US3560359A

US3560359A - Corrosion protection

Info

Publication number: US3560359A
Application number: US825121A
Authority: US
Inventors: Richard L Hood
Original assignee: RICHARD L HOOD
Current assignee: RICHARD L HOOD
Priority date: 1969-05-05
Filing date: 1969-05-05
Publication date: 1971-02-02
Anticipated expiration: 1988-02-02

Abstract

A SOURCE OF FREE ELECTRONS, SUCH AS IS SUPPLIED BY THE GRID OF A CONDUCTING VACUUM TUBE, IS CONNECTED BY A SINGLE ELECTRICAL CONDUCTOR TO A METALLIC STRUCTURE TO BE PROTECTED FROM CORROSION. NO RETURN PATH FOR ELECTRONS FROM THE METALLIC STRUCTURE TO THE SOURCE IS PROVIDED AND IT IS NOT REQUIRED THAT THE PROTECTED STRUCTURE BE EARTH GROUNDED.

Description

Feb. 2, 1971 V HOOD 3,560,359

CORROSION PROTECTION Fil ed May 5 1969 I N VE N TOR Q0/5 20 A. #000 HTTOFA/[Ki United States Patent O 3,560,359 CORROSION PROTECTION Richard L. Hood, 10333 Dolecetto, Rancho Cordova, Calif. 95670 Continuation-impart of application Ser. No. 384,376, July 22, 1964. This application May 5, 1969, Ser. No. 825,121

Int. Cl. C231? 13/ U.S. Cl. 204-147 7 Claims ABSTRACT OF THE DISCLOSURE A source of free electrons, such as is supplied by the grid of a conducting vacuum tube, is connected by a single electrical conductor to a metallic structure to be protected from corrosion. No return path for electrons from the metallic structure to the source is provided and it is not required that the protected structure be earth grounded.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 384,376, filed July 22, 1964, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to corrosion protection and, more particularly, to apparatus for protecting a metallic structure from corrosion by connecting the structure to a source of electrons.

Review of the prior art and the problems addressed by the invention The process of corrosion of a metallic structure is essentially an electrolytic process in that corrosion involves a loss of electrons from the structure, and the presence of an electrolyte is required before corrosion will occur. The loss of electrons from the metal structure is usually localized in that some areas of the structure become positive with respect to other areas. Those areas which are anodic with respect to the remainder of the structure corrode fastest, although corrosion occurs at all points of the structure which are in contact with the electrolyte. Anodic areas are deficient in electrons relative to other areas of the corroding structure.

Many methods have been used with varying degrees of success to counteract electron loss from a metallic structure in contact with an electrolyte, thereby to prevent or inhibit corrosion. The simplest method is to cover the structure with a protective coating, such as paint, which prevents contact between the structure and the electrolyte, thereby preventing one area of the structure from becoming anodic with respect to another area of the structure in the presence of the electrolyte. One basic method of corrosion protection requires that the whole of the structure to be protected be made cathodic with respect to a sacrificial electrode. For example, the protected structure may be made the cathode of a galvanic cell in which the anode, connected to the structure by an electrical conductor, is more active in the galvanic series than the metal protected; a specific example is provided where Zinc plates are bolted to the hull of a ship proximate a bronze propeller to be anodic relative to the propeller in the presence of water (normally salt water but frequently fresh water) and to corrode in preference to the more expensive propeller. An electrolyte, such as water with impurities or moist soil, serves as the return path for current flowing in such a cell, and the cathode is protected at the expense of the anode. Another related method requires that a direct current potential be impressed upon the protected structure to render the entirety of the protected structure cathodic relative to earth ground or to a sacrificial anode which, in the case of an impressed potential, may be defined by a metal occupying a position lower in the galvanic series than the metal of the protected structure.

It has long been believed that a complete circuit must be made between the anode and the cathode in any cathodic protection system so that current may flow. Considerable attention has been directed to the amount of current which must flow between the anode (either sacri ficial or that defined by an impressed potential) and the protected structure in order to completely overcome all traces of local anodization of the structure.

A more complete description of prior efforts in the area of cathodic protection are set forth in Corrosion Hand book" by Herbert H. Uhlig, John Wiley & Sons, 1948, page 923 et sequor, and in Corrosion and Corrosion Control by Herbert H. Uhlig, John Wiley & Sons, 1963, chapter 12.

PROPOSED THEORY OF THE INVENTION The method and apparatus of this invention are based upon the discovery that the problem of electrolytic corrosion may be overcome merely by connecting the structure to be protected with a source of free electrons. Neither a sacrificial anode nor a return conductive path from the protected article to the source of electrons is required; this feature of the invention is distinguished from prior methods of corrosion protection or inhibition wherein a sacrificial anode is connected by means of an electrical conductor to the protected structure and a return path is made from the structure to the anode through the electrolyte. It has been found that the invention is effective in protecting all parts of a structure, such as a well casing or the like, even though the structure itself has parts which are electrically insulated from one another.

The precise mechanisms which are at work between the protected structure and an electrolyte during operation of this invention are not known with certainty. It is believed, however, that one area of the structure may tend to become anodic with respect to another area of the structure by reason of the presence of ions in the electrolyte contacting the structure. That is, if a positive ion moves into proximity to an atom in the metallic structure, the metal atom may tend to lose a valence electron to the ion and thereby become chemically active suificient to participate in the corrosion process which is generally agreed to be an electrochemical process; it will be understood, too, that the atom, by loss of an electron, becomes positive and renders the area of the structure in its vicinity anodic with respect to other areas of the structure Where similar events have not occurred. The longer the active atom remains deficient in a valence electron (i.e., remains chemically active), the greater the probability that the atom will participate in the electrochemical corrosion process. This invention reduces the probability that a chemically active atom (an atom missing one or more valent electrons) will participate in the corrosion process by making it possible for such atom to replace the lost electrons from within the metallic structure itself rather than by reacting chemically or electrochemically with the electrolyte. This electron replacement process is accomplished in such a way that another atom of the metallic structure to be protected is not required to give up an electron and thereby become susceptable to participation in the corrosion process with the electrolyte. The corrosion inhibition or prevention effect of this invention is directly related to the speed with which lost electrons are replaced.

The structure to be protected is provided with an excess of electrons, i.e., is connected conductively to a source of free electrons which are not associated with an atomic nucleus. Therefore, an alternate theory recognizes that when a positive ion in the electrolyte moves into proximity to an atom in the metallic structure, a discrete amount of force must be exerted on the valence electrons of the atom to overcome the forces acting on such electrons within the atom before one of such atoms can be lost to the ion. The force required to draw an excess electron from the metallic structure is less than the force required to draw an electron from an atom. Thus, the electrolyte satisfies its electron demands from the source of free electrons via the metallic structure more readily than from the valence electrons of the atoms in the structure, with the result that a significantly diminished number of atoms in the structure become involved in the corrosion process.

It is believed that these alternate theories of the operation of the present invention are compatible with the principles at work in the prior corrosion prevention and inhibition methods described above. The sacrifical anode method relies upon there being some other metal in contact with the electrolyte to respond to the presence of an ion in the electrolyte faster than the atoms of the principal metal of interest, i.e., the metal to be protected from corrosion. In the sacrifical anode method, the anode metal is higher in the electromotive table than the principal metal so that a galvanic cell is produced and negative ions in the electrolyte prefer to gather at the sacrificial metal rather than at the principal metal. The impressed potential method renders the entire body of the principal metal cathodic relative to ground or to some other conductive body so that even though isolated atoms of the principal metal may respond to ions in the electrolyte and become anodic relative to other areas of the same metal, such atoms are still cathodic relative to the electrolyte.

SUMMARY OF THE INVENTION Generally speaking, the present invention provides apparatus for preventing corrosion of a metallic structure. The apparatus comprises a source of free electrons and means for conducting electrons from the source to the metallic structure to be protected so that electrons lost from the structure to an electrolyte in contact with the metallic structure are replaced from the source more readily than from the electrolyte. The source, except for the conducting means connected from the source to the metallic structure, is electrically isolated from the structure and from earth ground.

As used herein the term source of free electrons encompasses a source in which electrons are not constrained within a crystalline lattice or released by an electrochemical reaction. A preferred source is the grid of a conducting triode-type vacuum tube.

BRIEF DESCRIPTION OF THE DRAWINGS The above mentioned and other features of this in vention are more fully set forth in the following detailed description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an apparatus according to this invention; and

FIG. 2 is a schematic diagram of a second embodiment of this invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS OF THE INVENTION FIG. 1 illustrates a well casing pipe extending vertically through a geological formation 11. The pipe is exemplary of a metallic structure which is to be protected from corrosion. FIG. 1 also illustrates a corrosion protection apparatus 12 which is essentially a source of free electrons in combination with means for maintaining the source. The protection apparatus is electrically connected to a pipe 10 by a conventional electrical conductor 13.

Corrosion protection apparatus 12 includes an isola tion transformer 15 having a primary winding 16 and a secondary winding 17, the windings preferably having a turns ratio of 1:1. The primary winding is connected to a source of alternating current such as a 117 volt AC source. The isolation transformed provides that direct current does not flow between the primary and secondary thereof, but that a voltage is generated in the secondary only by reason of the flux linkage between the primary and secondary. Secondary winding 17 is connected across a rectifier 19, shown in FIG. 1 as a diode vacuum tube 20, although any suitable rectifier may be used. Rectifier 19 preferably is the diode portion of a triode-connected 117L7 vacuum tube. A filter capacitance 21, preferably having a value of about 20 microfarads where a 117L7 tube is used, is connected in series between the transformer secondary and the cathode of the diode across a load resistance formed by

series resistors

28 and 31. Where a 117L7 tube is used in apparatus 12,

resistors

28 and 31 may have values of 47 and 6800 ohms, respectively. Thus the rectifier acts as a power source of a direct potential developed across the load resistance.

A triode 22 having a cathode 23, a grid 24 and an anode 25 is powered by the direct current source provided by the diode 19 and preferably is the triode portion of a triode-connected 117L7 vacuum tube. A load impedance, in the form of an anode resistor 27 in series with the resistance 28, is coupled across the anode and cathode of triode 22. Resistor 27 may have a value of 560 ohms. A common terminal 29 between resistors 27 and 28 is connected to the rectifying circuit between the diode cathode and capacitance 21, and a return 30 from the triode circuit to the rectifier circuit is provided from the opposite end of resistor 28 to the rectifier circuit between capacitance 21 and isolation transformer secondary winding 17. This arrangement provides a low plate load for triode 22 relative to the internal impedance of the triode. The resistor 31 is provided in the return connection from the triode to the rectifier circuit and the grid 24 is connected by a large grid resistor 32 to the junction between the

resistors

28 and 31 to impart a small amount of self bias to the triode. The resistors 31 and 32 are not essential but protect the triode from being overloaded. In view of the low plate load and small self bias of the triode, the triode is heavily conducting and grid 24 is maintained in a heavy stream of free electrons.

As noted above, grid 24 is connected to the well casing 10 by a conductor 13. The grid functions as means for collecting free electrons from the electron stream flowing between the cathode and anode of the triode. As incipient corrosion is manifested at the casing by the loss of an electron from a given area of the casing surface, another electron is immediately withdrawn from the electron stream to take its place. A given replacement electron may be derived from the surrounding metal, but the number of electrons originally present in the casing is maintained by connection of the casing to the free electron collector so that anodization of the casing is prevented and corrosion is inhibited.

There is no direct connection of the corrosion protector circuitry to ground and preferably the circuitry is housed in a completely insulated enclosure. In other words, but for conductor means 13 which extends only from grid 24 to casing 10, an open electrical circuit exists between the corrosion protection circuitry and pipe 10 so no return path for electrons from pipe 10 to this circuitry is provided.

Electrons flow from grid 24 to casing 10 via conductor 13 only when a demand for electrons exists at the casing by reason of incipient corrosion. Because no impedance, other than the impedance of conductor 13 itself, is present in conductor 13 between the grid and the casing, the grid is at the same potential as the casing. The electron flow which exists in conductor 13 is very small and is not a current in the conventional sense because no return path exists from the casing back to the circuitry of apparatus 12. The absence of current flow back to apparatus 12 from casing 10 has been confirmed by coupling a special infinite input impedance vacuum tube voltmeter across the input and output terminals of transformer 15; no direct current potential of either polarity was detected.

FIG. 2 shows a corrosion protector 35 connected to a gasoline storage tank '36, the tank being another example of a structure which is to be protected from corrosion. The corrosion protector is comprised of a battery 37 connected in series with a load impedance 38, the battery and load impedance being connected across a triode 39 having a grid 40 connected to the storage tank by a single electrical conductor 41. No return path is provided from tank 36 to protector 35.

The source of electrons to which the protected structure is connected in the practice of this invention must be a source of electrons which are free of chemical or interatomic bonds. A storage battery is not an acceptable source since a battery produces electrons only when a circuit is completed across the terminals of the battery. Also, the electrons associated with a storage battery are produced by electrochemical reactions, and the threshold forces involved in such reactions are of the same order of magnitude or greater than the forces associated with removal of an electron from casing 10, for example, by a contacting electrolyte, with the result that there is no preference by the electrolyte for a battery electron as opposed to an electron from the casing. Similarly, it is believed that conducting transistor or other solid state device may not be an effective source of free electrons because, in such devices, electrons move in a crystalline environment and may not be readily removable from such environment.

*It has been found that merely connecting the protected structure to a source of free electrons, such as the grids of

triodes

22 and 39, prevents corrosion in the structure. In a test of apparatus according to this invention, two identical steel rods were buried in moist soil for a period of five days. One of the rods was connected to a corrosion protector according to this invention. After five days it was found that the rod connected to the grid of the corrosion protector bore no trace of corrosion whereas the other rod was severely corroded. In subsequent tests, the circuit of FIG. 1 has shown the ability to control corrosion of oil pipe lines for indefinite periods with no apparent oxidizing, scaling or pitting of the metal pipe.

In another test it was determined that an electron flow exists between the corrosion protection apparatus and the protected article by inserting a high resistance megohms) in series with the grid of the apparatus. A small voltage drop was detected across the resistance with the use of a very high impedance meter.

The invention described above has the feature that no sacrificial anode is provided and not only is no provision made for a return path for electron flow from the protected structure to the source of free electrons, but such a return path is intentionally eliminated. Apparatus provided by this invention is extremely economical to 1,186.

A single corrosion protector may be connected to several protected structures if desired.

What is claimed is:

1. Apparatus for inhibiting corrosion of a metallic structure comprising a source of free electrons, and means for conductively connecting the source to a metallic structure in which corrosion is to be inhibited for flow of electrons from the source to the structure, the source but for the connecting means being conductively isolated from the structure and earth ground, the apparatus being characterized by the absence of a metallic return path from the metallic structure in which corrosion is to be inhibited to the source of free electrons.

2. Apparatus according to claim 1 wherein the source comprises a vacuum tube having a cathode, an anode, and a grid disposed between the anode and the cathode, and means for energizing the tube to produce a flow of electrons between the cathode and anode of the tube, the connecting means being connected between the structure and the grid of the tube.

3. Apparatus according to claim 2 characterized by the absence of an effective conductive connection from the structure to the anode or the cathode of the tube.

4. Apparatus according to claim 2 wherein the tube energizing means is effectively conductively isolated from earth ground.

5. Apparatus according to claim 2 wherein the energizing means comprises an isolation transformer having a primary winding adapted for connection to a source of alternating current, means for rectifying a signal developed across the secondary winding of the transformer, and means for applying the rectifying means output across said anode and cathode.

6. Apparatus for inhibiting corrosion of a metallic structure comprising a source of free electrons, and means for conductively connecting the source to the structure for flow of electrons from the source to the structure and arranged to define an open electrical circuit between the source and the structure, the apparatus being characterized by the absence of a metallic return path from the metallic structure in which corrosion is to be inhibited to the source of free electrons.

7. A method for protecting a metallic structure from corrosion comprising the steps of providing a source of free electrons, and supplying electrons to the structure from the source via a single conductor connected from the source in the absence of a return path for flow of electrons from the structure to the source.

References Cited UNITED STATES PATENTS 2,021,519 11/1935 Polin 204-1 96 TA-HSUNG TUNG, Primary Examiner US, Cl. X.R. Z041.96