Environmental Degradation or Corrosion

Corrosion Deteriorative Mechanisms

1. In Metals  ↠  actual metal lost by corrosion (dissolution) or oxidation (formation of non-metallic scale or film).
2. In Ceramic Materials  ↠  resistant to deterioration but occurs at elevated temperature.
3. In Polymers (degradation)  ↠  may dissolve in liquid solvent or absorb solvent.

Corrosion in Metals

It is defined as, "the destructive or unintentional attack on a metal". 

• 5 % of income is spent on corrosion prevention and maintenance.

Corrosion is occasionally used as advantage:

• Etching (chemical reactivity of grain boundaries).
• Current develops in dry cell batteries.
  

Types of Corrosion

1. Uniform Attack  ↠  form of electrochemical corrosion that occurs with equivalent intensity over the exposed surface and often leaves behind a deposit.
2. Galvanic Corrosion  ↠  occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte.
3. Crevice Corrosion  ↠  due to the concentration differences of ions or dissolved gases in the electrolyte solution between two regions of metal piece. It can be prevented using welding instead of stagnant areas (riveted or bolted joints).
4. Pitting  ↠  localized corrosion in which small pits or holes form initiated by surface defect (scratch, slight composition variation). It can be prevented using polished surface.
5. Intergranular Corrosion  ↠  occurs preferentially along grain boundaries for some alloys and in specific environments.
6. Selective Leaching  ↠  occurs when one element in solid solution (alloy) is removed as a consequence of corrosion processes.
7. Erosion Corrosion  ↠  combined action of chemical attack and mechanical abrasion due to fluid motion. It is prevented by changing the design to eliminate fluid turbulence and impingement effects. For eg: bend elbows, propellers, turbine blades, valves, pumps.
8. Stress Corrosion Cracking  ↠  result of combined action of an applied tensile stress and a corrosive environment. Prevented by reducing stress magnitude.
9. Hydrogen Embrittlement  ↠  in response to applied or residual tensile stresses, brittle fracture occur catastrophically as crack grows and rapidly propagates.

Corrosion Prevention

• Inhibitors  ↠  reduce corrosion rate (normally used in closed systems).
• Physical Barriers  ↠  applied to surface (as films or coating) to reduce corrosion.
• Cathodic Prevention  ↠  supplying of electrons to metal from an external source to the metal to be protected.

Note : Get more info about Corrosion.

References:

• Material from Class Lectures + Book named Materials Science and Engineering: An Introduction by Callister and Rethwick + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube (Engineering Sights).