Mechanistic Insights of Dissolution and Mechanical Breakdown of FeCO₃ Corrosion Films

Abstract

Carbon steel is a universally used material in various transportation and construction industries. Research related to CO2 corrosion environments agree 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 to 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 trigonalpyramidal 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 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 hours changing also the local chemical structure of Fe3C under the film. Similarly, flow velocity of 1 m/s is capable to induce crystal removal at neutral pH inducing 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 localised 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.

Iron Calcium Carbonate Instability: Structural Modification of Siderite Corrosion Films

“…To further understand the changes in the structure of siderite when Ca 2+ is present, we quantified the geometrical changes in the structure of Fe 0.66 Ca 0.33 CO 3 compared to FeCO 3 through XANES analyses. As previously reported, the Fe K-edge XANES spectra of FeCO 3 corrosion films, grown under the same conditions of iron calcium carbonate films, have a fingerprint consisting of three peaks between 7110 and 7140 eV as observed in Figure c. ,, Although both spectra are very similar, there are clear differences in the pre-edge and in the feature after the white line (peak 3) (Figure c). For Fe 0.66 Ca 0.33 CO 3 , the pre-edge (1s-3d) is a flattened curve in comparison to that of FeCO 3 .…”

“…Initial observations evidenced the severe effect of low pH (3.6) and flow on the dissolution of the Fe 0.66 Ca 0.33 CO 3 crystals provoking substantial breakage and leaving behind a porous material (Figure a,b). The damage is more severe than films consisting of pure FeCO 3 in experiments performed under the same conditions . This points out that the incorporation of Ca 2+ in the structure of FeCO 3 increases the dissolution of the iron calcium carbonate crystals in comparison to the film composed of pure siderite.…”

“…The situation is more favorable for the Fe 0.66 Ca 0.33 CO 3 crystals at pH 7.0 as their morphology is still visible but they show signs of erosion and higher porosity expressed as rougher faces and deteriorated materials along the steps (Figure c,d). In addition, substantial dissolution of the Fe 0.66 Ca 0.33 CO 3 film at low pH led to the transformation of 65% of FeCO 3 from the top surface (∼10 nm) to Fe 2 O 3 , indicating higher chemical transformation than FeCO 3 films; in which under the same experimental conditions, 88% of FeCO 3 was preserved (Figure b) . At neutral pH, Fe 0 from the steel was detected probably from the gaps between the crystals but nevertheless showing that the film is not protecting the steel (Figure c).…”

“…Focusing on testing the behavior of a Fe 0.66 Ca 0.33 CO 3 corrosion film, we performed flow experiments as described in the Methods section (i.e., pH 3.6 and 7.0, flow velocity of 1 m/s of 1% NaCl solution at 80 °C for 24 h) using the same conditions as previously reported for FeCO 3 films . Initial observations evidenced the severe effect of low pH (3.6) and flow on the dissolution of the Fe 0.66 Ca 0.33 CO 3 crystals provoking substantial breakage and leaving behind a porous material (Figure a,b).…”

“…Most of the experimental studies regarding corrosion in CO 2 environments point out siderite (FeCO 3 ) as the main corrosion product although other Fe oxides coexist. , The focus on FeCO 3 is in part because of the fact that the majority of the studies aim to investigate the formation of this product alone but in most of the cases it seems that the purpose is to simplify the system. Real applications have more complex chemistries with the presence of other ions (e.g., Mg, K, Ba, and Sr) in the fluids that cannot only modify the corrosion products but also change the pathway of the reactions, the mechanisms, and kinetics associated.…”