Corrosion of stainless and carbon steel in aqueous amine for CO₂ capture
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Post-combustion carbon capture and storage with amine absorbents is a key technology needed to provide low-cost decarbonized electricity. Improving understanding of corrosion by amines may reveal a solvent system compatible with carbon steel, which would reduce plant capital costs. Corrosion of stainless and carbon steel in aqueous monoethanolamine (MEA) and piperazine (PZ) has been measured. High temperature amine corrosion was measured in a bench-scale pressure vessel and iron solubility in amines was screened in stirred reactors. Corrosion was measured at two PZ pilot plants and one MEA pilot plant, using coupons and electrical resistance probes. Corrosion products were characterized by SEM and powder X-ray diffraction. Carbon steel (C1010) often performs well in 5 molal PZ up to 150 °C due to the formation of a passivating FeCO₃ layer. This layer is promoted at high temperature, high CO₂ loading, low solution velocity, and in amines with low Fe²⁺ solubility. FeCO₃ formation is favorable at high temperature because Fe²⁺ solubility decreases and the kinetics of FeCO₃ formation are faster. This also means that FeCO₃ is not observed at low temperature. Despite this, carbon steel performs well at low temperature due to slower kinetics of metal oxidation. Depassivation and high corrosion of stainless steel (316L) can occur in amine solutions at high temperature (150 °C) when conditions are relatively anoxic and reducing. Performance of stainless at high temperature in PZ suggests that it can be pushed into and out of the passive state by small process changes, such as different flue gas O₂ concentrations. However, stainless performs well in both MEA and PZ in pilot plants at ≈120 °C. Fe³⁺ corrosion products are generated in the absorber, then reduced to Fe²⁺ in the high temperature, anoxic conditions of the stripper. In this way, carried-over Fe³⁺ is responsible for oxidation of amine and corrosion at high temperature. Certain highly corrosive amines also have high Fe²⁺ solubility. Ethylamines like MEA are likely the correct chain length to form stable complexes with Fe²⁺ in solution. Stable Fe²⁺-amine complexes cause high Fe²⁺ solubility, which prevents FeCO₃ formation and leads to high corrosion.