CO2 Corrosion of N-80 Steel at 71°C in a Two-Phase Flow System

Corrosion behavior of an American Petroleum Institute N-80 grade steel in 3% sodium chloride (NaCl) in distilled water was studied under a carbon dioxide (CO2) partial pressure of 1.03 mPa (150 psig) at 71°C (160°F). The pH was maintained constant at 3.4 for some tests but allowed to fluctuate for other tests. A two-phase, flow-loop test facility for studying effects of flow velocity produced two-phase flow conditions of CO2 gas and liquid in stratified, slug, froth, and mist flow patterns. A test cell for the flow loop allowed for corrosion rate measurements by electrochemical and weight-loss methods using removable pipe spools. Iron carbonate (FeCO3) formed only for low-velocity, single-phase flows where pH increased above 5.0. Corrosion rates after scale formation decreased by almost one order of magnitude. No FeCO3 scale formed in tests involving high-velocity, single-phase flows or two-phase flows. Corrosion rates when no scales formed were independent of pH and flow pattern.