and said resistor means.

4. A system as defined in claim 3, 'Wherein said circuit means comprises a source of electrical current and means for varying the magnitude of said electrical current to establish and maintain a flow of current through said circuit means substantially equal in magnitude to the flow of current between said first electrode member and said resistor means.

5. In a cathodic protection system for maintaining at a predetermined electronegative value of polarization potential a metal structure immersed in an electrolyte, in combination, at least one anode immersed in said electrolyte in spaced relation to saidstructure; direct current supply means having a positive terminal connected to said anode and a negative terminal connected to said structure for passing unidirectional current through said electrolyte from said anode to said structure, including means operable to vary the magnitude of said current; first and second reference electrodes of substantially equal and constant potential, immersed in said electrolyte and interposed between said anode and said structure in spaced relation thereto and to each other, said reference electrodes being so disposed that the potential drop represented by passage of current through said electrolyte from said first reference electrode to said structure is greater than the potential drop represented by passage of current through said electrolyte from said second reference electrode to said structure; a reference potential source providing a constant voltage equal to said predetermined value of polarization potential; a bridge potentiometer including resistor means having first and second terminals and providing an electrical resistance between said terminals, and tap means for dividing said resistance into first and second resistances in series, said tap means being adjustable to vary the magnitude of said first and second resistances relative to each other; a galvanometer connected to said tap means; control means for regulating said direct current supply means, to maintain the polarization potential of said structure substantially constant at said predetermined value, in response to presence and changes in direction of flow of a signal current, said control means including means for sensing presence and direction of said signal current, and said sensing means being connected to said tap means; and means for selectively connecting said second reference electrode to said tap means through either one of said gal'vanometer and said sensing means; said structure, said reference potential source, said resistor means and said first reference electrode being connected to constitute a series circuit wherein said reference potential source is interposed between said structure and said second resistance, and said first resistance is interposed between said second resistance and said first reference electrode; said signal current sensing means being adapted, when connected to said second reference electrode, to sense the presence and direction of current flow between said second reference electrode and said tap means resulting from differences in relative magnitude of said constant voltage and the polarization potential of said structure.

6. In a cathodic protection system for maintaining at a predetermined electronegative polarization potential a metal structure immersed in electrolyte, in combination, at least one anode immersed in said electrolyte in spaced relation to said structure; alternating current supply means; rectifier means conected to the output of said alternating current supply means and having a positive terminal connected to said anode and a negative terminal connected to said structure for passing unidirectional current through said electrolyte from said anode to said structure upon supply of alternating current to said rectifier means from said current supply means, said rectifier means further including means operable to vary the magnitude of said unidirectional current; first and second reference electrodes of constant potential, immersed in said electrolyte and interposed between said anode and said structure in spaced relation thereto and to each other, said first reference electrode being spaced further from said structure than said second reference electrode; each of said electrodes including a non-conductive vertically oriented vessel having a closed lower end and an open upper end and containing a salt solution, a metal electrode member disposed in the lower end of said vessel, means for making an electrical connection to said metal electrode member and a semipermeable electrically conductive membrane extending across the upper end of said vessel to separate said salt solution from said electrolyte, opposite sides of said membrane being respectively in contact with said salt solution and said electrolyte; a reference potential source providing a constant voltage and including means for adjusting said voltage to establish and maintain said voltage equal to said predetermined value of polarization, potential; a bridge potentiometer including resistor means having first and second terminals and providing an electrical resistance between said terminals, and tap means for dividing said resistance into first and second resistance in series, said tap and changes in direction of flow of a signal current, said control means including means for sensing the presence and direction of said signal current and means responsive to said sensing means for varying the output voltage of said alternating current supply means in accordance with the presence and direction of said signal current, said sensing means being connected to said tap means; and means for selectively connecting said second reference electrode to said tap means through either one of said galvanometer and said sensing means; said structure, said reference potential source, said resistor means and said first reference electrode being connected to constitute a series circuit wherein said reference potential source is interposed between said structure and said second resistance, and said first resistance is interposed between said second resistance and said first reference electrode; said Signal current sensing means being adapted, when connected to said second reference electrode, to sense the presence and direction of current flow between said second reference electrode and said tap means resulting from differences in relative magnitude of said constant voltage and the polarization potential of said structure; said first reference electrode further including a second metal electrode member in contact with said salt solution and means connected between the two metal electrode members of said first reference electrode for establishing and maintaining therebetween a flow of current equal and opposite to current fiow between said first reference electrode and said bridge potentiometer.

7. A method for cathodically protecting a metal structure immersed in a corroding electrolyte while maintaining the polarization potential of said structure substantially constant at a predetermined negative value, comprising continuously passing a flow of unidirectional current to said structure through said electrolyte from anode means immersed therein; establishing a control circuit comprising, in series, a first reference electrode immersed in said electrolyte, first and second resistances, a reference potential source providing a reference voltage equal to said predetermined negative value of polarization potential, said structure, and said electrolyte, and further in cluding a second reference electrode connected to said control circuit by a bridge at a point intermediate said first and second resistances and immersed in said electrolyte in such position that the potential drop due to passage of current through said electrolyte between said second electrode and said structure is less than the potential drop due to passage of current through said electrolyte between said first electrode and said structure, said first and second electrodes having substantially equal and constant potentials, and said first and second resistances being so proportioned that the ratio of said first resistance to the resistance of the current path through said electrolyte between said first and second electrodes equals the ratio of said second resistance to the resistance of the current path between said second electrode and said structure, sensing the presence and direction of current flow between said second electrode and said point on said control circuit through said bridge as representing a difference between the polarization potential of said structure and said reference voltage; and regulating the fiow of said unidirectional current in accordance with the sensed presence and direction of current flow through said bridge to adjust the polarization potential of said structure to substantial equality with said reference voltage.

8. A method according to claim 7, wherein said step of establishing a control circuit comprises establishing said reference voltage at said predetermined negative value, and after initiating said flow of unidirectional flow to said structure, varying the relative magnitude of said first and second resistances while varying the magnitude of said unidirectional current, and measuring the fiow of current through said bridge until the relative magnitudes of said first and second resistances are such that variation of said unidirectional current effects no change in magnitude of current fiow through said bridge.

9. In a system for maintaining at a predetermined value of potential a metal structure immersed in an electrolyte, in combination, at least one auxiliary electrode immersed in said electrolyte in spaced relation to said structure; direct current supply means having a first terminal connected to said auxiliary electrode and a second terminal connected to said structure for passing unidirectional current through said electrolyte between said auxiliary electrode and said structure; first and second reference electrodes of substantially equal and constant potential, immersed in said electrolyte and interposed between said auxiliary electrode and said structure in spaced relation thereto and to each other, said reference electrodes being so disposed that the potential drop represented by passage of current through said electrolyte between said first reference electrode and said structure is greater than the potential drop represented by passage of current through said electrolyte between said second reference electrode and said structure; a reference potential source providing a constant voltage equal to said predetermined value of structure potential; resistor means providing first and second resistances in series so proportioned that the ratio of said first resistance to the resistance of the current path through said electrolyte between said reference electrodes equals the ratio of said second resistance to the resistance of the current path through said electrolyte between said second reference electrode and said structure; and control means for regulating said direct current supply means, to maintain the potetial of said structure substantially constant at said predetermined value, in response to presence and changes in direction of an electrical signal of unbalance, said control means including means for sensing the presence and direction of said signal; said structure, said reference potential source, said resistor means and said first reference electrode being connected to constitute a series circuit wherein said reference potential source is interposed between said structure and said second resistance, and said first resistance is interposed between said second resistance and said first reference electrode; and said signal sensing means being connected between said second reference electrode and a point on said series circuit intermediate said first and second resistances for sensing the presence and direction of an electrical signal of unbalance between said second reference electrode and said point resulting from differences in relative magnitude of said constant voltage and the potential of said structure.

10. A system as defined in claim 9, wherein said first terminal of said direct current supply means is a negative terminal and said second terminal of said direct current supply means is a positive terminal.

11. A method for maintaining at a predetermined value of potential a metal structure immersed in an electrolyte, comprising continuously passing a flow of unidirectional current through said electrolyte between said structure and auxiliary electrode means immersed in said electrolyte; establishing a control circuit comprising, in series, a first reference electrode immersed in said electrolyte, first and second resistances, a reference potential source providing a reference voltage equal to said predetermined value of potential, said structure, and said eletcrolyte, and further including a second reference electrode connected to said control circuit by a bridge at a point intermediate said first and second resistances and immersed in said electrolyte in such position that the potential drop due to passage of current through said electrolyte between said second electrode and said structure is less than the potential drop due to passage of current through said electrolyte between said first electrode and said first structure, said first and second electrodes having substantially equal and constant potentials, and said first and second resistances being so proportioned that the ratio of said first resistance to the resistance of the current path through said electrolyte between said first and second electrodes equals the ratio of said second resistance to the resistance of the current path between said second electrode and said structure; sensing the presence and direction of current flow between said second electrode and said point on said control circuit through said bridge as representing a difference between the potential of said structure and said reference voltage; and regulating the flow of said unidirectional current in accordance with the sensed presence and direction of current flow through said bridge to adjust the potential of said structure to substantial equality with said reference voltage.

12. A method according to claim 11, wherein said structure is maintained anodic by said flow of unidirectional current.

References Cited UNITED STATES PATENTS 2,986,512 5/1961 Sabins 204--196 5 2,987,461 6/1961 Sabins 204-196 3,197,775 7/1965 Conger 204196 3,303,118 2/1967 Anderson 204147 3,371,023 2/1968 Banks, et a1 204-147 10 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

US. 01. X.R.