About Corrosion

Most metals in nature exist in form of compounds (oxides) since they are stable in this form. Metals that can be used in daily life and industrial applications are produced by changing the form of these compounds using external energy. This process requires energy, knowledge and appropriate materials. Once the form of metals change they become unstable since production processes infuse external energy. Due to that fact metals become unstable, metals try to return to their stable compound form by first reaction with other elements in environment and changing to metal ions then changing to oxide form which is their stable and initial form in nature by reacting with other elements in environment. These series of deterioration reactions, deterioration of materials physical, chemical, mechanical and electrical properties, are referred as CORROSION.

Corrosion is a spontaneous process in which metals are in a reaction with their environments without a need for external energy. There is an enormous amount of loss of material, energy, effort, knowledge and product due to corrosion. Besides the financial losses due to corrosion, it should not be forgotten that corrosion also brings threats to nature and human life. In Turkey the loss of material due to corrosion (excluding product) corresponds to %4-5 of gross national income. (Tübitak, The Scientific and Technological Research Council of Turkey, Prof. Dr Ali Fuat Çakır)

HOW CORROSION OCCURS

Corrosion occurs between two metallic regions or points which have electric and electrolytic contact and potential difference. On the surface region or point having more noble potential difference cathode reaction takes place. In moist environments oxidation (giving electrons) and reduction (taking electrons) reactions occurred as a result of electron exchange are referred as electro chemical reactions. In that sense reactions take place in water, at atmosphere and under soil are referred as electro chemical reactions. Corrosion occurs on surfaces of metallic structures under soil corrosion in the presence of water film and dissolute oxygen inside this film on surface of materials. At clayed soil since there are less pores inside the soil, corrosive agents such as water and oxygen could not easily penetrate on surfaces of metals. On the other hand at rocky and gravelly since there are more pores, water and oxygen could easily penetrate inside soil.

In pipelines passing through both clayed and gravelly soil, the part of pipeline which is under clayed soil behaves like an anode and subject to corrosion and the part of pipeline which is under gravelly soil behaves like a cathode. In this case corrosion occurs without a need for two different metallic structures. On the other hand soils physical, chemical and biological properties could be also considered as factors affecting corrosion under soil. Chaning characteristics of soil in order to prevent under soil corrosion are not a practical and economic application. Despite that fact sometimes clean sand is used for covering pipes located at a ditch. Coating pipelines is also an effective in prevention of corrosion. However the most effective and cheap method is applying cathodic protection with isolation.