HOW DOES HOSTILE GROUND
AFFECT GROUNDED ELECTRODES?

SUMMARY

Previous publications ('97#1, '98#1) summarize the effect of outside surroundings on materials of conductors as well as deterioration of their mechanism by corrosion. On this occasion we will make a brief examination of the hostility of the soils or of the fills of underground installations that attack the metals of Grounded Electrodes. In order to avoid long term deterioration, planning engineers when designing these installations, should consider precautions against this problem, since it leads to dangerously increased resistance to dispersal and the reduction of the useful life of the installation that normally does not admit substantial low cost renewals or repairs.

1.-   Introduction
2.- ¿How does corrosion apperar in the soil?
3.-   Corrosion mechanisms of grounded electrode.
4.- ¿How the soil or the fill participates in the corrosion?
5.-   Conclusions

1. INTRODUCTION

Grounded Electrodes are those buried in direct contact with the natural soil or through fills of fine earth mixed with conductor salts dissolved in water, so as to disperse with the minimum Electrical Resistance, in permanence, lesser currents of Static charge, faults, unstable or erratic equilibrium, etc. and also occasionally and for brief moments, others of greater magnitude such as induced currents, short circuits or lightning; the functioning of the conductor and disperser is then uninterrupted, assuring the protection of persons mainly against electric shocks and facilitating the reference of Zero Potential for the correct functioning of electric and electronic apparatus.

The described installation method, seen another way, becomes the immersion of a metallic object in a non-homogeneous electrolytic mass with different saline concentrations and oxygen which is susceptible to forming a dispersal plan of Basic micro and macro Corrosion Cells all over the contact surface.

2. HOW DOES CORROSION APPEAR IN THE SOIL?

Underground reinforced metallics, as in the case of Grounded Electrodes, are corroded by electrolysis; these are phenomena of an electrochemical nature implying movement of electrons from anodic zones (where there is oxidation) to the cathodic zones (where the Reduction occurs), through the parts of the metal not involved in the reaction, the electrolyte closing the circuit (damp or filled land) that is a solution characterized by its Ionic conductivity. Thus the basis electrochemical equation is as follows:

In this way, the corrosion called Electrogenetic or galvanic may occur making up Corrosion Cells.

- Galvanic: When dissimilar metals are present in a same electrolyte, or

- Electrolytic: In a same metal immersed in different electrolytes, or

- Concentrated: In a same metal immersed in different concentrations of a same electrolyte.

When metals are different, the most Cathodic or Passive (Noble) prevails at the cost of the permanent deterioration of the Anodic (Active) metal, which is gradually affected until it disappears, and when dealing with only one metal and different concentrations in a same electrolyte, the deteriorating parts are those suffering the greatest flow of continuous current going towards the electrolyte. Said spontaneous processes are permanent and conform to the existence of differences in potential of up to hundreds of millivolts, associated with the presence of the metals.

2.1 Potentials of the Metals themselves

The Refining of metallic minerals to obtain pure metals requires enormous thermal energy that, according to the structure of the metal, prevails in the form of its own electric potential, as a quality that permits a long term return to its natural state (metallic oxide); said parameter is measured in each case, exposing the pure metal to a solution that contains an atomic gram of weight of its respective ions, and this is related to the energy itself of removal of electrons to achieve an equilibrium according to the indicated basic electrochemical equation.

When comparing said typical potentials with a Reduction or Oxidation Pattern, a hierarchic classification can be established. Such is the case of the Electrochemical Series based on the Reduction of Hydrogen (H2/H+) to which the Zero Potential is arbitrarily assigned:

Thus, the place of each metal in the classification of Electrode Potentials is given by:

Thus, the metals most resistant to corrosion are those that produce fewer electrons than Hydrogen and therefore present a more positive electrode potential. In Table No. 1, Gold heads the Series with (V= + 1.5 Volts), while the earthy alkaline metals show greater negative potentials, Lithium closing the list with (V= -3.0 Volts).

TABLE N° 1

* Electrochemical Series of Electrode Potentials regarding the Hydrogen Electrode (H2/H+)

A more practical series results from utilizing a known electrolyte (sea water), some referential engineering metals and alloys, and an easily carried Reference Cell (Cu-SO4Cu), as appears in Table No. 2.

TABLE N° 2

* Potential Electrode Electrochemical Series regarding the electrode (Cu-S04 Cu).

2.2 Currents that Produce Corrosion

Deterioration is not only caused by the Continuous Current generated in Corrosion Cells; there are also so- called erratic currents circulating by different circuits that substantially increase the process by leaving the electrode. See Table No. 3. There also exist other very corrosive currents deriving from the functioning of devices with a so-called "non lineal" charge such as Rectifiers, Battery Chargers, UPSs, etc.

TABLE No.3

* Loss of weight from corrosion in Kg/Amp per Year of metals due to Erratic Currents.

Compared to these, Alternate Currents of identical magnitude cause only a small percentage of corrosion, and those of High Frequency scarcely begin the process. In this respect, Industrial Plants that have sources and charges of Continuous Current, would foster greater Underground Corrosion.

2.3 Activity in the Corrosion Cell

For corrosion to be possible it is necessary that four indispensable elements be present:

- The presence of one Anode and one Cathode
- An Anode-Cathode Potential difference
- A direct Anode-Cathode connection
- A common medium for Anode-Cathode immersion.

The elemental cell therefore has two electrodes, directly connected and immersed in one solution. The Anode dissolves and, on one side, sends positive (+) metallic ions towards the Cathode, through the Electrolyte, and on the other side negative electrons (-) towards the same Cathode, through the direct connection. When the Electrons and the Positive Ions meet in the Cathode, they mutually neutralize each other (Reduction) giving rise to the corrosion phenomenon localized in the Anode.

If the Potential Difference in a common Metal-Elelctrolyte system is greater than the Potential Anode-Cathode Difference of zones of a same metal or of two solidly joined different metals, there will be corrosion or dissolution of the metal in the Anode, while the reaction in the Cathode is generally a reduction of the Oxygen in the Electrolytes (Chart No. 2). Should the (pH) of the solution be neutral or alkaline (pH>7) there would be corrosion in the Fe for example:

If the solution (pH) were, on the other hand, acid (pH=1), this would result in the so-called Protons Reduction reaction:

02+ 2H20 + 4e- ---- 4OH-

If the solution (pH) were, on the other hand, acid (pH=1), this would result in the so-called Protons Reduction reaction:

2H+ + 2e- ---- H2

Should the solution (pH) on the acid side be near the neutral point (pH=6), that is, should there exist Oxygen and Hydrogen simultaneously, the reaction would be:

02 + 4H+ + 4e- ---- 2H20

3. CORROSION MECHANISMS OF A GROUNDED ELECTRODE

Corrosion occurring through micro or macro cells will be sufficient to produce between any two parts whatsoever of the Electrode the appearance of a Potential Difference, which may occur in various ways.

a. By difference in Oxygen concentration

This gives rise to a Differential Aeration Cell, that is related to the existence of zones deprived of Oxygen (Anodics), and others with abundant oxygen (Cathodic) which occur respectively between adjusted interface points, and outside the Pressure Clamps or between the lower and upper surface of the horizontal conductors in the middle of the Bedding, or perhaps due to the difference between little and greater grading of the soil strata in contact with the Vertical Electrodes.

b. Due to existence of Permanent Stress

A Permanent Tension Cell appears, related to the existence of mechanical traction stress zones presenting (anodic) behavior and others with mechanical compression stress behaving like cathodics. This generally occurs when the electrodes are installed by nailing in the ground,during which penetration they suffer forced deviation or deformation, or also by the doubling of horizontal electrodes.

c. Due to difference of Ground Resistivity

A Differential conductivity Cell arises from the existence of nonhomogeneous fills or ground strata with different Resistivity in those having an Electrode installed. In this case the Fills or the Low Resistivity (Anodic) strata and those of greater or high Resistivity are Cathodic. This generally occurs in soils leveled for Horizontal Electrodes and in those stratified for Vertical Electrodes.

3.1 Study of Alternate Materials for Electrodes

In the past, when carrying out Protection grounding, featuring large surface coverage and installation in the form of mesh networks, studies were made of different metallic materials as competitive alternatives at a lower cost than Copper, which also gave rise to Corrosion of steel structures and Iron piping installed together in the ground. On such occasions Applied Studies were made touching on considerations of a technical order (fusion, resistance, low corrosion) and of an economic nature (existence in the market, low cost). Said materials were basically galvanized steel, stainless steel, copper sheathed steel and anodized aluminum.

The models established for said initiatives of lower cost and initial performance equivalent to those of Copper, however, showed that with time they would be vulnerable to the corrosion in the ground.

3.2 Comparison of Corrosion of Basic Materials

The comparison of the performance of metallic materials, byproducts of Iron, that are the only competitive ones, can be made regarding Copper with the POURBAIX Diagrams that give a thermo-dynamic description of the Metal-Solution system indicating the final state of the tendencies of each metal under normal conditions, relating to the Electrode Potentials (Volts) and the nature of the solution (pH), and showing the stability zones of the different chemical species derived from same. Those corresponding to metal (Me-) are zones of IMMUNITY, those that indicate a tendency to dissolution of the metal (Me ⁿ⁺, MeO, etc.) are zones of CORROSION, and others indicating formation of solid products that offset the corrosion (MeOH) are zones of PASSIVITY.

a. The Case of Iron

This shows its instability in the presence of water, and it corrodes also in non-oxygenable aqueous solutions, giving off hydrogen. These reactions that are strong in acid media, are allayed with an increase of pH until they cease in the 10-13 interval, when the metal is covered with an oxide layer; however, over a pH higher than 13 the solutions, already free of oxidant agents, become corrosive again.

b. The Case of Copper

Two corrosion zones arise, referring to those of acid media and strong alkalines; they can be attacked slowly by Ammonia Salts (cultivated earth), by Chlorides, Sulfides and Oxidant Agents, in the acid medium with a pH below 5.4, the limit of corrosion rule is a horizontal line, by increasing the pH a change of direction is produced beginning formation of a layer of Cu₂O that protects the metal, forming, in addition, Cu (OH) and CuO. When the pH exceeds 11.6 the hydroxides and oxides do not retain their stability and dissolve, giving rise to the appearance of a second corrosion zone.

The joint evaluation shows the supremacy of Copper that does not corrode, giving off Hydrogen (H2) around the neutral pH ground, while the Iron is susceptible to it. Moreover it presents a small zone of corrosion but with greater Potentials on the alkaline side, where the Iron shows an acceptable performance in the range of lesser Potentials. Finally, the superiority of Copper is verified through the wide common part of the immunity zone with the Thermo-dynamic water stability zone, which is not shown for Iron.

Consequently, if one expects a long term duration of the Grounded Electrode buried with saline bedding, it would not be advisable to use other materials, even special alloys although these were protected by a Copper sheath, given that with the deterioration of said protection, the interior steel would dissolve fast, breaking the protective walls.

4. HOW THE SOIL OR THE FILL PARTICIPATES IN THE CORROSION.

Low Resistivity soils normally permit high Rates of Corrosion, the humidity in this playing an important role, due to the content and type of elements producing free Ions in the Electrolyte:

- Different concentrations of Oxygen.
- Different Resistivities between adjacent zones.
- Saline Content.
- Concentration of Hydrogen Ions.
- Degrees of Temperature.

4.1 The Kinetics of Corrosion in the Soil

The concentration of free Ions of Hydrogen (pH) and of dissolved Oxygen participate in the process of controlling Corrosion. An Acid Electrolyte (pH<7) contains an excess of Hydrogen Ions that promote the neutralization of electrons and stimulate the flow of corrosive current. In this way, the parts of the Electrode that are found in the acid zone of the Electrolyte are Anodic compared to those that are found in a greater pH zone.

A byproduct of Corrosion is the accumulation of a gaseous layer of Hydrogen on the Cathodic Surface (where the Corrosion Current enters); this Cathodic polarization intervenes, spontaneously reducing the Corrosion current, setting up an isolation barrier. However, the Oxygen dissolved in the Electrolyte can react to the Hydrogen, and back to forming water, which activity destroys the polarizing layer and permits the corrosion to continue, resulting in areas that have high dissolution of Oxygen tending to be cathodics.

4.2 The Control of Corrosion in the Ground

A study of the functioning of the Corrosion cells suggests the same methods of mitigation or reduction of damage, which are largely based on minimizing the Corrosion Currents (that abandon the Electrode), which contradict the functioning of the Grounding, whose Electrode should mainly lead to and disperse currents of all types, for which reason there are practically only three viable alternatives for preserving Grounding Electrodes without impairing the evacuation and dispersal of currents:

- Utilizing materials highly resistant to Corrosion, such as Copper, Stainless Steel or Nickel or Titanium alloys, and rejecting metals (Active).
- Utilizing the installation beds, stable neutral pH fills such as Bentonite that is both conductive and limits migration of corrosive salts.
- Utilizing Cathodic Protection.

Discarding the use of Cathodic Protection due to its cost and the need for maintenance, there remain only the possibilities of using Electrodes Resistant to Corrosion and stable and neutral conductor Fills, which demands are difficult to comply with strictly but can be carried out approximately and are possible to apply together in order to obtain an optimum long-term result.

4.3 The Performance of Stainless Steel and Copper as Electrodes.

At present this is the only material technically and economically competitive with Copper insofar as performance towards Corrosion is concerned, which is demanded for Grounded Electrodes. However, it is necessary to mention the conditions under which said material complies with its contribution, and when these do not work, in what way it suffers deterioration.

- It has an optimum performance in the atmosphere, due to the fact that there is Oxygen that it requires to maintain its protective Oxide layer. But when buried the surface ventilation is limited or is minimal, and it needs to be immersed in a special Electrolyte (Bedding) that acts permanently as an oxidant.

- Any absence of Oxygen, even in a localized form on the surface of the Electrode, gives rise to an Anodic zone which the Corrosion Process begins pitting, which is self-incentivating by constantly reducing the interior pH, while on the outside the Cathodic alkaline protector zone is maintained, until there is a physical breakage of the Electrode.

In the case of Copper, the Oxide protective layer is hard and, unless it becomes cracked, does not need to be renewed because it resists the corrosive attack by the common salts of the soil, except for the components of fertilizers contained in the cultivated earth, in which case the Corrosive process is an extensive and slow coverage.

Electrodes of Steel sheathed in Copper suffer violent internal corrosion and total destruction, when said layer is perforated by corrosion in its weakest parts, which occurs after a few years due to the use of Conductor Salts applied directly to the surface.

5. CONCLUSIONS

a. The hostility of the Soil or fills with salts on Grounded Electrodes, unlike Copper, are shown through the rapid and inevitable corrosion provided by the increase in their Resistance to Dispersal and their short or medium term destruction. Deterioration of Copper is slow.

b. Any installation that is grounded should adjust to the requirements that by their nature are favorable to the phenomenon of Corrosion, with others that are contrary and that are aimed at preserving Electrodes and their correct functioning.

c. In order to obtain a stable grounded installation, with a great and long term dispersal capacity, an electrode resistant to Corrosion (Copper) might be used. Isolated from the Salts by a neutral natural and conductive fill (Bentonite), that is, moreover, neither toxic nor despoiler.

This publication was prepared by Justo Yanque M., a Mechanical-Electric Engineer (UNI), M.Sc. App. (FPMS-Belgium), who is a Consultant specialist with extensive technical experience in electro-mechanical projects.

For more information on this subject, and for bibliographical references, please contact Procobre Peru.

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