Corrosion and Prevention- Chemistry- CHEM 4210- Physical Chemistry

Subject: Chemistry
Topic: Corrosion and Prevention 
Subject Code: CHEM  4201
Teacher Name: Md. Jahedul Islam
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Corrosion & Prevention

Definition of 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.

Attack of Environment on Materials

q Metals get oxidized.

q  Polymers react with oxygen and degrade.

q  Ceramic refractories may have dissolved in contact with molten materials.

q  Materials may undergo irradiation damage.

Oxidation

q Oxide is the more stable than the metal (for most metals).

q  Oxidation rate becomes significant usually only at high temperatures.

q  The nature of the oxide determines the rate of oxidation.

Effects of corrosion

Ø Reduced Strength.

Ø Downtime of equipment.

Ø Escape of fluids.

Ø Lost surface properties.

Ø Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal.  Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. 

Ø Underground corrosion

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs, by which time considerable damage may be done.

Ø Electronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections.  Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits

Ø Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped.  The high velocities in the pump accentuated the corrosion damage.

Ø Safety of aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet.  Any failure of a structural component of an aircraft can lead to the most serious results.

Ø Influence of corrosion on value

A very slight amount of corrosion may not interfere with the usefulness of an article, but can affect its commercial value.  At the points where these scissors were held into their plastic case some surface corrosion has occurred which would mean that the shop would have to sell them at a reduced price.

Ø Motor vehicle corrosion and safety

The safety problems associated with corrosion of motor vehicles is illustrated by the holes around the filler pipe of this petrol tank.  The danger of petrol leakage is obvious.  Mud and dirt thrown up from the road can retain salt and water for prolonged periods, forming a corrosive “poultice”.

Ø Corrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel.  This ship has suffered severe damage in the areas which are most buffeted by waves, where the protective coating of paint has been largely removed by mechanical action.

Ø “Corrosion” of plastics

Not only metals suffer “corrosion” effects.  This dished end of a vessel is made of glass fibre reinforced PVC.  Due to internal stresses and an aggressive environment it has suffered “environmental stress cracking”.