Mechanisms of CO(2) corrosion related to velocity in two-phase flow systems

This dissertation presents a study on carbon dioxide corrosion at CO{dollar}sb2{dollar} pressure of 150 psig and temperature of 160{dollar}spcirc{dollar}F. Steel corroding in certain CO{dollar}sb2{dollar} environments forms a corrosion product layer (iron carbonate, FeCO{dollar}sb3{dollar}) that can dramatically retard the corrosion rate. Factors affecting the protectiveness of this scale include the metal microstructure and electrolyte chemistry. Factors affecting the formation and/or removal of this scale are electrolyte pH, CO{dollar}sb2{dollar} partial pressure, temperature, flow conditions, and the presence of solid particles in the flow. Both protectiveness and formation are examined in this dissertation by means of experiments performed in an autoclave. Models for FeCO{dollar}sb3{dollar} formation and removal are presented. Formation and adherence of the scale depend on the microstructure of the steel specimens. Removal of scales were studied by means of an erosion apparatus and a two-phase flow loop. The erosion apparatus was designed to shoot slugs of liquid (with or without solid particles entrained) at iron carbonate layers formed on steel specimens. Liquid impingement erosion did not damage the FeCO{dollar}sb3{dollar} scale for the conditions tested. On the other hand, solid particle erosion caused damage to the scale after only a few impacts. The two-phase flow loop test facility for studying effects of flow velocity was designed to produce two-phase flow conditions with superficial gas velocities up to 130 ft/sec and superficial liquid velocities up to 8 ft/sec. A test cell for the flow loop was designed to allow for electrochemical measurements of corrosion rate using removable pipe spools. Performance testing of the flow loop facility verified that it can produce the desired two-phase flow regimes such as stratified, slug, froth, and mist flows. In tests performed in the flow loop, iron carbonate was formed only for low velocity single phase flows in which the pH was allowed to increase to 5.5. Corrosion rates after scale formation decreased by an order of magnitude. No iron carbonate scale was formed in tests involving higher single phase flow velocities or two-phase flow velocities. Corrosion rates in these tests were independent of whether pH was maintained constant at 3.4, or allowed to increase to about 5.5.