Impact of Prolonged Exposure to Sour Service on the Mechanical Properties and Corrosion Mechanism of Nace Carbon Steel Material Used In Wet Sour Gas Multiphase Pipeline in Qatar

Abstract

Oil and Gas industries involve severe corrosive/sour environmental conditions due to H2S and CO2 and moisture content. The National Association of Corrosion Engineers, NACE, has developed standards to enable the users to select suitable material for given sour conditions by means of laboratory testing. This thesis takes up the study of a failed (corroded) piping sample (API-5L-X65) removed from a pipeline after 15 years of service. Mechanical properties of the removed samples (both corroded as well as non-corroded) were evaluated and compared. Pitted samples were analyzed using a scanning electron microscope coupled with energy dispersive X-ray (SEM/EDS) for understanding the morphology of the deposits. EDS results were further compared using XPS analysis. Optical microscopy was used to compare the microstructure of the corroded sample near the surface exposed to the service environment and away from it. Micro-hardness was carried out on the cross-section of the removed sample to understand any evident hardness variation from ID to OD of the piping. The electrochemical behavior using Electrochemical Impedance Testing (EIS) of the corroded sample (with corrosion scales) was tested in 5% sodium sulfate solution, and the results were compared with the un-corroded part (polished) of the same sample. All the results suggest that prolonged service exposure has resulted in the development of micro defects, resulting in the reduction of strength and impact toughness along with the reduction in the hardness at the exposed surface of the corroded piping. Understanding the material behavior due to long-term exposure will surely help the industry in repair/replacement planning while extending the usable design life of the material. The results acheived during this thesis, will surely pave the way for oil & gas industry to develop further ways to monitor the changes in the critical material properties when exposed to sour services