US3098026A

US3098026A - Cathodic protection system

Info

Publication number: US3098026A
Application number: US766147A
Authority: US
Inventors: Edward P Anderson
Original assignee: Engelhard Industries Inc
Current assignee: Engelhard Industries Inc
Priority date: 1958-10-08
Filing date: 1958-10-08
Publication date: 1963-07-16
Anticipated expiration: 1980-07-16
Also published as: NL243852A; DE1150555B; DK110203C; FR1244505A; GB913895A

Description

July 16, 1963 E. P. ANDERSON CATHODIC PROTECTION SYSTEM Filed Oct. 8, 1958 2 Sheets-Sheet 1 Refe rev 0 e Half Cell [Hull I Il/z/ZZ lirmde] Reference Half Cell INVENTOR.

EDWARD P. ANDERSON BY VJ M ATT EYS July 16, 1963 E. P. ANDERSON 3,

CATHODIC PROTECTION SYSTEM Filed Oct. 8, 1958 2 Sheets-Sheet 2 FIG. 3

Hall I l/ull flrwae Reference Hall I 64 Reference flZ/ll flelf Cell INVENTOR. LL J M. EDWARD P. ANDERSON BY KM IM 5M 6". M) ATTOR EYS United States Patent 3,098,026 CATHQDKC PRGTECTEQN SYSTEM Edward P. Anderson, Livingston, N..l., assignor, by rnesne assignments, to Engelhard industries, inc Newark, NJ a corporation of Deiaware Filed Get. 8, 1958, Ser. No. 766,147 1 @laim. (CL 2ti4-1%) This invention relates to a system for cathodically protecting the hull of a ship and, more particularly, is concerned with such systems for small craft, etg. cabin cruisers below 100 feet in length, or the like.

Cathodic protection of ships or other metallic objects in contact with an electrolyte, commonly water, consists basically of applying an electrical current to an anode immersed in the Water and to the surface to be protected as the cathode, i.e. the hull. The hull surface potential is maintained cathodic with respect to the water, thus preventing corrosion of the surface. in the case of painted metallic surfaces such as ship hulls, however, when the impressed current density exceeds a certain amount, the paint film will become damaged Whereas an insufficient current results in inadequate protection against corrosion. Therefore, an optimum exists and cathodic protection systems have been suggested, in which the fluctuating potential on the metallic surface is continuously registered and the power output to the anodes is controlled accordingly, either by manually or automatically adjusting the output.

In order to register the potential on the hull, reference electrodes immersed in the water and insulatingly mounted on the Wetted portion of the hull are commonly used which perform, in cooperation with the hull surface, as a voltaic cell to generate a current the strength of which is a function of the condition of the hull and is utilized to adjust the output of the system. However, by constantly drawing current from this cell, the reference half cell electrode deteriorates within a short time and must be replaced. Furthermore, the reference half cell electrade is subjected to polarization which impairs the generated current to an extent beyond control so that such a reference electrode does not constitute a reliable means for registering the actual potential on the hull.

The present invention is concerned with a method and a system which avoids the aforementioned disadvantages and is designed for small craft such as cabin cruisers to meet the special requirements thereof.

Manual adjustment of the protective current supplied to one or more anodes is provided in such a fashion that one single electrical meter continuously indicates the current output. The reference half cell cooperating with the hull forms a control circuit which is a normally interrupted circuit and closed only when the same galvanometer is inserted therein to close it. The system for cathodically protecting the hull of a ship according to the invention therefore consists basically of two independent circuits, a protective current circuit and a control circuit, the latter being energized by the reference half cell. The meter can be connected alternately and selectively into one or the other of the two circuits. When forming a part of the protective current circuit, it is suitably connected across a series shunt resistance therein to perform as a voltmeter so that the protective current is not interrupted when the electrical meter is disconnected therefrom to form a part of the control circuit in series connection, thereby monitoring the condition on the hull surface. This is the case for a short time and necessary only from time to time when readjustment of the protective current is desired. However, the system can be handled by untrained personnel, r

since no erroneous connection can be made, and the protective current is not interrupted even during the ICC above mentioned short period, whereas the control circuit is closed exclusively during this period so that no current is drawn from the reference half cell during normal performance of the system.

The invention provides, further, the use of a second, substituting current supply which commonly consists of the equipment required for feeding the protective current circuit from a shore base network. Small craft such as cabin cruisers, when not in use, remain parked at a dock for extended periods of time and, in order to prevent corrosion without drawing current during this time from the inboard current source, cathodic protection is afforded by supplying the current from the shore base network by cable. The device on board comprises an automatically operated switch system in the form of one or more relays which disconnects the current source on board, usually a battery, upon connecting the system to the network and, when the network is disconnected, re-inserts the battery into the circuit such that continuous cathodic protection of the hull is ensured even when the network supply fails.

The invention will be further illustrated by reference to the accompanying drawing in which FIGURE 1 is a simplified schematic circuit diagram of a cathodic protection system,

FIGURE 2 is a similar circuit diagram of a cathodic protection system provided with a substituting power supply,

FIGURE 3 is a diagram of a modified circuit of FIG- URE 2 in which the substituting power supply can be used as a battery charger, and

FIGURE 4 is another modification of the circuit of FIGURE 2.

Referring to FIGURE 1, a cathodic protection system according to the invention basically comprises a protective current circuit which includes the hull, a current source e.g. a battery 10, a shunt resistance 12 and an adjustable resistor 14 and, as a circuit independent therefrom, a control circuit energized by the voltaic cell: reference half cell and hull. An electrical meter 16 is connected to the armature 18 of a double-pole, doublethrow switch 29 which has two positions. In the first position of the switch, the meter 16 is connected to the

terminals

22 and 24 of the shunt resistance 12, in the second position it is inserted into the control circuit in series. Since the control circuit is interrupted when the switch 20 is in its first position, no current will be drawn from the reference half cell during normal performance of the system, except for the comparatively short time required for reading the meter 16. Suitably, the switch 20 is a type which, urged by a spring, automatically returns in its first position when released, similarly to a push-button switch. On the other hand, the protective current circuit remains closed over the shunt resistance 12 in whatever position the switch 20 may be so that continuous cathodic protection is afforded to the hull. The circuits as shown in the drawing have been simplified by omitting elements which do not form part of this invention i.e. fuses, pilot lights, main switches etc. The resistor 24 in the control circuit serves for reducing the current therein to a range appropriate to be registered by the meter 16 and thereby additionally increasing the life of the reference half cell since less current is drawn therefrom.

The circuit shown in FIGURE 2 includes, additionally to the elements identical with that of FIGURE 1 and designated by the same reference numerals, a second, substituting power supply. In accordance with this modification, the terminals 36) and 32 of an isolation transformer 34 are adapted to be connected to the network of a shore base by means of a cable. Suitably, the transformer 34 is dimensioned such that the output voltage from the secondary winding is identical with that of the inboard cur- =2 rent source 10, commonly a 12 volt battery. The output from the transformer 34 is converted into a direct current, e.g. by a full-

wave rectifier

36, 38 and supplied to the protective current circuit through a relay 40 which immediately operates to disconnect the battery 10. Energized by the rectified output current through the coil 42 when connection is made with a shore base network, the relay armature 44 operates to break the contact with the contact point 46, thereby disconnecting the battery 10, and makes contact with the contact point 48. Since the rectified negative output pole is connected to the hull, the system is now energized only by the network current supplied to the terminals 30, 32. On the other hand, upon disconnection or failure of the shore base supply, the relay coil 42 is de-energized and cathodic protection from the battery It is restored.

In the circuit shown in FIGURE 3 the rectified network current is utilized additionally for recharging the inboard battery 10. This is accomplished by means of a second relay 50 which, when energized by the rectified output current from the transformer 34, connects the positive output terminal 52 to the positive pole 54 of the battery over the contact 56 and the armature 58 when the circuit is closed by actuating a switch 60. A resistor 62 delimits the charging current to an amount sufiiciently small to permit a protective current to pass through the relay 40,

resistors

12 and 14 and to the anode so that cathodic protection of the hull is ensured during the charging period. All other elements shown in FIGURE 3 are connected and perform as described in connection with FIGURES 1 and 2.

The phenomenon of cathodic protection is based on the fact that a polarized hydrogen film is formed on the cathodically charged hull. This hydrogen film is constantly removed by oxidation and this oxidation rate increases when water flows over the polarized surface. For this reason, greater current is required to maintain the proper potential on a hull moving through the water than on a stationary hull. A cathodic protection system which automatically compensates therefor is shown in FIGURE 4. There is an additional resistor 64 provided and connected across the variable resistor 14, the connection being made and interrupted by the armature 66 cooperating with a contact 68 of a relay 70. The

terminals

72 and 74 of the coil 76 of the relay are inserted into the ignition circuit (not shown) of the ship engine and therefore energized when the ship is in motion. Consequently, the resistor 64 will be connected over the contact 68 and the armature 66 in parallel with the adjustable resistor 14 to decrease the total resistance value in the protective current circuit and, as a result, the power output to the anode is increased.

Whereas any type of adjustable resistance may be used for the resistor 14, it has been found useful to provide a set of fuse-type plugs to permit the operator to select that which has the appropriate resistance value and to insert it in the fashion of a fuse at a convenient location on the panel of the apparatus. Thereby, an upsetting of the system by accident or error is eliminated to a large extent. In normal practice for operation in a given locality, a particular resistance plug can be used for months until paint failure on the hull requires the substitution of a plug having a lower resistance. The electrical meter forming part of the system is suitably an ammeter of a conventional type which performs as such when inserted in the control circuit, but performs as a voltmeter when forming part of the protective current circuit. In order to facilitate operation of the system by untrained personnel, the scale of the meter may be divided into appropriate color zones indicating insufiicient protective current, optimum current and excessive current. The fullwave rectifier, illustrated in FIGURES 2 and 3 comprised of the center-tapped secondary winding of the transformer 34 and the

rectifiers

36 and 38, can be replaced by a bridge-type rectifier. Furthermore, a pair of single contact relays can be used instead of the relay 40, whereby the first operates as a normally closed relay in such a manner that the battery it is inserted in the circuit, while the second relay is normally open and closes its contact only when energized by the rectified network current.

It will be obvious to those skilled in the art that many more modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

In a system for cathodically protecting the hull of a ship driven by an engine provided with an electrical ignition circuit, at least one anode, a cathodic protection current circuit connected to said anode, variable resistance means connected in series in the protective current circuit, an additional second resistor, and a relay responsive to energization of the ignition circuit for connecting said second resistor across said variable resistance means to increase the protective current.

References Qited in the file of this patent UNiTED STATES PATENTS 2,370,871 Marks Mar. 6, 1945 2,402,494 Hantzsch et al. June 18, 1946 2,483,397 Bonner Oct. 4, 1949 2,759,887 Miles Aug. 21, 1956 2,803,797 Cowlcs Aug. 20, 1957 2,903,405 Sabins Sept. 8, 1959 FOREIGN PATENTS 503,946 Great Britain July 9, 1937 OTHER REFERENCES Logan et al.: The Petroleum Engineer, Reference Annual 1943, pages 168-180.