What is Corrosion

Corrosion

Corrosion is the destruction or deterioration of metal by direct chemical and electrochemical reaction with its environment (Budinski, 1983). It is the gradual physiochemical destruction of materials by the action of environment. The word corrosion comes from the Latin word “corrodore”, which means to graw away. Corrosion usually begins at the surface of a material as a result of the combined action of atmospheric oxygen and water.


This destruction or deterioration of metal occurs because of the spontaneous tendency of the materials to return to their thermodynamic stable state or to one of the forms in which they are originally found (Uzorh, 2013). Metals are generally prone to corrosion because most of them occur naturally as ores, which is the most stable state of low energy, and there is a net decrease in free energy from metallic to oxidized state (Jaikumar et al., 2010). Most simply stated metallic corrosion is the reverse of electroplating. It is accelerated by the presence of carbon dioxide in the air. The most important example of metallic corrosion is that of iron, which is commonly called rusting.

The corrosion lake place at the metal medium phase boundary and therefore is a heterogeneous reaction in which the structure and condition of the metal surface have a significant role (e.g whether the surface is uncoated, coated with an adhesion, compact or loose process coasting, or whether its properties have been changed by machining and processing).
Therefore, material transport phenomena including free convection and diffusion into adjacent surface layers must also be taken into account (Idenyi and Ekuma, 2006). Corrosion also exists on alloy because it is an intentional mixing of two or more chemical elements that have at least one metallic property (Idenyi et al., 2006).

Types of Corrosion

There are four basic types of corrosion. These are general corrosion, localized corrosion, caustic agent corrosion, galvanic corrosion, putting corrosion and atmospheric corrosion.

General corrosion: 

General corrosion occurs as a result of rust. When metal, specifically steel, is exposed to water, the surface is oxidized and a thin layer of rust appears. Like galvanic corrosion, general corrosion is electrochemical. In order to prevent oxidation, a preventive coaling must interfere with the reaction.

Localized corrosion:

Localized corrosion occurs when a small part of a component experiences corrosion, or comes into contact with specific corrosion-causing stresses. Because the small "'local" area corrodes al a much faster rate than the rest of the component, and the corrosion works alongside other processes such as stress and fatigue, the end result is much worse than the result of stress or fatigue alone.

Caustic Agent Corrosion:

Caustic agent corrosion occurs when impure gases, liquids, or solids wear a material down. Although most impure gases do not damage metal in dry form, when exposed to moisture they dissolve to form harmful corrosive droplets. Hydrogen sulphide is an example of such caustic agent.

Galvanic Corrosion:

Galvanic corrosion is extraordinarily common, and occurs when two metals with different electrochemical charges are linked via a conductive metal. In this case, a corrosion resistant coating would be applied to prevent either the transfer of ions or the condition that causes it. Galvanic corrosion can also occur when one impure metal is present. If a metal contains a combination of alloys that posses different charges, one of the metals can become corroded. The anodized metals is the weaker, less resistant one, and loses ions to the stronger, positively charged cathodic metal. Without exposure to an electrical current, the metal corrodes uniformly - this is known as general corrosion.

Atmospheric corrosion:

This is know as try corrosion. It is probably the most common form of corrosion and its defined as the corrosion or degratation of materials exposed to the aim and its pollutants. Therefore, it is important to know the specific corrosion rate in given application environment in order to effectively use metals in out doors structure. Most of the board forms of corrosion occur in the atmosphere and some appear to be largely restricted to it. Since the corroding metal is not bathed in large quantity of electrocyte, most atmospheric corrosion operates in hgihly localized corrosion cells (Kenneth, 1970).

Pitting Corrosion:

Pitting is an extremely localized attack that results in a small or large hole in the metal or alloye. Pitting is the most destructive and insidious form of corrosion. At the anode, the basic reaction is corrosion or metal dissolution subsequently ions are released into the solution recordings to the reaction.
Zn = Z+n + Ze Zn+ + e-
Furthermore, oxygen evolution may occur as a completing anodic reaction under high anodic potential as show below.
2H20 = 02+ 4H+ + 4e-
At the cathode, the basic reaction involves either hydrogen evolution or reduction of dissolved oxygen or both.
2H+ + 2e-       H2
02 + 4H+ + 4e- = 2H2O (in acide solution)
02 + 2H20 + 4e- = 40H (in basic or metal solution).
However, metal ion reduction may occur as a completing cathodic reaction.
Hence
Cu2+ +2e- = Cu

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