Soil Corrosion and Earthing

TABLE OF CONTENT

1.0 Introduction
1.1 Corrosion in Soil

2.0 General Analysis
2.1 Material and Method

3.0 Result

4.0 Discussion

5.0 Recommendation

6.0 Conclusion

7.0 References

SOIL CORROSION AND EARTHING

1.1 INTRODUCTION

The purpose of earthing electrical equipment or an installation is to ensure safety. No matter the sensitivity of a protective device, without proper earthing good performance of the device cannot be ensured. One of the major problems of having a continued low earthing resistance is corrosion. Corrosion is not limited to earth electrode only but all electrical equipments, buried metals, undergrounds telecommunication system, etc.

The life expectancy of a material or equipment depends on the severity of corrosion in the area (Gonos and Stathopuslos, 2006, Liu et al, 2004). The processes of corrosion are not simple and their effects are not easily predicted. Therefore if the corrosion processes are not controlled it will result to frequent and costly replacement of materials and equipments.

Due to the geographic location, the closeness to the ocean, and the soil conditions, the soil of Niger Delta are corrosive. This is particular true with the coastal soil. For this reasons, it was necessary to carry out a study of various soil structure and their corrosivity. Such records when properly presented will serve as a guide in the selection of earthing materials as well as help in the control of corrosion on buried materials.

1.1 Corrosion in Soil

All materials both organic and inorganic can react with their environments (Ala and Di Silvestre, 2002, Gonos and stathopuslos, 2006) and may eventually lose their usefulness for a given application.

Corrosion is the result of electrochemical, chemical or biological reaction between a metal and its surroundings. Corrosion of metal in soil is primarily electrochemical in nature and results from the operation of numerous galvanic corrosion cells (Sekioka et al, 2006, Laver and Griffiths, 2001, Liu et al, 2004).

Each galvanic corrosion cell comprises the following:

1. An anode and cathode areas on metal surfaces
2. Soil electrolyte
3. Conducting path between an anode and cathode.

The quantity of metal lost by the corrosion process is directly proportional to the amount of direct current which flows through the corrosion cell. The weight loss was shown by Michael faraday as W=K*I*t

W= Weight loss in grams

K= Electrochemical equivalent in grams/ coulomb

I= Current in Amperes

t= Time in seconds.

For a given amount of current over a given period of time, the electrochemical equivalent (K) is the variable which determines the actual weight loss of the metal or material. Each metal has its own electrochemical equivalent which is a natural characteristic of that metal.

The thermodynamic instability of a metal depends on its amount of energy expended in extracting it from its natural ore (Ala et al, 2009, Saumade and Fontaine, 2008). The higher the energy expended, the higher is the thermodynamic instability, and the greater is the tendency of metal to migrate into electrotyle in ionic form. The thermodynamic instability of metals is expressed in terms of electrochemical potentials as shown in table1.

Table 1: Electrochemical Potential Series

illustration not visible in this excerpt

The magnitude of initial potential difference is influence by the electrochemical potential of metal and factors responsible for the formation of anodic areas. The greater the potential difference the higher the magnitude of corrosion current (Laver and Griffiths, 2001, Habjanic and Trlep, 2006).

illustration not visible in this excerpt

Eco = initial potential at cathode

Eca = initial potential at anode

R = Ohmic resistance of the galvanic cell.

Ohmic resistance of the galvanic cell depends upon the specific resistivity of the electrolyte, ratio of the areas of the anodic and cathode phases and geometric configuration.

For a given corrosion cell, the effect of ohmic resistance can be predicted on the basis of resistivity of soil and distance between anode and cathode (Ala and Di Silvestre, 2002, Poljak and Doric, 2006).

There are two distinguishable corrosion cells, these are the micro and macro corrosion cells (Gupta, 2005, Mohamad et al, 2006).

Micro cell which causes corrosion of electrode (conductor) are formed due to heterogeneity in the metal surface and its composition. Physicochemical properties of soils are uniform at anode and cathode and the ohmic resistance in such cells is generally the same as predicted on the basis of physiochemical properties of soils.