CORROSION BEHAVIOUR OF METALS/ALLOYS WITH AND WITHOUT COATING AT HIGH TEMPERATURE (200 – 1000OC) AND UNDER MARINE ENVIRONMENT

INTRODUCTIONCorrosion is the deterioration a material undergoes as a result of its interaction with its surroundings. Although this definition is applicable to any type of material, it is usually reserved for metallic alloys. Of the 105 known chemical elements, approximately eighty are metals , and about half of these can be alloyed with other metals, giving rise to more than 40,000 different alloys. Each of the alloys will have different physical, chemical, and mechanical properties, but all of them can corrode to some extent, and in different ways.Corrosion is a natural phenomenon. When newly made steel is first exposed to the air, its originally shiny surface will be covered with rust in a few hours. The tendency of metals to corrode is related to the low stability of the metallic state. Metals occur either in the pure metallic state, the zero oxidation state, or in the form of compounds with other elements (they acquire positive states of oxidation). In the natural world, most metals are found as compounds with other elements, indicating the greater stability of their oxidized forms. For this reason, to obtain the pure metal from one of its compounds, it is necessary to put in energy. The reverse is true when a metal is exposed to its environment: it tends to release this stored energy through the processes of corrosion. This is rather analogous to what happens when an object is suspended at a point above the ground (equivalent to the metallic state). When allowed to fall or reach a stable state, it returns to a position of minimum energy on the ground (equivalent to the metal's oxidized state).The chemical reactions that take place in corrosion processes are reduction-oxidation (redox) reactions. Such reactions require a species of material that is oxidized (the metal), and another that is reduced (the oxidizing agent). Thus the complete reaction can be divided into two partial reactions: one, oxidation; the other, reduction. In oxidation, the metal loses electrons. The zone in which this happens is known as the anode. In the reduction reaction, the oxidizing agent gains the electrons that have been shed by the metal, and the zone in which this happens is the cathode.
TABLE OF CONTENTS
Introduction 2 1.1 Definition of Terms 2.0 Forms of Corrosion 2.1 High Temperature Corrosion of Metals/Alloys 2.2 Corrosion Behaviour of Metals/Alloys with and without coating at High  Temperature (200 – 10000C) and under Marine Environment 2.3 Economic Impact of Corrosion         2.4 Prevention of Corrosion Conclusion References