Srdjan Nesic

An Electrochemical Model for Prediction of Corrosion of Mild Steel in Aqueous Carbon Dioxide Solutions

Abstract A predictive model was developed for uniform carbon dioxide (CO2) corrosion, based on modeling of individual electrochemical reactions in a water-CO2 system. The model takes into account the electrochemical reactions of hydrogen ion (H+) reduction, carbonic acid (H2CO3) reduction, direct water reduction, oxygen reduction, and anodic dissolution of iron. The required electrochemical parameters (e.g., exchange current densities and Tafel slopes) for different reactions were determined from experiments conducted in glass cells. The corrosion process was monitored using polarization resistance, potentiodynamic sweep, electrochemical impedance, and weight-loss measurements. The model was calibrated for two mild steels over a range of parameters: temperature (t) = 20°C to 80°C, pH = 3 to 6, partial pressure of CO2 (PCO2) = 0 bar to 1 bar (0 kPa to 100 kPa), and ω = 0 rpm to 5,000 rpm (vp = 0 m/s to 2.5 m/s). The model was applicable for uniform corrosion with no protective films present. Performance of...

A Mechanistic Model for Carbon Dioxide Corrosion of Mild Steel in the Presence of Protective Iron Carbonate Films—Part 1: Theory and Verification

Abstract A mechanistic model of uniform carbon dioxide (CO2) corrosion is presented that covers the following: electrochemical reactions at the steel surface, diffusion of species between the metal...

Kinetics of droplet evaporation

Abstract This paper presents research on evaporation of droplets containing dissolved or dispersed solids. Experiments on evaporation kinetics of droplets of water, colloidal silica, sodium sulphate and skimmed milk were performed. The experimental procedure was similar to that proposed by Charlesworth and Marshall (1960, A.I.Ch.E.J.6, 9–23). Individual droplets were suspended in a controlled air stream and their weight and temperature were measured as evaporation progressed. Video recording of the size and appearance of the droplets was made with magnification close to 100. The temperature was followed by a micro-thermocouple, while the mass was measured by using the deflection of a calibrated glass filament balance. Experimental data were compared with a computer simulation based on a model of heat and mass transfer, both within the droplet and from the droplet surface to surrounding air. Analytical and numerical solution of the set of equations was tested. A good agreement between experimental results and the numerical predictions for a variety of evaporation regimes is reported. Diffusion coefficients for three tested materials are determined. The model includes all stages of evaporation: initial heating and evaporation, quasi-equilibrium evaporation, crust formation and growth, boiling, and porous particle drying. Stage transition criteria are defined.

Role of Conductive Corrosion Products in the Protectiveness of Corrosion Layers

Abstract In carbon dioxide (CO2) corrosion of steels, the bicarbonate ion (HCO3−) is simultaneously the buffer for carbonic acid (H2CO3), the source of iron carbonate (FeCO3) precipitation, and the product of the cathodic reaction. In addition to spatial separation of the production of Fe2+ and HCO3−, galvanic coupling between the steel and cementite (Fe3C) layers is the principal cause of internal acidification in these layers, since the HCO3− ions are removed from the steel surface by electromigration. This can facilitate localized corrosion by lateral galvanic coupling. This mechanism explains the role of traces of free acetic acid (CH3COOH, or HAc) and the existence of multiple steady states. Transposition to corrosion of iron by hydrogen sulfide (H2S) or to corrosion of copper is discussed.

Effect of Impurities on the Corrosion Behavior of CO2 Transmission Pipeline Steel in Supercritical CO2−Water Environments

The corrosion property of carbon steel was evaluated using an autoclave under CO(2)-saturated water phase and water-saturated CO(2) phase with impurities (O(2) and SO(2)) at 80 bar CO(2) and 50 °C to simulate the condition of CO(2) transmission pipeline in the carbon capture and storage (CCS) applications. The results showed that the corrosion rate of carbon steel in CO(2)-saturated water was very high and it increased with adding O(2) in the system due to the inhibition effect of O(2) on the formation of protective FeCO(3). It is noteworthy that corrosion took place in the water-saturated CO(2) phase under supercritical condition when no free water is present. The addition of O(2) increased the corrosion rates of carbon steel in water-saturated CO(2) phase. The addition of 0.8 bar SO(2) (1%) in the gas phase dramatically increased the corrosion rate of carbon steel from 0.38 to 5.6 mm/y. This then increased to more than 7 mm/y with addition of both O(2) and SO(2). SO(2) can promote the formation of iron sulfite hydrate (FeSO(3)·3H(2)O) on the steel surface which is less protective than iron carbonate (FeCO(3)), and it is further oxidized to become FeSO(4) and FeOOH when O(2) is present with SO(2) in the CO(2)-rich phase. The corrosion rates of 13Cr steel were very low compared with carbon steel in CO(2)-saturated water environments with O(2), whereas it was as high as carbon steel in a water-saturated CO(2) phase with O(2) and SO(2).

A mechanistic model for carbon dioxide corrosion of mild steel in the presence of protective iron carbonate films-Part 3: Film growth model

Abstract A model of iron carbonate (FeCO3) film growth is proposed, which is an extension of the recent mechanistic model of carbon dioxide (CO2) corrosion by Nesic, et al. In the present model, the film growth occurs by precipitation of iron carbonate once saturation is exceeded. The kinetics of precipitation is dependent on temperature and local species concentrations that are calculated by solving the coupled species transport equations. Precipitation tends to build up a layer of FeCO3 on the surface of the steel and reduce the corrosion rate. On the other hand, the corrosion process induces voids under the precipitated film, thus increasing the porosity and leading to a higher corrosion rate. Depending on the environmental parameters such as temperature, pH, CO2 partial pressure, velocity, etc., the balance of the two processes can lead to a variety of outcomes. Very protective films and low corrosion rates are predicted at high pH, temperature, CO2 partial pressure, and Fe2+ ion concentration due to ...

A mechanistic model for carbon dioxide corrosion of mild steel in the presence of protective iron carbonate films: Part 2: A numerical experiment

Abstract A theoretical carbon dioxide (CO2) corrosion model was used to conduct numerical experiments, which allowed total insight into the underlying physicochemical processes. The focus was on fa...

Investigation of Carbon Dioxide Corrosion of Mild Steel in the Presence of Acetic Acid—Part 1: Basic Mechanisms

Abstract The corrosion behavior of mild steel in the presence of acetic acid (CH3COOH) and carbon dioxide (CO2) has been investigated using electrochemical techniques and weight-loss measurements. Electrochemical measurements have shown that the presence of acetic acid affects predominantly the cathodic reaction. The acetic acid effect is much more pronounced at elevated temperatures when catastrophic corrosion rates may be encountered at high concentrations. The undissociated form of acetic acid, present at lower pH, is responsible for the increases seen in the corrosion rate.

Large-eddy simulation of heat transfer downstream of a backward-facing step

Large-eddy simulation is used to predict heat transfer in the separated and reattached flow regions downstream of a backward-facing step. Simulations were carried out at a Reynolds number of 28 000 (based on the step height and the upstream centreline velocity) with a channel expansion ratio of 1.25. The Prandtl number was 0.71. Two subgrid-scale models were tested, namely the dynamic eddy-viscosity, eddy-diffusivity model and the dynamic mixed model. Both models showed good overall agreement with available experimental data. The simulations indicated that the peak in heat-transfer coefficient occurs slightly upstream of the mean reattachment location, in agreement with experimental data. The results of these simulations have been analysed to discover the mechanisms that cause this phenomenon. The peak in heat-transfer coefficient shows a direct correlation with the peak in wall shear-stress fluctuations. It is conjectured that the peak in these fluctuations is caused by an impingement mechanism, in which...

Electrochemical Study and Modeling of H2S Corrosion of Mild Steel

The internal corrosion of mild steel in the presence of hydrogen sulfide (H2S) represents a significant challenge in oil production and natural gas treatment facilities, but the underlying mechanisms involved in H2S corrosion are still not fully understood. This lack of knowledge makes the prediction, prevention, and/or control of aqueous H2S corrosion of mild steel much more difficult. In the present study, H2S corrosion mechanisms were experimentally investigated in short-term corrosion tests (lasting 1 h to 2 h), conducted in a 1 wt% sodium chloride (NaCl) solution at different pH (pH 2 to pH 5), at different temperatures (30°C to 80°C), under various H2S/N2 gaseous concentration ratios (0 to 10%[v]) and flow rates, using a X65 mild steel rotating cylinder electrode. Corrosion rates were measured by linear polarization resistance (LPR). Corrosion mechanisms were investigated by using potentiodynamic sweeps and by comparison with electrochemical modeling. LPR results showed that corrosion rates increase...