Corrosion+Mechanism


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 * The CO2 Corrosion Mechanism **

CO2 is naturally present in the ground, but it is also injected into wells as a method to increase oil recovery. The CO2 reacts with ground water (H2O), to produce carbonic acid (H2CO3):

CO2(g) ↔ CO2(aq)

CO2(aq) + H2O(l) → H2CO3(aq)

For the production of Carbonic Acid, the rate constant is K=1.70E-3 (at 25C, 1atm), so the reactants are favoured and only a small amount of CO2 is converted into H2CO3.

Carbonic Acid (H2CO3), which is diprotic dissociates into Bicarbonate (HCO3-) and then can dissociate into Carbonate (CO32-).

H2CO3(aq)+ e- → H+(aq)+ HCO3-(aq) (K=2.5E-4,E°=-0.21V), at 25C, 1atm

HCO3-(aq)+ e- → H+ (aq)+ CO32-(aq) (K=5.61E-11,E°=-0.61V ), at 25C, 1atm

The third cathodic reaction also includes the reduction of hydrogen ions that are present in the ground water.

2H+(aq)+ 2e- → H2(g) (E°=0.00V), at 25C, 1atm

The selectivity of the cathode reaction is determined by certain environmental factors.

The anode reaction is the dissolution of iron metal at the pipe surface into iron ions.

Fe(s) → Fe2+(aq)+ 2e- (E°=0.44V), at 25C, 1atm

The overall electrochemical reaction of CO2 corrosion is given by,

Fe(s) + CO2(aq) + H2O(l) → FeCO3(aq) + H2(g)

Where FeCO3 can further react to form a precipitate.

FeCO3(aq)+CO32-(aq) → Fe(CO3)2(s)


 * Sources **

===[Author unknown] Basic Chemical Engineering Operations: Carbon Dioxide CO2 Corrosion Mechanism (In pipe lines of natural gas and oil transportation) [Internet]. [Cited 2010 November 1]. Available from: [] ===

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