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Figure 6 Iron and chromium content in the amine solvent
in the regenerator tower, reboiler bundles, stainless steel piping, or stainless peripherals (pump internals or thermal probes). This appears to be caused by the pump installed at the regenerator column where several solids were circu- lated. The pump was installed because of corrosion at the outlet bucket of the regenerator. Harsh pitting While iron content has continued to decline up to August, the chromium content rose after attaining an equilibrium value of around 40 ppmw. High stainless steel corrosion without a correspondingly high carbon steel corrosion tendency is not normal for amine units. The reason stainless steel is chosen for amine unit metallurgy is because of corrosion resistance (H 2 S and CO 2 corrosion plus velocity-related corrosion). Corrosion resistance is a result of a thin (about 5 nanometers) oxide layer on the steel’s surface. The passivation layer forms because of the chromium added to the stainless steel surface. There are several reasons stainless steel will corrode in amine units. The first is aggressive chloride (Cl-) content in
the solvent. Depending on the supplier, the maximum chlo- ride content is 250-500 ppmw. As Figure 7 shows, the chlo- ride levels were often within the maximum range. Chlorides cause chloride stress corrosion cracking (CSCC) of stainless steel, sometimes described as ‘harsh pitting’. Carbon steel is often too soft to crack in the presence of chlorides. Another reason for stainless steel corrosion would be the presence of large amounts of foulant (possibly by corrosion products), causing a loss of local oxygen and subsequent loss of the protective oxide layer. Recently installed probes that corroded rapidly had no solids covering them when extracted. The probe likely suffered from distinct corrosion mechanisms (CO 2 pitting and CO 2 cavitation from CO 2 release). High chloride levels It is also possible that a stainless steel corrosion probe sitting in stagnant liquid, where the chloride content in the solvent was enough to disrupt the protective oxide film, resulted in pitting of the steel. The probe showed distinct rounded pits often associated with CO 2 attack. The high lean amine
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Figure 7 Chloride ions in solution
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Gas 2025
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