Corrosion Science Chemistry Research Article | Page 4

4
Corrosion: Understanding the Basics
Perhaps most dangerous of all is corrosion that occurs in major indus-
trial plants, such as electrical power plants or chemical processing
plants. Plant shutdowns can and do occur as a result of corrosion. This is
just one of its many direct and indirect consequences. Some conse-
quences are economic, and cause the following:
•
•
•
•
•
•
Replacement of corroded equipment
Overdesign to allow for corrosion
Preventive maintenance, for example, painting
Shutdown of equipment due to corrosion failure
Contamination of a product
Loss of efficiency—such as when overdesign and corrosion products
decrease the heat-transfer rate in heat exchangers
• Loss of valuable product, for example, from a container that has cor-
roded through
• Inability to use otherwise desirable materials
• Damage of equipment adjacent to that in which corrosion failure occurs
Still other consequences are social. These can involve the following is-
sues:
• Safety, for example, sudden failure can cause fire, explosion, release
of toxic product, and construction collapse
• Health, for example, pollution due to escaping product from corroded
equipment or due to a corrosion product itself
• Depletion of natural resources, including metals and the fuels used to
manufacture them
• Appearance as when corroded material is unpleasing to the eye
Of course, all the preceding social items have economic aspects also
(see the discussion that follows, “Economic Impact of Corrosion”).
Clearly, there are many reasons for wanting to avoid corrosion.
The Many Forms of Corrosion
Corrosion occurs in several widely differing forms. Classification is
usually based on one of three factors:
• Nature of the corrodent: Corrosion can be classified as “wet” or
“dry.” A liquid or moisture is necessary for the former, and dry corro-
sion usually involves reaction with high-temperature gases.
• Mechanism of corrosion: This involves either electrochemical or di-
rect chemical reactions.