INTRODUCTION
It is always a pleasant surprise to find remnants of past civilizations,
be they large unexplored ruins or small objects such as projectile-points. All such
remnants share a common factor in that they have been and are subjected to corrosion
processes. These processes are the result of wind, rain, soil burial and electrochemical
processes.
Two interesting cases relating archaeological remnants to cathodic protection are
presented in reference 5. The first case concerns the discovery of iron objects which were
buried in corrosive soil, yet these objects exhibited a high state of preservation. The
preservation, in this case, was directly related to the fact that the objects constituted
part of a human burial, which also included leather items and bones. The leather items
produced tannates which impede the growth of sulfate-reducing bacteria. The bones (animal
and human) produced phosphates, which in turn, through a complex chemical action, formed a
protective film around the iron objects. The second case involved the finding of a Viking
sword that was buried with a warrior. As was fairly customary during the Viking Age, a
warrior’s armament and equipment were "killed" prior to burial. The sword
in this case was bent, as a method of "killing." When found, the sword exhibited
corrosion only localized around the bend, the area of maximum stress. The rest of the
sword was well preserved. This phenomena relates to stress corrosion.
Although unplanned, both the above cases illustrate corrosion protection processes.
Corrosion of buried metallic objects, such as underground piping, is associated with both
the flow of electricity and the chemical processes within the soils surrounding the
objects. Twenty to thirty years ago an accepted and often used method of corrosion
protection for buried pipeline was to increase the wall thickness of the piping. This
simply had the effect of adding time to the corrosion process and extending the life of
the pipeline. Since that time, more economical and efficient corrosion control methods
have been developed.
The control of external corrosion on buried metallic objects is now generally achieved
by a combination of one or more complementary procedures of coating, backfilling and
cathodic protection. Geotechnical investigations play a very important role in determining
some of the design parameters for cathodic protection systems.
BASIC PRINCIPLES
The type of electrical circuitry which sustains corrosion processes in soils,
for example on a pipeline, is a function of differing soil properties along the route of
the pipeline, relationship of the pipe to other metallic objects in the same soils and the
type of metal used in the connections on the pipe route. This flow of electricity is
associated with an electrochemical process which can cause corrosion products, such as
rust, on the metallic surface which is discharging the electrical current. The
flow of electricity being discharged from one metallic surface, termed the anode, is
completed by the passage of the current from the electrolyte (in this case the soil) to
the same or other metal objects nearby. The metallic object receiving the electrical
current is termed a cathode. The total amount of metal removed from the pipe in the
corrosion process is directly related to the amount of electricity flowing in the
completed circuit.
CATHODIC PROTECTION
Cathodic protection is a process in which an electric current is forced to flow
from an auxiliary anode into the structure to be protected against corrosion, thus making
the entire structure a cathode. Cathodic protection, if necessary, is normally applied to
properly coated structures to minimize the cost of the total corrosion control system.
In practical terms cathodic protection may be applied by the use of sacrificial
anodes or power impressed groundbeds.
Sacrificial anodes are manufactured of materials which are more noble in the
electromotive series and would corrode preferentially when connected to mild steel or cast
iron. Such anodes, generally cast of magnesium, aluminum or zinc, are installed alongside
the pipelines in the trench line at intervals determined by design and connected to the
pipelines via a test post facility.
Power impressed systems comprise an a.c. powered transformer-rectifier which
provides a controllable d.c. output. The positive terminal is connected via a single core
cable to a groundbed consisting of high silicon iron or graphite anodes laid in a trench
or borehole located generally 50 to 150 meters from the structure to be protected.
When current is supplied, the anode will discharge electrical current into the
ground, the underground structure will receive the discharged current, thus protecting the
structure from corrosion. These systems can become very complex, requiring careful design
and construction.
In order to properly design cathodic protection systems, preconstruction
analysis of the following factors is necessary:
1. The type, grade, length and size of the piping to be used will determine the
electrochemical reaction processes that will take place when the pipe is placed in soils
of differing electrical characteristics.
2. A knowledge of the anticipated life of the piping will give an insight into
the type, and hence expense of the cathodic protection system most suitable for use.
3. The electrical properties of the soils along the pipeline route are analyzed
to provide information on the type of electrical reaction that may take place between the
pipe and the adjacent soils. As an example, the lower the soil resistivity, the higher the
current that will flow from the pipe into the soil.
4. The chemical and bacteriological characteristics of the soils along the
piping route are analyzed to determine the type of electrochemical reaction that may take
place and to determine the type of oxidation process that may exist. Oxidation processes
may promote corrosion.
5. The types of available materials applied as coatings to pipelines form an
essential criterion to the design of a cathodic protection system. Generally, cathodic
protection requirements may be reduced when a high quality coating is achieved.
6. Details of other corrosion control practices within the immediate area of
construction should be investigated. This is necessary so that a compatible cathodic
protection system can be designed for newer structures.
7. Existing facilities within the immediate area of construction should be
investigated in order that suitable precautions may be taken to design compatible cathodic
protection systems and to provide information on stray currents.
8. In order to maintain cathodic protection levels, the buried structure or pipe
must be electrically isolated from all other metalwork. Analysis of pipeline connections
to other structures will provide information relating to the electrical isolation
procedures that would be necessary prior to the application of cathodic protection.
Cathodic protection is a surface protection only, and if installed for the
external surface will have no effect on internal corrosion. The control of internal
corrosion cannot always be readily achieved. For large diameter pipes, internal coatings
can be readily applied and inspected. However, the internal coating of small diameter
pipes (less than.50-75 millimeters) is particularly difficult from a practical viewpoint
and inspection cannot easily be performed. A discontinuous coating may lead to localized
corrosion, creating a worse situation than having no applied coating.
Where internal corrosive conditions are anticipated, a corrosion monitoring
system may be used where test "coupons" are installed within the pipe and
regularly inspected to evaluate the corrosion rate.
COATINGS AND BACKFILL
Although these are not cathodic protection procedures, they are complimentary to
cathodic protection systems. The use of coatings and backfill will only be mentioned
briefly, as they were not considered in detail for the case history to be discussed.
The use of pipe coatings or wrappings reduces the area of the underground
structure exposed to the electrolyte (soil),
hence reducing corrosion. Coatings may take the form of paints, greases,
bitumen, or coal tar, which may be reinforced with fibrous material and epoxy resins.
Wrappings consist of protective materials in the form of plastic sheets or tapes. All
coatings and wrappings should have a high electrical resistance, should resist abrasion
and should be alkali resistant in certain types of soils. When applied to underground
structures, coatings and wrappings should adhere strongly to the surfaces to be protected,
should exhibit no blisters and should be applied in continuous form such that no gaps are
left on the structural surface. Coatings may be applied to the metallic structure either
prior to or upon arrival at site. It is preferable that the trench backfill materials are
chemically inert, for instance, washed sand, but economic and availability constraints at
construction sites may preclude its use.
MEASUREMENT OF EARTH RESISTIVITY
The common factor that appears to relate soils to the corrosion processes is the
resistivity of the soils. Measurement of soil resistivity is therefore a prime concern in
cathodic protection considerations.
In electrical resistivity surveying, the most common method used to measure
earth resistivity is to drive a current through the ground using galvanic contacts.
Normally a four electrode system is used, driving the current through two of the
electrodes and measuring the established potential in the earth with the other two
electrodes. The four electrode system offers an advantage over a two electrode system in
that resistivity measurements are heavily affected by the properties of the soils close to
the electrodes, and the use of four electrodes reduces this effect.
The choice of electrode configuration is governed by the type of investigation
desired. In studying the lateral variations in resistivity across a site, a fixed
electrode separation is used and maintained, and the whole electrode array is moved along
a line of profile. This electrode configuration is termed the Wenner array. In studying
the vertical variations of resistivity with depth, the spacings between the electrodes are
gradually increased, which enhances the effects of materials at depth with larger
electrode spacings. This latter electrode configuration is termed the Schlumberger array.
Normally, soil resistivity surveys for cathodic protection studies are performed
using the Wenner configuration. The Schlumberger array offers advantages over the Wenner
array in that the former is more convenient from an operational point of view, local
inhomogeneities close to the potential electrodes can be rapidly located on the apparent
resistivity curves and the theoretical computation or the apparent resistivity curves can
be performed more rapidly and with less assumptions than similar computations for the
Wenner array. The Schlumberger array does not provide for detailed information on lateral
changes in the resistivity values across a site. A schematic of the Schlumberger array is
shown on Figure 1. The use of this type of array produces a vertical electrical sounding
(VES).
For the specific case history to be discussed, the site was one of eleven fairly
large sites to be investigated within a rigid time schedule. Most of the sites
investigated were large enough to prohibit the use of the Wenner array due to the
excessive amount of time that would be required. Therefore the Schlumberger array was
selected for use in the field investigations. With the spacings used between successive
vertical electrical soundings, lateral changes in soil resistivity across each of the
sites can be measured with sufficient detail for the purposes of a cathodic protection
investigation.
EQUIPMENT
Instrumentation for most electrical resistivity surveying is relatively simple.
Current is normally provided by dry batteries in the form of a single long direct-current
surge. The current electrodes (A, B) are normally steel or copper-clad steel stakes driven
into the ground. Frequently in dry areas, the soil around the electrodes may have to be
moistened to insure proper contact with the ground.
The voltage between the potential electrodes (M, N), which are also made of
steel or copper-clad steel, is measured with a
potentiometer or a voltmeter. Contact with the soil of the potential electrodes
is not as important as with the current electrodes.
The equipment selected for this investigation consisted of a dry battery unit
capable of voltage outputs from 50-400 volts DC, an amperimetric unit capable of measuring
current intensity from 300 millamps to 3 Amps (with 1 percent accuracy), and a
millivoltmeter capable of measuring voltages from +1 millivolt to +100 volts at an
accuracy of 1 percent. This equipment was supplemented by using steel electrodes, and
lightweight, well insulated copper cables to connect all systems to the array. Spacings
used for this investigation are shown on Figure 1.
INTERPRETATION OF MEASUREMENTS
In the Schlumberger array, the depth to which the resistivity is averaged is
roughly equal to half the separation between the current electrodes. This resistivity is
computed using the formula:
p = p (L2) (deltaV)
(2l)
(I)
where L = the half-separation between the current electrodes (A,
B) measured in meters;
l = the half separation between the measuring electrodes (M, N)
in meters;
AV = the voltage at the measuring electrodes, in volts;
I = the current between the current electrodes, in amperes.
This formula is based on the assumption that the ratio V/l is approximately
equal to the voltage gradient, E, at the center of the electrode array.
The resistivity as calculated from the above equation is not necessarily equal
to the resistivity of the portion of the earth over which the measurement was made, due to
the influence of the electrical properties of one layer on another layer. For this reason
the value obtained from the above equation is termed the apparent resistivity. The
interpretational process consists of deducing a likely set of true resistivity values
which would be compatible with the observed apparent resistivity values. In many cases,
there exists no single set of true resistivities that correspond to a particular set of
apparent resistivities, and as such, the true resistivity cannot be uniquely determined.
The apparent resistivity which would be measured over a series of uniform
horizontal layers presents a fairly simple case, and since this is frequently a good first
approximation to gelogic conditions, the first step in the interpretation process normally
involves determining what layer thicknesses and resistivities can explain the measured
apparent resistivities.
Numerous investigations have produced resistivity curves for various geologic
conditions using theoretical data. These curves have been plotted in terms of
dimensionless variables and form the basis for the curve-matching technique of resistivity
interpretation.
A curve of the field data, in which the values of the observed apparent
resistivity are plotted against electrode spacing on logarithmic graph paper, will have
the same shape as the theoretical curves plotted in terms of dimensionless variables. By
plotting the field data on graph paper of the same scale as the theoretical curves, the
field data curve may be laid over the theoretical curves. The field curve is moved until
the field points correspond or match with the points of the theoretical curves. The only
requirement is that both sets of curve axes must be kept parallel. The depths and
corresponding true resistivities are then read off of the field matched theoretical curve.
It must be remembered, however, that there are available many differing sets of
theoretical curves for many types of geologic settings and the curve matching process may
become time consuming.
LABORATORY TESTING
Since corrosion processes are electrochemical in nature, it is
advisable to perform chemical analyses of soil samples from the vicinity of a proposed
buried object. Soil acidity (pH) should also be determined, as this can be related to the
corrosion potentials of soils. Finally, if possible, groundwater samples should also be
analyzed.
Laboratory values of electrical resistivity in soil samples can play a very
important role in determining cathodic protection design parameters. For example, dry
soils that may be periodically subjected to irrigational water may be enhanced in their
corrosion potential. It may be very difficult to study this effect in the field, as it may
be impractical to saturate the on-site materials. This effect, that of enhanced soil
corrosion potential as a function of saturation, can be practically studied in the
laboratory.
Bulk soil samples can be obtained of on-site materials. In-situ densities and
moisture contents should also be determined at the same time the sample is obtained.. The
bulk sample can then be compacted to its natural density and moisture content in a
specially designed tube. The tube is designed to allow for resistivity determinations. The
authors chose to use a tube made of PVC, as diagramed on Figure 2. The tube and soil
sample, after the determination of the dry apparent resistivity, are then soaked in water
for 24 hours. The apparent resistivity on the saturated soil sample is then determined.
CASE HISTORY
INTRODUCTION
A geotechnical investigation was conducted on eleven sites, scattered throughout
the Kingdom of Saudi Arabia, to assess soil corrosivity and the need for cathodic
protection at each of the sites. Proposed construction at each of the sites called for the
use of underground utility piping. At most of the sites, this piping would be subject to
irrigational waters, and hence at each site the potential corrosivity had to be assessed
for both dry and saturated soils.
For the purpose of this case history, results from such an investigation are
presented from two adjacent sites, near the city of Jeddah. On-site investigations
consisted of electrical resistivity surveying, collection of bulk soil samples for
laboratory analysis and in-situ density measurements at each soil sample site. Two water
samples were collected, one from each site, but in transit outside of the Kingdom, for
analysis, the sample containers were broken, and hence no analysis was performed.
The electrical resistivity surveys were performed using the Schlumberger array.
The electrode spacings were chosen such that a theoretical interpretation of the
electrical resistivity values could be made to a depth of 10 meters. This depth was
approximate in that it depended on the electrical properties and characteristics of the
site materials. The depth of 10 meters was considered to be a realistic maximum depth for
the proposed utility piping.
The field work took a total of four days, during which time 40
vertical electrical soundings were completed and three soil samples were obtained. The
results of each vertical electrical sounding were interpreted in the field, such that
errors could quickly be corrected and anomalous conditions could be checked on site.
SITE DESCRIPTION
The total site for the proposed construction activities consisted of an area of
approximately three kilometers by one kilometer. Within this area, the geotechnical
investigation was conducted on two smaller areas, approximately one kilometer by one
kilometer, and one kilometer by two kilometers in size. There were no existing permanent
facilities or underground piping in either of these areas at the time of the geotechnical
investigation.
A general site and vertical electrical sounding location map is shown on Figure
3. The site was located adjacent to the Red Sea. A detailed knowledge of the groundwater
system was unknown; however, it was assumed that the groundwater was highly saline.
On-site personnel mentioned that after periodic rainstorms, the groundwater system was
noted to rise 40-60 centimeters. This rise was believed due to capillary action.
Site soils information was available from a soils investigation conducted prior
to the geotechnical investigation. The original surface soils consisted of loose silty
sands mixed with some gravels. In lower areas, the sands exhibited some cohesion due to
cementing materials. Throughout the site, the sand material was leveled and fill material
was placed over it to bring the site to construction grades. There were approximately two
meters of fill material which consisted of dune sand throughout the site. The fill
material and original surface soils were underlain by fragmented limestones and corals.
EVALUATION CRITERIA
The exact size and nature of the proposed underground utilities for this site
were not known; therefore for the purposes of this investigation it was assumed that the
categories of buried materials which would be subjected to corrosion or deterioration
would generally be as follows:
1. cast or ductile iron water mains;
2. black steel chilled water air conditioning lines;
3. raw water lines and tanks (concrete);
4. thermal piping;
5. black steel POL lines and tanks;
6. assorted ferrous metal structures.
The deterioration effects of plastic piping were not investigated.
For the purposes of evaluating the corrosive electrical properties of the soil
in regard to the buried structures the following criteria were adopted:
Soil Resistivity
50 - 100 ohm-meter mildly corrosive
30 - 50 ohm-meter moderately corrosive
below 30 ohm-meter very corrosive
It should be noted that wide variations in soil resistivities within a site or
over the surface of a continuous structure can cause the formation of corrosion cells.
The resistivity readings used for the evaluation of the site data were taken on
an average of those values between depths of one meter to five meters. In preparing
recommendations for the necessity of cathodic or other protection, it was assumed that the
utility services would not be buried deeper than three meters.
RESULTS
The resistivity readings are tabulated on Figures 4 to 7.
These readings averaged for a depth of 3 meters can be summarized as follows:
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