Corrosion behavior and microstructural study of low carbon steel weldment within aqueous solution containing CO_2

摘要

Oil companies have experienced considerable corrosion in weld zones of oil and gas industry systems made of carbon and low alloy steel. In CO_2 containing environments corrosion failures are often explained by varying tenacity of protective corrosion scales adhesion on different carbon steel microstructures. Localised corrosion of weld metal (WM) and heat affected zone (HAZ) has mostly been explained by galvanic effects caused by both differences in alloy content and microstructure. An investigation of the corrosion behavior and microstructural features of parent metal (PM), WM and HAZ was carried out to understand microstructural effects on the corrosion behaviour of welded joints of low carbon steel. Corrosion testing within a 3.5% NaCl solution saturated with CO_2 under deoxygenated condition at ambient temperature, with pH of 4.0±0.3 was conducted. Characterisation of the corrosion behaviour was carried out by electrochemical techniques of open circuit potential, and linear polarisation resistance as well as general metallographic characterisation performed using optical microscopy. The main morphological phases found were acicular ferrite, retained austenite and martensite / bainite structures in WM. These were less prevalent in the HAZ. Significantly different electrochemical behavior and corrosion rates were found within the PM, WM and HAZ.