Effect of surface roughness and flow on corrosion inhibition of coiled tubing steel under matrix acidizing conditions

摘要

The effect of surface roughness on the corrosion behavior of low carbon steel in 4M hydrochloric acid at 80°C in the presence of corrosion inhibitor molecules has been examined using rotating cylinder electrodes under laminar (5 rpm) and turbulent (6000 rpm) flowing conditions. Glass bead blasted (BB) metal coupons as well as samples with P240 or P1200 grit surface finishes were studied. Rotating cylinder electrodes were used to simulate the effect of flow velocity on pipe corrosion. The corrosion rate and cathodic current density were determined by potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy. A commercially available corrosion inhibitor "A" comprising a mixture of polymerizable molecules (acetylenic alcohols), an oily phase and nonionic surfactants was used. Under laminar and turbulent flow conditions, the inhibited corrosion rate decreases in the following order: Bead blasted ＞ P240 ＞ P1200. The lowest corrosion rate is established on smooth surfaces (Ra =0.14 μm and 0.63 μm) and the highest rate on the rougher surface (Ra=5.26 μm). Turbulent flow has different effects on smooth and rough surfaces. On smooth surfaces (Ra ≤0.63 μm), the corrosion rate slightly increases with flow velocity which may be attributed to a decrease in the density of hydrogen bubbles formed on the surface of the metal and corrosion occurs in the diffusion sublayer. However, when the surface roughness (Ra=5.26 μm) is sufficient to interfere with the diffusion boundary layer (～2 μm at 6000 rpm), turbulent eddies enhance the flow-induced erosion of the inhibitor film and an increase in flow velocity results in a stronger increase in corrosion rate. For both flow regimes, the effect of exposure time on corrosion rate, open-circuit corrosion potential and the value of electrochemical double layer capacitance were studied.