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Cathodic Protection & Electrochemical Techniques For Corrosion Control

Corrosion is a deterioration of a material, usually metal, that results from a chemical or electrochemical reaction with its environment.

Three components responsible for corrosion:

  1. Material to be corroded
  2. Chemical or electrochemical reaction
  3. Surrounding environment
  1. Metal usually high carbon steel used in oil and gas pipeline
  2. Chemical or electrochemical reaction: Chemical or electrochemical reaction takes place between metal and environment that contains moisture, oxygen and other chemicals which results into decay in metal that is called corrosion.
  3. Environment: It is a deterioration of a material, usually metal, that results from a chemical or electrochemical reaction with its environment.

Three components responsible:

  1. Material to be corroded
  2. Chemical or electrochemical reaction
  3. Surrounding environment
  1. Metal usually high carbon steel used in oil and gas pipeline
  2. Chemical or electrochemical reaction: Chemical or electrochemical reaction takes place between metal and environment that contains moisture, oxygen and other chemicals which results into decay in metal that is called corrosion.
  3. Environment: The surrounding environment responsible for electrochemical reaction is called electrolyte containing moisture, oxygen and other chemicals.

CORROSION CELL

It is an electrochemical process involving the flow of electrons and ions. Electrons are negatively charged particles and ions are electrically charged atoms either positive or negative.

Electrochemical Corrosion is due to transfer of charged ions at metal/electrolyte interface which is an anode and another part of metal is called cathode.

It occurs at anode and cathode is protected.

It occurs within a corrosion cell that consists of four parts:

  1. Anode
  2. Cathode
  3. Electrolyte
  4. Metallic Path

If any of the above is absent then ceases to exist

Corrosion protection methods are based on isolating either one of them or stopping electrochemical reaction which causes corrosion.

Electrochemical Cell- Corrosion reaction at anode and cathode

Corrosion Reaction:

Fe ↔ Fe ++ + 2e- At anode: (Oxidation)

2e- + 2H+ ↔ 2H →H2 ↑ At cathode: hydrogen reduction (more common in an acidic environment where pH is below 4)

Or

H2O +2e- +½ O2 →2(OH -) Oxygen reduction (more common in a neutral environment where pH is 6-7)

At Anode

Fe++ + 2OH- ——-› Fe (OH)2

Fe (OH)2 + ½ O2 +½ H2 O——–› Fe (OH)3

[Green color]

2Fe (OH)3 ——–›2 Fe 2 O 3 +3 H2 O

[Rust, Brown color]

Pourbaix diagram(Electrode potential / PH diagram)

The concept of corrosion from pourbaix diagram:

The phenomenon of corrosion is expressed by the Pourbaix diagram which is showing the relationship between a potential at metal-electrolyte interface and pH

The solubility of iron decreases as the potential at the interface becomes more electronegative and pH of electrolyte increases or becomes alkaline

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