STUDY OF CATHODIC PROTECTION IN SOILS WITH THE USE OF SPECIFIC COUPONS

Nicolas Larché and Etienne Leroy French Corrosion Institute

F-29200 Brest, France

Jean Vittonato and Eric Agel Total

F-64000 Pau, France

François Castillon TIGF

F64010 Pau, France

ABSTRACT

Metallic coupons are generally used close to buried pipes under cathodic protection in order to evaluate potential and current demand at coating defects. The “ON/OFF” measurement technique (at interrupted current), is generally the preferred method. The “OFF” potential is supposed to give a realistic picture of the electrochemical potential of the coupon, excluding the effect of ohmic drops due to soil resistivity.This technique assumes that a metallic coupon is representative of a coating defect. However, considering the inhomogeneity of soils and their possible high resistivity, the coupon design and the measurement techniques must be carefully studied and understood to get reliable results. For this purpose, specific on-shore coupons have been designed, allowing adapted measurement at direct proximity of the virtual coating defect. These coupons are used in field soil for cathodic protection survey of buried pipes. However, the response depends on many parameters which are difficult to measure and control in the field (actual soil composition, humidity, soil compaction at coupon location, ageing of the coupon, etc.). In former programs, soil exposure cells have been developed and used at the Institut de la Corrosion to study soil corrosion of different materials. These cells allow controlling physicochemical parameters of the tested soils which potentially affect corrosion of materials (texture, humidity, salt composition, pH, etc.), and results were in very good line with field results. In the present study, these cells were adapted to study and characterize cathodic protection on the specific on-shore coupons. The main objective of the study was to better understand the polarization (polarization curves) of the used coupons as a function of soil nature and humidity. This paper presents the experimental technique that was developed to get exploitable polarization curves of carbon steel coupons in soils. It provides the polarization curves at different levels of humidity for two different types of soil. A rather good reproducibility was found which allowed studying the influence of significant parameters affecting cathodic protection in soils.

Key words: Cathodic Protection, Soil, Specific Coupons, Polarization Curves

INTRODUCTION

The use of metallic coupons to quantify the cathodic protection (CP) of buried pipelines is a common practice, although the field measurements results are often questionable and sometimes not reliable. Considering the complexity and expected high resistivity of soils, the design and understanding of on-shore coupons is of major importance to get trustable and workable measurements. The on-shore coupon should be designed to allow potential and cathodic current measurements of a bare steel surface representing a defect that can potentially exists on coated pipe. The “ON/OFF” measurement techniques on buried coupon have been widely used for potential measurements in soils to mitigate ohmic drop induced by soil resistivity. However, field measurements can remain tricky, and their interpretation depends on many uncontrolled parameters which are difficult to assess in field soil conditions (actual soil composition, humidity, soil compaction at coupon location, ageing of the coupon, etc.). Thus, from the large quantity of CP measurements performed using “on-shore” coupons, some unexpected data are collected. Among these data some general trends are regularly observed as listed below:

- During big rains, both the potentials (EOFF and EON) and the cathodic current demand can increase significantly to very high values

- In highly resistive sandy soils at low moisture content, buried pipelines polarization can remain insufficient despite high impressed current densities (e. g. > 1A/m²)

- In these resistive soils, efficient protection is easily obtained with low impressed current density (e. g.

Figure 1 shows the principle of these coupons. They are made of a carbon steel ring embedded in a plastic support. The head of the system is fixed in an PVC conduit that is dedicated to receive a portable reference electrode (Ag\AgCl or Cu\CuSO4). The metallic ring is connected to a copper cable that is completely insulated from the surrounding media. This specific “on-shore” coupon presents two main characteristics

1. the reference electrode is in the direct vicinity of the metallic coupon 2. the shape of the coupon is a ring and the reference electrode is located in in the centre of the

ring.

† Trade name These two specificities lead to the main following advantages compared to other usual types of coupons:

1. Once ON/OFF measurements are performed on coupon, only the link between the coupon and the pipeline is interrupted, not the surrounding transformer-rectifiers. Then, current still circulates inside the soil, leading to voltage gradients. Depending on the location of the coupon and the intensity of those gradients, there could be a non negligible ohmic drop between the coupon and the reference electrode if the electrode is located few cm away from the coupon. This could lead to significant errors in the potential measurements. Thanks to the design of the coupon, the distance between the coupon and the reference electrode is minimized (almost null), and as such the error becomes negligible.

2. The circular shape of the coupon fits almost perfectly with the reference electrode, and so there is a planar symmetry. Such symmetry reduces even to a smaller value the impact of the remaining electric field on the potential measured, thanks to the averaging of the measurement. As such, the potential measurement reliability is significantly enhanced.

Coupon size is determined depending from the desired steel surface to be exposed. In the present study, coupons with metallic surface of 10 cm² were used. The reference electrodes used in this study were Cu/CuSO4 specifically designed to fit with the “on-shore coupons”. The ceramic membrane

(electrolytic junction) has the same diameter of the electrode, which fits perfectly in the plastic cylinder extension of the on-shore coupon.

Figure 1: Principle of the specific « on-shore » coupon

The specific on-shore coupons are designed for field measurements, as schematically shown in Figure 2. They can be easily connected to buried pipelines under CP and allow realistic measurements of the potential of the pipe to assess if they can be considered as protected or not, according protection potential criteria. To better understand potential measurements and polarization of metals in soils, it is needed to control (or at least measure precisely) the soil characteristics, especially in terms of texture, water content, pH and chemistry. For this purpose, field exposure of coupons cannot be used due to uncontrolled soil characteristic, and controlled soil laboratory cells were used.

Coupon : Carbon Steel ring

Plastic support

PVC extension

Electrical contact with the coupon

Figure 2: Schematic drawing of on-shore coupon used in field conditions

Soil exposure cells and testing conditions

Specific cells have been designed to expose the coupons in the test soil. They are composed of a cylindrical plastic container whose bottom is constituted by a polymer permeable membrane, allowing a permanent humidification of the soil in the cylinder (i. e. stable water content gradient from the bottom to the top of the cell). The principle, the development and the qualification of these soil exposure cells are derived from previous studies.1,2 For polarization tests, a Pt-coated titanium mesh was placed in the soil cell, in front of the coupon, at about 10 cm from the metallic ring. The respective locations of the counter electrode and the coupon were expected to lead to uniform and symmetric current distribution in the soil cell. The cells are partially immersed in the selected electrolyte, at a controlled level which is kept constant during the exposures. This technique allows getting a constant water content gradient along the soil cell. The position of the coupons in the soil cell with respect to the water surface is thus determined depending on the desired moisture content, which also depends on the soil texture. For coupons to be exposed in an environment saturated with water and aerated, a system involving a permanent rain was used. A schematic drawing of the cell used for laboratory soil exposures is given in Figure 3.

Figure 3: Schematic drawing of the laboratory soil exposure cell

Two natural soils were used in this study. A sandy soil from Roquefort, France (40) and a clayey soil from Mouais, France (44). Both soils were directly extracted from their natural location and transported to Brest for laboratory investigations. The classical soil analyses were performed by a specialized laboratory in France. The soil resistivity as a function of water content was drawn in a soil box, using the 4 points Wenner method. The tested conditions in the soil cells are given in Table 1.

Table 1 Testing conditions

100% Water Saturation Rain = ON

(Flowing/forced aeration)

100% Water SaturationRain = OFF

(Static / diffusion)

15% Water Saturation Rain = OFF

(Static / diffusion)

Sandy Soil from Roquefort

x x x

Clayey Soil from Mouais

x - -

Measurement techniques and polarization curves with the use of specific on-shore coupons

For potential measurement of metal under CP in soils, the ohmic drop between the reference electrode and the coupon must be considered. A common method to minimize this effect is to make potential measurement at a known moment quickly after a punctual interruption of the cathodic current. Potential of the coupon is thus measured some milliseconds after disconnection of the CP current generator. The potential measured just after the current cut is called "Eoff", while the potential measured during CP and affected by ohmic drop is called "Eon". To optimize the Eoff measurement it is also of major importance to have the reference electrode as close as possible to the metallic coupon. In this way, the specific “on-shore” coupon is perfectly adapted.

The current interruptions and "Eoff" measurements on "on-shore" coupons under cathodic protection were automatically performed with the use electrical device which allows automatic interruptions / reestablishments of the current with measurements of Eoff some milliseconds after the punctual current cuts. The reference electrodes were regularly calibrated with saturated calomel electrode. The current applied to the coupon was adjusted with regulated current generator. The polarization curves were drawn step by step by increasing the applied current from open-circuit potential (current = 0) to potentials below -1000 mV/ Cu/CuSO4. The criteria to change current step was a stable Eoff at +/-10 mV over a duration of 48h. At least 3 independent replicates were used to draw polarization curves in order to consider the variability in reproducibility. This technique to draw polarization curve in soil is the results of several attempts using different techniques including the potentiostatic one It revealed that the more stable and reliable technique was the galvanostatic technique, as used in the present study.3

During the polarization tests, the dissolved oxygen content was measured at the vicinity of the coupon with the use of permanent LDO (Luminescence Dissolved Oxygen) probes. The diameter of the probes was 2 mm and they were introduced in the soil as illustrated in Figure 4.

Figure 4: Dissolved Oxygen measurement

RESULTS

Tested soils

Grain size analyses were performed on the two tested soils, indicating that Roquefort soil was sandy and Mouais soil was clayey. They are both represented in the soil ternary diagram given in Figure 5. The main characteristics of the two tested soils are given in Table 2. The pH is rather acidic with values of 5,3 and 5,1 respectively. The tested soils are not contaminated with chloride (< 20 ppm in both soils) and no Sulfate-Reducing Bacteria (SRB) were detected from bacteria analysis.

Figure 5: Ternary Diagram for the two tested soils

Table 2: Analysis of tested soils

Roquefort Mouais Analytical Technique

pH 5,3 5,1 NF ISO 10390 (X31-117) (1) (2)

Total organic carbon

1,0 g/kg 1,2 g/kg NF ISO 10694 (X31-409) (1) (2)

Organic Matter 1,7 g/kg 2,1 g/kg Calculated

Total nitrogen

Polarisation curves

The polarizations curves that were obtained in soil cells with the specific on-shore coupons exposed at water saturation (both static and flowing rain) in the Roquefort sandy soil are given in Figure 7. In a same soil it is clearly observed very different polarization behaviour depending on the tested conditions, namely at 100% of static water saturation and 100% of dynamic (rain) water saturation. To reach a polarisation of the coupons at -850 mV/ Cu/CuSO4 the required current density is in the ranged of 15-20 mA/m² in static condition and about 150-200 mA/m² when rain condition is ON. The dissolved oxygen content and the flow rates measured in the soil cells containing the sandy soil are given in Table 3. The low continuous flow induced by the rain allows keeping a 100% oxygen saturation at the vicinity of the coupon. In the static condition, the dissolved oxygen decreased significantly to low values, inferior to 10% of oxygen saturation. Regarding the low flow rate (0,4 mm/s), the dissolved oxygen content appears as the main factor affecting the polarization of the coupons.

Figure 7: Polarization curve in the Roquefort sandy soil, in water saturated soil, with and without rain (i. e. flowing and static).

Table 3 Oxygen and flow rates in soil cells containing Roquefort sandy soil

Oxygen[% of Saturation]

Flow rate[mm/s]

100% Water SaturationRain = OFF

(Static)

Figure 9: Polarization curve in the Mouais clayey soil, in water saturated soil (static conditions)

DISCUSSIONS

All the polarization curves obtained in soil cells at water saturation are given in Figure 10. When the oxygen was limited (i. e.

Figure 10: Polarization curves in soils tested at water saturation

At 15% of water saturation, much higher cathodic current was measured when compared to water saturated conditions, which is in very good line with the field measurements described in the introduction (i. e. high current densities in resistive soils with low moisture content). Below the water saturation, soils are gas/liquid/solid multiphase corrosion system, in which the liquid form of water will be the main electrolyte where electrochemical reactions may occur. Water film on a metal surface in soils will be discontinuous due to the dispersion of soil particles and some authors reported that water film thickness (i. e. thickness of the electrolyte on metal surfaces in soil) can range from 0 to 100 µm depending on soil texture.5 Some studies revealed that three-phase-boundary (TPB) zone involving thin electrolyte films on metal surfaces was a high-speed cathodic reaction zone, in which the diffusion rate of oxygen was much higher than that in bulk solutions.6–8 The TPB is illustrated in Figure 11. Also the oxygen will diffuse easier though the air phase of non-water saturated soils compared to diffusion in bulk water. Other studies confirm that higher corrosion rates are frequently observed at water contents below water saturation.9–15 This hypothesis may explain the very high cathodic current demand measured in the aerated sandy soil below water saturation. Assuming that this theory is correct, a decrease of oxygen content would lead to less cathodic current demand. It is probably the case in field situation where structures are buried deeper than in laboratory soil cells, i. e. with less access to oxygen to diffuse. This hypothesis should however be further investigated to be confirmed and other explanations could also be formulated (nature of oxide products, polarization of soil particles, etc.).

Figure 11: Schematic representation of the three-phase-boundary (TPB) zone in wet soils5

CONCLUSIONS

The specific design of the “on-shore” coupon is well adapted to minimize the main issues related to cathodic protection measurements in soils. It limits the effect of ohmic drops and inhomogeneous current distribution, thanks to the central location of the reference electrode. Field measurements are often difficult to analyze since many uncontrolled parameters can occurs in field soils, influencing significantly the measurements (inhomogeneity, actual water and oxygen content, local phenomena, compaction, etc.). Thus, laboratory soil cells were adapted to draw polarization curves in selected soils and to study the influence of selected parameters on the polarization of the coupons. It allowed reproducing some of the field measurements and to investigate scientific explanations in a controlled media and coupon. The main results are listed below:

- The oxygen content has very significant influence on the polarization of buried carbon steel. Together with the hydrodynamic effect of rains, it explains the large CP data variations that can be observed during big rains.

- In poorly aerated soils, a good correlation was found with current densities from the literature and results obtained with the developed technique using the specific on-shore coupons

- In oxygen saturated soils, the cathodic protection current increased significantly. It was higher in 15% water saturated soils than in 100% water saturated soil. This result, also observed in field measurement campaigns, is attributed easier oxygen access (through the air phase) and to much higher efficiency of the oxygen cathodic reduction on thin water films (wet condition) than in bulk water.

Regarding the major importance of oxygen content on polarization, potential and current measurements in soils could be analyzed in connection with oxygen content. For this purpose, the on-shore coupons could be equipped with permanent LDO probes to allow measurement in field configurations.

The results of this study and of the continuing work at laboratory scale will allow the further analysis and interpretation of the field results collected from the huge number of on-shore coupons already installed at the vicinity of buried structures under cathodic protection.

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