Srdjan Nešić

The limiting current density for the hydrogen evolution reaction in aqueous weak carboxylic acids was re-derived using a general approach that incorporates revised boundary conditions and a modified formulation for the mass transfer boundary layer. Unlike most previous derivations, the new expression is not restricted to rotating disk electrode geometry and is applicable to other flow configurations. Furthermore, it remains valid under conditions that extend beyond the previous restriction about the chemical reaction boundary layer, used in the classic studies on this topic. The limiting current densities calculated from the new expression were compared with numerical results and with reliable experimental data for aqueous acetic acid solutions, demonstrating improved agreement relative to earlier models. Parametric studies were also performed to examine the influence of buffering kinetics and weak carboxylic acid concentration on the limiting current density behavior. The resulting expressions for the limiting current density and for the mass transfer boundary layer thickness in aqueous weak carboxylic acid solutions take the following forms: i lim,H + buff,HA = z H + F D H + c b,H + δ m ( 1 + D HA c b,HA D H + c b,H + 1 + 1 λ D HA c b,HA D H + c b,H + ) δ m = c b,HA ( δ m,HA ) 3 + c b,H + ( δ m,H + ) 3 c b,HA + c b,H + 3 .

The influence of 1,000 ppmm of acetic acid on the corrosion mechanisms of mild steel with up to 10 mbar (10,000 ppmv) of H2S with and without CO2 at atmospheric pressure was investigated using electrochemical and surface analysis techniques. The results showed that anodic reaction retardation caused by acetic acid in CO2 corrosion at low temperatures was not observed when H2S was present in an H2S/CO2 environment. The potentiodynamic sweeps for H2S/CO2 corrosion showed a retardation of the corrosion rate, which is thought to be caused by a thin mackinawite layer that is known to form in these conditions.

In the present study, a quartz crystal microbalance with dissipation monitoring (QCM-D) was used to investigate the adsorption of a model corrosion inhibitor compound, tetradecyldimethylbenzylammoniumbromide (BDA-C14), on gold electrode. Sauerbrey’s equation was used to analyze the equilibrated normalized frequency change for estimation of the adsorbed mass and adsorbed layer thickness at different bulk inhibitor concentrations after careful validation. The average adsorbed layer thickness for BDA-C14 at tested experimental conditions lie in the range of 1 nm to 1.4 nm. Time-dependent part of the frequency change was analyzed using Langmuir adsorption isotherm to calculate the kinetic constants (kA = 0.075±0.02 mM–1·s–1, kD = 0.0023±0.0007 s–1, and KAD = 32.2 mM–1). Equilibrium surface coverage (θeq) was estimated at each bulk inhibitor concentration tested: 0.69 for 25 ppm(w), 0.74 for 50 ppm(w), and 0.91 for 100 ppm(w). Some theoretical calculations are also shown explaining the use of known molecular geometry and adsorption kinetics information from QCM-D analysis to reasonably speculate the predominant adsorbed layer configuration. A conscious effort is made to state and validate each assumption made for the analysis of the experimental results.

Aqueous CO2 corrosion of mild steel is one of the major problems in the oil and gas industry. While current understanding primarily focuses on cathodic reaction mechanisms, less attention has been given to the impact of aqueous CO2 on the anodic iron dissolution reaction. In contrast, the mechanism of iron dissolution in strongly acidic environments has been thoroughly investigated. Among the reaction mechanisms found in the open literature, a multipath mechanism was identified that could explain the iron dissolution in strong acidic sulfate solution; both in terms of steady-state polarization sweeps and impedance data at various pH values and current densities. However, the role of aqueous CO2 in solutions containing chlorides on the mechanism of iron dissolution remained an open question. The present study used electrochemical impedance spectroscopy (EIS) as the main technique, to study the mechanism of iron dissolution in strong acid chloride solution with and without the presence of CO2. Results showed that the presence of chloride ions (0.5 M) decreased the rate of iron dissolution by competing with hydroxide ions to adsorb on the metal surface, forming chloride-containing intermediate species that participate in the iron dissolution reaction. The resulting decrease in the availability of hydroxide intermediates, which are more effective at enhancing the reaction rate compared to chloride-containing intermediates, leads to an overall decrease in the rate of iron dissolution. While the presence of CO2 increases anodic current density, EIS investigation revealed that neither aqueous CO2 nor other carbonic species directly react on the bare metal surface to form adsorbed intermediates involved in the anodic reaction. EIS investigation suggested that aqueous CO2 may induce changes in the chemical composition of adsorbed species, rate constants, and surface coverage, thereby altering the kinetics of the underlying reactions.

The use of organic surfactant corrosion inhibitors provides an economical and effective way for internal corrosion control of oil and gas production and transportation pipelines. Corrosion inhibition measurements using electrochemical techniques can be used to evaluate the efficacy of corrosion inhibitors based on steady-state corrosion rates. The corrosion behavior with respect to time can further be utilized to quantify the adsorption kinetics of inhibitor molecules on metal and can further be integrated into the development of corrosion prediction tools. However, corrosion rate measurements can be very sensitive to experimental methodology in the presence of corrosion inhibitors in the test solution, and repeatability is often difficult to achieve if a robust methodology is not followed meticulously. In this study, the importance of the location of inhibitor addition in lab-scale corrosion experimentation is discussed.

Time-of-flight-secondary ion mass spectrometry (ToF-SIMS) 3D mapping and depth profiling were used to study the anodic iron dissolution mechanisms of mild steel in chloride-containing aqueous CO2 environments. The technique detected adsorbed hydroxide and chloride intermediates formed during the corrosion process, consistent with the proposed multipath reaction mechanism for anodic iron dissolution reaction. Despite the presence of aqueous carbonic species and their observed effect on the kinetics of iron dissolution, no additional adsorbed intermediates have been detected in aqueous CO2 environments, indicating that carbonic species do not directly participate in the iron dissolution reaction. ToF-SIMS 3D mapping results on characterization of the specimens immersed in a chloride-containing solution with and without CO2 suggest that one role of aqueous carbonic species CO2 could be to accelerate the adsorption of chloride ions and the formation of chloride intermediates.

The effect of H2S on the aqueous corrosion behavior of mild steel was evaluated at high-pressure and high-temperature (HPHT) conditions at a partial pressure of CO2 of 12 MPa and a temperature of 160°C. The corrosion rate of steel samples was determined by electrochemical and weight loss measurements. The surface and cross-sectional morphology and the composition of the corrosion product layers were analyzed by using surface analytical techniques (scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction). Results showed that the corrosion rate decreased with time and no significant difference was observed in the presence of 1,000 ppm and 2,000 ppm of H2S at HPHT CO2 conditions. Surface and cross-sectional analyses revealed that the corrosion process was governed by the formation of FeCO3 regardless of the presence of H2S. Furthermore, the corrosion behavior of mild steel in these conditions did not depend significantly on flow velocity.

The effect of a small amount (1 wt%) of Cr or Mo on aqueous CO2 corrosion of low-alloyed steel and the formation of protective FeCO3 corrosion product layers was investigated under controlled water chemistry conditions, where the bulk saturation value of FeCO3 was maintained at near-saturated condition. Changes in CO2 corrosion rate with exposure time were monitored by linear polarization resisitance measurements. The surface morphology and the composition of the corrosion product layers were analyzed by scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and transmission electron microscopy. Results showed that the emergence of a continuous Fe3C layer created favorable conditions at the surface of nonalloyed steel (containing no Cr and Mo) for semiprotective FeCO3 to form even though the bulk saturation value of FeCO3 was maintained at near-saturated condition. However, semiprotective FeCO3 was not observed on the surface of 1% Cr steel and 1% Mo steel, but rather discontinuous and porous corrosion product layer was formed. Due to the hydrolysis reactions of Cr3+ and Mo3+, and the discontinuous structure of the corrosion product layers, the surface conditions for 1% Cr steel and 1% Mo steel were not favorable for the formation of FeCO3 under the experimental conditions of this study.

Electrochemical atomic force microscopy (EC-AFM) experiments, including simultaneous linear polarization resistance (LPR) tests and in situ AFM imaging, under a CO2 atmosphere, were performed to investigate the adsorption characteristics and inhibition effects of a tetradecyldimethylbenzylammonium corrosion inhibitor model compound. When the inhibitor bulk concentration was at 0.5 critical micelle concentration (CMC), in situ AFM results indicated nonuniform tilted monolayer formation on the mica surface and EC-AFM results indicated partial corrosion of the UNS G10180 steel surface. At 2 CMC, a uniform tilted bilayer or perpendicular monolayer was detected on mica, and corrosion with UNS G10180 steel was uniformly retarded. Consistently, simultaneous LPR tests showed that corrosion rates decreased as the inhibitor concentration increased until it reached the surface saturation value (1 and 2 CMC). Molecular simulations have been performed to study the formation of the inhibitor layer and its molecular-level structure. Simulation results showed that at the initiation of the adsorption process, islands of adsorbed inhibitor molecules appear on the surface. These islands grow and coalesce to become a complete self-assembled layer.

In the present study, the implementation of a comprehensive mechanistic predictive model for corrosion of mild steel in the oil and gas transmission pipelines is described. The present model simultaneously accounts for all major corrosion scenarios, including CO2 corrosion, H2S corrosion, and corrosion in the presence of organic acid and also incorporates the effect of corrosion product layer formation, including iron carbonate and iron sulfide. With this approach, the present model mechanistically reflects the mainstream understanding of corrosion in such environments and can be readily used to predict the corrosion rates in industrial applications. The model was implemented by using a generalized mathematical and programming approach that has built-in flexibility to add new chemical species and additional reactions in the future. The model was designed to make it easy to extend and cover an even broader range of conditions than it currently does, such as higher temperatures and pressures, nonideal solutions, etc. The mechanistic nature of the model allows it to be readily coupled with other applications such as computational fluid dynamics codes, multiphase flow simulators, process design simulators, etc. In order to demonstrate the capabilities of this model, the calculated corrosion rates were compared with the experimental corrosion rate data across a broad range of environmental conditions and brine chemical compositions.