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Sahithyan's S1

Prevention of Corrosion

Corrosion results in loss of materials and money. It must be minimized or controlled.

To avoid corrosion

  • Use noble metals (Au, Pt)
  • Use purest possible metal - not possible in real life
  • Use corrosion resistant alloys
  • Use non-metals (like polymers or ceramics)

Cathodic protection

Corrosion can be prevented by making the metallic structure as cathode.

2 types of cathodic protection is available.

Galvanic protection

Metal to be protected is made cathode by connecting it to an anodic metal (which is called the sacrificial anode). Zn and Mg are commonly used as anodic metals. Aka. sacrificial anodic protection.

Uses

  • Protection from soil corrosion of buried pipelines, underground cables
  • Protection from marine corrosion of cables, ship hulls, piers
  • Prevention of rust in water boilers by inserting magnesium
  • Minimizing engine corrosion using calcium

Impressed current cathodic protection

Suppressing corrosion by supplying external current (opposite to corrosion). The applied current converts corroding metal from anode to cathode.

Usually current is derived from DC sources with an inert anode.

Modifying the environment

Corrosion can be prevented by adding inhibitors to the environment.

Examples:

  • Hydrogen evolution posions - used when cathodic reactions are HER.
  • Oxygen scavengers - eliminates oxygen from the corroding medium. used when cathodic reactions are ORR.

Protective coatings

Preventing corrosion by covering the surface by means of protective coatings.

The coating acts as a physical barrier between the coated metal surface and the environment.

2 types:

  • Inorganic
    • Metallic
    • Non-metallic
  • Organic
    • Paints

Galvanizing

The process of coating steel sheets with a thin coat of zinc to prevent them from rusting.

Steel article is:

  1. cleaned with diluted sulfuric acid
  2. washed with distilled water
  3. dried
  4. dipped in a bath of molten zinc

Now zinc is coated on the steel article.

Paints

Paint is a viscous, opaque mechanical dispersion mixture of one or more pigments (dye) in a vehicle (drying oil).

Corrosion inhabitors are added to paints.