The Effect of Low Frequency Cycling & Mill Scale on Stress Corrosion Cracking of Pipeline Steel in Simulated Fuel-Grade Ethanol
Elsayed, Omar Hesham
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Using non-renewable fossil fuel energy resources has become a major concern in modern day society. Major efforts have been made to decrease the effect of such hazardous materials. Ethanol (CH3CH2OH, or EtOH) has been proven to be a promising alternative option to fuel. Approximately 10-15% of the commercial fuel used nowadays is composed of fuel-grade ethanol (FGE). However, field failures due to stress corrosion cracking (SCC) of carbon steel pipelines and storage tanks used in FGE transportation have been reported. Leaks are found at stress concentration points, such as heat affected zones and geometric discontinuities. Prior research on the effect of EtOH chemistry and electrochemical conditions on crack initiation, growth and propagation behavior has shown that contaminants and/or additives are important factors in causing SCC in FGE pipelines. The role of low frequency stress fluctuations on SCC initiation and propagation on the inner surface in FGE pipelines was not understood. The main objective of this research is to evaluate low frequency cyclic effects on SCC behavior under simulated conditions, and thus use the obtained information to optimize productivity and prevent catastrophe. A four-point bend test on a pipeline section immersed in an ethanol solution will be used to simulate the service conditions experienced during operation. It has been conjectured that SCC is not experienced with a static applied load below or above yield stress levels, therefore indicating that dynamic or cycling loading is required for SCC to occur. A smoother surface finish results in significantly less SCC than a rougher surface, emphasizing the importance of surface roughness in SCC behavior. A smaller R-ratio results in lower crack density, nucleation rate and crack velocity than larger R-ratios, hence indicating the importance of fluctuating stresses in SCC behavior. A higher cyclic frequency results in increasing crack density, nucleation rate, velocity and crack length, with a possible threshold leading to crack propagation. Longer test durations resulted in reduced crack velocity, indicating that cracks grow slowly with time due to a number of factors, including crack shielding. Finally, oxygen supply is essential for SCC to occur, which support previously conducted research by X. Lou et. al and Sridhar et. al.
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