Assessment of Preferential Weld Corrosion of Carbon Steel Pipework in CO2-Saturated Flow-Induced Corrosion Environments

Abstract

Preferential weld corrosion (PWC) has posed a problem to the oil and gas industry for a number of years. The general consensus from authors is that environmental effects take precedence over the weld chemical composition and microstructure. Therefore, the primary approach is to ensure the correct corrosion inhibitor selection and application to prevent PWC. Although there has been considerable focus directed toward the effect of flow rate on PWC in inhibited carbon dioxide (CO2)-saturated environments, the consideration of a higher, localized turbulence over the weld material and the implications this has on PWC appears minimal. This paper considers this effect by reviewing the performance of a commercially available, film-forming oilfield corrosion inhibitor using a submerged impinging jet (SIJ). The dual nozzle arrangement of the SIJ allowed a carbon steel parent metal and a 1% Ni, 0.25% Mo weld material to be subjected to the same brine chemistry, but different flow conditions. The velocity over the parent metal was kept constant at 7 m/s while the weld material was subjected to velocities of 7, 8.8, and 11.4 m/s to simulate different severities of turbulence. Galvanic current and linear polarization resistance (LPR) measurements were used to assess the weld resistance to PWC. Although the weld was shown to be cathodic to the parent metal, increasing velocity and turbulence at the weld relative to the parent metal was found to reduce the galvanic current, suggesting increased susceptibility of the weld to corrosion. The addition of inhibitor at 100 ppm accentuated this effect and led to a reversal of the galvanic current when the relative velocities were 7 m/s and 11.4 m/s for the parent metal and weld, respectively. The paper discusses the power of this galvanic technique for prediction of the conditions conducive to accentuating PWC.