WHAT ARE THE MAIN DETERIORATION MECHANISMS FOR CONDUCTORS OF OVERHEAD ELECTRIC NETWORKS?

SUMMARY

The preceding publication, titled "What criteria are involved in the selection of material for conductors?", contains basic information that will be further developed in this article. Without entering into the well-known Engineering Design, it summarizes the essence of the benefits offered by overhead electric conductors, examining the elements and mechanisms of atmospheric corrosion on copper and aluminum, as well as the normal forms of wear, permitting a qualitative appreciation of these materials.

1. INTRODUCTION

Natural wear of metals in general is manifested as the relatively quick dissolution or loss of material, due to the local or general establishment of chemical mutations that have intrinsic electric manifestations. This electro-chemical process is known as corrosion. The phenomenon occurs independently of the kind of usage, form or designated use, which, although it certainly depends on the aggressiveness of the environment itself, also occurs with the spontaneous tendency of metals to return to the combined state they initially had as minerals. La publicación precedente bajo el título de "¿ Qué Parámetros del Entorno Participan en la Elección del Material de los Conductores ?" contiene, en parte la información de base para el desarrollo del presente artículo. Este, sin abordar el Diseño de Ingeniería que es ampliamente conocido, resume la esencia de las prestaciones que están llamadas a cubrir los conductores eléctricos aéreos, examinando los elementos y los mecanismos del ataque corrosivo atmosférico sobre los metales Cobre y Aluminio, y las formas usuales del deterioro, permitiendo la apreciación cualitativa de dichos materiales.

2. WHAT ARE THE MAIN ADVANTAGES INHERENT TO ELECTRIC CONDUCTORS FOR OVERHEAD NETWORKS?

The requirements to which electric conductors of Overhead Networks are submitted, are related to the transport requirements for Electric Energy, to the initial and exceptional mechanical conditions of the installation, and to the conditions imposed by the aggressiveness of the environment and the surroundings. These demands, which prevail over the projected useful lifetime of the infrastructure, establish the advantages that must be offered efficiently, which depends in great measure on the properties of the basic metals.

2.1 Primary Electrical Advantages
The electric and thermal conductivity with minimal energy loss and minimal reduction in tension, are the main qualities that must be covered. Specifically regarding copper and aluminum, between 16 and 95 mm2 caliber, it is apparent that for similar sections, copper has 28% greater transport capacity, with 77% greater thermal conductivity, which favors the transfer of heat. On the other hand, given the same load current and line length, aluminum has 60% more loss than copper, and for similar sections, load current and length of line, aluminum has a 45 to 55% greater loss in tension than copper. The technology of alloys for these metals only achieves slight variations in these results.

2.2 Primary Mechanical Advantages
The resistance to traction with minimal linear expansion and minimal permanent elongation, particularly under every day stress and other exceptional greater demands that may occur during the service, all constitute the primary qualities that must be covered. In this case, copper has more than twice the mechanical resistance to traction stress. In addition, given increases in temperature that occur periodically during the service, it shows a comfortable Linear Expansion coefficient that is 30% less than that of aluminum, a metal that due to its reduced Elasticity (55% less than copper) allows greater loosening with permanent deformation. However, it does have the advantage of having 70% lower specific weight than copper.

2.3 Primary Environmental Advantages
Resistance and durability against erosion derived from thermal cycles and daily radiation, and against the abrasion caused by solid particles carried by the wind, and the permanent establishment of various electro-chemical processes originating from contamination and humidity, are all the primary qualities that must be covered by overhead conductors. Regarding these natural demands, the metals generate a protective coating of oxidation, whose mechanical and electrical performance is related to its intrinsic vulnerability. In his case, copper patina has the qualities of great strength and compactness, as well as an excellent Electric Resistance that is not prone to corrosion.  

3. HOW THE ENVIRONMENT ATTACKS THE MATERIAL OF THE CONDUCTORS AND ACCESSORIES?

The interaction of the environment and surroundings, made up basically of external radiation and the variation of seasonal meteorological and atmospheric conditions, as well as the ground morphology and its topographical characteristics, and the presence of contamination sources, all directly affect exposed materials and cause their deterioration (aging). This translates into loss of weight and a reduction in the scope of qualities.

3.1 Attack by Erosion
The weathering of metals refers mainly to their permanent exposure to Cosmic Radiation and radiation derived from sunlight, particularly the ultraviolet range. These phenomena are related to daily thermal variations and the action of air currents. Given this incidence of factors of deterioration, metals in general have a high superficial resistance thanks to the Patina layer, the protective oxidation that metals spontaneously and immediately form, which lessens the attack and makes any consequences insignificant and very slow to appear in the long run.

3.2 Attack by Abrasion
The natural process of abrasion suffered by conductors and their accessories, presupposes the existence of a moving element (the wind), and a projectile element (light and heavy solid particles) that are lifted, transported and projected against the metallic surface. They either slowly wear down the protective layer of oxidation (with moderate winds) that simultaneously reconstitutes itself on the next lower level, or else they break it (with gusts of wind), which may cause localized processes of deep deterioration. In this case, the performance of the metals (aside from their physical qualities) and of the protective layer of oxidation, depends on the presence of predominant winds and unstable or desert ground and other electro-chemical conditions created by the environment. Save exceptional cases, having only natural abrasion will cause minimal wearing, but may be a catalyst for other kinds of wear.

3.3 Electro-chemical Attack
Atmospheric corrosion is basically an electro-chemical reaction to the Electrode Potential between a metal or a metallic alloy and the surrounding environment, creating localized deterioration. Chemically, the metallic atoms of the affected or ANODIC areas (where oxidation or acidification occurs) are transformed into Ions (Mn+), and give up their Electrons (ne-) to a non-metal that could be (H+) or (O2), through the healthy or CATHODIC areas (where reduction or alkalinization occurs). The circuit closes through the ohmic resistance of the Electrolyte (on the metal surface).

-  ANODIC process: Dissolution of the metal and production of electrons.
  M ---> MN+ + ne-
-  CATHODIC process: Preservation of the metal and consumption of electrons.
  Mn+ + ne- ---> nM
-  Hydrolisis: Complementary reaction of unstable metallic ions in water.
  M2 + H2O ---> M (OH) + H+

4. WHICH MECHANISMS OF CORROSION CAUSE THE DETERIORATION OF OVERHEAD CONDUCTORS?

The process of corrosion is established by the presence of the intrinsic potentials of the metals (Electrode Potential), which may be measured in relation to hydrogen (Standard), giving for copper (+0.340 volts) and for aluminum (-1.700 volts), values that correspond to the scale of Reaction to Reduction, according to which aluminum is apparently the most susceptible to dissolution or corrosion.

a. Formation of Electrolytes and the Protective Barrier.
The conductor surface that is covered irregularly by contamination, suffers the daily phenomenon of natural environmental dampening, which is very efficient in reduced thicknesses (at 80% Hr, from 30 um to 150 um), or else the phenomenon of condensation at greater thicknesses that could become dew with run-off, which could limit the process of the attack. Under these conditions, electrolytes form with different concentrations and dilutions. Through the covering, oxygen from the air enters and molecular hydrogen from the reaction exits, and the products of corrosion are also transported. The protective barrier is identical for all metals, and is a fine film of oxidation with specific characteristics that in each case determines the kinetics of the deterioration, under the influence of other factors such as temperature, the mechanical demands, and basically the kind of contamination.

b. Participation of Electric Current
When the conductors are energized, during the anodic cycle the process of corrosion is added – the emphasis on the formation of an alkaline medium in the electrolyte as a product of the concentration of (OH)- in the fault; or else, during the negative alternation or cathodic cycle, the acidification from the greater concentration of hydrogen as ions (H)+. Both phenomena affect the dissolution of the protective barrier, leaving the metal unprotected.

4.1 Corrosion of Copper Conductors
The derivatives of chlorine and sulfur contained in coastal contamination, originate the oxidation of copper by forming a blackish film called "Patina". It is very dense, has low solubility, high electric resistance and high resistance to the chemical attack and to corrosion. Thanks to this, it protects the underlying metal, gradually limiting the rate of penetration of corrosion to about 4.0 um/year near the coast and 1.0 um/year in areas with a clean atmosphere.

a. Compounds that form the protective barrier
The formation of the stable compounds of the Patina is variable, as a function of the contaminants (Cl Na, SO2, etc.) and the catalysts (CIH) that originate it. The (CuO and the (Cu2O) are found in the interface with the metal, to which the insoluble hydroxides (Cu(OH)) with different origins are superimposed; the following might predominate according to each case: - In contaminated coastal areas - Brochantites: CuSO4 3Cu(OH)2 old Patinas - Antlerites: CuSO4 2Cu(OH)2 recent Patinas

- In areas free of contamination - Malachites: CuCO3 Cu(OH)2, mainly - Azurites: 2CuCO3 Cu(OH)2,    occasionally
- In contaminated and rainy areas - Langites: CuSO4 3Cu(OH)2 2H2O - In agriculturally contaminated areas -   Basic Cu Nitrates: Cu(NO3)2 Cu(OH)2

b. Performance of Copper Conductors
The stated characteristics give copper a high resistance to corrosion, except in the presence of NH3 or NH4 that is contained mainly in agricultural contamination. Despite this, mechanical vibrations, abrasion, erosion and thermal variations may cause fissures and faults in the Patina. When this happens, the metal is uncovered, corrosion may occur, and electrolytes with low contents of (Cl) could enter, causing a dislocation of the passivity; otherwise, this may occur due to the deficit of oxygen that makes these areas Anodic, in exceptional cases in certain seacoast splash areas that are usually rare, and even so the foreseeable deterioration occurs over a long term.

4.2 Corrosion of Aluminum Conductors
The oxidation of aluminum is originated primarily by the (Cl.H) resulting from coastal contamination, forming a transparent gray Alumina film with varying thickness, composed of a thin, compact, hard, layer that has low solubility and high electric resistance, and acts as the main protective barrier of the metal. It is also composed of another thicker layer that is permeable and acts as an interface with the environment, providing limited further protection. As a whole, the penetration of coastal corrosion is lessened to an average rate of 16 um/year on the coast, and 3 um/year in the sierra.

a. Compounds that Form the Protective Barrier
The layer of protective oxide is also formed by soluble compounds that are formed as a function of aging. the stable Alumina (Al2 O3) is found in the interface of the metal, superimposed by Hydracilles (Al2 O23 H2O), and finally the covering of hydroxides (Al(OH)2 3H2O) that in the presence of the chlorides in coastal contamination, dissolves into acid or alkaline solutions. Said deposits form independently of the nature of the area, in this case the degree of contamination alone constitutes a catalyst. They are called:
- Bahomite : Al2 O3 H2O Rhomboidal
- Bayerite : Al2 O3 3H2O Monoclinic
- Hydracillite : Al2 O3 3H2O Monoclinic
- Hydroxides : Al(OH)3 3H2O

b. Performance of Aluminum Conductors and Alloys
According to the stated characteristics, because aluminum is a very alkaline and amphoteric metal, it is highly vulnerable to the aggressions of a saline environment, particularly in coastal areas with direct marine contamination. However, generalized superficial corrosion does not constitute the main problem. the main problem is the localized corrosion that can be particularly strong in the microfissures and faults in the layer of protective oxide, which mainly have mechanical causes associated with thermal variations, abrasion or vibration, uncovering the metal and allowing the entry of electrolytes that accelerate the corrosion either by the concentration of (OH)- or of (H)+, and extend the anodic areas without interruption.

4.3 Kinds of Corrosion that Affect Conductors
They are related to the different factors under which Electric Conductors are installed and operated. Without going into details of kinetics or morphology, we may mention the kinds of corrosion that commonly occur along the Peruvian Coast, which have been corroborated by field experience, particularly in certain cases exclusive to aluminum conductors.

a. Corrosion from galvanic connections
This is produced due to the difference in Electrode Potential when there is contact between different metals. This case demonstrates the deterioration between the conductor and the clamp that leads to the breakage; it also occurs in microcells when there are impurities in the manufacture of the conductor.

b. Corrosion from Differential Airing – Pitting
It occurs within the limit, between threads and inside the gaps on the surface, due to the tightness of the conductors, in deep places where the electrolyte has a greater concentration of oxygen, forming Anodic and Cathodic areas. The reduction of oxygen occurs in the lower areas that receive less air, producing a greater acidity in the electrolyte, accelerating the deep corrosive attack.

c. Corrosion under Mechanical Tension – Fatigue
This requires a mechanical tension (EDS of the installed conductors) to combine with other demands (wind, its own weight, cycles of contraction and expansion). In addition, it requires an alloy and a corrosive medium. If under these conditions there are structural dislocations within the crystal network or slippage within the grain of the metal, or else when there are gaps or transversal cracks in the wiredraw, a combined process of mechanical and electro-chemical corrosion will begin.

d. Corrosion from Concentration Cells
It occurs in the grooves of the wiredraw or in surface imperfections. Within each groove, the metal dissolves by acidification or alkalinization, becoming a solution. The same occurs on the outer part of the grooves, but quicker, creating a chemical imbalance between the metal and the electrolyte, which creates a flow of current of corrosion that establishes the process.

e. Inter-grain Corrosion
When the metal has a matrix phase and complementary precipitate phases, upon the failure of the thermal treatment of hardening by excessive artificial aging, an excess aging occurs that is characterized by the migration of the precipitates to the edges of the grain, pushing the corrosive process to the exterior, and making the threads brittle.This is because the phenomenon continues at room temperature. The residual tensions of its manufacture also contribute to this kind of corrosion.

f. Biological Corrosion
This is a metabolic process provoked by certain kinds of algae, fungus or lichens, as well as by single-cell bacteria that land on the surface of the conductors and actively decompose the organic and mineral matter from the contamination trapped in the gaps, fissures, cracks and interfaces of the threads.They produce enzymes, ammonia and sulfur and carbon dioxide compounds that modify the local pH and cause corrosion.

5. PREVENTION OF CORROSION AND MEASURES TO OFFSET IT

a. Prevention of Corrosion
The main goal of Engineering Design is to ensure the quality of service based on a trustworthy, safe and economic operation of the infrastructure, closely observing this performance for a minimum period equivalent to its designated useful lifetime (25 to 30 years); according to which the examination of the environment and surroundings will lead to the selection of appropriate materials – frequently based on a technical-economic analysis that will lead to the rejection of the minimum initial investment which in the long run will be counterproductive.

b. Ways to offset corrosion
There are ways to extend the inexorable period of conductor deterioration by corrosion; these technical resources are effective when the conditions of the location are only moderately aggressive, which depends on the content of the soluble matter in the atmospheric contamination. However, it is impossible to ensure the trustworthiness of the installations.
When a more severe level of corrosive aggressiveness is determined, artificial methods to counter corrosion in the installed infrastructure become merely palliative since the failures from collapse in conductors do not occur from generalized corrosion but rather from localized corrosion, which may happen at any point.

6. CONCLUSIONS

a. The most important attributes that electric conductors for Overhead Networks must have are mainly related to their performance given environmental and surrounding aggressiveness, in which case the correct selection of materials is fundamental.
b. The most common mechanisms of corrosion attacks on Overhead Conductors, all lead to the localized or specific deterioration of the threads, whereupon collapse is imminent. In these situations, copper ensures long duration because it is a cathodic (noble) metal, even in seaside (Splash Zone) installations.
c. The only way to ensure a trustworthy service and safe and economic operation in the long run for coastal Overhead Electric Networks, is to use the right material for the conductors. The justification of a lower priced but qualitatively inferior alternative is only an illusion.

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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