A mechanistic analysis was performed on preferential attack nucleating at the weld metal joining flange and pipe adjacent to a choke valve installed at the well head of a CO2 bearing gas condensate well. The main cause of the preferential attack of the weld metal was revealed to exist in the difference of the corrosion rate between the weld metal and the surrounding materials. The electrochemical analysis and the immersion test in the simulating environment for the well head condition indicated that the corrosion rates of the composing materials depended on their cathodic currents in their corrosion systems. The corrosion losses calculated based on polarization resistance indicated a good agreement with that of the immersion test. Based on the results of the analysis the case was concluded to be a typical uneven corrosion.
Keywords: preferential attack, choke valve, carbon dioxide, weld metal, cathodic current, microstructures, uneven corrosion