System for monitoring a cathodically protected structure

A corrosion monitoring system for measuring cathodic protection current flow. It is applicable to a buried structure that has cathodic protection applied to it. The monitoring system includes a microcoulometer connected between the buried structure and a small target electrode of a predetermined size.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns corrosion monitoring, in general. More 
specifically, it relates to a system for monitoring the effectiveness of a 
buried structure that is under cathodic protection for reducing corrosion. 
2. Description of the Prior Art 
Heretofore, there have been various proposals for making measurements or 
determinations relating to the rate of corrosion of a metal object or the 
like that is subjected to corrosive conditions. Thus, there are U.S. Pats. 
such as those to Marsh et al, No. 3,066,082, Nov. 27, 1962; Estes et al, 
No. 3,209,255, Sept. 28, 1965; Kanno et al, No. 4,130,464, Dec. 19, 1978; 
and Tejfalussy et al, No. 4,155,814, May 22, 1979. 
The Marsh et al patent does deal with determining conditions of protective 
coatings on a pipeline. However, it makes use of a specimen pipe that must 
be as similar as possible to a corresponding section of the pipeline. And, 
in addition, the specimen is separate and is electrically unconnected to 
the pipeline which is done in order to avoid complications relating to 
cathodic protection currents to which the pipeline is subjected. The 
applicant's invention, on the other hand, is a simple system that makes 
use of the cathodic protection current flow itself. And, it measures the 
quantity of current over an extended period of time in order to determine 
the effectiveness of the protection at a given location on a pipeline. 
The Estes et al patent merely discloses a particular type of current 
integrator. It employs a fritted disc in order to obtain osmotic current 
flow, and it basically acts as a coulometer per se which may be used in 
various ways. 
The Kanno et al patent deals with a relatively complicated electronic 
circuit arrangement for evaluating corrosion rates of metals and, 
consequently, is not relevant to the applicant's invention. 
The Tejfalussy et al patent provides for setting up a galvanic action 
circuit arrangement. And, it employs a plurality of electrodes situated 
near a metallic object that is to have the corrosion investigation thereof 
carried out. 
Thus, it is an object of this invention to provide a relatively simple yet 
highly effective system for monitoring the current flow that relates to a 
buried structure which is under cathodic protection. 
SUMMARY OF THE INVENTION 
Briefly, the invention concerns a system for monitoring a buried structure 
having cathodic corrosion protection applied thereto. It comprises a 
coulometer, and a target electrode buried adjacent to said structure. It 
also comprises circuit means for connecting said coulometer between said 
structure and said target electrode whereby said cathodic corrosion 
protection current density may be measured. 
Again briefly, the invention concerns a system having a buried structure 
that is subject to galvanic corrosion. The said structure is under 
cathodic protection which comprises an anode and a DC power supply 
connected between said structure and said anode for counteracting said 
galvanic corrosion. The invention concerns means for monitoring the 
quantity of current flow between said anode and structure, and it 
comprises a target electrode buried adjacent to said structure, plus a 
microcoulometer and low resistance circuit means for connecting said 
microcoulometer between said target electrode and said structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In connection with various pipelines and other buried metal structures 
where there is a cathodic protection system in operation, it is helpful to 
have some determination concerning the effectiveness of such protection. A 
cathodic protection system makes use of a DC power supply and includes a 
buried anode system. It causes current flow through the earth to the 
protected structure. When such cathodic protection is applied to a long 
pipeline, there are occasionally sections which suffer high corrosion 
rates and consequent rapid failure. And, by making use of a system 
according to this invention, such a location may be monitored to determine 
whether the cathodic protection system is functioning properly. 
Thus, with reference to the drawing FIGURE it will be noted that there is 
shown a buried pipeline 11 that is under cathodic protection. It has a DC 
power supply 12 electrically connected between an anode 15 and the 
pipeline 11. The electrical connection may be carried out by any feasible 
circuit connections, e.g. a circuit connection 16 that goes from the anode 
15 to the positive terminal of the DC power supply 12. Similarly, there is 
another circuit connection 17 that goes between the negative terminal of 
the power supply 12 and the pipeline 11. 
It has been determined that current densities which are required for 
protection of steel pipe will vary depending upon the protective coating 
in a range of several milliamperes per square foot, for bare steel, to a 
few microamperes or less for very well coated steel. And, of course, if 
the current density is known, it gives an indication that may be used in 
determining whether the cathodic protection being supplied is adequate. 
In order to monitor the current flow that is taking place due to the 
cathodic protection system described above, a system according to this 
invention may be employed. It makes use of a coulometer 20 that is 
connected between the pipeline 11 and a target electrode 21. Target 
electrode 21 is buried adjacent to the pipeline 11, and is between the 
pipeline and the anode 15 that is providing the cathodic protection 
current. 
The coulometer 20 may take various forms, but preferably it is a 
microcoulometer instrument. And, it may be like one manufactured by Curtis 
Instruments, Inc. of 200 Kisco Avenue, Mount Kisco, New York 10549. Such 
an instrument has a precision capillary tube (not shown) that is filled 
with two columns of mercury separated by an indicating gap of electrolyte. 
When direct current passes through the mercury column, the mercury plates 
from one column to the other at a rate that is governed by precise 
electro-chemical principles and, consequently, the gap moves along a scale 
and indicates the current-time integral. Structural details of such a 
coulometer are shown and described in a U.S. Pat. No. 3,665,308 issued May 
23, 1972. 
The target electrode 21 is made of a simple steel probe that has suitable 
surface area for a particular test to be made. It is inserted in the earth 
adjacent to the pipeline 11, and it intercepts some of the cathodic 
current so that the measurement it makes is a determination related to the 
current per square foot. Furthermore, because of the large difference in 
surface area between the pipeline 11 and the target electrode 21, the 
location of the target relative to the pipeline is not particularly 
critical. However, it is preferred that the target 21 should be located 
between one foot and fifteen feet from the pipeline 11. The exact location 
to be largely determined by the convenience in connection with burying the 
target 21. 
It will be noted that there are low resistance circuit connections 24 and 
25 that connect the pipeline 11 and the target electrode 21, respectively, 
with the coulometer 20. It will be appreciated that the physical 
connections to the pipeline 11 and to the target electrode 21 may be made 
in any convenient manner which will provide a low resistance electrical 
connection. 
As an example of the size of the current measurement which may be carried 
out by the system, it may be noted that if the system is to provide one 
milliampere of current per square foot to the pipeline 11 and the surface 
area of the target 21 is 0.01 square feet, then the target current (being 
measured by the coulometer 20) should be 10 microamperes. 
Since the microcoulometer 20 provides a current-time integral measurement, 
the scale reading after a known operating time may be converted to current 
density which may then be related to corrosion protection. 
It may be noted that there are nominal current densities for steel pipe 
corrosion protection, as was noted above. A table indicating such nominal 
current densities follows: 
______________________________________ 
CURRENT REQUIRED 
MILLIAMPERES PER 
CONDITIONS SQUARE FOOT 
______________________________________ 
Bare steel in earth 
1-3 
Poorly coated steel 
0.1 
Well coated steel 0.003 
Very well coated steel 
0.0003 or less 
______________________________________ 
It will be clear that by making use of a simple system according to this 
invention, the conditions at any particular location of a pipeline that is 
under cathodic protection, may be monitored so as to determine whether the 
cathodic protection is effective. Such monitoring is carried out by the 
simple system that connects a microcoulometer to the pipeline between it 
and a buried target electrode which has a predetermined surface area. 
Then, the coulometric measurement is made over an extended time period and 
it will provide the desired monitoring determination. This arrangement 
permits an unattended monitoring operation over a long time period, and 
consequently the cost of such monitoring is quite minimal. 
While a particular embodiment according to this invention has been 
described above in considerable detail in accordance with the applicable 
statute, this is not to be taken as in any way limiting the invention but 
merely as being descriptive thereof.