A Systematic Study of Film Formation in C02 Corrosion

COz corrosion is one of the major problems in oil and gas industry. The formation of
FeCOJ film in C02 corrosion is observed to provide corrosion protection; however,
the degree of protection is unclear. The main objective of this study is to understand
the mechanism of protective film formation in COz environment with the aim to
assess the potential as corrosion mitigation and possible interaction between
corrosion inhibitor and film formation. The film formation tests were conducted in
one-litre glass cell under typical C02 corrosion test set-up by two film formation
methods - natural film and induced film formation with 50 and lOOppm Fe2
+ ions
.The interaction of corrosion inhibitor and induced FeC01 film fromation were
studied for two cases; full protection of corrosion inhibitor dosage of 50ppm and
partial protection at 25ppm corrosion inhibitor. Both studies were done at pH 6.0 and
for 24 hours of immersion. Corrosion rates were measured electrochemically by
Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance
(LPR). The condition of the film formation was analyzed by both Scanning Electron
Microscopy (SEM) and EIS.
The result s showed that for the natural film formation, the corrosion rate at 80 ° C
was lower than at 50°C which were 0.34 mm/year and 1.78 mm/year respectively.
Same trend of corrosion rate obtained from induced film formation where at 80 ° C,
the corrosion rate was 0.98 mm/year and at 50°C the corrosion rate was 0.04
mm/year. With high concentration of corrosion inhibitor deployment, the corrosion
rate was low. By injecting 50ppm imidazoline, the corrosion rate was 0.03 mm/year
and at 25ppm imidazoline, the corrosion rate was 0.76 mm/year. Interaction of
FeC03 and imidazoline was possible where by induced 50ppm and lOOppm ofFe2
+,
the corrosion rate were at 0.06 mm/year and 0.02 mm/year respectively.

FeC03 film formation in C02 corrosion is due to solubility of FeC03 which depends
on functions like saturation, pH and temperature. Partial FeC03 formation as
produced at 80" C under natural film formation reduces corrosion rate to 0.34
mm/year. Protective film formation as produced for inducing method reduces
corrosion rate down to 0.04 mm/year. This proves that protective film could reduce
the corrosion rate and method to accelerate film formation could be beneficial to
mitigate corrosion.
The mechanism of corrosion inhibitor adsorption is different than FeC03 film
formation. Corrosion inhibitor with sufficient dosage will be able to reduce corrosion
rate to a low value and F eC03 film was not formed. A possible positive interaction
between corrosion inhibitor and FeC03 formation could occur whereby low
corrosion inhibitor dosage will be supplemented by the formation of FeC03 film to
reduce the corrosion rate to a low value.