Mechanisms of Iron Carbonate Formation on Mild Steel in Controlled Water Chemistry Conditions

Degradation of metals associated with hydrocarbon production, combustion processes, and carbon capture is a pernicious problem in the energy sector. Consequently, laboratory simulation of CO2-containing aqueous environments is of vital importance for the study of steel corrosion. However, discrepancies between predicted, experimental, and field corrosion rate data in a midrange of operating parameters have been reported. These discoveries necessitate improving the accuracy of corrosion prediction models. Based upon preliminary experiments, iron carbonate formation near its saturation condition was identified as a major factor observed corrosion rate discrepancies. Therefore, it is imperative to conduct a systematic study to gain more understanding of corrosion product layer formation mechanisms in controlled water chemistry conditions. Small scale CO2 corrosion experiments have lacked control of pH and ferrous ion concentration, potentially creating misleading conditions related to the growth of protective iron carbonate (FeCO3). An improved experimental apparatus to study steel corrosion and associated formation of FeCO3 was developed. The design incorporates a flow-through system, enabling control of water chemistry, and newly developed sample holders that eliminate non-uniformity of flow associated with hanging samples as well as centrifugal effects associated with rotating cylinder electrodes. Corrosion experiments were conducted in a conventional glass cell and the newly developed flow-through system. Significantly different corrosion product morphologies were observed in these different systems. In controlled water chemistry conditions, iron carbide (Fe3C) was found to play a crucial role in the development of the corrosion product as it provided a favorable environment for the formation of a FeCO3 layer at the steel surface.Three different kinds of material (X65 with tempered-martensitic microstructure, C1018 with ferritic-pearlitic microstructure, and 99.8% pure iron) were used to confirm the influence of iron carbide on the formation of corrosion products near saturation conditions. It was found that in the absence of iron carbide (Fe3C), pure iron produced no significant FeCO3 formation under the same operating conditions as the tests conducted on mild steels.Environmental parameters relating to the effect of FeCO3 saturation value, solution pH, and temperature on the formation of corrosion product layers were also investigated. Saturation value conferred the strongest effect on FeCO3 layer formation near saturation conditions among the three environmental parameters explored.