Abstract
Corrosion research related to CO2-containing environments has focused over the past few decades on siderite formation (FeCO3) as a main corrosion product on carbon steel, yet the influence of Ca and other ions on its chemical and structural characteristics is not fully understood. Metal-localized corrosion is the biggest industrial challenge because of the unknown and unpredictable character of this phenomenon that frequently leads to failure. We report here the role of Ca and formation of iron-calcium carbonate (FexCayCO3) through a spiral growth model as in the calcite system and quantify the replacement of Fe2+ by Ca2+ ions in the structure of FeCO3 to form FexCayCO3. The incorporation of Ca2+ inhibits the completion of spiral segments on the growth of the rhombohedral crystals of FeCO3, promoting an enlargement of its structure along the c-axis. This leads to distortions in the chemical structure and morphology affecting the chemical and mechanical properties. Under flow conditions over time in an undersaturated environment, Ca is leached out from the expanded structure of FexCayCO3 increasing the solubility of the crystals, weakening the mechanical properties of the resulting corrosion films and stimulating localized corrosion.
Highlights
Many industries use carbon steel (C-steel) for the construction of pipes, tanks, and pumps for transportation and distribution of bulk gas and liquids demanding an annual steel production of 1.8 billion tonnes.[1]
Initial observations of the crystals grown on C-steel in brine solutions containing Ca2+ showed a rounded morphology in comparison to FeCO3 crystals and evidenced a growth mode in kink sites along steps as reported to occur in calcite (Figure 1)
The morphology of these crystals resembles a distorted version of the of FeCO3 morphology.[20−22] They have rough and smooth surfaces in different directions indicating large anisotropy in bonding. This suggests that different rates of growth occurred in each direction when compared to siderite, in which smooth crystal faces grow at a slower rate than kinked
Summary
Many industries use carbon steel (C-steel) for the construction of pipes, tanks, and pumps for transportation and distribution of bulk gas and liquids demanding an annual steel production of 1.8 billion tonnes.[1]. Calcium as an additional ion (Ca2+) in the brine solution has been marginally studied in the corrosion field with only a few works published. These works lack of consensus regarding the effect of calcium on corrosion rates. Some authors have reported the increase of corrosion rates when Ca2+is present,[6−8] while others have documented reduction instead.[9,10] These discrepancies can be because of the use of different experimental conditions and lack of detail or control of them in the experiments
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