Corrosion behaviour and microstructural study of low carbon steel weldment within aqueous solution containing CO₂

Oil companies have experienced considerable corrosion in weld zones of oil and gas industry systems made of carbon and low alloy steel. In CO₂ containing environments corrosion failures are often influenced by the varying tenacity of protective corrosion scales adhesion on different microstructures of welded steel grade X70. Weld corrosion of the weld metal(WM) and heat affected zone (hAZ) has mostly been explained by galvanic effects caused by both differences in alloying content and microstructures. An investigation into the corrosion behaviour study and microstructural features of low carbon welded pipeline regions grade X70, e.g., parent metal (PM), heat affected zone (hAZ) and weld metal (WM) was carried out to understand the microstructural effects on the corrosion behaviour of this type of steel welds. Corrosion testing was performed in a 3.5% NaCl solution saturated with CO₂ under stagnant and deoxygenated conditions at ambient temperature, (20 °C ± 2), 45 °C and 65 °C where the pH ranges from 3.9 to 4.8. Characterisation of the corrosion behaviour was carried out by various electrochemical techniques including corrosion potential measurements (ecorr), polarisation resistance technique (Rp) and galvanic current measurements (gC). In addition, general metallographic characterisation was also performed using optical microscopy. The main morphological phases found were bainite, martensite and acicular ferrite structures in the WM and HAZ. However, ferrite and pearlite phases were found in the PM. For the microstructural study and morphology of WM and HAZ showed significant different electrochemical behaviours and corrosion rate than that of PM. This is an induction due to the crystalline protective film formations of FeCO₃, Fe₇C₃ and Fe₃C covered some of the electrode surface of PM, whilst less in WM and HAZ. However, these film formations confirmed and investigated using Cu-K alpha XRD system using energy voltage of 40 kV and current value of 40 mV.