Electrochemical measurements of corrosion in supercritical CO2 environments

Open Access
Author:
Beck, Justin Richard
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 26, 2012
Committee Members:
  • Serguei Lvov, Dissertation Advisor
  • Serguei Lvov, Committee Chair
  • Derek Elsworth, Committee Member
  • Semih Eser, Committee Member
  • Barbara Shaw, Committee Member
Keywords:
  • electrochemical measurements
  • supercritical CO2
  • corrosion
  • ion conductive membrane
Abstract:
An electrochemical system was designed and assembled for performing corrosion measurements in supercritical CO2 environments. Initial testing with a parallel plate electrode probe found that electrochemical measurements could only be performed if an aqueous phase was present between the electrodes to provide adequate ionic conductivity. Additionally, surface analysis of exposure coupons found that corrosion only occurred in localized regions where water had condensed or adsorbed onto the metal surface. These results led to the design of a flush mount probe that utilized an ion conductive membrane to provide electrolyte conductivity between the electrodes. This design allowed consistent measurements to be made without relying on natural wetting of the probe surface, which can be inconsistent and intermittent. Corrosion measurements were performed using linear polarization resistance, electrochemical impedance spectroscopy, and electrochemical frequency modulation. Samples were exposed to scCO2-containing environments until steady-state corrosion behavior was observed. It was observed with the prototype probes that the construction of the probe could have significant impact on the measured solution resistance and overall corrosion rate. Tests performed in supercritical CO2 at 50 °C and 10 MPa with 2000 ppm water vapor found corrosion rates between 10-4 and 10-2 mm y-1 for a black carbon steel sample. However, the charge transfer resistance measured using impedance spectroscopy was not found to vary greatly between probes. Kinetic corrosion rates using this value were between 0.05 and 0.1 mm y-1 for nearly all of the initial probes tested. Tests were also performed in the bulk aqueous phase saturated with scCO2 using probes with and without the ion conductive membrane to determine its impact on the corrosion process. Samples of X65 carbon steel saw overall corrosion rates ranging from 1 to 5 mm y-1, similar to what has been reported literature from weight loss measurements. Greater consistency was observed in the charge transfer resistance, with the kinetic corrosion rate for nearly all probes falling around 5 mm y-1. No major difference in behavior was observed between probes with the membrane coating and those with a bare surface. The final set of tests was performed with X65 carbon steel in scCO2 with 2000 ppm water vapor using probes with various membrane thicknesses. Both the overall corrosion rate and the charge transfer resistance were found to vary with the membrane thickness. Probes with thinner membrane coatings reported lower corrosion rates and larger solution resistance values. It is proposed here that the thickness of the membrane coating may have affected the thickness of the resulting aqueous film, with thicker membranes allowing for the formation of thicker layers of water on the probe surface. A reduction in the aqueous film thickness could have reduced the rate for replenishing carbonic acid in the water phase that is consumed in the corrosion process, which could have changed the solution pH and affected the charge transfer kinetics. Tests at 1000 ppm water saw a decrease in the observed corrosion rate, while tests at 3000 ppm did not differ from measurements at 2000 ppm water.