Prediction of CO2 Corrosion for Welded Joints

Most CO2 prediction models disregard the fact that the welded joints of pipelines are microstructurally different from the rest of the pipe material. The fact that the welded joints consist of different regions of microstructures indicates that they should behave differently due to the microstructure variation which then forms galvanic couplings. The aim of this research was to study the effect of different microstructure to the corrosion rate of welded A106 Grade B pipeline steel in 3% brines saturated with carbon dioxide at one bar pressure, under static condition, at a range of temperatures and also pH. Linear Polarization Resistance (LPR) technique is used to monitor the corrosion rate for each zone of the welds individually. The individual and galvanic corrosion rates for each of the weld region were measured electrochemically using the weld tester based on ZRA concept. The weld metal and HAZ were shown to be more anodic to the parent metal in pH 5 but there were some irregularities at high temperature and higher pH. The coupled HAZ and weld metal were found to be more anodic compared to the coupled HAZ and base metal thus showing the preferential weld corrosion at the weld metal and HAZ region. The galvanic effect between the microstructures is small since there are only slight microstructure differences between the regions. Findings showed that different microstructures of the different regions of the weld influenced the corrosion rates and also the preferential weld corrosion based on the LPR measurement and also galvanic current density measurement. Therefore it is concluded that the prediction models used to predict the corrosion rates of pipe line must consider the different regions of weld in its prediction.