US3242064A

US3242064A - Cathodic protection system

Info

Publication number: US3242064A
Application number: US85008A
Authority: US
Inventors: Paul B Byrne
Original assignee: Engelhard Industries Inc
Current assignee: BASF Catalysts LLC; Engelhard Industries Inc
Priority date: 1960-02-29
Filing date: 1961-01-26
Publication date: 1966-03-22
Anticipated expiration: 1983-03-22
Also published as: DE1446395A1; DE1446395B2; NL261794A; NL136909C; GB957467A; DK112841B

Description

March 22, 1.966 p. B, BYRNE 3,242,064

CATHODIC PROTECTION SYSTEM Filed Jan. 26, 1961 2 Sheets-Sheet 1 I6 35 FIG..

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J es 66 #5J/za 4 FQ? March 22, 1966 P. B. BYRNE 3,242,064

CATHODIC PROTECTION SYSTEM Filed Jan. 26, 1961 2 Sheets-Sheet .2

INVENTOR.

ATTORNEY United States Patent O 3,242,064 CATHODIC PROTECTION SYSTEM Paul B. Byrne, Passaic, NJ., assigner to Engelhard lndustries, Inc., Newark, NJ., a corporation of Delaware Filed lan. 26, 1961, Ser.. No. 85,008 4 Claims. (Cl. Zim- 196) T-his application is a continuation-impart of application Serial No. 11,593, filed February 29, 1960, now abandoned.

This invention relates to corrosion reduction systems in which t-he direct current supplied to the` surface to be protected, such as the h-ull of a ship, is automatically varied in accordance with the protective conditions on the hull, as monitored by a sensin-g half-cell.

rl'he use of cathodic protection systems for large oceangoing vessels is a well accepted technique for avoiding severe corrosion. One circuit arrangement for such vessels is disclosed in patent appli-cation Serial No. 732,275, by` E. M. Fry, filed May 1, y1958, now U.S. Patent No. 3,129,154. Y

In cathodic protection systems for small boats, however, the elaborate, powerful, and relatively expensive system disclosed `in the above-identified patent application is not necessary or practical. vMore specifically, in view of the high power capacity and the necessary control circuit, the cathodic protection system for large vessels as described in the Fry application -cited above must cost more than many small boats. Foi-,small craft, such as cabin cruisers with steel hulls andthe like, simpler cathodic protection systems have been proposed. One such system is presented in application Serial No. 766,- 147, by E. P. Anderson, led October 8, 1958, now U.S. Patent No. 3,098,026. In such sys-tems for smaller boats, a single anode and an inexpensive sensing halfecell are normally provided. Resistance networks have normally been employed to regulate the power supplied to the anode, and the sensing half-cell has 'been used only for initial setting purposes, in these small boat systems.

With the simplications noted above, adequate cathodic protection for small boats of the cabin cruiser type has been obtained. However, the systems have required readjustment upon changes in cathodic protection conditions, 4for example, when traveling between river Waters and the ocean. Similar readjustments are necessary when changes in the condition of the paint on the hull and the like occur. Variations in the speed of the craft also affect the protection on the hull thereof, and such changes could not readily be accommodated.

The aluminum hulls which are employed in many small boats present an additional and more complex cathodic protection problem. With aluminum hulls, the corrosion initially decreases with increasing protective current and subsequently increases sharply. Thus, if a resistance type small boat cathodic protection system were adjusted to a predetermined level for an aluminum boat, with changes in the conductivity of the Water, the cathodic protection conditions frequently change to such an extent that excessive protective current is supplied, which may :be termed overprotectiom resulting in appreciable corrosion of the aluminum hull. In fact, this corrosion could even be more rapid than without `any cathodic protection or too little protective current, termed under-protection. Because of these facts, none of the resistance type small boat cathodic protection systems have been recommended for use on craft with aluminum hulls.

Accordingly, a principal object of the present invention is the provision of a cathodic protection system, especially suitable for use in connection with small boats, which is simple and inexpensive, and wh-ich has a power output directly controlled by the sensing half-cell signals.

ice

Another object of the invention is the protection of aluminum surfaces, eg. small boat hulls, by cathodic protection techniques.

A further object of the invention is to increase the throwing power of cathodic protection systems.

Heretofore, in cathodic protection systems gene-rally a continuous current was supplied to the anodes and the current level was controlled in dependence on the signal `obtained from a sensing halfJcell. In some instances, periodically occurring interruption of the protective current was provided and the potential on the surface to be protected was monitored during the off-period of protective current. yDuring the on-period, the protective current level supplied was adjusted, with the` ratio and duration of on and off-periods being fixed and independent of cathodic protection :conditions and requirements. However, controlling the protective cur-rent level ygenerally involves complicated circuitry using relays and resistors which are selectively connected or disconnected into the output circuitbymeans of the relays. Such arrangements include moving parts and power consuming circuit elements, both` of which are objectionable forobvious reasons. Accordingly,- it is another object of this invention to increase the efficiency of cathodic protection systems by eliminating moving parts such as relays and power consuming resistors,

In accordance with the present invention, the foregoing objects may be achieved by the use of a cathodic protection system in which unidirectional pulses are supplied to the anodes, with the ratio of the pulse duration to the ytime between pulses applied to the anode being controlled in dependence on cathodic protection conditions by` means of a signal obtained from a sensing .half-cell. `When applying the fundamental principle `of adjusting the aforementioned ratio, an oscillator m-ay be employed for supplying pulses to the anode `or anodes. The current level of each pulse may be substantially constant, since the .total current supplied to the anode and to the surface to be protected is the `sum of pulse durations during any given period of time. The ratio of the pulse duration to the time between pulses therefore directly .controls the total protective current supplied.

This ratio, when maintaining constant `the length `of pulse duration, depends on the frequency of pulses or the time between pulses, as exemplified by one of the embodiments described below. Alternatively, the frequency tions prevail.

may be maintained constant and the duration` of pulses may be varied in order to control the total current output.

As a further alternative, illustrated and described in detail below in connection with FIGURE 3, the system, which constitutes a closed-loop system, may be designed to exhibit high response speed and sensitivity suflicient tection. A mean frequency corresponds to optimum conditions, and is maintained as long a optimum condi- In order to use a term describing the fundamental principle developed in the foregoing, the cath-odio protection system in accordance with the embodirnent under consideration is driven to a degree of over-sensitivity at which overshooting, generally considered objectionable, must necessarily occur so that" J constantly extremely small, and therefore harmless, changes between overand underprotection are impressed on the surface to be protected, with the changes appearing as oscillations at a frequency determined by cathodic protection condit-ions, or requirements.

In accordance with the first mentioned embodiment involving t-he use of an oscillator, the pulses may be of substantially constant current level, and their periodici-ty is increased or decreased to provide more or less cathodic protection in accordance with signals from the sensing half-cell. At this point, it should be noted that the term periodicity is used herein to define the frequency of pulses having equal length in time. The term frequency indicating only the number of pulses, regardless of their length, would not adequately include the necessary second magnitude -involved which is the duration of the pulses. In other words, and as a matter of course, the same ratio of pulse duration to the time between pulses may be obtained at any desired frequency and, then, the total current delivered would be the same. Controlling only the frequency would, consequently, not necessarily result in a control of the total output current supplied during a given time.

In one illustrative circuit, in accordance with the invention, a transistor having emitter and base control electrodes is employed as a preamplifier. Automatic regulation of the cathodic protection action is initiated at a predetermined level in accordance with a preset adjustable voltage level. A sensing half-cell is connected to one of the control electrodes of the transistor, while the reference voltage level is supplied to the other control electrode of the transistor. A pulse circuit is connected to supply metered increments of unidirectional current to the anode or anodes of the system. The periodicity of the pulse circuit is determined by the conduction level of the input transistor. Accordingly, when cathodic protection conditions change, e.g. on the hull in the case of ship protection, the frequency of current pulses increases or decreases to provide a proper increase or decrease in total current supplied to the surface of the hull during any given period of time, thereby restoring optimum cathodic protection conditions.

In accordance with one feature of the invention therefore, a direct current protection system includes an electrode, a sensing cell, circuitry for supplying unidirectional pulses to the electrode, and additional circuitry responsive to signals from the sensing cell for changing the ratio of pulse length to the period between pulses.

With constant pulse length, and in accordance with one embodiment of the invention, the frequency of the pulses is adjusted.

In accordance with another feature of the invention, an aluminum water craft is provided with a cathodic protection anode or anodes and a sensing half-cell. In addition, circuitry is provided for applying current of one polarity to the anode, and also for controlling the total current applied to the anode in accordance with the potential sensed at the half-cell.

In accordance with a further feature of the invention, the input electrodes of a transistor amplifier are connected between the sensing half-cell and a reference voutage source in an electrical corrosion prevention system, and circuit arrangements are provided for varying the unidirectional current applied to the protective electrode in accordance with the output of the transistor amplifier.

The system, in accordance with the present invention, has the advantage of accurately following the cathodic protection conditions on the surface of the hull, whether these conditions are a result of changes in conductivity of the water or of changes in speed of the craft, or other factors. As stated above, tests indicate that the throwing power of the system is significantly increased by the use of unidirectional pulses preferably at a steady level, as compared with either continuous current output or current interrupted at a constant frequency with the current level being adjusted in dependence on the sensing half-cell signals, as it is the case with systems used heretofore.

Other objects, features and advantages of the invention will be more readily apprehended from a consideration of the following detailed description and from the appended drawings, in which;

FIGURE 1 is a circuit diagram of a cathodic protection system in accordance with the present invention and using an oscillator;

FIGURE 2 is a plot which is useful in understanding the mode of operation of the circuit of FIGURE 1;

FIGURE 3 is a block diagram illustrating a cathodic protection system of high sensitivity and adapted to oscillate, under the action of naturally or normally occurring changes in cathodic protection conditions; and

FIGURE 4 is a detailed circuit diagram of a cathodic protection system in accordance with FIGURE 3.

With reference to FIGURE 1 of the drawings, the principal components of the circuit include a sensing halfcell 12, an anode 14 and a source of direct current (not shown) coupled to the power lead 16. The anode has a platinum surface or is otherwise of electrochemically inert material. The remainder of the circuit of FIGURE 1 is designed to apply pulses of current from lead 16 to the anode 14. These pulses are unidirectional of constant duration, and vary in frequency in accordance with signals from half-cell 12 indicating the level of cathodic protection on the hull18.

The electrical circuit includes five transistors, 21 through 25. The first two

transistors

21 and 22 are preamplifier transistors; the second two

transistors

23 and 24 form an oscillation circuit; and the last transistor 25 is the output or power transistor. It may be noted that

transistors

21 and 23 are npn type transistors, while

transistors

22, 24 and 25 are pnp type transistors.

Proper biasing potentials for operation of the transistors are provided by the various resistors included in the circuitry. The circuit resistors include

resistors

28 and 30 in the circuit of meter 31. Resistor 32 is connected in series with the cell 12 at the input to the base of transistor 21 and is sufficiently large to prevent burn out of transistor 21 if resistor 32 is inadvertantly connected to the voltage supply. To provide a preset adjustable voltage, a potentiometer 34 is connected in the emitter circuit of transistor 21. This voltage assumes the role of a bucking voltage in circuit opposition to the voltage developed by the sensing half-cell 12. It had been found that, among other advantages, such arrangement operates to increase the life of the sensing half-cell, as set forth in the above-mentioned copending application Serial No. 732,275, filed May 1, 1958. The silicon diode 36 provides an 0.7 volt drop (at 10 milliamperes) across the potentiometer 34. A Zener diode could also be used, in place of diode 36. A number of

resistors

38, 40, 42, 44 and 46 are connected from the 12 volt input lead 16 to various points in the circuitry of transistors 21 through 25 to supply the proper operating potentials. Resistor 48 is connected between the emitter of transistor 22 and the base of transistor 23. Similarly, resistor 50 is connected between the collector of transistor 23 and the base of transistor 24. A resistor 52 and a capacitor 54 are connected between the base of transistor 23 and the collector of transistor 24. Resistor 56 is connected from the collector of transistor 24 to ground.

Another capacitor 58 is connected between the emitter and collector of transistor 22. Capacitor 60 in the input circuit to transistor 21 forms, with resistor 32, a lowpass filter to prevent high frequency transients from affecting transistor 21 by shunting them to ground. With reference to the cathodic protection anode 14, it may be spaced from the hull 18 by a suitable insulating layer 62. The insulation 62 may extend around the anode 14 to,

f cover an area of the hull adjacent the anode, thus avoiding concentrating the protective current near the anode.

In operation, the boat owner would initially set the potentiometer 34 to a predetermined meter reading as shown on meter 31 with the switch arm 64 contacting terminal 66. This desired potential is a known function of the sensing half-cell material and the surface to be protected. Thus for example, when using an aluminum hull 18 and a steel sensing halfacell 12, the desired reading on the meter 31 is 400 millivolts. Accordingly, this voltage level is set on the variable resistor 34.

To obtain the figure of 400 millivolts specified in the preceding paragraph, it may be noted that steel has a galvanic potential in sea water of about 0.60 volt or 600 millivolts with respect to a saturated calomel halfcell, or a standard silver-silver chloride cell. Aluminum has an activity of about -750 millivolts with respect to a calomel half-cell. This gives an initial difference of 150 millivolts. When a proper level` of cathodic protection current is flowing, the protective hydrogen film on the surface to be protected increases the negative potential by 250 millivolts. The total desired reading on the meter is therefore 400 millivolts.

The exact mode of operation of the circuit will be set forth in detail below. From an overall standpoint, however, the

oscillator including transistors

23 and 24 starts oscillating at a high rate as soon as the circuit is turned on and continues to oscillate rapidly as cathodic protection builds up on the hull. Standard pulses are applied through transistor 25 to the anode 14. The frequency of these pulses varies in accordance with cathodic protection conditions. When the level of protection is high, pulses are only applied to the anode 14 infrequently, and the pulse repetition rate is low. Initially, however, when there is no protective film built up on the hull of the ship, a high rate of oscillation is maintained, thus providing maximum current to the anode 14. In the case of the circuit of FIGURE 1, the maximum rate of oscillation is approximately 300 pulses per second. This rate ranges down to about pulses per second as the lowest rate available with the circuit of FIGURE 1. However, normal cathodic protection needs under constant operating conditions for even the smallest boat will be greater than 10 pulses per second. With minor circuit changes other frequencies could readily be obtained.

Considering the nature of the oscillations in the

circuit including transistors

23 and 24, it will initially be assumed that transistor 24 is in the energized state. Transistor 23 is also energized to supply drive to the base of transistor 24. The base of the npn transistor 23 must be positive with respect to its emitter for it to conduct. Now, as time passes, capacitor 54 charges up, reducing the base drive to transistor 23. The base drive to transistor 24 is also reduced, and the collector current of this transistor 24 is decreased. The reduction of current ow through transistor 24 reduces the voltage across resistor 56 and provides a cumulative effect.

Transistors

23 and 24 then cutoff.

The time required for the discharge of capacitor 54 to a level where

transistors

23 and 24 are reactivated is determined by the discharge path through

resistors

48 and 52 and voltage on capacitor 58. Transistor 22 controls the voltage across capacitor 58. If the voltage across capacitor 58 is high (when maximum power output is desired)

transistors

21 and 22 are off. Under these conditions, capacitor 54 discharges quickly, the potential at the base of transistor 23 rises rapidly, and the off time is short. The resulting high pulse repetition rate through transistor 25 gives maximum power to the cathodic protection anode 14. Under the maximum pulse repetition rate conditions of 300 pulses per second, the specific circuit of FIGURE 1 has an off time which is about equal to the on time. Thus, the ratio of the pulse duration to the time between pulses is changed in accordance with the sensed input signals from the sensing half-cell 12.

The graph of FIGURE 2 represents a plot of the relative current supplied to anode 14 plotted against the departure of the half-cell 12 from the preset voltage on potentiometer 34. Assuming, for example, that the optimum voltage which is desired at sensing cell 12 is 400 millivolts, resistor 34 is initially set to give a 400 millivolt reading on meter 31. Initially, the sensing half-cell will have a much lower reading as no protective lm will have `built up on the hull. This corresponds to the portion 72 of the plot of FIGURE 2. As a protective film builds up on the hull, the voltage level of the sensing half-cell 12 builds up and the current applied to anode 14 is reduced, This situation corresponds to the section 74 of the curve of FIGURE 2. After equilibrium conditions have been established, an average operating point 76 will be reached, at which the voltage supplied by the sensing half-cell 12 will be equal to the voltage preset on resistor 34, when compared by switching meter 31 from terminal 66 to terminal 68. In the case of overprotection, the voltage at sensing half-cell 12 will exceed the preset voltage and the current applied to the anode 14 must be reduced. These conditions are represented by section 78 of the characteristic shown in FIGURE 2. This situation would exist, for example, when a boat is proceeding from the ocean into a river. The resulting decrease in conductivity of the water would produce overprotection, if the salt water level of cathodic protection current were maintained. Reduction in the speed of a craft causes the same result. Under such conditions, a new operating point at a lower level of current is produced. In terms of the circuit of FIGURE 1, this would mean that the oscillator would be operating at a somewhat lower frequency.

Applying this last example to the details of the circuitry of FIGURE 1, it will be assumed that the halfcell voltage increases to provide a greater level of protection than is desired. As the voltage applied to the base of transistor 21 exceeds that on the emitter of this transistor, the transistor 21 is energized to a higher level of conduction. Transistor 22 is similarly energized to a higher level of conduction. Under these conditions, the voltage at the base of transistor 23 is reduced, and the oit period of the oscillating circuit is increased. This, of course, reduces the periodicity of the oscillating circuit and also reduces the cathodic protection current applied to anode 14.

For completeness, the components in the circuit of FIG- URE l are as follows:

Resistor:

28 18,000 ohms.

30 12,000 ohms.

32 470 ohms.

34 500 ohms (variable).

38 1,000 ohms.

40 27,000 ohms.

42 4,700 ohms..

44 470 ohms.

46 20 ohms.

48 47,000 ohms.

50 270 ohms.

52 15,000 ohms.

56 10 ohms. Capacitor:

54 0.1 microfarad.

58 1.0 microfarad.

60 110 microfarads. Transistors:

21, 23 type 2N35.

22 type 2N184.

24 type 2N176.

25 type 2N1l63. Meter 31 50frnicroamperes,

2,000 ohms.

It may be noted that the resistor 28 differs from resistor 30. This difference is provided in order to equalize the meter readings when optimum conditions are obtained. The resistor 30 is different from resistor 28 by 6,000 ohms to compensate for the 0.1 volt drop across the input base and emitter electrodes of transistor 21. With this difference in resistance, the meter readings appear to be exactly the same when the system is at the desired operating point as shown at point 76 in FIGURE 2.

The use of the transistor 21 as the first preamplifier stage has a number of advantages. First, it is substantially free from drift, and will always conduct at the same level with the same difference in potential across the input base and emitter electrodes. It is unaffected by the vibration characteristic of shipboard installations. In addition, it has a low input impedance when back biased, and requires no biasing circuitry, in addition to the adjustable voltage which is preset on potentiometer 34.

A number of salient advantages of the present cathodic protection system will now be recapitulated. First, by the use of low frequency oscillation control circuitry, inexpensive transistors may be used. The resulting automatic small boat cathodic protection unit is out slightly more expensive than non-automatic resistance type units, and takes up no more space. In addition, tests indicate that, for a given level of protection as measured by a sensing half-cell, a lesser amount of current applied to the anode is required when this current is in the form of unidirectional pulses, and not a steady current at a lower voltage. The reduction in current appears to be in the order of fifteen to twenty-five percent. It is considered that this improvement may result at least in part from the increased throwing power of the higher voltage level applied to the anode or anodes when the direct current is in pulse form.

At this point, it should be noted that the term throwing power is used to designate the maximum distance from each anode at which the potential impressed at a given total current output is still sufficient to ensure satisfactory protection. The throwing power increases with the applied voltage and, since current is applied by the system of this invention in increments separated by off-periods, it will be apparent that a higher voltage level is associated with the current pulses supplied as compared to a continuous current supply operating at the same average current. In other words, an increased throwing power is achieved under otherwise identical conditions when using protective current supplied as pulses, interrupted by off-periods.

Referring now to the embodiment of FIGURES 3 and 4, the cathodic protection system shown is designed to exhibit high response speed and sufficient sensitivity to oscillate under normal operating circumstances between the conditions termed above underprotection and overprotection, In other words, instead of includin-g an oscillator, the closed-loop system oscillates as a whole with it natural frequency which depends on the naturally occurring cathodic protection conditions on the surface to be protected, such as a hull. At this point, it should be noted that most closed-loop systems, when deviating from optimum conditions, perform in a manner generally referred to as hunting, as a phenomenon necessary to reach optimum conditions at which the otherwise useless and objectionable hunting performance comes to a stop. In sharp contrast to such systems, that of the present invention is designed to continuously oscillate under any conditions, which includes optimum or balance conditions, in such a manner that oscillations of the system as a whole occur at a frequency of at least 1 per second, preferably 20 to 200 per second. This result may be achieved by the inclusion of at least one amplication stage driving the sensitivity of the system to a degree at which overshooting occurs under any circumstances, including optimum conditions. Accordingly, the system of FIGURES 3 and 4 is at no time in balance but, due to the intentionally induced overshooting, the system oscillates even under continuing optimum conditions. As a result, the protective anode current is a pulsating current, with the ratio of the pulse duration to the time between pulses being a function of the signal voltage of the sensing half-cell, and the frequency being dependent on naturally occurring changes in cathodic protection conditions, such as the hull polarization and the speed of the ship, in the case of ship protection.

Referring now particularly to FIGURE 3, the hull or other surface to be protected is designated by reference numeral 80. Between the hull and a sensing half-cell 82, a signal voltage is developed which is representative of cathodic protection conditions on the hull, as illustrated by the graph 84 with a high at 86 and a low at 88. The sensing circuit so described includes a bucking or reference voltage derived from a potentiometer 90. The sensing half-cell voltage and the reference voltage are compared by a first transistor of the amplification stage designated in FIGURE 3 by the symbol 92. The graph following the amplifier 92 at its right-hand side illustrates the shape of an amplified signal 94 which appears at this stage with reversed polarity. The following stage 96 is a conventional component frequently referred to as a Schmitt Trigger for translating the signal into the shape of distinct pulses designated 98 in the corresponding graph. The signal is applied to a driver transistor 100 (graph 102) which, in turn, actuates the power transistor 104 performing as a protective current switch in the output circuit branch connecting a power supply 106 with the anode or anodes 108. It will be noted that the output or protective anode current, represented in graph 110, is in opposite phase with respect to that of the driver output, graph 102. Therefore, it is in phase with the original signal 84 obtained from the sensing helf-cell. Since the higher level of a signal indicates overprotection, the corresponding phase of the output is that with the output transistor 104 being cut-off, so that no protective current is supplied to the anode 108.

FIGURE 4 is a detailed circuit diagram of the system schematically shown in FIGURE 3 and described in the foregoing paragraph. Identical components are designated by the same reference numerals.

Transistor 112, which also constitutes one component of the amplifier 92 of FIGURE 3, compares the reference half-cell voltage with a voltage set on potentiometer 90. The voltage across the potentiometer 90 is regulated by means of the diode 114.

The difference between the desired hull potential, set on potentiometer 90, and the voltage generated by the reference half-cell 82 is amplified by

transistors

112 and 116. This amplified voltage appears across resistor 118 and is applied to the base of transistor 120. When this voltage exceeds a predetermined value, in practice approximately 5 volts, transistor 120 will conduct and transistor 122 will be cut-off to asume its non-conductive state, due to the coupling between them, known as a Schmitt Trigger and designated 96 in the block diagram of FIGURE 3.

When, on the other hand, the voltage at the base of falls below a predetermined value, in practice about 4 volts, transistor 122 starts to conduct. Consequently, the voltage across resistor 124 increases to cause transistor 120 to cut-off. While transistor 122 is conducting, current can pass from the base of the driver transistor 100. This causes the voltage appearing across resistor 126 to be nearly equal to the voltage appearing at the positive terminal 128 of the power supply 106, generally a l2 volt battery on small boats. Therefore, no current can pass from the base of current switch transistor 130, and no current is supplied to the anode or anodes 108.

When the voltage at the reference half-cell falls to reach a low, as indicated at 88 in FIGURE 3, indicating underprotection, transistor 112 draws less current, tran` sistor 116 draws more current, and the voltage developed across resistor 118 and at the base of transistor 120 increases. This causes transistor 120 to become conductive, and

transistors

122 and 100 assume their nonconductive states. Now current can Ipass from the base of power switch transistor 130 through resistor 126, and current to the anode or anodes 108 is available.

On the other hand, in the case of over-protection with the voltage at the reference half-cell 82 rising to a value such as that illustrated at 86 in FIGURE 3, transistor 112 conducts more current, transistor 116 conducts less current, and the voltages across resistor 118 and at the base of transistor 120 decreases.

Transistors

120 and 122 are cut-off and transistor 100 becomes conductive. No current can pass from the base of output switch transistor 130, since it is at a larger positive voltage than the emitter. When such conditions prevail, the voltage drop across transistor 100 may be 0.2 volt, while the drop across the rectifier 132, used to provide a voltage drop at the emitter of transistor`130, is 0.5 volt. As a result, no protective current is supplied to the anodes 108.

Rectifier 132 affords, additionally, temperature stability at transistor 130 for the off or non-conducting state.

Another rectifier, 134, and capacitor 136 are used to filter the voltage derived from power supply 106.

The remaining resistors, 140`158 and

capacitors

160 and 162 are used for providing proper potentials at different points of the circuits, and for forming filters, respectively. According to the embodiment of FIGURE 4, the meter 164 is continuously connectedto read the sensing half-cell voltage, and is used to set the reference voltage on potentiometer 90.

For completeness, the components in the clrcuit of FIGURE 4 are as follows:

0.001 microfarad.

Reslsiiir 50o Ohms.

11S 4,700 ohms. 124 470 ohms.

126 150 ohms. 140 18,000 ohms. 142 470 ohms. 144 27,000 ohms. 146 1,000 ohms. 148 100 ohms. 150 820 ohms. 152 1,500 ohms. 154 2,200 ohms. 156 470 ohms. 158 1,000 ohms.

Rectllle' type 1Nl692. 132 type 1N1128. 134 type 1N1692.

Tranllsor.. type 2Nl102. 116 type 2N1102. 120 type 2N1102. 122 type 2N1102. 100 type 2N1303. 130 type 2N1557.

Capalcllto 100 microfarads. 160 110 microfarads. 162 0.001 microfarad.

Meter 164 50 microamperes,

'2,000 ohms.

Power Supply 106 12 volt battery.

With the circuit of FIGURE 4, and considering that the system is designed to exhibit a sufficiently high power points, as illustrated at 86 and 88 in FIGURE 3. The

corresponding change of voltage at the base of transistor is then about l Volt. Under such circumstances, and with the constantly occurring fiuctuations of cathodic protection conditions on a hull supplying the sensing halfcell voltage to produce oscillations, thesystem illustrated in detail in FIGURE 4 performs such oscillations at a rate of about 20 per second. However, tests have shown that an oscillation rate of less than 1 per second is still sufiicient for successfully protecting a ships hull, while, on theother hand, no upper limit of frequency exists, theoretically speaking. With a small craft at rest, little current would be needed, so relatively short pulses at long intervals would be sufficient; when the craft is in motion, however, the length of ythe pulses would increase, and the off time would decrease, thus increasing the ratio of pulse time to o time.

It is to be understood that the above described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without. departing from the spirit and scope of the invention. i

What is claimed is:

1. A system for cathodic protection of a surface comprising an anode insulated from the surface, a source of electric power, a transistor connected to the source of power and t-o the anode to pass current to the anode in the form of pulses as the transistor is intermittently switched between its conductive and non-conductive states, a sensing half-cell insulated from said surface, and electric circuit means connected between the sensing half-cell and the transistor to switch the transistor to its conductive state and to vary the ratio of the duration of the pulses to the time between in proportion to the response of the half-cell when the negative potential of said surface decreases to a first predetermined level relative to the potential of the sensing half-cell and to switch the transistor to its non-conductive state when the negative potential of the surface increases to a second predetermined level relative to the potential of the half-cell, the negative potential of said second level being greater that the negative potential of said first level.

2. A system for cathodic protection of a surface comprising an anode insulated from the surface, a source of electric power, a rst PNP transistor having its emitter connected to the source of current and its collector connected to the anode to pass current to the anode in the form of pulses, a sensing half-cell insulated from said surface, a first NPN transistor having its collector connected to the source of current and its base connected to the sensing half-cell to become conductive in response to a difference in electric potential between the sensing half-cell and said surface when the potential on the surface exceeds a desired level, and circuit means between the first NPN transistor and the base of the first PNP transistor to energize the PNP transistor to its conductive state and to vary the ratio of the duration of the pulses to the time between in proportion to the response of the half-cell when the conductive level of the first NPN transistor falls below a first predetermined level and to switch the first PNP transistor to its non-conductive state when the conductive level of the first NPN transistor exceeds a second predetermined level which is higher than said first level.

3. A system according to claim 2 in which said circuit means comprises a second NPN transistor having its base connected to the source of power and to the collector of the first NPN transistor, said second NPN transistor having its collector connected to the source of power and its emitter connected to ground through a rst resistor, a third NPN transistor having its collector connected to the source of power and its base connected to the emitter of the second NPN transistor between said emitter and said rst resistor, a fourth NPN transistor having its collector connected to the source of power and its emitter connected to ground through to a second resistor, the third NPN transistor having its emitter connected to the emitter of the fourth NPN transistor between the emitter of the fourth NPN transistor and the second resistor, a capacitor and a third resistor connected in parallel between the collector of the third NPN transistor and the base of the fourth NPN transistor, and a second PNP transistor havings its base connected through a fourth resistor to the collector of the fourth NPN transistor, said second PNP transistor having its emitter connected t0 the source of power and its collector connected to the base of the first PNP transistor and through a fifth resistor to ground.

4. A system according to claim 2 in which a potentiometer is connected between the emitter of the rst NPN transistor and the source of power to apply a voltage through the sensing half-cell in opposition to the voltage produced through it by a difference in potential between the half-cell and said surface.

References Cited by the Examiner UNITED STATES PATENTS 2,021,519 11/1935 Polin 204--196 2,659,850 11/1953 Phillips et al. 318-341 2,752,308 6/1956 Andrus 204-196 2,759,887 8/1956 Miles 204-196 2,832,734 4/1958 Eckfeldt 204--195 2,912,635 11/1959 More 307-885 2,932,783 4/1960 Mohler 323-22 2,939,824 6/1960 Greenfield 204-67 2,962,651 11/ 1960 MCNamee 32322` 2,982,714 5/1961 Sabins 204-196 3,127,337 3/1964 Conger et al 204-196 FOREIGN PATENTS 657,392 9/1951 Great Britain.

OTHER REFERENCES Ewing: Gas Age-Record,7 March 9, 1935, pp. 219-222.

Fitzgerald et al.: Basic Electrical Engineering, 2nd ed., 1957, pages 414-417- NRL Report, No. 131, Ernest E. Nelson, March 1953.

25 JOHN H. MACK, Primary Examiner.

JOSEPH REBOLD, JOHN R. SPECK, WINSTON A.

DOUGLAS, Examiners.

Claims

1. A SYSTEM FOR CATHODIC PROTECTION OF A SURFACE COMPRISING AN ANODE INSULATED FROM THE SURFACE, A SOURCE OF ELECTRIC POWER, A TRANSISTOR CONNECTED TO THE SOURCE OF POWER AND TO THE ANODE TO PASS CURRENT TO THE ANODE IN THE FORM OF PULSES AS THE TRANSISTOR IS INTERMITTENTLY SWITCHED BETWEEN ITS CONDUCTIVE AND NON-CONDUCTIVE STATES, A SENSING HALF-CELL INSULATED FROM SAID SURFACE, AND ELECTRIC CIRCUIT MEANS CONNECTED BETWEEN THE SENSING HALF-CELL AND THE TRANSISTOR TO SWITCH THE TRANSISTOR TO ITS CONDUCTIVE STATE AND TO VARY THE RATIO OF THE DURATION OF THE PULSES TO THE TIME BETWEEN IN PROPORTION TO THE RESPONSE OF THE HALF-CELL WHEN THE NEGATIVE POTENTIAL OF SAID SURFACE DECREASES TO A FIRST PREDETERMINED LEVEL RELATIVE TO THE POTENTIAL OF THE SENSING HALF-CELL AND TO SWITCH THE TRANSISTOR TO ITS NON-CONDUCTIVE STATE WHEN THE NEGATIVE POTENTIAL OF THE SURFACE INCREASES TO A SECOND PREDETERMINED LEVEL RELATIVE TO THE POTENTIAL OF THE HALF-CELL, THE NEGATIVE POTENTIAL OF SAID SECOND LEVEL BEING GREATER THAT THE NEGATIVE POTENTIAL OF SAID FIRST LEVEL.