Testing of Carbon Dioxide Corrosion Inhibitor Performance at High Flow Velocities in Jet Impingement Geometry. Effects of Mass Transfer and Flow Forces

The performance of three continuous treatment carbon dioxide (CO₂) corrosion inhibitor products was tested in an impinging jet apparatus (40°C, CO₂ partial pressure 1 bar, pH 5, 3 wt% sodium chloride [NaCl] brine). The objective was to test the performance of the inhibitors at very high flow velocities; the jet flow velocity was varied in the range from 0.5 m/s to 20 m/s. Methods for characterization of mass transfer and calculation of wall shear stress are described. To assess how dissolved oxygen (O₂) biased the test results, the dissolved oxygen (O₂) concentration was intentionally held within two different ranges: 2 ppb to 4 ppb and 8 ppb to 30 ppb. Since flow velocity and O₂ concentration were varied independently, a flow-dependent inhibited corrosion rate observed in the higher O₂ concentration range could be attributed to flow-dependent O₂ corrosion; the corrosion current was close to the O₂ diffusion-limited current. The inhibited corrosion rate was not affected by the flow as long as O₂ reduction did not contribute significantly to the corrosion rate. It could be concluded that flow forces as such did not reduce the inhibitor performance in the tested range of flow velocities. Fundamental concepts of energies involved in fluid flow and thermal motion of molecules were discussed, as well as the properties of the hydrodynamic boundary layer. The discussion and the present experimental findings provide arguments to question whether flow forces can affect CO₂ corrosion inhibitors within practical ranges of flow velocities.