Corrosion Inhibitor Adsorption and Desorption Kinetics in CO₂-Containing Aqueous Environments

This thesis examined how key parameters affected the adsorption and desorption
kinetics of a surfactant corrosion inhibitor on X65 carbon steel in CO₂-saturated brine.
The work focused on continuous-dosing systems in the energy and carbon abatement
sectors where a fall in inhibitor concentration due to limited inhibitor availability or
dosing interruptions may occur. During such cases, operators often assumed no
protection, yet surfactant inhibitors exhibit a property known as persistency, the ability
of an inhibitor to remain on the metal surface for a defined period after dosing stops
before desorbing.

The study evaluated the persistency of a commonly studied surfactant corrosion
inhibitor, benzyldimethyltetradecylammonium chloride (BAC-C14) with two main
experiments. First, a modified rotating cylinder electrode (RCE) setup with linear
polarization resistance measurements was used for corrosion rate monitoring. The
test comprised four stages: (1) pre-corrosion of X65 for a specified period; (2) an
adsorption period with the inhibitor at the desired concentration; (3) a dilution stage in
which the inhibited, concentrated test brine was replaced with fresh, uninhibited brine
to reduce the inhibitor concentration to below 1 ppm; and (4) a persistency period to
track the desorption profile. Second, a quartz crystal microbalance with dissipation
(QCM-D) was run under similar conditions to provide real-time mass uptake and loss
on inert (gold) and reactive (iron) substrates, as well as information on the
viscoelasticity of the surface film.

The investigation covered concentrations expressed as fractions of the critical micelle
concentration (CMC), specifically below, at, and above the CMC, along with variations
in surface conditions (via different pre-corrosion periods) and metallurgy (X65 carbon
steel versus high-purity iron). Films established at 1.00xCMC on X65 delivered the
highest inhibition efficiency and persistency. At 0.75xCMC and 2.00xCMC, protection
declined rapidly after dilution, indicating a highly reversible monolayer below the CMC
and a micelle-biased partitioning effect above the CMC that hindered formation of a
protective film. Metallurgy had the largest influence on persistency: high-purity iron
showed the highest inhibition efficiency and partially irreversible persistency at every
concentration tested.

A pseudo-first-order (PFO) Langmuir model was used to predict persistency from the
adsorption segment of the data for rapid screening. The model successfully captured
monolayer-like cases (seen with gold and carbon steel at 0.75x and 2.00xCMC) but
failed for films with bilayers or strong, partially irreversible binding, thereby defining the
PFO model limits.