What is Corrosion

Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals. The most widely used metal is iron (usually as steel) and the following discussion is mainly related to its corrosion.

 

It results from oxidation of metal.

• First the metal releases electrons to form positive ions.
• Atmospheric water, condenses over the metallic surface dissolves oxygen.
• Oxygen + Water results in formation of OH ion. 
• OH (-) and Fe(+) combine to for iron hydroxide.
• Iron hydroxide reacts with oxygen to give iron oxide (rust)

We infer from the above steps that 

• Ions are involved and need a medium to move in (usually water)
• Oxygen is involved and needs to be supplied
• The metal has to be willing to give up electrons to start the process
• A new material is formed and this may react again or could be protective of the original metal
• A series of simple steps are involved and a driving force is needed to achieve them

To avoid corrosion read How to avoid corrosion

 

Types of Corrosion

1. Uniform Corrosion
2. Localized Corrosion 
• Galvanic Corrosion 
• Pitting Corrosion
• Selective Attack
• Stray Current Corrosion
• Microbial Corrosion
• Intergranular corrosion
• Concentration Cell Corrosion
• Thermogalvanic Corrosion
• Corrosion caused by combined action
• Corrosion Fatigue
• Fretting Corrosion
• Stress Corrosion Cracking
• Hydrogen Attack