Study On Corrosion Mechanism Of Heat Exchanger In MDEA Solution Saturated By H₂S

This theme comes from the research project of YANGZI PETROCHEMICALCOMTANY LIMITED which research corrosion cause and mechanism of heatexchanger pipeline in MDEA solution saturated by H2S.Uniform corrosion, localcorrosion and SCC (stress corrosion cracking) are the main corrosion forms of theheat exchanger. In this paper electrochemical test, metallic samples test, SSRT (slowstrain rate test) have been used to research the corrosion of the heat exchangerpipeline.Firstly, the corrosion of 20 steel was proceeded under simulated work conditionin our equipment. It was found that the corrosion of the steel in the vapor phase wasserious, and corrosion in the liquid phase and vapor/liquid phase was insignificant.Secondly the electrochemical polarization was proceeded under the simulated workcondition. It showed that 20 steel in the MDEA solution without H2S was stable, andthat the steel was passivated in the MDEA solution with H2S. But with thetemperature of the solution increasing, the tendency of the passivation became weak.The experiment of crevice corrosion susceptivity indicated that susceptivity of 20steel was high. At last SCC susceptivity and hydrogen blistering susceptivity of 20steel were studied. The test result showed that the susceptivity under different solutionin a descending order as: the standard solution of NACE, the solution saturated byH2S,the MDEA solution saturated by H2S and the air. The susceptivity of hydrogenblistering under different solution was also studied. The result indicated that thehydrogen blistering can be found on the surface of 20 steel in the standard solution ofNACE and in the solution saturated by H2S, but in the MDEA solution there was nohydrogen blistering.By the methods of electrochemical test, metallic samples test and SSRT (slowstrain rate test), the corrosion process could be presented as follows: regenator wasfull of the gas of H2S under the work condition. When it stopped, the temperature ofregenator fell and the pipelines of the heat exchanger exposed to the H2S solution,which caused hydrogen blistering on the surface of 20 steel. The place where the IIhydrogen blistering existed would become the origin of the crevice corrosion. Whenregenator worked again, the pipelines with hydrogen blistering went on corroding onthe site of crevices, then these crevices became the pit. In the end, the pittingcorrosion resulted in perforation of heat exchanger pipelines.