Study On Corrosion Behavior Of Carbon Steel In Methyldiethanolamine Solution Containing Heat-stable Salts

Methyldiethanolamine (MDEA) is one of the most important alkanolamine used in acid gas treating process. MDEA degradation causes the formation of heat-stable salts (HSS), which will lead to corrosion of equipment. In the present work, the corrosion behavior of carbon steel in MDEA solutions with HSS was investigated by electrochemical method, such as anodic polarization curve, Tafel curve and electrochemical impedance spectroscopy (EIS). Moreover, modern surface analytical techniques, like scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS) were also used. Several factors including MDEA concentration, HSS anion, HSS cation, HSS concentration, liquid temperature and dissolved oxygen were studied. The rate-determining step of carbon steel eroded in MDEA solution containing HSS was investigated too. The present work will offer theoretic analysis and experimental date for corrosion control in amine system.There are five parts in the dissertation:In chapter 1, firstly, the alkanolamine used in acid gas treating process was summarized and the excellent character of MDEA was pointed out. Secondly, the equipment will be eroded seriously as the, degradation of MDEA and the formation of HSS. Thirdly, the principles of anodic polarization curve, Tafel curve, EIS and static weight loss test were explained. Lastly, the design and the innovation of the dissertation were introduced.In chapter 2, the effect of MDEA concentration on corrosivity was investigated by anodic polarization curve, electrochemical impedance spectroscopy and static weight loss test. It was showed that the trend of corrosion rate of carbon steel in various concentration of MDEA was relied on the quantity of HSS.(1) In the blank solutions or MDEA solution containing small quantity of HSS, the corrosion rate was increased at first and then decreased when the concentration of MDEA increased. The maximum corrosive solution was 30 mass% MDEA.(2) In the simulated solutions containing a large amount of HSS, the corrosion rate was decreased monotonously with an increase in MDEA concentration.In chapter 3, the effect of HSS species and concentration on corrosivity was investigated by anodic polarization curve, electrochemical impedance spectroscopy and static weight loss test. It was indicated that:(1) The pitting crrosion will occur in MDEA solutions containing chloride ions at normal temperature. The corrosive ability of the solution enhanced as the concentration of the chloride increased.(2) The formate, acetate, propionate, sulfate and oxalate anion showed weak corrosive ability, their corrosive ability would enhance slightly when their concentration increased.(3) Neutralizing MDEAH+ with caustic (which converts the amine salt to free amine along with the sodium salt that stays in solution) could increase the pH value of the solution, it was benefit for the passivation of carbon steel, and then the corrosivity of the solution was decreased.In chapter 4, 30 mass% MDEA solution containing seven kinds of HSS anions were prepared. The corrosion process and the rate-determining step of carbon steel in the solution were investigated by anodic polarization curve, Tafel curve, EIS, SEM, EDS and static weight loss test. It was indicated that:(1) The corrosion rate of carbon steel in HSS simulated solution was doubles higher than that in the solution without HSS since their lower pH value and a large amount of HSS anions. Carbon steel might result in pitting corrosion in HSS simulated solutions containing small quantity of chloride ions.(2) The rates of cathodic reduction and anodic dissolution were both determined by both electrochemical reaction and diffusion process in 30 mass% MDEA solution. As in HSS simulated solution, the rate of cathodic reduction retained unchanged, the rate-determining step of the anodic dissolution was electrochemical reaction only. (3) As anodic polarization potential was increased, the EIS of carbon steel in HSS simulated solution showed characteristics of inductive shrinkage, negative impedance and Warburg-response, which implied pitting corrosion, transition from active state to passive state and passive state, respectively.In chapter 5, 30 mass% MDEA solution containing seven kinds of HSS anions were prepared. The effects of dissolved oxygen and liquid temperature on the corrosion behavior of carbon steel were investigated by EIS, Tafel curve and SEM. It was indicated that:(1) The corrosion rate of carbon steel in solution containing dissolved oxygen was higher than that in solution removed of dissolved oxygen before cathodic activation. They were reversed after cathodic activation.(2) The formation of passive film was hindered in HSS simulated solution due to its lower pH value and quantities of HSS anions.(3) The general corrosion rate was increased as well as the macro pitting corrosion was inhibited at high temperature.