US2943027A

US2943027A - Method and apparatus for determining current density

Info

Publication number: US2943027A
Application number: US689123A
Authority: US
Inventors: Schaschl Edward; Glenn A Marsh
Original assignee: Pure Oil Co
Current assignee: Pure Oil Co
Priority date: 1957-10-09
Filing date: 1957-10-09
Publication date: 1960-06-28
Anticipated expiration: 1977-06-28

Description

June 28,v 1960 E. SCHASCHL Er. 2,943,027

METHQD AND APPARATUS FOR DETERMINING CURRENT nsnsm 2 Sheets-Sheet 1 Filed Oct. 9. 1957 et mat Stu uOZEhE l :71 I l .l I I I I I l I I I I I l I I I l I I I m INVENTORS EWARD SCHASOHL GLENN A. MARSH June 28, 1960 E. SCHASCHL ETAL 2,943,027 uz'mon AND APPARATUS FOR DETERMINING CURRENT mzusm Filed Oct. 9. 1957 ZSheets-Sheet 2 INVENTORS EDWARD SOHASOHL Y GZENN A. MARSH ATTORN METHOD AND APPARATUS FOR DETERMINING CURRENT DENSITY Edward Schaschl and Glenn A. Marsh, Crystal Lake, Ill., assignors to The Pure Oil Company, Chicago, 111., a corporation of Ohio Filed Oct. 9, 1957, Ser. No. 689,123

14 Claims. (Cl. 204-1) This invention relates to new and useful improvements in methods and apparatus for measuring current density and corrosion rate on cathodically protected metal structures, such as pipes and the like, buried underground.

Any metallic structure, such as pipe, cable, tanks, rods, etc., when placed in an electrolyte such as wet soil or salt water, sets up potential differences between one portion and another, or between other metals adjacent thereto. Electrolytic action (i.e., current flow) resulting from the potential differences causes parts of the metal to go into solution thus causing pitting and electrolytic corrosion. One of the most eflicient methods of counteracting electrolytic corrosion is to make the buried metal structure cathodic instead of anodic by connecting it to a sacrificial metal anode of a more active metal such as aluminum or magnesium, or any other metal more active in the electrochemical series than the metal being protected.

In protecting buried or submerged metal'structures against corrosion, the submerged or buried portion of the structure is usually wrapped or otherwise coated with an impermeable material before installation. These coatings, however, are seldom completely effective since they unavoidably contain small holes, cracks, cuts and other imperfections which expose isolated areas of indeterminate size and location to the corrosive environment. In order to compensate for these imperfections and assure complete protection against corrosion, cathodic protection is employed. But heretofore it has been impossible to determine the amount of cathodic protection required, and in many installations over-protection has probably been resorted to in attempting to assure complete protection, with unnecessarily high expenditures resulting. On the other hand, it has been impossible to ascertain the corrosion rate at the exposed areas and it is also probable that unnecessary replacement costs have been experienced because of inadequate cathodic protection at isolated locations on the metalstructure. As a result, it has been impossible to evaluate the quality of workmanship in applying protective coatings and in designing cathodic protection except by noting the longterm effectiveness of the protection as'measured by the occurrence or non-occurrence of corrosion over a period of several years.

It is, therefore, one object of this invention to provide an improved method and apparatus for determining the current density at any point on the surface of a submerged or buried, coated or uncoated metallic structure.

Another object of this invention is 'to provide an improved method for determining the current density on a submerged or buried rnetallic structure and converting such determinations into corrosion rate, area of metal surface exposed to the corrosive environment, and/or current required for cathodic protection. v I A feature of this invention is the provision of a method andapparafus for determining current density on an un- I lyte to provide cathodic protection.

der'grou'nd metal structure located in an electrolyte, such as 'wet soil or salt water, and conected to a sacrificial anode buried adjacent .thereto, whichincludes an electrode of the samemetal as the metal structure buried adjacent to the metal structure and electrically connected to apoint on the surface thereof, means for impressing a voltage on the electrode to maintain the same at the same potential asthe point of connection relative to said anode, andmeans for measuring the current flow from the electrode to the anode, whereby the current flow thus measured divided by the surface area of the electrode is equal to the current density on the metal structure.

Another feature of this invention is the provision of a process and'apparatus which includes, in combination with the apparatus just described for measuring current density, a half-cell connected to the buried electrode and an inert electrode buried adjacent to the electrode, and means to impress a selected potential difference between the half-cell and the first-named electrode corresponding to th potential occurring with complete cathodic protection, and measuring the current flow between the inert electrode and the first-named electrode, the difference between the current measurement obtained in determining current density and the current measurement obtained in the last-named reading being proportional to the corrosion rate on the buried metal structure.

Other objects and features of this invention will become apparent from time to time throughout the specilication and claims as hereinafter related.

In the accompanying drawings, to be taken as a part of this specification, there are clearly and fully illustrated three preferred embodiments of this invention, in which drawings, v

Figure l is a schematic wiring diagram of an apparatus for measuring the current density on cathodically protected buried metal structures,

Figure 2 is a schematic Wiring diagram of apparatus for measuring current density and corrosion rate on cathodically protected buried metal structures, and

Figure 3 is a schematic diagram of a modification of the apparatus shown in Fig. 2 for measuring current density and corrosion rate ona buried metal structure.

This invention is based upon our discovery of an improved apparatus and method for determining current density on an underground metal structure located in an electrolyte such as wet soil .or salt water and con.- nected to'a sacrificial metal anode buried in the electro- In this apparatus and method an electrode ofthe same metal as the buried metal structure is positioned in the electrolyte in close proximity to the surface of the underground metal structure. The electrode is then connected to an adjacent point on the metal structure and to parallel circuits which include a high-resistance voltmeter in one leg and a battery,va'ri'able resistor, and an ammeter inthe other leg. The variable resistor is adjusted to maintain the electrode and the point of connection on the underground metal structure at the same potential relative to the anode. The current flowing from the electrode to the anode, as measured by the ammeter, divided by the surface area of the electrode is equal to the current density on the metal structure. In embodiments of the invention in which corrosion rate is to be determined, the electrode is connected through a second voltmeter to a coppercopper sulfate half-cell and is also connected through a variable resistor to one side of a battery, the other side of which is connected to an ammeter which is in turn connected to a buried inert electrode. When the variable resistor in this circuit is adjusted to maintain a potential between the half-cell and the first-mentioned electrode, corresponding to the potential, relative to the half-cell when the metal structure is "provided with complete protection, the current flowing between the two electrodes, as measured by the second ammeter, differs from the current measured by the first ammeter by an amount proportional to the rate of corrosion on the underground metal structure.

Referring to Fig. 1, there is shown an application of our method and apparatus to the determination of the current density at area 1 of a metal pipe 2. The pipe 2 has been provided with a suitable coating 13, buried, and connected to a sacrificial metal anode 3 by lead wire 4. Area 1 on pipe 2 is connected by lead wire 10 to terminal 14 on instrument 5, known as a zero-resistance ammeter. The zero-resistance ammeter 5 consists of a vacuum tube voltmeter 6 connected in parallel with a battery or other DC). power source 7, variable resistor 8, and ammeter 9. The parallel circuits are joined at

terminals

14 and 15. Terminal 15 of zeroresistance ammeter 5 is connected by lead wire 12 to electrode 11 which is made of a metal substantially identical to that of pipe 2 and buried in the electrolyte in close proximity to area 1 on pipe 2.

In employing this method and apparatus, variable resistor 8 is adjusted so that voltmeter 6 registers zeropotential between

terminals

14 and 15. When this condition exists, area 1 on pipe 2, and electrode 11 are at the same potential with respect to anode 3. Anode 3 normally functions with the metal of pipe 2 as a battery, indicated diagrammatically by the dotted lines 16 having the polarity as indicated. Under these conditions, the circuit results in a flow of current from electrode 11 to anode 3 through wire 4, pipe 2, wire 10, zero-resistance ammeter 5 and wire 12. The current flowing in this circuit from electrode 11 into the electrolyte or corrosive environment is indicated by ammeter 9. This current, 1 is proportional to the potential, E of electrode 11 with respect to anode 3, and is inversely proportional to the surface resistance, R of the electrode. This is represented by:

Also, R is inversely proportional to the surface area of the electrode:

(Equation 1) n (Equation 2) B where k is a constant dependent upon the metal of which electrode 11 is made and the environment in which it is contained. Combining equations 1 and 2 gives 11 A11 (Equation 3) I and rearranging gives 7 In k11- (Equation 4) Eu =11 I Current density, CD, on electrode 11 is defined as the total current divided by the total surface area, 01',

and, because the metals of area 1 and electrode 11 are similar and are contained in the same environment,

(Equation 10) k =k Therefore, (Equation 11) CD: CD.

Knowing the surface area, A, of electrode 11 and the total current, 1 flowing from it (as indicated by ammeter 9), CD. can be calculated from the expression,

In C.D.

Thus, our method permits determination of the current density at area 1, orat any other given point on the surface of pipe 2. In making a determination of current density using this method and apparatus it should be noted that full sensitivity and accuracy is obtainable only when the small openings in the coating 13 on pipe 2 are sufficiently large so that they are not the limiting resistance in the circuit. Thus, this method tends to be somewhat inaccurate when the pipe is exposed to the electrolyte only at very minute points. In such a case, the resistance to electrical flow within the minute openings is such as to introduce indeterminate errors into the method.

As an example of the utility of determining current density at exposed portions of pipe of indeterminate size, it should be noted that the determination of current density may be used to determine the amount of surface actually exposed to the electrolyte or corrosive environment. To obtain this value, the total current flowing to pipe 2 from anode 3 through lead wire 4 is determined by means ofan ammeter (not shown). The average current density along the entire length of pipe between (Equation 12) adjacent anodes is determined by applying this apparatus .and method at a number of spaced intervals. The exposed area (A due to imperfections in coating 13, is then calculated by dividing the total current, I by theaverage current density, C.D. determined by the several measurements with this apparatus,

From this value the percent of total pipe surface area exposed due to imperfections in coating 13 may be calculated by dividing the exposed area, A by the calculated total area of the pipe. This measurement can be used in determining the quality of coating 13 applied to pipe 2 and may even be used as a contract specification of required quality for pipe installations.

In Fig. 2 there is shown another embodiment of this invention in which the apparatus described and shown in Fig. l-is provided with a switching means, a coppercopper sulfate'half-cell, an inert electrode, and a corrosion-rate meter for determining corrosion rate at area 1 on pipe 2. In this embodiment of the invention, area 1 on pipe 2 is connected by lead wire 10 to terminal 14 on zero-resistance ammeter 5, as in Fig. 1. Zero-resistance ammeter 5 consists of vacuum-tube voltmeter 6, connected in parallel with battery or other DC. power source 7, variable resistor 8, and ammeter 9, both of which are connected to

terminals

14 and 15. From terminal 14, lead wire 12 extends to electrode 11. In this embodiment of the invention, however, lead wire 12 is broken and provided with switch 17 for connecting or disconnecting electrode 11 to and from the remainder of the current-density measuring circuit. From lead wire 12 there is provided a connection to one side of voltmeter 25, the other side of which is connected to a copper-copper sulfate half-cell 26 which functions in combination with electrode 11 as a battery 31, indicated by-the dotted lines and having the indicated polarity. Lead wire 12 is also connected to one side of corrosionrate meter 22, the other side of which is connected iii,

through switch 21 to inert electrode 23. Corrosion-rate meter 22 includes variable resistor 24 and ammeter 27 connected on opposite sides of battery or other D.C. power source 28. i

In operating this apparatus to measure corrosion rate;

switch 17 is first closed and switch 21 opened so that electrode 11 is connected to the current-measuring circuit in the identical manner shown and described in connection with Fig. 1. The variable resistor 8 is then adjusted to establish a zero potential between

terminals

14 and 15 as measured by voltmeter 6. The current reading between electrode 11 and anode 3, as indicated by ammeter 9, may then be used for calculating current density at area 1 of pipe 2.

Switch 17 is then opened and switch 21 closed to connect electrode 11 in circuit with electrode 23. Variable resistor 24 is then adjusted to establish a potential dif-' ference of 0.85 v. between inert electrode 23 and halfcell 26, is indicated by voltmeter 25. Ammeter 27 then indicates the corrosion'rate of electrode 11 when converted to the proper units. The arithmetic difference between the measured readings of meters 27 and 9, when converted to the proper units, is the true corrosion rate at area 1 on pipe 2. While this apparatus is described in connection with a copper-copper sulfate half-cell 26, other half-cells can be used provided that the potential difference between half-cell 26 and inert electrode 23 corresponds to the potential produced relative to the half-cell when the metal structure is provided with complete cathodic protection.

In Fig. 3 there is shown a modification of the apparatus described in'Fig. 2. In this embodiment of the invention switches 17 and 21 are replaced by single-pole double-throw switch 17a which is connected to electrode 11 by Wire 12a. When switch 17a is closed against contact 12b electrode 11 is connected in circuit with the current-density measuring circuit and measurements are takenas described for Fig. 1. When switch 17a is closed against terminal 120, electrode 11 is connected in circuit with electrode 23 and corrosion-rate measurements are taken as described in connection with Fig. 2. All of the other parts of the apparatus in Fig. 3 are identical to those described and identified in Fig. 2 and are given the same reference numerals.

While we have fully and completely described our invention, including several preferred embodiments thereof, as required by the patent laws, we wish it understood that within the scope of the appended claims this invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method for determining current density on an underground metal structure located in an electrolyte and connected to an anode buried in said electrolyte to provide cathodic protection comprising, connecting an electrode of the same metal and having the same electrical resistance constant as said metal structure for a given electrolyte to a point on the surface of the underground structure and positioning the electrode in the electrolyte in close proximity to said point of connection, impressing a voltage on said electrode to maintain the same at the same potential as said point of connection relative to said anode, and measuring the current flow from said electrode to said anode, said current flow divided by the surface area of said electrode being equal to the current density on said metal structure in the region of said point of connection.

2. A method according to claim 1 in which the metal structure is at least partially covered with a corrosion-resistant non-conductive protective coating, and the ratio of the measured current density to the current produced by the anode independently of the impressed voltage rep 3. A method according to claim 1 in which there is tially zero-resistance ammeter, connected in series be-- tween said electrode and the point of connection on said metal structure, a voltmeter connected inparallel with said battery, variable resistor, and ammeter, said resistor being adjustable to impress the required potential on said electrode, and the ammeter measuring the flow of current from said electrode to said anode.

4. A method for determining the rate of corrosion on an underground metal structurelocated in an electrolyte and connected to an anode buried in said electrolyte to provide cathodic protection comprising, connecting an electrode of the same metal and having the same electrical resistance constant as said metal structure for a given electrolyte to a point on the surface of the metal structure and positioning the electrode in the electrolyte in close proximity to said point of connection, impressing a voltage on said electrode to maintain the same at the same potential as said point of connection relative to said anode, measuring the current flow from said electrode to said anode, connecting said electrode to a standard reference half-cell and positioning said half-cell in said electrolyte adjacent to said electrode, connecting said electrode to an inert electrode and positioning said electrode in said electrolyte adjacent to said first-named electrode, impressing a voltage between said first-named electrode and said inert electrode to establish a predetermined pocurrent flow between said inert electrode and said firstnamed electrode at said predetermined potential, with the difierence between said first-measured current and said last-measured current being proportional to the rate of corrosion of the metal structure. V

5. A method according to claim 4 in which said halfcell is a copper-copper sulfate half-cell and said predetermined potential is 0.85 v. I

6. A method according to claim 4 in which said inert electrode is disconnected from said first-named electrode during said first-named current measurement and said first-named electrode is disconnected from said metal structure and connected to said inert electrode during said last-named measurement.

7. A method accordingto claim 4 in which there is provided a first circuit portion including a battery, variable resistor, and substantially zero-resistance ammeter, connected in series between said first-named electrode and the point of connection on said metal structure, and a voltmeter connected in parallel with said battery, variable resistor, and ammeter, said resistor being adjustable to impress the required potential on said first-named electrode, and the ammeter measuring the flow of current to' said anode, a second circuit portion including a battery, variable resistor and ammeter, connected in series between said first-named and said inert electrodes, and a voltmeter and said half-cell connected to said first-named electrode in parallel with said second circuit portion, said variable resistor in said second circuit portion being adjustable to establish said predetermined potential difference between said half-cell and said first-named electrode as measured by said last-named voltmeter, and the ammeter in said second circuit portion measuring the current flow between said inert electrode and said firstnamed electrode.

8. A method according to claim 7 in which there is provided switch means for disconnecting said inert electrode from said first-named electrode during said firstnamed current measurement, and for disconnecting said named current measurement. I

9. In combination with an underground metal structur located in an electrolyte and connected to an anode buried in said electrolyte to provide cathodic protection therefor, an apparatus for determining current density on the metal structure comprising, anelectrode of the same metal and having the same electrical resistance constant as said metal structure connected by parallel circuits to a point on the surface of said metal structure and positioned in said electrolyte adjacent to the surface of said metal structure, one leg of said parallel circuits including a battery, variable resistor, and substantially zero-resistance ammeter connected in series between said electrode and said metal structure, and the other leg of said parallel circuits including a voltmeter connected between said metal structure and said electrode in parallel with said battery, resistor, and ammeter, the current reading of said ammeter divided by the surface area of said electrode being equal to the current density on said metal structure when said resistor is adjusted to establish said electrode at the same potential as said point of connection relative to said anode.

10. An apparatus according to claim 9 in which the metal structure is at least partially covered with a corrosion-resistant non-conductive protective coating, and the ratio of the measured current density at said established potential to the current produced by the anode independently of the impressed voltage represents the surface area of the metal structure not covered by the protective coating. 7

11. In combination with an underground metal structure located in an electrolyte and connected to an anode buried in said electrolyte to provide cathodic protection therefor, an apparatus for determining current density and rate of corrosion on the metal structure comprising, an electrode of the same metal and having the same electrical resistance constant as said metal structure, connected by parallel circuits to a point on the surface of, said metal structure and positioned in said electrolyte adjacent to the surface of said metal structure, one leg of said parallel circuits including a battery, a variable resistor, and substantially Zero-resistance ammeter, connected in series between said electrode and said metal structure, the other leg of said parallel circuits including a voltmeter connected between said metal structure and said electrode in parallel with said battery, resistor and ammeter, the current reading of said ammeter divided by the surface area of said electrode being equal to the current density on said metal structure when said re- Sister is adjusted to establish said electrode at the same potential as said point of connection relative to said anode, an inert electrode positioned in the electrolyte adjacent said first-named electrode, a battery, variable resistor and ammeter connected in series between said electrodes, a standard reference half-cell positioned in the electrolyte adjacent said electrodes, a voltmeter connected to said half-cell and to said first-named electrode, and switch means connecting said first-named electrode to said metal structure and disconnecting the same from said last-named electrode during measurement with said firstnamed voltmeter and ammeter, said switch means connecting said electrodes together and disconnecting the same from said metal structure during measurements with the last-named voltmeter and ammeter, the current flow indicated by said last-named ammeter when said firstnamed electrode and said half-cell are maintained at a selected potential difference diifering from the current flow indicated by said first-named ammeter by an amount proportional to the rate of corrosion of said metal structure.

12. Apparatus according to claim 11 in which said half-cell is a copper-copper sulfate half-cell and the selected potential difference between the half-cell and said first-named electrode is 0.85 v. v

13. Apparatus according to claim 11 in which said switch means comprises a pair of switches, one switch being connected between said first-named electrode and said metal structure, and the other switch being connected between said electrodes.

14. Apparatus according to claim 11 in which said switch means comprises a single-pole double-throwswitch with said first-named electrode connected to the pole of the switch and the switch terminals connected in circuit with the metal structure and the inert electrode, respectively.

References Cited in the file of this patent UNITED STATES PATENTS 2,697,673 Rice Dec. 21, 1954 2,795,759 Rezek June 11, 1957 2,803,797 Cowles Aug. 20, 1957 OTHER REFERENCES Metallic Corrosion, Passivity and Protection, Evans, Edward Arnold & Co., London, 1948, page XXXIH.

Claims

1. A METHOD FOR DETERMINING CURRENT DENSITY ON AN UNDERGROUND METAL STRUCTURE LOCATED IN AN ELECTROLYTE AND CONNECTED TO AN ANODE BURIED IN SAID ELECTROLYTE TO PROVIDE CATHODIC PROTECTION COMPRISING, CONNECTING AN ELECTRODE OF THE SAME METAL AND HAVING THE SAME ELECTRICAL RESISTANCE CONSTANT AS SAID METAL STRUCTURE FOR A GIVEN ELECTROLYTE TO A POINT ON THE SURFACE OF THE UNDERGROUND STRUCTURE AND POSITIONING THE ELECTRODE IN THE ELECTROLYTE IN CLOSE PROXIMITY TO SAID POINT OF CONNECTION, IMPRESSING A VOLTAGE ON SAID ELECTRODE TO MAINTAIN THE SAME AT THE SAME POTENTIAL AS SAID POINT OF CONNECTION RELATIVE TO SAID ANODE, AND MEASURING THE CURRENT FLOW FROM SAID ELECTRODE TO SAID ANODE, SAID CURRENT FLOW DIVIDED BY THE SURFACE AREA SAID ELECTRODE BEING EQUAL TO THE CURRENT DENSITY ON SAID METAL STRUCTURE IN THE REGION OF SAID POINT OF CONNECTION.