1.0 Introduction
1.1 Background Of Study
1.1.1 Corrosion
Corrosion is defined as a natural process, which converts refined metal to their more stable oxide. It is the gradual destruction or degradation of materials (usually metals) by chemical reaction with their environment which are most likely inevitable. Corrosion is a natural and costly process of destruction like earthquakes, tornados, floods and vocanic eruptions, with one major difference. Whereas we can be only a silent spectator to the above processes of destruction, corrosion can be prevented or at least be controlled.
Despite different definitions, it can be observed that corrosion is basically the result of interaction between materials and their environment. Up to the 1960s, the term corrosion was restricted only to the metals and their alloys and it did not incorporate ceramics, polymers, composites and semiconductors in its regime. The term corrosion now encompasses all types of natural and man – made materials including biomaterials and nanomaterials, and it is not confined to metals and alloys alone. The scope of corrosion is consistent with the revolutionary changes in materials development witnessed in recent years.
Figure 1.1: Corrosion Attack On An Old Ship – the diagram is in the complete material which you will get before the end of this page
1.1.1.2 Corrosion And Its Mechanism
In nature, metals are not found in free state due to their reactivity. Metals are generally in high energy state because some energy is added during their manufacturing process from the ores. Low energy – state ores are more stable than the high energy – state metals. As a result of this uphill thermodynamic struggle, the metals have a strong driving force to release energy and go back to their original form. Hence the metals revert to their parent state or ore under a suitable corrosive environment. The electrochemical process involved in corrosion by nature is opposite to the extractive metallurgy involved in manufacturing of the metals. Therefore, corrosion is sometimes considered as the reverse process of extractive metallurgy as can be seen below:
Figure 1.2: The Energy Cycle Of Iron Indicating Its Extractive Metallurgy – the diagram is in the complete material which you will get before the end of this page
According to electrochemistry, the corrosion reaction can be considered as taking place by two simultaneous reactions:
The oxidation of a metal at an anode (a corroded end releasing electrons) and the reduction of a substance at a cathode (a protected end receiving electrons). In order for the reaction to occur, the following conditions must exist:
- Two areas on the structure must differ in electrical potential.
- Those areas called anodes and cathodes must be electrically interconnected.
- Those areas must be exposed to a common electrolyte.
- An electric path through the metal or between metals be available to permit electron flow.
When these conditions exist, a corrosion cell is formed in which the cathode remains passive while the anode deteriorates by corrosion. As a result of this process, electric current flows through the interconnection between cathode and anode. The cathode area is protected from corrosion damage at the expense of the metal, which is consumed at the anode. The amount of metal lost is directly proportional direct current flow. Mild steel is lost at approximately 20 pounds for each ampere flowing for a year. (Thomas, 1994) .
Figure 1.3: The Component Of An Electrochemical Corrosion Cell – The Diagram Is In The Complete Material Which You Will Get Before The End Of This Page
At the anode, metals are oxidized and the electrons are liberated from the metal to form positive metal ions. The liberated electrons dissolve into the electrolyte, and deposition is formed on the cathodic metal. Anode corrodes while the cathode remains intact.
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