US3442779A - Anodic protection of metals - Google Patents

Description

May 6 1959 GAR. HoEY 3,442,779A

ANODIC PROTECTION OF METALS Filed July 26. 1965 Sheet of 4 CONT/POIL INVENTOR Gordon Randolph HOEY AGENT G. R. HOEY ANoDIc PROTECTION o'F 'METALS May 6, 1969 Z of 4 Sheet Filed July 26, 1965 INVENTOR v Gordon Randolph HOEY AGENT May 6 1969 G. R. Hor-:Y

ANODIC PROTECTION OF METALS Sheet Filed July 26, 1965 K7 www AGENT Sheet Filed July 26, 1965 CONTROL/ff? 50 /ME/MM/UTES/ INVENTOR Gordon Randolph HOEY AGENT United States Patent O Claims priority,

Inf. c1. czsf 13/00 U.S. Cl. 204-147 3 Claims ABSTRACT OF THE DISCLOSURE Corrosion of a metal in a corrosive medium is minimized by applying a predetermined direct current potential between the metal and an inert cathode immersed in the medium, the potential being applied in a pulsed form when the nobility of the metal decreases below a predetermined value suicient to maintain it in a passive state.

This invention relates to an improved method for minimizing corrosion in vessels containing corrosive solutions.

It is known to control the corrosion of metallic vessels by various solutions through the use of corrosion inhibitors. Theseprotective agents dissolve in the corrosive liquids and by their action inhibit the dissolution of the metallic material. This method of inhibiting corrosion has the disadvantage, however, that a separate material must be added to the soluti/on and it is not always possible to find suitable inhibitors.

It is also known to protect a metallic object against corrosion by making it the cathode in an electrical circuit. This method has been used for the protection of iron in near neutral solutions such as sea water but it has not been found effective tor protection against acid solutions such as sulphuric acid. However, it has been found that if the metallic object is made the anode in an electrical circuit, corrosion may be minimized by inducing passivity as disclosed by C. Edeleanu, at page 739 of vol. 173 of the 1954 issue of Nature. In this article, it is disclosed that if the potential difference between the metallic object and an inert cathode is maintained within a certain range, the metal becomes passive. It is stated that passivity willegrist only within a certain range of potentials. Below and above this potential range, passivity will not exist and corrosion will take place. Edeleanu suggest the use of a potentiostat to maintain the potential difference between the metal and the inert cathode at a constant value. This requires complicated apparatus and it is not always feasible to use such a method in commercial practice.

It is an object of this invention to provide an improved method for minimizing corrosion of metals in acid solutions. An additional object is to provide an apparatus adapted to minimize corrosion by vthe technique of anodic protection. Additional objects will appear hereinafter.

The improved method of this invention comprises insertingan inert cathode in the corrosive medium which is in contact with the metal to be protected and applying a predetermined direct current potential between said inert cathode and said metal, said potential being applied in pulsed form when the nobility of the metal to be protected as measured by a reference electrode decreases below a predetermined value, said predetermined value being sulicientto maintain the metal in a passive state.

' The improved method of this invention is illustrated by reference to the accompanying drawings wherein:

FIGURE 1 is a diagram-matic view of an apparatus suitable for applying pulses of electrical current to a tank requiring anodic protection;

3,442,779 Patented May 6, 1969 ICC FIGURE 2 is a diagrammatic view of an electronic relay and control system suitable for use inthe method of this invention;

FIGURE 3 is a schematic diagram of an electronic controllery suitable for controlling the application of pulses of current to the tank to be protected;

FIGURE 4 illustrates the application of the invention to a steel tank;

FIGURE 5 is a diagrammatic view of an alternative embodiment of the apparatus wherein a single electrode is employed both as cathode and reference electrode;

FIGURE 6 illustrates the operation of the apparatus of FIGURE 1 compared to operation of the apparatus of FIGURE 5. 1

Referring now to FIGURE l, the metal tank containing corrosive liquid is shown at 1 and theA inert cathode is shown at 2. A battery 3 is so connectedthat the tank becomes the anode with respect to the inert cathode 2. Application of electrical current to the tank is brought `about by a relay 4 which is controlled by a controller 5. The controller 5 operates the relay t9 provide a pulse of electrical current whenever the potential difference between a reference electr-ode 6 and the tank 1 reaches a predetermined value, such predetermined value being set on 7. The circuit including ythe batterycontains a fuse 8, a manually operated switch 9 and a resistance 10.

A suitable controller comprises a high resistance moving coil meter tted with limit controls which can be adapted to actuate a latching relay-timing circuit whenever the impressed voltage reaches a predetermined value.

In FIGURE 2, an alternative type of controller and associated relays are shown in diagrammatic fashion. The voltage from a reference electrode 6 is compared with a voltage of opposite polarity and of predetermined value provided by a standard cell 11. The resultant electric current is interrupted by an electromechanical chopper 12, the output from which is amplified by an AC amplifier 13. The output from the amplifier 13 is applied to a phase sensitive demodulator 14, the output frdm which is either positive or negative direct current. This direct current is amplified in a DC amplifier 15, the output from which is fed to a trigger circuit 16. The trigger circuit becomes operative only when a positive signal is applied to it. This positive signal will be produced only when the potential difference between the reference electrode 6 and the metal tank has fallen below the value at which the tank is in a passive condition with respect to corrosion. The pulse from the `trigger circuit causes the multivibrator to generate a rectangular pulse of predetermined width. This pulse acts through a relay driver 18 to close relay 19 which, through the intermediary of an intermediate relay 20, operates a power relay 21 to apply current between the cathode 2 and the tank 1 being protected from corrosion. 22 is a counter to permit the recording of the amounts of power employed for anodic protection.

In FIGURE 3 is shown a schematic diagram of a controller suitable for the production of pulses of electrical current when the nobility of the object being passivated, as determined by means of a reference electrode falls below the value corresponding to the passive state. In the diagram 23 is a connection to a reference electrode. Current from the reference electrode circuit passes through a lter system comprising a resistor 24, and inductance 25 and two capacitors 26 and 27, and is then opposed to the potential from a standard cell 28 reduced to a predetermined -value by a voltage divider 29. The setting of 29 is such that it corresponds to a potential difference between the object being anodically protected and the reference electrode at which the object being protected is in 4a passive state. The resultant current passes through a resistor 30 to an electromechanical chopper 31 where it is converted into a 60 cycle square wave suitable for AC amplification. The output from the chopper is applied to the grid of an electron tube 32 which, with its associated circuitry, serves to amplify the signal. The output from this amplifier is applied to the grids of electron tube 33. This tube serves to amplify the signal still further. The plate circuit of tube 33 comprises a phase sensitive demodulator including transformers 34 and 35. The output from the demodulator is direct current, either positive or negative. The direct current output from the demodulator is fed to the grid of the first section of an electron tube 36, the resistance and capacitance of the circuit containing 36 being such that only positive direct current will drive it into conduction. The output from the electron tube section 36 is applied to a neon tube 37 through a diode 38. When a positive pulse is applied to tube 36 this causes 37 through its associated circuitry to act as a relaxation oscillator. The negative-going pulses from this oscillator are amplified by the second section of the electron tube, 36'. The amplified positive pulses from 36 are used to drive a trigger tube 39 which energizes the one-shot multivibrator comprised of tube 40 and its associated circuitry. This multivibrator provides a single rectangular Wave pulse, positive-going, which is then applied to the grid of the relay driver consisting of the second half of the electron tube, 39. The duration of the pulse from the multivibrator can be adjusted by a variable resistance 41. When a positive pulse is applied to the grid of electron tube 39', the tube becomes conductive and actuates relay 42. Relay 42 is a double pole double throw type; one set of contacts is connected to actuate an intermediate relay shown as 20 in FIGURE 2 and the other set of contacts, when closed, will short the grid of electron tube 36 to ground. This shorting of the grid of electron tube 36 ensures that a second pulse will not be applied to the multivibrator during the time that a pulse is being generated by it.

It has been found that suitable value for the components of a controller of the type shown in FIGURE 3 are as follows. Electron tubes:

32 Dual triode, type 5691. 33, 36-36', 39-

39', 40 Dual triode, type 5692. Silicon diodes:

38, 43, 44 Silicon diodes type IN2071. Resistors:

29 200,000 ohms, 100 turn Helipot.1 41 5 megohms, variable. 24 100,000 ohms, 1 watt. 30 15,000 ohms, l watt. 45, 48, 49, 51, 54, 59 and 76 1 megohm, 1 watt. 46, 47 3,900 ohms, 1 watt. 50 4,700 ohms, 1 watt 52, 53 2,200 ohms, 2 watts. 55 560,000 ohms, 1 watt. 56 150,000 ohms, 1 watt. 57, 58, 70, 71,

72, 73 22,000 ohms, 2 watts. 61, 62, 74 220,000 ohms, l watt. 60 3,300 ohms, l watt. 69 100,000 ohms, 2 watts. 68 10 megohms, 1 watt. 67 20,000 ohms, 20 watts. 66 68,000 ohms, 2 watts. 63, 64 27,000 ohms, 1 watt. 65 100,000 ohms, 1 watt. 68,000 ohms, l watt. 77 15,000 ohms, 2 watts.

Capacitors:

26, 27 5 mfd., 200 volts, paper. 78, 79 25 mfd., 25 volt. 80 10 mfd., 150 volts, paper. 81 25 mfd., 50 volts.

82 0.5 mfd., 600 volts.

4 Capacitors:

83, 85, 88, 98 0.1 mfd., 600 volts. 84, 97 2.0 mfd., 600 volts. 86 1.0 mfd., 600 volts. 87 100 picafarad. 89 0.04 mfd., ceramic. 90 4 mfd., 450 volts. 91 40 mfd., 450 volts. 92, 93, 94 0.05 mfd., 600 volts.

Neon lamps:

37 type NE2. 95, 96 type NES l. Inductance:

25 60 henrys.

1 Helically Wound, continuously variable resistor.

It is to be understood that other types of controllers may be employed in this invention. For example the output from the reference electrode may be amplified by a DC amplifier with adjustable amplification, the amplified output actuating a latching relay-time delay relay system. The controller provides the means for closing the power relay for a set time interval whenever the reference electrode indicates that a predetermined voltage has been reached.

The method of this invention can be carried out by using the same electrode alternately :as both current-carrying cathode during the on period of operation and as reference electrode during the off -period of operation.

The alteration in the electrical circuit required to accomplish this different mode of operation comprises disconnecting from reference electrode 6 (FIGURE l) the electrical connection linking the reference elecUode 6 to controller 5 and attaching said connection to electrode 2 which serves to carry the passivating current. The modified circuit is illustrated in FIGURE 5 wherein the connection from controller 5 to electrode 2 is made at point 61 in the pulsed current-carrying circuit. The electrode 2 may be of the same or of different material from that of the metal being protected. The difference in electrical potential between the electrode 2 and the metal 1, necessary for operation of controller 5, may arise from a difference in nobility between electrode and metal, or from a standard potential applied to the electrode by the controller. When the metal being protected is iron, a platinum coated electrode is suitable. Also it is necessary that the controller employed have a high internal resistance of such value that only a minor current will flow through the controller during operation.

Since the signal derived from the reference electrode serves only to initiate the pulse of passivating current and since the controller is designed in such a manner that one pulse must be completed before another pulse can be initiated, the two functions of the same electrode do not interfere.

The method of this invention is most suitable for the protection of metals such as steel, stainless steel and titanium when used in acid service. Although its utility is not restricted to one particular acid, it has been found that this invention is most useful for protection of metals against corrosion by sulphuric acid.

The cathode employed for the passivation of the metal in the method of this invention may be more noble than the metal being protected. Suitable materials for use in steel protection are carbon and platinum. It is of course necessary that the cathode material itself be inert and do not dissolve in the corrosive medium.

The reference electrode should be one capable of measuring potential differences in the range at which the metal is in a passive state. The silver-silver chloride electrode and the calomel cell are suitable.

It is necessary that the reference electrode be inert to the corrosive liquid. It has been found that the silversilver chloride electrode is very suitable since it may be immersed directly in the acid medium.

In the carrying out of the method of this invention for the protection of a steel tank containing acid, an inert electrode such as carbon is inserted in the tank and insulated from the tank shell; the tank itself is connected to the power source acting as an anode. Also inserted in the tank and insulated therefrom will be the reference electrode. A high capacity source of electric power such as a group of storage batteries can conveniently be employed as power source. This power source is connected through an electrically operated switch to the tank and the inert cathode. The reference electrode is connected to the controller of the type described hereinbefore. The control mechanism must be adjusted so that when the potential as indicated by the reference electrode falls below the value at which the tank is in a passive state, the controller will actuate this switch to apply a pulse of power to the tank. It has been found that a suitable duration for the pulse is one second. However, pulse duration as low as 1/10 second to 2 or 3 seconds would be suitable. In estimating the pulse duration, it is necessary to consider the operation of the high capacity switches used for carrying the current.

Although it is known that if a continuous excessive voltage is applied in a method of anodic protection, the corrosion will increase, this disadvantage has not been experienced when power is applied in a pulsed form. In the method of this invention, when the power is applied to the circuit comprising the object to be protected and the inert electrode it is possible that the potential rises above the value at which passivity is produced on the metal surface. However, due to the pulsed nature of the application of this excess voltage, it would appear that corrosion is not increased.

Employing the apparatus described hereinbefore for applying pulsed power for anodic protection, the duration of the pulse is manually controllable, the pulse spacing being dependant upon the rate of fall of tank potential with respect to the reference electrode. The voltage applied depends upon the power source. It is of course possible to vary this voltage by adjustment of the resistance of FIGURE l.

The invention is further illustrated by the following examples:

EXAMPLE l A mild:v steel tank of volume equivalent to 7,300 cubic centimeters and internal area of 2,100 square centimeters was used containing sulphuric acid of concentration 66.0 B. (96%). During the experiment the temperature of the acid was in the range of 25 to 50 C. The cathode employed was platinum. The reference electrode was a silver-silver chloride cell immersed in the sulphuric acid. The power source was a 4 volt battery. The circuit cmployed was similar to that shown in FIGURE l using as controller a high resistance (20,000 ohms) moving coil meter with adjustable limit controls actuating a latching relay. The controller was set so that pulses of l second duration would be applied between the platinum cathode and the steel tank whenever the tank potential fell below 0.7 volt with respect to the silver-silver chloride reference electrode. The ofi-time (when the circuit was opened) depended upon the timerequired for the tank potential to decay to 0.7 volt with respect to the silver-silver chloride reference electrode. Using a 4 volt battery the pulses would apply a voltage of 3.3 volts (vs. Ag-AgCl electrode) to the tank during the on period. The anodic passivation experiment was maintained over a period of 4 days. During this time, the iron concentration of the acid was determined. Determination of the iron content of the acid was also made after anodic protection had ceased. The change in concentration of iron in the sulphuric acid during the 4 days of anodic protection and in the period after anodic protection had been discontinued is shown in FIGURE 4 and it can be seen that the method of this invention maintains corrosion at a low level in comparison to corrosion if protection is not given. During the experiment itis noted that the off-time increased with increasing time. In other words at the beginning it required more current to maintain a passive condition than after the first passive state had been reached. Thus, the electrical charge consumed for unit time is high at the commencement of anodic protection change, but decreases with anodic protection time.

The following example illustrates an alternative mode of carrying out the method of this invention.

EXAMPLE 2 A mild steel model tank containing 7000 cc. of 96% sulphuric acid at 23 C. was employed. The area exposed to the acid was 2100 cm?.

As comparison, anodic protection of the tank was carried out using bot-h the system of FIGURE l and the system of FIGURE 5. The duration of the on period of operation was set at 2 seconds and current during the on period was approximatelyI 0.3 ampere during both experiments.

In the system of FIGURE 1 a platinum cathode and silver-silver chloride reference electrode were employed, the controller being adjusted so that the potential of the tank interior with respect to the silver-silver chloride electrode was maintained equal to or greater than 0.51 volt. The potential of the tank was 0.28 volt with respect to the silver-silver reference electrode before passivation. 'I'he tank potential was therefore more noble after passivation than before passivation.

In the system of FIGURE 5 a platinum electrode was employed to serve both as current carrying cathode and reference electrode. A silver-silver chloride electrode separate from the passivating circuit was employed as standard for setting the value of tank potential at which the controller actuated the switch. The controller thus was adjusted so that the potential of the tank interior with respect to the silver-silver chloride electrode was maintained equal to or greater than 0.47 volt. During the experiment the silver-silver chloride electrode was employed to monitor the potential of the tank and thereby veried that the tank interior was maintained in a passive state.

A comparison of the effectiveness of the two systems was made by recording the duration of the time between the on periods of operation for both systems over an interval of 2%. hours. The results are shown in FIGURE 6. It can be seen that the progress of passivation is substantially the same in both cases.

I claim:

1. A method for minimizing corrosion of a metal in a corrosive medium which comprises applying between an inert cathode immersed in said corrosive medium and said metal a fixed-duration pulse of direct electric current of potential at least equivalent to the potential at which said metal is in a passive state, said pulse being applied only when the nobility of said metal as measured by a reference electrode falls below a value suicient to maintain said metal in a passive state the measurement of nobility by said reference electrode being made when no current is being applied to passivate said metal.

2. An apparatus for minimizing corrosion of a metal immersed in a corrosive medium which comprises an inert electrode for immersion in said medium; a source of direct electric current; a circuit means connecting said direct current source to said inert electrode and said metal in such direction as to make the metal the anode; a switching means in said circuit; a reference electrode for immersion in said corrosive medium and adapted to indicate the potential difference between said reference electrode and said metal; and connected to vsaid reference electrode and said switching means a controller means adapted to provide, in response to the indication by said reference electrode of a predetermined potential difference, fixedduration control pulses for actuating said switching means to apply corresponding ixed-duration pulses of current to said circuit, each control pulse being initiated in response to the indication of said reference electrode, each control pulse being initiated after the termination of the preceding current pulse.

3. An apparatus for minimizing corrosion of a metal immersed in a corrosive medium which comprises an electrode for immersion in said medium; a source of direct electric current; circuit means connecting said direct current source to said inert electrode and said metal in such direction as to make the metal the anode; a switching means in said circuit; and connected to said electrode, metal and switching means a controller means adapted to provide, in response to the attainment of a predetermined potential difference between said electrode and said metal, fixed-duration control pulses for actuating said switching means to apply corresponding fixed-duration pulses of current to said circuit, said electrode acting alternately as reference electrode and as conductor of pulses of electric current, each control pulse being initiated in response to the attainment of a predetermined potential difference between the electrode and the metal, each con- 8 trol pulse being initiated after the termination of the preceding current pulse.

References Cited UNITED STATES PATENTS 3,242,064 3/1966 Byrne 204-196 3,280,020 10/1966 Conger 204-196 3,317,415 5/1967 Delahunt 204-196 OTHER REFERENCES Sudbury et al.: Corrosion, vol. 16, no. 2, February 1960, pp. 47t-54t.

Ewing: Gas Age-Record, Mar. 9, 1935, pp. 219-222.

HOWARD S. WILLIAMS, Primary Examiner.

T. TUNG, Assistant Examiner.

U.S. Cl. X.R.

Images