Determining the behavior of sulfur compounds in controlling preferential weld corrosion in CO2-saturated brine

This paper presents the application of white light interferometry as a new complimentary technique in addition to zero resistance ammetry (ZRA) and the linear polarization resistance (LPR) technique to determine regions of selective attack on 1% Ni welds in CO2-saturated environments. Un-segmented electrodes consisting of a parent material, heat affected zone (HAZ) and weld were subjected to long duration immersion tests and subsequently 3D profiled using white light interferometry to assess which regions (if any) preferentially corroded. In parallel tests, a multi-array electrode was introduced, consisting of a segmented carbon steel parent material, HAZ and 1% Ni weld. Each electrode facilitated electrical connections, allowing each individual segmented to be electrically connected externally via a potentiostat, simulating a fully un-segmented sample. A multi-channel ZRA arrangement was adopted to simultaneously quantify the current transfer between the areas of interest. The intrinsic corrosion rates of the materials were also measured using the linear polarization resistance technique in an effort to determine whether the total corrosion rates of each weld region could be calculated and compared with the profilometry data. One week experiments were conducted in 3 wt.% NaCl uninhibited and inhibited (5 ppm of 2-mercaptoethanol and 5 ppm sodium thiosulphate) environments. The profiles obtained are further supplemented with scanning electron microscopy (SEM) images to assess the surface damage in more detail. The results indicate that white light interferometry can act as a valuable complementary technique by helping understand the extent of localized/pitting corrosion on the material surface. The technique also allows verification of the relative differences in total corrosion rates of the parent, HAZ and weld, thus helping to validate the electrochemical responses obtained. Finally, a case study is presented to demonstrate how the application of white light interferometry was used to determine the propensity for PWC to occur on a North Sea pipeline.