BY G. Clark Davenport, Edward E. Rinne, and Alfonso Maldonado Zamora

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:

The results of the electrical and chemical analyses of the soil samples are presented on Table 1.

A lack of correlation was noted between the dry resistivity values as determined in the field and in the laboratory. The probable reason for this lack of correlation was the change in internal structure and soil-water system of a partially saturated soil between its in-situ and recompacted state. This change was significant even though the recompacted laboratory specimens were compacted to the in-situ water content and unit weight.

For this reason, the dry resistivity values as determined in the laboratory were often erratic or unobtainable.

Vertical electrical soundings were performed in.wet soils on the sites where possible. Laboratory resistivity values for saturated samples in the same soils correlated well to the resistivity values determined in the field. In a fully saturated state the soil-water system is more closely reproduced, i.e., the amount of interstitial water in a laboratory sample is very close to the amount of interstitial water in a fully saturated in-situ sample.

RECOMMENDATIONS

From these results it was apparent that any underground metallic piping or structures on this site would be subjected to a corrosive environment. It was recommended that cathodic protection should be installed with any such piping or structures.

Once the characteristics of the underground piping or structures have been selected, specific cathodic protection design recommendations could be made from the results of this investigation.

The soil pH in this area did not indicate soil acidity to be a corrosion factor.

The soil had a high sulfate content and this would have to be considered when specifying materials which are to be buried. This might apply in the case of concrete or asbestos-cement.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the United States Army Corps of Engineers, U.S. Army Engineer Division, Middle East for their assistance and support during the course of the geotechnical investigation. we also wish to thank the same group and Dames & Moore for their permission and encouragement in the sharing of the principles and results of the investigation.

Table 1
Soil Analyses --JEDDAH