CO Corrosion of API N-80 Steel in Two-Phase Flow Systems
Authors: Palacios C.A. , Shadley J.R. 
Published: October 2008

This paper presents a study on carbon dioxide corrosion of American Petroleum Institute (API) N-80 grade steel at CO2 pressure of 150 psig (11 bars) and temperature of 160 °F (71°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.

INTRODUCTION

Velocity enhanced corrosion problems in oil and gas production equipment are common. These often occur when produced fluids are accompanied by carbon dioxide (sweet corrosion) and/or hydrogen sulfide (sour corrosion). In recent years the presence of carbon dioxide in the produced fluid is encountered more frequently due to the use of enhanced oil recovery techniques involving C02 injection into reservoirs and also due to the occurrence of sweet gas production from deeper wells.

Carbon dioxide dissolves in the presence of a water phase forming a weak acid (carbonic acid) which ionizes, thus reducing the pH and corroding carbon steel pipes. As the carbon steel corrodes, the steel forms a corrosion product scale (ferrous carbonate) which provides a degree of protection of the steel from further corrosion. The protectiveness of the scale depends on environmental factors and characteristics of the steel. Environmental factors are those factors that define the medium of the corroding steel such as temperature, carbon dioxide partial pressure, solution chemistry, fluid velocity, single-phase or multi-phase flows, pipe geometry, and solution pH [1-7]. Steel characteristics include chemical composition and heat treatment, which determine the microstructure of the steel [3, 8, 9].

Velocity enhanced carbon dioxide corrosion occurs by accelerating the process of dissolution of a protective film such as the corrosion product scale, or by erosion of the film by the mechanical forces impressed on the film by the moving fluid. Once the protective film has been removed, the protectiveness of the scale is lost, and the steel pipe again corrodes freely. Scale growth and scale removal affect the rate at which the steel corrodes. Effects of flow velocity on corrosion are more severe and most frequently seen in two-phase or multi-phase flow systems.