A Study of Preferential Weld Corrosion in the Presence of Carbon Dioxide and Acetic Acid

The presence of carbon dioxide (CO2) gas in oil and gas steel pipeline is a major concern in the industry. CO2 gas dissolves in water to form carbonic acid which will further dissociate to form free hydrogen ions that can cause rapid corrosion to steel material. In addition, the presence of organic acid such as acetic acid contributes to the additional sources of free hydrogen ions. What is more critical is the fact that these thousand miles of pipelines are connected through welds, which are very susceptible to galvanic corrosion, causing preferential weld corrosion (PWC). Galvanic corrosion occurs due to the difference in compositions and microstructures of the weldment. The primary objective of this study is to investigate the weldment structure and the microstructures of parent metal region, heat-affected zone and weld metal region of an API 5L X52 grade carbon steel pipe. This study also aims to study the effects of varying pH levels and acetic acid concentrations at elevated temperatures to the corrosion behavior of different weldment regions in the presence of acetic acid and CO2 corrosion. A welded section of an old API 5L X52 pipe which had been exposed to CO2 corrosion was used as the test samples in this study. Critical literature review has been done regarding the pipe material, structure of weldment, carbon dioxide and acetic acid corrosion as well as the experimental setup and procedures according to ASTM G5-94 and NACE Standard TM0169-2000. An attainable test matrix has been designed as a guide for the experimental study to achieve the objectives. An electrochemical test by using Linear Polarization Resistance (LPR) was used to conduct the corrosion measurement analysis. Results from Zero Resistance Ammeter (ZRA) show that the weld metal and heat affected zone metal always behave anodically compared to parent metal. Based on the Linear Polarization Resistance, the corrosion rates increased for all three metals due to the acidity level in low pH condition that inhibits the formation of protective film