CARBON DIOXIDE CORROSION OF API N-80 STEEL IN TWO-PHASES FLOW SYSTEMS

This paper presents a study on carbon dioxide corrosion of American Petroleum Institute (API) N-80 grade steel at carbon dioxide pressure of 150 psig (11 bars) and temperature of 160 degrees F (71 degrees C) in a 3% NaCl deaerated brine. The pH was maintained constant at 3.4 for some tests and was allowed to change for other tests. A 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 150 ft/s (45 m/s) and superficial liquid velocities up to 8 ft/s (2.4 m/s). A test cell for the flow loop was designed to allow for electrochemical measurements of corrosion rate using removable pipe spools. The loop is capable of producing 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 velocity single phase flows or two-phase flows. Corrosion rates in these tests were independent of whether pH was maintained constant at 3.4, or allowed to increase.