| Today, ethanol, as well as other biofuels, has been increasingly gaining popularity as a major alternative liquid fuel to replace conventional gasoline for road transportation. Currently, less than 10% of fuel-grade ethanol (FGE) is blended into the gasoline to reduce the pressure from gas shortage. One of the key challenges for the future use of bioethanol is to increase its availability in the market via an efficient and economic way. Pipelines are normally used for transporting and distributing traditional petroleum products. However, ethanol, so far, is transported primarily by tanker trucks. One major concern in using the existing gas-pipelines to transport fuel-grade ethanol or blended fuel is the potential corrosion and stress corrosion cracking (SCC) susceptibility of carbon steel pipelines in these environments. The key technical challenge for ethanol fuel transportation is to understand the mechanisms of metal degradation in such an environment.;In this thesis, both phenomenological and mechanistic investigations have been carried out in order to address the possible degradation phenomena of X-65 pipeline carbon steel in simulated fuel-grade ethanol (SFGE). Firstly, the susceptibilities of stress corrosion cracking of this steel in SFGE were studied. Ethanol chemistry of SFGE was shown to have great impact on the stress corrosion crack initiation/propagation and the corrosion mode transition. Inclusions in the steel can increase local plastic strain and act as crack initiation sites. Secondly, the anodic behavior of carbon steel electrode was investigated in detail under different ethanol chemistry conditions. General corrosion and pitting susceptibility under unstressed condition were found to be sensitive to the ethanol chemistry. Low tendency to passivate and the sensitivity to ethanol chemistry are the major reasons which drive corrosion process in this system. Oxygen plays a critical role in controlling the passivity of carbon steel in ethanol. Thirdly, the detailed study was carried out to understand the SCC mechanism of carbon steel in SFGE. A film related anodic dissolution process was identified to be a major driving force during the crack propagation. Fourthly, more detailed electrochemical impedance spectroscopy (EIS) studies using phase angle analysis and transmission line simulation reveal a clearer physical picture of the stress corrosion cracking process in this environment. Fifthly, the cathodic reactions of carbon steel in SFGE were also investigated to understand the oxygen and hydrogen reactions. Hydrogen uptake into the pipeline steel and the conditions of the fractures related to hydrogen embrittlement were identified and studied.;This study, for the first time, systematically investigated the stress corrosion cracking and corrosion of carbon steel in simulated fuel-grade ethanolic environment, which provides useful information on material degradation and selection in biofuel environments. |
