Effects of Ca2+ Ions on the Localized Corrosion of Carbon Steel Influence of the Associated Anion

The corrosion rate of carbon steel under the attack of electrolytic mediums saturated by CO2 at a working temperature of 65 degrees C under dynamic conditions is studied in this paper. The effect of Ca2+ ions on corrosion rates and their dependency on the associated anion (Cl- or CO32-) with respect to the Ca2+ cation and Na+ are also assessed. A potentiostat/galvanostat associated with a rotor stator in an AISI 1040 steel-ring configuration, rotating at 600 rpm, was used. A linear polarization-resistance technique was applied to calculate the corrosion rate. It is found that the corrosion rate depends not only on the presence of Ca2+, but also on the salt anion added to the electrolytic medium. There was a strong inhibitory effect of the corrosion in the presence of Ca2+ in chloride-free mediums because of an ankerite deposition with a lower solubility compared to siderite. Moreover, it was observed that, even if the corrosion rate increased together with the Cl- concentration, its value depended on the cation introducing the anion. It was higher when Cl- was associated with Na+ compared to when it was associated with Ca2+. We found the following decreasing order in the carbon steel corrosion rate: 2% NaCl + 1380 ppm CaCl2 > 2% NaCl > 2% NaCl + 1360 ppm Na2CO3 > 2% NaCl + 1.25 g/L CaCO3 > 1450 ppm of NaCl > 1380 ppm CaCl2 > 1360 ppm Na2CO3 > 500 ppm CaCl2 > 424 ppm Na2CO3 >> 1.250 g/L CaCO3.