Corrosion Product Structures of Steel Samples Exposed to Amine Solutions Used for H2s Removal.

We present our results on the corrosion effects of heat stable salts (HSS), i.e., amine salts and amino acids that occur during removal of H₂S from petroleum and natural gas with alkanolamines. The salts include heat stable salts and amine degradation products. Corrosion was measured using coupon weight loss in liquid and vapor phases in stirred solutions that model amine plant environment (e.g., in refinery systems). We have developed a chamber that enables exposure of samples to the gas and liquid phases of amine solutions. At increasing concentration of HSS the corrosion products show well defined structures in the nanometer scale in the solutions exposed to the liquid phase. However, this was not observed in the vapor phase. Overall, the corrosion products include amorphous, columnar and disk-shaped structures depending on the exposure conditions. We discuss the implication of the products formed on the corrosion mechanism under this environment and we propose that the substrates become passivated and that localized corrosion takes place under the conditions of our study that mimic the HSS buildup in a plant.

The corrosion products were characterized by scanning electron microscopy and X-ray powder diffraction. We report our findings for the corrosion rate measured in amine solutions with progressive additions of H₂S and additional amine salts. In summary, we found the weight loss to be significantly different for samples exposed to the liquid phase and to the vapor phase: all of our liquid samples had corrosion rates < 10 mils per year (mpy), while the samples in the head space corroded at ³ 10 mpy. Our measurements were carried out in a closed environment with the liquid composition under control. For all of our experiments, the solution was purged with N₂ and then H₂S was dosed to achieve a concentration of 0.01 M/M of H₂S/MDEA . The corrosion rates in the head space do follow a trend with respect to the composition in the liquid. Most notably, the corrosion rates in both liquid and gas increase when bicine is added.  In addition the corrosion rate in both liquid and gas phases increased when the HSS level increased from 1.4 to 5%. This suggests that some of the heat stable salts are present in the head space, possibly from partitioning via vaporization or splashing into the head space.