In situ SR-XRD study of FeCO3 precipitation kinetics onto carbon steel in CO2-containing environments: The influence of brine pH

Experimental Investigation on Corrosion Behavior of X80 Pipeline Steel under Carbon Dioxide Aqueous Conditions

A combined steady-state and transient approach is employed to investigate the corrosion behavior of X80 pipeline steel in carbon dioxide-saturated brines. Continuous bubbling of carbon dioxide into a test vessel with 1 liter capacity is performed to simulate the flowing condition. The measurement of time-dependent open-circuit potential, polarization resistance, and electrochemical impedance spectroscopy (EIS) is conducted to interpret the evolution of dissolution processes at the corroding interface. Three distinguishing stages are observed at a temperature of 60 °C during a whole exposure of 144 h. Analyses mainly based on the consecutive mechanism show that after the first stage of the active-adsorption state, the anodic reaction is significantly retarded by the accumulation of (FeOH)_ads on the iron surface, causing a sharp increase in the polarization resistance and the open-circuit potential, as well as the disappearance of the inductive loop in EIS. At the third stage, the formation of the corrosion product layer similarly reduces both the anodic and cathodic reactions, which arouses a linear increase in the polarization resistance with time and a capacitive loop in EIS but changes the open-circuit potential slightly. An increase in salinity in this study reduces the polarization resistance and enhances iron dissolution by promoting the formation and relaxation of (FeOH)_ads; however, it brings little change to the developing time of the three stages obviously. At a low temperature of 20 °C, a protective product layer is not observed in carbon dioxide-saturated brine, and the dissolution of iron is mainly under activation control during the whole exposure. A notable enlarged polarization resistance and different interfacial processes are observed in an alkaline solution compared with those in acidic environments, which is deduced to be resulted from an impedance in the relaxation of (FeOH)_ads by increasing pH. The observations in this study support well that the iron dissolution reaction at the initial stage exposed in carbon dioxide aqueous environments is dominant by water adsorption on the iron surface, and further investigation should be performed on the role that carbon dioxide plays in the evolution of corrosion products and the formation of a protective film on the steel surface by taking into account local water chemistry.

Neutralization of bauxite residue with high calcium content in abating pH rebound by using ferrous sulfate

The high alkalinity of bauxite residue and its sustained release impose major limitation on its reuse and ecological disposal. It has been confirmed from sustained rehabilitation that gypsum can effectively reduce the alkalinity of bauxite residue by continuously releasing Ca²⁺ to react with carbonate and hydroxide. However, the combined bauxite residue with high calcium content exhibits stubborn alkalinity for most alkaline reduction methods employing cations to consume carbonate. In this study, we have aimed to address this knowledge gap by investigating the dose-response relationship in the alkaline reduction induced by ferrous sulfate (FS) neutralization. The pH, exchangeable sodium percentage (ESP), and CO₃^2-/HCO₃^- of bauxite residue decreased from 10.6, 44.1%, and 42.7/24.5 mg/kg to 8.1, 27.7%, and 0.7/18.0 mg/kg, respectively. Approximately 20-55 days were required for the neutralization reaction to reach equilibrium. The FS induced an increase in free iron oxide (Fe_d) and amorphous iron oxide (Fe_o), and partial dissolution of alkaline minerals including calcite, cancrinite, and kaolinite in bauxite residue. Further, addition of FS also affected the kinetic dissolution process of bauxite residue; the acid neutralization capacity of bauxite residue to pH 7 decreased from 0.21 mol H⁺/kg solid to 0.02 mol H⁺/kg solid. The results showed FS to be a potential candidate for improving the characteristics of the combined bauxite residue, and guide the FS application for the disposal of the combined bauxite residue.

Substrate Protection with Corrosion Scales: Can We Depend on Iron Carbonate?

Controlling corrosion with naturally occurring corrosion scales is potentially a more environmentally sustainable alternative to current approaches, including dosing of organic corrosion inhibitors. We report operando grazing incidence X-ray diffractograms correlated with electrochemical measurements to elucidate the growth and corrosion protection properties of a corrosion scale composed of FeCO₃ crystallites, which is encountered in various key energy industry applications. Data, acquired as a function of time from high-purity iron immersed in CO₂-saturated deionized H₂O at pH 6.8 and T = 80 °C, show that the FeCO₃ scale not only prevents corrosion of the covered substrate but also acts as a significant interfacial diffusion barrier for corrosion reagents and/or products once sufficient coverage is achieved. Most notably, from a corrosion engineering perspective, however, it is determined that corrosion occurring in gaps between scale crystallites remains appreciable; this important insight is gained through the analysis of electrochemical impedance spectra to estimate the variation in electrochemically active surface area as scale coverage increases. These results indicate that naturally occurring FeCO₃ scales are not a tenable solution for corrosion protection, as even in their intact state they are highly likely to be, at best, semiprotective.

Use of a Gemini-Surfactant Synthesized from the Mango Seed Oil as a CO2-Corrosion Inhibitor for X-120 Steel

A gemini surfactant imidazoline type, namely N-(3-(2-fatty-4,5-dihydro-1H-imidazol-1-yl) propyl) fatty amide, has been obtained from the fatty acids contained in the mango seed and used as a CO₂ corrosion inhibitor for API X-120 pipeline steel. Employed techniques involved potentiodynamic polarization curves, linear polarization resistance, and electrochemical impedance spectroscopy. These tests were supported by detailed scanning electronic microscopy (SEM) and Raman spectroscopy studies. It was found that obtained gemini surfactant greatly decreases the steel corrosion rate by retarding both anodic and cathodic electrochemical reactions, with an efficiency that increases with an increase in its concentration. Gemini surfactant inhibits the corrosion of steel by the adsorption mechanism, and it is adsorbed on to the steel surface according to a Langmuir model in a chemical type of adsorption. SEM and Raman results shown the presence of the inhibitor on the steel surface.

Mechanistic Insights of Dissolution and Mechanical Breakdown of FeCO3 Corrosion Films

Carbon steel is a universally used material in various transportation and construction industries. Research related to CO₂ corrosion environments agrees on the occurrence of siderite (FeCO₃) as a main product conforming corrosion films, suggested to impart protection to carbon steel. Identifying and understanding the presence of all corrosion products under certain conditions is of greatest importance to elucidate the behavior of corrosion films under operation conditions (e.g., flow, pH, temperature), but information regarding the nature and formation of other Fe corrosion products apart from FeCO₃ is lacking. Corrosion products in CO₂ environments typically consist of common Fe minerals that in nature have been demonstrated to undergo transformations, forming other Fe phases. This fact of nature has not been yet explored in the corrosion science field, which can help us to describe mechanisms associated with industrial processes. In this work, we present a multiscale and multidisciplinary approach to understand the mechanisms occurring on corrosion films under the key factors of flow and pH through the combination of molecular techniques with imaging. We report that certainly siderite (FeCO₃, cylindrical with trigonal-pyramidal caps) is the main product identified under the conditions used (representative of brine transport at 80 °C), but wustite (FeO) and magnetite (Fe₃O₄) minerals also form, likely from the de-carbonation of FeCO₃ → FeO → Fe₃O₄, depending on pH under the action of flow. These minerals exist across the corrosion films evidencing a more complex nature of the three-dimensional layer not currently accounted for in the mechanistic models. A relatively low flow velocity (1 m/s), as recognized for industrial operations, is enough to produce chemo-mechanical damage to the FeCO₃ crystals, causing breakage at low pH where dissolution of FeCO₃ occurs with a rapid crystal size reduction of the cylindrical FeCO₃ geometry of ∼80% in just 8 h, changing also the local chemical structure of Fe₃C under the film. Similarly, a flow velocity of 1 m/s is capable of inducing crystal removal at neutral pH, promoting further degradation of the steel, compromising the protectiveness assumption of FeCO₃ corrosion films. The chemo-mechanical damage and Fe phase transformations will affect the critical localized corrosion, and therefore, they need to be accounted for in mechanistic models aiming to find new avenues for control and mitigation of carbon steel corrosion.

Fundamental aspects of the corrosion of N80 steel in a formation water system under high CO2 partial pressure at 100 °C

The corrosion behavior of N80 carbon steel in a simulated formation water system saturated with CO₂ under high pressure at 100 °C was investigated. The effect of the CO₂ partial pressure on the electrochemical behavior and surface morphologies of the N80 carbon steel was analyzed by in situ electrochemical methods and surface analysis, combined with a series of thermodynamic calculations of the potential of anodic/cathodic reactions. While an increase in the CO₂ partial pressure did not alter the corrosion mechanism of the N80 steel, it resulted in higher concentrations of H⁺ and HCO₃⁻ ions, thereby significantly enhancing the rate of the cathodic reactions. The precipitation rate of FeCO₃ increased with the CO₂ partial pressure, with small and fine grains nucleating and growing on the steel surface with poor protectiveness.

The corrosion behavior of N80 carbon steel in a simulated formation water system saturated with CO₂ under high pressure at 100 °C was investigated.